Electronic states and photodissociation dynamics of hydroxyalkyl radicals

UnrestrictedThe predissociation of vibrationally excited hydroxymethyl radical and the ionization spectroscopy and the photodissociation dynamics of 1-hydroxyethyl radicals from excited Rydberg states are described.; The OH-stretch overtone spectroscopy and dynamics of the hydroxymethyl radical, CH2OH, are reported in the region of the second and third overtones. The second overtone spectrum at 10484 cm–1 is obtained by double resonance IR-UV resonance enhanced multiphoton ionization (REMPI) spectroscopy via the 3pz electronic state. The third overtone spectra of CH2OH and CD2OH are observed at ~13600 cm–1 by monitoring H-atom photofragments while scanning the excitation laser frequency. Dissociation via tunneling is proposed. No isomerization to methoxy is observed.; The electronic spectroscopy and photodissociation dynamics of the CH3CHOH radical in the region 19400-37000 cm-1 were studied in a molecular beam using resonance-enhanced multiphoton ionization (REMPI), photofragment yield spectroscopy, and time-of-flight (TOF) spectra of H and D fragments. The onset of thetransition to the Rydberg 3s state, the lowest excited state, is estimated at 19600. The 3s state dissociates fast, and no REMPI spectrum is observed. The origin band of the transition to the 3pz state, identified by 2+2 REMPI, lies at 32360 cm-1.; By comparison of the TOF distributions of the isotopologs CH3CHOH, CHCHOD, and CDCHOH, it is concluded that two major product channels dominate the photodissociation, one leading to acetaldehyde and the other to vinyl alcohol (enol) products. There is no indication of isomerization to ethoxy.; On the basis of electronic structure calculations, we offer a physical explanation of the observed large decrease (0.9 eV) in ionization energies (IE) in going from hydroxymethyl to hydroxyethyl radical. The effect is attributed to hyperconjugative interactions between the #CH orbitals of the methyl group in hydroxyethyl, the singly occupied p orbital of carbon, and the lone pair p orbital of oxygen. Analyses of vertical and adiabatic IEs and hyperconjugation energies computed by the natural bond orbital (NBO) procedure reveal that the decrease is due to the destabilization of the singly occupied molecular orbital in hydroxyethyl radical as well as structural relaxation of the cation maximizing the hyperconjugative interactions