Benchmark ab initio thermochemistry of the isomers of diimide, N_2H_2,   using accurate computed structures and anharmonic force fields
  A benchmark ab initio study on the thermochemistry of the trans-HNNH, cis-HNNH, and H$_2$NN 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: accurate geometries and anharmonic force fields were thus obtained as by-products. Our best computed $\Delta H^\circ_{f,0}$ for trans-HNNH, 49.2 \pm 0.3 kcal/mol, is in very good agreement with a recent experimental lower limit of 48.8 \pm 0.5 kcal/mol. CCSD(T) basis set limit values for the isomerization energies at 0 K are 5.2 \pm 0.2 kcal/mol (cis-trans) and 24.1 \pm 0.2 kcal/mol (iso-trans). Our best computed geometry for trans-HNNH, $r_e$(NN)=1.2468 \AA, $r_e$(NH)=1.0283 \AA, and $\theta_e$=106.17$^\circ$, 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 H$_2$NN are dominated by very strong Coriolis and Fermi resonances. In addition, the NH stretches in H$_2$NN 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 Schr\"odinger equation. 
Jan M.L. Martin, Peter R. Taylor