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 51 and 92 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 92 is in fact lower in energy than 51. This is largely based on examining the intensity ratio of transitions with the same rotational quantum numbers between the 92-91 and 51-91 hot bands, which indicates that the 51 [92] state has ∼31% 92 [51] character. We also examined the torsional spectrum of trans-HCOOD, and find that the intensity ratios are consistent with 92 instead being higher in energy than 61 (which is analogous to 51 in trans-HCOOD), as previously determined from higher energy spectra.

Increasing the weights in the molecular work-out of cis- and trans-formic acid: extension of the vibrational database via deuteration

“…Considering that Raman scattering cross-sections of overtones (or combination bands) are typically about two orders of magnitude lower than those of fundamentals, 78 whereas this difference typically amounts to about one order of magnitude in the infrared, it is more plausible that the infrared n 9 overtone is brighter than the n 5 fundamental. This is in line with the suggested label switch of the resonance partners by Hull et al, 27 which was also proposed in the VCI and MCTDH studies from 2016 11 and 2018. 12 As aforementioned, the n 5 /2n 9 Fermi resonance is also present in DCOOH with a slightly larger splitting (93 cm À1 versus 86 cm À1 for HCOOH) and a similar intensity ratio (7(2) : 1).…”

“…This resonance is part of a larger resonance polyad involving half a dozen of states 26 and problems with the n 5 assignment were noted early on. In 2019, Hull et al presented convincing experimental evidence that the overtone 2n 9 is -contrary to previous beliefs -lower in energy than the fundamental n 5 , 27 in agreement with recent high-level calculations. 11,12 As we will highlight in this contribution, one of the most crucial and insightful aspects that improves our understanding of near-degeneracies in these systems is the comparison of vibrational spectra of all four H/D isotopologues.…”

“…In a high resolution study of the respective hot bands in 2019, Hull et al showed that the labels of the two resonance partners at 1306 cm À1 and at 1220 cm À1 need to be switched, as the overtone was shown to be lower in energy than the fundamental. 27 This Fermi resonance is also present in trans-DCOOH as well as in trans-HCOOD but seemingly absent in trans-DCOOD. For the cis-conformers, this Fermi resonance is not predicted, [11][12][13] which makes a combination of both species valuable for vibrational benchmarking.…”

Vibrational states of deuterated trans- and cis-formic acid: DCOOH, HCOOD, and DCOOD

“…Mode assignment is given as ν=stretching, δ=bending, γ=rocking, and def.=deformation.Available rare-gas matrices measurements for the cis isomer agree with the theoretical results within the maximum spectral shifts in comparison with the gas-phase measurements observed in the case of trans-formic acid; the matrix-induced shifts for trans-formic acid are given in Tables IV and VI. The present MCTDH prediction for the ν 9 fundamental of HCOOD (374.8 cm −1 ) is in very good agreement with the value (378.9 cm −1 ) predicted recently using a 1-dimensional torsional potential computed at the CCSD(T) level of theory extrapolated to the complete basis set limit 38. A Fermi resonance between the ν C=O stretch and the 2τ DCO overtone in rare-gas matrices was reported by Maçôas et al11 for cis-DCOOH Assignment Mode Ne 63 Ar 63 Kr 63 Xe 63 MCTDH VPT2…”

“…29,32,[34][35][36][37] We use the same method as Richter and Carbonnière 5 , their PES, and the (block-) improved relaxation method [29][30][31][32][33] implemented in the MCTDH Heidelberg package which proved to be successful in assigning vibrational states for formic acid (HCOOH), 5 including the recently confirmed re-assignment of the Fermi-coupled ν 5 and 2ν 9 states. 38…”