Highly excited symmetry-breaking infrared and THz transitions in methanol-D1

Abstract

In this paper, symmetry-breaking transitions have been identified in the Fourier transform infrared (FTIR) and Terra Hertz (THz) fast scan spectra of asymmetrically deuterated methanol CH2DOH involving three quanta of the lowest lying vibrational state (ν1). These transitions have an upper state belonging to highly excited torsional state with the torsional quantum number v=11 (e5) (with the axial component of total rotational angular momentum K=5 and 6) in the Internal Axis Method (IAM) formalism developed by Quade and his co-workers [J. Mol. Spectrosc. 146 (1991) 238; J. Mol. Spectrosc. 146 (1991) 252], which was later extended by Mukhopadhyay [Spectrochim. Acta A 53 (1997) 2457; Spectrochim. Acta A 53 (1997) 1947] to include highly excited torsional–rotational states. The originating lower states belong to the o1 (v=3) with K=4 and 5. In analogy with pure methanol these transitions terminate to the third excited torsional state, which has very small torsional matrix elements to be observable in normal FTIR spectra. The location of the e5 states suggests that the calculated energy levels using the parameters of Liu and Quade [J. Mol. Spectrosc. 146 (1991) 252] are reasonable and very helpful to assign such highly excited transitions, considering that their studies include low angular momentum states. In addition the very existence of these transitions proves that the matrix elements calculated by Mukhopadhyay [Spectrochim. Acta A 53 (1997) 1947] are very useful and dependable. Thus they represent a valuable tool for entangling the complex spectrum of this asymmetrically deuterated methanol. In order to provide confirmatory evidence the THz spectra obtained using the Fast Scanning Submillimeter Spectroscopy Technique (FASSST) at the Ohio State University [Rev. Instr. 68 (1997) 1675; Anal. Chem. 70 (1998) 719A] were searched for the ground state transitions that can be calculated precisely from IR combination relations. All the transitions that can be predicted with K=4 and 5 in the o1 states are indeed identified in the FASSST spectrum. To our knowledge this is the first reported work involving direct transitions to such highly excited torsional state of CH2DOH. This work will enable the determination of higher order barrier terms and provide enough understanding of the energy levels for the identification of many unidentified transitions. To our knowledge, this is the first time such high frequency symmetry-breaking transitions have been observed in asymmetrically deuterated methanol.