Accurate ab initio quartic force field for trans-HNNH and treatment of resonance polyads

The quartic force field of trans-HNNH (trans-diazene, Irans-diimide) has been computed ab initio at the CCSD(T)/cc-pVTZ level. A comprehensive set of formulas for 1-1, 2-2, and 1-3 resonance constants of asymmetric tops in the contact transformed vibrational Hamiltonian is presented, and an automatic procedure for setting up and solving the relevant resonance eigensystems is presented. Resonance polyads are found to be very important in trans-diazene: the fundamentals of HNNH and DNND each involve both a resonance tetrad and a triad, while a triad plus a separate Fermi type 1 resonance exist in HNND. For HNNH, HNND, and DNND, agreement be,tn:een computed and observed fundamentals is within the customary accuracy of 10 cm-l or better at this level of theory, except for the NN stretch where significant basis set and core correlation effects make for a slightly larger error. The experimentally proposed nu(1) value of 3128 cm(-1) is a misassignment and actually corresponds to nu(2) + nu(3), although strong mixing between these two levels and 2 nu(3) makes the distinction largely semantic. Performance of the resonance treatment is found to be quite satisfactory. (C) 1997 Elsevier Science B.V.