摘要:

A1H,19F and2H nuclear magnetic resonance study of eight isotopomers of fluoromethane (CH3F) has been carried out under virtually identical experimental conditions. An analysis of the observed anisotropic couplings on the basis of bond additivity has been performed and the underlying assumption of bond additivity has been found to be adequate.In an accurate description of the solute orientation ‘non-rigid’ contributions such as vibration–rotation interaction, and harmonic and especially anharmonic vibrational corrections have to be included. The present study indicates the presence of a contribution to the observed anisotropic carbon–fluorine couplings due to an anisotropy inJCF.

ISSN:0026-8976    

DOI:10.1080/00268979650026569

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A quantum scattering calculation of collision cross-sections and energy transfer of HCN and HNC in collisions with Ar is reported. The HCN (HNC) rotation is treated in the infinite order sudden approximation, and the HCN (HNC) vibrations are treated by the coupled-channel method. A sum of two-body Lennard-Jones potentials is used to describe the Ar–HCN/HNC interaction, and the HCN/HNC intramolecular potential is a global fit to extensive, high qualityab initiocalculations. Vibrational state-to-state cross-sections and state specific energy transfer for HCN and HNC states are calculated as functions of initial relative translational energy.

ISSN:0026-8976    

DOI:10.1080/00268979650026578

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A theoretical investigation is presented of the weakly bound complex formed between benzene and water. Diffusion quantum Monte Carlo methods are used to describe the nuclear motion plus two potentials which give quite good agreement with DZP/MP2ab initiocalculations, and simulations were performed for four isotopomers of C6H6H⋅⋅⋅H2O. Although the minimum energy structure can be considered to have only a single hydrogen bond, vibrational averaging renders the hydrogens indistinguishable, a prediction in agreement with the experimental observation that the complex is a symmetric top. The results include zero-point energies, vibrationally averaged structures, rotational constants and wavefunctions. By calculating transition states and rearrangement mechanisms, it is possible to characterize the tunnelling dynamics and calculate the associated tunnelling splittings.

ISSN:0026-8976    

DOI:10.1080/00268979650026587

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A non-perturbative computational approach is developed to treat resonances attributable to non-adiabatic radiationless decay into several electronically distinct channels. Expressions for both the radiative and radiationless decay rates are developed. An alternative derivation of the classic multichannel line shape formula of U. Fano and J. W. Cooper (1965,Phys. Rev.A,137, 1364) is obtained. Although the approach is non-perturbative, the line-width is expressed in a golden rule type formula. The resonance energy is obtained from the iterative solution of an eigenvalue problem in the bound state space, avoiding the tedious characterization of theSmatrix as a function of scattering energy. These attributes enable the efficient determination of both narrow line widths and large resonance energy shifts.

ISSN:0026-8976    

DOI:10.1080/00268979650026596

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Certain difficulties with the usual one-centre multipole expansion of long-range intermolecular interaction energies can be circumvented by multicentre multipole expansions using several expansion sites in each molecule, such as, e.g., the nuclear positions. Based on the topological partitioning of the molecular volume provided by Bader's ‘atoms in molecules’ theory, a method has been developed for calculating the required atomic multipole moments and polarizabilities. The performance of these topologically partitioned electric properties is examined for the calculation of multipole expanded first-order electrostatic and second-order induction energies by comparing their convergence behaviour with that of the corresponding one-centre expansions. The homomolecular dimers of the water, carbon monoxide, cyanogen, and urea molecules serve asexamples.The results show thatdistributedelectricproperties calculated within the topological partitioning scheme indeed solve the ‘shape’ convergence problem, which arises in the calculation of interaction energies of large non-spherical molecules via multipole expansions.

ISSN:0026-8976    

DOI:10.1080/00268979650026604

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

There appears to be a conflict between theory and experiment concerning the vibrational frequencies of dioxirane, a CH2O2isomer. Here we employ sophisticatedab initioquantum mechanical methods with large basis sets to determine geometry, harmonic vibrational frequencies, infrared (IR) intensities and isotopic shifts of dioxirane. At the highest level of theory, the CCSD(T) approach is used together with a cc-VTZ2P+f,d basis set. Best predictions for CO asymmetric and OO stretching (or OCO deformation) harmonic frequencies are 931 (b2)cm-1and 759 (a2)cm-1, respectively. The IR intensities of these two peaks are predicted to be 19 km mol-1and 1 km mol-1. An examination of the experimental vibrational frequencies of substituted dioxiranes is also presented. Calculated frequencies, intensities, and isotopic shifts all imply that the two experimental IR features (with their observed intensity ratio of 2⋅8) assigned to CO stretching (839⋅0 cm-1) and OCO deformation (800.9 cm-1) frequencies by J. R. Sodeau and L. J. Whyte (1991,J. chem. Soc. Faraday Trans.,87, 3725) should be re-addressed.

