High-accuracy estimates for the vinylidene-acetylene isomerization energy and the ground state rotational constants of:C=CH₂

Highly accurate calculations are reported for properties of vinylidene (H₂C=C:), specifically the position of its zero-point vibrational level relative to that of acetylene and its equilibrium structure and ground state rotational constants. The isomerization energy of vinylidene calculated at the HEAT-456QP level of theory is 43.53 ± 0.15 kcal mol^-1, in agreement with the previous best estimate, but associated with a much smaller uncertainty. In addition, the thermochemical calculations presented here also allow a determination of the H₂CC-H bond energy of the vinyl radical at the HEAT-345(Q) level of theory, which is 77.7 ± 0.3 kcal mol ^-1. The equilibrium structure of vinylidene, estimated with an additivity scheme that includes treatment of correlation effects beyond CCSD(T) as well as relativistic and adiabatic (diagonal Born-Oppenheimer correction) contributions, is r_CC = 1.2982 ± 0.0003 Å, r _CH = 1.0844 ± 0.0003 Å, and θ_CCH = 120.05 ± 0.05, with zero-point rotational constants (including vibrational contributions and electronic contributions to the moment of inertia) estimated to be A₀ = 9.4925 ± 0.0150 cm^-1, B ₀ = 1.3217 ± 0.0017 cm^-1, and C₀ = 1.1602 ± 0.0016 cm^-1.