Skip to main content
NCBI home page
ACS AuthorChoice logoLink to ACS AuthorChoice
. 2022 Apr 28;126(18):2802–2810. doi: 10.1021/acs.jpca.2c01376

Benchmark Ab Initio Characterization of the Abstraction and Substitution Pathways of the Cl + CH3CN Reaction

Petra Tóth 1, Tímea Szűcs 1, Gábor Czakó 1,*
PMCID: PMC9109142  PMID: 35482972

Abstract

graphic file with name jp2c01376_0008.jpg

We investigate the reaction pathways of the Cl + CH3CN system: hydrogen abstraction, methyl substitution, hydrogen substitution, and cyanide substitution, leading to HCl + CH2CN, ClCN/CNCl + CH3, ClCH2CN + H, and CH3Cl + CN, respectively. Hydrogen abstraction is exothermic and has a low barrier, whereas the other channels are endothermic with high barriers. The latter two can proceed via a Walden inversion or front-side attack mechanism, and the front-side attack barriers are always higher. The C-side methyl substitution has a lower barrier and also a lower endothermicity than the N-side reaction. The computations utilize an accurate composite ab initio approach and the explicitly correlated CCSD(T)-F12b method. The benchmark classical and vibrationally adiabatic energies of the stationary points are determined with the most accurate CCSD(T)-F12b/aug-cc-pVQZ energies adding further contributions of the post-(T) and core correlation, scalar relativistic effects, spin–orbit coupling, and zero-point energy corrections. These contributions are found to be non-negligible to reach subchemical accuracy.

1. Introduction

One of the main goals of chemistry is to understand the mechanisms of reactions at the deepest atomic and molecular levels. For small, few-atom systems, such as the reactions of the Cl atom with H2, H2O, NH3, and CH4 molecules, modern quantum chemical methods are capable of determining the stationary-point energetics with subchemical accuracy, thereby revealing possible reaction pathways and guiding various experiments, potential energy surface (PES) developments, and reaction dynamics studies.1−20 In addition to the above-mentioned fundamental benchmark systems, reactions of more complex molecules such as C2H6,21−25 CH3OH,26−29 and CH3NH230−32 have also attracted significant attention. The main reaction pathway of these processes is hydrogen abstraction forming a HCl molecule and a radical product. However, recent studies23,24,32 revealed that several other product channels are also possible such as H + CH2ClNH2, H + CH3NHCl, NH2 + CH3Cl, and CH3 + NH2Cl in the case of the Cl + CH3NH2 reaction.32 Quantum chemical computations can determine the reaction enthalpies and barrier heights for these processes, thereby revealing their thermodynamic and kinetic requirements, respectively. Even if some of these pathways may proceed over high barriers and cannot occur at standard thermal conditions, they may be accessible by crossed-beam techniques at hyperthermal collision energies as Minton and co-workers33 did in the case of the O(3P) + CH4/C2H6/C3H8 reactions revealing novel reaction pathways such as H substitution, CH3 substitution, etc.

In the present electronic structure study, we focus on the Cl + CH3CN reaction, which has also attracted considerable experimental and theoretical attention in the past few decades.34−42 Rate constants for the Cl + CH3CN → HCl + CH2CN process were measured using different experimental techniques and were computed applying transition state theory.34−36,38−40,42 Furthermore, photo-detachment of an electron from the Cl–···CH3CN anion complex was also studied both experimentally and theoretically, thereby probing the neutral system as well.37,41 The theoretical studies usually employed the density functional theory, MP2, QCISD(T), and CCSD(T) methods with a triple-zeta basis and investigated only the H abstraction channel.40−42 Thus, we have multiple goals in the present study. First, we aim to uncover novel reaction pathways and product channels for the Cl + CH3CN reaction and determine their energetics requirements. Second, we plan to move beyond the accuracy of the previous work40−42 by using explicitly correlated coupled-cluster theory and basis sets up to quadruple-zeta quality as well as considering effects of correlation beyond the gold-standard CCSD(T) level, core electron correlation, scalar relativity, and spin–orbit coupling. These high-level ab initio results will anchor the energetics of the PES, thereby guiding future analytical PES developments and reaction dynamics simulations. Such dynamics studies have not been reported for the title reaction but are available for the F + CH3CN brother system43−46 and also for the Cl + CH4, C2H6, and CH3NH2 cousins.7−22,25,30 In these reactions, the HF or HCl rotational distributions are the key dynamics properties, whose accurate determination challenged theory for many years, especially for the HCl product.14,25,47 This adds to our motivation to investigate the title reaction to see how the CN ligand affects the HCl rotational distributions. As a first step toward this direction, in the present paper, we give the details of the high-level composite ab initio computations in Section 2, discuss the results on the stationary-point properties along the different reaction pathways in Section 3, and provide the Summary and Conclusions in Section 4.

2. Computational Details

The important stationary-point geometries of the potential energy surface of the Cl + CH3CN reaction are first determined with the restricted open-shell second-order Møller–Plesset perturbation theory (RMP2)48 using the correlation-consistent aug-cc-pVDZ basis set.49 The initial structures of the geometry optimizations are based on chemical intuition and previous studies.23,24,32 To further optimize the obtained minima and saddle-point structures and compute the harmonic vibrational frequencies, we use the restricted open-shell Hartree–Fock (ROHF)-based unrestricted explicitly correlated coupled-cluster singles, doubles, and perturbative triples (CCSD(T)-F12b)50 method with the aug-cc-pVDZ and aug-cc-pVTZ basis sets. We also compute CCSD(T)-F12b/aug-cc-pVQZ single-point energies using the most accurate geometries obtained at the CCSD(T)-F12b/aug-cc-pVTZ level of theory. To achieve subchemical accuracy, we explore further energy contributions as detailed below.

The correction of post-CCSD(T) correlations is obtained as follows:

2. 1
2. 2

where unrestricted CCSD(T),51 CCSDT,52 and CCSDT(Q)53 methods are used with the aug-cc-pVDZ basis set and the unrestricted Hartree–Fock (UHF) reference.