ISSN:0026-8976    

DOI:10.1080/00268979650026613

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The [Xtilde]2A1and Ã2B1electronic states of the BH2molecule become degenerate (2Π) when the molecule is linear and the electronic angular momentum has an important effect (the Renner effect) on the rovibronic term values. This paper reports the results of anab initiocalculation of the two potential surfaces, and of calculating the rovibronic energies allowing for the effect of the electronic angular momentum and for spin-orbit coupling. A direct calculation is made of the rovibronic term values from the three-dimensional potential surfaces of the two electronic states and from the value of the spin–orbit coupling strength. The theoretical approach used is that described previously (Jensen, P., Brumm, M., Kraemer, W. P., and Bunker, P. R., 1995,J. molec. Spectrosc.,171, 31) but an improvement is made to the basis set contraction scheme used in a manner that is particularly important for quasilinear linear Renner systems like BH2and CH+2. The experimental data available for BH2come from a flash photolysis absorption spectrum for the Ã2B1← [Xtilde]2A1band system (Herzberg, G., and Johns, J. W. C., 1967,Proc. R. Soc. Lond.A,298, 142). These data yield only rotational term values for the ground vibronic state and rotation–vibration term values for some bending excited vibrational levels of the excited electronic state. Theab initiobarrier is adjusted to linearity in the electronic ground state, and the spin–orbit coupling strength also is modified, in order to improve agreement with available experimental data. As a result, the predictions herein should be useful indicators of the rovibronic energies and spin splittings, and in particular they should assist in a more detailed experimental characterization of the ground electronic state.

ISSN:0026-8976    

DOI:10.1080/00268979650026622

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Effects of the solvent on the optical limiting action of C60solutions are investigated under conditions of sequential two-photon absorption. C60transmission measurements are reported both at constant wavelength and at constant linear ground state absorbance for a set of solvents using visible nanosecond laser radiation. The limiting action is shown to depend not only on the ground state absorbance of the solute but also on the local field created by the solvent.

ISSN:0026-8976    

DOI:10.1080/00268979650026631

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The study is reported of the solvation of a deuterated hydrogen fluoride molecule (DF) inheavy water (D2O)using the Car–Parrinello density functional theoryab initiomolecular dynamics method. In dilute aqueous solution, the DF molecule is found to form a strongly bound complex, dynamically fluctuating between F—D⋅⋅⋅D2O and F⋅⋅⋅D3O structures. Vibrational frequencies calculated for the complex correlate well with features observed in the experimental infrared spectrum.

ISSN:0026-8976    

DOI:10.1080/00268979650026640

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A selected ion flow tube (SIFT) study of the O-+ CH2F2reaction is reported which gives the following negatively charged products at 298 K: CF2(37%), OH-(25%) and electrons (38%).Ab initiomolecular orbital calculations are carried out at various levels of sophistication to investigate the reaction paths and the relative energies of the possible reaction channels.Both the reactant-like (O-.CH2F2) and product-like (H2O.CF-2) ion–molecule intermediates are located. However, it is found that the OH-.CHF2product-like intermediate is not a stationary point on the (P)UMP2/6-31 + + G** hypersurface. A proton abstraction from CHF2by OH-gives the H2O.CF-2intermediate without a barrier, which in turn gives rise to the product channel H2O + CF-2. Extensive transition state searches, followed by intrinsic reaction coordinate (irc) path calculations, locate a number of transition state structures and minima, but none of them is connected with any of the observed products. Nevertheless, one transition state search leads to the surface for abstraction of a proton from CH2F2by O-. A careful search on the intersection between the OH.CHF-2and the OH-.CHF2surfaces indicates that these two surfaces cross in a region with the OC distance between 2⋅8Å and 3⋅2Å at energies close to the total reactant energy (at the UMP2/6-31 G** level). Optimization and transition state search on the OH-.CHF2surface in the region near the crossing with the OH.CHF2surface lead to the H2O.CF2intermediate and the products, OH-and CHF2, respectively. These observations suggest that both the negative ion products observed, OH-and CF-2, are produced from the same reaction surface. Initially OH.CHF-2is produced via proton abstraction by O-. This is followed by an electron transfer between OH and CHF-2at the crossing between the OH.CHF-2and OH-CHF2surfaces. Later in the reaction, the OH-.CHF2surface is crossed by the H2O.CF-2surface. The only exothermic product channels which could produce electrons from the O-+ CH2F2reaction are found to be H2O.CF2+e-and H2O + CF2+e-. Negative ion and electron relative concentrations determined as functions of temperature in SIFT experiments could be understood in terms of these reactions.

ISSN:0026-8976    

DOI:10.1080/00268979650026659

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor