The anomalies found in the spin splitting of a number of 2 Sigma states of diatomic molecules are explained by taking the rotational stretching (centrifugal distortion) of the molecule into account by using an adequate quantum-mechanical treatment. This calculation also yields both the true rotational dependence of the parameters p and q describing the Lambda doubling of the 2 Pi states, and the centrifugal distortion correction terms to the rotational energy. For the 2 Pi states the dependence of the spin-orbit coupling constant A on the internuclear distance gives rise to an additional correction term to the energy of the form A J J(J+1). An expression for the parameter A J in terms of spectroscopic quantities where values are usually known is derived, and tested for several examples.
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L. Veseth (1970) studied this question.