The core electron correlation is defined as the difference between the frozen-core (FC) and all-electron (AE) energies:

2. 3

where the FC approach correlates the valence electrons only, whereas the AE computations also correlate the 1s2 (C and N) and 2s22p6 (Cl) electrons. The FC and AE energies are obtained at the ROHF-UCCSD(T)-F12b/cc-pCVTZ-F12 level of theory.50,54

The scalar relativistic effect is calculated with the following formula:

2. 4

where second-order Douglas–Kroll (DK)55 relativistic energies are computed at the AE-CCSD(T)/aug-cc-pwCVTZ-DK level of theory.51,56

Spin–orbit (SO) corrections are determined utilizing the interacting-states approach57 using the Davidson-corrected58 all-electron multireference configuration interaction59 (MRCI+Q) method with the aug-cc-pwCVTZ basis set60 and with an active space of 5 electrons in 3 spatial 3p-like orbitals. Higher-order correlation energy effects are estimated by the Davidson correction (+Q). The SO eigenstates are provided by diagonalizing a 6 × 6 SO matrix, where the corrected MRCI energies replace the diagonal elements. The SO corrections are defined as follows:

2. 5

where SO1 and non-SO1 are the SO and non-SO ground-state energies, respectively.

All computations are carried out with the Molpro program package,61 except the CCSD(T), CCSDT, and CCSDT(Q) computations, where the energies are obtained with MRCC62,63 interfaced to Molpro.

The benchmark classical relative energies are calculated as follows:

2. 6

The benchmark adiabatic relative energies are computed as follows:

2. 7

where ΔZPE is the zero-point energy correction, which is obtained at the CCSD(T)-F12b/aug-cc-pVTZ level of theory from the harmonic frequency computations.

3. Results and Discussion

The seven pathways of the Cl(2P3/2) + CH3CN reaction are shown in Figure 1 with the benchmark classical and adiabatic energies of the stationary points relative to the reactants. The most important structural parameters of the stationary-point geometries, determined at different levels of theory, are given in Figure 2. The only exothermic reaction channel is the hydrogen abstraction (H Abs) leading to HCl and CH2CN. This channel has the benchmark classical (adiabatic) relative energy value of −2.68 (−7.42) kcal mol–1. The reaction has a small barrier of 8.15 (3.76) kcal mol–1 and a N···HCl bonded complex (H Abs MIN) in the product channel with a De (D0) dissociation energy of 5.26 (4.02) kcal mol–1. All the other reaction pathways are endothermic.

Figure 1.

Figure 1

Open in a new tab

Schematic of the potential energy surface of the Cl(2P3/2) + CH3CN reaction pathways showing the benchmark classical (adiabatic) relative energies of the stationary points.

Figure 2.

Figure 2

Open in a new tab

Structures of the stationary points corresponding to the different pathways of the Cl + CH3CN reaction showing the most important bond lengths (Å) and angles (degree) obtained with the MP2/aug-cc-pVDZ (green), CCSD(T)-F12b/aug-cc-pVDZ (blue), and CCSD(T)-F12b/aug-cc-pVTZ (red) levels of theory.

After the hydrogen abstraction, the carbon-side methyl substitution (CH3 Sub C-side) has the lowest barrier, 31.36 (28.98) kcal mol–1. This channel leads to CH3 and ClCN products with ΔE (ΔH0) = 24.43 (20.13) kcal mol–1. However, the nitrogen-side methyl substitution (CH3 Sub N-side) products (CH3 and CNCl) have the highest relative energy, 68.25 (63.31) kcal mol–1; thus, this is the most endothermic pathway. The barrier of the N-side reaction is higher by 30.79 (31.59) kcal mol–1 than that of the C-side methyl substitution, whereas the De (D0) values of the CH3 Sub complexes are quite similar, 1.24 (0.85) and 1.08 (0.69) kcal mol–1.

In the case of the hydrogen substitution (H Sub), leading to H and ClCH2CN, the relative energy of the products is 30.12 (24.94) kcal mol–1. This reaction can proceed via a Walden inversion (W) transition state (TS) or a front-side attack (FS) TS. The barrier height of the W TS is 50.69 (46.62) kcal mol–1, while that of the FS TS is substantially higher, 66.05 (61.80) kcal mol–1. The classical relative energy differences between the products and product-like minimum complexes are only 0.35 (W) and 0.42 (FS) kcal mol–1; however, considering the ZPE corrections, the D0 dissociation energies of the complexes are even closer to zero (−0.01 (W) and 0.03 (FS) kcal mol–1). Thus, it seems that these shallow product wells do not support stable vibrational ground-state complexes. Similarly to the H Sub, the cyanide substitution (CN Sub) via the Walden TS has a lower barrier than the FS pathway. Moreover, the latter one has the highest barrier of 74.08 (72.45) kcal mol–1 among the mechanisms studied in this work. This reaction leads to the CN and CH3Cl products with ΔE (ΔH0) = 38.96 (37.36) kcal mol–1, and the De (D0) value of the ClCH3···CN complex along the FS pathway is 3.90 (3.01) kcal mol–1. Note that we have not found the W MIN for CN Sub.

The diversion of the above-mentioned product–complex dissociation energies is caused by the difference in the chemical interactions between the fragments. For example, the H Abs complex (H2CCN···HCl) contains a dipole–dipole interaction that is stronger than the dipole–induced dipole interaction in the CH3 Sub complexes (ClCN/CNCl···CH3); thus, the De (D0) value of the H Abs complex is higher. In the case of the H Sub reaction, because of the extremely low polarizability of the hydrogen atom, the ClH2CCN···H complexes are quite unstable and their dissociation energies are also low. Both the H Abs (H2CCN···HCl) and CN Sub (ClCH3···CN) complexes have dipole–dipole interactions, but the dipole moment of the HCl molecule is larger than that of the CN radical; therefore, the H Abs complex is more stable.

