Gas-phase alkyl and N-alkylamino cation affinities of anionic alpha-oxygen nucleophiles (HnXO−; X = N, P, As, O, S, Se, F, Cl, Br; n = 0–2): a theoretical G2(+)M study

In this work, we studied anionic alpha-oxygen nucleophiles in which there was an atom of with one or more unshared lone-pair electrons from groups 15–17 and periods 2–4 of the periodic table positioned adjacent to the negative attacking-oxygen (i.e., HnXO−; X = N, P, As, O, S, Se, F, Cl, Br; n = 0–2); these nucleophiles were termed “alpha-agents.” Specifically, we investigated the gas-phase simple alkyl cation affinities (ACAs) and the simple N-alkylamino cation affinities (NAAMCAs) of these alpha-agents theoretically via the modified G2(+)M method. Our calculations indicate that the O–C bond of CH3OXHn is somewhat similar to that of iPrOXHn (n = 0, 1, 2; X = N, P, As, O, S, Se, F, Cl, Br). Also, steric repulsion is an important influence on these cation affinities. It appears that the kinetic gas-phase alpha effect in the SN2 reactions of interest cannot be explained by the thermodynamic proton affinity or the affinity (SuA) of the anionic or neutral nucleophile (Nu) for the substrate cation, as the gas-phase ACA (NAAMCA) was found to be linearly correlated with the PA for the alpha-agents, similar to the previously reported linear correlation of the gas-phase ACA (NAAMCA) with the PA for normal nucleophiles.