No abstract

Infrared spectroscopy has been utilized to examine the structure and vibrational decay dynamics of CH4–OH complexes that have been stabilized in the entrance channel to the CH4+OH hydrogen abstraction reaction. Rotationally resolved infrared spectra of the CH4–OH complexes have been obtained in the OH fundamental and overtone regions using an IR-UV (infrared-ultraviolet) double-resonance technique. Pure OH stretching bands have been identified at 3563.45(5) and 6961.98(4) cm−1 (origins), along with combination bands involving the simultaneous excitation of OH stretching and intermolecular bending motions. The infrared spectra exhibit extensive homogeneous broadening arising from the rapid decay of vibrationally activated CH4–OH complexes due to vibrational relaxation and/or reaction. Lifetimes of 38(5) and 25(3) ps for CH4–OH prepared with one and two quanta of OH excitation, respectively, have been extracted from the infrared spectra. The nascent distribution of the OH products from vibrational predissociation has been evaluated by ultraviolet probe laser-induced fluorescence measurements. The dominant inelastic decay channel involves the transfer of one quantum of OH stretch to the pentad of CH4 vibrational states with energies near 3000 cm−1. The experimental findings are compared with full collision studies of vibrationally excited OH with CH4. In addition, ab initio electronic structure calculations have been carried out to elucidate the minimum energy configuration of the CH4–OH complex. The calculations predict a C3v geometry with the hydrogen of OH pointing toward one of four equivalent faces of the CH4 tetrahedron, consistent with the analysis of the experimental infrared spectra.

Electron impact (70 eV) mass spectra of a series of C 1 -C 6 alcohols encased in large superfluid liquid helium nanodroplets (B60 000 helium atoms) have been recorded. The presence of helium alters the fragmentation patterns when compared with the gas phase, with some ion product channels being more strongly affected than others, most notably cleavage of the C a -H bond in the parent ion to form the corresponding oxonium ion. Parent ion intensities are also enhanced by the helium, but only for the two cyclic alcohols studied, cyclopentanol and cyclohexanol, is this effect large enough to transform the parent ion from a minor product (in the gas phase) into the most abundant ion in the helium droplet experiments. To demonstrate that these findings are not unique to alcohols, we have also investigated several ethers. The results obtained for both alcohols and ethers are difficult to explain solely by rapid cooling of the excited parent ions by the surrounding superfluid helium, although this undoubtedly takes place. A second factor also seems to be involved, a cage effect which favors hydrogen atom loss over other fragmentation channels. The set of molecules explored in this work suggest that electron impact ionization of doped helium nanodroplets does not provide a sufficiently large softening effect to be useful in analytical mass spectrometry.

A hydrogen-bonded complex between the hydroxyl radical and acetylene has been stabilized in the reactant channel well leading to the addition reaction and characterized by infrared action spectroscopy in the OH overtone region. Analysis of the rotational band structure associated with the a-type transition observed at 6885.53(1) cm(-1) (origin) reveals a T-shaped structure with a 3.327(5) A separation between the centers of mass of the monomer constituents. The OH (v = 1) product states populated following vibrational predissociation show that dissociation proceeds by two mechanisms: intramolecular vibrational to rotational energy transfer and intermolecular vibrational energy transfer. The highest observed OH product state establishes an upper limit of 956 cm(-1) for the stability of the pi-type hydrogen-bonded complex. The experimental results are in good accord with the intermolecular distance and well depth at the T-shaped minimum energy configuration obtained from complementary ab initio calculations, which were carried out at the restricted coupled cluster singles, doubles, noniterative triples level of theory with extrapolation to the complete basis set limit.

Predissociation of the B 3Σu− state of S2 has been investigated by a combination of cavity ring-down spectroscopy and model calculations. The experimental spectra of the B 3Σu−−X 3Σg−(v′,0) bands for 10⩽v′⩽22 span the wavenumber range 35 480–39 860 cm−1. Extensive variation is observed in the degree of rotational structure within the vibrational bands because of lifetime broadening caused by predissociation. Fits to the band contours give homogeneous linewidths for transitions to the B-state vibrational levels for 10⩽v′⩽17 that vary from ⩽1 cm−1 for the (10,0) band to 7±1 cm−1 for the (17,0) band with a maximum linewidth of 14±1 cm−1 for the (13,0) band. For v′⩾18, all bands are completely diffuse, indicating linewidths in excess of 15 cm−1. The experimental results are compared with the results of a theoretical model that uses a Rydberg–Klein–Rees (RKR) potential for the B 3Σu− state, ab initio calculations of the repulsive potentials that cross the B state, and Fermi golden rule calculations of the predissociation rates for the different repulsive potentials. Minor adjustments to the ab initio potentials, and an estimate of the spin-orbit coupling between the bound and repulsive states enable us to calculate predissociation rates in excellent agreement with the experimental observations. We deduce that the predissociation for v′⩽16 is predominantly via a 1Πu state, whereas for v′⩾17, coupling to a second repulsive state, suggested to be either a 5Σu− or 5Πu state, provides the primary mechanism for predissociation.