A Computational Investigation on HCN2+ Isomeric Structures: Implications for the Chemistry of the Titan's

The structure and stability of various HCN2 + isomeric structures have been investigated at the complete active space SCF (CASSCF) and multireference–configuration interaction [MR-CISD( Q)] levels of theory with the 6-31G(d) and 6-311G(d,p) basis sets. The investigated species include the singlet (S) and triplet (T) open-chain HNCN+ ions 1S, 1S’, and 1T, the open-chain H CNN+ ions 2S, 2S’, and 2T, the HCN2 + cyclic structures 3S and 3T, and the HNCN+ cyclic structures 4S and 4T. All these species have been identified as true energy minima on the CASSCF(8,7)/6-31G(d) potential energy surface, and their optimised geometries, refined at the CASSCF(8,8)/6-31G(d) level of theory, have been used to perform single point calculations at the [MR-CI-SD(Q)]/6-311G(d,p) computational level. The most stable structure was the HNCN+ ion 1T, whose absolute enthalpy of formation at 298.15 K has been estimated as 333.9 2 kcal mol1 using the Gaussian-3 (G3) procedure. The two species closest in energy to 1T are the triplet HCNN+ ion 2T and the singlet diazirinyl cation 3S, whose G3 enthalpies of formation at 298.15 K are 343.52 and 340.62 kcal mol1, respectively. Finally, we have discussed the implications of our calculations for the detailed structure of the HCN2 + ions formed in the reaction between N3 + and HCN, experimentally observed by flowing afterglow- selected ion flow/drift tube mass spectrometry and possibly occurring in Titan’s atmosphere.