摘要:

A mathematical procedure to derive in a systematic manner exact quantum mechanical Hamiltonians forN-particle systems is presented. As an application, exact quantum mechanical Hamiltonians are obtained for the motion of three atoms, with the help of various sets of curvilinear internal coordinates and for various body-fixed frames of reference.

ISSN:0026-8976    

DOI:10.1080/00268979100100011

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

We demonstrate that Longuet-Higgins' molecular symmetry (MS) group for describing non-rigid molecules allows deduction of the transition state for an intramolecular rearrangement and that the level of symmetry of the transition state is governed by very simple rules. Key pieces of information are the order of the MS group and the number of distinctly labelled forms represented by it. We also show that the local symmetry at stationary points on the potential energy surface is important and introduce natural definitions of narcissistic reactions and pathways using the laboratory-fixed inversion operation, giving examples of each. Inspection of normal modes is used to depict motion across the potential energy surface between a minimum-energy structure and a transition state. This analysis is applied to acetylene trimer, a recently observed van der Waals cluster. We elucidate the relationships between the stationary points identified by our earlierab initiowork. There are two transition state structures that together allow full interconversion between the 16 distinctly labelled forms of the global minimum.

ISSN:0026-8976    

DOI:10.1080/00268979100100021

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

A full Hamiltonian matrix coherent with a phase convention leading to e/f labellings and to positivel-doublingqparameters is constructed to describe the rovibrational energy levels in a linear polyatomic molecule with two bending vibrations. This model is used to investigate quantitatively the effect of the rotational and vibrationallresonances on level mixings and intensity features in C2H2.

ISSN:0026-8976    

DOI:10.1080/00268979100100031

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The properties of a complex linear absorbing barrier, recently employed to avoid the coordinate transformation in reactive scattering, are analysed by reduction to a one-parameter model. Limiting values of the slope and width of the barrier, for 0·1% reflection and 99% absorption, are derived and confirmed by numerical tests.

ISSN:0026-8976    

DOI:10.1080/00268979100100041

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Reactive scattering of F atoms with (CH3)3CI molecules has been studied at an initial translational energyE≈ 38 kJ mol-1using a supersonic beam of F atoms seeded in He buffer gas and atE≈ 12 kJ mol-1with Ne buffer gas. Centre-of-mass angular distributions of IF scattering show peaking in the forward and backward directions that is consistent with reaction via a persistent F-I-C(CH3)3complex with a lifetime greater than one rotational period. The product translational energy distribution for scattering in the forward and backward directions is pitched at higher energy than the sideways scattering forE≈ 38 kJ mol-1, but the variation of product translational energy with scattering angle is much more limited forE≈ 12 kJ mol-1. An extended microcanonical theory is used to interpret the scattering at higher initial translational energyE≈ 38 kJ mol-1in terms of the precessional motion of F-I-C(CH3)3product transition states formed in collisions at large impact parameters coupled with a phase space description of scattering from small impact parameters. For scattering at low initial translational energyE≈ 12 kJ mol-1the phase space description proves adequate over the full range of impact parameters. At high initial translational energy the forward and backward scattering indicates that only low-frequency transitional modes of the transition state are strongly coupled to motion along the reaction coordinate in large-impact-parameter collisions.

ISSN:0026-8976    

DOI:10.1080/00268979100100051

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Neutron diffraction and heat capacity measurements are combined to study the structures and phase transitions occurring in a monolayer of fluoromethane physically adsorbed on graphite. Neutron diffraction reveals two solid phases with a phase transition between 100 and 105 K and melting of the layer between 120 and 130 K. The unit cells of both phases are determined to be commensurate with the underlying substrate (3 × √3) with lattice parametersa= 0·738 nm,b= 0·426 nm and β = 90·0°, and both contain two molecules in the unit cell. The heat capacity is determined for the monolayer between 0·5 and 150 K. There is a sharp anomaly corresponding to melting at 124 K and a small anomaly around 100 K corresponding to an order-order transition, both consistent with the diffraction results. An additional anomaly is observed around 140 K, where the film is in the liquid phase. At very low temperatures there is evidence of tunnelling splitting of the librational ground state with a value of 10 μeV, smaller than that in bulk CH3F. The low-temperature heat capacity also gives a value of 30 K for the ‘gap’ in the phonon density of states arising from the commensurate structure of the layer. The normal heat capacity is also analysed to obtain average characteristic temperatures for the different molecular motions.

ISSN:0026-8976    

DOI:10.1080/00268979100100061

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

A general theory for the calculation of the elastic constants of molecular crystals has been developed within the pair-potential rigid-molecule approximation. Interatomic rather than intermolecular interactions are considered explicitly, thus taking more angular dependence into account. Model calculations for CO2have been carried out, the results being in good agreement with those obtained using the crystal packing programme PCK83, with the same set of potentials. Both methods yield lower values than the corresponding intermolecular potential calculations for thec11,c12andc44tensor components of CO2.

ISSN:0026-8976    

DOI:10.1080/00268979100100071

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The Dunham parameters of the ground state of the CO molecule are determined by a least squares fit of recent high-quality microwave and heterodyne frequency measurements. It is shown that these new parameters are more accurate than others obtained recently. Revised equilibrium molecular constants have been deduced from these new Dunham parameters and the mass-scaling coefficients determined by Guelachviliet al.The most important constants are ωe= 65 047 645(7) MHz, ωexe= 398 365·1(14) MHz, ωeye= 312·11(14) MHz,Be= 57 907·98(6) MHz, αe= 524·791(13) MHz,De= 0·183 520 8(57) MHz andRe= 1·128 243(1) Å. The new ground state parameters allow precise determination of the transition frequencies, which can now be used as secondary standards in the microwave and infrared ranges.

ISSN:0026-8976    

DOI:10.1080/00268979100100081

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

ISSN:0026-8976    

DOI:10.1080/00268979100100091

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Finite-difference algorithms for Nosé-Hoover canonical molecular dynamics (NH-MD) are investigated. In general, NH-MD functions excellently, but it is demonstrated that time symmetry is broken in NH-MD algorithms and that this results in irreversible thermostatting, which can destroy the canonical sampling. However, the traditional ‘leapfrog’ algorithm, extended to NH-MD, works almost perfectly and equilibrates correctly, even for a system of a single butane chain, which is found to be very sensitive to numerical approximations. The MD conserves the total momentum—and for a single molecule also the angular momentum. NH-MD can run with or without these conservation constraints, and it is stable with respect to round-off errors, at least for the first 109time steps for the present system.

ISSN:0026-8976    

DOI:10.1080/00268979100100101

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor