摘要:

The intermolecular potential energy surface of the electronic ground state of the ammonium-argon ionic dimer, NH+4-Ar, is calculated byab initiomethods using different levels of theory (MP2, MP4) and basis sets (aug-cc-pVXZ, X = D/T/Q). The deformation of the ammonium ion in the complex is shown to be small and its geometry is therefore fixed in these calculations to the tetrahedral structure optimized for the bare ion. The global minimum of the potential corresponds to a proton-bound structure with C3vsymmetry (Re≈ 3.4 Å,De≈ 950 cm−1) and the barrier to internal rotation between the four equivalent minima is around 200 cm−1. The three-dimensional potential is expanded in tetrahedral harmonics whose radially dependent coefficients,Vi(R), are compared for the considered levels of theory. The rotation-intermolecular vibration Hamiltonian is solved using a two-dimensional fixed-Rrepresentation of the calculated potentials,Vi, ≡Vi(Reff), where the effective intermolecular separation,RReff, is determined from the experimental rotational constants of the complex. The accuracy of these parametrized potential energy surfaces is judged by their ability to reproduce the hindered rotor subband structure in the experimentalv3(t2) infrared band of the complex. The simulations using the potentials calculated at the MP2/aug-cc-pVTZ or higher levels of theory reproduce the coarse structure of the experimental spectrum well. Further improvement could be achieved by least-squares fitting the potential parameters to the observed subband positions. The fittedV3andV4parameters remain in close agreement with those determined from theab initiocalculations but the anisotropy of the potential is significantly different from that in a previous least-squares fit ofV3alone.

ISSN:0026-8976    

DOI:10.1080/00268970009483270

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The mid-infrared spectrum of thev3,(t2) transition of the NH+4-Ar complex has been recorded at rotational resolution using photofragmentation spectroscopy. The spectrum is divided into perpendicular and parallel subbands corresponding to transitions between different hindered internal rotor states. The P and R branches of the strongest perpendicular subbands are rotationally resolved providing rotational and centrifugal distortion constants. The widths of individual rotational lines are limited by the laser bandwidth of 0.02 cm−1, giving a lower limit of 250 ps for the lifetime of the excited states. Effective intermolecular separations for each internal rotor state are determined from its rotational constant, after correction for the contribution due to Coriolis coupling between the internal and total rotational angular momenta. The absolute energies, rotational and distortion constants for the first few intermolecular bending and stretching levels of the ground intramolecular vibrational state are determined in a numerical solution to the rotation-intermolecular vibration Hamiltonian, employing a three-dimensionalab initiointermolecular potential. The results are compared with the experimental constants in order to assess the accuracy of the calculated potential. The relative energy levels from this calculation are also compared with those from a two-dimensional representation of the potential energy surface (‘fixed-R’ model) in order to judge directly the influence of the radial dependence of the potential.

ISSN:0026-8976    

DOI:10.1080/00268970009483271

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Critical constants of pure fluids (as important reference data in constructing vapour-liquid phase diagrams and basic input of various estimation methods) were determined for systems of non-spherical Kihara molecules; values of the critical temperature, density, compression factor and pressure of fluids composed of prolate and oblate molecules were evaluated from the fourth-order virial expansion. The second and third virial coefficients of the Kihara molecules were determined by applying the recently proposed method in which the effect of molecular core geometry and functional dependence of a pair interaction on the surface-surface distance are factorized and the former contribution determined from a formula for the corresponding hard convex body virial coefficient. The virial expansion for non-spherical Kihara molecules is applied to determine the critical constants ofn-alkanes (methane to octane) and cyclic hydrocarbons (cyclopentane, cyclohexane, benzene and naphthalene); a fair agreement with experimental data was found.

ISSN:0026-8976    

DOI:10.1080/00268970009483272

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The present work describes a novel mathematical scheme for the evaluation of multi-dimensional time domain integrals that arise in the calculation of resonance Raman excitation profiles and the optical absorption spectra when the electronic transitions take place between displaced, distorted, and rotated harmonic potential energy surfaces. A recipe is provided for formulating the time domain integrals in terms of a tractable summation of products involving hermite polynomials. Expressions for the special cases (displaced and/or distorted oscillators) are written in terms of mathematical special functions which are computationally easier to deal with. Relations with the so-called transform theory are discussed. Resonance Raman excitation profiles and the optical absorption spectrum for a 7-mode system are calculated as an illustration of our expressions.

ISSN:0026-8976    

DOI:10.1080/00268970009483273

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The spectrum of the weakly-bound van der Waals complex Ar-CO has been observed for the first time in the region of the CO vibrational overtone, using a long-path (180 m) low-temperature (64 K) absorption cell and a Fourier transform infrared spectrometer. The appearance of the spectrum was very similar to that studied previously in thevco= 1 ← 0 region. However, it was very much weaker, so a detailed analysis was only possible for the relatively uncongestedK= 1 ← 0 and 0 ← 1 subbands. The parameters determined here for these sub-bands should help to constrain the C-O bond length dependence of the Ar-CO intermolecular potential in future theoretical models.

ISSN:0026-8976    

DOI:10.1080/00268970009483274

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The confinement of a lattice fluid in a set of slitlike pores separated by semipermeable walls with a finite width has been studied. The walls are modelled by a square-well repulsive potential with a finite height. The thermodynamic properties and the phase behaviour of the system are evaluated by means of Monte Carlo simulations. For some states theoretical calculations have been made using a mean-field-type theory. These investigations confirm previous findings for confined Lennard-Jones fluids, obtained from a density functional approach. For intermediate and low potential barriers that separate the pores, the isotherms exhibit two hysteresis loops and the liquid-vapour coexistence curve divides into two branches describing condensation inside the pore and inside the permeable wall. These two branches are separated by a triple point. At temperatures lower than the triple point temperature, the condensation takes place instantaneously in both the pore and inside the permeable wall. It was found that when the temperature is scaled by the bulk critical temperature, the phase diagram emerging from this simple mean-field treatment is close to the phase diagram obtained from simulation.

ISSN:0026-8976    

DOI:10.1080/00268970009483275

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor