ISSN:0005-9021    

DOI:10.1002/bbpc.19910950302

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1991

数据来源: WILEY

摘要:

AbstractKramers' diffusion model in the energy controlled low viscosity and the momentum controlled high viscosity range is confronted with reality for elementary unimolecular reactions and radical associations in dense media. Collisional energy transfer appears to be much more complicated than described by the idealized model. On the other hand, there are examples where the Kramers‐Smoluchowski equation well describes the transition to high viscosity behavior. In other cases, solvent shifts of the reaction barriers are pronounced and superimposed on the transport effects described by Kramers' mode

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950303

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1991

数据来源: WILEY

摘要:

AbstractThe pressure and temperature dependence of the rate coefficient for the singlet state photoisomerization of trans‐stilbene and all‐trans‐diphenylbutadiene in supercritical and liquid alkane solvents reveal the importance of specific solvent effects modifying the reaction path on the potential energy surface. In the gas‐liquid transition range the barrier height for the reaction seems to be lowered due to increasing solvation of the reactants in solute‐solvent clusters. At higher pressures, the transition to the Smoluchowski‐limit within each solvent is well described by Kramers' model. Differences between solvents can be attributed to a solvent‐dependent barrier shape. Deviations from this description only appear for stilbene at higher viscosities. In part, they seem also be due to a specific influence of the compressed solvent on the reaction path and to a frequency dependence of the friction. Solvent‐size dependent micro‐friction effects do not seem to be responsible for the observed solvent dependence of the reaction rates. Manifestations of multidimensional barrier crossing show up in the strong temperature dependence of the rate coefficient at constant solvent self‐diffusion coefficient. Apparently, the reaction path on the energy surface changes with temperature leading to an effectively temperature dependent height and shape of the barrier for the reaction with increasing excitation of “perpendicular” modes. Possible reasons for the striking difference of the friction dependence of the rate coefficient between stilbene and diphenylbutadiene at intermediate to high fric

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950304

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1991

数据来源: WILEY

摘要:

AbstractAdvantages of NMR techniques to obtain the rate data for simple isomerizations in liquid solutions and advantages of using high pressure to change the viscosity of the solution are discussed. After a brief overview of our experiments on cyclohexane, 1,1‐difluorocyclohexane, and N,N‐dimethyltrichloroacetamide, the discussion focuses on the ethylene rotation in μ‐cyclopentadienylethylenetetrafluoroethylene‐rhodium in several solvents. – The experimental data, as interpreted in terms of stochastic models of isomerization reactions, indicate a Kramers turnover for the pressure dependence of the rotation of coordinated ethylene in the Rh complex in solution. In fact, the observation of the energy‐controlled regime in this system may be the consequence of the so‐called heavy metal atom bottleneck effect which reduces the intramolecular energy transfer within the molecule. The experimental dependences of the rates upon solvent viscosity and/or Enskog collision frequency show that solvent shear viscosity represents only an approximative measure of the coupling of the reaction coordinate

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950305

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1991

数据来源: WILEY

摘要:

AbstractFemtosecond laser studies on cis‐stilbene photoisomerization in alkane solvents show a weak friction dependence on the excited state lifetime. A possible barrier crossing process is indicated when these results are compared with various potential models. The anisotropy values of the transient absorptions allow the assignment of the various excited states to A type in C2symmetry. Anisotropy measurements in the regions of product ground state absorptions demonstrate a high alignment between reactant cis and product trans and a low alignment between cis and product dihydrophenanthrene (DHP) transition dipoles. These results indicate a significant angular displacement of the ethylene bond during isomerization. In addition, time resolved absorption studies detecting product trans fluorescence estimate an initial internal temperature of 725 ± 100 K which decays with ca. 14 ps time consta

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950306

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1991

数据来源: WILEY

摘要:

AbstractTwo aspects of barrierless processes are discussed. Adiabatic processes are discussed briefly in the context of diffusion to a sink. Second, a multi‐level Redfield theory is developed as a model for rate processes where vibrational relaxation and dephasing occur on the time scale of the electronic process. The results of numerical calculations are presented and deviations from the Golden Rule predictions discusse

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950307

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1991

数据来源: WILEY

摘要:

AbstractComparison of the fluorescence kinetics of two dialkylaminobenzonitriles with different hindrance to planarity is used to extract information on the polar‐solvent induced shape changes of the excited‐state potential surface for TICT formation. The conical intersection between B*and A*states is shown to move to smaller twist angles as solvent polarity increa

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950308

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1991

数据来源: WILEY

摘要:

AbstractRecently we applied, for the first time, the general theory of dilation analyticity (or: Complex Scaling Method, CSM) to quantum statistics of nonequilibrium. A novel kind of (universal) coherence was revealed from first quantum theoretical principles. The corresponding irreducible structures we called coherent‐dissipative structures, since they represent a short‐lived and spatially restricted cooperative phenomenon. The crucial points of the theory are stressed. Similarities as well as differences with (i) the BCS‐states of superconductivity, (ii) the dissipative structures of the Brussels school, and (iii) Yang's concept of ODLRO are mentioned. Very recently the general theory was applied to different dynamical processes in amorphous condensed matter, and specific and quantitative predictions were made. In connection with current work, we discuss here in some detail two new predicted effects concerning protonic delocalization in water (and other materials) and proper experiments to test them. These effects are: (1) a novel relation between proton transfer rates and proton mobility in water; (2) an anomalous decrease of proton mobility in H2O/D2O mixtures. Both effects are shown to contradict the predictions of all thus far existing theories or models. Current experimental results verify undoubtedly our theoretical predic

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950309

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1991

数据来源: WILEY

摘要:

AbstractFlash photolysis experiments on carbonmonoxymyoglobin over wide ranges in time and temperature provide information about the rate processes involved in the rebinding reaction. The non‐exponential rebinding at low temperatures shows that the myoglobin molecules are frozen into a large number of conformational substates with different enthalpic barriers. Above 160 K we observe a relaxation process that shifts the peak of the barrier distribution from ∼ 10 kJ/mol to ∼ 21 kJ/mol. This process is non‐exponential in time and does not obey the Arrhenius law. Above 220 K equilibrium fluctuations between the conformational substates lead to an averaging of the binding rate distribution and to the opening of pathways for the ligands to escape from the protein mo

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950310

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1991

数据来源: WILEY

摘要:

AbstractThe model of Agmon and Kosloff for two‐dimensional diffusive barrier crossing is extended. We demonstrate how an increase in stiffness of the potential perpendicular to the reaction coordinate leads to viscosity dependent rates which are closer to the one‐dimensional Kramers result. The range over which the dynamics are truly two dimensional is characterized by a fractional viscosity dependence of reaction rates and viscosity dependent activation energies. It is a transition region between Kramers behaviors observed in the two extreme limits. A simple kinetic approximation rationalizes these observations as arising from two competing pathways. It shows surprisingly good agreement with the full calculat

ISSN:0005-9021    

DOI:10.1002/bbpc.19910950311

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1991

数据来源: WILEY