Volume 214, Issue 11 - 11

Scaling models for the polyatomic potential energy surface occupy a ground intermediate between ab initio surfaces and random matrix models. They are useful for the study of vibrational energy redistribution (IVR) in large molecules, and have been shown to reproduce many features of more accurate spectroscopically fitted or ab initio potential surfaces. A previous analysis of potential constants and coupling matrix elements [J. Chem. Phys. 106 (1977) 5874] considered modes that maintain vibrational character up to the dissociation limit. Here we discuss factorization and scaling properties of the vibrational Hamiltonian in the presence of an internal rotor. We find that the “rotor effect” is most pronounced for delocalized skeletal vibrations, potentially resulting in increased IVR of “bath” states compared to “bright” states. For localized vibrational modes that involve atomic displacements near the rotor, the local density of states is enhanced, while for more remote localized modes, the effect is negligible.

Simple relations between the experimentally observable thermodynamic properties and the molecular density of states are utilized to provide a method of accurate evaluation of the vibrational density of states taking into account both quantum and anharmonic effects. The result is a smooth function of energy that represents a local average of the exact density of states. Alternatively, it is shown that the anharmonic vibrational density of states may, at least for the small molecules NO 2 , HO 2 and H 2 O, be obtained from information on the normal mode frequencies and bond energies if one represents the molecule as a set of uncoupled Morse and harmonic oscillator, where the Morse oscillators correspond to the stretching vibrations.

The kinetics and mechanism of the title heterogeneous halogen exchange reactions of potential atmospheric importance have been studied under molecular flow conditions in a FEP Teflon-coated Knudsen flow reactor on HX (X = Cl, Br, I) - doped ice condensed from the vapor phase under conditions of several formal monolayers of HX coverage at approximately 200 K. In addition, the halogen exchange reactions involving the expected primary reaction products BrCl, ICl and IBr of the above mixed anhydrides with HX-doped ice have been studied at 200 K as well. The uptake coefficient γ for the heterogeneous reaction ClONO 2 + HBr on ice is 0.56±0.11 and Cl 2 and Br 2 are formed in yields of 100% and 66 to 80%, respectively, in the range 180 to 200 K. The γ value for the reaction ClONO 2 + HI on ice is 0.30±0.02 at 200 K with Cl 2 being the main product appearing after an induction time. The primary product ICl is formed at the same time as Cl 2 whereas HOCl appears at a later time under conditions of waning HI supply. The γ value for the reaction BrONO 2 + HI on ice is 0.40±0.02 at 200 K with Br 2 being the main product observed after a short delay. The primary product IBr is observed without delay, whereas HOBr is observed at a later time once HBr has reacted. The mechanism of the reactions of the interhalogens BrCl, ICl and IBr with HX on ice at 200 K involves the formation of trihalide ions at the ice interface which is consistent with the observed significant negative temperature dependence for the reaction ICl + HBr on ice in the range 180 to 205 K as well as for the reaction BrCl + HBr between 190 and 200 K. The uptake coefficient γ for each of the interhalogens increases from ice to HI-doped ice in the order of increasing molecular weight of HX with the exception of ICl, whose g attains a limiting value of γ = 0.32±0.05. A halogen exchange reaction on ice has been observed in cases where the most stable hydrohalic acid could be formed: HCl > HBr > HI. A propensity for the formation of the homonuclear halogen molecule in the reactions of halogen nitrates with HX-doped ice has been explained by the occurrence of fast secondary reactions of the primary interhalogen product at the HX/ice interface.

Time-resolved Fourier transform IR emission spectroscopy, capable of 10 -8 s and 0.1 cm -1 spectral resolution, has been used to study the collisional deactivation of highly vibrationally excited SO 2 by bath-gas molecules Ar, N 2 , O 2 , CO 2 and SF 6 . The vibrationally excited SO 2 were initially prepared with 32,500 cm -1 energy in the X˜ 1 A 1 state by the pulsed 308 nm laser excitation followed by internal conversion. The entire collisional deactivation process of the excited SO 2 was monitored by time-resolved IR emission spectra through the IR active transitions. The average energy, <E>, of excited SO 2 was extracted from the IR emission bands using known vibrational constants and selection rules. <E>is further used to derive the average energy loss per collision, <E>, by each of the bath-gas molecules. The results show that <E> increases from mono- and di-atomic quenchers to more complex polyatomic molecules, as V-V energy transfer contributes to V-T/R. For all bath molecules, <E> increases with <E> and displays a marked increase at <E> ≈ 20,000 cm-1. The observed threshold behavior most likely arises from intramolecular vibronic coupling within SO 2 and implies the importance of long range interaction in intermolecular energy transfer.

