摘要:

A kinetic energy analysis of total energy differences in 822 atomic multiplets arising fromsmdn(m = 0,1,2;n = 2–8) electronic configurations is performed within the nonrelativistic, restricted Hartree–Fock framework. For the 444 multiplets arising from thednands2dnconfigurations of 27 atoms in groups 2–10, a very good linear correlation between the total energy difference and the kinetic energy difference of the outermostd-electrons is demonstrated. For the 378 multiplets arising from thesdnconfiguration, on the other hand, a good linear correlation is obtained provided that the multiplets are classified into groups based on spin multiplicity.Keywords: kinetic energy, atomic multiplets,smdnconfigurations, Hartree–Fock approximation.

ISSN:0008-4042    

DOI:10.1139/v96-085

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

The dynamic second hyperpolarizability for real frequencies,in the limit ωi → 0 can be expressed aswhere ωL2 = ωσ2 + ω12 + ω22 + ω32andis the frequency-independent (static) quantity; the parallel sutsscriptindicates that the polarization and electric fields all lie along the same axis. In this paper the coefficientis evaluated exactly for the H atom and very accurately for H−, He, and Li+. A similar analysis is carried out forin the limit ω → ∞.Keywords: nonlinear optics, hyperpolarizabilities, dispersion formulas.On peut exprimer la deuxième hyperpolarisabilité dynamique de fréquences réelles,dans la limiteωi → 0, peut être exprimée sous la forme, dans laquelle ωL2 = ωσ2 + ω12 + ω22 + ω32etest la quantité (statique) indépendant de la fréquence; l'indice parallèleindique que la polarisation et les champs électriques sont situés dans le même axe. Dans ce travail, on a fait une évaluation exacte depour l'atome d'hydrogène et d'une façon précise pour H−, He et Li+. On a effectué une analyse semble pourdans la limite ω → ∞.Motsclés: optique non linéaire, hyperpolarisabilités, formules de dispersion. [Traduit par la rédaction]

ISSN:0008-4042    

DOI:10.1139/v96-086

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Properties of the bond critical points, including the electron density and its Laplacian, and distances to bonded nuclei, for all species (reactants, transition states, and products) in the reactions of the hydroxyl radical with the series C2HnF6−nn = 1–6 were calculated using Bader's atoms-in-molecules methodology. The electron density and its Laplacian at the bond critical points correlate with bond strength, as measured by bond dissociation energies. The positions of the bond critical points vary with the electronegativity of surrounding atoms. Charge development in the course of the reactions was monitored using Mulliken population analysis at the HF/6-31G(d), HF/6-311G(d,p)//HF/6-31G(d), MP2/6-311G(d,p)//HF/6-31G(d), and HF/6-311G(d,p)//MP2/6-31G(d,p) levels of theory, and natural population analysis and Bader population analysis at the highest common level of theory, MP2/6-311G(d,p)//HF/6-31G(d). In general, there is a buildup of charge in the transition states, concentrated near the reaction centre, which dissipates somewhat as the reaction proceeds to products. The description of charge transfer varies somewhat with the three methods.Keywords: topological properties, atomic charges, charge development, reactions of the hydroxyl radical with fluorinated ethanes.

ISSN:0008-4042    

DOI:10.1139/v96-087

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Ab initio calculations at the MP2/3-21G(*) level of theory have been carried out for the magnesium–anthracene complexes 9,10-magnesiumanthracene•3H2O (1) and the 9-methyl (2), dimethyl (3), and 9,10-bis(methylsilyl) (4) substituted derivatives. The theoretically predicted geometries of the anthracene ligands are also reported. The calculated geometries of1–4are in very good agreement with experimental values for the corresponding THF complexes. The Mg—C9,10bonds of the bridged structures are rather long and the anthracene ligands are folded by ~40° along the C9–C10line in the complexes. Analysis of the electronic structure shows clearly that the Mg—C9,10bonds should be considered as purely ionic. This is revealed by topological analysis of the electron density distribution and its associated Laplacian. The electron density at the Mg—C9,10bond critical points ρ(rb) is very low and the Laplacianand the energy densityHbhave positive values. The ionic nature of the Mg—C9,10bond is also indicated by the natural bond order (NBO) analysis, which gives a Lewis structure with two lone pairs at C9and C10but no Mg—C9,10bonds. The NBO method gives a charge donation from Mg to the anthracene ligand of nearly two. The theoretically predicted NMR chemical shifts using the GIAO method give13C resonances for the complex1and for anthracene and anthracene dianion that are in good agreement with experimental values.Keywords: magnesium–anthracene complexes, ab initio calculations, analysis of magnesium–carbon bonding.

