摘要:

A general expression is developed for the dipole-dipole nuclear spin-lattice relaxation timeT1in molecules whose constituents are undergoing an arbitrary number (n) of superimposed reorientations about different molecular symmetry axes. In addition, the calculation takes into account isotropic tumbling of the whole molecule for the case of a liquid, or, a powder average for the case of a powder or polycrystalline sample. As an example, the case ofn= 3 (three superimposed reorientations plus tumbling or a powder average) is investigated. The incentive for treating then= 3 case is the need to interpret relaxation measurements (reported in the accompanying paper) in 4,4′-methylenebis (2,6-di-t-butylphenol) where, in addition to methyl and t-butyl reorientation, the benzene rings may be reorienting with respect to one another. The calculation draws upon the general theory of spin-lattice relaxation and on existing calculations concerned with the effects that superimposed reorientations have on correlation functions. The resulting expression forT1is much more general than previous calculations which have only been done forn= 2 and even then only for specific geometries. It is shown that the present expression forT1reduces to those determined previously in the appropriate special cases.

ISSN:0026-8976    

DOI:10.1080/00268978000103511

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

We have measured the proton spin-lattice relaxation rateRin 4,4′-methylenebis(2,6-di-t-butylphenol) (MBDBP) and MBDBP-OD (deuteriated hydroxyl proton) between 88 K and 380 K. The experimental results are compared with those from a previous study in the closely related but simpler molecule 4-methyl-2,6-di-t-butylphenol (MDBP). The present experiment is analysed in terms of relaxation resulting from intramolecular reorientation modulation of the proton spin-spin interaction. This intramolecular reorientation involves different combinations of superpositions of methyl, t-butyl, aromatic ring and hydroxyl group reorientation and a discussion is presented concerning the effects on the relaxation of superimposing these motions. The MBDBP-OD data is best fitted with the assumption that only methyl and aromatic ring reorientations occur. In MBDBP it is found that, in addition, an OH flip-flop motion is superimposed on the aromatic ring reorientation.

ISSN:0026-8976    

DOI:10.1080/00268978000103521

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

A new method is presented for computing ground and excited states potential energy surfaces, within the context of the valence bond (VB) theory, retaining the possibility of describing electronic molecular changes in terms of valence bond structure concepts. The separated electron pair (SEP) theory in the rank two geminal approximation is reformulated and compounded with the straightforward VB formalism: the set of non-orthogonal orbitals generated by the SEP optimization is used to construct VB structures, which allow a compact and correct description of the wavefunction. The procedure is applied to the study of the ground and excited states of LiH of Σ and Π symmetry (A1Σ+,C1Σ+,a3Σ,b3Π,B1Π). All the qualitative features of these states are reproduced, including the peculiarities of theA1Σ+state. Each state is described by one to four structures, from small values of the internuclear distance up to dissociation: a clear interpretation of the wavefunction is therefore provided.

ISSN:0026-8976    

DOI:10.1080/00268978000103531

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

Cartesian polarizability derivatives are defined as derivatives of the molecular polarizability with respect to the displacement of an atom in a space-fixed axis system. These derivatives are used to describe the intensity of molecular vibrational and rotational Raman scattering. The number of independent derivatives is easily determined and is shown to equal the number of intensity parameters which may be obtained from experiment. The cartesian polarizability derivatives thus form a complete, minimal set of parameters for describing Raman intensities.

ISSN:0026-8976    

DOI:10.1080/00268978000103541

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

The interaction of an electronicE-term and two degenerate normal vibrations of the same symmetry, leading to the so-called two-modeE⊗(ϵ + ϵ) Jahn-Teller effect, is investigated. Absorption spectra are calculated numerically via the Lanczos algorithm and are shown to exhibit new interesting structures. These two-mode spectra are seen to differ markedly from the spectra obtained by simply convoluting the constituting one-modeE⊗ϵ Jahn-Teller spectra. To classify the spectra we introduce, in addition to the coupling strengths of the individual normal vibrations, a hopping parameter which serves as a measure for the interaction of the normal vibrations via the electronic state. An effective one-modeE⊗ϵ Jahn-Teller hamiltonian is introduced which reproduces the envelope of the two-mode Jahn-Teller spectrum to a high accuracy.

ISSN:0026-8976    

DOI:10.1080/00268978000103551

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

Single crystal E.P.R. and cobalt ENDOR measurements onN,N′-ethylenebis(acetylacetonatiminato)Co(II), Coacacen, diluted in Niacacen·1/2 H2O are reported. Forbidden ΔmCo= 1,2 transitions in the E.P.R. spectra have been observed. Theg-tensor (E.P.R.) and the cobalt hyperfine and quadrupole tensors (ENDOR) have been determined. Theg- and cobalt hyperfine tensors are discussed. They support the |2A2,yz⟩ groundstate proposed for four coordinated low-spin Schiff base complexes of cobalt(II). The measured quadrupole coupling is also compatible with a |2A2,yz⟩ groundstate, if anisotropic contraction of the cobalt 3dorbitals is taken into consideration.

ISSN:0026-8976    

DOI:10.1080/00268978000103561

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

The rotational behaviour of NOF3is derived from the band contour analysis of the Raman spectra recorded over the liquid phase. The angular momentum correlation times derived from theJdiffusion model show us that thisC3vmolecule rotates almost isotropically. The band profiles are interpreted in terms of the rough hard sphere model.

ISSN:0026-8976    

DOI:10.1080/00268978000103571

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

The idea that the molecular orbitals of a cluster of atoms can be usefully classified according to their nodal structure, so that the energy increases with the number of nodes, is quantified and shown to apply in its usual form only to molecular orbitals constructed from atomic σ orbitals. The method is extended to deal with atomic π and δ orbitals by the use of vector and tensor surface harmonics respectively. In all cases the orbitals can be classified approximately in terms of angular momentum quantum numberslandm, but in the π and δ cases there are two orbitals, with different parity, for eachlm, and the energy is determined primarily by the parity. The method provides a classification scheme and an approximate energy ordering which does not depend on any point-group symmetry that the cluster may have, and therefore provides a useful framework for discussion of the bonding in cluster compounds such as the boron hydrides and the transition metal cluster carbonyls.

ISSN:0026-8976    

DOI:10.1080/00268978000103581

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

We present results of calculations of the equilibrium density profile and surface tension for the liquid-vapour interface of the restricted primitive model of a molten alkali halide. In this model the anion and cation are represented by hard spheres of equal diameterRand opposite charges; there is no electrical double layer at the interface. Our calculations are based on a square gradient approximation to the free energy of an inhomogeneous charged fluid. We find that the shape of the total density profile is different from that calculated for a Lennard-Jones fluid and that the interfacial thickness is somewhat sharper at low temperatures; the ‘10–90’ thickness is ≈Rnear the melting temperature. The surface tension decreases linearly with temperature until near the critical point, in a manner similar to that found for the Lennard-Jones fluid. Our results are in qualitative agreement with the results of a recent computer simulation for a model of KCl. We have also compared our result for the surface tension with experimental data for those alkali halides where the anion and cation are of similar size. This involves specifyingRfor each salt. The theory gives a reasonable description of both the magnitude and the temperature dependence of the measured surface tension.

ISSN:0026-8976    

DOI:10.1080/00268978000103591

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

The properties of some infrared and Raman bands of OH and SH vibrators engaged in weak hydrogen bonds in the liquid phase at room temperature are studied experimentally as well as theoretically. The differences between isotropic Raman, anisotropic Raman and infrared band profiles are explained in terms of electrical coupling of theXH andX … Bstretching modes.

ISSN:0026-8976    

DOI:10.1080/00268978000103601

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor