Infrared action spectroscopy and time-resolved dynamics of the OD–CO reactant complex

Thermodynamics of the hydroxyl radical addition to isoprene

Oxidation of isoprene by the hydroxyl radical leads to tropospheric ozone formation. Consequently, a more complete understanding of this reaction could lead to better models of regional air quality, a better understanding of aerosol formation, and a better understanding of reaction kinetics and dynamics. The most common first step in the oxidation of isoprene is the formation of an adduct, with the hydroxyl radical adding to one of four unsaturated carbon atoms in isoprene. In this paper, we discuss how the initial conformations of isoprene, s-trans and s-gauche, influences the pathways to adduct formation. We explore the formation of pre-reactive complexes at low and high temperatures, which are often invoked to explain the negative temperature dependence of this reaction's kinetics. We show that at higher temperatures the free energy surface indicates that a pre-reactive complex is unlikely, while at low temperatures the complex exists on two reaction pathways. The theoretical results show that at low temperatures all eight pathways possess negative reaction barriers, and reaction energies that range from -36.7 to -23.0 kcal x mol(-1). At temperatures in the lower atmosphere, all eight pathways possess positive reaction barriers that range from 3.8 to 6.0 kcal x mol(-1) and reaction energies that range from -28.8 to -14.4 kcal x mol(-1).

Quantum force molecular dynamics study of the reaction of O atoms with HOCO

The reaction of HOCO with O atoms has been studied using a direct ab initio dynamics approach based on the scaling all correlation UCCD/D95(d,p) method. Ab initio calculations point to two possible reaction mechanisms for the O+HOCO-->OH+CO2 reaction. They are a direct hydrogen abstraction and an oxygen addition reaction through a short-lived HOC(O)O intermediate. The dynamics results show that only the addition mechanism is important under the conditions considered here. The lifetime of the HOC(O)O complex is predicted to be 172+/-15 fs. This is typical of a direct and fast radical-radical reaction. At room temperature, the calculated thermal rate coefficient is 1.44 x 10(-11) cm(3) mol(-1) s(-1) and its temperature dependence is rather weak. The two kinds of reactive trajectories are illustrated in detail.

The relaxation of OH (v=1) and OD (v=1) by H2O and D2O at temperatures from 251 to 390 K

We report rate coefficients for the relaxation of OH(v=1) and OD(v=1) by H2O and D2O as a function of temperature between 251 and 390 K. All four rate coefficients exhibit a negative dependence on temperature. In Arrhenius form, the rate coefficients for relaxation (in units of 10(-12) cm3 molecule-1 s-1) can be expressed as: for OH(v=1)+H2O between 263 and 390 K: k=(2.4+/-0.9) exp((460+/-115)/T); for OH(v=1)+D2O between 256 and 371 K: k=(0.49+/-0.16) exp((610+/-90)/T); for OD(v=1)+H2O between 251 and 371 K: k=(0.92+/-0.16) exp((485+/-48)/T); for OD(v=1)+D2O between 253 and 366 K: k=(2.57+/-0.09) exp((342+/-10)/T). Rate coefficients at (297+/-1 K) are also reported for the relaxation of OH(v=2) by D2O and the relaxation of OD(v=2) by H2O and D2O. The results are discussed in terms of a mechanism involving the formation of hydrogen-bonded complexes in which intramolecular vibrational energy redistribution can occur at rates competitive with re-dissociation to the initial collision partners in their original vibrational states. New ab initio calculations on the H2O-HO system have been performed which, inter alia, yield vibrational frequencies for all four complexes: H2O-HO, D2O-HO, H2O-DO and D2O-DO. These data are then employed, adapting a formalism due to Troe (J. Troe, J. Chem. Phys., 1977, 66, 4758), in order to estimate the rates of intramolecular energy transfer from the OH (OD) vibration to other modes in the complexes in order to explain the measured relaxation rates-assuming that relaxation proceeds via the hydrogen-bonded complexes.

Time-Dependent Wave Packet Studies on the Cl + HCl Hydrogen Exchange Reaction

The initiation of the hydrogen exchange reaction Cl((2)P)+HCl --> ClH+Cl((2)P) by excitation of the HCl molecular stretch to v=2 is studied for total angular momentum quantum number J=(1)/(2) and both even and odd parity. The calculations were performed using a time-dependent propagation from an initial quasi-bound state and employed all three relevant potential energy surfaces and the nonadiabatic couplings between them. Coriolis and spin-orbit coupling were also taken into account. The electronic and HCl rotational distributions of the products in both dissociation channels are analyzed, and the results are interpreted using features of the potential energy surfaces.

Quantum Molecular Dynamics Study of the Reaction of O2 with HOCO

The reaction of O2 with HOCO has been studied by using an ab initio direct dynamics method based on the UB3PW91 density functional theory. Results show that the reaction can occur via two mechanisms: direct hydrogen abstraction and an addition reaction through a short-lived HOC(O)O2 intermediate. The lifetime of the intermediate is predicted to be 660 +/- 30 fs. Although it is an activated reaction, the activation energy is only 0.71 kcal/mol. At room temperature, the obtained thermal rate coefficient is 2.1 x 10(-12) cm3 molecule(-1) s(-1), which is in good agreement with the experimental results.

Fluorescence-dip infrared spectroscopy and predissociation dynamics of OH AΣ+2 (v=4) radicals

Fluorescence-dip infrared spectroscopy, an UV-IR double-resonance technique, is employed to characterize the line positions, linewidths, and corresponding lifetimes of highly predissociative rovibrational levels of the excited A (2)Sigma(+) electronic state of the OH radical. Various lines of the 4 <--2 overtone transition in the excited A (2)Sigma(+) state are observed, from which the rotational, centrifugal distortion, and spin-rotation constants for the A (2)Sigma(+) (v = 4) state are determined, along with the vibrational frequency for the overtone transition. Homogeneous linewidths of 0.23-0.31 cm(-1) full width at half maximum are extracted from the line profiles, demonstrating that the N = 0 to 7 rotational levels of the OH A (2)Sigma(+) (v = 4) state undergo rapid predissociation with lifetimes of < or =23 ps. The experimental linewidths are in near quantitative agreement with first-principles theoretical predictions.