The most important structural parameters of the stationary points of the Cl + CH3CN reaction are shown in Figure 2. They are determined at different levels of theory: MP2/aug-cc-pVDZ, CCSD(T)-F12b/aug-cc-pVDZ, and CCSD(T)-F12b/aug-cc-pVTZ. The distances obtained with MP2 and CCSD(T)-F12b mainly differ by about 0.01 Å, while the difference between the CCSD(T)-F12b bond lengths determined with the aug-cc-pVDZ and aug-cc-pVTZ basis sets is usually just about 0.001 Å. Greater, even by orders of magnitude, differences can appear in the case of large intermolecular distances because these distance deviations do not cause a significant change in the relative energy values.

The convergence of the relative energies is shown in Table 1 and Figure 3. The average deviation of the MP2/aug-cc-pVDZ and the corresponding CCSD(T)-F12b relative energies is about 3–4 kcal mol–1, but in some cases, it can also be even higher than 9 kcal mol–1. Consequently, the use of the CCSD(T)-F12b method is needed to reach chemical or subchemical accuracy. In the case of CCSD(T)-F12b, the average absolute difference for the aug-cc-pVDZ relative energies with respect to the aug-cc-pVQZ values is about 1 kcal mol–1, whereas the average deviation for the aug-cc-pVTZ relative energies is just about 0.04 kcal mol–1, showing the outstanding basis convergence of the explicitly correlated CCSD(T)-F12b method.

Table 1. Energies (kcal/mol) at Different Levels of Theory and Their Auxiliary Corrections (kcal/mol) for the Stationary Points and Product Channels of the Cl(2P3/2) + CH3CN Reaction Relative to the Reactants.

  MP2 CCSD(T)-F12b
                
stationary points aVDZa aVDZb aVTZc aVQZd δ[T]e δ[(Q)]f Δcoreg Δrelh ΔSOi classicalj ΔZPEk adiabaticl
H Abs TS 7.28 7.43 7.98 7.93 –0.36 –0.26 –0.04 +0.07 +0.80 8.15 –4.39 3.76
CH3 Sub C-side TS 33.90 29.78 31.09 31.08 –0.64 –0.36 +0.28 +0.18 +0.83 31.36 –2.38 28.98
H Sub W TS 51.62 49.71 50.31 50.27 –0.23 –0.43 +0.21 +0.05 +0.81 50.69 –4.07 46.62
CN Sub W TS 57.51 54.73 54.82 54.83 –1.32 –0.67 +0.65 +0.01 +0.78 54.27 –1.73 52.54
CH3 Sub N-side TS 64.25 60.28 60.90 61.02 –0.13 –0.09 +0.77 –0.11 +0.68 62.15 –1.58 60.57
H Sub FS TS 67.41 65.12 65.93 65.92 –0.39 –0.48 +0.18 +0.00 +0.83 66.05 –4.25 61.80
CN Sub FS TS 77.14 74.40 75.05 75.07 –1.49 –0.86 +0.64 –0.10 +0.81 74.08 –1.63 72.45
H Abs MIN –6.95 –9.26 –8.33 –8.43 –0.45 –0.09 –0.07 +0.27 +0.83 –7.94 –3.50 –11.44
CH3 Sub C-side MIN 25.53 20.72 22.29 22.25 +0.02 –0.23 +0.12 +0.20 +0.83 23.19 –3.90 19.28
H Sub W MIN 26.00 27.29 28.83 28.80 +0.15 –0.23 +0.04 +0.18 +0.83 29.77 –4.83 24.95
CH3 Sub N-side MIN 75.18 64.14 65.81 65.87 –0.15 –0.05 +0.51 +0.16 +0.83 67.17 –4.55 62.62
H Sub FS MIN 25.98 27.11 28.76 28.73 +0.15 –0.23 +0.04 +0.18 +0.83 29.70 –4.79 24.91
CN Sub FS MIN 38.82 34.44 35.21 35.22 –1.17 –0.45 +0.52 +0.11 +0.83 35.06 –0.71 34.35
HCl + CH2CN –0.78 –3.82 –3.11 –3.21 –0.48 –0.06 +0.00 +0.23 +0.83 –2.68 –4.74 –7.42
ClCN + CH3 27.06 22.05 23.55 23.48 +0.02 –0.22 +0.13 +0.20 +0.83 24.43 –4.30 20.13
ClCH2CN + H 26.30 27.65 29.19 29.14 +0.15 –0.22 +0.04 +0.18 +0.83 30.12 –5.18 24.94
ClNC + CH3 76.55 65.41 66.89 66.91 –0.16 –0.03 +0.52 +0.17 +0.83 68.25 –4.94 63.31
CH3Cl + CN 44.32 38.94 39.51 39.44 –1.59 –0.48 +0.60 +0.15 +0.83 38.96 –1.60 37.36
Open in a new tab
a

MP2/aug-cc-pVDZ relative energies obtained at MP2/aug-cc-pVDZ geometries.

b

CCSD(T)-F12b/aug-cc-pVDZ relative energies obtained at CCSD(T)-F12b/aug-cc-pVDZ geometries.

c

CCSD(T)-F12b/aug-cc-pVTZ relative energies obtained at CCSD(T)-F12b/aug-cc-pVTZ geometries.

d

CCSD(T)-F12b/aug-cc-pVQZ relative energies obtained at CCSD(T)-F12b/aug-cc-pVTZ geometries.

e

CCSDT – CCSD(T) obtained at CCSD(T)-F12b/aug-cc-pVTZ geometries with the aug-cc-pVDZ basis set.

f

CCSDT(Q) – CCSDT obtained at CCSD(T)-F12b/aug-cc-pVTZ geometries with the aug-cc-pVDZ basis set.

g

Core correlation corrections obtained as the differences between all-electron and frozen-core CCSD(T)-F12b/cc-pCVTZ-F12 relative energies at CCSD(T)-F12b/aug-cc-pVTZ geometries.

h

Scalar relativistic effects obtained as the difference between DK-AE-CCSD(T)/aug-cc-pwCVTZ-DK and AE-CCSD(T)/aug-cc-pwCVTZ relative energies at CCSD(T)-F12b/aug-cc-pVTZ geometries.

i

Spin–orbit (SO) corrections obtained as the differences between the SO and non-SO ground-state MRCI+Q/aug-cc-pwCVTZ relative energies at CCSD(T)-F12b/aug-cc-pVTZ geometries.

j

Benchmark classical relative energies obtained as CCSD(T)-F12b/aug-cc-pVQZ relative energies + δ[T] (e) + δ[(Q)] (f) + Δcore (g) + Δrel (h) + ΔSO (i).

k

Zero-point energy (ZPE) corrections obtained at CCSD(T)-F12b/aug-cc-pVTZ.

l

Benchmark vibrationally adiabatic relative energies obtained as classical relative energies (j) + ΔZPE (k).

