THE PRESENT STATUS OF THE VIBRONIC ANALYSIS OF THE $VISIBLE^{*}$ BAND SYSTEMS OF s-TETRAZINE $VAPOR^{1}$

$^{1}$ Supported by the National Science Foundation and in collaboration with Professors P.C. Cross and K. B. Wiberg under Air Force Office of Scientific Research, Air Research and Development Command. Contract AF 18(600)-1522 with the University of Washington, Seattle 5, Washington. $^{2}$ National Science Foundation Postdoctoral Fellow, Harvard University, 1958-59; after September 15, 1959, at Department of Chemistry and Chemical Engineering, Stanford University, Stanford, California.Author Institution: Department of Chemistry, Harvard UniversityThe red color that visually characterizes a room temperature assembly of s-tetrazine $(C_{2}H_{2}N_{4})$ molecules is thought to be the collective, chromatic consequence of the lowest singlet $n\rightarrow\pi^{*}$ transitions. Using the third diffraction order of a 21 foot grating spectrograph, vapor path lengths from 0.05 meters, and vapor temperatures from -70 to $70^{\circ} C$, we have photoelectrically record high resolution spectra of s-tetrazine vapor. The frequencies of 514 of the best resolved absorption peaks were measured to within one $cm^{-1}$ in the region 4500 to 6100 \AA. The outstanding feature of the visible spectrum is a set of four vibronic band progressions, each of which is propagated by the consecutive excitation of the same fundamental vibration. This vibration, believed to be the totally symmetric mode 6a, has a ground electronic state frequency of 737 and $720 cm^{-1}$ for s-tetrazine-$d_{0}$ and s-tetrazine-$d_{2}$, respectively; in the $n\rightarrow\pi^{*}$ states, these frequencies drop to about 700 and $690 cm^{-1}$, respectively, Although a successful vibronic analysis has not been obtained yet, the presence of three relatively intense $n\rightarrow\pi^{*}$ singlet transitions that are electronically forbidden by molecular symmetry as well as the allowed $^{1}B_{tu}\rightarrow ^{1}A_{g}$ transition is strongly suspected. Theoretical studies pertaining to the electronic-vibrational interaction of the lowest singlet, almost accidentally degenerate $n\rightarrow\pi^{*}$ states of the azines have been initiated and will be discussed in terms of simple MO theory