The first excited singlet state of s-tetrazine: A theoretical analysis of some outstanding questions

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Abstract
The equation-of-motion coupled cluster method for excited electronic states (EOMEE-CC) is applied to study the structure and selected properties of the first excited singlet state of s-tetrazine. Adiabatic S1←S0 excitation energies obtained with large basis sets containing up to 270 functions are uniformly somewhat above the experimental 0–0 value of 2.238 eV, but nevertheless are the most accurate calculations reported to date for this quantity. The equilibrium geometry of S1 predicted in this study is in excellent agreement with another high-level calculation, and moreover is quantitatively consistent with both the intensity of vibrational progressions observed in absorption and measured rotational constants for S1. The EOMEE-CC harmonic force field of S1 is the first to satisfactorily describe the low frequency in-plane b3g bending mode, notably its marked reduction in frequency upon excitation and characteristic positive anharmonicity. Both of these effects are due to a strong second-order Jahn–Teller interaction between S1 and the nominal S2(1Au) state, which is also investigated superficially in this work. Finally, results presented for the static dipole polarizabilies of the S1 state join others in calling for a reinvestigation of the experimentally determined parameters.