Figure 3.

Figure 3

Open in a new tab

Convergence of the relative energies for stationary points and products of the Cl + CH3CN reaction, obtained with different levels of theory: the MP2 method with the aug-cc-pVDZ basis set and the CCSD(T)-F12b method with the aug-cc-pVDZ, aug-cc-pVTZ, and aug-cc-pVQZ basis sets.

Despite the fact that the large-basis CCSD(T)-F12b computations provide high accuracy, to approach the “exact” energies, some additional energy contributions need to be considered. These auxiliary corrections are shown in Table 1 and Figures 4 and 5. The post-CCSD(T) correlation corrections are usually found to be negative contributions. The δ[CCSDT] correction is typically larger than the δ[CCSDT(Q)] correction, and in most cases, the absolute contribution is around 0–0.5 kcal mol–1. However, for the CN Sub, the δ[CCSDT] corrections are above 1 kcal mol–1. The core correlation corrections (Δcore) are usually small and positive contributions in the range from −0.06 to 0.77 kcal mol–1. The scalar relativistic effects (Δrel) are around 0.1 kcal mol–1, and generally, these contributions are smaller than the core correlation corrections. The absolute Δcore contributions are always larger for saddle points than minima, except for H Abs, whereas in the case of the Δrel corrections, the opposite is true without exception. The contribution of the relativistic spin–orbit interaction in the Cl atom is also needed to be taken into account. The energy of the Cl atom decreases with about 0.8 kcal mol–1 due to spin–orbit coupling; thus, the relative energy of the stationary points increases with this value. Some differences can appear in the case of the saddle points; for example, for the nitrogen-side CH3 Sub TS, the contribution is just 0.68 kcal mol–1. This correction will be discussed in more detail later in this section.

Figure 4.

Figure 4

Open in a new tab

Energy contributions of the post-CCSD(T) (eqs 1 and 2) and core (eq 3) correlations, scalar relativistic effects (eq 4), and spin–orbit corrections (eq 5) for stationary points and products of the Cl + CH3CN reaction.

Figure 5.

Figure 5

Open in a new tab

Zero-point energy corrections for the stationary points and products of the Cl + CH3CN reaction.

Although some corrections have a value greater than 1 kcal mol–1, they often balance each other due to the opposite sign. Nevertheless, they are not negligible since in the case of nitrogen-side CH3 Sub, for example, the amount of corrections is 1.12–1.33 kcal mol–1, so it does not reach chemical accuracy. The harmonic ZPE correction is needed to be considered to get experimentally observable quantities. This correction has the largest effect on the relative energy values, compared to the contributions of the other auxiliary corrections. The absolute ZPE contribution is in the range of 0.7–5.2 kcal mol–1 and has an average of 3.5 kcal mol–1. The ZPE corrections turn out to be negative in all cases, and the products usually have larger corrections than the corresponding saddle points and exit-channel minima (see Table 1 and Figure 5).

Because of the above-mentioned spin–orbit (SO) interaction, the non-relativistic ground state of the Cl atom (2P) splits into two energy levels, 2P1/2 and 2P3/2. The ground 2P3/2 state has a lower energy by ε/3 than 2P, and the excited 2P1/2 state is higher in energy by 2ε/3, where ε is the SO splitting. As the Cl atom approaches the molecule, the spherical symmetry changes; thus, the fourfold degenerate ground 2P3/2 state splits into SO ground (SO1) and excited (SO2) states, and the sixfold degenerate non-relativistic 2P state also splits into a non-SO ground (non-SO1) and two non-SO excited (non-SO2 and non-SO3) states. All the resulting states are twofold degenerate. Only the SO1 and non-SO1 ground states are reactive, i.e., correlate adiabatically with electronic ground-state products. As shown in Figure 6, we consider five different directions of the Cl atom approaching the CH3CN molecule. The depth of the van der Waals wells depends on the direction. The shallowest well with a depth of 0.7/0.6 kcal mol–1 with/without the SO interaction belongs to the case when the Cl atom approaches the methyl group, while the deepest one with a depth of 1.8/2.4 kcal mol–1 is when the Cl atom attacks the molecule from the N atom. As seen, SO coupling plays an important role in the entrance channel and decreases the well depths if the interaction energy is substantial. Furthermore, as also shown in Figure 6, the gap between the SO1 and non-SO1 potentials decreases as the Cl atom approaches the CH3CN molecule, and at small intermolecular distances, the two potentials merge; as a consequence, the SO1 – non-SO1 energy differences tend to zero.

Figure 6.

Figure 6

Open in a new tab

Potential energy curves of the CH3CN···Cl system obtained at the MRCI+Q/aug-cc-pwCVTZ level of theory considering five different separation directions: The Cl atom is approaching CH3CN from the methyl group along the C3 axis (first row), approaching one H atom of the methyl group (second row), approaching perpendicularly the C–C bond (third row) and the C–N bond (fourth row), and approaching the N atom along the C3 axis (fifth row). The CH3CN unit is kept frozen at its equilibrium geometry. The distance dependence of the difference between the spin–orbit (SO) and non-SO ground-state energies is shown on the right panels.

We have used the Active Thermochemical Tables (ATcT)64,65 to compare the computed benchmark vibrationally adiabatic relative energies with the available “experimental” data. Three reaction enthalpies can be compared: The ATcT (present computed) 0 K reaction enthalpies are −7.01 ± 0.17 (−7.42), 20.10 ± 0.13 (20.13), and 38.65 ± 0.09 (37.36) kcal mol–1 for H Abs, CH3 Sub, and CN Sub, respectively. The CH3 Sub reaction gives the best agreement, and the difference between the experimental and the present computed values is only 0.03 kcal mol–1. For the H Abs, this deviation is 0.41 kcal mol–1, which is still well below chemical accuracy. In the case of CN Sub, the absolute deviation is much greater; it is 1.29 kcal mol–1, indicating some issue with both/either theory and/or ATcT.

