Hydrogen Bonding Interactions of m-Chlorotoluene with 1-Alkanol Analyzed by Thermodynamic, Fourier Transform Infrared Spectroscopy, Density Functional Theory, and Natural Bond Orbital

Abstract

Fourier transform infrared spectroscopy (FT-IR) has been employed to obtain information about the nature of interactions in the liquid solutions of pure solvents and their mixtures of m-chlorotoluene (MCT) with 1-alkanol systems at different mole fractions. Furthermore, densities (ρ) and speeds of sound (u) of binary mixtures of MCT with a set of five 1-alkanols, namely, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, and 1-heptanol, were measured as a function of composition at 298.15 K. From the experimental quantities, the excess volumes (V^E), isentropic compressibility (k_s), and excess isentropic compressibility (k_s^E) were calculated for the binary mixtures over the entire composition range and under the atmospheric pressure. These excess properties (V^E) and (k_s) were correlated with the Redlich–Kister polynomial equation. Additionally, theoretical density functional theory calculations and natural bond orbital analyses were carried out to further discern the nature and strength of interactions between MCT and 1-alkanols. Moreover, the recorded FT-IR spectra-derived excess properties and quantum chemically derived data revealed the presence of interactions between component molecules in binary liquid solutions.

1. Introduction

Over the last several years, hydrogen bonding comprising the component molecules is found as the major path for academic and industrial communities to analyze the nature of the molecular interactions in solution chemistry as well as design of applications in engineering and pharmaceutical sciences. The information on the phase and interfacial properties are vital and necessary to operate and design the processes, i.e., separation or reaction steps in chemical processes.¹ However, there is widespread literature data on thermophysical and excess thermodynamic properties, which provides information about intra- and intermolecular interactions in pure liquids and their mixtures. But there are still some unfilled gaps in understanding the nature of different molecular interactions that exist in liquid mixtures. This is partly due to scarcity of spectral evidences and sound theoretical data.¹⁻⁴

In this context, 1-alkanols are polar and self-associated molecules that form a network of H bonds in solution. Therefore, in the pure liquid state, alkanol molecules are associated with hydrogen bonding as linear and chain aggregates, as can be seen in Figure S1. Also, 1-alkanols are cheap and used as solvents in a variety of chemical and technological processes.⁵⁻⁷ Furthermore, the knowledge of excess thermodynamic properties of binary liquid mixtures containing halogenated hydrocarbons is very staple due to their practical applications in various fields, including detergents, rubber, plastics, and aerosol propellants.⁸ The m-chlorotoluene (MCT) solvent is one of the methyl substituted halo-benzenes that is used as an intermediate in pesticide pharmaceutical and dye industries.⁹

Extensive survey of the literature showed that few studies have been carried out on measurements of thermophysical properties of binary mixtures containing MCT with dimethylformamide,⁹ 1-decanol,¹⁰ tetrahydrofuran,¹¹ chloroalkanes,¹² aromatic hydrocarbons,¹³ and acetophenone.¹⁴ In view of the aforementioned points, the present study mainly focused on thermophysical and excess thermodynamic properties of MCT with 1-alkanols (C₃–C₇). The extent of self-association of 1-alkanols and the effect of mixing with MCT at different mole ratios was investigated using Fourier transform infrared spectroscopy (FT-IR) spectroscopy. Density functional theory (DFT) calculations were carried out to probe into the nature of molecular interactions between isolated molecules of MCT and 1-alkanols in a 1:1 dimer. Eventually, both experimental and computational studies explicitly explained the intermolecular interactions between MCT and 1-alkanol binary mixtures.

2. Materials and Methods

2.1. Experimental Details

2.1.1. Materials

The percentage purities of all of the liquids obtained from S D Fine-Chem Ltd., India are as follows: MCT (99.5%), 1-propanol (99.5%), 1-butanol (99.5%), 1-pentanol (99.0%), 1-hexanol (98.5%), and 1-heptanol (99.5%). The source, purity, and water content of solvents used in this work are listed in Table 1. Prior to making the experimental measurements, all of the liquids were double distilled and partially degassed with a vacuum pump in an inert atmosphere. The purity of all of the solvents was adjudged by comparing their measured densities, speed of sound, heat capacities, and thermal expansion coefficient to those of corresponding pure liquids available in the literature.^{3,8,10,15−39} The comparative data are listed in Table 2.

Table 1. Provenance and Purity of the Materials Used.

component	CAS number	source	mass fraction purity (%)	water content (%)
m-chlorotoluene	108-41-8	S D Fine-Chem, India	99.5	0.041
1-propanol	71-23-8	S D Fine-Chem, India	99.5	0.036
1-butanol	71-36-3	S D Fine-Chem, India	99.5	0.038
1-pentanol	71-41-0	S D Fine-Chem, India	99.0	0.042
1-hexanol	111-27-3	S D Fine-Chem, India	98.5	0.048
1-heptanol	111-70-6	S D Fine-Chem, India	99.5	0.034

Table 2. Density (ρ), Speed of Sound (u), Heat Capacity (C_p,i) and Thermal Expansion Coefficient (α_i) Data of Pure Liquids at 298.15 K^a.

