Gas phase enthalpies of formation, isomerization, and disproportionation of mono- through tetra-substituted tetrahedranes: A G4MP2/G4 theoretical study

Gas phase (298.15 K, 1 atm) enthalpies of formation (ΔfH°(g)), enthalpies of disproportionation to two corresponding acetylene molecules (ΔrxnH°(g),Td→acet), and enthalpies of isomerization from a tetrahedrane geometry to a 1,3-cyclobutadiene structure (ΔisomH°(g),Td→CBD) were calculated for the monothrough tetra-substituted hydro, fluoro, chloro, bromo, methyl, ethynyl, and cyano carbon tetrahedrane derivatives at the G4MP2 and G4 levels of theory. All derivatives have endothermic ΔfH°(g) indicative of the cage strain in these systems. In all cases, ΔrxnH°(g),Td→acet and ΔisomH°(g),Td→CBD are predicted to be substantially exothermic. High quality linear regression fits within a homologous series were obtained between the number of substituents and the G4MP2/G4 estimated ΔfH°(g). Via calculations on lower homolog members, this strategy was employed to allow extrapolated G4 and/or G4MP2 ΔfH°(g) (as well as some ΔrxnH°(g),Td→acet and ΔisomH°(g),Td→CBD) to be obtained for the monothrough tetra-substituted t-butyl, trifluoromethyl, and trimethylsilyl carbon tetrahedrane derivatives.