The rotational dependence of the Renner-Teller interaction: a new term in the effective Hamiltonian for linear triatomic molecules in Π electronic states

An additional term in the effective Hamiltonian for a molecule in a 2S+1Π state subject to a small Renner—Teller effect is derived. It takes the form of a rotational dependence of the Renner—Teller operator and is assigned the parameter ∊ω2,D . This term reproduces two characteristics of the spin—rotational levels of Renner—Teller molecules, both of which are well documented by experimental examples but are not explicable in terms of the previous effective Hamiltonian. These are the small difference between the B values of different vibronic components of the same bending vibrational level and the anomalously strong dependence of the spin—rotation parameter γ on ν2 for molecules with S > 0. These effects have been explained by previous workers using perturbation theory but in a less general and more complicated fashion.