Theoretical Determination of Geometrical Structures of the Nitric Oxide Dimer, (NO)2

Geometrical structures for the dimerization of (NO)2 from (NO + NO) have been calculated using ab initio Har-tree-Fock (SCF), second-order Mo \ ller-Plesset perturbation (MP2), and coupled cluster with the single, double, and triple substitution [CCSD(T)] methods with a triple zeta plus polarization (TZP) basis set including diffuse Rydberg basis functions. The structure of (NO)2 can be described by two interactions (NN, NO). One is the ONNO structure with an (NN) interaction. In this structure, acyclic cis-ONNO with C2v-symmetry, acy-clic trans-ONNO with C2h, and cyclic ONNO with trapezoidal structure (C2v) are optimized at the MP2 level. The other structure is the ONON structure with an (NO) interaction. In the structure, acyclic cis-ONON with Cs-symmetry and cyclic ONON of the rectangular (C2h), square (D2h), rhombic (D2h), and parallelogramic (D2h) geometries are also optimized. It is found that acyclic cis-ONNO (1A1) is the most stable structure and cyclic ONNO (3A1) is the least stable. Acyclic trans-ONNO (3A1) with an (NN) interaction, acyclic trans-ONON and bicyclic ONON (C2v) with (NO) interaction, and acyclic cis- and trans-NOON with an (OO) interac-tion can not be optimized at the MP2 level. Particularly, acyclic trans-ONNO with C2h-symmetry can not be optimized at the CCSD(T) level. Meanwhile, acyclic NNOO (1A1, Cs) and trianglic NNOO (1A1, C2v) formed by the (ON) interaction between O2 and N2 are optimized at the MP2 level. The binding energies and the relative energy gaps among the isomers are found to be relatively small