Three Conformers of 2‑Furoic Acid: Structure Changes Induced with Near-IR Laser Light

Conformers of 2-furoic acid were studied using the matrix-isolation technique combined with narrow-band near-IR excitations with tunable laser light. Two conformers of the compound were trapped from the gas phase into low-temperature Ar or Ne matrixes with the population ratio of nearly 1:1. The two forms differ from each other by 180° rotation of the carboxylic group with respect to the furan ring. In both structures, the OH group adopts the cis orientation, with its H atom directed toward the CO bond of the OC–O–H group. Narrow-band near-IR excitations of the OH stretching overtone vibrations resulted in transformation of one of the initially observed conformers into a third conformational structure. This near-IR-induced isomerization concerned rotation of the OH group from the initial cis orientation to the trans conformation with the hydrogen atom directed toward the oxygen atom of the furan ring. In the photoproduced conformer, the hydrogen-bond-like O–H···O interaction (between O–H and the oxygen atom of the furan ring) is rather weak. Nevertheless, this interaction stabilized the structure so that it was present in the matrix for several hours after the near-IR-induced generation. The spontaneous conversion of the photogenerated, higher-energy form back into the more stable conformer with the carboxylic group in cis orientation was monitored for 2-furoic acid isolated in Ar and Ne matrixes. The speed of this process was found to be dependent on temperature and on the matrix material. The experimentally determined half-life times of this conformational conversion occurring in the dark are t 1/2 = 1390 min (Ar, 5.5 K); t 1/2 = 630 min (Ar, 15 K); t 1/2 = 240 min (Ne, 5.5 K). The three conformers of 2-furoic acid observed in the present work were identified by comparison of their infrared spectra with the spectra theoretically calculated for the candidate structures.