Methylene: ab initio vibronic analysis and reinterpretation of the spectroscopic and negative ion photoelectron experiments

The potential curves from extensive ab initio configuration interaction calculations on the three lowest states of methylene (³B₁, ¹A₁, and ¹B₁) and one state (²B₁) of CH^-₂ are reported. The Franck-Condon factors from the vibrational wave-functions obtained using these potential curves lead to excellent agreement with the observed ¹B₁-¹A₁ spectrum (Herzberg) and also lead to an excellent fit with the photoelectron spectra of CH^-₂ (Lineberger), showing that the lowest three bands in the observed negative ion spectrum are hot bands (from vibrationally excited CH^-₂). Reassignment of the observed spectrum based on these calculations leads to the prediction of a ¹A₁-³B₁ splitting of 0.39 ± 0.05 eV (9 kcal); theoretical value: 0.45 eV.