title: A computational investigation of HCN2+ isomeric structures: implications for the chemistry of Titan's atmosphere.
abstract: The structure and stability of various HCN2+ isomeric structures have been investigated at the complete active space SCF (CASSCF) and multireference-configuration interaction [MR-Cl-SD(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 H-N-C-N+ ions 1S, 1S', and 1T, the open-chain H-C-N-N+ ions 2S, 2S', and 2T, the HC-N2+ cyclic structures 3S and 3T, and the HN-CN+ 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-Cl-SD(Q]/6-311G(d,p) computational level. The most stable structure was the H-N-C-N+ ion 1T, whose absolute enthalpy of formation at 298.15 K has been estimated as 333.9 +/- 2 kcalmol(-1) using the Gaussian-3 (G3) procedure. The two species closest in energy to 1T are the triplet H-C-N-N+ ion 2T and the singlet diazirinyl cation 3S, whose G3 enthalpies of formation at 298.15 K are 343.5 +/- 2 and 340.6 +/- 2 kcalmol(-1), 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 after-glow-selected ion flow/drift tube mass spectrometry and possibly occurring in Titan's atmosphere.
authors.affiliation: Dipartimento di Chimica Generale ed Organica Applicata Università degli Studi di Torino C.so M. D'Azeglio, 48, 10125 Torino (Italy). paola.antoniotti@unito.it
authors.forename: Paola
authors.initials: P
authors.lastname: Antoniotti
authors.forename: Stefano
authors.initials: S
authors.lastname: Borocci
authors.forename: Nicoletta
authors.initials: N
authors.lastname: Bronzolino
authors.forename: Felice
authors.initials: F
authors.lastname: Grandinetti
mesh_terms: D001272:Atmosphere / Q000737:chemistry*; D003198:Computer Simulation; D003486:Cyanides / Q000737:chemistry; D005118:Extraterrestrial Environment / Q000737:chemistry; D006838:Hydrocarbons / Q000737:chemistry*; D007536:Isomerism; D009584:Nitrogen / Q000737:chemistry*; D018539:Saturn
chemical_list: D003486:Cyanides; D006838:Hydrocarbons; D009584:Nitrogen