Thiamin deprotonation mechanism: Carbanion development stabilized by the LUMOs of thiazolium and pyrimidylimine working in tandem and release governed by a H-bond switch

The analysis of orbital contributions to canonical molecular orbitals (CMOs) of six abbreviated azolium analogs of the carbanion was carried out to understand the unique function of sulfur by applying natural bond orbital (NBO) theory. The analysis has suggested the operation of an H-bonding scheme that leads to the formation of a cryptic C2 carbanion that is not accumulated and the shielding of the carbanion by the N4'-H hydrogen bond that is weakened by N1'-H deprotonation.