High-resolution rovibrational spectroscopy of trans-formic acid in the v1 OH stretching fundamental: Dark state coupling and evidence for charge delocalization dynamics

Abstract

High-resolution infrared (IR) reduced-Doppler absorption spectra of jet-cooled gas phase trans-formic acid in the v1 OH stretching fundamental region are reported for the first time, obtained by supersonically expanding trans-formic acid/Ar mixtures through a slit jet nozzle source and rotationally cooling to Trot ≈ 10.9(5) K, with absorption signals recorded by high-resolution difference-frequency IR absorption spectroscopy. Two a/b-type rovibrational bands of comparable intensity, one ∼10-fold weaker b-type band, and one ∼6-fold weaker a-type band are observed, with vibrational band origins at 3570.493(5), 3566.793(5), 3560.032(9), and 3534.6869(2) cm−1, respectively. Based on previous Raman jet spectroscopic work by Nejad and Sibert [A. Nejad, E.L. Sibert III, The Raman jet spectrum of trans-formic acid and its deuterated isotopologs: Combining theory and experiment to extend the vibrational database, J. Chem. Phys. 154(6) (2021) 064301.], these four rovibrational bands have been assigned to v1, (v2 + v7), (v6 + 2v7 + 2v9), and 2v3, respectively. Specifically, two of the three upper dark states (2171 (a′) and 617292 (a′)) are close enough to the “bright” 11 (a′) state to facilitate strong anharmonic resonance interactions, which results in intensity mixing into the two zero-order bands that would otherwise be “dark”. Furthermore, our high-resolution spectral analysis reveals that there are local rotational crossings between these zero-order 11 and 2171 states resulting in extra lines (i.e., some upper levels in the nominally v1 band have majority zero-order 2171 state character). This motivates development of a 3 coupled state (11, 2171, and 617292) picture to aid in the spectral analysis, which is able to match all 3 observed band origins and relative band intensities, as well as indicate the necessity of multistate (> 2) coupling. Though limited by range of J and Ka levels (J' ≤ 9 and Ka' ≤ 3) populated at supersonic jet temperatures, this work offers first precision spectroscopic analysis of trans-formic acid in the v1 OH stretch region, which should aid in assignment of the more complete yet highly congested room temperature FTIR spectra [D. Hurtmans, F. Herregodts, M. Herman, J. Liévin, A. Campargue, A. Garnache, A. Kachanov, Spectroscopic and ab initio investigation of the νOH overtone excitation in trans-formic acid, J. Chem. Phys. 113(4) (2000) 1535–1545.]. Finally, and in sharp contrast to the spectral complexity in the three predominantly b-type bands, the lone a-type 2v3 rovibrational band at 3534.6869(2) cm−1 is well described by a simple, rigid asymmetric top Hamiltonian.