Calculation of infrared absorption intensities of combination bands of cyclic acid dimers by vibrational second order perturbation theory

Vibrational second order perturbational theory (VPT2) was applied for anharmonic calculation of vibrational frequencies and infrared absorption intensities of combination bands of OH bending modes in cyclic dimers of carboxylic and sulfonic acids. Simulated IR spectra with calculated frequencies and intensities could reproduce changes in fine structures in the lower wavenumber side of OH stretching region of the measured IR spectra of acetic acid and its deuterated and fluorinated analogues. A weak band in the measured IR spectra of trifluoromethanesulfonic acid was successfully identified as a combination band of its cyclic dimer.