Volume 214, Issue 8 - 8

The adiabatic channel interaction potentials for a system charge + dipole-open-shell-quadrupole linear rotor are calculated in the perturbed rotor and slightly anharmonic Renner-Teller limits. In both cases, first order charge-dipole and charge-quadrupole interactions are not additive; this leads to novel expressions for the energy levels. The result can be used in the construction of the adiabatic correlation diagrams between free rotor states with account for the Λ -doubling of a diatom and the linear anharmonic Renner-Teller triatomic system.

A recent set of experimental data which gave new state-to-state He-CO rotationally inelastic integral cross sections at two different collision energies is examined in the present study using a potential energy surface obtained by us which combines Density Functional Theory (DFT) methods with dispersion interaction corrections. Computations with exact quantum dynamical coupling indicate that the DFT-DISP approach generates a model potential which agrees with the experiments more closely than two of the most accurate potential functions which already exist for the title system. The implications of the present results are extended by also calculating state-to-state differential cross sections and rotational relaxation/excitation rates at low temperatures. The quality of the potential well produced by the present model is further tested by calculating the structure of its rovibrational bound states.

Collision-induced dissociation (CID) of the [OCS · C 2 H 2 ] + complex ion with both Xe and Ar over an energy range of 0 to 10 eV in the center of mass frame is studied using a guided ion beam tandem mass spectrometer. The cross sections of the ionic products observed (C 2 H 2 S + , OCS + , C 2 H 2 + , and S + ) are analyzed by taking into account reactant energy distributions, multiple collisions, lifetime effects and competition. A recently devised statistical model for the simultaneous analysis of competitive product channels is used to analyze three channels for the first time, with good results. Thresholds for product formation at 0 Kelvin are 0.33±0.07 eV for C 2 H 2 S + , 0.95±0.07 eV for OCS + , 1.22±0.08 eV for C 2 H 2 + , and an upper limit of 4.26 eV for S + . These results are comparable to available literature thermochemical data within experimental errors. Competitive shifts are significant, about 0.3 eV for both OCS + and C 2 H 2 + . Ab initio calculations at the QCISD/6-311+G**//MP2/6-311+G** and CCSD//6-31G*//CCD/6-31G* levels are performed on the system. The reaction coordinates of the potential energy surface of the system is quantitatively mapped using results from CID and ab initio calculations. The identity of the C 2 H 2 S + product is suggested to be the cyclic ethylene sulfide cation on the basis of the results of calculations and previous kinetic energy release measurements. Product branching ratios as a function of energy are analyzed and compared to those determined in previous photodissociation and bimolecular reaction experiments.

The temperature dependence of the gas-phase collisional relaxation of highly vibrationally excited aromatic molecules has been studied using large scale classical trajectory calculations. The investigations have focused on azulene collisions with different colliders (He, Ar and N 2 ) as well as pyrazine self-collisions providing the moments of energy transfer <Δ E > and <Δ E 2 >in the temperature range 50-1500 K. The interaction well depth ε eff / k B is found to be the key factor controlling the observed T dependence of collisional energy transfer. Systems with a relatively deep interaction well (pyrazine + pyrazine, azulene + Ar, azulene + N 2 ) show a pronounced negative dependence of -<Δ E > when T < ε eff / k B (in the systems studied here roughly at T < 300-400 K). The increased efficiency of collisional energy transfer at low T is due to additional contributions from collisions at large impact parameters. In systems with a very shallow well (azulene + He), however, a positive T dependence is found in the low temperature regime (< 300 K) due to the dominant contributions from impulsive, adiabatic collisions at short impact parameters. At higher temperatures ( T > 300-400 K) - when the temperature is well above ε eff / k B - all systems behave qualitatively similar, showing only a very weak, slightly negative T dependence, as long as one is still far away from thermal equilibrium.

We report high level ab initio calculations (treating correlation by second order Møller-Plesset perturbation theory, MP2) of various two-dimensional (2D), three-dimensional (3D) and four-dimensional (4D) normal coordinate subspaces of the full 12D coordinate space of trans formic acid, HCOOH. With the resulting potential and electric dipole hyper-surfaces accurate vibrational variational calculations are carried out using a discrete variable representation (DVR) for the anharmonically coupled modes. The full absorption spectra are calculated and the overtone spectra of the OH chromophore, which clearly dominates the entire absorption spectrum, are analyzed in detail with respect to their anharmonic resonance dynamics. We have investigated the OH/CH stretching coupling, which is an example for a coupling across bonds, in terms of direct and indirect coupling terms. A close resonance coupling of 5 ν OH (5 ν 1 ) with 4 ν OH + ν CH (4 ν 1 + ν 2 ) leads to short time (250 fs) intramolecular vibrational redistribution (IVR). Using the various multidimensional normal coordinate calculations we have investigated in detail the role of the "skeletal" modes. A sequence of calculations including various subspaces (two-dimensional up to four-dimensional) leads to the conclusion that the resonance includes important contributions from coupling to the CH and OH bending modes and C-O stretching mode in addition to the direct coupling of the CH- and OH stretching modes. The time dependent population dynamics further demonstrate that the IVR cannot be explained by a simple direct high order potential coupling. Important resonances are predicted between OH stretching and CH bending, OH bending, and C-O stretching modes. These resonances significantly contribute to IVR at higher levels of OH stretching excitation. The results provide an example for a more general "through-bond" coupling mechanism with wider implications for the understanding of IVR in multiple chromophore systems.

