Benchmark ab initio thermochemistry of the isomers of diimide, N2H2, using accurate computed structures and anharmonic force fields

A benchmark ab initio study on the thermochemistry of the trans-HNNH, cis-HNNH, and H2NN isomers of diazene has been carried out using the CCSD(T) coupled cluster method, basis sets as large as [7s6p5d4f3g2h/5s4p3d2f1g], and extrapolations towards the 1-particle basis set limit. The effects on inner shell correlation and of anharmonicity in the zero-point energy were taken into account: thus accurate geometries and anharmonic force fields were obtained as by-products. The best computed ΔHof0 for trans-HNNH, 49.2 ± 0.3 kcal mol-1, is in very good agreement with a recent experimental lower limit of 48.8 ± 0.5 kcal mol-1. CCSD(T) basis set limit values for the isomerization energies at 0 K are 5.2 ± 0.2 kcal mol-1 (cis-trans) and 24.1 ± 0.2 kcal mol-1 (iso-trans). Our best computed geometry for trans-HNNH, re(NN) = 1.2468 Å, re(NH) = 1.0283 Å, and θe = 106.17°, reproduces the precisely known ground state rotational constants of trans-HNNH to within better than 0.1%. The rotation—vibration spectra of both cis-HNNH and H2NN are dominated by very strong Coriolis and Fermi resonances. In addition, the NH stretches in H2NN are so strongly anharmonic that vibrational perturbation theory breaks down, and the molecule appears to be an excellent test case for variational treatments of the vibrational Schrodinger equation.