An estimation of the isomerization energy of acetylene

An accurate prediction of the adiabatic energy difference separating acetylene and vinylidene is presented. The electronic energy contribution is estimated on the basis of CCSD(T) calculations using up to 400 contracted Gaussian basis functions together with extrapolation to the basis set limit; an additional correction is applied to account for residual electron correlation effects. Zero-point vibrational corrections are based on quartic force fields of both acetylene and vinylidene calculated at the CCSD(T) level. Also included are contributions for relativistic effects and the Born–Oppenheimer diagonal correction. The isomerization energy is estimated to be 15200±205 cm−1, which is somewhat lower than the best previous theoretical prediction.