摘要:

Bond dissociation energy curves have been calculatedab initiousing the valence bond self-consistent field (VBSCF) method for CH3-Y molecules, with Y = F, OH, NH2, CH3, BH2, CN and NO. Basis sets of at least double-zeta polarization quality for first row atoms have been used. The VB wavefunction is expanded in terms of one covalent (CH3:Y) and two ionic (CH3+Y-and CH3-Y+) structures. All results were referenced to the perfect pairing generalized valence bond method. The VB curves for CH3-F were calculated using both the same orbitals for the different structures (SODS) and tailored (different) orbitals for the three different structures (TODS). Each of these model levels was examined further using purely localized fragment (CH3and Y) orbitals (L-SODS and L-TODS) and allowing delocalization between the CH3and Y fragments among the passive orbitals (D-SODS and D-TODS), where only the electron pair bond undergoing homolytic dissociation uses purely localized fragment orbitals. Only the two SODS model levels were used for the other CH3-Y molecules.

ISSN:0026-8976    

DOI:10.1080/002689796173769

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The Hamiltonian for a three-centre reaction in a polyatomic collision complex or molecule is derived, the classical Hamiltonian is given, and the dynamics of a specific system the HM2+OH → H2O+H reaction is considered.

ISSN:0026-8976    

DOI:10.1080/002689796173778

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

We address a central question in the realm of the dynamics of high-n(= 40- 250) Rydberg states of diatomics and large molecules. What is the coupling responsible for the 'global'lmixing, which results in the breakdown of then3scaling law for the non-radiative lifetimes and for the lifetime lengthening (by two to four orders of magnitude) of these states? To explore the implications of intramolecular interactions onlmixing and on electronic-rotational energy exchange we analysed the intramolecular couplings of the ion core dipole, quadrupole and (anisotropic) polarizability with a non-penetrating (l⩾ 3) Rydberg electron, in conjunction with the energy gaps between proximal pairs of energy levels. Calculations of the energy gaps and the couplings were performed for the high-nnon-penetrating Rydberg states of NO and for model 'light' (B= 19 cm-1) and 'heavy' (B = 0⋅05 cm-1) polar molecules. All the intramolecular interactions are of the form of a power law proportional tol-η, with η being determined by the nature of the long-range coupling, by theldependence of the quantum defects for multipole couplings and by theldependence of angular integrals. We established a bottleneck effect for the intramolecular couplings between non-penetrating (l⩾3) states. Forn- and N+-changing dipole, quadrupole and polarizability interactions the energetics ofthe proximalpairs of levels, in conjunction with the bottleneck effect,prohibit the |n,l,N+,N> - |n',l',N+',N>,l(⩾ 3) couplings (withn╪n') and the electronic-rotational energy exchange. Forn- andN+-conserving quadrupole and polarizability interactions, thel(⩾ 3) mixing (which prevails only forlN++N) is also prohibited by the bottleneck effect. 'Global' intramolecular l mixing (with bothn╪n' andn=n') in diatomics and in large molecules is precluded, implying that the dramatic lengthening of the non-radiative lifetimes of high-nRydberg states can be induced only by exterior electric field coupling.

ISSN:0026-8976    

DOI:10.1080/002689796173787

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The detailed stereodynamics of the H+CO2system have been studied using polarized Doppler resolved laser induced fluorescence to state selectively probe the OH product. The lambda-doublet specific differential cross-sections and translational energy distributions were measured, along with the scattering angle dependence of the rotational angular momentum polarization. The observed centre-of-mass differential cross-section has significant contributions in both the forward and backward hemispheres, with a small preference for backward scattering, and the measured fraction of available energy released as translational energy is greater than that predicted by statistical models. The rotational angular mometum vector of the OH product displays a polarized angular distribution, measured with respect to the reagent relative velocity vectorkand thekk' scattering plane, with the π(A'') lambda-doublet component being significantly more strongly polarized than the π(A') level. In addition, those products scattered backwards are seen to have a different polarization from those scattered forwards. The unprecedented detail of the information obtained is explained in terms of rather stringent steric constraints in the entrance channel and the formation of short-lived collision complexes.

ISSN:0026-8976    

DOI:10.1080/002689796173796

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Rare gas atom dimers from (He)2to (Xe)2were studied by beyond Hartree-Fock methods and the results obtained were compared with experimental values. In the case of (He)2, the CCSD(T), CCSDT, and FCI results enable estimation of the role of triple and quadruple excitations. CCSD(T) already yields reliable stabilization energy values. Passing from CCSD(T) to CCSDT leads to only a small increase in the stabilization energy; a very similar value was surprisingly obtained for the (Ne)2dimer. For the (Ne)2and (Ar)2dimers, the all-electron and the effective core potential (ECP) results were compared. Correcting the ECP CCSD(T) characteristics for (Kr)2and (Xe)2by the factors found for (Ne)2and (Ar)2resulted in very good agreement with the experimental values.

