Theoretical study of rectangular (D2h) N4

The N 4 molecule in a rectangular geometry (D 2h ) has been studied by means of ab initio calculations based on the GAUSSIAN 94 computer code. Whereas various single configuration approaches adequately described N 4 at its equilibrium geometry, a multiconfigurational approach, such as the complete active space SCF, was required to describe the transition-state region and the dissociation to two N 2 molecules. The barrier towards dissociation to 2N 2 from (D 2h ) N 4 is calculated to be 2.5 kcal mol −1 , which is much less than the corresponding value for tetrahedral N 4 . It is, however, sufficient to render (D 2h ) N 4 stable at room temperature. At the same level of theory, the quadratic (D 4h ) N 4 is not stable. It is found to be a saddle point of second order.