The Vibronic Structure of Methyl Heteroatom Radicals: Spectroscopic Investigations and Theoretical Considerations.

The vibronic structure of the A Leftrightarrow X band system of CH_3X radicals provides insight into interactions between a saturated alkyl group and adjacent open-shell atoms. One interaction involves chemical bonding and has structural consequences. Another consists of mixed-mode Jahn-Teller coupling between nuclear and electronic motion. Discrepancies between ab initio and experimental molecular structures have motivated a reexamination of emission from the A ^3E Leftrightarrow X ^3A _2 electronic band system of rotationally cold methylnitrene (CH_3N), its deuterated analogue (CD_3N), and its carbon-13 isotopomer (^{13} CH_3N). Signal-to-noise is enhanced by damping electrical oscillations in the Corona Excited Supersonic Expansion, and by increasing both mass flow and current density. This permits observation and inspection of the weak, as well as the strong, vibronic features in the spectra. Analysis of the progressions yields all six ground-state vibrational frequencies for both CH_3N and ^ {13}CH_3N, along with three for CD_3N. The formally forbidden e-type fundamentals are activated via Jahn-Teller distortion in the A ^3E state. The frequencies, along with the empirical geometry, define a ground-state force field and normal coordinates. The resulting force constants are consistent with unusually strong C-N bonds and weak C-H bonds. The observation of anomalous intensities in the vibronic spectra of methyl heteroatom radicals--intermediates in which the Jahn-Teller vibrational center is spatially distant from the Jahn-Teller electronic center--has prompted an investigation of the parametric dependence of E otimes e Jahn-Teller coupling upon a totally symmetric displacement. Inspection of the static potential reveals an increase in the Jahn-Teller distortion accompanying displacement along the symmetric coordinate when the modes are coupled. The exact solution to the dynamic problem is outlined using the techniques of second quantization and is investigated numerically for examples of weak to moderate coupling between the Jahn-Teller mode and the symmetric mode. Model spectra involving a degenerate E excited state and a nondegenerate A-type ground state are presented.