	ρ (g cm–3)		u (m s–1)
component	exp.	lit.	exp.	lit.	Cp,i (J mol–1 K–1)	αi (kK–1)
m-chlorotoluene	1.06721	1.06729[10]	1297.0	1298.0[10]	171.73[17]	0.8396
		1.06723[15]		1296.5[16]
1-propanol	0.79965	0.79960[18]	1206.5	1205.93[19]	144.47[19]	1.0004
		0.79966[20]		1206.17[22]
		0.79962[21]		1206[20]
1-butanol	0.80595	0.80593[19]	1238.9	1239.28[19]	173.80[23]	0.9355
		0.80591[24]		1238.94[3]
		0.80597[25]		1239.22[23]
1-pentanol	0.81082	0.81077[26]	1276.2	1275.00[27]	208.14[27]	0.9151
		0.81080[28]		1276.23[3]
		0.81084[24]		1275.18[29]
1-hexanol	0.81531	0.81524[30]	1303.2	1303.6[31]	241.64[8]	0.8757
		0.81532[32]		1303.2[34]
		0.81530[33]		1303.3[35]
1-heptanol	0.81879	0.81877[36]	1326.5	1326.10[38]	273.24[8]	0.8598
		0.81875[24]		1326.48[3]
		0.81878[37]		1327.01[39]

^aStandard uncertainties s are s(ρ) = ±0.00004 g cm^–3, s(u) = ±0.5 m s^–1.

2.1.2. Apparatus and Procedure

The water content of organic solvents used in the present study was measured using Analab (Micro Aqua Cal 100) Karl Fischer Titrator and Karl Fisher reagent from Merck.⁴⁰ It can detect water content from less than 10 ppm to 100% by conductometric titration with dual platinum electrode. All of the binary liquid mixtures were prepared by weighing appropriate amounts of pure liquids on an electronic balance (Afcoset, ER-120A, India) with a precision of ±0.1 mg. Each component was syringed into airtight stopper bottles to minimize losses due to evaporation, which are shaken to ensure complete homogeneity of the two solvents.⁴⁰ The uncertainty of the mole fraction was ±0.0005. Densities of the pure liquids and their mixtures were measured by using a Rudolph Research Analytical Digital Densitometer (DDM-2911 model) with an accuracy of ±0.02 K in temperature and ±0.00005 g cm^–3 in density.⁴⁰ The speed of sound measurements was measured using a commercially available single-crystal ultrasonic interferometer (model F-05) from Mittal Enterprises, New Delhi, India, at 2 MHz frequency at various temperatures with accuracy of ±0.5 m s^–1.⁴⁰ For all speed of sound measurements, the temperature was controlled at ±0.02 K by circulating the water from a constant temperature bath (model MV F-25 Julabo/Germany).⁴⁰

2.2. FT-IR Measurements

To study the nature and sites of interactions of 1-alkanols of different chain lengths and MCT, at different compositions, FT-IR spectra were recorded and the interactions were probed by comparing the IR bands of the pure compounds and the mixtures. The infrared spectra of pure liquids and their bubble-free homogeneous binary liquid mixtures were recorded within the range of 500–4000 cm^–1 at room temperature by using Cary 670 series FT-IR from Agilent Technologies, South Africa. The device has 0.002 cm^–1 precision, and 110 scans were performed at 16 cm^–1 resolution and averaged.

2.3. Computational Details

To provide further insights on the nature and strength of interactions between 1-alkanols and MCT, isolated dimers of MCT/1-alkanols were modeled and studied quantum chemically. Initial geometries of 1-alkanols and MCT were modeled with the aid of Gauss View 5.0 and used as input for density functional theory (DFT) calculations. Optimized geometries of MCT and each alkanol were used as input geometries for 1:1 MCT/1-alkanol complexes. It is worth mentioning that the 1:1 complex is only a representation of possible interactions within the binary system. However, this assumption of the 1:1 complex is supported by previous studies on interactions of alkanols with different organic molecules such as alkylmethacrylates⁴¹ acrylic esters.^42,43 Full geometry optimization was carried out on each of the studied 1-alkanols and MCT. Geometry optimization was then carried out on MCT/1-alkanol complexes. Force constant calculations were carried out on each optimized structure to verify that they correspond to the true energy minimum. Thermodynamic parameters were derived from the vibrational frequencies of the molecules.

The model chemistry employed was B3LYP/6-31++G(d,p), which is a combination of the Becke 3-parameter exchange functional, the Lee–Yang–Parr correlation functional (B3LYP),^44,45 and the 6-31++G(d,p) basis set. Liu et al.⁴⁶ had proven and enlisted B3LYP as one of the hybrid DFT functionals that produces excellent description of H-bonding interactions between water molecules. The 6-31++G(d,p) basis set has been used in Hartree–Fock and MP2 calculations to produce satisfactory results for some H-bond energies.⁴⁷ The utilization of B3LYP/6-31++G(d,p) model in the present work is also in relation to its success in a study on the correlation between bond lengths in OHN and OHCl fragments,⁴⁸ in which the same type of H bond considered in the present study was suitably treated. Malik et al.⁴⁹ had also used a similar model (B3LYP/6-311++G(d,p)) to predict the internuclear distances between interacting atoms in mixtures of piperidine and some cyclic ketones.

Furthermore, exploration of donor–acceptor interactions between MCT and 1-alkanols was carried out using the natural bond orbital (NBO) analysis calculations. NBO analysis allows a probe into the interplay of hyperconjugation and hybridization that accompany MCT/1-alkanol complex formation. Hyperconjugation interaction energy (E(2)) was calculated from the solution to the second-order Fock matrix according to the equation^50,51

1

where ⟨σ_i|F|σ_j⟩² or F_ij² is the off diagonal Fock matrix element between the i (donor) and j (acceptor) NBOs, ε_i and ε_j, are the energies of i and j NBOs, and n_i is the occupancy or population of the i (donor) NBO.

Windows-based Gaussian 09 software⁵² was used for all calculations, whereas NBO version 3.1 implemented in Gaussian 09 was used for the bond orbital analyses.

3. Results and Discussion

3.1. Volumetric Behavior

Densities (ρ) of pure and liquid mixtures are used to solve various problems in industries, such as quality control in the production of industrial liquids, concentration determination in the food industries, and measuring the concentration of sugar and alkanols. The experimental and calculated thermodynamics properties, such as densities (ρ), speed of sound (u), excess volumes (V^E), isentropic compressibilities (k_s), and excess isentropic compressibilities (k_s^E) for all the binary mixtures of MCT with 1-alkanol at 298.15 K are given in Table 3. The excess volume data were calculated from the densities of pure liquids and their mixtures using the following equation

2

where x, M, and ρ are mole fraction, molar mass, and density, respectively. The subscripts i and m represent pure components and mixture, respectively.

Table 3. Mole Fraction of MCT (x₁), Densities (ρ), Excess Volumes (V^E), Speed of Sound (u), Isentropic Compressibilities (k_s), and Excess Isentropic Compressibilities (k_s^E) for Binary Liquid Mixtures of MCT with 1-Alkanol at 298.15 K^a.

x1	ρ (g cm–3)	VE (cm3 mol–1)	u (m s–1)	ks (TPa–1)	ksE (TPa–1)
m-Chlorotoluene(1) + 1-Propanol(2)
0.0000	0.79965	0.000	1206.5	859.1	0.0
0.0876	0.83514	–0.028	1217.4	807.9	–11.6
0.1898	0.87224	–0.037	1228.8	759.2	–18.6
0.2937	0.90599	–0.031	1238.4	719.7	–20.1
0.3862	0.93314	–0.019	1245.6	690.7	–18.3
0.4841	0.95938	–0.002	1252.2	664.7	–14.5
0.5935	0.98602	0.023	1257.7	641.1	–7.7
0.6863	1.00672	0.041	1263.2	622.6	–2.7
0.7880	1.02783	0.045	1270.0	603.2	2.0
0.8915	1.04770	0.038	1280.5	582.1	3.3
1.0000	1.06721	0.000	1297.0	557.1	0.0
m-Chlorotoluene(1) + 1-Butanol(2)
0.0000	0.80595	0.000	1238.9	808.3	0.0
0.0884	0.83520	–0.025	1245.8	771.4	–9.3
0.1914	0.86732	–0.031	1251.7	735.9	–14.3
0.2896	0.89616	–0.020	1255.9	707.5	–15.0
0.3883	0.92357	–0.001	1258.1	684.0	–12.0
0.4938	0.95131	0.023	1260.0	662.1	–7.1
0.5894	0.97517	0.045	1261.6	644.3	–1.8
0.6928	0.99985	0.058	1264.8	625.2	3.1
0.7865	1.02123	0.061	1270.0	607.1	5.8
0.8890	1.04380	0.039	1280.0	584.7	5.2
1.0000	1.06721	0.000	1297.0	557.1	0.0
m-Chlorotoluene(1) + 1-Pentanol(2)
0.0000	0.81082	0.000	1276.2	757.2	0.0
0.0898	0.83589	–0.020	1279.0	731.4	–6.8
0.1930	0.86405	–0.023	1279.0	707.5	–9.1
0.2873	0.88923	–0.012	1277.8	688.7	–8.4
0.3897	0.91603	0.009	1275.9	670.6	–5.6
0.4884	0.94134	0.033	1273.3	655.2	–1.2
0.5918	0.96746	0.052	1272.1	638.8	2.9
0.6933	0.99269	0.065	1273.0	621.6	5.7
0.7899	1.01637	0.067	1275.0	605.2	8.1
0.8880	1.04023	0.047	1282.3	584.6	6.3
1.0000	1.06721	0.000	1297.0	557.1	0.0
m-Chlorotoluene(1) + 1-Hexanol(2)
0.0000	0.81531	0.000	1303.2	722.2	0.0
0.0885	0.83663	–0.017	1302.7	704.3	–4.5
0.1872	0.86053	–0.019	1300.1	687.5	–6.2
0.2895	0.88546	–0.004	1295.3	673.1	–4.6
0.3920	0.91063	0.018	1290.0	659.9	–1.5
0.4894	0.93482	0.041	1284.5	648.3	2.8
0.5904	0.96014	0.066	1279.8	635.9	7.1
0.6846	0.98410	0.078	1277.5	622.6	9.7
0.7938	1.01231	0.076	1278.4	604.4	10.4
0.8879	1.03710	0.054	1283.0	585.8	8.4
1.0000	1.06721	0.000	1297.0	557.0	0.0
m-Chlorotoluene(1) + 1-Heptanol(2)
0.0000	0.81879	0.000	1326.5	694.1	0.0
0.0884	0.83753	–0.020	1323.1	682.0	–2.4
0.1921	0.86012	–0.022	1317.8	669.5	–3.0
0.2869	0.88135	–0.006	1311.0	660.1	–1.0
0.3903	0.90526	0.017	1303.0	650.6	2.4
0.4879	0.92860	0.046	1296.3	640.8	5.4
0.5891	0.95365	0.074	1289.0	631.1	9.5
0.7108	0.98517	0.089	1284.0	615.7	11.7
0.7918	1.00706	0.087	1283.8	602.5	10.9
0.8893	1.03447	0.064	1286.0	584.5	8.6
1.0000	1.06721	0.000	1297.0	557.0	0.0

