A density functional theory study of dimers of hydrophosphoryl compounds and proton transfer in them

The structures of dimers of several types of dimethylphosphinous acid (CH3)(2)POH and dimethylphosphine oxide (CH3)(2)P(O)H and dimers of the corresponding perfluorinated derivatives (CF3)(2)POH and (CF3)(2)P(O)H were studied in detail by density functional theory with the PBE gradient-corrected functional and the TZ2p basis set. Fairly strong dimeric associates (2.50-10.5 kcal/mol) were shown to form thanks to O-H center dot center dot center dot O, O-H center dot center dot center dot P, and C-H center dot center dot center dot O H-bonds and dipole-dipole interactions of polar phosphoryl groups P -> O of two monomer molecules. The existence of C-H center dot center dot center dot O and the absence of P-H center dot center dot center dot O H-bonds in (CH3)(2)P(O)H dimers was substantiated by an AIM (atoms in molecules) analysis of their structures according to Bader. The reaction coordinates were calculated for synchronous transfer of two protons in (CH3)(2)POH and (CF3)(2)P(O)H dimers. Both rearrangements were shown to occur via symmetrical six-membered planar transition states with activation barriers of less than 20 kcal/mol, which was much lower than for intramolecular transfer in the corresponding monomers (47 kcal/mol for the (CH3)(2)P(O)H -> (CH3)(2)POH pair). The tautomeric transitions between the phosphinous acid and phosphine oxide forms observed experimentally in nonpolar media under mild conditions in the absence of molecules that could act as proton carriers were shown to proceed as bimolecular reactions with the intermediate formation of the corresponding dimers.