Conformation resolved induced infrared activity: Trans-and cis-formic acid isolated in solid molecular hydrogen

摘要

We report combined experimental and theoretical studies of infrared absorptions induced in solid molecular hydrogen by different conformers of formic acid (HCOOH, FA). FTIR spectra recorded in the H_2 fundamental region (4120 - 4160 cm~(-1)) reveal a number of relatively strong trans-FA induced Q-branch absorptions that are assigned by studying both FA-doped parahydrogen (pH_2) and normal hydrogen (nH_2) samples. The induced H_2 absorptions are also studied for HCOOD doped nH_2 crystals for both the trans and cis conformers that show resolvable differences. Samples containing >90% of the higher energy cis-HCOOD conformer are produced by in situ IR pumping of the OD stretching overtone of trans-HCOOD using narrow-band IR light. Minimum energy structures for 1:1 complexes of H_2 and FA are determined using ab initio methods. The measured differences in the cis- versus trans-HCOOD induced spectra are in qualitative agreement with the frequencies and intensities calculated for the identified cluster structures as discussed in terms of the model of specific interactions.