S-tetrazine. I. High-resolution vapor-phase study of the visible n → π*vibronic absorption band systems

Abstract

The low-resolution electronic spectrum of s-tetrazine vapor has been measured down to about 190 mμ, and the complex systems of vibronic bands that comprise the n→ π*visible absorption spectrum have been studied under higher resolution. These latter spectra were photoelectrically recorded in the third order of a 21-ft grating spectrograph, using path lengths from 0.05 to 48 m and temperatures from -70° to 70°C. In the absorption region, 4500 to 6100 A, the spectrum consists of over 500 sharp, vibronic band peaks. Their collected Q-branch maxima have been measured to the nearest cm-1, but no discrete rovibronic structure was resolved. The outstanding feature of the visible spectrum is a set of four intense progressions, each of which is propagated by the consecutive excitation of the same vibration (believed to be the totally symmetric mode 6a). In the ground state, the frequency of this vibration is 737 cm-1 for s-tetrazine-do and 720 cm -1 for s-tetrazine-d2; these frequencies drop to about 700 and 690 cm-1, respectively, in the excited n→ π*state (s). From simple molecular orbital theory and the Herzberg-Sponer-Teller selection rules, a plausible, preliminary, singlet absorption model is developed and is qualitatively applied to the spectral data. Because there may be an "almost accidental degeneracy" of some of the n→ π*states of the azines (excluding pyridine), the need for a more elaborate model that will properly treat the electronic-vibrational interaction problem is emphasized. Although a complete vibronic analysis has not yet been obtained, the presence of three electronic transitions forbidden on the basis of D2h, excited state molecular symmetry, as well as the electronically allowed n→ π* transition; is suspected.