Theoretical prediction of the vertical electronic spectrum of the C2H radical

The vertical transition energies and oscillator strengths from the $\text{X}\text{?}{}^2\text{Σ}{}^{\mathrm{+}}$ ground and $\text{A}\text{?}{}^2\text{Π}$ excited states of the ethynyl radical C₂H to all higher-lying states resulting from excitation out of π and σ into $\text{π}{}^1$ and $\text{σ}{}^1$ valence-shell MO's, respectively, as well as into 3s and 3p Rydberg species, are calculated by large-scale CI techniques. It is found that the first excited states all result from $\text{π → π}{}^1$ excitations (the lowest three with quartet character), and not from the 4σ → 5σ counterparts favored in the case of isoelectronic CN. This distinction can be explained on the basis of orbital stability differences caused by the effects of hydrogen mixing. The first six states of the C₂H⁺ ion are also treated, and the correspondence with the various associated Rydberg series is discussed. Dipole moments for the $\text{X}\text{?}{}^2\text{Σ}{}^{\mathrm{+}}$ and $\text{A}\text{?}{}^2\text{Π}$ states are also calculated.