Conformational Mobility and Proton Transfer in Hydrogen-Bonded Dimers and Trimers of Phosphinic and Phosphoric Acids.

The monomers, H-bonded cyclic dimers, and trimers of five acids were studied by density functional theory calculations, such as hypophosphorous acid (H2POOH, 1), dimethylphosphinic acid (Me2POOH, 2), phenylphosphinic acid (PhHPOOH, 3), dimethylphosphoric acid ((MeO)2POOH, 4), and diphenylphosphoric acid ((PhO)2POOH, 5). Particular attention was paid to the conformational manifold existing due to the internal degrees of freedom: proton transfer (PT), puckering ("twist") within the ring of H-bonds, and mobility of the substituents (namely, -Ph, -OMe, and -OPh rotations). For acid 3, the number of conformers is additionally increased because of the varying relative orientation of nonequivalent substituents in cyclic complexes. We show that 31P NMR chemical shifts (δP) are very sensitive to the details of the conformation, spanning ranges from ca. 1 ppm (for trimers of acids 1 and 2) to ca. 12 ppm (for trimers of 4). The energy barriers for the transitions between conformers are rather low (<6 kcal/mol for PTs, <2.5 kcal/mol for puckerings, and ca. <3 kcal/mol for rotations of substituents), such that the fast exchange regime in the NMR timescale and subsequent δP averaging are expected. Correlations are proposed linking the change of average δP with the H-bond energy, showing the slope of ca. 4 ppm per kcal/mol. The sensitivity of δP to the OPO angle and the OPOH dihedral angle and the geometries of both H-bonds formed by the POOH moiety are analyzed.