The electronic spectra and structure of complexes between s-tetrazine and acetylene

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Abstract
Complexes between s-tetrazine and acetylene have been prepared in a supersonic free jet and their fluorescence excitation spectra have been observed. Analysis of the rotational structure in the electronic spectra of the complexes was used to determine the geometry of the complexes. Four bands due to complexes were observed at shifts of +15, +80, +201, and +208 cm−1 with respect to the origin of uncomplexed tetrazine at 18 128 cm−1. The bands at +15 and +80 cm−1 were assigned to two different isomers of a complex containing one molecule of tetrazine and one molecule of acetylene. The +15 band was due to a planar structure where the proton of acetylene forms a hydrogen bond with the nitrogen lone pair of tetrazine and the π cloud of the acetylene interacts with the hydrogen atom of the tetrazine ring. The +80 cm−1 band is due to a stacked complex with the acetylene molecule above and parallel to the plane of the tetrazine. In this isomer, the axis of the acetylene bisects the N–N bonds of the tetrazine. The band at 208 cm−1 is due to a complex consisting of two acetylene molecules and one tetrazine molecule with one acetylene above and parallel to the tetrazine ring and the other at the side of but perpendicular to the acetylene ring. The band at +201 cm−1 is assigned to a larger complex containing more than two acetylene molecules, but further analysis of this band was not possible.