摘要:

Realistic two-valued potential energy surfaces for the reaction C(3P) + CH(X2Π) → C2+ H have been constructed from a set of high levelab initiodata describing the first two2A′ electronic states of the C2H system. These states have linear equilibrium configurations, known as theX2Σ+and A2Π states, and are coupled by a conical intersection. They lead to the formation of C2(X1Σ+g) and C2(a3Πu) considering an adiabatic dissociation process. Theab initiocalculations are of the multireference configuration interaction variety and were carried out using a polarized triple-zeta basis set. Using theab initioadiabatic energies and the matrix elements of the dipole moment, a 2 × 2 diabatic representation of the electronic Hamiltonian was built. Each element of this Hamiltonian matrix was expressed within the double many-body expansion (DMBE) scheme which is based, in this case, on the extended Hartree-Fock approximate correlation energy model (EHFACE). The analytical adiabatic potential energy surfaces are then obtained as the eigenvalues of this matrix, and display correctly the Σ/Π conical intersection. Moreover, the non-adiabatic couplings given by our analytical model are compared with theab initioones, and good qualitative agreement is observed.

ISSN:0026-8976    

DOI:10.1080/00268970009483396

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The valence shell photoelectron spectrum of cyanogen chloride has been studied using HeI and synchrotron radiation. In the outer valence region the molecular orbital model of ionization holds, and the main bands can be associated with single-hole states. However, in the inner valence region electron correlation effects become important, and these result in complex satellite structure being observed. Vertical ionization energies and spectral intensities have been computed using the Green's function approach, and the results have facilitated an interpretation of the experimental spectra. Photoelectron angular distributions and branching ratios have been measured and have been used to assess the bonding characteristics of the outer valence molecular orbitals. The experimental data for the 8σ orbital display an energy dependence which suggests that photoionization from this orbital may be influenced by the chlorine 3p Cooper minimum. The extent to which the 8σ orbital can be considered as a chlorine atom lone-pair is discussed. Vibrational structure has been observed in the [Xtilde]2Π, Ã2Σ+and [Btilde]2Π photoelectron bands recorded with HeI radiation, and has been assigned to progressions involving the ν+1and ν+3modes.

ISSN:0026-8976    

DOI:10.1080/00268970009483397

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

We examine the relative efficiencies of three- algorithms for performing Brownian Dynamics simulations without many-body hydrodynamics. We compare the conventional Brownian Dynamics algorithm of Ermak (CBD), Smart Monte Carlo (SMC) which incorporates Boltzmann sampling into essentially a CBD procedure, and the Stochastic Runge Kutta (SRK) method. We show, using the repulsive potential φ(r) = ϵ(σ/r)n, wheren= 36 and 72, that the SRK algorithm gives the most accurate short-time dynamics for the mean-square displacements. The SRK algorithm static and dynamical properties converge better with a reducing time step to the exact values, than those generated by the CBD algorithm; giving efficiency gains typically of a factor of 3–4. Both CBD and SMC have the incorrect sign for the first correction term to the mean square displacement in a time step, whereas the SRK algorithm gives essentially the exact solution to order Δt2, where Δtis the simulation time step. In fact, these correction terms are almost equal and opposite in sign. Expressions for these terms were derived in terms of the average interaction energy per particle. The force, shear and bulk stress autocorrelation functions were calculated. The average energy per particle and time correlation functions at short time have values in excess of the exact values, while the corresponding quantities for SRK are below this. This difference in behaviour can be traced back to the extent of compliance of the particle trajectories with the exact expansion of the Smoluchowski equation. The accuracy, at a given value of the time step, of the stochastic algorithms can significantly depend on the form of the interaction potential between particles. It is also demonstrated that the long time limits of various correlation functions are fairly insensitive to a particular scheme (SRK or CBD) used in the simulations. All the correlation functions have a stretched exponential region at intermediate to long times, and the values of the exponents on density and force law steepness have been determined.

ISSN:0026-8976    

DOI:10.1080/00268970009483398

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Hetero-association of the anthracycline drug, daunomycin (DAU), with typical mutagens, the acridine dyes proflavine (PF) and acridine orange (AO), has been studied by 500 MHz1H NMR spectroscopy as a function of concentration and temperature in 0.1 mol dm−3phosphate buffered aqueous solutions at pD = 7.1. The results have been analysed in terms of a statistical-thermodynamical model of hetero-association of aromatic molecules, described previously [Davies, D. B., Veselkov, D. A., and Veselkov, A. N., 1999,Molec. Phys.,97, 439], but generalized in this work, so that there is no limitation on the magnitudes of the self-association constants of the interacting molecules. Expressions suitable for the analysis of NMR parameters of both components in the mixed solution have been developed enabling both the structural and thermodynamic properties of hetero-association to be determined. The magnitude of the equilibrium constant for hetero-association of PF + DAU is found to be substantially higher than the self-association constants of these molecules, whereas that for hetero-association of AO + DAU is intermediate between the equilibrium constants of self-association of AO and DAU. Intermolecular cross-peaks observed in 2D-ROESY spectra of PF + DAU mixed solutions are consistent with formation of a hetero-association complex in which an intermolecular hydrogen bond can form between either of the 3,6-diamino groups of the PF chromophore and the 9-MeCO group of DAU, which is in contrast to AO + DAU hetero-association, where such hydrogen bonds are unable to form. Quantitative structural and thermodynamical analysis of PF + DAU complexation is consistent with an intermolecular hydrogen bond contributing to the stability of the hetero-complex in aqueous solution. The NMR results show that hydrophobic interactions play a substantial role in the stabilization of the AO-DAU complex, characterized by a relatively small entropy change on complexation, compared to the PF-DAU hetero-complex, which is mainly stabilized by hydrogen bond and dispersive van der Waals interactions.

ISSN:0026-8976    

DOI:10.1080/00268970009483399

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Ab initioquasirelativistic calculations on the B0+u− X0+g, A1u(1Πu)-X0+gtransition dipole moment functions for the I2molecule are reported. The computational approach combines a description of relativistic effects through core pseudopotentials with correlation treatment by many-body multipartitioning perturbation theory. The resulting functions are used to simulate the absorption spectra of I2and to estimate the radiative decay rates of the B0+u,v′ states.

ISSN:0026-8976    

DOI:10.1080/00268970009483400

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The Karplus equation has been investigated byab initiocomputation of the spin-spin coupling constants for a series of rotated ethane geometries. The couplings have been calculated at the self-consistent field (SCF) level as well as using the second-order polarization propagator approximation (SOPPA) and the second-order polarization propagator approximation with coupled cluster singles and doubles amplitudes (SOPPA(CCSD)) and have been compared with results of previous calculations. The four principal components of the coupling constants rather than just the Fermi-contact have been calculated, and the common supposition that the Fermi-contact term is totally dominant has been confirmed. The derivatives of the orbital paramagnetic and orbital diamagnetic terms are significant but opposite in sign for the case of this rotation in ethane. It is found that the coefficients in the Karplus equation are largely overestimated at the SCF level, whereas the SOPPA(CCSD) results are in good agreement with coefficients derived from experimental coupling constant data or the results of multi-configurational self-consistent field (MCSCF) calculations. It is further observed that extending the Fourier series in the Karplus equation to include cos(3θ) and cos(4θ) terms neither significantly improves the quality of the fit nor significantly changes the values of the other coefficients. In order to simulate the Abraham and Pachler equation, calculations varying the nuclear charge on hydrogen have been performed. These will allow an abstract but flexible prediction of the effect of electronegative substituents.

ISSN:0026-8976    

DOI:10.1080/00268970009483401

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

This paper presents correction terms for the effective potential for the translation and rotation of an impurity atom or molecule solvated in a helium nanodroplet that were previously published (Lehmann, K. K., 1999,Molec. Phys.,97, 645). The correction arises from changes in the total He-He potential energy of the displaced liquid as a function of the solute position within the droplet. For the alkali atoms, this buoyancy type correction removes the large barrier to ejection of the atom from the droplets, which is predicted if this term is neglected.

ISSN:0026-8976    

DOI:10.1080/00268970009483402

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor