Infrared Spectra of Matrix-Isolated Acetic Acid Monomers

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Abstract
Infrared spectra of monomeric acetic acids [CH3COOH(D) and CD3COOH(D)] isolated in Ar and N2 matrices near 4°K are reported. A large anharmonic potential‐energy contribution is evident from the observed isotopic frequency shifts. It is proposed that this arises primarily from the double‐minimum potential‐energy curve for tunneling of the H atom between O atoms via the COH angle bending coordinate. In all cases, important coupling between the COH angle bending and C–O stretching coordinates is observed, with a strong addition of CD3 (umbrella) angle deformation in the case of CD3COOH. Exceptional matrix effects are observed for the coupled vibrations of the –OH molecules. A complete assignment is proposed for these acetic acids on the basis of the new data. As additional supporting evidence, the spectrum of matrix‐isolated CH3COF and of acetic acid vapor at low pressure and long path length are reported.