Proton relaxation and intermolecular structure of liquid formic acid: a nuclear magnetic resonance study

Proton magnetic relaxation rates in liquid formic acid have been measured and rate contributions have been separated and assigned. The assignment has been ascertained by H/D isotopic dilution series and by temperature-dependent measurements. Proton magnetic relaxation in liquid formic acid between 25degreesC and 55degreesC is dominated by magnetic dipole dipole interactions between protons, with a minor contribution due to spin rotation interactions. Suppositions in previous work of substantial additional relaxation mechanisms could not be confirmed. The intermolecular dipolar relaxation rate contribution has been used in conjunction with a theoretical model to obtain the closest intermolecular distances of proton approach and to construct the formic acid pair configuration of maximum occurrence probability in the liquid. The planar pair configuration is characterised by two intermolecular hydrogen bonds, a strong OH...O and a weaker CH...O. The pair configuration introduced could be a building block for further molecular association and chain-like structures.