Severe perturbation and observation of forbidden transitions in asymmetrically deuterated methanol (CHD2OH) and atlas of very accurate torsional rotational spectrum obtained using Synchrotron radiation

Abstract

High-resolution spectral studies have been continued for the doubly deuterated methanol species (CHD2OH) in the entire region from far infrared (FIR) to Infrared (IR). The spectrum has been obtained using a conventional Fourier transform spectrometer and the state-of-the-art Synchrotron radiation-based spectrometer with unprecedented resolution and very high signal-to-noise (S/N). During the investigation severe perturbation was encountered. The perturbation has been characterized as due to first and second-order Coriolis interaction. The interaction affected a wide range of states and caused the spectral study quite challenging. The perturbation caused a variety of mixing of states producing hybrid states. This hybridization resulted in producing a large number of transitions which should not be allowed by the dipole selection rules via the “intensity borrowing effect”. It was possible to identify a large number of such forbidden transitions that gain sufficient strength through the “Intensity borrowing” effect. The transitions connecting to these perturbed states have been analyzed using standard Coriolis interaction formulae. The possibility of “Fermi” interaction has been excluded as the interaction is highly dependent on the rotational quantum number J. In general, this kind of transition should be excluded from global analysis and should be treated in isolation. It is pertinent to note that some previously published works studied these states using un-substantiated assignments being perhaps unaware of the perturbation. Corrections have been suggested for the analysis and understanding of the properties of the energy levels which have subtle differences with the energy levels of normal asymmetric molecules. In all, available microwave (MW) millimeter wave (MMW) far infrared (FIR), and infrared (IR) assignments have been put together to prepare the most accurate and and complete atlas of about 12,000 spectral lines. This atlas (Appendix A) should be considered a work-in-progress supplement to this paper and supersedes all other lists known to date on this species can be had from one of us (IM) or via Researchgate.