Relatives of cyanomethylene: replacement of the divalent carbon by B−, N+, Al−, Si, P+, Ga−, Ge, and As+

Abstract

The lowest lying singlet and triplet states of HBCN⁻, HCCN, HNCN⁺, HAlCN⁻, HSiCN, HPCN⁺, HGaCN⁻, HGeCN, and HAsCN⁺ were studied using the CCSDT(Q)/CBS//CCSD(T)/aug-cc-pVQZ level of theory. Periodic trends in geometries, singlet–triplet gaps, and barriers to linearity were established and analyzed. The first row increasingly favors the triplet state, with a singlet–triplet gap (ΔE_ST = E_singlet − E_triplet) of 3.5 kcal mol⁻¹, 11.9 kcal mol⁻¹, and 22.6 kcal mol⁻¹, respectively, for HBCN⁻, HCCN, and HNCN⁺. The second row increasing favors the singlet state, with singlet–triplet gaps of −20.4 kcal mol⁻¹ (HAlCN⁻), −26.6 kcal mol⁻¹ (HSiCN), and −26.8 kcal mol⁻¹ (HPCN⁺). The third row also favors the singlet state, with singlet–triplet gaps of −26.8 kcal mol⁻¹ (HGaCN⁻), −33.5 kcal mol⁻¹ (HGeCN), and −33.1 kcal mol⁻¹ (HAsCN⁺). The HXCN species have larger absolute singlet–triplet energy gaps compared to their parent species XH₂ except for the case of X = N⁺. The effect of the substitution of hydrogen with a cyano group was analyzed with isodesmic bond separation analysis and NBO.