Theoretical Analysis on the Fundamental and Overtone OH Stretching Spectra of Several Simple Acids and Alcohols

We calculated the fundamental and overtone OH stretching vibrational spectra for the following alcohols and acids - methanol, ethanol, 1-propanol, 2-propanol, tert-butyl alcohol, 2,2,2-trifluoroethanol, acetic acid, trifluoroacetic acid, and nitric acid - under the local mode model. We obtained the potential energy surface (PES) and the dipole moment function (DMF) by hybrid density functional theory method and performed vibrational calculation using the grid variational method. The theoretical results were in good agreement with the experimental observations. It was found that the molecular shape, such as the rotational conformation, is very important in the description of the OH stretching vibrational spectra. For alcohols with rotational conformers, such as ethanol, 1-propanol, and 2-propanol, we found that the isomer with the alkyl group in the trans position of the vibrating OH bond has a larger transition energy and a slightly stronger absorption intensity. We analyzed the first and second derivative terms of the DMF of these molecules to obtain insight on the difference in the absorption intensities. In addition, for the fundamental spectra, we investigated the difference between the local and normal mode vibrational calculation results.