Tetrazine, electronic spectra

X4)2 (s-tetrazine dimer) electronic spectrum, 397tf... 

Figure 24-14 Typical ultraviolet absorption spectra. The compound is 1,2,4,5-tetrazine. In (a), the spectrum is shown in the gas phase, where many lines due to electronic, vibrational, and rotational transitions are seen. In a nonpolar solvent (b), the electronic transitions can be observed, but the vibrational and rotational structure has been lost. In a polar solvent (c), the strong intermolecular forces have caused the electronic peaks to blend together to give only a single smooth absorption peak. (From S. F. Mason, J. Chem.

A selective interaction with adsorbed tetrazine molecules is revealed when the adsorbent is brought into contact with the vapors of nitrobenzene, or benzoquinone. The spectrum is radically changed (52). This is presumably due to the formation of intermolecular addition compounds between the electron donating and accepting molecules in the adsorbed state. 

A few years ago, a powerful version of molecular optical spectroscopy with supersonic beams and jets was developed by Smalley, Wharton and Levy . Supersonic expansion of molecules in an inert carrier gas yields an ideal spectroscopic sample. As a result of the expansion, the translational temperature of the carrier gas decreases to extremely low values (below O.I K). The flow is collisionless so that even extremely unstable species survive. Special attention was paid to fluorescence excitation spectroscopy but the technique is by no means limited to this type of spectroscopy. (Because of fundamental difficulties, however, direct measurement of light absorption in molecular beams is not easy.) Cooled molecules in the beam are electronically excited with a tunable dye laser. The emitted fluorescence is detected and plotted against the wavenumber of the exciting radiation. The obtained fluorescence excitation spectrum is generally very similar to the corresponding absorption spectrum. The technique was used for analysis of the spectra of interesting vdW molecules He. .. NOj, He... Ij, X. .. tetrazine and Xj. .. tetrazine (X = He, Ar, H ) complexes . 

As can be seen in Figure 14-la. the visible absorption spectrum for 1.2,4.5-ictrazine vapor shows the line structure that is due lo the numerous rotational and vibrational levels associated wiih the excited electronic states of this aromatic molecule. In the gaseous state, the individual tetrazine molecules are suflicicntly separated from one another to vibrate and rotate freely, and the many individual absorption lines appear as a rcsuli of the large number ol vibrational and rotational energy states. In the condensed slate or in solution, however, the lelrazine molecules have little freedom to rotate, so lines due to dil ferencos in rotational cnergv... 

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