The reaction CN + NO + M ↔ NCNO + M was studied in the bath gas helium at temperatures between 200 and 600 K and in the pressure range between 1 and 100 bar. CN radicals were generated by laser flash photolysis of BrCN at 193 nm in the presence of NO and high helium pressures. The concentration of CN radicals was detected by recording their non-resonant fluorescence yield at 420 nm after delayed excitation in the (0, 1)-band of the X 2 ∑ + → B 2 ∑ + transition near 387.8 nm. Thermal second order rate constants were extracted from the fluorescence-yield time profiles under pseudo-first order conditions. We obtained access to wide parts of the falloff range and constructed complete falloff curves using the Troe formalism. We expressed the low and high pressure limiting rate constants as k 1,0 = [He] (2.1±0.7)x 10 -30 (T/300 K) -(2.1±0.3) cm 6 s -1 and as k 1,∞ = (5.4±1.4)x 10 -11 (T/300 K)(0.0±0.2) cm 3 s -1 and gave a broadening factor F C (He) = exp(-T/590 K) + exp(-947 K/T). In separate experiments we monitored the non-resonant fluorescence yield of CN at 418.5 nm in the presence of NO after delayed excitation in the (1, 2)-band of the B 2 ∑ + X 2 ∑ + transition near 386.6 nm. The rate constant for collisional deactivation of vibrationally excited CN radicals in the electronic ground state CN(ν=1) + NO →CN(ν=0) + NO is strongly related to the high pressure limiting recombination rate constant. We determined its rate constant to be k 2 = (5.3±1.3)x 10 -11 (T/300 K)(-0.2±0.2) cm 3 s -1 in excellent agreement with our high pressure limiting recombination rate constant.

Electron ionization cross sections for (N 2 O) n clusters have been investigated near the threshold with a newly constructed crossed beams apparatus using a hemispherical electron monochromator (HEM) to monochromatize the primary electron beam. Exploratory test measurements including electron attachment studies to CCl 4 and CO and appearance energy (AE) determinations for some rare gases (Ar, Kr, Xe) and molecules (N 2 , O 2 , N 2 O, O 2 ) measured for calibration purposes showed that the accuracy of the energy scale is better than 10 meV exhibiting a good linearity and that AE´s derived for these compounds agree within 20 meV with known photoionization data when using a novel data handling procedure (involving a simultaneous non-linear weighted least-squares fit of two functions). Comparison of the presently derived appearance energies with earlier photoionization data shows a distinct difference in corresponding values attributed to different ionization mechanisms. Both data sets can be fitted successfully by a n -1/3 dependence yielding by extrapolation a value for the bulk photoelectric threshold (11.25 eV in the case of the photoionization data).

The reaction NH 2 + H 2 → NH 3 + H (11) was investigated behind shock waves in the temperature range from 1360 to 2130 K. Time histories of NH 2 were measured by frequency modulation (FM) spectroscopy. The rate constant was determined to be k 11 = 4.5 · 1013 · exp[-61 kJ mol -1 / RT ] cm 3 mol -1 s -1 (Δlog k 11 = ±0.15, Δ E a = ±9 kJ/mol). According to recent literature values the reverse reaction NH 3 + H → NH 2 + H 2 (-11) can be described by k -11 = 1.5 · 10 7 · T 2.0 · exp(-47 kJ mol -1 / RT ) cm 3 mol -1 s -1 for the temperature range of 400 to 2500 K. On the basis of k -11 and by using the enthalpy of formation of Δ f H o 298 Κ (NH 2 ) = 190 kJ/mol, which was determined in a previous study [1], via detailed balance the following expression for the forward reaction (11) was obtained for a temperature range of 400 to 2200 K: k 11 = 1.5 · 10 4 · T 2.6 · exp(-25 kJ mol -1 / RT ) cm 3 mol -1 s -1 . This expression is in very good agreement with the reported experimental values for k 11 .