Synthesis and DFT computations on structural, electronic and vibrational spectra, RDG analysis and molecular docking of novel Anti COVID-19 molecule 3, 5 Dimethyl Pyrazolium 3, 5 Dichloro Salicylate

Abstract

Prospective Anti-viral compound 3, 5 Dimethyl Pyrazolium 3, 5 Dichloro Salicylate (DPDS) was synthesized and characterized using FT-IR, FT-Raman, UV and NMR spectra. To escort the experimental results, computational methods were performed using B3LYP with 6-311G (d, p) basis set expending Gaussian09w package to attain geometry of the molecule. Vibrational assignments for all the vibrational modes have been made of PED results obtained from SQM method. On contrary, FMO analysis, global chemical reactivity descriptors, Aromaticity and Natural charge analysis were studied. Molecular stability and bond strength have been inquired by executing NBO analysis. Topological features of DPDS were intended by MEP, ELF and LOL maps. UV–vis spectrum was predicted by TD-DFT method in gaseous phase and compared with the experimental spectrum for displaying the involved electronic transitions in the compound. The interactions within the DPDS molecule were investigated via RDG analysis. Molecular docking was performed with SARS-CoV-2 proteins and docking parameters were obtained. Drug likeness was carried out based on Lipinski's rule of five and the ADMET factors were also predicted.

Graphical abstract

1. Introduction

In recent times, viral infections are reflected as one of the prime extortions to social existence and health global. Controlling of such viral infections epitomizes a rich field of scientific research because of viruses’ mutability that springs new drug-resistant strains. Pyrazole derivatives have attracted more attention due to their expediency in the pharmaceutical research. Naturally acquired pyrazoles exhibits organic activities and used as efficient chemotherapeutics. Among azoles, pyrazoles are rarely found in nature probably due to their difficulty in formation of N-N bond by living organisms [1]. Pyrazole derivatives are also important in treating muscle pain, fever and arthritis but the recent literature shows these derivatives are not only analgesic and anti-inflammatory but also parades anti-viral, anti-bacterial, anti-tumor and anti-microbial activities.The competency to kill infectious agents and preventing their proliferation inside the living organisms and environment is the anti-microbial properties. Thus, multitude of compounds is investigated for their potential applications as therapeutic agents in treating infectious diseases especially those caused by multi-resistant virus and bacteria [2]. Salicylic acid is an organic acid used in pharmaceutical laboratories and is the formative material for making drugs like aspirin [3] and is the natural and safest chemical used for postharvest quality maintenance in the field of horticulture [4]. 3, 5-dichloro salicylic acid has extensive applications and has been recognized as a potential enzyme inhibitor and burgeoning usage in cosmetic industries [5]. Effect of pyrazole is studied and reveals that the pyrazolium anion is less reactive towards nucleophiles and is more to electrophiles whereas it has broad spectrum of biological activity comprising anti-viral, anti-bacterial and anti-tumor to engender novel primes possessing pyrazole nucleus with high efficacy. Since pyrazole a heterocyclic moiety, it epitomizes the fundamental configuration for the number of drugs and establishes a pertinent synthetic path in curative diligence [6].

Literature screening divulges that the crystal structure of 3, 5-Dimethyl Pyrazolium 3, 5-Dichloro Salicylate (DPDS) has not been reported so far and this meagerness observed in the literature stimulated us to make vibrational spectroscopic research based on the molecule, to give a precise consignment of the fundamental bands in FT-IR and FT-Raman spectra. Geometry of the organic structures requires diverse techniques and approaches en route for their strength in gaseous state. Redistribution of electron density was designed using NBO (Natural Bond Orbital) analysis. MO (Molecular Orbital) analysis was accomplished to intend the biological activity of the molecule and the impact of transition of electrons from π →π* were studied using UV-vis (Ultraviolet Visible) spectrum. Further Aromaticity and Natural Charge Analysis were also counted in order to determine the aromatic character and charge distribution of the molecule correspondingly. Distribution of electrons and reactive sites on the surface were analyzed using ESP (Electrostatic potential), ELF (Electron localization function) and LOL (Localized orbital locator). Repulsive, attractive, and van der Waals strong and weak interactions in DPDS were investigated by RDG (Reduced Density Gradient) analysis. Drug likeness and molecular docking methodology were empowered to form drug potential and bioactivity of DPDS. Insilico ADMET (Absorption, Distribution, Metabolism, Excretion and Toxicology) analysis was also anticipated to crisscross whether the compound is a vigorous drug to treat SARS-CoV-2.

2. Experimental details

2.1. Synthesis

Slow evaporation- solution growth technique was used to grow single crystal. 3, 5 –Dichloro Salicylic acid (DS) and 3, 5-Dimethyl Pyrazole (DP) were taken in 1:1 stoichiometric proportions and dissolved homogeneously using methanol as a solvent. Their reaction mixture was thoroughly mixed together using mechanical stirrer up to 3 h at room temperature to get a clear solution. The synthetic scheme of DPDS was depicted in Scheme 1 . This solution was filtered through a quantitative filter paper (Whatman no.40) and filtrate was kept aside without any disturbance for the growth of crystal in a dust free environment at ambient temperature. Well defined, transparent crystals were collected at the end of 6th day and the collected crystals were recrystallized using dry methanol to get good quality crystals.

Scheme 1.

Synthetic scheme for DPDS.

2.2. Characterization techniques

FT-IR spectrum of DPDS was recorded on a Perkin Elmer FT-IR 8000 spectrophotometer in the range of 4000–400 cm⁻¹using KBr pellet technique at room temperature. FT-Raman spectrum has been recorded on a Bruker RFS 27: Standalone FT-Raman Spectrometer with Nd: YAG laser source at 1064 nm and resolution 2 cm⁻¹ in the region 4000–50 cm⁻¹. Electronic absorption spectrum was measured in methanol using JASCO UV-Vis spectrophotometer in the range of 200–800 nm.¹³Cand ¹H NMR spectra were recorded employing a Bruker AV III 400 MHz spectrometer in deuterated dimethyl sulphoxide (DMSO) as solvent using TMS as an internal standard.

3. Computational details

Theoretical calculations for DPDS were carried out with the aid of Gaussian09w package [7] using B3LYP method in conjunction with 6–311G (d, p) basis set. Natural bond orbital (NBO) analysis [8] was also achieved and the molecular visualization was ended by the Gauss view6 program [9]. MOLVIB programming was preferred for the vibrational spectral assignments by utilizing potential energy distribution (PED) analysis [10]. MO analysis and Molecular electrostatic potential (MEP) mapping were calculated using the same B3LYP level of theory. Auto Dock Suite 4.2.1 was used to find the minimum binding energy, inhibition constant and various parameters of the ligand-protein docking interactions [11].The ligand-protein binding sites have been visualized using PYMOL graphic software [12]. ELF, LOL and the RDG and sign (λ2)ρ functions were computed using Multiwfn [13] and the RDG scatter graph was drawn with the VMD (Visual Molecular Dynamics) program [14].

4. Results and discussions

4.1. Geometry optimization

Optimized molecular structures of DS, DP & DPDS with atomic symbols and labels are shown in Fig. 1, respectively.

Fig. 1.

Optimized Molecular Structures of DS, DP and DPDS.

The self-consistent field energy of DPDS -1720.34 a.u is combined with the energy of DS and DP refers to -1415.40 a.u and -304.92 a.u, respectively. BSSE and counterpoise corrected energy for DPDS have been calculated as 0.004 a.u and -1720.34 a.u, respectively. Wavenumbers in the solid phase must be higher than the gaseous phase due to the isolation of molecule in the solid state. Optimized Bond length with bond angles of DPDS are tabulated in Table 1 and dihedral angles of DPDS are tabulated in Table S1 which are compared with the XRD data of 3,5 Dichlorosalicylic acid [16] and Dichlorobis(3,5-dimethylpyrazole)copper(II) [18] besides their RMS values are calculated.

