Infrared absorption of CH3O/CD3O radicals produced upon photolysis of CH3ONO/CD3ONO in a p-H2 matrix

The methoxy radical, \chem{CH_3O}, has attracted much attention because of its important molecular structure and also as a reaction intermediate in combustion and atmospheric chemistry. Previous investigations include laser-induced fluorescence, laser magnetic resonance, and stimulated emission pumping. High-resolution infrared spectrum of jet-cooled \chem{CH_3O}, produced by laser photolysis of \chem{CH_3ONO}, in the C-H stretching region 2850-2940 cm$^{-1}$ has been reported.\footnote { J.-X.~Han, Y. G.~Utkin, H.-B.~Chen, L. A.~Burns and R. F.~Curl, \textit{J. Chem. Phys.} \underline{\textbf{117}}, 6538 (2009).} However, direct infrared absorption spectrum of \chem{CH_3O} other than the C–H stretching region remains unreported. Irradiation of a ${p}$-H$_{2}$ matrix containing \chem{CH_3ONO} at 3.2 K with UV light produced main features at 1365.4, 1427.5 (2$_{1}$$^{-}$, 2$_{1}$$^{+}$), 1041.8 (3$_{1}$$^{-}$), 1346.8, 1427.5, 1520.9, 1520.9 (5$_{1}$$^{-}$, 5$_{1}$$^{+}$, 5$_{1}$$^{-}$, 5$_{1}$$^{+}$), and 689.3/694.9, 945.9/951.7, 1233.5, 1235.9 cm$^{-1}$ (6$_{1}$$^{-}$, 6$_{1}$$^{+}$, 6$_{1}$$^{-}$, 6$_{1}$$^{+}$); labels 2-6 in parentheses indicate transitions to vibrational states attributable to the umbrella, C–O stretching, \chem{CH_2} scissoring, and HCO deformation modes of \chem{CH_3O}, respectively. These features appeared upon photolysis and diminished after five minutes; formation of \chem{CH_2OH} was observed as \chem{CH_3O} decayed. The assignments were based on comparison of observed vibrational wavenumbers with those predicted with the quadratic potential energy force field and quadratic dipole moment expansion calculated with the CCSD(T)/cc-pVTZ method.\footnote { J.~Nagesh and E. L.~Sibert III, \textit{J. Phys. Chem. A} \underline{\textbf{116}}, 3846 (2012).} Jahn-Teller and anharmonic vibrational contributions were included in the full Hamiltonian to estimate the correlation diagram connecting the harmonic eigenvalues to those of the fully coupled problem. Similarly, lines of \chem{CD_3O} were observed upon UV photolysis of \chem{CD_3ONO}, but became diminished within five minutes. These observations demonstrates the advantage of diminished cage effect of solid ${p}$-H$_{2}$; \chem{CH_3O} and \chem{CD_3O} are produced via ${in situ}$ UV photodissociation of \chem{CH_3ONO} isolated in ${p}$-H$_{2}$, but not in Ar or Ne