All electron calculations of open-shell polyatomic molecules. III. Perturbation treatment of the spin polarization in vinyl and methyl radicals

A first order perturbation treatment starting with SCF-MO 'S in canonical or equivalent quasi-localized form is presented for the hyperfine coupling constants of vinyl and methyl radicals. The spin-polarisation contribution to hyperfine splittings is found to be large, negative for the proton of the radical center in both radicals and positive for the β protons of vinyl.     

Structure électronique et spectres de resonance paramagnétique électronique des radicaux vinyle et cyclopropyle

Résumé Une méthode de calcul dérivant des théories de champ self-consistant est utilisé pour l'étude des radicaux vinyle et cyclopropyle. L'énergie électronique et les écarts hyperfins dûs aux hydrogènes et au ¹³C ont été évalués en fonction de la géométrie du carbone radicalaire. L'angle C=-H du radical vinyle a été trouvé de l'ordre de 150°. Il est montré que les radicaux vinyle et cyclopropyle ont une structure électronique similaire qui ne peut être totalement expliquée ni par un modèle ni par un modèle classique, chaque radical présentant à la fois les deux caractères.

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A method of calculation derived from self-consistent field theories is used for a study of vinyl and cyclopropyl radicals. Electronic energy, hydrogens and ¹³C hyperfine splittings are evaluated as a function of the carbon geometry. The C=-H bond angle is found to be about 150°. It is shown that vinyl and cyclopropyl radicals have a similar electronic structure which cannot be completely interpreted neither by a classical model nor by a one, each radical presenting both and character.

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Zusammenfassung Nach einer auf dem SCF-Verfahren basierenden Methode werden Elektronenenergie, H- und ¹³C-Hyperfeinaufspaltung in Vinyl- und Cyclopropylradikalen in Abhängigkeit vom Bindungswinkel am berechnet, der sich im ersten Fall zu 150° ergibt. Es zeigt sich, daß die Elektronenstruktur der beiden behandelten Radikale ähnlich ist und daß sie weder durch ein reines - noch ein reines -Modell zu beschreiben ist.

     

Comprehensive photokinetic and NMR study of a biphotochromic supermolecule involving two naphthopyrans linked to a central thiophene unit through acetylenic bonds

A photophysical and photochemical study of a biphotochromic compound where two naphthopyran units are linked by an acetylene-thiophene-acetylene bridge has been carried out in toluene. Both fluorescence and intersystem crossing to the triplet manifold were found to compete with the photocoloration process. Two photoproducts (transoid-trans and transoid-cis stereoisomers), absorbing at approximately 480 nm and corresponding to the opening of a single photochromic unit, were detected by spectrophotometric analysis after short irradiation time in diluted solution and identified by 1H-nuclear magnetic resonance (NMR) spectroscopy. After prolonged irradiation at 228 K of highly concentrated solutions (up to 3 x 10(-3) mol dm(-3)), two additional isomers, absorbing at approximately 550 nm, were formed. Their NMR spectra indicate the opening of both photochromic units. An interesting effect of selective vibronic excitation was found, showing that the photoreaction is favored at excited vibronic levels to the detriment of the radiative relaxation.     

Force field for platinum binding to adenine

The antitumor drug cis-diamminedichloroplatinum(II) (cisplatin) binds preferentially to GpG and ApG sequences of DNA, forming N7,N7 intrastrand chelates. Molecular modeling of the intrastrand adducts have been handicapped, so far, by the lack of force-field data describing the Pt–guanine and Pt–adenine binding. We used ab initio calculations with relativistic pseudopotentials to evaluate three important parameters for the platinum–adenine model complex [Pt(NH₃)₃(Ade)]²⁺: (1) the force constant for the Pt? N7 bond bending out of the adenine plane; (2) the energy profile for the torsion about Pt? N7; (3) a set of fractional atomic charges that reproduce the ab initio potential for a number of space points placed around the adduct. A population analysis and comparative study on the tetrammine complex [Pt(NH₃)₄]²⁺ have shown that for platinum adenine is a better σ-donor than NH₃, but its capacity as a π-acceptor is weak. © 1993 John Wiley & Sons, Inc.     

Calcul de constantes de couplage nucléaires spin-spin par la méthode des orbitales moléculaires à l'aide de fonctions d'onde non-empiriques

Résumé Un calcul complet de constantes de couplage nucléaires J _HH, J _XH et J _XX (X = C, N, P) a été fait pour une série de molécules organiques dont les fonctions LCAO SCF MO exactes sont connues. L'influence de la charge nucléaire efficace des orbitales atomiques de base a été étudiée dans le cas particulier du méthane et de l'éthane. La décomposition des constantes de couplage en éléments de symétrie est employée pour la comparaison des résultats théoriques et expérimentaux.

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Numerical calculations of spin-spin coupling constants with exact LCAO SCF MO functions

Complete calculations of nuclear spin-spin coupling constants J _HH, J _XH and J _XX (X = C, N, P) have been carried out for some organic molecules the exact LCAO SCF MO functions of which are known. The results obtained using atomic basis sets with different orbital exponents are discussed in the case of methane and ethane. Theoretical values are compared to experimental data in terms of components arising from the symmetry analysis of actual coupling constants.

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Zusammenfassung VollstÄndige Berechnungen der Spin-Spin-Kopplungskonstanten J _HH, J _XH und J _XX (X = C, N, P) werden für einige organische Moleküle durchgeführt, deren genaue LCAO-SCF-MO-Funktionen bekannt sind. Der Einflu\ der effektiven Kernladungszahlen der Basisfunktionen wurde im Falle von Methan und Äthan untersucht. Die theoretischen und experimentellen Werte werden nach ihrer Zerlegung in Symmetriekomponenten miteinander verglichen.

     

Pressure‐Induced Intersite Bi?M (M=Ru, Ir) Valence Transitions in Hexagonal Perovskites

Pressure‐induced charge transfer from Bi to Ir/Ru is observed in the hexagonal perovskites Ba_3+nBiM_2+nO_9+3n (n=0,1; M=Ir,Ru). These compounds show first‐order, circa 1 % volume contractions at room temperature above 5 GPa, which are due to the large reduction in the effective ionic radius of Bi when the 6s shell is emptied on oxidation, compared to the relatively negligible effect of reduction on the radii of Ir or Ru. They are the first such transitions involving 4d and 5d compounds, and they double the total number of cases known. Ab initio calculations suggest that magnetic interactions through very short (ca. 2.6 Å) M M bonds contribute to the finely balanced nature of their electronic states.     

Almost automorphic solutions of evolution equations

This paper is concerned with the existence of almost automorphic mild solutions to equations of the form

where generates a holomorphic semigroup and is an almost automorphic function. Since almost automorphic functions may not be uniformly continuous, we introduce the notion of the uniform spectrum of a function. By modifying the method of sums of commuting operators used in previous works for the case of bounded uniformly continuous solutions, we obtain sufficient conditions for the existence of almost automorphic mild solutions to in terms of the imaginary spectrum of and the uniform spectrum of .

     

Resonance expansions in quantum mechanics

The goal of this contribution is to discuss various resonance expansions that have been proposed in the literature. Presented at the 3rd International Workshop “Pseudo-Hermitian Hamiltonians in Quantum Physics”, Istanbul, Turkey, June 20–22, 2005.