摘要:

The dynamics of the highly excited H2S molecule is investigated. Calculations up toJ= 40 are performed for the bending-rotational Hamiltonian by applying a self-consistent methodology. Centrifugal destabilization and stabilization of the molecular geometry is discussed. The appearance of fourfold clusters of energy levels as a consequence of molecular-geometry stabilization is predicted. It is established that the lowest value ofJfor which clusters emerge is 24.

ISSN:0026-8976    

DOI:10.1080/00268979000102591

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The13C CPMAS NMR spectra of four crystalline forms ofp-aminobenzenesulphonamide (sulphanilamide) were recorded at room temperature. Three of these forms (α, β, and γ) showed doublings in the resonances of the carbon atomsorthoto the amino group, but only a single signal was obtained from thoseorthoto the asymmetric sulphonamide group. A variabletemperature study allowed the interconversion of the α and β forms to the γ form to be monitored. Changes were also observed in the spectrum of the γ form as the temperature was increased, and were ascribed to the presence of 180° flips of the phenyl rings about theirparaaxis. This interpretation was confirmed by analysis of the broadenings introduced by the assumed motion on the centreband and sidebands in the13C CPMAS NMR spectrum of the exchanging nuclei. Variable-temperature spectra of the γ form were simulated in order to obtain information about the geometry, the rates and the activation parameters involved in the process. These calculations were in good agreement with the experimental data. The possible relevance that the observed doublings and ring motion may have for the mode of action of sulphonamides is also discussed.

ISSN:0026-8976    

DOI:10.1080/00268979000102601

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A new basis set for describing multipulse NMR experiments is presented and discussed. It is shown that unit spherical tensorsÛkq(I1I2) can be used to simplify the description of AA and AB coupled spin-1/2 systems evolving between radio-frequency pulses, in the presence of chemical shift and scalar coupling. For comparison the new results are compared with those obtained using coupled irreducible tensor operators[Tcirc]kq(k1k2). It is shown that 86 (respectively 94) terms are required to describe the evolution of the general density matrix for the AB system if unit spherical tensors (respectively irreducible tensor operators) are used. For the AA system the gains are more substantial in that only 26 (respectively 54) terms are required. Such reductions have clear advantages for handling systems of three or four coupled spins. It is also argued that in dealing with coupled spin systems of three or more spins, the use of the unit spherical tensorsÛkq(Iij) offer significant mathematical advantages over concatenated tensor operators[Tcirc]kq(k1…kn). In the case of theÛkq(IiIj) tensors only one Clebsch-Gordan coefficient is required, regardless of the number of coupled spins. However, for the[Tcirc]kq(k1…kn) tensors, it is necessary to evaluate ‘nested’ Clebsch-Gordan coefficients repeatedly.

ISSN:0026-8976    

DOI:10.1080/00268979000102611

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The ᵬ1B1-ã1A1spectrum of the CH2radical exhibits singlet-triplet perturbation of the ã and the ᵬ states, even though the direct first-order interaction between the ᵬ1B1and [Xtilde]3B1states is forbidden. We have derived a model for such interactions that involves both Renner-Teller and spin-orbit coupling. The results of calculations made using this model show that the effect of the Renner-Teller coupling is to generate1B1-3B1interactions that areas largeas those between the1A1and3B1states. These effects are much larger than those normally expected whensecond-order singlet-triplet coupling occurs. From these calculations, we have identified the triplet-state levels responsible for the ᵬ-[Xtilde] perturbations, and have refined the triplet-state potential.

ISSN:0026-8976    

DOI:10.1080/00268979000102621

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Methods for the calculation of atom-molecule dispersion coefficients are reviewed, with emphasis on approximate techniques. New results are reported for the interaction of several S-state atoms with ground-state molecular hydrogen using the Padé bounding technique. The scarcity of data for the dependence on the intramolecular coordinate imposes the use of approximate schemes to interpolate and extrapolate the dispersion coefficient for regions where it is unknown. Several approaches are suggested based on the Unsöld approximation and the Kirkwood variational method.

