Vibrational spectroscopy of cis- and trans-formic acid in solid argon

Abstract

Absorption spectra of cis and trans conformers of formic acid (HCOOH) isolated in solid argon are analyzed in the mid-infrared (4000-400 cm À1 ) and near-infrared (7800-4000 cm À1 ) regions. The HCOOH absorption spectrum reveals matrix-site splitting for the trapped molecule. Narrowband tunable infrared radiation is used to pump a suitable vibrational transition of the trans conformer in order to promote site-selectively the conversion to the cis conformer and separate the spectral features of each site group. Several anharmonic resonances are identified for both conformers. The results of anharmonic vibrational ab initio calculations (CC-VSCF) for the trans and cis conformers of formic acid are reported and compared with the experimental spectra.

Internal Rotation in Propionic Acid: Near-Infrared-Induced Isomerization in Solid Argon

“…Similar splitting of the νC-O or δCOH bands due to Fermi resonance was reported for formic and acetic acids. 6,38 In the 3300-1100 cm -1 spectral window available for observation of the C t conformer, the bands assigned to this form are observed at ∼1800 cm -1 (νCdO), ∼1360 cm -1 (ωCH 2 ), and ∼1156 cm -1 (νC-O), in good agreement with the computational values of 1773, 1361, and 1134 cm -1 , respectively (Figure 4). In addition, two doublets are observed at ∼1276 and ∼1248 cm -1 , which are ascribed to δCOH, predicted at 1254 cm -1 .…”

“…The observed shifts are also in agreement with the available data for acetic acid, where a change in conformation from the trans to the cis arrangement around the C-O bond leads to blue shifts in these modes of 44 and 30 cm -1 , respectively. 6,38 In the case of T g ( , both the νOD and νCdO modes were predicted to be shifted from those of T t by less than 1 cm -1 . Accordingly, the bands assigned to this conformer are observed at 2627 (νOD) and 1780-1760 cm -1 (νCdO, quartet), which are close to the corresponding bands of T t observed at 2630/2631 and 1770-1762 cm -1 , respectively.…”

“…Conformational isomerization was induced site selectively by using tunable narrow-band radiation as found for HONO 37 and formic acid. 2,38 The site selectivity upon excitation of the 2νOH mode of PA-OH is demonstrated in Figure 3, which shows holes burned in the νOH absorption envelope of T t upon pumping at four different frequencies.…”

“…This is due to the fact that the pumping efficiency of various matrix sites depends on the pumping frequency, as discussed elsewhere. 6,38 However, very interestingly, the HF excitation of the 2νOD mode of the T t conformer more efficiently promotes the T t f T g ( conversion, whereas the LF excitation promotes the T t f C t conversion with a relatively higher efficiency. Although the bands of both photoproduced conformers (T g ( and C t ) appear as a result of both HF and LF excitation, their relative intensities are strongly affected by the excitation frequency.…”

“…Similar deviations (30-90 cm -1 ) between the observed and calculated νC-O frequencies were previously observed for formic and acetic acids. 6,38 The assignment of the C g ( bands is based on the spectral changes induced by the excitation of T g ( , although the relatively low concentration of the C g ( conformer limits the number of observed bands of this form. As expected from the calculations, the observed bands of C g ( appear to be slightly shifted but clearly distinguishable from those of the C t form (Table 2), and they fit the most intense bands predicted by the calculations (νOD at 2670, νCdO doublet at 1798/1784, and νC-O doublet at 1238/1216 cm -1 ).…”