David T. Anderson

Exploring the OH+CO reaction coordinate via infrared spectroscopy of the OH–CO reactant complex

A hydrogen-bonded complex of OH with CO is identified along the reaction coordinate for the OH+CO↔HOCO→H+CO2 reaction. The existence of this linear OH–CO complex is established by infrared action spectroscopy, which accesses vibrational stretching and bending modes of the complex. Complementary electronic structure calculations characterize the OH–CO and OH–OC complexes, the transition state for HOCO formation, and the reaction pathways that connect these complexes directly to the HOCO intermediate.

Infrared spectroscopy and time-resolved dynamics of the ortho-H2–OH entrance channel complex

The rotationally resolved infrared spectrum of the prereactive o-H2–OH complex in its ground electronic state is obtained in the OH overtone region at ∼1.4 μm using an IR-UV double resonance fluorescence enhancement technique. The pure OH overtone band of o-H2–OH is observed as well as approximately 20 additional rovibrational transitions extending out to the OH (Xu20092Π,v=2)+o-H2(Xu20091Σg+) dissociation limit. These transitions are assigned as combination bands involving the simultaneous excitation of the OH vibrational overtone and intermolecular bending (internal rotor) states. The assignment of the experimental spectrum is aided by a detailed comparison with the bound states computed for the ab initio potential of Clary, Werner, and co-workers [Mol. Phys. 83, 405 (1994)]. The infrared spectroscopy results also verify the topology of this ab initio potential in the entrance channel to the OH+H2 hydrogen abstraction reaction. Direct time-resolved experiments indicate that the lifetime of the vibrationally act...

State-to-state inelastic scattering from vibrationally activated OH–H2 complexes

State-selective infrared excitation of o-H2–OH via the pure OH overtone transition has been used to induce a half-collision inelastic scattering event between the OH radical and ortho-H2 under restricted initial orientation conditions. The time evolution and final state distribution of the OH products from vibrational predissociation have been evaluated by ultraviolet probe laser-induced fluorescence measurements. The half-collision scattering takes place with ∼3350 cm−1 of energy available to the OH (v=1)+o-H2 products, an energy that exceeds the classical barrier to reaction. The OH (v=1) products are preferentially populated in high rotational levels with a distribution that is consistent with an energy gap law. A significant fraction of the OH fragments are promoted to the excited spin–orbit state in the predissociation process. A strong lambda-doublet propensity is also found, indicating that the OH unpaired pπ orbital is preferentially aligned perpendicular to the rotational plane of the OH products...

Probing reactive potential energy surfaces by vibrational activation of H2-OH entrance channel complexes

This review article presents an overview of recent experimental and theoretical studies of reactant complexes composed of OH and H2 that have been stabilized in the entrance channel to the OH + H2

Reactive quenching of electronically excited OH radicals in collisions with molecular hydrogen

The hydrogen atom products of the OH Au200a2Σ+u200a(v=0)+H2→H+H2O quenching reaction have been characterized by Doppler spectroscopy. The translational energy distribution of the products is bimodal, with the two components accounting for approximately 3% and 40% of the 4.72 eV of available energy.

Infrared spectroscopy of OHH2 entrance channel complexes

Abstract The rotationally-resolved infrared spectrum of the ground electronic state of OHue5f8H2 has been observed near the OH gn = 2 ← 0 origin by an infrared-ultraviolet fluorescence depletion technique. The experimental spectrum agrees remarkably well with a fully ab initio infrared spectrum originating from the lowest intermolecular level of ortho-H2ue5f8OH. The lack of measurable homogeneous line broadening in the spectrum indicates that neither vibrational predissociation nor chemical reaction is occurring faster than 45 ps. The magnitudes of the depletions, however, suggest that these processes are taking place on the nanosecond time scale.

