Study of the Overtone C-O Stretching Band of Methanol by Multiple Resonance Spectroscopy

Author Institution: National Institute of the Standards and Technology, Gaithersburg, MD 20899 U.S.A.; Institute for Defense Analyses, 1801 N. Beauregard Street, Alexandria, VA 22311 U.S.A.; University of New Brunswick, Saint John, NB, E2L 4L5, Canada.Two microwave-sideband $CO_{2}$ lasers have been used with a molecular-beam electric-resonance spectrometer to study the overtone C-O stretching vibration of methanol. Infrared-infrared double-resonance results have been obtained for levels involving the K = 1 and 2, A symmetry, and the $K = 2, E_{2}$ symmetry species. In the A torsional symmetry case, radiofrequency-infrared multiple resonance was used to obtain accurate asymmetry splittings for the $\nu_{co} = 1$ and 2, C-O stretching states. The asymmetry splitting constants have been determined for these states, are in good agreement with the literature values for the first excited C-O stretching states. However, the nearly factor-of-two change in the K = 2 asymmetry splitting constant for the $\nu_{co} = 2$ level compared to the $\nu_{co} = 0$ and 1 level results suggests that this state is weakly pertubed. The overtone transition frequencies obtained in this work were combined with previous overtone Fourier-transform results in a global fit to a torsion-rotation Hamiltonian to refine the fundamental molecular constants for the second-excited C-O stretching state. The $\nu_{co} = 2$ torsional barrier height is found to be $372.227(3) cm^{-1}$ or $374.984(7) cm^{-1}$ depending on data set used. In the analysis the overtone vibrational energy origin is constrained to $2054.83113 cm^{-1}$. This barrier can be compared to the $\nu_{co} = 0$ and 1 values of $373.5421 cm^{-1}$ and $392.35 cm^{-1}$. respectively