ISSN:0026-8976    

DOI:10.1080/002689797170888

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Relativistic effective core potential calculations have been employed in the framework of a spin–orbit configuration interaction to compute the lowest-lying electronic states of the CaBr2+and CaI2+dications, and the results are compared with the data for the isovalent CaCl2+system studied earlier. The ground X2Π state in all three dications arises from a strong polarization of X(2P°)(X= Cl, Br or I) by the Ca2+(1S) ion and is bound by 0·96–1·55eV with respect to the corresponding diabatic dissociation limits. It is split by the spinorbit interaction into the X12Π3/2and X22Π1/2components, with the energy splittings calculated to be 647 cm-1(CaCl2+), 2115 cm-1(CaBr2+) and 3544 cm-1(CaI2+). The X1and X2states are found to be thermodynamically stable in CaCl2+and CaBr2+, while in CaI2+the lowest dissociation limit, Ca++(2S)+ I+3P2), lies 1700 and 5200 cm-1lower than the X1and X2minima respectively. The X1and X2states in CaI2+are extremely long-lived, however, owing to the high and very broad potential barriers to dissociation. The first electronic excited state, A2σ+, is also bound in all the above systems, although it is pre-dissociated in CaBr2+and CaI2+at large internuclear distances. All other low-lying electronic states of CaX2+are repulsive. Electric-dipole moments are calculated for the A→ X1, X2transitions. The corresponding radiative lifetimes are found to be very long in CaCl2+: τ(A→X1) = 19·3 ms and τ(A→X2) = 9·9 ms (the values are given forν'= 0), and become very significantly shorter for CaBr2+and CaI2+because of the stronger spin-orbit interaction in the heavier systems. This effect is especially noticeable for the A →X2transitions, for which the values are computed to be 364 μs in CaBr2+and 50·3 μs in CaI2+. The theoretical data obtained should aid in the future spectroscopic detection of these species. To data no experiment of this type has been successfully carried out for any of the thermodynamically stable diatomic dications.

ISSN:0026-8976    

DOI:10.1080/002689797170897

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The mechanisms of several types of azoxy compound (XN(O)NY) rearrangement reactions have been studied using density functional theory (DFT) with the B3LYP exchange-correlation potential. The substituents X and Y are taken from the set H, CH3,F,C6H5, Cl and CN. The 6-311 + G(d,p) basis set was used to optimize up to ten equilibrium and transition state structures for a given pair of X and Y substituents; except for azoxybenzene where a 6-311(+)G(d) basis set was used. All geometric structures were characterized by a frequency calculation. The reaction path for converting XNα(O)NβY to XNαNβ(O)Y via a concerted intramolecular shift of the oxygen atom from Nαto Nβinvolves three equilibrium and two saddle-point structures. The relatively high (about 70-80 kcalmol1) calculated barrier height to reaction and its independence of the nature of the X and Y substituents is attributed to characteristic orbital, atomic charge and structural factors along the reaction path.Cis↔transisomerization across N=N in the NH(O)NH azoxy compound and across N–N in the ring XNONY oxadiaziridine intermediate is found to have a barrier height of at least about 30 kcalmol-1. Single-point CCSD(T)/DFT energy differences are found to be somewhat smaller than the DFT calculated values.

ISSN:0026-8976    

DOI:10.1080/002689797170905

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Atoms and molecules subjected to a constant circularly polarized light beam carry a timeindependent current density proportional to the intensity. The direction of the current is opposite for right and left polarization. As a result there is an induced magnetic moment, which is the inverse Faraday effect, and a magnetic field at a nucleus. Formulae are derived for the current density and the induced magnetic field at a nucleus; an equivalence is established between this field and the antisymmetric polarizability resulting from the nuclear magnetic moment. The shifts in the NMR frequencies of1H,13C,19F and205Tl due to a continuous-wave 10 W cm-2beam are estimated to be 0·2 to 50 μHz for right circular polarization at transparent wavelengths, with larger shifts for near-resonance conditions. If the full width at half maximum height of an electric-dipole allowed transition were 10 cm-1, the circularly polarized radiation at resonance would cause shifts of the order of a few hundred hertz for19F. Shifts are expected for all magnetic nuclei. In a beam of sodium atoms the 10 W cm-2circularly polarized light, whose frequency is 6 GHz from resonance with a yellow D line, is predicted to induce an NMR shift of the order of 1 MHz.

ISSN:0026-8976    

DOI:10.1080/002689797170914

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The size dependence of ionization potentials, electron affinities and cohesive energies of small Hgn(n= 3,4,5,6,8,13, 15) clusters has been studied using a combination of relativistic energy-consistent pseudopotentials supplemented by core-polarization potentials and highly correlated valence wavefunctions. In order to ensure a size extensive treatment of electron correlation, coupled-cluster and quantum Monte Carlo calculations have been performed, and excellent agreement observed between the results obtained with the two methods. The experimentally observed systematic trends of these properties with respect to increasing cluster size are reproduced well.

ISSN:0026-8976    

DOI:10.1080/002689797170923

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A total of 22 substituted benzene radicals are investigated by means of density functional theory (DFT). Two different functional schemes, the B3LYP hybrid DFT-HF functional and the ‘pure’ gradient corrected DFT functional PWP86, previously employed successfully in calculations of radical hyperfine parameters, are employed. The results are compared with experimental data. The same strategy is used for all calculations: initial geometry optimization using a double-ζ basis set, followed by single point hyperfine calculations using the larger IGLO-III, 6-311G(2df, p) and aug-cc-pCVTZ basis sets. It is found that of the two functionals, the PWP86 functional gives the more homogeneous data, with nearly constant underestimations in HFCCs by 5–15%. With the B3LYP approach the smaller couplings, found at the meta positions, generally are overestimated considerably. The best overall performance is observed at the PWP86/6-311G(2df, p) level.

ISSN:0026-8976    

DOI:10.1080/002689797170932

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Hybrid quantum mechanical/molecular mechanical potentials have proved to be powerful tools for the simulation of many processes in condensed phase systems and, as a result, there is much current research into how they can be improved. An area of recent attention has been the inclusion of polarization effects on the atoms in the molecular mechanical region which have been shown to be important for the calculation of certain properties. One way to treat such effects is with a dipole polarizability model. However, an alternative and, in many ways, simpler method is available that is based upon the principle of electronegativity equalization. In this paper a number of hybrid potentials are presented that employ a fluctuating charge model and electronegativity equalization to describe the electrostatic interactions in the molecular mechanical region of the system. The models are straightforward to implement and their use is illustrated with applications to some simple solution phase systems.

ISSN:0026-8976    

DOI:10.1080/002689797170941

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

An exchange functional is proposed which is conceptually similar to the established Becke functional and satisfies the same asymptotic properties. The one-parameter variant A of this new functional yields atomic exchange energies which are very close to those from the optimized potential model (OPM) for the light elements (up to Ne), whereas the three-parameter variant B reproduces the OPM reference values for all elements (H to Xe) more accurately than other established exchange functionals. For molecular applications, the new exchange functional is combined with a new non-local correlation functional based on a Coulomb hole model. Detailed results are reported for the molecules from the G2 data base. For each of the four basis sets considered, variants A and B yield essentially the same atomization energies, ionization potentials, proton affinities and molecular geometries. The mean absolute deviation between the calculated and experimental atomization energies for the G2 reference molecules is 3 kcalmol-1. In an overall assessment, the present results seem comparable in quality with those from the B3LYP hybrid functional, even though our new functional does not include any Hartree–Fock exchange. Some illustrative results for transition-metal compounds are also given.

ISSN:0026-8976    

DOI:10.1080/002689797170950

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A full configuration interaction study on the BH molecule is presented. The potential energy curves of 20 different electronic states have been calculated correlating the four valence electrons. On the two most important states, i.e. the X1Σ+and A1Π states, a complete study has been performed. This includes the effect of core electron correlation, estimated via truncated configuration interaction techniques. The dissociation energy of the molecule in the two states and the height of the predissociative barrier in the A1Π state have been determined with basis sets of increasing quality.

ISSN:0026-8976    

DOI:10.1080/002689797170969

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The local temperatureT(r) is studied in molecules and compared with the Politzer average local orbital energy index. Computed values of the two functions are plotted along various spatial directions for the atoms C and O and for the molecules CO, CO2, C2H2, C2H4, HCN. Except for the bond regions the two properties are found to behave similarly, characterizing the structures of the molecules and suggesting thatT(r) is also a possible indicator of chemical reactivity towards electrophiles.T(r) is also found to be more 'sensitive' to the bond regions and to distinguish between double bonds and triple bonds.

ISSN:0026-8976    

DOI:10.1080/002689797170978

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor