Theoretical study of HNXH (X = N, P, As, Sb, and Bi) isomers in the singlet and triplet states

The lowest singlet and triplet potential energy surfaces for all group 15 HNXH (X = N, P, As, Sb, and Bi) systems have been explored through ab initio calculations. The geometries of the various isomers were determined at the QCISD/LANL2DZdp level and confirmed to be minima by vibrational analysis. In the case of nitrogen and phosphorus, the global minimum is found to be a singlet trans-HN=XH structure. Moreover, both singlet and triplet H2NX (X = N and P) species are also found to be local minima surrounded by large activation barriers, so that they should be observable. For arsenic, theoretical investigations demonstrate that the stability of HNAsH isomers decreases in the order triplet H2NAs > singlet trans-HN=AsH ≈ singlet cis-HN=AsH > singlet H2NAs > triplet HNAsH > triplet H2AsN > singlet H2AsN. For antimony and bismuth, the theoretical findings suggest that the stability of HNXH (X = Sb and Bi) systems decreases in the order triplet H2NX > singlet H2NX > singlet trans-HN=XH ≈ singlet cis-HN=XH > triplet HNXH > triplet H2XN > singlet H2XN. Our model calculations indicate that the relativistic effect on heavier group 15 elements should play an important role in determining the geometries as well as the stability of HNXH molecules. The results obtained are in good agreement with the available experimental data and allow a number of predictions to be made.