4. Summary and Conclusions

In this study, the different pathways of the Cl + CH3CN reaction have been investigated. In addition to the already studied H abstraction, we have also examined the H substitution, CH3 substitution, and CN substitution reaction paths. The first mentioned H Abs, leading to the HCl and CH2CN products, is the only exothermic (ΔH0 = −7.42 kcal mol–1) reaction; however, it also has a small classical (adiabatic) barrier of 8.15 (3.76) kcal mol–1. The H substitution (with the products of H and ClCH2CN) can proceed via a Walden-inversion transition state and also a front-side attack TS. The Walden-inversion pathway has a lower barrier by about 15 kcal mol–1. This case is similar to the CN substitution, leading to CN and CH3Cl products (ΔH0 = 37.36 kcal mol–1): The Walden-inversion barrier height is 54.27 (52.54) kcal mol–1, whereas the front-side attack TS has a higher relative energy of 74.08 (72.45) kcal mol–1. Two reaction channels are possible in the case of the CH3 substitution: The Cl atom can bind to the molecule at the C atom or at the N atom of the CN group, so the products can be CH3 + ClCN (ΔH0 = 20.13 kcal mol–1) or CH3 + CNCl (ΔH0 = 63.31 kcal mol–1). The stationary-point properties have been investigated using a high-level composite ab initio method. We have determined the benchmark classical and vibrationally adiabatic energies using the most accurate CCSD(T)-F12b/aug-cc-pVQZ values refined with the contributions of the post-(T) and core correlation, scalar relativistic effects, spin–orbit coupling, and ZPE corrections. The post-(T) correlation effects are substantial, especially for the CN Sub reactions. The core and scalar relativistic corrections are usually small and positive contributions, while the SO coupling effect almost always means a relative energy increase of 0.8 kcal mol–1. Based on this study, in the future, it is possible to develop a global analytical potential energy surface for the Cl + CH3CN system, allowing dynamic investigations to gain deeper insight into the atomic-level mechanisms of this multichannel reaction.

Acknowledgments

We thank the National Research, Development and Innovation Office–NKFIH, K-125317; the Ministry of Human Capacities, Hungary grant 20391-3/2018/FEKUSTRAT; project no. TKP2021-NVA-19, provided by the Ministry of Innovation and Technology of Hungary from the National Research, Development and Innovation Fund, financed under the TKP2021-NVA funding scheme; and the Momentum (Lendület) Program of the Hungarian Academy of Sciences for financial support.

The authors declare no competing financial interest.

