Structured electron transfer transition state. Valence bond configuration mixing analysis and ab initio calculations of the reactions of formaldehyde radical anion with methyl chloride

Abstract

Ab initio computations are performed on the competing electron transfer (ET) and substitution (SUB) pathways between formaldehyde radical anion and methyl chloride. The results are followed by a valence bond configuration mixing (VBCM) analysis. A variety of computational procedures starting from UHF and ROHF, with four different basis sets, on to higher levels up to UQCISD optimization, establish the existence of two transition states (TSs). One having a general (OH₂C---CH₃---Cl)•- structure corresponds to the ET pathway, and the other with the general (CH₂O---C---Cl)•- structure corresponds to the O-alkylation (SUB(O)) pathway. No TS could be located for the direct C-alkylation pathway, and the C-alkylated product is formed in a stepwise manner. The ET-TS is bonded and has a distinct stereochemistry compared to the analogous SUB(O)-TS. A detailed path-following study, starting from the ET-TS in the direction of products, shows that the mode with the negative force constant (the reaction vector) along the reaction coordinate changes its character from a C---C approach to a C---C recoil. The recoil is triggered by the flapping mode of the two hydrogens of the formyl group which opposes the C---C approach and directs the final destiny of the path toward dissociative ET. A systematic computational study establishes that the ROHF/6-31G* level is a consistent level for geometry optimization, while a single point calculation UCCSD(T)/6-31+G*//ROHF/6-31G* is sufficient for obtaining reliable energetics. The points along the IRC path at UHF and UMP2 levels are highly spin contaminated and lead to erroneous conclusions when using MW (mass-weighted) coordinates. The mechanistic details and the TS stereochemistries established for the molecular calculations in vacuum persist when the calculations are repeated with the SCRF method based on the simple reaction field solvent model. Orbital selection rules are derived from the VBCM analysis to understand the structural features of ET- and SUB(O)-TSs and also the absence of a TS for the direct C-alkylation pathway. Probes are proposed to test the bonded nature of the ET-TS and its distinct stereochemistry compared with the SUB(O)-TS.