Theoretical interpretation of the infrared lineshapes of the H- and D-bonds in liquid formic acid

A full quantum theoretical approach has been used to study the νOH experimental IR line shapes of liquid formic acid. For this purpose, a previous model [Benmalti et al., 2009], which accounts for the proportion of cyclic dimers, has been successfully adapted. The present model thus incorporates the strong anharmonic coupling between the high frequency mode and the H-bond bridge, the Davydov coupling between the excited states of the two moieties, multiple Fermi resonances between the νOH (Bu) mode and combinations of some bending modes, together with the quantum direct and indirect dampings. This model reproduces satisfactorily the main features of the experimental lineshapes of liquid hydrogenated and deuterated formic acid, by using a minimum set of independent parameters.