  A complete theoretical model of the nuclear spin conversion in 13CH3F induced
by intramolecular ortho-para state mixing is proposed. The model contains
parameters determined from the level-crossing spectra of the 13CH3F spin
conversion. This set of parameters includes the ortho-para decoherence rate,
the magnitude of the hyperfine spin-spin interaction between the molecular
nuclei and the energy gap between the mixed ortho and para states. These
parameters are found to be in a good agreement with their theoretical
estimates.
