  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.
