The formic acid dimer (HCOOH)2 probed by time-resolved structure selective spectroscopy

Abstract

Rotationally resolved spectra of the formic acid dimer (HCOOH)2 have been recorded by femtosecond degenerate four-wave mixing (fs DFWM) in the gas cell at room temperature (≈10 mbar, 298 K) and under supersonic jet conditions (≈60 K). Rotational recurrences of both, the monomeric and the dimeric species have been identified and assigned by fitted simulations of the fs DFWM spectra. For the first time the study of molecular clusters by fs DFWM is reported. From the fitted simulation based on a general nonrigid asymmetric rotor model the rotational and centrifugal distortion constants, and parametrized polarizability were derived for the dimeric structure of type O–H⋯O/O⋯H–O as well as for the formic acid monomer. With the assumption of unperturbed monomers a center-of-mass distance of R=2.990±0.001 Å for the monomers within the dimer is calculated from the spectroscopic results. The structure of the formic acid dimer and a possible isomer of the type C–H⋯O/O⋯H–O is discussed and compared to recent ab initio calculations.