ISSN:0026-8976    

DOI:10.1080/002689796173804

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A new gradient-corrected exchange functional (G96) is introduced. While similar to Becke's B88 functional, it is much simpler and its potential in finite systems is asymptotically unbounded. The mean absolute deviations of the B88 and G96 exchange energies from the corresponding Hartree-Fock values for the atoms H to Ar are 12⋅5 and 8⋅5 mEh, respectively. In combination with the LYP correlation functional, it yields a density functional (G-LYP) that performs similarly to B-LYP on the standard G2 benchmark and we infer that the usefulness of a functional for molecular calculations does not depend critically on its asymptotic behaviour.

ISSN:0026-8976    

DOI:10.1080/002689796173813

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Simulations of clusters formed with equal proportions of Ar and Kr atoms suggest that under certain conditions a regular CsCl structure will be formed on cooling. Comparison with similar studies of pure Kr systems, both as clusters and in the bulk, strengthens the reliability of these model calculations. Whether the proposed CsCl structure can actually form is presented as a challenge to the experimentalists, especially those doing molecular beam experiments. The formation of the CsCl structure by slow annealing has been the most successful, but considerable order is apparent even when the liquid cluster undergoes a very rapid quench.

ISSN:0026-8976    

DOI:10.1080/002689796173822

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

In order to account for the extra vibrational Raman bandwidths observed specifically for electronically excited molecules in solution, a new model, the dynamic polarization model, has been introduced. In this model, the vibrational frequency of a highly polarizable electronically excited molecule in solution is stochastically modulated by the solvent-induced polarization. An olefinic C = C double bond is considered as the simplest prototype, for which the carbon-carbon stretch frequency can take values between the unpolarized (C = C) and the completely polarized (C+-C-) frequencies. The two-frequency exchange model is first introduced, followed by more general models. A focus is placed on the asymmetric exchange limit in which the solvent-induced polarization can be treated as a perturbation, so that the effect of the dynamic polarization manifests as a shift of the peak position and an increase in the bandwidth of the original unperturbed C = C stretch vibrational band. The experimental results on S1trans-stilbene are discussed, based on the formulae derived.

ISSN:0026-8976    

DOI:10.1080/002689796173831

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Spectroscopic constants (Re, ωe,De,D0) for the3Δ ground-state potentials of the diatomic ions CoAr+, CoKr+, CoXe+, and CoRn+are calculated using coupled-cluster theory, flexible basis sets, and relativisticab initiopseudo-potentials for the heavy rare-gas atoms. This standard approach yieldsc. 90% of the measured interaction energies and overestimates the bond distances byc. 0⋅05 bohr. Bonding is shown to be intrinsically electrostatic. For the case of CoAr+, possible sources of error in the calculation are investigated by studying the effects of core correlation,scalar and spin-orbit relativistic effects, and basis set superposition errors. Correlating the 3s and 3p electrons in Co and the inclusion of scalar relativistic terms increase the bond strengths and decrease the equilibrium distance in CoAr+, while accounting for spin-orbit interaction and correction for the basis set superposition errors lead to the opposite effects. Including all these corrections, the theoretical predictions of the spectroscopic parameters for the3Δ3ground state of CoAr+are (experimental values in parentheses)Re= 4⋅460 (4⋅509 ± 0⋅009) bohr; ωe= 198 cm-1;Do= 0±4874 (0⋅5097 ± 0⋅0006) eV.

ISSN:0026-8976    

DOI:10.1080/002689796173840

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

We present a simple treatment of supercritical fluid extraction for binary (solvent-solute) and ternary (solvent-solute-cosolvent) systems. Solubility predictions are made using the virial equation of state. The solute chemical potential is calculated up to fourth order in the fluid density. There are no adjustable parameters: the pressure mixture virial coefficients Bα β, Cα β γand Dα β γ δare computed directly from the potentials representing the interactions of solvent, solute and cosolvent molecules. For the purposes of illustration we assume these are of Lennard-Jones form; however, the method is considerably more general, and may be applied even when the potentials are anisotropic. Computer simulations have also been performed, and confirm the accuracy of this approach at densities up to twice the critical density.Our analysis of ternary systems helps to identify the nature of the interactions primarily responsible for entrainer effects.

ISSN:0026-8976    

DOI:10.1080/002689796173859

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor