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, cisHNNH, 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 innershell 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 ∆H f,0 for trans-HNNH, 49.2±0.3 kcal/mol, is in very good agreement with a recent experimental lower limit of 48.8±0.5 kcal/mol. CCSD(T) basis set limit values for the isomerization energies at 0 K are 5.2±0.2 kcal/mol (cis-trans) and 24.1±0.2 kcal/mol (isotrans). Our best computed geometry for trans-HNNH, re(NN)=1.2468 Å,