Theoretical study of nitrogen-rich CN3− anion and related salts M+[CN3]− (M = Li, Na, K)

Abstract

Hetero-doped nitrogen-rich compounds are of great interest for potential use in high energy density materials (HEDM) fields. In this paper, we report in detail the structures and stabilities of a tetra-atomic cluster CN₃⁻, which is isoelectronic to the well-known N₄. A series of higher-level energetic calculations were carried out at the CCSD(T)/6-311+G(d)//B3LYP/6-311+G(d), CASPT2(12,12)/6-311+G(d)//CASSCF (12,12)/6-311+G(d) and CASPT2(12,12)/aug-cc-pVTZ//CASSCF(12,12)/aug-cc-pVTZ levels. The kinetic stability of CN₃⁻ was evaluated for the first time by studying the dissociation, isomerization and intersystem crossing barriers, as well as the Born–Oppenheimer molecular dynamic (BOMD) simulations. We found that the ground state isomer, i.e., chain-like triplet NCNN⁻(C_s), and a higher-energy isomer, i.e., tetrahedral-like singlet CN₃⁻(C_3v), are both kinetically stable, even when the intersystem crossing (ISC) is included. Therefore, the two isomers, ³NCNN⁻ and ¹CN₃⁻ (C_3v), could be observable in future laboratory studies. By comparison with the isoelectronic N₄ system, we conclude that the carbon-doping for XN₃ (X = N to C⁻) greatly enhances the kinetic stability of the chain-like triplet and the tetrahedral-like singlet structures. Finally, for more practical use, our studies on the related salts M⁺[CN₃]⁻ (M = Li, Na, K) showed that both the chain-like ³NCNN⁻ and tetrahedral-like ¹CN₃⁻ could act as promising building blocks for novel HEDMs.