STRUCTURED ELECTRON-TRANSFER TRANSITION-STATE - VALENCE-BOND CONFIGURATION MIXING ANALYSIS AND AB-INITIO CALCULATIONS OF THE REACTIONS OF FORMALDEHYDE RADICAL-ANION WITH METHYL-CHLORIDE

Ab initio computations are performed on the competing electron transfe r (ET) and substitution (SUB) pathways between formaldehyde radical an ion and methyl chloride. The results are followed by a valence bond co nfiguration mixing (VBCM) analysis. A variety of computational procedu res starting from UHF and ROHF, with four different basis sets, on to higher levels up to UQCISD optimization, establish the existence of tw o transition states (TSs). One having a general (OH2C-CH3-Cl)(.-) stru cture corresponds to the ET pathway, and the other with the general (C H2O-C-Cl)(.-) structure corresponds to the O-alkylation (SUB(O)) pathw ay. No TS could be located for the direct C-alkylation pathway, and th e C-alkylated product is formed in a stepwise manner. The ET-TS is bon ded 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 con stant (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 o pposes the C-C approach and directs the final destiny of the path towa rd 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 suf ficient for obtaining reliable energetics. The points along the IRC pa th at UHF and UMP2 levels are highly spin contaminated and lead to err oneous conclusions when using MW (mass-weighted) coordinates. The mech anistic details and the TS stereochemistries established for the molec ular 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 unders tand the structural features of ET- and SUB(O)-TSs and also the absenc e 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 c ompared with the SUB(O)-TS.