Nuclear spin conversion in molecules: Experiments on (CH3F)-C-13 support a mixing-of-states model

Using light-induced drift as a tool to separate nuclear spin isomers, we have studied the ortho-para conversion process for gaseous (CH3F)-C-13 interacting with O-2, N-2, CH3Cl, and CH3F itself. It is found that the conversion rate is significantly smaller for collisions with O-2 or N-2 than with CH3Cl or CH3F. The data show that intermolecular magnetic interactions do not play a leading role in the conversion. In order to test an alternative model based on mixing of states due to intramolecular interactions, pressure broadening has been measured for these systems. The observed correlation between conversion rates and broadening coefficients supports this mixing-of-states model.