摘要:

The effectiveness of Van Vleck direct and Raman processes and of Orbach processes of electron spin relaxation in solution and isotropic crystals is discussed with reference to the modulation of the hyperfine coupling constant by the collision process. The mechanism is compared with that discussed by Kivelson [1] which involved electronic excitation: the present mechanism may be more effective in some situations.

ISSN:0026-8976    

DOI:10.1080/00268976700100261

出版商:Taylor & Francis Group    

年代:1967

数据来源: Taylor

摘要:

The importance of a relaxation mechanism involving the modulation of the isotropic hyperfine interaction tensor by phonon mechanisms is discussed on the basis of a previous theory [1]. The hyperfine interaction energy for the molecule is obtained from both molecular orbital and valence bond calculations. The line-widths arising from conventional mechanisms are calculated and it is found that in solution the dominant mechanism is due to a spin-rotation interaction but the vibrational modulation of the isotropic hyperfine interaction gives a contribution comparable with that due to thegand hyperfine tensor anisotropies. The calculations also enable one to predict the temperature dependence of the isotropic hyperfine interaction: a negligible increase of about 2 m gauss deg-1at 300°kis expected.

ISSN:0026-8976    

DOI:10.1080/00268976700100271

出版商:Taylor & Francis Group    

年代:1967

数据来源: Taylor

摘要:

The density-matrix method of analysing relaxation effects in nuclear magnetic double resonance experiments is extended to the case of large molecules containing several groups of equivalent nuclei giving rise to fist-order single resonance spectra. The difficulty of using this theory for such systems possessing degenerate energy levels is circumvented by assuming that the relaxation in the system occurs through an istropic random field with complete correlation between interactions within each group and no correlation between interactions with different groups. This relaxation mechanism preserves the equivalence of the spins in each group signified by the spin Hamiltonian. This assumption, along with those used in the general theory, not only permits, in suitable cases, the use of a convenient approximation originally proposed by Bloch for simplifying the computations involved in the theory, but also separates the problem into a linear superposition of several independent sub-problems of much smaller dimension. The method is applied to proton double resonance experiments in C2H5F, which show some features arising from relaxation in the system. It is estimated from the analysis that the spin relaxation of fluorine in C2H5F is about four times as efficient as that of the protons.

ISSN:0026-8976    

DOI:10.1080/00268976700100281

出版商:Taylor & Francis Group    

年代:1967

数据来源: Taylor

摘要:

Unrestricted Hartree-Fock wavefunctions have been calculated for a number of conjugated azine anion radicals. Pi-electron spin densities are obtained by projection of the desired doublet state.

ISSN:0026-8976    

DOI:10.1080/00268976700100291

出版商:Taylor & Francis Group    

年代:1967

数据来源: Taylor

摘要:

The signs and magnitudes of the reduced coupling constants between carbon and selenium and tellurium in several compounds have been determined by heteronuclear double resonance techniques. In all cases the reduced coupling constantKc-xis negative andKH…xis positive. It appears that the hybridization of X is an important factor in determining the magnitude of the coupling constant.

ISSN:0026-8976    

DOI:10.1080/00268976700100301

出版商:Taylor & Francis Group    

年代:1967

数据来源: Taylor

摘要:

Measurements have been taken on a glass trennschaukel of eight tubes for the Ar-He system as a function of composition and with its hot and cold bulbs at 100°cand 0°c. For each mixture, several speeds for the mechanical push-pull motion of the gas which cover a wide enough range were tried. The results have been interpreted to determine the thermal diffusion factorαT, in conventional fashion as well as on the basis of a theory characterizing the low speed operation, developed here. Four such different possibilities ofαtevaluation are discussed and values compared with the traditional ones obtained from the convection-free two-bulb apparatus. Lastly we discuss also in brief the prospect of estimating the diffusion coefficient as an important byproduct from measurements on trennschaukel runs.

ISSN:0026-8976    

DOI:10.1080/00268976700100311

出版商:Taylor & Francis Group    

年代:1967

数据来源: Taylor

摘要:

Intersystem-crossing in aromatic molecules is caused by spin-orbit coupling. This mechanism is expected to be highly selective for a specific spin-component of each triplet state. Experimental evidence is given for quinoxaline proving that spin polarization indeed occurs when this molecule is optically excited into its lowest triplet state. A remarkable coherence phenomenon, reminiscent of level-crossing situations in atomic spectroscopy, seems to be observed following flash excitation in a magnetic field.

ISSN:0026-8976    

DOI:10.1080/00268976700100321

出版商:Taylor & Francis Group    

年代:1967

数据来源: Taylor

摘要:

Calculations of the coupling constants in the fluoromethanes and other first-row fluorides are reported. The theoretical basis is the one electron valence shell MO theory of Pople and Santry. Coulomb integrals were taken as Skinner-Pritchard valence state ionization potential values, corrected for drift of electrons from less electronegative to more electronegative atoms. The amount of correction was estimated from trial calculations on BF3. Calculations on F2showed that it was necessary to use the Mulliken approximation for estimating resonance integrals. The effect of varying the scale factor in the Mulliken approximation was examined, as was the effect of varying the Slater exponent of hydrogen. Best agreement with experimental bonded coupling constants came from a scale factor of 0·75 and a hydrogen Slater exponent of 1·3. Calculated geminal couplings were in poor agreement with experiment.

ISSN:0026-8976    

DOI:10.1080/00268976700100331

出版商:Taylor & Francis Group    

年代:1967

数据来源: Taylor

摘要:

The method of using pre-dissociation data to determine interatomic forces suggested by Bernstein is applied to the case of HF where the Rydberg-Klein-Rees (R-K-R) curve of the molecule is known. Fitting the potential curve toc/r6at internuclear distances greater than 2regives a value forcof 8·41 × 10-59erg cm6, in agreement with that calculated from the limiting curve of dissociation, 8·04 × 10-59erg cm6. A limitation of the Bernstein method is discussed. The long-range interatomic forces in the halogens are also studied using R-K-R data and certain anomalies in the results pointed out.

ISSN:0026-8976    

DOI:10.1080/00268976700100341

出版商:Taylor & Francis Group    

年代:1967

数据来源: Taylor

摘要:

13C satellites in proton and fluorine N.M.R. spectra have been used to obtain the coupling constantsJ13c-handJ13c-ffor a number of ortho-, meta- and para-substituted fluorobenzenes. These coupling constants are found to depend on the substituent as well as the position of substitution. The13C-H coupling constants, for a particular position, in general, increase with increase of the electron withdrawing power (expressed through Hammett σ-parameters) of the substituents. For a particular substituent, the values of13C-H coupling constants increase with decrease of σ values and are found to change with the position of the substitution in the order meta < ortho < para. Such regularities are not found for13C-F coupling constants.

ISSN:0026-8976    

DOI:10.1080/00268976700100351

出版商:Taylor & Francis Group    

年代:1967

数据来源: Taylor