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A uniform and systematic approach to the calculation of the capture rate constant is presented in the case of most typical long-range intermolecular interactions. We consider ion-linear dipole rotor, ion-axially symmetric quadrupole rotor and two dipole linear rotors with additional isotropic ion-induced dipole and induced dipole-induced dipole interactions. The results for the capture rate constants are given in analytical form for nonadiabatic and adiabatic capture of classical as well as quantum rotors in very wide range of dynamic and kinematic parameters of the capture events.

The collisional deactivation of vibrationally highly excited azulene was studied in equimolar supercritical mixtures of xenon and ethane at 385 K from gas to liquid phase densities. Azulene with an energy of 18000 cm -1 was generated by laser excitation into the S 1 - and subsequent internal conversion to the S σ * ground state. The loss of vibrational energy was monitored by transient absorption at the red edge of the S 3 ←S 0 absorption band at 290 nm. Transient signals were converted into energy-time profiles using hot band absorption coefficients from shock wave experiments for calibration and accounting for solvent shifts of the spectra. Under all conditions, the decays were monoexponential. At densities below 10 -3 mol/cm 3 , the observed collisional deactivation rate constants k c of the mixture were equal to the sum of the individual contributions of ethane and xenon collisions as expected from simple gas kinetics. At mixture densities above 2x10 -3 mol/cm 3 , k c is smaller than the deactivation rate constant found in neat ethane at half the density. This behavior can be rationalized by a model employing an effective collision frequency which is related to the finite lifetime of collision complexes; the required parameters follow from experiments in neat xenon and ethane.

Fine structure and hyperfine structure effects can influence the kinetics of unimolecular and complex-forming bimolecular reactions. The corresponding energetic effects are analyzed in the framework of the statistical adiabatic channel model (SACM). The reactions O+OH O2 + H, OH + CO H + CO2, C+ + OH products, C+ + NO products, and NO2 O + NO serve as examples.

Compact approximate equations for rovibrational densities of states ρ(E,J) for harmonic oscillator-rigid rotor molecular systems are derived. Linear molecules, spherical tops, prolate and oblate symmetric tops are considered. Vibrational densities of states are represented by the Whitten-Rabinovitch approximation which then is convoluted with the contribution from the K-rotor (for a symmetric top). Likewise, numbers of states W(E,J) are approximated for prolate symmetric top molecules. The derived expressions allow for quick calculations of specific rate constants k(E,J) of unimolecular reactions.

The rotational relaxation of t-stilbene and t-diphenylbutadiene in the first electronically excited singlet state was measured by picosecond anisotropic absorption spectroscopy. The viscosity η in n-alkane solvents was changed by increasing the pressure up to several kilobars. In all cases the viscosity dependence of the rotational relaxation time τor could be described by a modified form of the Stokes-Einstein-Debye equation. No solvent dependence was found for η ≤ 0.8 cP. Therefore we attribute the observed solvent dependence of photoisomerisation rates of t-stilbene and t-diphenylbutadiene measured under the same conditions to specific solvent-solute interactions that modify the path on the potential energy surface.

Femtosecond two-photon fluorescence was used to redetermine the S1-lifetime of azulene and perdeutero-azulene in solution. A value of 1.0 (±0.l) ps was obtained in both cases, which is a factor of two smaller than the lifetime measured previously with lower time resolution.

The quantum yield of the photolysis of NO2 (6 Torr) under continuous irradiation has been measured in the presence of N2 in the pressure range 0 ≤ P(N2) ≤ 1000 atm at temperatures 296 ≤ T ≤ 428 K and wavelengths 313 ≤ λ ≤ 416 nm. The primary quantum yield is separated from the contribution of secondary reactions. The dependence of the primary quantum yield on pressure, temperature and wavelength is analyzed in terms of the specific rate constants k(E) for photolysis and detailed rate constants for collisonal energy transfer. Die Quantenausbeute der stationaren Photolyse von NO2 (6 Torr) wurde in Gegenwart von N2 im Druckbereich 0 ≤ P(N2) ≤ 1000 atm bei Temperaturen 296 ≤ T ≤ 428 K und Wellenlangen 313 ≤ λ ≤ 416 nm gemessen. Die primare Quantenausbeute wird von dem Beitrag der Folgereaktionen abgetrennt. Die Abhangigkeit der primaren Quantenausbeute von Druck, Temperatur und Wellenlange wird anhand der spezifischen Geschwindigkeitskonstanten k(E) fur die Photolyse und von detaillierten Geschwindigkeitskonstanten fur Energieubertragung in Stosen analysiert.

Product distributions in the UV photolysis of cyclooctatetraene vapour have been measured. Two parallel reaction channels leading to decomposition into benzene + acetylene and to isomerization to dihydropentalene or semibullvalene have been observed. Equal isomer yields have been found at pressures of 2–3 Torr; semibullvalene formation is, however, favoured at increasing pressures, and dihydropentalene formation becomes more important at decreasing pressures. Absolute quantum yields and specific rate constants k(E) for isomerization and decomposition of cyclooctatetraene have been determined under steady state photolysis conditions. Direct measurements of k(E) under collision-free conditions have been performed in the laser flash photolysis of cyclooctatetraene. In addition the steady state photolysis of dihydropentalene (main primary reaction product semibullvalene) has been studied; again quantum yields and specific rate constants have been determined. Die Produktverteilung bei der UV-Photolyse von Cyclooctatetraen in der Gasphase wurde gemessen. Dabei wurden die parallel ablaufenden Reaktionen zu den Zerfallsprodukten Benzol + Acetylen und den Isomeren Dihydropentalen bzw. Semibullvalen beobachtet. Beide Isomere entstehen mit etwa gleicher Ausbeute bei Drucken um 2-3 Torr; bei ansteigendem Druck wird die Bildung von Semibullvalen und bei geringerem Druck die Bildung von Dihydropentalen bevorzugt. Absolute Quantenausbeuten und spezifische Geschwindigkeitskonstanten k(E) fur Isomerisierung und Zersetzung von Cyclooctateuaen wurden unter den Bedingungen der stationaren Photolyse bestimmt. Mit Hilfe der Laser-Blitzlicht-Photolyse von Cyclooctatetraen wurden zudem direkte Messungen von k(E) unter stosfreien Bedingungen durchgefuhrt. Weiterhin wurde die stationare Photolyse von Dihydropentalen untersucht. Als uberwiegendes Primarprodukt wurde das Isomer Semibullvalen gefunden. Auch in diesem System wurden Quantenausbeuten und spezifische Geschwindigkeitskonstanten bestimmt.

The photoisomerization of trans-stilbene after picosecond excitation at 308 nm has been studied by picosecond absorption at 500 or 616 nm in ethane and propane. Experiments were performed in gaseous ethane (30–54 bar), liquid ethane (42–6000 bar), and liquid propane (9–4050 bar). The pressure dependence of the reaction is modelled from isolated molecule conditions in low pressure gases up to diffusion control in high pressure liquids. In dense media marked solvent shifts of the reaction threshold energy are postulated which can be described by simple cluster — and cage — solvent shift models. UV spectra show similar solvent shifts as isomerization rates.

Product distributions in the flow system pyrolysis of cyclooctatetraene and dihydropentalene have been measured in the temperature range 700–900 K. Furthermore the thermal reactions of cyclooctatetraene leading to the isomer dihydropentalene or the decomposition products benzene + acetylene have been studied in shock wave experiments. Rate constants of the two parallel reaction channels have been determined in the temperature range 1000–1400 K. By the same technique, the isomerizations of dihydropentalene to styrene (T = 1200–1600 K) and of semibullvalene to dihydropentalene (T = 740–900 K) have been investigated; rate constants of these reactions have also been determined. The thermal rate data in particular of the reactions of cyclooctatetraene are compared with results of our study of reactions of photoactivated cyclooctatetraene molecules (Part II) by means of unimolecular reaction rate theory. Bei der Pyrolyse von Cyclooctatetraen und Dihydropentalen in einem Stromungssystem wurden Produktverteilungen im Temperaturbereich von 700–900 K gemessen. Weiterhin wurden die thermischen Reaktionen von Cyclooctatetraen zum isomeren Dihydropentalen und zu den Zerfallsprodukten Benzol + Acetylen in Stoswellenexperimenten untersucht. Geschwindigkeitskonstanten der beiden Parallelreaktionen wurden im Temperaturbereich von 1000–1400 K bestimmt. Mit derselben Methode wurden die Isomerisierungen von Dihydropentalen zu Styrol (T = 1200–1600 K) und von Semibullvalen zu Dihydropentalen (T = 740–900 K) untersucht: die Geschwindigkeitskonstanten dieser Reaktionen wurden ebenfalls gemessen. Die thermischen Geschwindigkeitskonstanten, insbesondere fur die Reaktionen des Cyclooctatetraens, werden verglichen mit den Ergebnissen unserer Untersuchung der Reaktionen photoaktivierter Cyclooctatetraen-Molekule (Teil II) mit Hilfe der Theorie unimolekularer Reaktionen.

The maximum free energy criterion of canonical transition state theory is discussed in terms of the statistical adiabatic channel model developed previously [2]. A temperature dependent location of the transition state at maximum free energy is shown to provide the best canonical approximation (upper limit) to the true high pressure limiting rate constant of unimolecular reactions. For the practical evaluation, a simple model is proposed which uses a one parameter interpolation between known reactant and product properties without arbitrary assumptions about the transition state. Empirical values for the interpolation parameter are suggested, allowing for an estimation of rate constants from spectroscopic and thermochemical data of stable molecules only. The model is illustrated with calculations for the recombination-dissociation kinetics of XNO (X = F, Cl, Br, I, H, O), H2O, O3, ClNO2, HNO3, CH4, and C2H6. The results are compared with experiment and with previous calculations from the statistical adiabatic channel model. General dynamical concepts like the limiting cases of loose and tight transition states are discussed briefly. Im Rahmen des fruher entwickelten statistischen Modells adiabatischer Reaktionskanale [2] wird die Lokalisierung des Ubergangszustandes an der Stelle maximaler freier Energie erlautert. Es wird gezeigt, das sich hierbei die beste kanonische Naherung fur die thermische Hochdruckgeschwindigkeitskonstante der unimolekularen Reaktion ergibt. Fur praktische Zwecke wird ein einfaches Modell entwickelt, das durch eine einparametrige Interpolation zwischen bekannten Reaktanten- und Produkteigenschaften willkurliche Annahmen uber Eigenschaften des Ubergangszustandes vermeidet. Empirische Regeln fur die Interpolation erlauben eine quantitative Abschatzung von Geschwindigkeitskonstanten mit Hilfe spektroskopischer und thermochemischer Daten stabiler Molekule. Als Beispiel wird die Dissozations-Rekombinationskinetik von XNO (X = F, Cl, Br, I, H, O), H2O, O3, ClNO2, HNO3, CH4, und C2H6 berechnet und mit Experimenten und fruheren Rechnungen auf der Basis des statistischen Modells adiabatischer Reaktionskanale verglichen. Auch allgemeine dynamische Konzepte, wie die Grenzfalle loser und starrer Ubergangszustande werden kurz besprochen.

Vibrationally highly excited azulene (E ≤ 17000 cm-1) was prepared by light absorption in the S1 band and subsequent fast internal conversion. Collisional energy transfer in 22 bath gases was measured in the temperature range 300–800 K. In the investigated energy range, the average energies 〈ΔE〉 transferred per collision nearly linearly depend on the excitation energy.

HNO3 has been pyrolyzed in shock waves in the presence of NO2. The formation of HO2 in the reaction HO + NO2 HO2 + NO has been monitored by the UV absorption. From the yields and lifetimes of HO2, a rate constant (4.5 ± 1) · 1012 cm3 mol−3 s−1 of the reaction HO2 + NO HO + NO2 has been derived in the temperature range 1350–1700 K. HNO3 wurde in Gegenwart von NO2 in Stoswellen pyrolysiert. Die Entstehung von HO2 in der Reaktion HO + NO2 HO2 + NO wurde uber seine UV-Absorption nachgewiesen. Aus den HO2-Ausbeuten und Lebensdauern wurde eine Geschwindigkeitskonstante von (4, 5 ± 1) · 1012 cm3 mol−1 s−1 fur die Reaktion HO2 + NO OH + NO2 zwischen 1350 und 1700 K abgeleitet.

One-parameter fit versions of unimolecular rate theory for complex elimination and simple bond fission reactions are compared with detailed experiments. The agreement generally is so good that no further extension of this operational version of the theory is required. However, the relation between the fit parameter and properties of the potential surface not always is transparent. The article discusses the implementation of ab initio calculations of the potential by constructing adiabatic channel potential curves. Explicit results for charge-dipole interactions are presented. SACM-modified PST applies for selected conditions. Strategies for handling more complex potentials are outlined. The treatment closes the gap between operational fit parameters and potential properties.

Nach der Theorie des aktivierten Komplexes werden spezifische Geschwindigkeitskonstanten k(E) fur Zerfallsreaktionen bei verschiedenen Anregungsenergien E berechnet. Zur Lokalisierung der Lage aktivierter Komplexe wird das Prinzip des Minimums der Zustandsdichte systematisch angewandt. Wegen der starken Kopplung zwischen Reaktionskoordinate und Molekulrest liegen die aktivierten Komplexe weit vor den Zentrifugalmaxima. Ihre Lage hangt stark von der Anregungsenergie, jedoch nur schwach vom Rotationszustand ab. Bei zu hohen Anregungsenergien existieren aktivierte Komplexe im definierten Sinne nicht mehr. Die Berechnungen von k(E) stehen in guter Ubereinstimmung mit Experimenten zur Photolyse von NO2 sowie zur thermischen Dissoziation und Rekombination O + NO NO2 im Hochdruckbereich. Specific rate constants k(E) for dissociation processes at different excitation energies E have been calculated by the use of transitionstate theory. Activated complexes have systematically been localized by the minimum density of states criterion. Because of strong coupling between the reaction coordinate and the rest of the molecule, activated complexes are found at interfragment distances much smaller than those of the centrifugal maxima. Their position strongly depends on energy, but only weakly on the rotational state. At too high excitation energies, activated complexes in the defined sense do not exist. The calculated values of k(E) agree well with experimental data from the photolysis of NO2 and the thermal dissociation and the recombination O + NONO2 in the high pressure limit.

Photolysis quantum yields and atom recombination rate constants were measured for iodine and bromine at pressures of up to 7 kbar in both the dense gas and liquid phase employing a variety of solvents. The results are analyzed in terms of a simple model which uses empirical parameters to describe cage break-out, competition between diffusive separation of atom pairs and cage energy relaxation, and stabilization in halogen molecule-solvent clusters. This enables us to single out the dominant processes governing escape versus recombination efficiencies in the different density regimes.