摘要:

A theoretical approach to investigate the effects of the breakdown of the Born-Oppenheimer approximation in vibration-rotational spectra of polyatomic molecules in their electronic ground state is developed. The treatment is based on the derivation of an effective nuclear Hamiltonian in a space-fixed frame including adiabatic and non-adiabatic terms involving isotopically invariant functions of internuclear distances. At the stage of the transformation of this Hamiltonian into the molecular frame, the treatment is restricted to linear molecules, and adiabatic and non-adiabatic corrections to the potential and vibration-rotational kinetic energies respectively are calculated in the triatomic case. The advantages of this approach relative to that formerly used for diatomic molecules from the point of view of deriving mass independent spectroscopic parameters are discussed.

ISSN:0026-8976    

DOI:10.1080/00268979300100981

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The orientationally disordered phase of the two-dimensional hard cyclic pentamer system has been studied by Monte Carlo simulations. Various structural quantities have been calculated at several densities. The singlet distribution function, studied in detail, reveals considerable coupling between translational and rotational molecular motions. The coupling, stronger at higher densities, is present also at lower densities, down to melting, and cannot be neglected in any structural analysis of the system. An orientational transition connected with a change observed in the translational-orientational coupling is suggested.

ISSN:0026-8976    

DOI:10.1080/00268979300100991

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Exact statistical mechanical sum rules are combined with thermodynamic scaling arguments to determine the leading-order singular contributions to the transverse moments of the density-density correlation functionGof a simple fluid undergoing a complete wetting phase transition, from off-bulk coexistence, at a structureless substrate (wall). Contrary to our earlier suggestion, capillary-wave-like fluctuations do manifest themselves throughout the wetting film and lead to a transverse correlation length ξ‖(a), for both particles near the wall, which diverges with the same critical exponent as ξ‖, the correlation length appropriate for both particles in the depinning liquid-gas interface. Explicit results from a mean-field (density functional) calculation are consistent with our general predictions. In the weak fluctuation regime (d< 3), scaling methods are used to derive the short distance expansions of the density profile and moments ofGfor short ranged forces. These expansions, which describe singular contributions for distances ≪¯l, the average film thickness, are considerably richer than those for critical wetting at bulk coexistence. Using sum rule arguments and the results of a density functional calculation for the case of complete drying at a hard wall, we identify specific corrections to the leading order singular behaviour of the moments ofG. We show these corrections are not accounted for by the simplest capillary-wave-based ansatz forG. More significantly, they are not accounted for by the standard interfacial Hamiltonian description of complete wetting. We show, using sum rule arguments for interfacial Hamiltonians, that singular correction terms can be incorporated if the interfacial stiffness Σ (T) in the standard Hamiltonian is allowed to depend on the (fluctuating) film thicknessl(R). For short range forces we require the increment ΔΣ(l) to be proportional to (T-Tw)le-l/ξb, whereTwis the temperature of the critical wetting transition. This result is the same as the (leading order) result for ΔΣ(l) derived recently using an integral criterion to definel. We discuss other consequences forGof modifying the standard interfacial Hamiltonian.

ISSN:0026-8976    

DOI:10.1080/00268979300101001

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We report the gas phase microwave observation and subsequent analysis of the ring puckering level splitting of indoline and its N-D isotopomer. The spectra of the four lowest vibrational states are modelled through a two-dimensional vibrational Hamiltonian to furnish a coherent picture. According to this new picture, five-membered rings with a single double bond and five-membered rings in which the double bond is fused with a benzenic ring share a similar low ring puckering barrier. Discrepancies with previous work are ascribed to the use of a one-dimensional vibrational Hamiltonian.Ab initiocalculations concur with the present analysis.

ISSN:0026-8976    

DOI:10.1080/00268979300101011

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Rare-events molecular dynamics techniques are used to study the interconversion between the two half-chair isomers of cyclohexene (C6H10), in a solvent modelled through a stochastic bath, in order to investigate dynamic solvent effects on the isomerization rate. Adopting the torsional angle around the C-C sigma bond opposite to the double bond as the reaction coordinate, we calculate the equilibrium distribution of this coordinate (using umbrella sampling), and estimate the isomerization rate, including the transmission coefficient κ. The paper also contains methodological developments. A variant of Andersen's stochastic collision method (canonical ensemble sampling) is developed for molecules with constraints: by resampling Cartesian velocities of a localized subgroup of atoms of the molecule and leaving all other atomic velocity components unchanged, one mimics the collision of a virtual gas molecule with a subpart of the molecule of interest. To evaluate the transmission coefficient κ, the initial conditions for trajectories ‘crossing the top’ are automatically generated during the run, using a biased potential to obtain the probability of being at the saddle point.

ISSN:0026-8976    

DOI:10.1080/00268979300101021

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor