Quantum Chemical Calculations of the Cl⁻ + CH₃I → CH₃Cl + I⁻ Potential Energy Surface

Abstract

Electronic structure theory calculations, using MP2 theory and the DFT functionals OPBE, OLYP, HCTH407, BhandH, and B97-1, were performed to characterize the structures, vibrational frequencies, and energies for stationary points on the Cl(-) + CH(3)I --> ClCH(3) + I(-) potential energy surface. The aug-cc-pVDZ and aug-cc-pVTZ basis sets, with an effective core potential (ECP) for iodine, were employed. Single-point CCSD(T) calculations were performed to obtain the complete basis set (CBS) limit for the reaction energies. DFT was found to give significantly longer halide ion/carbon atom bond lengths for the ion-dipole complexes and central barrier transition state than MP2. BhandH, with either the aug-cc-pVDZ or aug-cc-pVTZ basis sets, gives good agreement with the experimental structures for both CH(3)I and CH(3)Cl. The frequencies of CH(3)I and CH(3)Cl, obtained with the different levels of theory and basis sets, are in excellent agreement with experiment. The major difference between the MP2 and DFT frequencies is for the imaginary frequency of the central barrier. Using the aug-cc-pVTZ basis the MP2 value for this frequency ranges from 1.26 to 1.59 times larger than those for the DFT functionals. Thus, the MP2 and DFT theories have different PES shapes in the vicinity of the [Cl--CH(3)--I](-) central barrier. The CCSD(T)/CBS energies are in good agreement with experiments for the complexation energies and reaction exothermicity, with a small 1 kcal/mol difference for the latter. The CCSD(T)/CBS central barrier height is lower than values deduced by using statistical theoretical models to fit the Cl(-) + CH(3)I --> ClCH(3) + I(-) experimental rate constant, which is consistent with the expected nonstatistical dynamics for the reaction. The BhandH energies are in overall best agreement with the CCSD(T) values, with a largest difference of only 0.7 kcal/mol.

Comparative Computational Study of Hydrogen Abstraction Reactions of CY3H + XO− (X, Y = F, Cl, and Br)

“…Usually, the principal difference between the frequencies of the DFT methods lies in the central barrier's imaginary frequency [44]. Our theoretical findings suggest that for a given transition state the DFT functionals, both B3LYP and BHandHLYP show a common structure and provided the single imaginary frequency, which turned out to be real (Figs.…”

“…Moreover, to obtain more reliable energies, high-level single-point energy calculations were performed at the CCSD(T) level of theory [19]. The CCSD(T) calculations were performed with the correlation consistent Gaussian basis sets, denoted by augcc-pVXZ (X = D, T, and Q) [20,21], based on MP2/ aug-cc-pVDZ minimum geometries [22,23]. These energies were extrapolated to the complete basis set (CBS) limit using the formula proposed by Peterson et al (like Eq 1) [24] EðnÞ…”

Fifty years of nucleophilic substitution in the gas phase

“…Stimulated by the advent of crossed‐beam reactive scattering experiments, the reactions of , and with have been extensively studied by Hase and coworkers using direct dynamics simulations (Mikosch, Trippel, et al, 2008; Xie et al, 2015, 2013; Zhang & Hase, 2010; Zhang et al, 2009, 2013, 2010). In the next section these results are discussed together with the experimental findings.…”

Gas-phase reaction of ClO−with CHnCl4-n(n= 0, 1, 2, 3) and CX3H (X = F, Cl and Br): Substituent effect from a comparative study

“…This is the standard mechanism, with in general lower energy, and is well-accepted, although recently an alternative has shown it to be "roaming". 13,14 ClO − was chosen as the nucleophile, because it has received considerable attention, 15,16 such as the reactions of ClO − with RCl (R = methyl, ethyl, isopropyl, and tert-butyl). To simplify comparisons and emphasize trends, the reaction ClO − + ClCH n Cl 3-n ¡ ClOCH n Cl 3-n + Cl − (n = 0, 1, 2, 3) was studied.…”