s-Tetrazine in Aqueous Solution:  A Density Functional Study of Hydrogen Bonding and Electronic Excitations

Using density functional theory based methods we studied vertical and adiabatic excitations of the s-tetrazine molecule, small clusters with water molecules and a single s-tetrazine molecule within 60 water molecules using periodic boundary conditions. We therefore achieve a consistent description of s-tetrazine from the isolated molecule to full solvation in water. The explicit treatment of solvent molecules allows for an accurate treatment of solute−solvent interactions. For the isolated s-tetrazine molecule a comparison with earlier high level ab initio calculations and other density functional calculations is made. In accordance with experiment the most favorable two−water-adduct displays a homodromic feature, i.e., a chain of hydrogen bonding from the nitrogen of the s-tetrazine to its methenyl (CH) group. Radial distribution functions calculated from a Car−Parrinello molecular dynamics simulation of the aqueous solution clearly show an unexpected preference of water for hydrogen bonding to the C−H group over the nitrogen lone pairs. Only infrequent and short-lived hydrogen bonds from water molecules to the nitrogen atoms are found. Calculations of vertical excitations using time-dependent density functional theory showed that the solvent shifts can be explained from the polarization of the Kohn−Sham orbitals of the solute. Hydrogen bonding has only a minor effect on the solvent shifts of low lying states of s-tetrazine.