Table 1.

Comparison of optimized bond length and bond angle of DPDS with the XRD data of 3,5 Dichlorosalicylic acid and Dichlorobis(3,5-dimethylpyrazole)copper(II).

Bond length	Theoretical value (Å)		Expt (Å)	RMS (Å)	Bond Angle	Theoretical value (°)		Expt (°)	RMS (°)
	DPDS	DS/DP				DPDS	DS/DP
C1-C2	1.401	1.409	1.393	0.00006	C2-C1-C6	120.58	120.58	120.22	0.1296
C1-C6	1.415	1.415	1.399	0.00025	C2-C1- C13	120.70	120.40	119.83	0.1296
C1-C13	1.482	1.485	1.474	0.00006	C6-C1- C13	118.70	118.66	119.93	1.5129
C2-C3	1.380	1.381	1.376	0.00001	C1-C2-C3	119.77	119.69	119.82	1.5129
C2-H8	1.080	1.055	0.930	0.02250	C1-C2- H8	119.18	119.13	120.11	0.8649
C3-C4	1.395	1.394	1.376	0.00036	C3-C2- H8	121.04	121.17	120.07	0.8649
C3-Cl9	1.760	1.745	1.733	0.00072	C2-C3-C4	120.86	120.80	121.12	0.0676
C4-C5	1.384	1.386	1.380	0.00001	C2-C3-Cl9	120.03	120.00	119.01	0.0676
C4-H7	1.081	1.088	0.931	0.02250	C4-C3-Cl9	119.10	119.18	119.87	0.5929
C5-C6	1.408	1.398	1.390	0.00032	C3-C4-C5	119.52	119.51	118.65	0.5929
C5-Cl10	1.748	1.745	1.737	0.00012	C3-C4- H7	120.61	120.63	120.65	0.0016
C6-O11	1.332	1.331	1.342	0.00010	C5-C4-H7	119.85	119.81	120.7	0.0016
O11-H12	0.989	0.982	0.921	0.00462	C4-C5-C6	121.47	121.82	122.2	0.5329
H12-O14	1.689	1.689	1.912	0.04972	C4-C5-Cl10	119.55	119.54	119.43	0.5329
C13-O14	1.232	1.239	1.232	0.00000	C6-C5-Cl10	118.97	118.13	118.37	0.3600
C13-O15	1.316	1.315	1.305	0.00012	C1-C6-C5	117.77	117.62	117.98	0.3600
O15…H21	1.013	-	0.820	0.03724	C1-C6- O11	123.08	122.46	123.85	0.5929
C16-C17	1.381	1.380	1.390	0.00008	C5-C6- O11	119.14	118.10	118.16	0.5929
C16-N20	1.35	1.350	1.368	0.00048	C6- O11-H12	106.51	106.91	106.42	0.0081
C16-C23	1.494	1.494	1.496	0.00000	C1- C13-O14	122.03	122.42	122.3	0.0081
C17-C18	1.415	1.419	1.352	0.00396	C1- C13-O15	114.82	114.82	114.7	0.0144
C17-H24	1.078	1.078	0.931	0.02160	O14- C13-O15	123.13	123.30	123	0.0144
C18-N19	1.331	1.329	1.347	0.00003	C13-O15…H21	111.75	-	109.54	4.8841
C18-C25	1.496	1.497	1.501	0.00002	C17- C16-N20	105.56	105.59	106.28	0.5184
N19-N20	1.355	1.354	1.342	0.00016	C17- C16-N23	131.75	131.85	130.75	1.0000
N19-H21	1.690	-	-	-	N20- C16-N23	122.68	122.65	121.94	1.0000
N20-H22	1.007	1.007	2.010	1.00600	C16- C17-C18	106.17	106.17	107.01	0.7056
C23-H26	1.094	1.094	0.960	0.01795	C16- C17-H24	126.56	126.87	126.45	0.7056
C23-H27	1.094	1.090	0.959	0.01822	C18- C17-H24	127.26	127.68	126.53	0.5329
C23-H28	1.089	1.084	0.960	0.01664	C17-C18-N19	110.00	110.58	110.91	0.5329
C25-H29	1.093	1.093	0.959	0.01795	C17-C18-C25	128.70	128.73	128.31	0.1521
C25-H30	1.089	1.084	0.960	0.01664	N19- C18-C25	121.28	121.96	121.78	0.1521
C25-H31	1.093	1.093	0.959	0.01795	C18-N19-N20	105.61	105.60	105.29	0.1024
					C18-N19-H21	134.62	134.53	134.85	0.1024
					N20-N19-H21	119.75	119.65	-	-
					C16-N20-N19	112.63	112.68	111.5	1.2769
					C16-N20-H22	127.94	127.96	127.81	0.0169
					N19-N20-H22	119.41	119.60	120.57	0.0169
					O15…H21-N19	168.23	-	-	-
					C16-C23-H26	111.57	111.61	110.5	1.1449
					C16-C23-H27	111.57	111.71	109.45	0.0144
					C16-C23-H28	109.97	109.71	109.55	0.0144
					H26-C23-H27	107.85	107.02	109.51	0.1156
					H26-C23-H28	107.85	107.70	109.4	0.1156
					H27-C23-H28	107.85	107.75	109.4	0.2025
					C18-C25-H29	110.72	110.75	109.36	0.2025
					C18-C25-H30	110.49	111.40	109.38	1.2321
					C18-C25-H31	110.73	110.66	109.4	1.2321
					H29-C25-H30	108.69	108.40	109.58	0.7921
					H29-C25-H31	107.39	107.41	109.63	0.7921
					H30-C25-H31	108.69	108.41	109.47	0.0484

Deliberated bond length O₁₅…H₂₁ is feeble than other wave numbers, indicates the formation of intermolecular hydrogen bonding O₁₅…H₂₁-N₂₁ with improved stabilization. Hydrogen bond length O₁₅…H₂₁ is obtained as 1.013 Å which is emphatically deficient than that of van der Waals radii (2.75 Å) [15] and is extraordinary compared to the experimental value 0.820 Å [16] with a RMS value surpassed around 0.03724 Å at very high resolution. Also geometrical bond angles O₁₅...H₂₁-N₁₉ spectacles extreme of 168.23 ° which is terribly amplified from the trigonal angle 120 ° owed to intermolecular interactions between pyrazole and benzene ring moieties. These results boot the contingency of intermolecular hydrogen bond formation which gets a closer comprehension and enterprise the molecules with enhanced biological contour.

Bond angles C₁-C₆-O₁₁ (123.08 °) and C₅-C₆-O₁₁ (119.14 °) with experimental values 123.85 ° and 118.16 °, respectively are diverged from the intended trigonal angle owing to the influence of hydroxyl group in the phenyl ring. This divergence scanted the bond length O₁₁-H₁₂ (0.9898 Å) and bond angle C₆- O₁₁-H₁₂ (106.51 °) which are deflated to radical values due to the deprotonation of salicylic acid to form hydrogen bond between oxygen in the carboxylate anion and hydrogen in the alcohol group, thereby forming intramolecular interaction H₁₂…O₁₄ with bond length 1.689 Å with a RMS value 0.04972 Å at high resolution stimulates the bioactive nature of DPDS molecule [17]. RMS value for the bond angle C₁₃-O₁₅…H₂₁ gets increased to 4.8841 ° due to the formation of salicylate anion at very low resolution leads to the inter-molecular interaction.

Adjoining these interactions C₃-Cl₉ (1.760 Å) & C₅-Cl₁₀ (1.748 Å) holds the extreme bond length ensuring their experimental values 1.733 Å & 1.737 Å, respectively which are in line for the virtue of electron withdrawing nature of chlorine which increases the double bond character by redistributing π electrons in maximum shared orbitals, thus deteriorate the bond length of chlorine as compared to other bond lengths. Bond length assigned to C₁₈-C₂₅ (1.4962 Å) & C₁₆-C₂₃ (1.4941 Å) remains shrinking owing to the resonance of dimethyl group annexed to the pyrazole ring. Bond length for C₁₈-N₁₉ (1.3319 Å) is subordinate to the bond length C₁₆-N₂₀ (1.3573 Å) which designates the effect of double bond in the pyrazole ring. RMS value increases to 1.0 Å amidst N₂₀-H₂₂ with bond distance 1.007 Å and C₁₆-N₂₀-N₁₉ with bond angle 112.63 ° obligating experimental value 2.010 Å and 111.5 °, respectively [18] which is hefty and random since the diffraction terms ought to slight or no impact on the geometry at low resolution [19].

Bond lengths C₁₈-N₁₉ (1.331 Å) and C₁₆-N₂₀ (1.357 Å) were stretched while equated to their experimental values 1.347 Å and 1.368 Å, respectively designates substantial electron delocalization in the pyrazole ring structure. Bond angles H₂₁-N₁₉-C₁₈ (134.62 °) and H₂₁-N₁₉-N₂₀ (119.75 °) are exaggerated distortedly in line for the incidence of nitrogen in the pyrazole ring whereas scale down in bond angles C₁₇- C₁₆-N₂₀ (105.56 °) and C₁₈-N₁₉-N₂₀ (105.56 °) stand owed to the steric interaction in virtue of intermolecular hydrogen bonding interactions.

Dihedral angle is an imperative constraint to unfold the conformation of voluminous organic and bioorganic molecules. Furthermost of the dihedral angles of phenyl ring are about virtually -179.99 ° and 0 ° point towards structure of salicylate ion is planar. Carboxylate anion devoted to the phenyl ring has antiperiplanar conformations which are flagged by the torsional angles C₂-C₁-C₁₃- O ₁₄ (-179.98 °) & C₆-C₁- C₁₃-O ₁₅ (-179.98 °). The dimethyl groups attached to the pyrazole ring are found to be out of plane are indicated by the torsional angles C₁₇-C₁₆-C₂₃-H₂₆ (-119.97 °), C₁₇-C₁₆-C₂₃-H₂₇ (-119.31 °), N₂₀-C₁₆-C₂₃-H₂₆ (-60.06 °), N₂₀-C₁₆-C₂₃-H₂₇ (60.65 °), C₁₇-C₁₈-C₂₅-H₂₉ (59.32 °), C₁₇-C₁₈-C₂₅-H₃₁ (-59.68 °), N₁₉-C₁₈-C₂₅-H₂₉ (-120.68 °) and N₁₉-C₁₈-C₂₅-H₃₁ (120.30 °) [20].

4.2. NBO profile

NBO analysis contributes the proficient schemes to examine the chemical characteristics of the bonding and different properties like intra and intermolecular charge transfer, basicity, stability, reactivity and correlation between donor and acceptor. The interactions between filled and vacant orbitals are tabulated in Table 2 .

Table 2.

Second order perturbation theory analysis of Fock matrix.

Donor (i)	ED(i) (e)	Acceptor (j)	ED(j) (e)	E(2)a (kcal/mol)	E(j)-E(i)b (a.u)	F(ij)c (a.u)
σ(C1 – C2)	1.96303	σ*(C1 – C13)	0.06263	2.25	1.12	0.045
		σ*(C3 – Cl9)	0.03318	5.60	0.84	0.061
LP(1)O14	1.95987	σ *(O11 - H12)	0.05796	4.30	1.10	0.062
π (C1 – C2)	1.65916	π*(C5 – C6)	0.44564	24.93	0.26	0.074
σ(C1 – C6)	1.96778	σ*(C1 – C2)	0.02188	4.29	1.26	0.066
π(C5 – C6)	1.60297	π*(C3 – C4)	0.42826	27.78	0.28	0.080
σ(O11 – H12)	1.98573	σ*(C5 – C6)	0.04282	5.82	1.25	0.077
LP (1)Cl9	1.99215	σ*(C3 – C4)	0.03087	1.51	1.47	0.042
LP (3)Cl9	1.93777	π*(C3 – C4)	0.42826	11.21	0.33	0.060
LP (2)Cl10	1.96664	σ*(C4 – C5)	0.02608	4.02	0.88	0.053
LP (3)Cl10	1.92703	π*(C5 – C6)	0.44564	11.60	0.32	0.060
LP (1)O11	1.97308	σ*(C1 – C6)	0.03779	7.79	1.10	0.083
LP (2) O11	1.81257	π*(C5 – C6)	0.44564	40.21	0.31	0.107
LP (2) O14	1.85119	σ*(C13 – O15)	0.06444	24.60	0.72	0.121
LP (1) O15	1.96086	σ*(C13 – O14)	0.02806	8.82	1.11	0.089
LP (2) O15	1.74384	π*(C13 – O14)	0.33800	64.50	0.30	0.126
σ*(C13 –O15)	0.02806	LP (1) H21	0.56515	13.93	1.16	0.133
LP (3) O15	1.65021	LP*(1) H21	0.56515	437.42	0.67	0.511
π* (C18 - N19)	1.90252	π* (C16 - C17)	0.05796	89.77	0.03	0.071
LP (1) N19	1.73150	LP*(1) H21	0.56515	286.67	0.61	0.404

^aE(2) means energy of hyper conjugative interaction (stabilization energy).

^bE(j) - E(i) is the energy difference between donor i and acceptor j.

^cF(i,j) is the Fock matrix element between i and j NBO orbital's.

Scope of hyper conjugation is governed by the contributing capability of Lewis type orbitals and the acquiescent capability of non-Lewis type orbitals which is measured in terms of interaction energy E(2). Inordinate hyper conjugative interaction energy advance the level of delocalization which consequently grander the stability of molecular system [21]. Overlapping of orbitals σ(C₁ – C₂) to σ*(C₁ – C₁₃) and σ*(C₃ – Cl₉) leads to the interaction energy of 2.25 kcal/mol and 5.60 kcal/mol, respectively. Utmost pivotal interactions in DPDS is stuck between the lone pairs LP(3)O₁₅ and LP*(1) H₂₁ ensuing very high stabilization energy 437.42 kcal/mol which forms intermolecular hydrogen bonding O₁₅…H₂₁-N₁₉ resulting in intermolecular charge transfer. This loftier energy shows that the hyper conjugative interaction transpires amongst the electron donating and the acceptor group which enriches the bioactivity of the DPDS molecule [22].

Anti-bonding orbital σ*(C₁₃ –O₁₅) overlaps with the lone pair orbital LP (1) H₂₁ outcomes with a negative hyper conjugation with a stabilization energy 13.93 kcal/mol and is illuminated using inductive effect of oxygen atom. Interaction between two lone pair electrons LP (1) N₁₉ and unfilled p-orbital LP*(1) H₂₁ grades to stronger stabilization with an interaction energy 286.67 kcal/mol and leads to reduction in hybridization and the non-periodic power of these bonds ensure inferences for the exposure of molecules [23]. Orbital overlapping between lone pair LP (1) O₁₄ to anti-bonding orbital σ *(O₁₁ - H₁₂) results in the stabilization energy of 4.30 kcal/mol triggering intra-molecular charge transfer and leads to hyper conjugative intra-molecular hydrogen bonding interaction [24]. Rehybridization plays a dominant role in weakening and elongation of hydrogen bond observed in Table 3 . NBO analysis of DPDS in comparison with DS and DP clearly shows the sign for the formation of strong H-bonded interactions.

Table 3.

Composition of H-bonded NBOs in terms of natural atomic hybrids.

Bond (A-B)	DS/DP							DPDS
	EDA %	EDB %	spn	A		B		EDA %	EDB %	spn	A		B
				s%	p%	s%	p%				s%	p%	s%	p%
C13-O15	66.79	33.21	sp2.41	25.08	74.72	35.03	64.90	66.74	33.26	sp2.16	29.31	70.49	34.35	65.57
C18-N19	59.35	40.65	sp1.97	29.61	70.28	38.85	61.06	61.19	38.81	sp1.94	28.75	71.15	41.25	58.69
N19- N20	45.30	54.70	sp2.86	30.59	69.33	22.39	77.46	47.24	52.76	sp2.85	22.96	76.92	29.83	70.09

The influence of rehybridization results in destructive effects, leads to contraction and strengthening of C₁₃-O₁₅ bond by the impact of s-character of the hybrid orbital decreases from sp^2.41 to sp^2.16. Though the contraction and elongation of bonds is due to the effect of rehybridization and hyper conjugation, rehybridization dominates and overshadows the latter. This is well reflected in the geometry as bond C₁₃-O₁₅ which contracts by 0.25 Å with respect to DS and manifests the delocalization of electron density. The s-character of C₁₈-N₁₉ hybrid orbital falls from sp^1.97 to sp^1.96 hints to the contraction of C₁₈-N₁₉ bond due to the reduction in s-character of carbon and polarization of the C-N bond in the progression of N-H…O inter-molecular hydrogen bond formation. Polarization promulgates over bonds leads to rehybridization with hybrid orbital declines from sp^2.86 to sp^2.85 and p-character increases to 69.33% with s-character 30.59% in the N-N hybrid bond. Because the total s- and p-characters at every nitrogen atom are sealed, this shrinkage in the s-character leads to a reflex upsurge in the other hybrid orbitals at N including the N-H bond. This escalation leads to inter-molecular hydrogen bonding by shortening and strengthening N-H bonds.

4.3. Electronic properties

4.3.1. Frontier molecular orbitals

Highest occupied molecular orbitals (HOMO) and Lowest unoccupied molecular orbitals (LUMO) are the Frontier molecular orbitals which are vibrant to depict kinetic stability and chemical reactivity by their energy gap. The positive and negative has shown in red and green congealed segments, respectively. HOMO is related to the more reactive molecule with electrophiles whereas the lower energy profile related to less reactive molecule with nucleophiles [25]. A molecule with small frontier orbital gap is more polarisable and is generally associated with high chemical reactivity [26]. Ordinarily the conjugated molecules are characterised by less H→L separation, which is significant to the gradation of intra-molecular charge relocation from the end-capping electron-donor groups to the proficient electron-acceptor group [27].

Fig. 2 illustrates the Frontier Molecular Orbital transitions within the molecule. It is inferred from the figure that HOMO localised over the phenyl ring and hydroxyl group and LUMO localised over the phenyl ring and carboxylic acid group besides DS. HOMO→LUMO transition implies that electron density has spread over and charge transfer occurs from hydroxyl group to the carboxylic acid group, thereby no charge interactions in DP moiety. Calculated HOMO and LUMO energies are -9.12983 eV and -4.89177 Ev, respectively while the energy gap serves as a stability index. In fact, a small HOMO-LUMO gap implies high molecular stability in the sense of virtuous reactivity in chemical reactions [28]. The figured band gap energy is 4.23806 eV which embodies the low energy electronic transitions inherently belongs to π→π* transitions and is chemically more reactive [29] affords biological delineation with a trifling band gap. The overlying orbital loops has occurred in the benzene ring of DS moiety and the HOMO LUMO transition endorses the revelation of resonance expedited with hydrogen bonding in course of electron density transfer. HOMO-1→ LUMO+1 and HOMO-2→LUMO+2 transitions were also implemented with their band gap energies 5.38738 eV and 5.84955 eV, respectively.

Fig. 2.

Plot of Frontier Molecular Orbitals of DPDS.

4.3.2. Global reactivity descriptors

Molecular descriptors of global reactivity act as mediators between the kinetic stability and chemical reactivity of the molecule and were calculated from Frontal Orbital energy using Koopman's theorem [30].The global descriptors are Ionisation Potential (IP), Electron Affinity (EA), Electronegativity (χ), Global hardness (ɳ), Softness (σ), Chemical Potential (µ) and Global Electrophilicity index (ω) [31] can be calculated using B3LYP/6-311G (d, p) and listed in the Table S2.

Following Parr and Pearson [32] Chemical potential describes the emerging inclination of electron from a firm system and is -7.01080 eV which notifies the stability does not disintegrate impulsively into its fundamental at its least possible assessment. The hardness insinuates the resistance towards the dislocation of electron cloud in the chemical systems under slight perturbations that stumble during chemical course of action. DPDS molecule is more reactive and highly polarizable due to lanky hardness of 7.01080 eV and stumpy softness value 0.14263 eV [33]. Global electrophilicity index is the better descriptor of global chemical reactivity which is 3.50539 eV describes the biological activity of DPDS compound [34] for DPDS and measures the equilibrium in energy when the system acquires additional electronic charge from the entourage. It also encompasses the dexterity of an electrophile to acquire additional electronic charge and the resistance of the system to swap electronic charge with the entourage.

4.4. UV-visible spectral analysis

UV absorption spectra for the optimized molecule DPDS were calculated with the aid of computational method to determine the low-lying excited states of DPDS theoretically [35] and recorded in gas phase. The electronic transitions confirm the absorption peaks experimentally and theoretically are shown in Fig 3 . NBO analysis indicates that molecular orbitals are generally composed of π and σ atomic orbitals but in UV-vis region molecules allows strong π →π* transitions with the high extinction coefficients and higher the coefficients, more wavelength is absorbed [36]. The absorption spectra exhibits an intense peaks at 270 nm which affirms the protonation of nitrogen in the pyrazole ring due to π→π* transitions.

Fig. 3.

UV spectra of DPDS.

Excitation energies, absorbance and oscillator strength (f) for DPDS molecule were tabularized in Table 4 using Gauss Sum [37]. Absorption peak (λ_max) in the UV-vis spectrum predicts electronic transition supreme at 265 nm with an oscillator strength f = 0.0576 with a major contribution of 86% from HOMO to LUMO shows good agreement with the experimental data at 270 nm. Other wavelengths obtained via computational method are 235.88, 232.97, 229.33, 225.12 and 219.05 nm (in gas). In view of calculated absorption spectra the maximal oscillator strength is 0.0722 at the wavelength 225 nm with a major contribution of about 52% from HOMO to LUMO+1 transitions. Minimal oscillator strength is zero at wavelength 235 nm with a major contribution 60% from HOMO to LUMO+4 electronic transition.

Table 4.

UV-vis excitation energy and oscillator strength for DPDS.

Experimental		Energy	Theoretical		Oscillator strength	Symmetry	Major Contributions
λmax (nm)	Band gap (eV)		λmax (nm)	Band gap (eV)
270	4.23806	37720	265	4.67924	0.0576	Singlet-A	H-1→L+1 (12%), HOMO→LUMO (86%)
		42394	235	5.27659	0.0000	Singlet-A	HOMO→L+2 (25%), HOMO→L+3 (12%), HOMO→L+4 (60%)
		42922	232	5.34482	0.0008	Singlet-A	HOMO→L+3 (83%), HOMO→L+4 (12%)
		43603	229	5.41484	0.0060	Singlet-A	HOMO→L+1 (16%), HOMO→L+2 (56%), HOMO→L+4 (26%)
		44420	225	5.51111	0.0722	Singlet-A	H-1→LUMO (30%), HOMO→L+1 (52%), HOMO→L+2 (15%)
		45650	219	5.66210	0.0014	Singlet-A	H-4→L+3 (31%), H-1→L+3 (63%)

4.5. Vibrational spectral analysis

Calculated frequencies of molecular structures are paralleled with the observed frequencies by NCA based on SQMFF (Scaled Quantum Mechanical force field) calculations recommended by Rauhat and Pulay [38]. Root mean square (RMS) error of scaled wavenumber is 8 cm⁻¹ and this scaling factor is used to correct the anharmonicity and neglected part of electron correlation [39]. The detailed assignments stipulated by the potential Energy Distribution (PED) are depicted in Table S3 and the experimental and theoretical FT-IR & FT-Raman spectra are shown in the Figs. 4 and 5 .

Fig. 4.

FT-IR spectra of DPDS.

Fig. 5.

FT-Raman spectra of DPDS.

4.5.1. C-H vibration

Aromatic hetero structure offers C-H stretching vibrations in the region 3100–3000 cm⁻¹ [40]. In DPDS, the C-H stretching vibration emerges at the medium range 3067 cm⁻¹ in the Raman spectrum where it shows the stretching mode flashes with strong Raman intensity and 99% PED contribution. Vibrations assigned to aromatic C-H stretching in the region 3076 cm⁻¹ predicted theoretically are in good alliance with the experimental assignment 3067 cm⁻¹. DPDS molecule possesses a dimethyl group in the pyrazole ring and the CH stretching expose at lower frequencies for about 3000–2800 cm⁻¹ than those of the aromatic ring are typically downshifted owing to electronic possessions [41]. CH₃ in-plane and out-of-plane stretching displays around medium region 2960 cm⁻¹ in the FT-IR and at the weak 2840 cm⁻¹ in the Raman region. Scaled frequencies for the in-plane and the out-of-plane stretching occur at 2960 & 2955 cm^−1, respectively and are correlated with the experimental values.

4.5.2. C – C and C=C vibrations

C-C and C=C stretching modes are expected in the range from 1650–1200 cm⁻¹ [42]. The strong bands at 1586 and 1448 cm⁻¹ belongs to tetra-substituted benzene ring which reveals the hyper conjugative interaction between the rings with the C – C stretching. The weak band 1220 cm⁻¹ in the FT-Raman region shows C-C stretching which contributes 91% PED. The bands 1548, 1382 & 1256 cm⁻¹ belong to the C=C stretching in the pyrazole group effects to hyperconjugation. C=C in-plane deformation prevails in the range 760–600 cm⁻¹ and out-of-plane deformation turns below 600 cm⁻¹ endorses by Shimanouchi et.al [43] which is higher frequencies than the out-of-plane vibrations. C-C-C in-plane bending vibration is observed at 752 cm⁻¹ and out-of-plane bending vibrations of the ring is observed as a strong peak at 395 cm⁻¹ with 86% PED contribution. In addition, these frequencies show the substitutions in the ring to some extend which affect the ring modes of vibration with good agreement by the literature.

4.5.3. C-Cl vibration

C-Cl stretching vibrations are expected in the range 765–505 cm⁻¹ [44] and authorizes that the CH bond has the sign of polarity opposite to that of the carbon‐halogen bonds. In DPDS, strong IR spectral band is observed at 731 cm⁻¹ and the medium Raman bands observed at 730 and 593 cm⁻¹which is well correlated with scaled wavenumbers at 757, 725 and 592 cm⁻¹ with 69% PED contribution. C-Cl in-plane bending vibrations are assigned as strong Raman bands at 284 and 144 cm⁻¹ and the C-Cl out-of-plane bending vibration is assigned at medium band 361 cm⁻¹ which are good parallelism with the scaled wavenumbers.

4.5.4. C-N & N-N vibrations

For the aromatic compound which bears a C-N group, a band of good intensity has been absorbed in the region 1400–1200 cm⁻¹ [45] where the identification of C-N stretching is very much entangled since the mixing of bands is possible in this region [46]. For the DPDS, the bands observed at 1383 and 1256 cm⁻¹ in IR region and at 1382 and 1257 cm⁻¹ in Raman region while scaled values at 1378 and 1247 cm⁻¹ coincides well with the experimental values. This result affirms the values are hard up in the direction of the higher end of the range which is due to the interaction of C-C modes. Similarly for the C=N stretching, Demir et.al observed the C=N stretching in pyrazole ring is predictable at 1610 cm⁻¹ [47]. In DPDS, C=N stretching mode for the pyrazole ring is observed in the region 1548 cm⁻¹. This is rather least in their wavenumber and may be due to the intermolecular interaction C=N-H…O between the rings. N-N stretching vibrations transpires at the weak bands 1112 cm⁻¹ in IR and Raman spectra and its scaled frequency has supposed at 1125 cm⁻¹ with 87% PED contribution and are in good line with experimental.

4.5.5. COO⁻ vibrations

Carbonyl stretching frequency is very sensitive to the factors that disturb the nature of carbonyl group and its precise frequency is characteristic to the type of the carbonyl compound being studied. C=O group of saturated aromatic carboxylic acid absorbs strongly in the region 1730–1680 cm⁻¹ [48]. In DPDS, theoretically predicted wave number 1724 cm⁻¹ is identified as C=O stretching vibration and equated with the experimental band as a very strong at 1700 cm⁻¹ in FT-IR spectrum and 1710 cm⁻¹ in FT-Raman spectrum gets correlated. Broadening and red-shifted nature of this band depicts the level of intermolecular hydrogen bonding N-H…O with the formation carboxylate anion by donating hydrogen atom to the pyrazole moiety [49]. For the DPDS, the aromatic C-O band observed in the FT-IR region at 1275 cm⁻¹ and the scaled at 1276 cm⁻¹ and related to the expected in the range 1320–1210 cm⁻¹. C=O in-plane deformation is observed as medium at 360 cm⁻¹in FT-IR and strong in FT-Raman at 361 cm⁻¹ and is correlated by the scaled frequency at 362 cm⁻¹ with 72% PED contribution. C=O out-of-plane deformation is weakly observed in FT-IR at 752 cm⁻¹ and scaled frequency 757 cm⁻¹ which is inactive in Raman spectrum due to the C=O in the vicinity of halogen substituted aromatic structure.

4.5.6. N-H vibration

For secondary amine formation in pyrazole ring, N-H stretching and N-H bending vibration occurs in the range 3500–3300 cm⁻¹ and 1580–1490 cm⁻¹, respectively [49]. For DPDS, symmetric stretching mode of NH group is observed at 3449 cm⁻¹ as a medium in Raman and scaled value at 3450 cm⁻¹ with 97% PED contribution. O…H-N bending occurs as a strong peak at 869 cm⁻¹ in IR and 870 cm⁻¹ in Raman spectrum with the scaled value at 872 cm⁻¹. N₂₃-H₁₂ stretching and bending proceed in the range below 100 cm⁻¹ with a contribution of 58% and 42%, respectively and this lower magnitude be existent due to the intermolecular N-H…O bonding between two ring moieties.

4.5.7. OH vibration

Hydroxyl bonds are very important in dipole interactions to stabilize the molecular structures and have higher stretching frequencies when its relative intensity increases due to the differences in force constants. [50]. For the DPDS, intramolecular hydrogen bonding occurs in the hydroxyl group and their stretching vibration expected at 3590–3400 cm⁻¹ [51]. Therefore stretching occurs at band 3448 cm⁻¹ in FT-IR spectrum which is strong and unaffected by concentration changes. In-plane deformation the band occurs in 1586 cm⁻¹ which is strong in IR and medium in Raman spectra related to the scaled values 1588 cm⁻¹ with 35% PED contribution. This deformation exceeds the expected range due to the intramolecular interaction O-H…O.

4.6. Electrostatic potential map analysis

ESP surfaces show the charge distributions of molecules three dimensionally and their interaction of molecules with one another can be determined. It can also be used to determine the nature of the chemical bond [52]. ESP is to probe the relative electron density in a molecule and to understand the concepts of Lewis acids and bases on top of hydrogen bonded interactions amidst the molecules [53]. ESP contour mapping is a substantial tool in molecular modeling studies to predict the interactions of distant geometries.

ESP view is mapped up with the optimized geometry using B3LYP computational method and shown in Fig. 6 with colour code mapped in the range between -0.06882e and 0.06882e. Electron rich zone is the negative region concentrated over the carboxylic oxygen atoms and indicate that oxygen atom is surrounded by greater surface of positive charge and is a site for electrophilic attack. Positive potential sites is the electron poor zone mainly focused over the hydrogen atoms of methyl group attached to the carbon atom of the pyrazolium moiety and be susceptible to nucleophile attack. The nucleophilic and electrophilic reactive sites illustrate the binding site [54] of the molecule besides act as a tool for giving information about intermolecular hydrogen interaction amongst salicylate and pyrazolium ions over and above leads to biological activity of the molecule.

Fig. 6.

Electrostatic potential for DPDS.

4.7. Aromaticity

Aromatic compounds have a cyclic and conjugated set of p-orbitals that embodies the π-system. Aromaticity quantification is positioned by the geometric indexes that are agitated in the drift of aromatic compounds making equal distances of the bonds in an aromatic ring [55]. There are many structural criteria to evaluate the aromaticity for the six-membered and five-membered heterocycles. For DPDS HOMA (Harmonic Oscillator Model of Aromaticity) index is preferred to estimate the delocalization of the molecules. HOMA divided into geometric and energetic terms that are not correlated with each other and the index of aromaticity is described [56]. Energetic and Geometric terms are in least number explains the decrease of resonance energy and slight increase of bond length alteration which is caused by the contingency of C-H through hydroxyl and carboxylic acid group, which derange the electron density distribution within the ring and inhibits the aromaticity. The HOMA value for supreme benzene molecule is unity but the phenyl ring shows lesser striction in this value. Experimental and theoretical HOMA indexes values of phenyl ring are 0.9873 and 0.9778, respectively which emphasize benzene belongs to aromatic compound.

By virtue of magnetic and geometric criteria, pyrazoles and triazoles are 80–85% aromatic relative to benzene in the midst of heterocyclic compounds [57] and for the pyrazole derivatives the disparity of supremacy is perceived. Analysis of the geometric data reveal that experimental HOMA value of the pyrazole derivatives is 0.4729 and the theoretical value is 0.8693. The theoretical HOMA exhibits pyrazole is an electron withdrawing substituent which attract π-electrons from the phenyl ring, prominent to the formation of structures not gratifies the Huckel's rule 4n + 2 [58]. The depreciate HOMA value on trial could not be premeditated as non-aromatic [59]. Owing to the fact they undergo other chemical reactions such as dimethyl substitution and intermolecular interaction N-H…O. It is witnessed that π-electron delocalization is very sensitive to the nature of the substituent and to intermolecular interactions. By this the observed HOMA value emphasizes pyrazole ring belongs to aromatic compound.

4.8. Natural charge analysis

To study the charge distribution of DPDS, it is better to use Natural Charge Analysis since it do not exhibit dependence on basis set [60]. The atomic charges are calculated by natural population analysis by using B3LYP/6-311G (d, p) method are plotted in Fig. 7 and tabulated in Table S4.

Fig. 7.

Natural charge Distribution for DPDS.

C₁₃ shows maximum positive charge (0.83017 e) than other atoms due to the charge formation over the electronegative oxygen atoms. In carboxylic acid group, carbon atom progress a partial positive charge and oxygen atom develops partial negative charge. Deprotonation takes place in the acid group and becomes carboxylate anion which makes the carbon atom more positive. Minimum positive charge over Cl₁₀ is due to the substitution reaction in which hydrogen atom is replaced by chlorine atom in the benzene ring which grades the polarization of chlorine by the Lewis base. Maxima negative charge (-0.70089 e) is stated in the atom O₁₅ of the carboxylate anion which interacts with nitrogen of pyrazole ring through intermolecular interactions. Since oxygen and nitrogen are electronegative atoms, they are electrostatically attracted to the hydrogen atom and bear a large negative charge. The minima negative charge (-0.00196 e) is observed in the atom Cl₉ and is more electronegative than carbon which pulls the bonded electrons closer to itself. Substitution of hydrogen atom by chlorine atom makes the carbon more electropositive and befalls minima negative charge.

4.9. NMR chemical shift analysis

¹H and ¹³C NMR spectra contribute a tectonic data with disparate hydrogen and carbon atoms in a molecule and their chemical shifts are shown in Fig. 8. For reliable calculation of magnetic properties it is acknowledged that literal prognosis of molecular geometries are important [61]. Chemical shifts are very advantageous to afford the facts of diverse protons and carbons extant in the molecule. Chemical shifts in the spectrum are due to the de-shielding and shielding by electrons.

Fig. 8.

Experimental NMR spectra of DPDS.

¹H NMR spectrum of DPDS advertises distinct type of protons in which signal at δ 2.164 ppm is is due to proton impurity in the DMSO – d₆ solvent. Hydrogen peaks in the ¹H spectrum develops in the range 6.95–7.40 ppm which reciprocates to the aromatic region whereas greater shielding is owing to the anisotropy of aromatic ring π electrons [62]. In DPDS, ¹H spectrum of pyrazole ring reveals a wide-ranging signal at δ 5.091 ppm which is the symptomatic of two identical hydrogen atoms in amino group (H₂₂ and H₂₁), whereas it appears in deshielded region because of adjacent amino group (NH⁺). The carboxylate group primarily arises in the range 10–12 ppm [63] however in DPDS H₂₁ exhibits a peak at δ 5.866 ppm which is deshielded in supreme because of the electronegative oxygen atom in the carboxylate anion. ¹H spectrum shows doublet signals at δ 7.68 ppm and δ 7.618ppm which are assigned to H₈ and H_{7 ,} respectively. These signals are deshielded due to the adjacent chlorine atoms in the salicylate group moiety. The signals at δ 2.55 ppm, δ2.35 ppm and δ2.164 ppm are attributed to the methyl groups attached to the pyrozolium moiety where it acquires electron donating resonance effect and shows a substantial lowering in chemical shift.

¹³C NMR spectrum of DPDS advertises ten distinct carbon signals to indicate carbon points present in the compound. The signal for DMSO solvent is assigned at δ 39.40 ppm. The presence of C₁₃ signal at δ 170.73 ppm in the highly deshielded region delivers the emergence of carboxylate anion in the compound. The C₆ (δ 157.72 ppm) and C₁₆ (δ 143.74 ppm) signals are intensely deshielded by virtue of O-H grouping and the transacted methyl group in the pyrazole ring moiety, respectively. The emergence of prime intensity signals in the region δ 120–130 ppm is indicative of the carbons in the aromatic ring. The signals in the low field region (– 20–50 ppm) manifest the being of methyl, methylene and methane carbons present in the compound [64]. Signals at C₃ (δ 133.25 ppm) and C₅ (δ 128.46 ppm) are granted to phenyl ring bordening on the electronegative chlorine atom which leads to deshielding of carbon by attracting all electron clouds towards chlorine atom and results in the increase of chemical shift value. Correspondingly signal at δ 118.81 ppm is due to the aromatic carbon C₁ and the downfield signal at δ 104.37 ppm is due to the carbon atoms attached to the methyl groups in the pyrazolium moiety. The upfield signal at δ 12.04 ppm is attached with methyl group of base moiety of C₂₃ and C25.

