Conformers of butyric acid were studied using matrix isolation infrared absorption spectroscopy combined with quantum chemical calculations. The results of Møller–Plesset perturbation theory (MP2) calculations indicate that the most stable conformer of butyric acid is the one with a nonplanar aliphatic chain, which is in contradiction with the B3LYP results, predicting the most stable conformer to have the aliphatic chain in one plane. Infrared absorption spectra of butyric acid isolated in argon and neon matrices have complicated band structures, suggesting the existence of multiple structural forms of butyric acid trapped in the matrices. The application of MP2 anharmonic calculations has enabled a precise assignment of spectral bands. The importance of Fermi resonance and the need for anharmonic calculations for the correct assignment of C O stretch vibrations of butyric acid are demonstrated. Comparison between the calculated and experimental spectra led to the conclusion that three conformers are trapped in argon and neon matrices. The conformer with the aliphatic chain in one plane TTT, along with the nonplanar ones namely G± TT, were found to coexist in similar amounts isolated in noble gas matrices.
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