Proton transfer in the molecular complexes of phosphorus acids with DMSO

Proton transfer processes in various H-bonded complexes of phosphoric (H3PO4), phosphorous (H3PO3) and methylphosphonic (H2MePO3) acids with dimethyl sulfoxide (DMSO), i.e., (acid)(2)-DMSO, acid-(DMSO) (n) for n = 1, 2 and H3PO4-(DMSO)(3) have been studied. The potential energy surface (PES) for proton transfer was investigated using the B3LYP level of theory, and the solvent effect on the PES was included using the conductor polarized continuum model. The energy curves for proton transfer from acid to DMSO oxygen atom in all investigated complexes represent single-well potentials, if no constraints are imposed on the system. The PES obtained by optimizing the geometry of each complex at fixed O center dot center dot center dot O distances has two nonsymmetric minima with respect to the energy barrier. In these cases, the distance at which a barrier starts to rise is slightly different. The energy barrier for proton transfer in all considered complexes increases in the series of acids: H3PO4 < H3PO3 < H2MePO3. The energies associated with proton transfer in the complexes of all investigated acid with DMSO become higher with increasing number of DMSO molecules. For all complexes, the effect of DMSO environment favors a proton transfer.