A stochastic search of the potential energy surface for the formic acid dimers results in 21 well-defined minima. A number of structures are reported here for the first time, others have already been experimentally detected or computationally predicted. Four types of different hydrogen bonds (HBs) are at play stabilizing the clusters: primary C=O⋯ H-O and H-O⋯ H-O and secondary C=O⋯ H-C and H-O⋯ H-C HBs corresponding to well-characterized bonding paths are identified. A novel C=O⋯ C stabilizing interaction is also reported. The double proton transfer reaction is calculated to occur in a synchronous fashion, with an energy barrier smaller than the energy needed to break up the dimers.

A wide variety of descriptors of the evolution of bonding, rooted in the formalism of quantum mechanics, but otherwise conceptually and methodologically independent of each other (based on the quantum theory of atoms in molecules and natural bond orbitals), consistently indicate that in the mechanism of the salt-free Wittig reaction, regardless of the nature of the ylide, regardless of the nature of the transition state, and regardless of the positioning of the substituents around the reactive center, the degree of advance in the formation of the emerging C–C bond as early as at the transition state for the oxaphosphetane formation step is firmly tied to the stereochemistry of the final alkene. In addition to the fast evolution of the emerging C–C bond, very early in the reaction, a long range, weak interaction between a lone pair in the oxygen atom of the carbonyl group and an empty p orbital in the phosphorous atom, resulting from the polarization of the PC bond in the ylide (n O → πPC *), clamps the PC and CO bonds to the positions required for the subsequent formation of oxaphosphetanes, thus explaining the formation of cyclic intermediates rather than betaines. Each step of the Wittig reaction is a highly asynchronous process.

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