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

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 T_rot ≈ 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 K_a levels (J' ≤ 9 and K_a'  ≤ 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 ν_OH 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.