Highly Accurate Quartic Force Field and Rovibrational Spectroscopic Constants for the Azirinyl Cation (c-C₂NH₂⁺) and Its Isomers

Abstract

The azirinyl cation is an aromatic cyclic molecule that is isoelectronic with cyclopropenylidene, c-C3H2, and c-C3H3 +. Cyclopropenylidene has been shown to be ubiquitous, existing in many different astrophysical environments. Given the similar chemistry between C and N, and the relative abundances between C and N in astrophysical environments, it is expected that there should be aromatic ringed molecules that incorporate N in the ring, but as yet, no such molecule has been identified. To address this issue, the present study uses high levels of electronic structure theory to compute a highly accurate quartic force field (QFF) for the azirinyl cation and its two lowest lying isomers, the cyanomethyl and isocyanomethyl cations. The theoretical approach uses the singles and doubles coupled-cluster method that includes a perturbative correction for connected triple excitations, CCSD­(T), together with extrapolation to the one-particle basis set limit and corrections for scalar relativity and core-correlation. The QFF is then used in a second-order vibrational perturbation theory analysis (VPT2) to compute the fundamental vibrational frequencies and rovibrational spectroscopic constants for all three C2NH2 + isomers. The reliability of the VPT2 vibrational frequencies is tested by comparison to vibrational configuration interaction (VCI) calculations, and excellent agreement is found between the two approaches. Fundamental vibrational frequencies and rovibrational spectroscopic constants for all singly substituted 13C, 15N, and D isotopologues are also reported. It is expected that the highly accurate spectroscopic data reported herein will be useful in the identification of these cations in high-resolution experimental or astronomical observations.

Azapyramidinium Ion and iso-Azapyramidinium Ion: Mechanisms for their Synthesis, Interconversion, and Decomposition

“…Azirenium ion 9 has been characterized by several high-level computational studies and it appears to have been prepared in the gas phase. 27,28 However, there seem to be few, if any, established solution routes to 9. Similar nitrenium ions have been made through oxidation of the corresponding amines and/or through photolysis of N-aminopyridinium ions.…”

Energy Landscape and Structural and Spectroscopic Characterization of Diazirine and Its Cyclic Isomers

“…Interstellar clouds are well-known to be a reservoir of molecules with unusual structures, some of these molecules are nitrogenous. Although, nitrogenous heterocyclic organics form the basis of many biomolecules, no nitrogen containing cyclic molecule in which nitrogen is part of the cyclic structure has ever been detected in the gas phase ISM despite numerous observational and theoretical studies. − Noncyclic nitrogen-containing molecules such as methyl cyanide, methyl isocyanide, and etheneimine have been detected. ,− Some linear isomers of diazomethane (CH 2 N 2 ) have been identified in the interstellar medium although diazomethane itself has not been identified thus far. ,, Azirines (C 2 H 3 N) are cyclic molecules isomeric with methyl cyanide, which has been observed in many astronomical environments . Cyclic diazirines (c-H 2 NCN) are isomeric with cyanamide, NH 2 CN, which has been detected in the interstellar medium, isomeric with diazomethane (CH 2 N 2 ), and isoelectronic with methyl cyanide (CH 3 CN).…”