Vibrational predissociation in argon complexes of 3-amino-s-tetrazine and 3-amino-6-methyl-s-tetrazine: Evidence for extreme mode-selectivity

Abstract

We have investigated the vibrational predissociation of argon complexes of 3-amino-s-tetrazine (AT) and 3-amino-6-methyl-s-tetrazine (AMT). Twelve vibrational levels of complexes of the form AT–Arn, where n=1, 2, and 3, were examined. The dissociation rate of the binary AT–Ar complex varied from 3×106 s−1 to >4×108 s−1 depending on the initially excited mode, with the fastest rate corresponding to the lowest energy vibration. Even levels with vibrational energies over five times that needed to break the van der Waals bond still showed significant amounts of fluorescence from the unfragmented complex. Photodissociation of the AT–Ar2 complex can yield two different dissociation products, AT and AT–Ar. The ratio of these photoproducts varied greatly among different vibrational modes. The 16b2 level of AT–Ar2 at +462 cm−1 gives primarily the AT photoproduct, while the 16a2 level at +505 cm−1 gives exclusively the AT–Ar photoproduct. The two levels of AT–Ar3 which were studied both appear to show only partial fragmentation. We examined the basic spectroscopy of AMT using vibrationally resolved fluorescence excitation spectroscopy, and assigned a number of vibrational levels in both the ground and excited states. We then used this information to measure the branching ratios and dissociation rates for seven vibrational levels of AMT–Ar. These vibrational predissociation rates show strong qualitative differences from similar data for tetrazine–Ar and aminotetrazine–Ar complexes. Finally, these results were compared to similar work on other tetrazine derivatives to attempt to account for this strong mode-selective behavior.