Ming-Wei Chen

The spectroscopic characterization of the methoxy radical. I. Rotationally resolved à A21–X̃ E2 electronic spectra of CH3O

New experimental data have been obtained for the methoxy radical by observing at high-resolution laser-induced fluorescence (LIF) and stimulated emission pumping (SEP) transitions between the X (2)E and lowest excited A (2)A(1) state. The SEP transitions were from the A state (pumped from the X (2)E(3/2) spin-orbit component) to the X (2)E(1/2) spin-orbit component. These data for the first time directly connect with high precision the spin-orbit components of the X (2)E ground state of CH(3)O. Surprisingly these new SEP observations are inconsistent with predictions of the X state structure based on long-standing analyses primarily based on the microwave spectra of ground state CH(3)O. It is found that all the experimental data can be understood consistently when the previously accepted value of the spin-orbit coupling constant is adjusted and the reflection parity assignments in the X state are reversed. The latter action changes the sign of a number of reflection-parity-dependent parameters in the X state. The ramifications of the changes and the physical interpretation of the resulting parameters are discussed in some detail.

The spectroscopic characterization of the methoxy radical. III. Rotationally resolved Ã2A1–X̃2E electronic and X̃2E submillimeter wave spectra of partially deuterated CH2DO and CHD2O radicals

Rotationally resolved laser induced fluorescence and stimulated emission pumping Ã(2)A(1)-X̃(2)E spectra, along with pure rotational spectra in the 153-263 GHz region within the E(3/2) component of the ground state in asymmetrically deuterated methoxy radicals CH(2)DO and CHD(2)O have been observed. The combined data set allows for the direct measurement with high precision of the energy separation between the E(1/2) and E(3/2) components of the ground state and the energy separation between the parity stacks in the E(3/2) component of the ground state. The experimentally observed frequencies in both isotopologues are fit to an effective rotational Hamiltonian accounting for rotational and spin-rotational effects arising in a near-prolate asymmetric top molecule with dynamic Jahn-Teller distortion. Isotopic dependencies for the molecular parameters have been successfully implemented to aid the analysis of these very complex spectra. The analysis of the first and second order contributions to the effective values of molecular parameters has been extended to elucidate the physical significance of resulting molecular parameters. Comparisons of measured parameters, e.g., spin-orbit coupling, rotational and spin-rotation constants, are made among the 5 methoxy isotopologues for which data is now available. Comparisons of experimental results, including the derived geometric structure at both the C(3v) conical intersection and at the Jahn-Teller distorted minima, are made with quantum chemistry calculations.

Detection and Characterization of Products from Photodissociation of XCH2CH2ONO (X = F, Cl, Br, OH)

Alkyl nitrites have been used previously to produce alkoxy radicals, which are important intermediates in the oxidation of alkanes in atmospheric and combustion processes. Substituted alkoxy radicals, particulary hydroxyalkoxy radicals, are also important intermediates in the atmospheric oxidation of alkenes and combustion of alcohols. In order to produce substituted alkoxy radicals we have photolyzed at 351 nm substituted alkyl nitrites, XCH(2)CH(2)ONO (X = F, Cl, Br, OH). Using laser-induced fluorescence only in the case of X = F do we observe the spectrum of substituted alkoxy radical, XCH(2)CH(2)O; but we always observe the electronic transitions of formaldehyde, HCHO, and vinoxy radical, CH(2)CHO. HCHO can be formed by the dissociation of XCH(2)CH(2)O in its ground state as the barrier to C-C bond dissociation is less than the photon energy remaining after O-NO bond breakage. However, the barrier along the reaction path directly leading from XCH(2)CH(2)O to CH(2)CHO + HX is much higher than the available energy remaining after O-NO bond breakage. A roaming mechanism, involving a frustrated dissociation of X followed by HX extraction, might explain the apparent paradox. Under the conditions of our observations vinoxy retains considerable vibrational excitation but the observed rotational temperatures of both HCHO and CH(2)CHO are ≲7 K.

Spectroscopic studies of the Ã-X̃ electronic spectrum of the β-hydroxyethylperoxy radical: structure and dynamics.

The jet-cooled Ã-X̃ near IR origin band spectra of the G(1)G(2)G(3) conformer of four β-hydroxyethylperoxy isotopologues, β-HEP (HOCH(2)CH(2)OO), β-DHEP (DOCH(2)CH(2)OO), β-HEP-d(4) (HOCD(2)CD(2)OO), and β-DHEP-d(4) (DOCD(2)CD(2)OO), have been recorded by a cavity ringdown spectrometer with a laser source linewidth of ~70 MHz. The spectra of all four isotopologues have been analyzed and successfully simulated with an evolutionary algorithm, confirming the cyclic structure of the molecule responsible for the observed origin band. The analysis also provides experimental à and X̃ state rotational constants and the orientation of the transition dipole moment in the inertial axis system; these quantities are compared to results from electronic structure calculations. The observed, broad linewidth (Δν > 2 GHz) is attributed to a shortened lifetime of the à state associated with dynamics along the reaction path for hydrogen transfer from the OH to OO group.