Theoretical study of the triplet N-4 potential energy surface

The N-4 molecule in its triplet state has been studied using the DFT-B3LYP, CASSCF, and CCSD(T) computational methods. The previously reported C-s ((3 )A ") minimum was not found to be stable using these methods. A D-2d ((3)A( 1)) minimum was found to be the lowest energy triplet with a closed structu re. This minimum is 20.2 kcal/mol higher in energy than T-d ((1)A(1)) N-4 a t the CCSD(T)/cc-pVTZ//CCSD(T)/DZP level of theory. The barrier to dissocia tion has been estimated to ca. 7.5 kcal/mol from CAS(8,8)+MR-ACPF and CAS(8 ,8)+MR-AQCC calculations. CASSCF(12,12) calculations indicate that the open -chain N-4 Of C-2h (B-3(u)) symmetry is not stable. The open-chain C-s((3)A ") structure has been found to be a true minimum at all investigated level s of theory. It is 13.4 kcal/mol lower in energy than T-d ((1)A(1)) N-4 at the CCSD(T)/cc-pVTZ//CCSD(T)/DZP level.