摘要:

AbstractThis overview paper provides a short introduction into the topics discussed at the International Autumn Bunsen Discussion meeting 1997 on “Hydrogen Transfer Theory and Experiment” which took place in Berlin during 10–13 September 1997 under the auspices of the Deutsche Bunsengesellschaft für Physikalische Chemie and the Freie Universität

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020302

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

年代:1998

数据来源: WILEY

摘要:

AbstractThe laser induced isomerisation of malonaldehyde derivatives is studied using a two‐dimensional model Hamiltonian. The two selected coordinates describe the H‐atom motion coupled to a heavy atom mode which modulates the reaction barrier. On the basis of the solutions of the time‐dependent Schrödinger equation pulse characteristics are determined which yield an efficient switching of the H‐atom wavepacket between the two isomers within a pump‐dump scheme. The influence of various system parameters such as the heavy‐light atom coupling and the direction of the dipole moment is

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020303

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

年代:1998

数据来源: WILEY

摘要:

AbstractProton transfer is examined in HO2−and HF2+. These species are isoelectronic and a direct comparison of their electron densities is possible. Electron densities are calculated at the QCISD/6‐311++G(2d,2p) level for the nonlinear equilibrium geometry and the C2vsaddle point and linear saddle point geometries. Bader's theory of atoms in molecules is applied to partition the electron density into its atomic components and atomic and molecular properties are calculated. These quantities are used to characterize the proton dynamics as being like internal rotat

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020304

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

年代:1998

数据来源: WILEY

摘要:

AbstractThe observed temperature‐dependent hydrogen transfer rate constants for the H, D, and T isotopic species of the porphin anion have been reproduced by two different quantum dynamical models. The approach is based on the master equation for the system‐state populations. The models involve adapted localization properties and a simplified interaction with a quantum mechanical heat bath. A one‐dimensional model describes a hindered circular transfer motion of the hydrogen atom between its four stable sites in the plane of the porphin ring framework. With five parameters that are common to the three isotopic species it yields agreement with the observed data. One of the parameters, the height of the fourfold potential energy barrier, predicts an acceptable estimate for the in‐plane N‐H wagging frequency. Interactions of the transfer motion with individual vibrations have been studied for a series of two‐dimensional model systems. There the circular transfer motion is combined with ring framework modes having properties similar to those obtained for porphin by Li and Zgierski [J. Phys. Chem.95, 4268 (1991)]. For vibrational modes near or above 960 cm−1, strong interactions are found to be unlikely, as they would yield kinetic isotope effects and/or apparent activation energies in disagreement with the experimental data. For low frequency modes of the ring framework, however, sizeable couplings with the transfer cannot be ruled out. An interaction of the type suggested by quantum chemical results reported by Vangberg and Ghosh [J. Phys. Chem. B.101, 1496 (1997)] has been included in a two‐dimensional model involving a rectangular displacement of the pyrrole rings. With low vibrational frequency, a very large coupling strength, and a barrier close to the predicted one, this model has also been found to be compatible

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020305

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

年代:1998

数据来源: WILEY

摘要:

AbstractDouble proton transfer in the hydrogen bonds of carboxylic acid dimers has been investigated via measurements of the dispersion of the proton spin‐lattice relaxation time,T1, using magnetic field‐cycling NMR. A plot of the spin‐lattice relaxation rate,T1−1, as a function of magnetic field, provides a direct measurement of the motional spectrum and hence of the proton transfer rate. In the materials studied, these NMR experiments are optimised for the low temperature region where the dynamics are dominated by incoherent quantum tunnelling. Measurements of the proton transfer rate at higher temperatures have been made using quasi‐elastic neutron scattering (QENS) and these facilitate the characterisation of the transition between quantum and classical dynamics in the intermediate temperature region. The role of tunnelling in the excited vibrational states of the double minimum potential (DMP) which characterises the system is revealed in these measurements and has been analysed according to the behaviour of a particle in a DMP which is coupled to a bath of phonons. The complementarity between the NMR and QENS techniques is emphasised and discussed, and the dynamic range accessible to the field‐cycling NMR technique is explored through studying a range of relate

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020306

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

年代:1998

数据来源: WILEY

摘要:

