The Raman spectrum of gaseous acetic acid at 21 °C

Abstract

The Raman spectrum of gaseous acetic acid at 21 °C is presented and assigned. It is suggested that a reassignment of the in-plane C–C–O deformation of the monomer to ∼450 cm−1 is appropriate, and that the infrared band contours between 700 and 400 cm−1 be analyzed in detail to confirm this. The Raman-active O–H stretching mode of the dimer could not be identified. The intermonomer coupling of the other intramonomer modes in the dimer is small except for the C=O stretching and C–C–O in-plane deformation modes. Two Raman-active intermonomer modes were observed for the first time, at 155 and 99 cm−1. The former is ν13(Ag), the O⋅⋅⋅O stretching mode, and allows the previously [J. Chem. Phys. 76, 886 (1982)] unobserved corresponding mode of (HCOOH)2 to be placed near 180 cm−1. Frequencies are deduced of all of the intermonomer modes, and the infrared doublet at 167/187 cm−1 is reassigned to ν42(Bu) in Fermi resonance with the combination state ν21(Bg)+ν28(Au) at ∼179 cm−1, instead of the previously assigned combination state ν29(Au)+ν21(Bg) which is now placed at 147 cm−1. The Fermi resonance explanation of the infrared doublet at 167/187 cm−1 invokes significant anharmonic interactions between in-plane and out-of-plane displacements of the hydrogen bond. It is favored by the experimental facts over the explanation that is based on coupling between the O–H and O⋅⋅⋅O stretching coordinates [J. Chem. Phys. 52, 4403 (1970)], which is not clearly compatible with the C2h symmetry of the dimer. Our assignment of the intermonomer bands of the gaseous dimer is similar to that deduced previously for matrix-isolated dimers [J. Chem. Phys. 54, 4111 (1971)] and for liquid acetic acid [J. Chem. Phys. 77, 3878 (1982)]. However, the band at 55 cm−1 in the Raman spectrum of the liquid has no analog in the spectrum of the gas, and is reassigned to an intermolecular vibration of the ‘‘liquid lattice’’. Further, our assignment differs from that for the matrix-isolated dimers in a way that is unfavorable to the interpretation of the O–H stretching band that led to the assignment of the matrix-isolated dimers. Finally, manual correction of the Raman intensities of gaseous formic and acetic acids, for frequency- and temperature-dependent factors, suggests that the vibrations below 150 cm−1 may have frequencies higher by up to 20 cm−1 than the values measured from the Raman spectra.