ISSN:0026-8976    

DOI:10.1080/00268979000102631

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The absolute Raman intensities of all fundamental bands of CH3I, CH2DI, CHD2I and CD3I, previously reported, are used to obtain the derivatives of the molecular polarizability with respect to the symmetry coordinates. Special attention is paid to anisotropic scattering, and the influence of data from the high-resolution spectra is discussed both from theoretical and practical points of view. The bond-polarizability model is used for interpreting the derived Raman intensity parameters ∂α/∂Si. While for trace scattering the results are consistent, we have found that the ratio between diagonal and non-diagonal components of ∂α/∂Siis not reproduced well by the model.

ISSN:0026-8976    

DOI:10.1080/00268979000102641

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Microcanonical transition-state theory has been employed to predict angular distributions of reactive scattering that arise from a long-lived collision complex dissociating via a transition state that approximates to a symmetric top when the direction of dissociation is perpendicular to the top axis. Bimodal peaking in the sideways direction is predicted for a prolate-symmetric-top transition state, while forward and backward peaking is predicted for an oblate-top transition state. In both cases the parameters determining the angular distributions depend upon the structure and energetics of the transition state. Sideways peaking is predicted in good agreement with reactive-scattering data for F + C2H4at higher initial translational energyE≳ 10 kJ mol-1when the transition state precesses as a prolate top and H-atom displacement occurs perpendicular to the plane of the heavy atoms. The emergence of forward and backward scattering at lower initial translational energyE≈ 5 kJ mol-1is attributed to transition states formed in collisions at the largest impact parameters precessing as slightly asymmetric tops.

ISSN:0026-8976    

DOI:10.1080/00268979000102651

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Neutron diffraction with isotope substitution of nitrogen, chlorine and hydrogen nuclei is used to investigate the hydration region of both ions and the possibility of ion pairing in aqueous solutions of tetramethylammonium chloride (TMACl) at 0·5m, 1·0mand approximately 2·0mconcentration. The use of a maximum-entropy method to calculate the radial distribution functions is discussed. The H-H correlation function for the water in the 1·0msolution is measured and compared with that for pure water. The nitrogen-substitution results are also compared with a hard-sphere model calculation. The results show that the TMA hydration is apolar rather than ionic, and is consistent with a disordered water-cage structure, while the water in the hydration sphere does not appear to be more ordered than in bulk water. There is no significant change in the hydration structure measured at the different concentrations, although at the highest concentration there may be evidence of the close approach of the anion and cation.

ISSN:0026-8976    

DOI:10.1080/00268979000102661

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The diatomic X ≡ Y and the triatomic X = Y = Z species containing phosphorus and second-row atoms are screened for possible new compounds, mostly at the HF and MP2/6-31G* level. The heats of formation are estimated and compared for nitrogen and phosphorus compounds, E = N and E = P, with the species EO-3and EO3-4included in the comparison. The CO4-4and OO2-4neighbours of the known NO3-4have weaker bonds. One predicted species, PBP3-, has been made independently by von Schneringet al., and itsReandvevalues agree with those predicted. The so-far unknown species PCP2-, PCO-and OPO+are proposed as possible new solid-state species, while CP-is found to be thermodynamically less favoured. The experimentally unknown CP-, SiP-, PS+, PF2+, PNe3+, PBF-, PCO-, PNN-, NPN-, PNP-, PPN-, OPO+and PNF+are proposed as new gas-phase species.

ISSN:0026-8976    

DOI:10.1080/00268979000102671

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The absorption spectrum of vapour-phase chloromethane-d2has been measured between 500 and 11400 cm-1using FTIR spectroscopy, and between 12800 and 17100 cm-1using laser photoacoustic spectroscopy. A total of 46 fundamental and overtone vibrational bands have been assigned, and for 11 of these bands the coarse rovibrational structure has been assigned and rotational parameters measured. The experimental data may be fitted successfully to a model local-mode Hamiltonian that includes the harmonic (1 : 1) resonance between the C-H and C-D bonds, but neglects the anharmonic (1 : 2) C-H stretch/bend Fermi resonance. The model suggests that the predominant extra structure in the high-overtone spectra can be ascribed to C-H/C-D interbond coupling. The ratio of the fitted interbond coupling parameters of the CHD and CD2groups is consistent with the value calculated from the structural data with the assumption that kinetic coupling dominates over potential coupling. It is suggested that the weaker structure in the high-overtone spectra is due to C-H stretch/bend Fermi resonance, and an order-of-magnitude estimate of the relevant coupling parameter is given.

ISSN:0026-8976    

DOI:10.1080/00268979000102681

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor