H/D isotope effect observed in the isotropic Fermi contact interaction offormyl radical: Experimental and theoretical analysis of the inter- and intra-moleculardynamics

An unexpected deficit of the computed hyper fine coupling constant in normal formyl HCOradicals, compared to that expected based on the proton to deuteron gyromagnetic ratio ofthe DCO species, was observed in experimental electron paramagnetic resonance studies ofHCO and DCO radicals in a CO solid matrix at cryogenic conditions. Still, the matrix wasfound to have only a small effect on the anisotropic parts of the magnetic parametertensors. The underlying isotope effect between the lighter proton and the heavier deuteronon the motional dynamics was verified and elucidated by quantum chemical calculations. Theexperimental results obtained within a temperature range of 1.4 K–4.2 K require specialattention due to the tunneling motions of the molecule and its constituent particles. Theeffect from vibrational, rotational, and librational motion observed in the molecularstates of formyl as a probe, averaged over the dynamics of the low temperature CO matrixisolation, reveals a clear proton isotope effect under both classical and quantumconditions.