^aStandard uncertainties s are s(ρ) = ±0.00004 g cm^–3, s(V^E) = ±0.005 cm³ mol^–1, s(u) = ±0.5 m s^–1, s(k_s) = ±0.9 TPa^–1, and s(k_s^E) = ±0.9 TPa^–1.

The excess volume data, which are plotted in Figure S2 for all of the mixtures over the entire composition range at 298.15 K, display an inversion sign for the binary mixtures of MCT with 1-alkanols, such as 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, and 1-heptanol. When the alkanol molecules are mixed with polar molecules like MCT, there could be mutual dissociation of the hydrogen-bonded structures present in pure alkanols with subsequent formation of intermolecular hydrogen bond (Cl···HO) between the chlorine atom of MCT molecule and hydroxyl group of 1-alkanols and electrostatic interactions. These interactions will have different resultant contributions to the volumes of the mixtures.

It can be seen from Figure S2 that the excess volumes of the MCT in the 1-alkanol binary mixture show negative to positive values over the entire mole fraction region and most negative V^E values were observed at the higher concentration region of 1-alkanols and positive V^E values in the MCT concentration-rich region. This could be attributed to the stronger interactions between the hydroxyl group of 1-alkanol and electronegative chlorine atom on the toluene molecule through hydrogen bonding at the rich concentration region of alkanols. Furthermore, the negative excess volume tends to shift toward positive values with increasing mole fraction of MCT due to rupturing of hydrogen bonding between unlike molecules in the mixture. The algebraic value of V^E for the mixtures of MCT with 1-alkanols following the order is: 1-propanol < 1-butanol < 1-pentanol < 1-hexanol < 1-heptanol. This order implies that MCT shows that the strong H-bonding interactions with 1-propanol than with those for the rest of the 1-alkanols due to the nonpolar nature is increased with increasing the −CH₂ group. The higher negative V^E values of MCT in the mixture of MCT with the 1-propanol system show that the stronger solvent–solute interactions at the rich concentration region of alkanols as compared to the lower negative V^E values of MCT in the mixture of MCT with the rest of the 1-alkanols form 1-butanol to 1-heptanol, which show comparatively weaker solvent–solute interactions at the rich concentration region of alkanols.^3,5,53−55

The measurement of the speed of sound has been successfully employed to understand the nature of molecular interactions in pure liquids and their liquid mixtures. Speed of sound measurements are highly sensitive to molecular interactions and can be used to provide qualitative information about the nature of physical and chemical strength of the interaction in liquid mixtures.^38,56 The isentropic compressibilities (k_s) were calculated from the Newton–Laplace equation given below

3

where ρ is the density and u is the speed of sound of the binary mixtures.

The values of excess isentropic compressibility (k_s^E) were calculated from the following relations recommended by Benson and Kiyohara⁵⁷ and Douheret et al.⁵⁸

4

5

Here, C_p,i and α_i are molar heat capacity and thermal expansion coefficient, respectively, for the ith component of the mixture. The values of α_i were calculated from the equation α_i = (∂V/∂T)_P/V = −1/ρ(∂_ρ^–1/∂T)_P and are listed in Table 2. The volume fraction ϕ_i was calculated as

6

The plots of excess isentropic compressibility (k_s^E) with mole fraction (x₁) for the binary mixtures of MCT with 1-alkanol (C₃–C₇) at 298.15 K are depicted in Figure S3. According to Fort and Moore,⁵⁹ the negative excess isentropic compressibilities are indicative of strong interactions in the liquid mixtures, which can be attributed to charge transfer, dipole–dipole, and dipole–induced dipole interactions between component molecules. However, the positive sign indicates weaker interactions that are attributed to dispersion (London) forces, which are likely to be operative in every case. In the present study, the (k_s) values exhibit inversion behavior at all temperatures over the whole composition range. The (k_s^E) values in Figure S3 reveal that as the chain length of alkanols increase, the negative excess isentropic compressibilities decrease. This is due to weaker dipole–dipole interactions in higher alkanols as a result of decrease in polarizability.⁶⁰ The algebraic values of (k_s) for the mixtures of MCT with 1-alkanol fall in the order: 1-propanol < 1-butanol < 1-pentanol < 1-hexanol < 1-heptanol.

The excess property (y^E) was fitted by the Redlich–Kister type polynomial equation⁶¹

7

where y^E is V^E or k_s^E and x₁ and x₂ refer to the mole fractions of the pure components. The corresponding standard deviations σ were computed using the relation

8

where y_calc^E is the calculated value of excess properties, n is the number of data points, and p is the number of coefficients in eq 7, and the standard deviations of all of the binary mixtures are presented in Table 4.

Table 4. Coefficients a_i of the Redlich–Kister Type Polynomial, Equation 7 and the Corresponding Standard Deviations σ (y^E) for the Binary Mixtures MCT with 1-Alkanols at 298.15 K^a.

excess function	a0	a1	a2	σ (yE)
m-Chlorotoluene(1) + 1-Propanol(2)
VE (cm3 mol–1)	0.017	0.447	0.024	0.002
ksE (TPa–1)	–52.7	110.8	–0.3	0.3
m-Chlorotoluene(1) + 1-Butanol(2)
VE (cm3 mol–1)	0.106	0.451	–0.073	0.002
ksE (TPa–1)	–26.5	105.9	–3.0	0.2
m-Chlorotoluene(1) + 1-Pentanol(2)
VE (cm3 mol–1)	0.138	0.452	–0.019	0.001
ksE (TPa–1)	–3.3	89.7	–6.0	0.4
m-Chlorotoluene(1) + 1-Hexanol(2)
VE (cm3 mol–1)	0.181	0.478	–0.005	0.001
ksE (TPa–1)	12.6	87.0	4.6	0.2
m-Chlorotoluene(1) + 1-Heptanol(2)
VE (cm3 mol–1)	0.198	0.559	0.021	0.001
ksE (TPa–1)	24.329	72.8	8.1	0.4

^aStandard uncertainties s are s(ρ) = ±0.00004 g cm^–3, s(V^E) = ±0.005 cm³ mol^–1, s(u) = ±0.5 m s^–1, s(k_s) = ±0.9 TPa^–1, and s(k_s^E) = ±0.9 TPa^–1.

3.2. FT-IR Studies

Figures S4–S6 show the FT-IR spectrum of binary liquid mixtures of MCT and 1-alkanols at different mole fractions at the range of wavenumbers, which is reported in Table 5. From the spectrum, it is important to analyze the −O–H, −C–H, −C–O, and −C–Cl stretching bands and predict the possible molecular interactions in liquid mixtures.

Table 5. Experimental Stretching Frequency of the Functional Groups in Pure Liquids and Change of Stretching Frequency after Mixing of MCT in the Pure Solvents at Equimolar Composition (≈0.5).

	stretching frequencies (cm–1)
pure solvents	–OH (3200–3600)	–C–H (2850–2950)	–C–O (1050–1150)	–C–Cl (600–700)
MCT		2934		680
1-propanol	3320	2948	1059
1-butanol	3321	2944	1057
1-pentanol	3322	2939	1051
1-hexanol	3318	2927	1048
1-heptanol	3323	2925	1050
MCT + 1-propanol	3331	2947	1063	674
MCT + 1-butanol	3325	2942	1061	672
MCT + 1-pentanol	3328	2938	1062	675
MCT + 1-hexanol	3332	2934	1060	673
MCT + 1-heptanol	3330	2927	1061	670

3.2.1. O–H Stretching Region

Figure S4 shows that the FT-IR spectrum of pure 1-alknols (C₃–C₇) and their binary mixtures with MCT over the entire range of mole fractions in the range of 2700–3700 cm^–1. A broad band appears at 3320, 3321, 3322, 3318, and 3323 cm^–1 for pure 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, and 1-heptanol, respectively. This is a clear evidence for self-association of 1-alkanol involved in the hydrogen bonding in the pure state. When the composition of MCT is added to 1-alkanols incrementally, the O–H stretching band of pure 1-alkanol shifted to around (3321–3340) cm^–1 in all of the binary liquid mixtures. From the above interpretations, it can be concluded that the intermolecular hydrogen bonds in pure 1-alkanol molecule was broken gradually by increasing the concentration of MCT and the −OH group of the 1-alkanol molecule is directly involved in the formation of hydrogen bonding between component molecules. As can be seen in Figure S4, the sharp O–H stretching band of pure 1-alkanols diminishes and disappears completely with increasing mole fraction of MCT in all the binary mixtures. This implies that self-associated H-bond formation within pure 1-alkanol molecules decrease with increasing mass fraction of MCT.