Examples of reactions which are termolecular near room temperature and gradually switch over to complex-forming bimolecular reactions with increasing temperature are discussed. The Sb/O 2 system, which is known from D. Husain´s work to be termolecular at 300 K, has been studied here from 1165-1495 K at 12 to 52 mbar in a high-temperature fast-flow reactor (HTFFR). Sb was vaporized from a crucible and atomic resonance absorption spectrometry was used to measure the rates of Sb consumption. Under these conditions the reaction, identified as Sb + O 2 → SbO + O, is pressure-independent with k(T) = 6.3×10 -11 exp (-9107 K/T) cm 3 molecule -1 s -1 . The 2σ precision limits are ±8% and the corresponding estimated accuracy limits are ±24%. The results suggest that D 0 (Sb-O) = 444±28 kJ mol -1 . The results are compared to those of other endothermic metal-atom O 2 abstraction reactions, all of which have pre-exponentials on the order of 10-10 cm 3 molecule -1 s -1 and activation energies for abstraction close to the endothermicity. This allows assessments of the relative importance of bi-, and ter- molecular kinetics, as illustrated for As + O 2 .

A method to predict temperature and pressure-dependent rate coefficients for complex bimolecular chemical activation and unimolecular dissociation reactions is described. A three-frequency version of QRRK theory is developed and collisional stabilization is estimated using the modified strong-collision approximation. The methodology permits analysis of reaction systems with an arbitrary degree of complexity in terms of the number of isomer or product channels. Specification of both high and low pressure limits is also provided. The chemically activated reaction of vinyl radical with molecular oxygen is used to demonstrate the approach. Subsequent dissociation of the stabilized vinyl peroxy radical is used to illustrate prediction of dissociation rate coefficients. These calculations confirm earlier results that the vinoxy + O channel is dominant under combustion conditions. The results are also consistent with RRKM results using the same input conditions. This approach provides a means to provide reasonably accurate predictions of the rate coefficients that are required in many detailed mechanisms. The major advantage is the ability to provide reasonable estimates of rate coefficients for many complex systems where detailed information about the transition states is not available. It is also shown that a simpler 1-frequency model appears adequate for high temperature conditions.

The structure, charge distribution and intermolecular binding energy of stable neutral and ionic aggregates of m-difluorobenzene and m-fluorochlorobenzene with one and two ammonia molecules are calculated on the MP2/6-31 G** level. The R2PI spectra of m-difluorobenzene/ammonia aggregates are reported. The origin of the size dependence of chemical reactions observed in the ionic aggregates is discussed.

With 3,5-dimethyl-4-(methylamino)benzonitrile (MHD) in n-hexane and n-hexadecane at 25°C, relatively small values are obtained for the fluorescence quantum yield Φ f (0.006) and the lifetime τ (0.103 ns). From Φ f and the yield of intersystem crossing Φ ISC (0.024), it follows that MHD in these solvents undergoes efficient internal conversion with a yield Φ IC of 0.96 and an IC rate constant k IC of 9×10 9 s -1 . An IC activation energy of 12 kJ/mol is obtained from the measurement of τ as a function of temperature in n-hexane. The amino group of MHD in the electronic ground state S 0 is twisted relative to the benzonitrile moiety, as deduced from quantum chemical calculations supported by experiments. It is assumed that the IC reaction is a consequence of the planarisation of the amino group upon excitation to the relaxed S 1 state and passes through a conical intersection. In support of this mechanism, efficient IC is not observed with the planar derivative 4-amino-3,5-dimethylbenzonitrile (HHD), in contrast to the strongly twisted 3,5-dimethyl-4-(dimethylamino)benzonitrile (MMD).