High-Resolution Vibrational Spectroscopy of trans-Formic Acid in Solid Parahydrogen

We report high-resolution vibrational spectra of six normal modes of trans-formic acid (FA) in rapid vapor deposited solid parahydrogen (pH(2)) with particular emphasis on the carbonyl stretching mode (nu(3)) at approximately 1770 cm(-1). Infrared spectra in the nu(3) and 2nu(3) regions reveal that even in 99.99% enriched pH(2) samples, residual orthohydrogen (oH(2)) present in the solid preferentially clusters to FA producing measurable shifts in the nu(3) transition frequency. The individual FA(oH(2))(n) cluster peaks in the size range from n = 0 to n = 5 are resolved, permitting unambiguous assignment of the nu(3) and 2nu(3) transition frequencies and linewidths for FA with a first solvation shell (n = 0) of only pH(2) molecules. This n = 0 feature is well fit by a Lorentzian line shape with a line width of 0.214(6) cm(-1) and 0.45(2) cm(-1) for nu(3) and 2nu(3), respectively, which is surprisingly broad for a small nonrotating molecule trapped in solid pH(2). Implications of the broad FA nu(3) Lorentzian line shape in terms of homogeneous and inhomogeneous broadening mechanisms are discussed.

The Cl + H2 → HCl + H Reaction Induced by IR + UV Irradiation of Cl2 in Solid para-H2: Experiment†

We report IR + UV coirradiation photolysis experiments conducted on Cl(2)-doped para-hydrogen (p-H(2)) crystals at 1.8 K, using pulsed 355 nm UV radiation and cw broad-band near-IR light from a FTIR tungsten source. The amount of HCl photoproduct is monitored using FTIR spectroscopy as a function of the IR + UV exposure time. Detailed analysis of the HCl growth kinetics reveals that the reaction Cl + H(2)(v=1,J=0) --> HCl + H is playing a significant (15%) role in the in situ photochemistry. In contrast, UV-only photolysis experiments conducted under similar conditions produce almost exclusively (99%) isolated Cl atom photofragments, indicating the reaction Cl + H(2)(v=0,J=0) --> HCl + H is not readily occurring. This combination of photolysis experiments confirms that under these conditions, the Cl + H(2) reaction probability increases by a factor greater than 25 for Cl atom reactions with H(2)(v=1) versus H(2)(v=0). These results are therefore consistent with the expectation that vibrational excitation of the H(2) reagent lowers the reaction threshold and increases the reaction cross section for the Cl + H(2) reaction. These experimental studies were motivated by and are compared to the quantum model simulations reported by Korolkov, Manz, and Schild in the accompanying paper.

Mapping the OH + CO → HOCO reaction pathway through IR spectroscopy of the OH–CO reactant complex

A hydrogen-bonded complex composed of the OH and CO reactants has been identified along the OH + CO-->HOCO reaction pathway. IR action spectroscopy in the OH overtone region has been used to examine the vibrational modes of the linear OH-CO complex, including intermolecular bending modes that probe portions of the reaction path leading to HOCO. The spectroscopic measurements have accessed highly excited intermolecular levels, with energies up to 250 cm-1 above the zero-point level, which lie in close proximity to the transition state for reaction. The OH-CO binding energy, D0 < or = 430 cm-1, has also been established from the quantum state distribution of the OH fragments following vibrational predissociation of the OH-CO complex. Complementary electronic structure calculations have been performed to characterize the OH-CO and OH-OC complexes, the transition state for HOCO formation, and the direct reaction path that connects the experimentally observed OH-CO complex to the HOCO intermediate.

Transient H2O Infrared Satellite Peaks Produced in UV Irradiated Formic Acid Doped Solid Parahydrogen.

We report newly identified satellite features of the R(0) rovibrational transition of all the fundamental modes of HDO and the ν3 mode of H2O measured via FTIR spectroscopy immediately after the 193 nm in situ photolysis of formic acid (HCOOH and DCOOD) in solid parahydrogen. The intensities of these satellite features decay slowly with a time constant of τ = 121(7) min after photolysis, even when the sample is maintained below 2 K. We propose that the van der Waals complex H···H2O (H···HDO) is the carrier of the satellite peaks and that these metastable complexes are produced after the low-temperature tunneling reaction of the OH (OD) photoproduct with the parahydrogen host.