3.2.2. C–H Stretching Region

In Figure S4, it was found that a sharp band appears at around 2850–2950 cm^–1, which is assigned to the C–H stretching vibrational frequency of pure 1-alkanol. When 1-alkanol was mixed with MCT, the shape of C–H stretching bands changed significantly to a weak band but did not disappear completely. This observation is attributed to the rupturing of van der Waals interactions in pure alkanols by increasing the concentration of MCT.

3.2.3. C–O Stretching Region

Generally, in pure 1-alkanol molecules, the oxygen atom is the polar part, which ensures dipole interactions with other solvent molecules. Figure S5 shows the C–O stretching frequencies of pure 1-alkanols, MCT, and their five binary mixtures in the range of 1000–1200 cm^–1. However, the C–O stretching frequency of pure 1-alkanols (C₃–C₇) appears at 1059, 1057, 1051, 1048, and 1050 cm^–1, respectively. Nonetheless, the C–O stretching frequencies increase as the mole fraction of MCT increases in the mixture of 1-alkanols. Undoubtedly, this apparently discloses that the formation of ion-induced dipole interactions between MCT and 1-alkanol mixtures due to the interruption of dipole–dipole interactions existing in 1-alkanol molecules.

3.2.4. C–Cl Stretching Region

Figure S6 shows the C–Cl stretching frequencies of pure MCT and binary mixtures with 1-alkanols over the entire mole fractions within 600–700 cm^–1. The band observed at 680 cm^–1 for MCT is assigned to the C–Cl stretching vibration coupled with the C–C–C in-plane deformation.⁶² Consequently, it shows that the C–Cl stretching frequencies decrease by increasing the concentration of 1-alkanol molecules in liquid mixtures. It can be observed that the C–H···Cl type of hydrogen bonds are rupturing gradually by the addition of alkanol and the chlorine atom forms weak hydrogen bonding with the hydroxyl group of 1-alkanol molecules.

3.3. DFT Explanations of Interactions between MCT and 1-Alkanols

The lowest energy configurations of 1:1 complexes of MCT with C₃–C₇ 1-alkanols are shown in Figure S7. Atom numbering in Figure S7 was arbitrary and only for the purpose of explanation of results. Results revealed that the interactions between MCT and the studied 1-alkanols feature two types of H bonds, which are OH···Cl and HO···HC. The OH···Cl bond lengths in the complexes are in the range of 2.661–2.687 Å, with maximum bond length observed for OH···Cl involving 1-propanol. Similar results have been reported for OH···Cl elsewhere.⁴⁸ The trend of OH···Cl bond lengths suggests manifestation of a stronger bond by the longer chain alkanol, although the trend is slightly tainted at 1-hexanol. The HO···HC bond lengths are in the range of 2.397–2.459 Å. The HO···HC bond length, unlike OH···Cl, does not follow a definite pattern with increasing alkanol chain length. However, the smallest HO···HC bond was observed for the dimer involving 1-propanol. The lowest energy configurations of the dimers differ in the orientation of MCT, leading to the variation of the C–H that interacts with the O–H of alkanols. The H atom on C₄ of MCT (on the basis of the numbering pattern in Figure S7) was used in interacting with 1-propanol and 1-pentanol, whereas the one on C₂ was used in 1-butanol, 1-hexanol, and 1-heptanol. No definite explanation could be provided for this inversion of MCT orientation toward 1-alkanols.

The effect of donor–acceptor interactions between MCT and 1-alkanols on neighboring bond lengths was investigated. Simultaneous lengthening and shortening of bonds directly involved in charge acceptance and donation in intermolecular H-bond formation have been reported in the literature.⁶³ The bond length of O–H (H9–O10) in isolated 1-alkanols was ca. 0.965 Å, whereas the C–Cl (C₃–Cl₈) bond length in isolated MCT was ca. 1.762 Å. Upon donor–acceptor interactions via OH···Cl (lone pair (LP)) donation from Cl atom of MCT and acceptance by antibonding orbital of alkanol (O–H) and CO···HC (donation from O atom of 1-alkanol and acceptance by the H atom of C–H bond in MCT), O–H (H9–O10) bond length slightly elongated to ca. 0.967 Å, whereas the C–Cl (C₃–C_l8) bond length was elongated to ca. 1.771 Å. The C–H bond of MCT (C₄–H₄ or C₂–H₂) involved in interaction with the O atom of alkanol is also lengthened in each case (ca. 1.086 Å for C₄–H₄, 1.085 Å for C₂–H₂). These bonds were 1.085 and 1.084 Å, respectively, in isolated MCT monomer.

