Structures and properties of [CN⋯CN −] coupling system in doublet state

Abstract

The structures and the properties of CN −(X 1Σ +)⋯CN(X 2Σ +) coupling system are investigated in detail in present paper with density functional theory (DFT) method at UB3LYP level employing the aug-cc-pVDZ basis set. The vertical electron affinity of CN is determined at the same level. Geometrical optimizations indicate that there are eight possible stable isomers (EC1: 2A′, C s; EC2: 2A′, C s ; EC3: 2B u, C 2h ; EC4: 2A′, C s ; EC5: 2A′, C s ; EC6: 2A′, C s; EC7: 2B u, C 2h ; EC8: 2A, C 2 ), which correspond to the genuine minima on the global potential energy surfaces (PES) for the doublet spin states. On the basis of the optimized geometries, the stabilization energies of the complexes are determined with a correction using counterpoise corrections method for the basis set superposition error (BSSE) and the Zero-point vibrational energy (ZPVE), and the relative stability of the coupling complexes, CN⋯CN −, are compared. The stabilization energy order of these complexes is EC3>EC4>EC5>EC2>EC1>EC6>EC7>EC8. The stabilization energy of the most stable complex, EC3, is 62.08 kcal/mol. The most interesting isomer is the EC8, which energy is higher than the sum of total energies of the two separated monomers. The electronic states of these complexes are all determined. Then the vibrational frequency analysis and the assignment of the vibrational frequency modes have been performed. The natural bond orbital analyses of these isomers have also been carried out in succession. The transition state searches and the intrinsic reaction coordinate (IRC) analysis have indicated that there are five transition states, in which TS1 correlates with EC1 and EC2, and TS3 correlates with EC4 and EC5, respectively. The forward and reverse activation energies are 5.37, 5.46 kcal/mol for TS1 and 8.26, 6.00 kcal/mol for TS3, respectively. No direct correlation is found for EC6 with the other complexes. The self isomerization of EC7 is more difficult to take place than the self isomerization processes of the most stable complex, EC3. TS5 is the transition state of EC8 in the dissociation process.