References

  1. Persky A. Quasiclassical Trajectory Studies of the Chlorine–Hydrogen System. I. Cl + H2 → HCl + H. J. Chem. Phys. 1977, 66, 2932–2942. 10.1063/1.434362. [DOI] [Google Scholar]
  2. Kettwich S. C.; Raston P. L.; Anderson D. T. The Cl + H2 → HCl + H Reaction Induced by IR + UV Irradiation of Cl2 in Solid para-H2: Experiment. J. Phys. Chem. A 2009, 113, 7621–7629. 10.1021/jp811206a. [DOI] [PubMed] [Google Scholar]
  3. Jiang B.; Guo H. Control of Mode/Bond Selectivity and Product Energy Disposal by the Transition State: X + H2O (X = H, F, O(3P), and Cl) Reactions. J. Am. Chem. Soc. 2013, 135, 15251–15256. 10.1021/ja408422y. [DOI] [PubMed] [Google Scholar]
  4. Zhao B.; Sun Z.; Guo H. Communication: State-to-State Dynamics of the Cl + H2O → HCl + OH Reaction: Energy Flow into Reaction Coordinate and Transition-State Control of Product Energy Disposal. J. Chem. Phys. 2015, 142, 241101. 10.1063/1.4922650. [DOI] [PubMed] [Google Scholar]
  5. Monge-Palacios M.; Espinosa-Garcia J. Reaction-Path Dynamics Calculations of the Cl + NH3 Hydrogen Abstraction Reaction: The Role of the Intermediate Complexes. Phys. Chem. A 2010, 114, 4418–4426. 10.1021/jp911664t. [DOI] [PubMed] [Google Scholar]
  6. Moradi C. P.; Xie C.; Kaufmann M.; Guo H.; Douberly G. E. Two-Center Three-Electron Bonding in ClNH3 Revealed via Helium Droplet Infrared Laser Stark Spectroscopy: Entrance Channel Complex along the Cl + NH3 → ClNH2 + H Reaction. J. Chem. Phys. 2016, 144, 164301. 10.1063/1.4947089. [DOI] [PubMed] [Google Scholar]
  7. Espinosa-García J.; Corchado J. C. Analytical Potential Energy Surface for the CH4 + Cl → CH3 + ClH Reaction: Application of the Variational Transition State Theory and Analysis of the Kinetic Isotope Effects. J. Chem. Phys. 1996, 105, 3517–3523. 10.1063/1.472219. [DOI] [Google Scholar]
  8. Kim Z. H.; Alexander A. J.; Bechtel H. A.; Zare R. N. Comparison of Near-Threshold Reactivity of Ground-State and Spin-Orbit Excited Chlorine Atoms with Methane. J. Chem. Phys. 2001, 115, 179–183. 10.1063/1.1378042. [DOI] [Google Scholar]
  9. Yoon S.; Henton S.; Zivkovic A. N.; Crim F. F. The Relative Reactivity of the Stretch–Bend Combination Vibrations of CH4 in the Cl(2P3/2) + CH4 Reaction. J. Chem. Phys. 2002, 116, 10744–10752. 10.1063/1.1476318. [DOI] [Google Scholar]
  10. Castillo J. F.; Aoiz F. J.; Bañares L. Quasiclassical Trajectory Study of the Cl + CH4 Reaction Dynamics on a Quadratic Configuration Interaction with Single and Double Excitation Interpolated Potential Energy Surface. J. Chem. Phys. 2006, 125, 124316. 10.1063/1.2357741. [DOI] [PubMed] [Google Scholar]
  11. Yan S.; Wu Y. T.; Zhang B.; Yue X.-F.; Liu K. Do Vibrational Excitations of CHD3 Preferentially Promote Reactivity Toward the Chlorine Atom?. Science 2007, 316, 1723–1726. 10.1126/science.1142313. [DOI] [PubMed] [Google Scholar]
  12. Wang F.; Lin J.-S.; Liu K. Steric Control of the Reaction of CH Stretch-Excited CHD3 with Chlorine Atom. Science 2011, 331, 900–903. 10.1126/science.1199771. [DOI] [PubMed] [Google Scholar]
  13. Greaves S. J.; Rose R. A.; Abou-Chahine F.; Glowacki D. R.; Troya D.; Orr-Ewing A. J. Quasi-Classical Trajectory Study of the Dynamics of the Cl + CH4 → HCl + CH3 Reaction. Phys. Chem. Chem. Phys. 2011, 13, 11438–11445. 10.1039/c0cp02694h. [DOI] [PubMed] [Google Scholar]
  14. Czakó G.; Bowman J. M. Dynamics of the Reaction of Methane with Chlorine Atom on an Accurate Potential Energy Surface. Science 2011, 334, 343–346. 10.1126/science.1208514. [DOI] [PubMed] [Google Scholar]
  15. Meng F.; Yan W.; Wang D. Quantum Dynamics Study of the Cl + CH4 → HCl + CH3 Reaction: Reactive Resonance, Vibrational Excitation Reactivity, and Rate Constants. Phys. Chem. Chem. Phys. 2012, 14, 13656–13662. 10.1039/c2cp41917c. [DOI] [PubMed] [Google Scholar]
  16. Wang F.; Liu K.; Rakitzis T. P. Revealing the Stereospecific Chemistry of the Reaction of Cl with Aligned CHD3(v1 = 1). Nat. Chem. 2012, 4, 636–641. 10.1038/nchem.1383. [DOI] [PubMed] [Google Scholar]
  17. Czakó G.; Bowman J. M. Reaction Dynamics of Methane with F, O, Cl, and Br on ab Initio Potential Energy Surfaces. J. Phys. Chem. A 2014, 118, 2839–2864. 10.1021/jp500085h. [DOI] [PubMed] [Google Scholar]
  18. Li Y.; Suleimanov Y. V.; Green W. H.; Guo H. Quantum Rate Coefficients and Kinetic Isotope Effect for the Reaction Cl + CH4 → HCl + CH3 from Ring Polymer Molecular Dynamics. J. Phys. Chem. A 2014, 118, 1989–1996. 10.1021/jp501043z. [DOI] [PubMed] [Google Scholar]
  19. Fu B.; Shan X.; Zhang D. H.; Clary D. C. Recent Advances in Quantum Scattering Calculations on Polyatomic Bimolecular Reactions. Chem. Soc. Rev. 2017, 46, 7625–7649. 10.1039/C7CS00526A. [DOI] [PubMed] [Google Scholar]
  20. Pan H.; Wang F.; Czakó G.; Liu K. Direct Mapping of the Angle-Dependent Barrier to Reaction for Cl + CHD3 Using Polarized Scattering Data. Nat. Chem. 2017, 9, 1175–1180. 10.1038/nchem.2858. [DOI] [PubMed] [Google Scholar]
  21. Kandel S. A.; Rakitzis T. P.; Lev-On T.; Zare R. N. Dynamics for the Cl + C2H6 → HCl + C2H5 Reaction Examined Through State-Specific Angular Distributions. J. Chem. Phys. 1996, 105, 7550–7559. 10.1063/1.472581. [DOI] [Google Scholar]
  22. Rangel C.; Espinosa-Garcia J. Full-Dimensional Analytical Potential Energy Surface Describing the Gas-Phase Cl + C2H6 Reaction and Kinetics Study of Rate Constants and Kinetic Isotope Effects. Phys. Chem. Chem. Phys. 2018, 20, 3925–3938. 10.1039/C7CP07592H. [DOI] [PubMed] [Google Scholar]
  23. Papp D.; Gruber B.; Czakó G. Detailed Benchmark ab Initio Mapping of the Potential Energy Surfaces of the X + C2H6 [X = F, Cl, Br, I] Reactions. Phys. Chem. Chem. Phys. 2019, 21, 396–408. 10.1039/C8CP06445H. [DOI] [PubMed] [Google Scholar]
  24. Gruber B.; Czakó G. Benchmark ab Initio Characterization of the Abstraction and Substitution Pathways of the OH + CH4/C2H6 Reactions. Phys. Chem. Chem. Phys. 2020, 22, 14560–14569. 10.1039/D0CP02560G. [DOI] [PubMed] [Google Scholar]
  25. Papp D.; Tajti V.; Győri T.; Czakó G. Theory Finally Agrees with Experiment for the Dynamics of the Cl + C2H6 Reaction. J. Phys. Chem. Lett. 2020, 11, 4762–4767. 10.1021/acs.jpclett.0c01263. [DOI] [PMC free article] [PubMed] [Google Scholar]
  26. Lu D.; Behler J.; Li J. Accurate Global Potential Energy Surfaces for the H + CH3OH Reaction by Neural Network Fitting with Permutation Invariance. J. Phys. Chem. A 2020, 124, 5737–5745. 10.1021/acs.jpca.0c04182. [DOI] [PubMed] [Google Scholar]
  27. Weichman M. L.; DeVine J. A.; Babin M. C.; Li J.; Guo L.; Ma J.; Guo H.; Neumark D. M. Feshbach Resonances in the Exit Channel of the F + CH3OH → HF + CH3O Reaction Observed Using Transition-State Spectroscopy. Nat. Chem. 2017, 9, 950–955. 10.1038/nchem.2804. [DOI] [PubMed] [Google Scholar]
  28. Lu D.; Li J.; Guo H. Comprehensive Investigations of the Cl + CH3OH → HCl + CH3O/CH2OH Reaction: Validation of Experiment and Dynamic Insights. CCS Chem. 2020, 2, 882–894. 10.31635/ccschem.020.202000195. [DOI] [Google Scholar]
  29. Roncero O.; Zanchet A.; Aguado A. Low Temperature Reaction Dynamics for CH3OH + OH Collisions on a New Full Dimensional Potential Energy Surface. Phys. Chem. Chem. Phys. 2018, 20, 25951–25958. 10.1039/C8CP04970J. [DOI] [PMC free article] [PubMed] [Google Scholar]
  30. Rudić S.; Murray C.; Harvey J. N.; Orr-Ewing A. J. The Product Branching and Dynamics of the Reaction of Chlorine Atoms with Methylamine. Phys. Chem. Chem. Phys. 2003, 5, 1205–1212. 10.1039/b211626j. [DOI] [Google Scholar]
  31. Taylor M. S.; Ivanic S. A.; Wood G. P. F.; Easton C. J.; Bacskay G. B.; Radom L. Hydrogen Abstraction by Chlorine Atom from Small Organic Molecules Containing Amino Acid Functionalities: An Assessment of Theoretical Procedures. J. Phys. Chem. A 2009, 113, 11817–11832. 10.1021/jp9029437. [DOI] [PubMed] [Google Scholar]
  32. Szűcs T.; Czakó G. Benchmark ab Initio Stationary-Point Characterization of the Complex Potential Energy Surface of the Multi-Channel Cl + CH3NH2 Reaction. Phys. Chem. Chem. Phys. 2021, 23, 10347–10356. 10.1039/D0CP06392D. [DOI] [PubMed] [Google Scholar]
  33. Garton D. J.; Minton T. K.; Troya D.; Pascual R.; Schatz G. C. Hyperthermal Reactions of O(3P) with Alkanes: Observations of Novel Reaction Pathways in Crossed-Beams and Theoretical Studies. J. Phys. Chem. A 2003, 107, 4583–4587. 10.1021/jp0226026. [DOI] [Google Scholar]
  34. Poulet G.; Laverdet G.; Jourdain J. L.; Le Bras G. Kinetic Study of the Reactions of Acetonitrile with chlorine (Cl) and hydroxyl Radicals. J. Phys. Chem. 1984, 88, 6259–6263. 10.1021/j150669a041. [DOI] [Google Scholar]
  35. Olbregts J.; Brasseur G.; Arijs E. Reaction of Acetonitrile and Chlorine Atoms. J. Photochem. 1984, 24, 315–322. 10.1016/0047-2670(84)80013-1. [DOI] [Google Scholar]
  36. Kurylo M. J.; Knable G. L. A Kinetlcs Investigation of the Gas-Phase Reactions of Cl(2P) and OH(X2Π) with CH3CN: Atmospheric Significance and Evidence for Decreased Reactivity between Strong Electrophiles. J. Phys. Chem. 1984, 88, 3305–3308. 10.1021/j150659a033. [DOI] [Google Scholar]
  37. Markovich G.; Perera L.; Berkowitz M. L.; Cheshnovsky O. The Solvation of Cl–, Br–, and I– in Acetonitrile Clusters: Photoelectron Spectroscopy and Molecular Dynamics Simulations. J. Chem. Phys. 1996, 105, 2675–2685. 10.1063/1.472131. [DOI] [Google Scholar]
  38. Tyndall G. S.; Orlando J. J.; Wallington T. J.; Sehested J.; Nielsen O. J. Kinetics of the Reactions of Acetonitrile with Chlorine and Fluorine Atoms. J. Phys. Chem. 1996, 100, 660–668. 10.1021/jp9521417. [DOI] [Google Scholar]
  39. Tyndall G. S.; Orlando J. J.; Wallington T. J.; Hurley M. D. Products of the Chlorine-Atom- and Hydroxyl-Radical-Initiated Oxidation of CH3CN. J. Phys. Chem. A 2001, 105, 5380–5384. 10.1021/jp004318p. [DOI] [Google Scholar]
  40. Li Q. S.; Wang C. Y. A Theoretical Study on the Two Reactions of Acetonitrile with Atomic Chlorine and Bromine. J. Phys. Chem. A 2002, 106, 8883–8890. 10.1021/jp0144707. [DOI] [Google Scholar]
  41. Rode M. F.; Roszak S.; Szymczak J. J.; Sadlej J.; Leszczynski J. The Effect of Electron Detachment on the Structure and Properties of the Chlorine-Acetonitrile Anionic Complex. J. Chem. Phys. 2004, 121, 6277–6281. 10.1063/1.1788653. [DOI] [PubMed] [Google Scholar]
  42. Pei K.; Li H. Direct ab Initio Dynamics Studies of Hydrogen Abstraction Reaction: Cl + CH3CN → HCl + CH2CN. J. Mol. Struct.: THEOCHEM 2004, 677, 67–71. 10.1016/j.theochem.2004.02.017. [DOI] [Google Scholar]
  43. Bogan D. J.; Setser D. W. HF Infrared Chemiluminescence: Energy Disposal and the Role of the Radical Fragment in the Abstraction of Hydrogen from Polyatomic Molecules by F Atoms. J. Chem. Phys. 1976, 64, 586–602. 10.1063/1.432249. [DOI] [Google Scholar]
