The Raman jet spectrum of trans-formic acid and its deuterated isotopologs: Combining theory and experiment to extend...

Revisiting recently published Raman jet spectra of monomeric formic acid with accurate high order perturbative calculations based on two explicitly correlated coupled-cluster quality potential energy surfaces from the literature, we assign and add 11 new vibrational band centers to the trans-HCOOH database and 53 for its three deuterated isotopologs. Profiting from the synergy between accurate calculations and symmetry information from depolarized Raman spectra, we reassign eight literature IR bands up to 4000 cm^-1. Experimental detection of highly excited torsional states (ν₉) of trans-HCOOH, such as 4ν₉ and ν₆ + 2ν₉, reveals substantial involvement of the C-O stretch ν₆ into the O-H bend/torsion resonance ν₅/2ν₉, which is part of a larger resonance polyad. Depolarization and isotopic C-D substitution experiments further elucidate the nature of Raman peaks in the vicinity of the O-H stretching fundamental (ν₁), which seem to be members of a large set of interacting states that can be identified and described with a polyad quantum number and that gain intensity via resonance mixing with ν₁.