Singlet-triplet energy gaps of divalent five-membered ring R2C4H2M: steric effects

Carbenes are important intermediates in a variety of the chemical reactions. They have been extensively studied both experimentally and theoretically [1–3]. Moreover, there has been significant synthetic interest in the production of the silylene and germylene analogues of the carbene [4, 5]. The five-membered cyclic conjugated carbene, cyclopentadienylidene, has been observed in the interstellar space [6]. In this work, the thermal internal energy gaps, ∆Es–t; enthalpy gaps, ∆Hs–t; Gibbs free energy gaps, ∆Gs–t, between singlet (s) and triplet (t) states of R2C4H2M (M = C, Si and Ge) were calculated at B3LYP/6-311++G** level of theory. The ΔGs-t of R2C4H2C was increased in the order (in kcal/mol): R = -CH3 (-10.51) > -H (-9.59) > i-Pr (-9.51) > t-Bu (-8.98). While, the ΔGs-t of R2C4H2Si and R2C4H2Ge were increased in the order (in kcal/mol): -CH3 (17.01) > i-Pr 15.30) > -H (15.26) > t-Bu (14.35) and -H (22.79) > -CH3 (22.69) > i-Pr (21.66) > t-Bu (21.01), respectively. The results of this work would be presented and discussed.