4.10. Thermal (TG/DTA) analysis

Thermal characteristics of DPDS crystal was studied by using TG and DTA under nitrogen atmosphere at a heating rate of 10 °C /min from 30 to 500 °C. TG-DTA thermo gram is shown in the Fig. 9 . The compound DPDS was stable up to 95 °C whereas the endothermic dip at 105 °C in DTA indicates the melting point of the compound. Single stage decomposition is observed between 90 °C and up to 230 °C, which corroborate the bulk degradation of the material and was further established by a broad endothermic dip at 200 °C in DTA. The weight loss in the bulk decomposition is around 80% when the temperature was increased from 160 to 250 °C. Volatile fragments such as CO, CO_2, NO, CH₄ are the decomposition products. Thus, the thermal analysis validates the applicability of DPDS for any biotic purpose.

Fig. 9.

TG/DTA spectrum of DPDS.

4.11. ELF and LOL analysis

ELF (electron localization function) and LOL (localized orbital locator) maps concede the zones of molecular space and their colour shade maps are presented in Fig. 10 . This topological surface analysis is based on the covalent bonds where the revelation of electron pair is high. The chemical content of LOL is analogous to that of ELF, as both counts upon the kinetic-energy density. ELF is dredged up owing to the fact of electron pair density and LOL decodes the gradients of localized orbitals inflamed when overlap [65].

Fig. 10.

(a) ELF and (b) LOL colour filled maps for DPDS.

The value of ELF, τ(r) ranges from 0.0 to 1.0, where relatively comprehensive values in the interval 0.5 and 1.0 indicate the contours containing bonding and nonbonding localized electrons. The diminutive values (< 0.5) describe the contours where electrons are intended to be delocalized [66]. LOL, ƞ(r), attains large values (> 0.5) in contours where the electron density is dominated by electron localization [67]. High localization of electrons is due to the endurance of covalent bond in the contours with comprehensive values [68]. From the Fig. 10a, the high ELF regions are seen around hydrogen atoms H₂₇and H₂₉ indicating the presence of highly localized bonding and nonbonding electrons and the blue regions around few chlorine atoms Cl₉ and Cl₁₀ show the delocalized electron cloud around it. From the Fig. 10b, it is seen that the central region of a hydrogen atom is white; indicating that electron density exceeds the upper limit of colour scale (0.80 au.) and the blue circles around hydrogen atoms (H₂₇ and H₂₉) shows the electron depletion region between inner shell and valance shell. The most part of the covalent region present between the carbon and chlorine atoms are indicated by green colour.

4.12. Reduced density gradient na

A visual approach known as RDG analysis is implemented to make the non-covalent interactions more specific and to explore intra and inter non-bonded interactions in a molecular system [69]. The gradient isosurfaces and scatter graphs of DPDS molecule are shown in Fig. 11 .

Fig. 11.

Gradient isosurfaces and scatter graphs of DPDS.

RDG scatter graph is generated between RDG versus sign (λ2)ρ, where sign(λ2)ρ is the second Eigen value of the electron density which provides useful information regarding the strength and nature of the interactions. The value and sign of sign (λ2)ρ are used to explain the nature of interactions,sign (λ2)ρ > 0 for repulsive interaction, sign (λ2)ρ < 0 for attractive interaction and sign (λ2)ρ nearly zero for vander Waals weak interaction [70]. The function of λ2(r) ranges between −0.035 and0.02 0 a.u in RDG scatter spectra which is divided in to three colours red, green and blue. In the RDG isosurfaces, the red spike shows the steric repulsion observed in the centres of aromatic and pyrazole rings. The RDG scatter graph manifests the red contour between 0.02 and 0.05 au clarifies the higher repulsive exchange contribution. This plot illustrates the steric repulsive force between the aromatic carbon atoms between the rings. The blue spike manifests the strong attraction between N-H…O constitutes inter-molecular hydrogen bonding. In the RDG scatter graph blue contour between -0.05 and -0.03 au confirms the presence of strong hydrogen bonding. The red-green mixed spikes are observed near the C-Cl interactions in the aromatic ring. The red-green mixed flaky area between 0.00 and 0.015 au confirms the absence of hydrogen bonding and the C-H formation in the aromatic ring are due to the interaction between hydrogen atoms and the π-electrons. The RDG graph results confirm the interacting regions in the molecular structure DPDS.

4.13. Molecular docking analysis

Molecular docking leads to further drug development with the bio molecular interactions for the rational drug design [71,72]. It is an enticing framework to be acquainted which is used to predict the binding energy, free energy and stability of the complexes. PDB structures of the target proteins are downloaded from RCSB (Research Collaboratory for Structural Bioinformatics) protein data bank. The ID's for downloaded SARS-CoV-2 hydrolase inhibitor proteins are 7JRN, 6XBI, 6XMK and 6XFN with various drug molecules. Fig. 12 represents the Ramachandran plots for the four centred proteins and the preparation of ligand with minimal energy for docking was done.

Fig. 12.

Ramachandran plots of SARS-CoV-2 hydrolase inhibitor proteins.

By observing the Ramachandran plots for all the four proteins, 85–95% of the amino acids lie in the allowed region (red) and only very few lie in the partially allowed vicinity (yellow) and therefore 7JRN, 6XBI, 6XMK and 6XFN have 88.3%, 90.6%, 90.7% and 91.7% of the amino acids in the allowed region, respectively. Majority of the amino acids in the allowed regions indicate the stability of the proteins chosen for the binding interaction.

Auto Dock Tools is used to remove the ligand and water molecules present in the target proteins, which was also used to add the polar hydrogen bond and Geisteger charges. The docking parameters such as binding energy and refRMS of the molecule with respect to the targeted proteins are listed in Table 5 . The interactions of ligand DPDS with SARS-CoV-2 proteins are shown in Fig. 13 and the yellow dotted interactive line in the figure indicates the formation of intermolecular hydrogen bond between DPDS and SARS-CoV-2 hydrolase inhibitor proteins.

Table 5.

Docking parameters of DPDS docked into SARS-CoV-2 hydrolase inhibitor proteins.

Protein (PDB ID)	Bonded residues	Bond distance (Å)	Estimated inhibition constant (milli Molar)	Binding energy (kcal/mol)	Inter molecular energy (kcal/mol)	Reference RMSD
7JRN	LEU`80	2.1	48.22	-5.89	-7.38	31.92
	PRO`77	2.5
	ARG`65	2.2
6XBI	THR`198	2.1	242.51	-4.93	-6.42	33.88
	LEU`208	2.2
	PHE`219	2.7
6XMK	GLN`189	2.8	377.41	-4.67	-6.16	36.69
6XFN	GLY`146	1.7	465.11	-4.55	-6.04	18.93

Fig. 13.

Molecular Docking of SARS-CoV-2 hydrolase inhibitor proteins.

Amongst the targeted protein inhibitors, 7JRN protein exhibits least binding energy -5.89 kcal/mol with three amino acids LEU`80, PRO`77 and ARG`65 encompassing N-H…O and O…H interactions with an intermolecular energy -7.38 k cal/mol and prominent RMSD value 31.92. At this juncture, the energy released due to the bond formation or relatively interaction of ligand and the protein is termed as the binding energy whereas higher the binding energy, the stronger the interaction. The intermolecular energy is estimated for the combination of ligand and protein in their bound conformation which shows that higher the interaction energy lowers the bond stiffness. This shows the relation between binding energy and the interaction energy between ligand and the protein [73,74]. It can be seen that the hydrogen bond is formed between the hydrogen atom and oxygen atom from the ligand and the hydrolase inhibitor. Looking into the docking conformation of the ligand with the proteins reveals that the atoms N₁₉, H₂₁ and O₁₅ are the only active sites undergoing hydrogen bond formation. In general hydrogen bonds are formed between the hydrogen which is bound to a more electronegative atom like nitrogen or oxygen and another atom bearing a lone pair of electron. This similarity has been observed between the DPDS and the SARS-CoV-2 hydrolase inhibitor proteins. Ligand DPDS shows good binding affinity towards all the four targeted proteins, indicating anti-viral features and the reactive site analysis with topological studies complements DPDS assured towards amino acids and awards to be a noble antiviral drug.