AbstractTwo‐proton exchange along the two hydrogen bonds mediates the tautomerization in benzoic acid (BA) dimers. Optical spectroscopy and quasi‐elastic neutron scattering (QENS) have been employed to characterize the proton dynamics in doped and pure BA crystals. The proton motion in BA is governed by a multidimensional potential energy surface (PES), and recent theoretical methods, based on a perturbative instanton approach, to describe tunneling in such PES are presented. This PES is also modulated by the interaction with the solid state environment as manifest by the energy difference between the otherwise equivalent tautomers. The value of this energy difference in pure crystals is an important parameter in the data analysis of NMR and QENS. Both methods give mutually consistent values that differ significantly from earlier determinations via infrared and13C NMR as well as a recent evaluation using neutron diffraction data. The energy difference between tautomers is altered for dimers in the vicinity of impurity molecules. This is the basis for the optical spectroscopic methods, which enable a direct and accurate determination of the level structure and tautomerization dynamics of these coupled dimers in the limit of very low temperatures, where coherent tunneling is also observed in some cases. Measurements with new impurity molecules make it possible to monitor simultaneously at least 10 different tautomer configurations and prove that the influence of the probe molecules on the proton dynamics is small. The transition to thermally activated barrier crossing at higher temperatures is accessed via the width of the QENS line that is determined by the inverse of the proton correlation time. The quantitative data analysis of the scattered intensity as a function of temperature and scattering angle yields the energy difference,A, between the two wells (A/kB=90+20 K), the length (0.686 Å), and direction of the proton jump vector. These measurements complement NMR investigations presented in the preceding p

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020307

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

年代:1998

数据来源: WILEY

摘要:

AbstractThe temperature dependent solid state15N CPMAS NMR spectra of15N enriched salicylideneaniline derivatives,N‐salicylideneaniline (SA), N,Nî‐bissalicylidene‐p‐phenylenediamine (BSP) and N,Nî‐di(2‐hydroxy‐1‐naphthylidene)‐p‐phenylenediamine (DNP) were measured in order to study possible proton transfer processes in the NHO hydrogen bonds. For DNP a remarkable change of the15N chemical shifts of more than 50 ppm was observed between 26 and 388 K by using the CPMAS NMR technique down to cryogenic temperatures. The result clearly indicates very small enthalpy differences among the four tautormers associated with the combination of two possible forms, OH and NH form, of the two NHO hydrogen bo

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020308

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

年代:1998

数据来源: WILEY

摘要:

Abstract35Cl spectroscopy was applied to study the hydrogen bond symmetry as well as the isotope and pressure effects on it in a wide range of (2:1) salts of chloroacetic trichloroacetic and difluorochloroacetic acids. The NQR results were discussed qualitatively on the base of a variational correlated ground state wave function theory of the single hydrogen bond

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020309

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

年代:1998

数据来源: WILEY

摘要:

AbstractThe ruthenium IV trihydride complex Cp*RuH3(Ppy3) (1) accommodating the π‐accepting tris(pyrollyl)phosphine ligand has been synthesized by standard methods. This complex exhibits average exchange couplingsJof the coherent dihydrogen exchange and average rate constantskof the incoherent dihydrogen exchange which are significantly larger than the corresponding triphenylphosphine and tricyclohexylphosphine analogs. No kinetic HH/HD isotope effects on the incoherent exchange are observed within the margin of error. Using a simple kinetic model involving incoherent vibrational excitation it is shown that in principle both the coherent and the incoherent processes should be related. However, whereas the energy of activation ofkcorresponds to the barrier of rotation, the energy of activation of the coherent dihydrogen exchangeJis much smaller i.e. determined by states well below the barrier. Nevertheless, it is found that chemical modification which reduces the energy of activation ofJalso reduces the energy of activation ofkwhich demonstrates the relation between both kinds of process

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020310

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

年代:1998

数据来源: WILEY

摘要:

AbstractIn this paper we attempt to explain the unusual isotope effect displayed by the1H‐NMR scalar H‐H coupling of the hydrides in complexes [CpIrL(H)3]+ (L=various phosphine and phosphite ligands) when one of the equivalent protons is turned into a deuteron (D.M. Heinekey, J.M. Millar, T.F. Koetzle, N.G. Payne, and K.W. Zilm, J. Am. Chem. Soc.112, 909 (1990)]. It is found that in the case of the perprotio species the system can be described as a three‐coupled well system, but when a deuterium is introduced the system is better described as a double‐coupled well. Finally, it is shown that the coupling in the case of a double‐well system is greater than in an equivalent triple‐well system, in this way giving a plausible explanation to the experimental fac

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020311

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

年代:1998

数据来源: WILEY