ISSN:0008-4042    

DOI:10.1139/v96-088

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Dithia- and diselena-diazolyl radicals (HCN2E2E = S, Se) and dimers are important building blocks in the design of low-dimensional molecular conductors. Research on the tellurium-based analogues is much rarer. This work reports the molecular and electronic structures of the cation, radical, and radical dimers of 1,2,3,5-ditelluradiazolyl using ab initio theory including electron correlation by Møller–Plesset perturbation theory up to partial fourth order (MP4SDQ). A face-to-faceC2vdimer is predicted to be bound with respect to two radicals by approximately 18 kcal/mol. AC2h, dimer also has been studied and is ca. 2 kcal/mol less stable than theC2vconformer. Relaxing symmetry constraints on the dimers led to more energetically stable structures at the Hartree–Fock level but theC2vstructure remains the most stable at a level of theory including electron correlation effects. The results for the Te compounds along with our earlier research on the S and Se analogues provide predictions for the geometries, vibrational frequencies, and ionization potentials for the Te species to assist in future experiments.Keywords: tellurium, ab initio, ditelluradiazolyl, dimers, binding.

ISSN:0008-4042    

DOI:10.1139/v96-089

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

A general theory of the effect of dynamics (relaxation and (or) exchange) on NMR spectra is presented. This theory is based on a reexamination of the transition probability. The classic expression for this is as the square of the transition moment, but we feel it is useful to separate the square into two separate terms. In the generalization presented here, we show that one of these terms corresponds to the share of the initial magnetization that each spin coherence receives at the start of the experiment. The second term is how much that coherence contributes to the total detected signal. The final intensity is the product of these two factors. For a static spectrum, these two terms are complex conjugates, so the product is real and we recover the standard transition probability. When there is dynamics, the product becomes complex, so the time evolution includes oscillatory and dispersive terms. This means that a dynamic spectrum is still a sum of individual transitions, but the lineshapes are distorted in phase, intensity, position, and linewidth by the dynamic process. In this paper we develop the general theory, and illustrate it with a calculation of the classic problem of mutual exchange in an AB spin system.Keywords: NMR spectroscopy, transition probability, chemical exchange, kinetics.

ISSN:0008-4042    

DOI:10.1139/v96-090

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

New results on nonadditive effects on small beryllium clusters are presented using the LCAO-MO-SCF method including variational and perturbational (MP-4) configuration effects. Three- and four-body contributions to the interaction energy of these clusters are dominant, and are used to explain the very different behaviour of bulk beryllium as compared to liquid helium. This is relevant because the atomic ground states of Be and He are strikingly similar (1s22s2:1S0and 1s2:1S0) and also He2and Be2share the honour of being the two hardest-to-detect dimers in the whole periodic system. The similitude is lost for larger Henand Bensystems because the former, as was shown by R.F.W. Bader 25 years ago, have very small three-body energies (≤ 1%). Benclusters will be shown here to have three-body energies that actually outweigh the pairwise additive ones. The consequences of this are discussed.Keywords: intermolecular forces, non-additivity, He and Be clusters.

ISSN:0008-4042    

DOI:10.1139/v96-091

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Using differential geometry, we propose the total curvature of interatomic surfaces to characterize bonds. In this way visual interpretations of interatomic surfaces are now rigorously quantified. The analysis of the intrinsic geometry of an interatomic surface is implemented in the programMORPHY2.0. It is shown that the total curvature depends on anionic polarizability, electronegativity differences, and steric effects determined by the total chemical environment of the bonded atoms in question. In general the proposed index measures the external chemical distortion of an atom in a molecule. It can be used in the context of uniform electric fields and in conformational studies.Keywords: interatomic surfaces, differential geometry, total curvature.

ISSN:0008-4042    

DOI:10.1139/v96-092

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

All Diels–Alder reactions between 1,3-butadiene and cyclopentadiene or 2H-phosphole have been examined at the MP4SDQ/6-31G*//HF/6-31G* level. There is remarkable similarity between the two systems. The thermodynamic product is the bicyclo[4.2.0]nonadiene while the kinetic product is the norbornene product. There is a slight kinetic preference for theendoaddition and for the butadiene to be in thes-transconformation. Except for the case where butadiene is the diene component and addition isendo, the reactions are concerted and synchronous. In these other two cases, the reaction is stepwise with a diradical intermediate.Keywords: phosphole, Diels–Alder reaction, topological electron density analysis.

ISSN:0008-4042    

DOI:10.1139/v96-093

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Oscillator strengths for C 1s, N 1s, and O 1s excitation spectra of aniline, nitrobenzene, and the isomeric nitroanilines have been derived from inner-shell electron energy loss spectroscopy recorded under low momentum transfer conditions (> 2.5 keV impact energy and small scattering angle, θ ≤ 2°). Extended Hückel Molecular Orbital (EHMO) calculations carried out within the equivalent core analogy are used to aid spectral interpretation. These spectra are used to investigate the sensitivity of core excitation spectroscopy to charge transfer interactions in aromatic molecules that have both electron-donating and electron-withdrawing substituents. Strong multielectron excitation features were not found, although these had been anticipated from photoemission studies. The C 1s → π* and N 1s (NH2) → π* spectral features of the nitroanilines are found to be strongly dependent on the substitution pattern (ortho,meta, orpara).Keywords: electronic structure, inner-shell excitation, nitroanilines, EHMO calculations.

ISSN:0008-4042    

DOI:10.1139/v96-094

出版商:NRC Research Press    

年代:1996

数据来源: NRC