4.14. Drug likeness

To unmask an active drug, reckoning of drug likeness is efficacious for which Lipinski's rule of 5 was used. In consonance with this rule, either ligand is considered as a pharmaceutic drug if it is apt for marked preconditions counting molecular weight < 500 Dalton, number of H-bond acceptor < 10, number of H-bond donors < 5 and lipophilicity laid out as log P < 5 [75]. Utterly these limitations deputize the drug likeness trial and the values akin to the antiviral drug DPDS is tabulated in Table 6 . Resultantly, the number of hydrogen bond donors and hydrogen bond acceptors for DPDS found to be 3 and 4 for each. Values of log P was perceived to be 1.32, which is a shade of lipophilic feature and Molar refractivity raised as 72 and settled within the appropriate range. DPDS ligand accede this rule and firmly established as a vigorous anti-viral drug.

Table 6.

Drug likeness parameters of DADS.

Descriptors	Calculated	Expected
Molecular mass(Dalton)	301	<500
Hydrogen bond donor	3	<5
Hydrogen bond acceptor	4	<10
Log P	1.32	<5
Molar refractivity	72	40-130

4.15. ADMET contour

To uncover a neoteric drug and its buildup, researchers must assay the liveliness of a drug in the body to gage welfare and toxicity. Drug assimilation and pharmacokinetics studies, such as ADME and toxicology studies, are the perilous phase in this process. Records poised states if a drug is doable and affords explicit goals for the future study and progression [76] and ADMET factors are tabulated in Table 7 .

Table 7.

ADMET Factors.

ADMET	Factors	DPDS
Absorption	Water solubility (log mol/L)	-2.987
	Caco-2 permeability (log Papp in 10−6cm/s)	0.909
	Human intestinal absorption (%)	98.692
	P-glycoprotein substrate	YES
	P-glycoprotein I/II inhibitors	NO
Distribution	CNS permeability (log PS)	-2.426
	Human VDss (log L/kg)	-2.018
	Fraction unbound (human)(Fu)	0.307
Metabolism	CYP substrates/inhibitors	NO
Excretion	Total clearance (log ml/min/kg)	0.2
Toxicity	AMES toxicity	NO
	Max. tolerated dose (human)(log mg/Kg/day)	1.177
	Oral rat Acute Toxicity (LD50)(mol/kg)	2.651
	hERG l/ll inhibitors	NO
	Hepatotoxicity	NO

Amidst absorption all the constituents of the ligand trot out water solubility (log S) greater than -5, which reflect their solubility in water at 25 ˚C [77] and the predicted water solubility of a compound results as -2.987 (log mol/L). Alike absorbance in the small intestine is one of the cardinal evolution to incline bioavailability of drug in the rear oral profit [78]. Thus the prediction for the fraction of DPDS that absorbed by virtue of the human small intestine is 98.6%. Conjointly Caco-2 permeability value presumes the logarithm of permeability coefficient 0.909 in Caco -2 monolayer of the cells (log Papp in 10⁻⁶ cm/s). DPDS compound is not a p-glycoprotein I/II inhibitors and act as substrates which functions as a biological barrier by extruding toxins and xenobiotics out of cells. While distributing the drug throughout the body, most drugs in plasma abide in serenity by unbounding or bounding to serum proteins. Bounding to the serum proteins in less would transverse cellular membranes [74]. DPDS compound results less bound to the serum proteins and is impotent to infiltrate the CNS. The steady state volume of distribution (VDss) is the complete dose of a drug needed to be circulated homogeneously in blood and plasma. It is considered low if log VDss is less than -0.15 and high if it is more than 0.45. Higher VDss value grants better drug distribution in tissue rather than plasma and cause renal failure and dehydration [78].DPDS spectacles VDss IS -2.081 (log L/kg) which is less and scattered homogeneously in blood plasma and tissues. Total clearance enumerates the amputation of drug from blood or plasma where the drug riddance practice grades from kidney and liver [79]. ADMET result foresees the total clearance as 0.2 (log ml/min/kg). Toxicity of the compound can be tested by AMES toxicity which predicts if the compound has mutagenic potential or not [77]. ADMET result predicts that the compound has flimsy inhibitor and non-inhibitor of hERG inhibition and is non-mutagenic which does not act as a carcinogen. Also the compound is not hepatotoxic as it does not intrude the normal function of liver. Maximum tolerated dose is freaked out and is about 1.177 (log mg/Kg/day) which is an estimate of the toxic dose inception in humans. ADMET results interpreted that DPDS compound is an innocuous incitation to treat SARS-CoV-2.

5. Conclusion

DPDS was synthesized by slow evaporation method and their molecular structure has been optimized using the DFT technique and the calculated bond length, bond angle and dihedral angle sustains a good stability with good agreement with the experimental values. Rehybridization overshadows hyper conjugation and is well reflected in the geometry of DPDS as bond O₁₁-H₁₂ contracts with respect to DS due to on intra-molecular hydrogen bonding O₁₁-H₁₂…O₁₄. Charge transfer interactions were dissected to scrutinize hydrogen bonding networks and the strong N-H…O inter-molecular hydrogen bonding get earlier perception into these interactions to project the molecules with biotic profile. Second order perturbation theory results the transfer of electron density from lone pair nitrogen atom to the anti-bonding orbital of O-H bond which produces a strong evidence for inter-molecular hydrogen bonding. HOMO-LUMO energy gap (ΔE) value is demoted and directs that DPDS molecule yields virtuous biological activity of the molecule. Electron distribution and reactive sites on the surface of the DPDS were analysed using ESP, ELF and LOL contour maps. Colour code for the ESP was mapped and confirmed the concepts of Lewis acids and bases with in the molecule. Electronic spectra was evaluated using DFT method displays that the absorption spectra exhibits an intense peaks at 360 transitions. Aromaticity evaluated and agreed the phenyl ring and the pyrazole ring belongs to the aromatic compounds while Natural Charge analysis confirmed that C₁₃ and O₁₅ are the most electropositive and electronegative atoms respectively. Thermal analysis showed that the DPDS crystal can retain its stability up to 95 °C which is recognized using TG/DTA curve. Structure elucidation of DPDS using IR, Raman and NMR spectroscopic techniques were used for the data analysis and spectral interpretation which contributes the outcome of the functional groups in DPDS. In this context, predictions of ¹H and ¹³C NMR chemical shifts and scaled frequencies have been demonstrated to be a viable strategy for the relative configuration of new molecules with good linearity amongst scaled and experimental vibration frequencies was also observed. Broadening and red-shifted nature of the bands depicts the level of intermolecular hydrogen bonding O-H…N with the molecular structure in gaseous nature. RDG approach allowed analyzing the weak attractive interactions, strong attraction, and steric repulsion existed in DPDS. Molecular docking study with SARS-CoV-2 hydrolase inhibitor protein 6XA4 shows substantial anti-viral activity exhibiting least binding energy -5.89 kcal/mol in the active sites. From docking result, it is evident that nitrogen atom in the pyrazole moiety involved in hydrogen bond formation with the target enzyme and concludes pyrazolium moiety acts as a good anti-viral medication. Drug Likeness and ADMET analyses analyzes revealed that DPDS is superlative in absorption and total clearance rate which high spot the consequential aptitude for antiviral activity counter to SARS-CoV-2. These consummations propound supplemental powerful assays for the declaration of activity of antiviral constituents contrast to SARS-CoV-2 affords evidence concerning drug pharmacokinetics.

CRediT authorship contribution statement

X.D. Divya Dexlin: Conceptualization, Data curation, Formal analysis, Writing – original draft. J.D. Deephlin Tarika: Investigation, Data curation. S. Madhan Kumar: . A. Mariappan: Methodology, Supervision. T. Joselin Beaula: Supervision, Writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.