Equilibrium Structures of Pyrazine, s‑Triazine, and s‑Tetrazine

In recent years, accurate equilibrium (r_e) structures have been determined for pyridine, pyridazine, and pyrimidine. Here, we report accurate r_e structures for the structurally related molecules pyrazine, s-triazine, and s-tetrazine, which were obtained using a composite approach based on explicitly correlated coupled-cluster theory (CCSD­(T)-F12b) in conjunction with a large correlation-consistent basis set (cc-pCVQZ-F12) to take core–valence electron correlation into account. Additional terms were included to correct for the effects of iterative triple excitations (CCSDT), noniterative quadruple excitations (CCSDT­(Q)), and scalar relativistic contributions (DKH2-CCSD­(T)). The performance of this computational procedure was established through test calculations on selected small molecules. For s-triazine, accurate experimental ground-state rotational constants (B₀) of the parent molecule and six D_3h isotopologues from the literature were used to determine a semiexperimental r_e structure, which was found to be essentially identical with the best estimate from the current composite approach. The presently recommended equilibrium structural parameters of s-triazine are r_e(CH) = 108.17 pm, r_e(CN) = 133.19 pm, and θ_e(NCN) = 125.95°, with estimated uncertainties of ±0.10 pm and ±0.10°, respectively. The predicted equilibrium geometries for pyrazine and s-tetrazine are expected to be of the same accuracy. We recommend for pyrazine: r_e(CH) = 108.16 pm, r_e(CN) = 133.34 pm, r_e(CC) = 139.07 pm, θ_e(CNC) = 115.60°, and θ_e(HCC) = 120.75°; and for s-tetrazine: r_e(CH) = 107.95 pm, r_e(CN) = 133.39 pm, r_e(NN) = 132.01 pm, and θ_e(NCN) = 126.59°.