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 spectromet ry 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. Deu terated methoxy radicals, CD3O, were also generated via the reaction o f CD3 with NO2. Photoionization of the radicals was achieved using hig h intensity, dispersed synchrotron radiation, and ionization energies (IE) of these radicals were derived from the thresholds of the PIE spe ctra: 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 c ompared to those of a previous photoionization study, and differences are discussed. Integration of previously published photoelectron spect roscopy data for CD2OH yields a curve quite similar to our PIE spectru m. Empirical estimates of IE(CH2OH) and IE(CH3O) are given to corrobor ate our assignments. Attempts to detect CH3O+ from direct ionization o f CH3O, which was generated by two methods (CH3OD + F and CH3 + NO2), were unsuccessful. However, HCO+, presumably formed along with H-2 fro m CH3O dissociative ionization, was detected at a threshold 8.73 eV. T his 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 i on species : were determined using literature values for the proton af finity of CH2O and integrated heat capacities and the IE's measured in this laboratory, and an energy level diagram was developed for T = 29 8 K. For hydroxymethyl and methoxy radicals the following values were obtained: Delta(f)H degrees(298)(CH2OH) = -20.4 kJ mol(-1) (which agre es 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 compa red with previously reported results.