# Rotational and fine structure of open-shell molecules in nearly degenerate electronic states. (arXiv:1801.03923v1 [physics.chem-ph])

An effective Hamiltonian without symmetry restriction has been developed to

model the rotational and fine structure of two nearly degenerate electronic

states of an open-shell molecule. In addition to the rotational Hamiltonian for

an asymmetric top, this spectroscopic model includes energy separation between

the two states due to difference potential and zero-point energy difference, as

well as the spin-orbit (SO), Coriolis, and electron spin-molecular rotation

(SR) interactions. Hamiltonian matrices are computed using orbitally and fully

symmetrized case (a) and case (b) basis sets. Intensity formulae and selection

rules for rotational transitions between a pair of nearly degenerate states and

a nondegenerate state have also been derived using all four basis sets. It is

demonstrated using real examples of free radicals that the fine structure of a

single electronic state can be simulated with either a SR tensor or a

combination of SO and Coriolis constants. The related molecular constants can

be determined precisely only when all interacting levels are simulated

simultaneously. The present study suggests that analysis of rotational and fine

structure can provide quantitative insights into vibronic interactions and

related effects.