High-Resolution Vibrational Spectroscopy of trans-Formic Acid in Solid Parahydrogen

We report high-resolution vibrational spectra of six normal modes of trans-formic acid (FA) in rapid vapor deposited solid parahydrogen (pH(2)) with particular emphasis on the carbonyl stretching mode (nu(3)) at similar to 1770 cm(-1). Infrared spectra in the nu(3) and 2 nu(3) regions reveal that even in 99.99% enriched pH(2) samples, residual orthohydrogen (oH(2)) present in the solid preferentially clusters to FA producing measurable shifts in the nu(3) transition frequency. The individual FA(oH(2))(n) cluster peaks in the size range from n = 0 to n = 5 are resolved, permitting unambiguous assignment of the nu(3) and 2 nu(3) transition frequencies and linewidths for FA with a first solvation shell (n = 0) of only pH(2) molecules. This n = 0 feature is well fit by a Lorentzian line shape with a line width of 0.214(6) cm(-1) and 0.45(2) cm(-1) for nu(3) and 2 nu(3), respectively, which is surprisingly broad for a small nonrotating molecule trapped in solid pH(2). Implications of the broad FA nu(3) Lorentzian line shape in terms of homogeneous and inhomogeneous broadening mechanisms are discussed.