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 (B-0) 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 theta(e)(NCN) = 125.95 degrees, with estimated uncertainties of +/- 0.10 pm and +/- 0.10 degrees, 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, theta(e)(CNC) = 115.60 degrees, and theta(e)(HCC) = 120.75 degrees; and for s-tetrazine: r(e)(CH) = 107.95 pm, r(e)(CN) = 133.39 pm, r(e)(NN) = 132.01 pm, and theta(e)(NCN) = 126.59 degrees.