The energy gaps between frontier molecular orbitals (FMOs) of MCT and 1-alkanols were calculated and listed in Table 6. The relative ease of electron transfer from MCT to 1-alkanols and converse was gauged by the values of ¹ΔE_LUMO–HOMO and ²ΔE_LUMO–HOMO. The results in Table 6 showed that 1-hexanol and 1-heptanol have the least values of both ¹ΔE_LUMO–HOMO and ²ΔE_LUMO–HOMO, suggesting that the feasibility of 1:1 complex formation between MCT and 1-alkanols increases with increasing chain length of alkanols. This is converse to the strength of interactions between MCT and 1-alkanols obtained from the experimental study.

Table 6. Frontier Molecular Orbital (FMO) Energies of 1-Alkanols and MCT^a.

molecule	EHOMO (eV)	ELUMO (eV)	1ΔELUMO–HOMO (eV)	2ΔELUMO–HOMO (eV)
1-propanol	–7.579	–0.299	6.862	6.506
1-butanol	–7.570	–0.304	6.853	6.502
1-pentanol	–7.566	–0.304	6.849	6.502
1-hexanol	–7.564	–0.306	6.847	6.500
1-heptanol	–7.563	–0.305	6.846	6.500

^a1ΔE_LUMO–HOMO is the difference between the E_LUMO of MCT (−0.717 eV) and E_HOMO of 1-alkanol, while ²ΔE_LUMO–HOMO is the difference between the E_LUMO of 1-alkanol and E_HOMO of MCT (−6.806 eV).

Stabilization energy (ΔE) and entropy change (ΔS) as a result of dimer formation were calculated as the difference between the corresponding values of the dimers and those of the isolated monomers (i.e., ΔE = E_AB – E_A – E_B, ΔS = S_AB – S_A – S_B). The values of ΔE and ΔS are listed in Table 7. The values of ΔE in Table 7 also correspond to interaction energies. Negative ΔE is indicative of energetically favorable dimer.⁴⁶ Likewise, a negative value of ΔS implies entropy favorability of dimer formation due to increased orderliness of the dimer compared to that of the isolated monomers. The results in Table 7 showed increasing magnitude of ΔE with increasing alkanol chain length. This corresponds to an increasing degree of interactions between MCT and 1-alkanols with increasing alkanol chain length. Also, the degree of orderliness in the dimer increases with increasing chain length. It is worthy of mention, however, that the effect of chain length of the alkanols on their interactions with MCT is less pronounced for alkanols that differ by only one carbon atom (−CH₃). In other words, 1:1 MCT/1-alkanol dimers of 1-propanol and 1-butanol have nearly the same values of ΔE, whereas those of 1-pentanol and 1-hexanol are also the same.

Table 7. Interaction Energies for MCT/1-Alkanol Complexes.

complex	–ΔE (kJ mol–1)	–ΔS (kJ mol–1)
m-chlorotoluene/1-propanol	7.178	96.470
m-chlorotoluene/1-butanol	7.199	110.868
m-chlorotoluene/1-pentanol	7.296	108.985
m-chlorotoluene/1-hexanol	7.296	110.566
m-chlorotoluene/1-heptanol	7.446	105.554

NBO analysis has been used in the literature to describe the nature of donor–acceptor interactions involving intermolecular hydrogen bonds.⁶³ Results of second-order perturbation (interaction) energies (E(2)) for donor–acceptor interactions in 1:1 MCT/1-alkanol dimers are listed in Table 8. The values of E(2) for the two types of lone pair (LP) to antibonding (BD*) interactions involved in the dimers, as listed in Table 8, revealed that LP(2)C_l8 → BD*(1)H9–O10 interaction is generally stronger than LP(1)O10→BD*(1)C₂–H₂ (or LP(1)O10 → BD*(1)C₄–H₄) interaction. The strength of LP(2)C_l8 → BD*(1)H9–O10 interaction increases with increasing alkanol chain length, whereas the strength of LP(1)O10 → BD*(1) C₂–H₂ (or LP(1)O10 → BD*(1)C₄–H₄) interaction does not follow a definite pattern.

Table 8. Second-Order Perturbation Energy for the Interactions of 1-Alkanols with MCT.

donor–acceptor orbitals interactionsa	E(2) (kJ mol–1)
m-Chlorotoluene/1-Propanol
LP(1)O10 → BD*(1)C4–H4	6.90
LP(2)Cl8 → BD*(1)H9–O10	9.33
m-Chlorotoluene/1-Butanol
LP(1)O10 → BD*(1)C2–H2	7.07
LP(2)Cl8 → BD*(1)H9–O10	10.00
m-Chlorotoluene/1-Pentanol
LP(1)O10 → BD*(1)C4–H4	4.90
LP(2)Cl8 → BD*(1)H9–O10	10.38
m-Chlorotoluene/1-Hexanol
LP(1)O10 → BD*(1)C2–H2	5.61
LP(2)Cl8 → BD*(1)H9–O10	10.46
m-Chlorotoluene/1-Heptanol
LP(1)O10 → BD*(1)C2–H2	5.69
LP(2)Cl8 → BD*(1)H9–O10	10.84

^aA → B implies that A is the donor orbital and B is the acceptor orbital.

The overall results of quantum chemical calculations obtained in the present study suggest that the strength of interactions between MCT and 1-alkanols increases with increasing alkyl chain length of alkanol. This, however, is contrary to experimental observations. Although the effect of chain length on such interactions is not significant between successive alkanols. The dissention between experimental and theoretical results might be due to the existence of other interactions that might exist in solution, which were not taken into consideration in the molecular modeling. However, the theoretical study provides more insights into the donor–acceptor interactions between MCT and 1-alkanols and the nature and strength of H bonds involved (Table 8).

4. Conclusions

The present study investigates the intermolecular interactions between straight-chain 1-alkanols of C₃–C₇ chain length and m-chlorotoluene (MCT) at various compositions of binary liquid mixtures. Volumetric techniques, such as densitometer, ultrasonic interferometer, and FT-IR methods, were used for the investigations. In addition, density functional theory (DFT) method was employed to provide theoretical insights into the 1-alkanol/MCT interactions. The values of excess volume (V^E) and excess isentropic compressibility (k_s^E) were determined for binary liquid mixtures containing MCT and 1-alkanols (C₃–C₇) at 298.15 K. Inversion deviations from ideality were observed for all of the binary systems over the entire composition range. The Redlich–Kister polynomial was used to correlate the results and provide a good description for V^E and k_s with respect to composition for all systems. The results revealed that there is a strong bond-making intermolecular interaction between 1-alkanol and MCT and the strength of this interaction decreases with increasing alkanol chain length. FT-IR spectra revealed that the strength of intermolecular hydrogen bonding interactions between self-associated alkanol–alkanol molecules and MCT–alkanol depends on aggregation, size, or successive increment of the methylene group (−CH₂−) of 1-alkanol molecules. The decreasing strength of intermolecular interactions between successive 1-alkanols and MCT is associated with a prospective gradual strain of the intermolecular hydrogen bond with increasing alkyl chain length of 1-alkanols. DFT calculations also showed the nature and strength of prevailing H bonds in 1:1 dimers of MCT and 1-alkanols. The DFT results predicted that the decreasing strength of the interactions of 1-alkanols with MCT follows the trend: 1-propanol > 1-butanol > 1-pentanol > 1-hexanol > 1-heptanol. Consequently, the results generally showed that a larger 1-alkanol has relatively less favorable interactions with MCT as compared to a smaller 1-alkanol. More so, the conformational properties of 1-alkanols in MCT mixtures appeared to be mainly governed by the strength of the interactions between the hydroxyl group of the alkanol and the chloro group of MCT.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01834.

• Formation of hydrogen bonding interactions of 1-alkanols (Figure S1); curves of excess volumes (V^E) vs mole fraction (x₁) for the binary mixtures of MCT + 1-propanol (■), 1-butanol (▲), 1-pentanol (◆), 1-hexanol (□), and 1-heptanol (Δ) at 298.15 K (Figure S2); curves of excess isentropic compressibilities (k_s^E) vs mole fraction (x₁) for the binary mixtures of MCT + 1-propanol (■), 1-butanol (▲), 1-pentanol (◆), 1-hexanol (□), and 1-heptanol (Δ) at 298.15 K (Figure S3); FT-IR spectrum of −OH and C–H stretching vibrations in the range of 2700–3700 cm^–1 for the binary mixtures of MCT with 1-alkanols: (a) 1-propanol, (b) 1-butanol, (c) 1-pentanol, (d) 1-hexanol, and (e) 1-heptanol at different mole fractions of MCT; 0.0 (red), 0.1 (black), 0.2 (green), 0.3 (blue), 0.4 (cyan), 0.5 (magenta), 0.6 (olive), 0.7 (orange), 0.8 (violet), and 0.9 (purple) (Figure S4); FT-IR spectra of C–O stretching vibration in the range of 1000–1200 cm^–1 for the binary mixtures of MCT with 1-alkanols; (a) 1-propanol, (b) 1-butanol, (c) 1-pentanol, (d) 1-hexanol, and (e) 1-heptanol at different mole fraction of MCT; 0.0 (red), 0.1 (black), 0.2 (green), 0.3 (blue), 0.4 (cyan), 0.5 (magenta), 0.6 (olive), 0.7 (orange), 0.8 (violet), and 0.9 (purple) (Figure S5); FT-IR spectra of C–Cl stretching vibration in the range of 600–700 cm^–1for the binary mixtures of MCT with 1-alkanols; (a) 1-propanol, (b) 1-butanol, (c) 1-pentanol, (d) 1-hexanol, and (e) 1-heptanol at different mole fraction of MCT; 0.1 (red), 0.2 (black), 0.3 (green), 0.4 (blue), 0.5 (cyan), 0.6 (magenta), 0.7 (olive), 0.8 (orange), 0.9 (violet), and pure MCT (purple) (Figure S7); gas phase-optimized structures of MCT/1-alkanol complexes (Figure S7) (PDF)

The authors declare no competing financial interest.