Synchrotron-based infrared spectroscopy of formic acid: Confirmation of the reassignment of Fermi-coupled 8 μm states

Abstract

The empirically derived assignment of the strongly interacting 5 1 and 9 2 vibrational states of trans-HCOOH has recently been reassigned on the basis of anharmonic frequency calculations, and this, in turn, affects the assignment of many higher energy states. Here, we investigate the high-resolution synchrotron-based torsional spectrum of trans-HCOOH, and find experimental confirmation that the proposed reassignment is indeed correct, i.e., that 9 2 is in fact lower in energy than 5 1 . This is largely based on examining the intensity ratio of transitions with the same rotational quantum numbers between the 9 2 -9 1 and 5 1 -9 1 hot bands, which indicates that the 5 1 [9 2 ] state has ∼31% 9 2 [5 1 ] character. We also examined the torsional spectrum of trans-HCOOD, and find that the intensity ratios are consistent with 9 2 instead being higher in energy than 6 1 (which is analogous to 5 1 in trans-HCOOD), as previously determined from higher energy spectra.