Anharmonicity and vibrational Stark fields in phosphinic acid dimers

Phosphinic acid is unique among oxyacids of phosphorus, as it has the ability to form cyclic dimers via intermolecular hydrogen bonding, analogous to carboxylic acid dimers with exceptional stability and higher dimerization enthalpies due to stronger hydrogen bonding interactions. The strength of the hydrogen bond with different combinations of substituents on the monomeric units can be effectively studied by evaluation of electric fields along the hydrogen-bonded OH donor groups. The correlation between OH stretch vibrational frequency and electric field was linear with average Stark tuning rates of 45.6 and 11.8 cm-1 (MV cm-1)-1, respectively, for the primary and secondary substitution effects, while the corresponding values for carboxylic acid dimers 21.9 and 5.7 cm-1 (MV cm-1)-1. Furthermore, the Stark tuning rate for the anharmonic O-D frequency shifts on average was about 40-50% higher compared to the corresponding harmonic O-D frequency shifts, which suggests the presence of strong anharmonicity of O-H/O-D oscillators in phosphinic acid dimers.