’’Free’’ O–H overtone absorption of methanols in the visible region by thermal lensing spectroscopy

Absorption spectra of CH3OH, CD3OH, CH3OD, and CD3OD in the region 15 800–17 400 cm−1 are reported as seen by thermal lensing spectroscopy. Dilution studies of the protonated methanols reveal a relatively narrow (FWHM=200 cm−1) absorption peak at 16 500±10 cm−1, which is assigned to the fourth overtone (v=5) of the nonhydrogen-bonded O–H stretching vibration. The energy of this overtone is in good agreement with the predictions made by a simple one dimensional anharmonic oscillator model.