Abstract
An unexpected deficit of the computed hyper fine coupling constant in normal formyl HCO radicals, compared to that expected based on the proton to deuteron gyromagnetic ratio of the DCO species, was observed in experimental electron paramagnetic resonance studies of HCO and DCO radicals in a CO solid matrix at cryogenic conditions. Still, the matrix was found to have only a small effect on the anisotropic parts of the magnetic parameter tensors. The underlying isotope effect between the lighter proton and the heavier deuteron on the motional dynamics was verified and elucidated by quantum chemical calculations. The experimental results obtained within a temperature range of 1.4 K–4.2 K require special attention due to the tunneling motions of the molecule and its constituent particles. The effect from vibrational, rotational, and librational motion observed in the molecular states of formyl as a probe, averaged over the dynamics of the low temperature CO matrix isolation, reveals a clear proton isotope effect under both classical and quantum conditions.
Highlights
Being an intermediate chemical species, the formyl radical plays an important role in a wide range of chemical reactions, such as the formation of complex organic molecules in the interstellar ices,1 the chemistry of the hydrocarbon flames and combustion processes,2 and the atmospheric chemistry involving organic molecules,3 among others
Despite a long history of studying matrix-isolated formyl radicals, which started from the first IR-investigation of HCO in solid CO,7 little has been done in the electron paramagnetic resonance (EPR) of HCO and DCO radicals trapped at low temperatures
As the rovibrational/librational averaging of the EPR magnetic parameters of formyl are not incorporated in the quantum chemistry computations, a significantly smaller Fermi contact interaction is obtained for the lighter hydrogen nucleus in the HCO isotopomer than what is observed in experiments in the solid CO matrix at cryogenic temperatures
Summary
Being an intermediate chemical species, the formyl radical plays an important role in a wide range of chemical reactions, such as the formation of complex organic molecules in the interstellar ices, the chemistry of the hydrocarbon flames and combustion processes, and the atmospheric chemistry involving organic molecules, among others. At 5 K, the HCO in CD4 did yield a signal of axially symmetrical g- and A-tensors, suggesting a fast uniaxial rotation around the axis close to the C–O bond At such a low temperature, solid CH4 is partially orientationally ordered with 5/6 of the methane molecules having fixed symmetrical axes and 1/6 of the molecules rotating freely. At 29 K, the appearance of the HCO spectrum changed to an almost isotropic doublet showing no line splitting At this temperature, both solid methane matrices are orientationally disordered being composed of rotating molecules making a possible formyl rotation around the three principal axes of inertia. The CH3 tunneling rotation for A- and E-symmetry radicals was studied in three orientational phases of solid CD4.15 Holmberg performed an EPR study of HCO in a gamma irradiated single crystal of formic acid. The spectrum showed a rhombic anisotropy of both the A- and g-tensors, respectively; suggesting that even at as high temperature as 77 K, the HCO radical is not allowed to undergo a rapid rotation (or to rotate at all)
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