摘要:

In this paper we present a unified treatment of non-radiative decay processes in large molecules which involve either electronic relaxation between two electronic states or unimolecular rearrangement reactions in excited electronic states. The present treatment is analogous to the formalism previously applied for the line shape problem in nuclear recoil and in the optical spectra of solids. We were able to derive theoretical expressions for the non-radiative decay probability so that an arbitrary number of different molecular vibrations can be incorporated in the vibrational overlap factors. The general expressions obtained herein can be reduced to analytical form for two limiting cases, which we call the strong coupling case (which corresponds to a substantial horizontal displacement of the potential energy surfaces of the two electronic states) and the weak coupling limit (whereupon the relative horizontal displacement of the two potential energy surfaces is small). Quantitative criteria for the applicability of these two coupling limits are provided. In the strong coupling limit the transition probability is determined by a gaussian function of the energy parameter (ΔE-EM), where ΔEis the energy gap between the origins of the two electronic states and 2EMis the Stokes shift. This limit exhibits a generalized Arrhenius type temperature behaviour whereupon the transition probability depends exponentially on the energy barrier for the intersection of the two potential surfaces. At low temperatures the transition probability is determined by the mean vibrational frequency and is thus expected to reveal only a moderately weak deuterium isotope effect. The weak coupling limit reveals an exponential (or rather superexponential) dependence of the transition probability on the energy gap ΔE. In this limit the transition probability is dominated by the highest vibrational frequency (e.g. the C-H or C-D vibrations) and thus will reveal a marked isotope effect. When semi-empirical estimates of the pre-exponential factors are provided, the approximate theoretical expression for the weak coupling limit is found to be consistent with the available experimental data on electronic relaxation in large organic molecules.

ISSN:0026-8976    

DOI:10.1080/00268977000100171

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor

摘要:

A more detailed analysis is given here of the singularities in the E.S.R. spectrum of the trifluoromethyl radical in a polycrystalline matrix. Some of the unexplained ‘extra’ lines are interpreted as resulting from orientations such that some of the resonance fields are stationary as a result of ‘non-crossing’ of the manifolds of resonance fields due to second-order splitting. A calculation of the polycrystalline spectrum for a planar axial model of·CF3is presented, and shows that the second-order effect is of the right order of magnitude.

ISSN:0026-8976    

DOI:10.1080/00268977000100181

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor

摘要:

The semi-empirical approximations of Pariser, Parr and Pople (PPP) are used in the valence bond (VB) method. Simple formulas are presented for the calculation of the diagonal and off-diagonal elements of the energy matrix based on the complete set of covalent and polar structures. The method has been applied to butadiene and benzene. In order to interpret theu.v.spectra of these molecules a very large number of polar structures have to be taken into account, whereas in the molecular orbital (MO) theory only a small number of excited configurations are needed. The main advantage of the VB method in comparison with the MO method, using in both cases the PPP approximations, is the simplicity of the expressions for the matrix elements between the VB structures. For the performance of complete calculations therefore the VB method is to be preferred.

ISSN:0026-8976    

DOI:10.1080/00268977000100191

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor

摘要:

A proton magnetic resonance coordination number and ion-pairing study of GaCl3, GaBr3, GaI3, Ga(NO3)3and Ga(ClO4)3in acetone-water mixtures has been completed. By cooling solutions of these salts to about -50°c, proton exchange is slowed and separate resonance signals are observed for bulk water molecules and water molecules in the Ga3+solvation shell. The integration of these signals provides an accurate measure of the cation hydration number. In the case of Ga(NO3)3and Ga(ClO4)3, Ga3+is hydrated by six water molecules even at extremely high acetone concentrations. Also, only one signal is observed for the complexed water molecules. In contrast to this result, the hydration number of Ga3+decreases from 6 to about 1·4 for GaCl3, 1·6 for GaBr3and 2·5 for GaI3as large amounts of acetone are added to aqueous solution, and more than one complexed water signal is observed in the spectra arising from the GaCl3solutions of low water content. In addition to providing evidence for contact ion-pairing, the results are also indicative of octahedral and tetrahedral complexes of Ga3+.

ISSN:0026-8976    

DOI:10.1080/00268977000100201

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor

摘要:

The principal values of the19F hyperfine tensor for the ·CF(CONH2)2radical in aγ-irradiated single crystal of difluoromalonamide have been determined to be 200, -1, -10gperpendicular to the radical plane, parallel to the C-F bond, and perpendicular to the C-F bond in the radical plane, respectively. The deviation from axial symmetry of the hyperfine tensor is discussed in comparison with that in other related compounds and it is concluded that the spin polarization of 2pσelectrons in the C-F bond plays an important role. The polarization factors,QcffandQfcf, for the 2pσelectrons have been estimated to be -45 and +233gso as to account for the hyperfine tensors of a series of compounds.

ISSN:0026-8976    

DOI:10.1080/00268977000100211

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor

摘要:

The force constants of the planar skeleton vibrations of benzene are calculated according to an SCF method which should be more generally useful than the one given recently by Tric.

ISSN:0026-8976    

DOI:10.1080/00268977000100221

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor

摘要:

By the N-decoupling technique the signal of the N-H proton in pyrrole was clearly unveiled. Using the full spectrum, including the N-H band, the N.M.R. parameters for all five protons of the system were accurately determined. Assuming an AA′BB′X system, a computer simulation was performed; assignments were made for the observed lines. Based on the assignments, the N.M.R. parameters were determined with high accuracy by an iteration method. By using a partial decoupling experiment, the relative signs of the coupling constants between the N-H proton and the ring protons were determined to be the same.

ISSN:0026-8976    

DOI:10.1080/00268977000100231

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor

摘要:

The gas-phase electron resonance spectrum of the CF radical in itsX2π3/2electronic state is described. Analysis of the spectrum gives values of the rotational constantB0and fine structure constantAwhich are in good agreement with those previously obtained from the ultra-violet emission spectrum. The axial component of the19F magnetic hyperfine interaction is derived, and significant contributions from the non-axial components are discussed. The electric dipole moment of CF is obtained from measurements of the first-order Stark effect.

ISSN:0026-8976    

DOI:10.1080/00268977000100241

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor

摘要:

Self-consistent field molecular orbitals are used to describe two interacting conjugated molecules with overlappingporbitals. Perturbation theory is used to determine intermolecular orbitals which could serve as starting point for a self-consistent field calculation for the composite system. The total interaction energy is obtained as an expansion in powers of the overlapSrrbetween interacting atomic orbitals and of the net electronic charge Δqron the atoms. The energy expression contains an important term for net charge interactions. Expressions are also obtained when one of the interacting molecules is excited. Configuration interaction is examined in this case and the behaviour of singlets and triplets compared.

ISSN:0026-8976    

DOI:10.1080/00268977000100251

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor

摘要:

The Gibbs-Bogoliubov inequality is used to develop a first-order perturbation theory that provides an upper bound on the free energy. Charged systems as well as a system of Lennard-Jones particles are discussed, and detailed numerical estimates of the bounds are presented.

ISSN:0026-8976    

DOI:10.1080/00268977000100261

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor