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An effective Hamiltonian without symmetry restriction has been developed tomodel the rotational and fine structure of two nearly degenerate electronicstates of an open-shell molecule. In addition to the rotational Hamiltonian foran asymmetric top, this spectroscopic model includes energy separation betweenthe two states due to difference potential and zero-point energy difference, aswell as the spin-orbit (SO), Coriolis, and electron spin-molecular rotation(SR) interactions. Hamiltonian matrices are computed using orbitally and fullysymmetrized case (a) and case (b) basis sets. Intensity formulae and selectionrules for rotational transitions between a pair of nearly degenerate states anda nondegenerate state have also been derived using all four basis sets. It isdemonstrated using real examples of free radicals that the fine structure of asingle electronic state can be simulated with either a SR tensor or acombination of SO and Coriolis constants. The related molecular constants canbe determined precisely only when all interacting levels are simulatedsimultaneously. The present study suggests that analysis of rotational and finestructure can provide quantitative insights into vibronic interactions andrelated effects.

Rotational and fine structure of open-shell molecules in nearly degenerate electronic states