Infrared absorption spectra of partially deuterated methoxy radicals CH₂DO and CHD₂O isolated in solid para-hydrogen

The investigation of partially deuterated methoxy radicals is important because the symmetry lowering from C_3v to C_s provides new insights into the couplings between rovibronic states via Jahn-Teller and spin-orbit interactions. The vibrational spectrum of the partially deuterated methoxy radical CH₂DO in a matrix of p-H₂ has been recorded. This species was prepared by irradiating a p-H₂ matrix containing deuterated d₁-nitritomethane (CH₂DONO) at 3.3 K with laser light at 355 nm. The identification of the radical is based on the photochemical behavior of the precursor and comparison of observed vibrational wavenumbers and infrared (IR) intensities with those predicted from a refined quartic, curvilinear, internal coordinate force field calculated with the coupled-cluster singles and doubles with perturbative triples/cc-pVTZ method. CH₂DO reacts with H₂ with a rate coefficient (3.5 ± 1.0) × 10^-3 s^-1. Predominantly c-CHDOH and a negligibly small amount of t-CHDOH were produced. This stereoselectivity results from the reaction H + C_s-CH₂DOH, which was demonstrated by an additional experiment on irradiation of a CH₂DOH/Cl₂/p-H₂ matrix with ultraviolet and IR light to induce the H + CH₂DOH reaction; only c-CHDOH was observed from this experiment. Even though the energies of transition states and products for the formation of c-CHDOH and t-CHDOH differ by only ∼10 cm^-1, the selective formation of c-CHDOH can be explained by tunneling of the hydrogen atom via an optimal tunneling path. Similarly, the vibronic spectrum for the partially deuterated specie d₂-methoxy radical (CHD₂O) was obtained upon irradiation of d₂-nitritomethane (CHD₂ONO) at 355 nm. Lines associated with the fundamental vibrational modes were observed and assigned; line positions agree with theoretically predicted vibrational wavenumbers. CHD₂O reacts with H₂ with a rate coefficient (6.0 ± 1.4) × 10^-3 s^-1; CD₂OH was produced as a major product because the barrier for the formation of CHDOH from H + CHD₂OH is greater by ∼400 cm^-1. Rate coefficients of the decays of CH₃O, CH₂DO, CHD₂O, and CD₃O and their corresponding potential energy surfaces are compared.