The Δν = 3, 4, and 5 Vibrational Overtones and Conformations of the Hydroxyl Group of Isobutyl Alcohol

The O−H stretching Δν = 3, 4, and 5 vibrational overtone spectra of isobutyl alcohol (isobutanol, 2-methyl-1-propanol) are measured by using cavity ring-down spectroscopy (CRDS), where three bands are observed for each vibrational level. They are assigned to the O−H stretching vibrational absorption of the three kinds of conformations of the hydroxyl group and further confirmed by density function theory (DFT) calculations using highly accurate B3LYP/6-31+G(d,p) and 6-311+G(d,p) levels. The experimental mechanical frequencies ω e, anharmonicities ω e χ e, and dissociation energies D are evaluated by the local mode theory. The band intensities of each band are obtained as well. The calculated O−H stretching Δν = 1, 3, 4, and 5 spectra are simulated by Lorentzian−Gaussian sum function in order to fit the experimental spectra. The molecular vibrational abundance and O−H stretching frequencies of different vibrations of the calculated molecular conformations are in agreement with the experimental results.