  44. Smith D. J.; Setser D. W.; Kim K. C.; Bogan D. J. HF Infrared Chemiluminescence. Relative Rate Constants for Hydrogen Abstraction from Hydrocarbons, Substituted Methanes, and Inorganic Hydrides. J. Phys. Chem. 1977, 81, 898–905. 10.1021/j100524a019. [DOI] [Google Scholar]
  45. Dehe K.; Heydtmann H. HF Infrared Emission from the Reactions of Atomic Fluorine with Methylcyanide, Methylisocyanide, Dimethylsulfide and Dimethyldisulfide. Chem. Phys. Lett. 1996, 262, 683–688. 10.1016/S0009-2614(96)01148-7. [DOI] [Google Scholar]
  46. Pratihar S.; Ma X.; Xie J.; Scott R.; Gao E.; Ruscic B.; Aquino A. J. A.; Setser D. W.; Hase W. L. Post-Transition State Dynamics and Product Energy Partitioning Following Thermal Excitation of the F···HCH2CN Transition State: Disagreement with Experiment. J. Chem. Phys. 2017, 147, 144301. 10.1063/1.4985894. [DOI] [PubMed] [Google Scholar]
  47. Rakitzis T. P. Transition States and Spin-Orbit Structure. Science 2021, 371, 886–887. 10.1126/science.abg3184. [DOI] [PubMed] [Google Scholar]
  48. Amos R. D.; Andrews J. S.; Handy N. C.; Knowles P. J. Open-Shell Møller–Plesset Perturbation Theory. Chem. Phys. Lett. 1991, 185, 256–264. 10.1016/S0009-2614(91)85057-4. [DOI] [Google Scholar]
  49. Dunning T. H. Jr. Gaussian Basis Sets for Use in Correlated Molecular Calculations. I. The Atoms Boron Through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007–1023. 10.1063/1.456153. [DOI] [Google Scholar]
  50. Knizia G.; Adler T. B.; Werner H.-J. Simplified CCSD(T)-F12 Methods: Theory and Benchmarks. J. Chem. Phys. 2009, 130, 054104 10.1063/1.3054300. [DOI] [PubMed] [Google Scholar]
  51. Raghavachari K.; Trucks G. W.; Pople J. A.; Head-Gordon M. A Fifth-Order Perturbation Comparison of Electron Correlation Theories. Chem. Phys. Lett. 1989, 157, 479–483. 10.1016/S0009-2614(89)87395-6. [DOI] [Google Scholar]
  52. Noga J.; Bartlett R. J. The Full CCSDT Model for Molecular Electronic Structure. J. Chem. Phys. 1987, 86, 7041–7050. 10.1063/1.452353. [DOI] [Google Scholar]
  53. Kállay M.; Gauss J. Approximate Treatment of Higher Excitations in Coupled-Cluster Theory. J. Chem. Phys. 2005, 123, 214105. 10.1063/1.2121589. [DOI] [PubMed] [Google Scholar]
  54. Hill J. G.; Mazumder S.; Peterson K. A. Correlation Consistent Basis Sets for Molecular Core-Valence Effects with Explicitly Correlated Wave Functions: The Atoms B–Ne and Al–Ar. J. Chem. Phys. 2010, 132, 054108 10.1063/1.3308483. [DOI] [PubMed] [Google Scholar]
  55. Douglas M.; Kroll N. M. Quantum Electrodynamical Corrections to the Fine Structure of Helium. Ann. Phys. 1974, 82, 89–155. 10.1016/0003-4916(74)90333-9. [DOI] [Google Scholar]
  56. de Jong W. A.; Harrison R. J.; Dixon D. A. Parallel Douglas–Kroll Energy and Gradients in NWChem: Estimating Scalar Relativistic Effects Using Douglas–Kroll Contracted Basis Sets. J. Chem. Phys. 2001, 114, 48–53. 10.1063/1.1329891. [DOI] [Google Scholar]
  57. Berning A.; Schweizer M.; Werner H.-J.; Knowles P. J.; Palmieri P. Spin-Orbit Matrix Elements for Internally Contracted Multireference Configuration Interaction Wavefunctions. Mol. Phys. 2000, 98, 1823–1833. 10.1080/00268970009483386. [DOI] [Google Scholar]
  58. Langhoff S. R.; Davidson E. R. Configuration Interaction Calculations on the Nitrogen Molecule. Int. J. Quantum Chem. 1974, 8, 61–72. 10.1002/qua.560080106. [DOI] [Google Scholar]
  59. Werner H.-J.; Knowles P. J. An Efficient Internally Contracted Multiconfiguration–Reference Configuration Interaction Method. J. Chem. Phys. 1988, 89, 5803–5814. 10.1063/1.455556. [DOI] [Google Scholar]
  60. Peterson K. A.; Dunning T. H. Jr. Accurate Correlation Consistent Basis Sets for Molecular Core–Valence Correlation Effects: The Second Row Atoms Al–Ar, and the First Row Atoms B–Ne Revisited. J. Chem. Phys. 2002, 117, 10548–10560. 10.1063/1.1520138. [DOI] [Google Scholar]
  61. Werner H.-J.; Knowles P. J.; Knizia G.; Manby F. R.; Schütz M.; Celani P.; Györffy W.; Kats D.; Korona T.; Lindh R., et al. Molpro, version 2015.1, a package of ab initio programs, see http://www.molpro.net (accessed February 25, 2022).
  62. Kállay M.; Nagy P. R.; Mester D.; Rolik Z.; Samu G.; Csontos J.; Csóka J.; Szabó B. P.; Gyevi-Nagy L.; Hégely B., et al. Mrcc, a quantum chemical program suite, see www.mrcc.hu (accessed February 25, 2022).
  63. Kállay M.; Nagy P. R.; Mester D.; Rolik Z.; Samu G.; Csontos J.; Csóka J.; Szabó P. B.; Gyevi-Nagy L.; Hégely B. The MRCC Program System: Accurate Quantum Chemistry from Water to Proteins. J. Chem. Phys. 2020, 152, 074107 10.1063/1.5142048. [DOI] [PubMed] [Google Scholar]
  64. Ruscic B.; Pinzon R. E.; Morton M. L.; von Laszewski G.; Bittner S.; Nijsure S. G.; Amin K. A.; Minkoff M.; Wagner A. F. Introduction to Active Thermochemical Tables: Several “Key” Enthalpies of Formation Revisited. J. Phys. Chem. A 2004, 108, 9979–9997. 10.1021/jp047912y. [DOI] [Google Scholar]
  65. Ruscic B.; Bross D. H.. Active Thermochemical Tables (ATcT) values based on ver. 1.122r of the Thermochemical Network, 2021, available at ATcT.anl.gov (Accessed February, 25, 2022).

Articles from The Journal of Physical Chemistry. a are provided here courtesy of American Chemical Society

RESOURCES