Dimers in jet-cooled s-tetrazine vapor: Structure and electronic spectra

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Abstract
Dimers with two distinct conformations form as s‐tetrazine vapor (C2N4H2) is cooled in a supersonic jet expansion. Rotationally resolved laser‐induced fluorescence excitation spectra of both hydrogen and deuterium substituted dimers have been obtained with single frequency scanning dye lasers. Dimer structures were determined by a combined rotational contour generating program and nonlinear least squares fitting procedure. One dimer conformation is planar with two translationally equivalent monomer units. In this conformation one hydrogen of each ring is in close proximity to the nitrogen lone pair of the other, allowing for stabilization by weak hydrogen bonds. The center‐to‐center ring separation is 5.6 Ȧ. The other dimer conformation is T shaped with symmetrically inequivalent rings giving rise to two electronic transitions with different rotational band types. In this configuration, one hydrogen of the first monomer is in close proximity to the π cloud of the second monomer, and the ring centers are separated by 4.4 Ȧ. The two electronic transitions are due to two close lying electronically excited states where the electronic excitation is localized on one or the other of the two rings. Additional spectral features are attributed to dimer vibrations and larger clusters.