Theoretical study of N4X (X = O, S, Se) systems.

A series of N(4)X (X = O, S, Se) compounds have been examined with ab initio and density functional theory (DFT) methods. To our knowledge, these compounds, except for the C(2v) ring and the C(3v) towerlike isomers of N(4)O, are first reported here. The ring structures are the most energetically favored for N(4)X (X = O and S) systems. For N(4)Se, the cagelike structure is the most energetically favored. Several decomposition and isomerization pathways for the N(4)X species have been investigated. The dissociation of C(2v) ring N(4)O and N(4)S structures via ring breaking and the barrier height are only 1.1 and -0.2 kcal mol(-1) at the CCSD(T)/6-311+G*//MP2/6-311+G* level of theory. The dissociation of the cagelike N(4)X species is at a cost of 12.1-16.2 kcal mol(-1). As for the towerlike and triangle bipyramidal isomers, their decomposition or isomerization barrier heights are all lower than 10.0 kcal mol(-1). Although the C(S) cagelike N(4)S isomer has a moderate isomerization barrier (18.3-29.1 kcal mol(-1)), the low dissociation barrier (-1.0 kcal mol(-1)) indicates that it will disappear when going to the higher CCSD(T) level.