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

Abstract

We investigate the reaction pathways of the Cl + CH 3 CN system: hydrogen abstraction, methyl substitution, hydrogen substitution, and cyanide substitution, leading to HCl + CH 2 CN, ClCN/CNCl + CH 3 , ClCH 2 CN + H, and CH 3 Cl + 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.

Photochemical and Single Electron Transfer Generation of 2′-Deoxycytidin-N4-yl Radical from Oxime Esters

“…When the exothermic tautomerization of 1-MeCyt-Nπ · to the iminyl radical (1-MeCyt-Nσ · ) is taken into account, the N–H bond dissociation enthalpy decreases to ∼104 kcal/mol (DFT) and 103 kcal/mol (G4) kcal/mol (Figure S8) which are in agreement with previous results . Bond strengths of the C–H bonds in ACN and methanol are less than this. − Thus, hydrogen atom abstraction from these organic cosolvents by dC· is exothermic, and these could be the sources of the hydrogen atom required for dC formation. Furthermore, dC· should be able to abstract hydrogen atoms from the 2′-deoxyribose backbone and thymine methyl group in DNA even more readily. , …”

“…Consistent with previous work, the DFT calculations predict that the N–H bond dissociation enthalpy of the parent (1-MeCyt) to π-type aminyl radical (1-MeCyt-Nπ · ) is 107 kcal/mol, and the G4 level calculations agree with a value of 106 kcal/mol (Scheme and Figure S8). , …”

“…46 Consistent with previous work, the DFT calculations predict that the N−H bond dissociation enthalpy of the parent (1-MeCyt) to π-type aminyl radical (1-MeCyt-Nπ • ) is 107 kcal/mol, and the G4 level calculations agree with a value of 106 kcal/mol (Scheme 3 and Figure S8). 48,49 When the exothermic tautomerization of 1-MeCyt-Nπ • to the iminyl radical (1-MeCyt-Nσ • ) is taken into account, the N−H bond dissociation enthalpy decreases to ∼104 kcal/mol (DFT) and 103 kcal/mol (G4) kcal/mol (Figure S8) which are in agreement with previous results. 46 Bond strengths of the C−H bonds in ACN and methanol are less than this.…”

Benchmark ab initio potential energy surface mapping of the F + CH3NH2 reaction

“…What comes after the previous work on the title reaction? First, it is important to note that the previous computations only considered the H-abstraction channels, whereas our recent studies on the F/Cl/Br/I/OH + C2H6 20,21 and Cl + CH3NH2/CH3CN 22,23 reactions revealed several other channels such as hydrogen and methyl substitution for the reactions of C2H6 and the picture is even more complex in the cases of CH3NH2 and CH3CN. Thus, in the present study, we plan to characterize all the possible, chemically relevant pathways of the title reaction, thereby providing new qualitative insights into the mechanisms of the F + CH3NH2 process.…”