PHOTOIONIZATION OF HYDROXYMETHYL (CD2OH AND CD2OD) AND METHOXY (CD3O) RADICALS - PHOTOION EFFICIENCY SPECTRA, IONIZATION ENERGIES, AND THERMOCHEMISTRY

Photoion efficiency (PIE) spectra were obtained for CD2OH, CD2OD, and CD3O radicals using the discharge flow-photoionization mass spectrometry technique. The radicals were generated in a flow tube via reaction of F atoms with the appropriate methanol isotopomers (CD3OH --> CD2OH, CD3OD --> CD2OD, and CD3OH --> CD3O), which were in large excess. Deuterated methoxy radicals, CD3O, were also generated via the reaction of CD3 with NO2. Photoionization of the radicals was achieved using high intensity, dispersed synchrotron radiation, and ionization energies (IE) of these radicals were derived from the thresholds of the PIE spectra: IE(CD2OH) = 7.54 +/- 0.02 eV, IE(CD2OD) = 7.53 +/- 0.02 eV, and IE(CD3O) = 10.74 +/- 0.02 eV. The PIE spectra for CD2OH and CD3O are compared to those of a previous photoionization study, and differences are discussed. Integration of previously published photoelectron spectroscopy data for CD2OH yields a curve quite similar to our PIE spectrum. Empirical estimates of IE(CH2OH) and IE(CH3O) are given to corroborate our assignments. Attempts to detect CH3O+ from direct ionization of CH3O, which was generated by two methods (CH3OD + F and CH3 + NO2), were unsuccessful. However, HCO+, presumably formed along with H-2 from CH3O dissociative ionization, was detected at a threshold 8.73 eV. This appearance energy corresponds to a barrier of 17.6 kJ mol(-1) for the process CH3O* --> HCO+ + H-2. Heats of formation for neutral and ion species : were determined using literature values for the proton affinity of CH2O and integrated heat capacities and the IE's measured in this laboratory, and an energy level diagram was developed for T = 298 K. For hydroxymethyl and methoxy radicals the following values were obtained: Delta(f)H degrees(298)(CH2OH) = -20.4 kJ mol(-1) (which agrees well with the recent results of Traeger and Holmes, J. Phys. Chem. 1993, 97, 3453); Delta(f)H degrees(298)(CH3O) = 12.2 kJ mol(-1). Bond dissociation energies were also computed: D degrees(0)(H-CH2OH) = 393.1 kJ mol(-1) and D degrees(0)(CH3OH) = 426.6 kJ mol(-1). The measured ionization energies and the derived thermodynamic quantities are compared with previously reported results.