Unimolecular processes in CH_2OH below the dissociation barrier:O-H stretch overtone excitation and dissociation

摘要

The OH-stretch overtone spectroscopy and dynamics of the hydroxymethyl radical,CH_2OH,are reported in the region of the second and third overtones,which is above the thermochemical threshold to dissociation to H + CH_2O(D_0=9600 cm~(-1)).The second overtone spectrum at 10 484 cm~(-1)is obtained by double resonance IR-UV resonance enhanced multiphoton ionization(REMPI)spectroscopy via the 3p_z electronic state.It is rotationally resolved with a linewidth of 0.4 cm~(-1)and displays properties of local-mode vibration.No dissociation products are observed.The third overtone spectra of CH_2OH and CD_2OH are observed at approx13 600 cm~(-1)by monitoring H-atom photofragments while scanning the excitation laser frequency.No double resonance REMPI spectrum is detected,and no D fragments are produced.The spectra of both isotope analogs can be simulated with a linewidth of 1.3 cm~(-1),indicating dissociation via tunneling.By treating the tunneling as one dimensional and using the calculated imaginary frequency,the barrier to dissociation is estimated at about 15 200 cm~(-1),in good agreement with theoretical estimations.The Birge-Sponer plot is linear for OH-stretch vibrations 1v_1-4v_1,demonstrating behavior of a one-dimensional Morse oscillator.The anharmonicity parameter derived from the plot is similar to the values obtained for other small OH containing molecules.Isomerization to methoxy does not contribute to the predissociation signal and the mechanism appears to be direct O-H fission via tunneling.CH_2OH presents a unique example in which the reaction coordinate is excited directly and leads to predissociation via tunneling while preserving the local-mode character of the stretch vibration.