摘要:

The concept of the spin-correlated radical pair has been used for some time in the interpretation of the results of magnetic polarization experiments in NMR and in ESR (CIDNP and CIDEP), and of the effects of magnetic fields on chemical reactions. It has, however, a much wider general significance as a reaction intermediate in all radical reactions, including photochemistry. Here the nature of the pair is introduced, and the evidence for it reviewed, and it is further shown how it can be treated theoretically, before some of the consequences of its existence are pointed out. Its recognition, and in particular the understanding of the processes which occur within it, notably the interplay of spin-mixing and diffusion, allow the design of new experiments in the absence or presence of an external magnetic field, and the optimization of reaction yields.

ISSN:0026-8976    

DOI:10.1080/00268979100101181

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Potential functions have been derived for the bulk phases of C, Si and Ge which reproduce the lattice energies, lattice spacings, elastic constants and the main features of the phonon dispersion curves of the diamond structures. These potentials all make the diamond lattice more stable than the simple cubic, body-centred cubic and face-centred cubic structures; for Si and Ge this is the order or decreasing stability. The potentials have a form suitable for use in studies of melting, defect structures, surface properties, etc.

ISSN:0026-8976    

DOI:10.1080/00268979100101191

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The structure of molten polyiodides, Et3SIx(l) (Et = ethyl;x= 3, 4, 5 and 7), have been investigated by liquid x-ray scattering and Raman spectroscopy techniques at room temperature. The predominant iodine-containing species in Et3SI3(l) is a centrosymmetric I3-ion with a closest I-I distance of 2·915(2) Å. The Raman spectra indicate a large bond flexibility of the triiodide ion. The structural results of the iodine-rich melts Et3SIx(l),x> 3, are consistent with a three-dimensional network of interconnected I3-ions and I2molecules. The short-range order bears close similarities to that of pure liquid and solid iodine. The triiodides are on the average solvated by one, two and three iodine molecules forx= 4, 5 and 7, respectively. The coordination mode is flat-on with the I3-and I2units almost parallel but slightly tilted away from each other like the nearest-neighbour contacts in pure iodine. Previously published conductivity results are consistent with such a structure model.

ISSN:0026-8976    

DOI:10.1080/00268979100101201

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

We report excitation profiles for producing vacuum ultraviolet radiation via third harmonic generation in a range of molecular gases, including C2H2, C2D2, CS2, H2S, CO, CH3I, CH3Cl, HCl and BrCN. We also present the results of model calculations which provide a qualitative rationale for the way in which these excitation functions are observed to vary with sample pressure and with laser focusing conditions, and conclude by pinpointing factors which are likely to encourage third harmonic generation in molecular gases.

ISSN:0026-8976    

DOI:10.1080/00268979100101211

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The infinite order sudden (IOS) approximation is used to calculate rate constants for rotationally inelastic scattering of two N2molecules. A new approximation, which is a modified version of the breathing sphere approximation, is introduced. Rate constants obtained using this approximation are compared with those of the IOS approximation. Both sets of rate constants are then summed up over all final rotational levels of the perturbed molecule in order to determine Raman Q-branch spectral linewidths. These theoretical linewidths are compared with experimental linewidths. Good agreement is obtained for higher temperatures.

ISSN:0026-8976    

DOI:10.1080/00268979100101221

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Complete sets of harmonic and cubic force constants of water, methanol and silanol are calculated by theab initioSCF method using double zeta plus polarization type basis sets. Nuclear motion corrections to equilibrium geometries are derived. The OH bonds become longer by about 1·7 pm and the XOH angles wider by about 0·15°. Vibrationally averagedrαandrgstructures are calculated which agree with observed data within experimental limits of uncertainty.

ISSN:0026-8976    

DOI:10.1080/00268979100101231

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Time domain techniques using pulse methods in microwave spectroscopy have been employed to determine the population and coherence decay rates, 1/T1and 1/T2, for theJ= 0–1 rotational transition of15N2O. The pressure dependence of both relaxation rates has been investigated for the pure gas. Pressure broadening of the line (∼ 1/T2) has also been studied for mixtures with O2, N2, air and the rare gases He, Ne, Ar, Kr and Xe. The observed transient emission signals were analysed by the standard model of constant coherence decay rates and also assuming dependence of the rates on the speed of the absorber molecules. The experimental results are compared with predictions from theoretical treatments of the binary collisions.

ISSN:0026-8976    

DOI:10.1080/00268979100101241

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Equations for three intermolecular correlation functions for athermal hardsphere chain fluids are derived in the context of the Percus-Yevick (PY) integral equation theory. The approach employed here is based on a particle-particle description of chain molecules developed in part I of this series. Specifically, we obtained equations for the site-site total correlation functionhαβ(r), the single index average total correlation functionhα(r), and the average total correlation functionh(r) form-mer homonuclear hard-sphere chains. It is found that the PY theory does not yield a closed equation for the average total correlation functionh(r). Solvingh(r) requires additional information on the chain-end correlation functions. Two approximations (beyond the PY theory) which yield a single closed equation forh(r) are proposed and examined. Analytic expressions for these average correlation functions at contact are obtained as a function of the chain lengthmand hard-sphere site volume fraction η. Numerical solutions forg(r) are obtained for 4-mer and 8-mer chains and compared with computer simulation data. It is found that the PY theory is able to predictg(r) for 4-mer chains accurately; however, only qualitative agreement is obtained for 8-mer chains.

ISSN:0026-8976    

DOI:10.1080/00268979100101251

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

For the clusters H(H2O)n+,n= 1 to 5, geometry optimization and analytical second-derivative calculations have been carried out with two basis sets, 6-31G* and 6-31 + G + +, at both the Hartree-Fock and MP2 levels. Minimum energy structures, harmonic vibrational frequencies and stepwise hydration enthalpies have been obtained and compared with available experimental data. The computed minimum energy geometries of the protonated water clusters show a balance between covalent and ion-dipole hydrogen bonding, with dispersion effects being more important in the heavier clusters. Allowance for the effects of basis set superposition error (BSSE) in the calculations has been examined. The choice of methodology and basis sets in the calculations has been discussed and particular attention has been given to the incorporation of the gradient method in the counterpoise procedure for calculation of interaction energies.

ISSN:0026-8976    

DOI:10.1080/00268979100101261

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Following the results of a previous measurement, new neutron diffraction data were taken in order to extract the intermolecular partial correlation functions of liquid H2S. The experimental results are successfully compared with the results of a molecular dynamics simulation. Comparison with the pair correlation functions of liquid water at the same thermodynamic state is also discussed.

ISSN:0026-8976    

DOI:10.1080/00268979100101271

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor