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  • 11
    ISSN: 0947-6539
    Keywords: ab initio calculations ; aluminum ; boron ; pi interactions ; sulfur ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The structures of H2X—Y—XH2 compounds (X = B, Al; Y = O, S) have been studied by means of ab initio calculations at the MP4/6-311 G**/MP2/6-311G** level. The potential energy surface (PES) of the aluminoxane species H2Al—O—AlH2 is rather flat: the energy differences between the various located extrema are less than 1.5 kcal mol-1, and this shows that the observed geometrical preference is not due to electronic factors. For the sulfur analogues H2X—S—XH2 (X = B, Al), three minima are located on the PES: a planar C2v structure, a Cs structure in which one hydrogen atom bridges the two X atoms, and a C2v structure with two bridging hydrogen atoms. For H2Al—S—AlH2, the Cs structure is the most stable, and the planar C2v and the dibridged structures lie 3.2 and 6.7 kcal mol-1 higher, respectively. Qualitatively similar results are obtained for H2B—S—BH2: the Cs and planar C2v structures are nearly isoenergetic, and the dibridged minimum lies 19.4 kcal mol-1 above the planar C2v minimum. These results are rationalized by analyzing the ability of these systems to bend, depending on the nature (oxygen or sulfur) of the central Y atom. The conjugation in these species is also discussed, and calculations on model systems H2X—YH (X = B, Al; Y = O, S) are presented.
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  • 12
    ISSN: 0947-6539
    Keywords: ab initio calculations ; inorganic polymers ; mechanistic studies ; titanium compounds ; titanoxanes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The controlled ionization of the linear [Cl-Ti-O-Ti-Cl] skeleton allowed the generation of the [Cl-Ti-O-Ti]+ dimer, which is nonsymmetrical as a consequence of extended Cl-Ti-O π interactions. The [Ti=O] unit thus formed is a building block for a variety of titanoxane structures. This chemistry has been investigated from a theoretical point of view by ab initio MO analysis of the [Cl-Ti-O-Ti-Cl] and [Cl-Ti-O=Ti]+ fragments. These calculations lead to the conclusion that single ionization generates the [Ti=O] unit, whereas double ionization does not affect the μ-oxo bonding mode in [Ti-O-Ti]2+ or [S-Ti-O-Ti-S]2+ (where S is a pure σ-donor ligand or solvent). This observation has been confirmed experimentally by ionizing the following model complexes: [(Cl)(acacen)-Ti-O-Ti(acacen)(Cl)] (3) (acacen = N,N'-ethylenebisacetylacetoneiminato dianion) and [(Cl)(salen)Ti-O-Ti(salen)-(Cl)] (4) (salen = N,N'-ethylenebissalicylideneiminato dianion), where the linear Cl-Ti-O-Ti-Cl unit is assured by the square-planar bonding mode of the tetradentate Schiff base ligand. The double ionization of 3 with AgNO3 gave the conventional μ-oxo derivative [(acacen)(η1-ONO2)Ti-O-Ti(acacen)(η1-ONO2)] (5). In contrast, the stepwise ionization of 3 and 4 with NaBPh4 in THF led to the nonsymmetrical [Cl-Ti-O=Ti]+ intermediates, which are the parent compounds for a variety of linear titanoxanes. The following species containing a Ti=O unit have been isolated from the NaBPh4-assisted ionization of 3: [(acacen)Ti=O-BPh3] (6) and [(L)(acacen)Ti=O-Ti(acacen)-O-(acacen)Ti-O=Ti(acacen)(L)]2+2 BPh-4 (L = THF, 7; L = none, 8). The same reaction carried out on 4 led to [(THF)-(salen)Ti=O - Ti (salen) - O - (salen)Ti-(THF)]2+2 BPh-4 (9) and [(L)(salen)-Ti=O-Ti(salen) - O - (salen)Ti-O=Ti-(salen)(L)]2+ 2BPh-4 (L = THF, 10; L = Py, 11; L = none, 12, polymeric form). A scheme is proposed to explain the formation of the species derived from the single ionization of 3 and 4, where the origin and the binding properties of the [Ti=O] unit play a major role.
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  • 13
    ISSN: 0947-6539
    Keywords: ab initio calculations ; binding studies ; crystal packing ; drug design ; hydrogen bonds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In the design of novel protein ligands one of the major challenges is the replacement of functional groups to modify and improve the binding characteristics. Often nitrogen- and oxygen-containing groups are exchanged, or both atoms occur in a competitive situation. We have investigated the hydrogen-bonding abilities of oxygen atoms covalently bound to two non-hydrogen atoms of which at least one is formally assigned to an sp2-type hybridization. In particular, examples in which such oxygen atoms compete with nitrogen atoms in the same molecular segment have been studied. Based on interaction energies obtained from ab initio calculations for complexes of these molecules with water, the oxygen atoms can be classified as rather weak hydrogen-bond acceptors; nitrogen atoms present in the same fragment exhibit much stronger interaction energies. The ab initio results are confirmed by the relative frequencies with which oxygen and nitrogen atoms are found to be involved in hydrogen bonding in the crystal structures of organic molecules containing the fragments of interest.
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  • 14
    ISSN: 0947-6539
    Keywords: ab initio calculations ; annulenes ; semiempirical calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The interconversion between the valence tautomers 1,6-methano[10]-annulene (1) and dinorcaradiene (2) was computationally investigated by qualitative extended Hückel molecular orbital methods (with CACAO visualization) and quantitative semiempirical and ab initio methods. The fundamental bonding interactions are described in terms of perturbation theory arguments for both tautomers, the influence of π-acceptor or π-donor substituents at the C11 position is rationalized, and bonding changes during the interconversion are monitored. The electronic molecular structural preferences can be modified by through-space interactions, and residual C1-C6 σ bonding remains even at the 〉2.1 å separations of the annulenic structures. Hitherto unprecedented calculations of the geometries of C11 homo-disubstituted derivatives of 1 and 2 have been carried out at semiempirical and ab initio levels to identify the more stable tautomer and find whether two stable minima may exist for each derivative. Searches at the AM 1 level to determine transition-state structures for the interconversion of 1 to 2 and of their C11 homo-disubstituted derivatives are also reported.
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  • 15
    ISSN: 0947-6539
    Keywords: ab initio calculations ; gold ; isolobal relationship ; sulfonium trication ; superelectrophiles ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The pentahydridosulfonium trication SH3+5 was found by ab initio MP2/ 6-31G** and QCISD(T)/6-311G** levels of calculation to be a stable minimum with Cs symmetric structure 1. It is isostructural with the parent pentacoordinate carbonium ion CH+5. Structure 1 resembles a complex between SH3+3 and molecular hydrogen forming a 2e-3c bond. This structure involving a pentaco-ordinate sulfur atom is unprecedented. Rotation of the H2 unit around the pseudo-C3 axis of SH3 is facile. The structure of SF3+5 was also calculated and compared with that of 1. In addition, our studies at the ab initio MP2/6-31G** level showed that the parent pentacoordinate oxonium trication OH3+5 is not a minimum on its potential energy surface, and dissociation into OH2+4 and H+ occurs upon optimization.
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  • 16
    ISSN: 0947-6539
    Keywords: ab initio calculations ; benzyldiazonium ion ; dative bonding ; electrostatic interactions ; gas-phase chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Diazonium ions are reactive intermediates in deamination reactions pertinent to chemical carcinogenesis. While methyldiazonium ion has been shown to exist as a short-lived intermediate in the gas phase and in solution, benzyldiazonium ions have never been observed, and the reaction intermediates in deaminations of benzyl systems remain a matter of debate. We therefore studied the benzylcation-dinitrogen interaction by ab initio methods; several important conclusions resulted. Analysis of the potential energy surface at the level MP4(sdtq)/6-31 G*/ MP 2(full)/6-31 G* + ΔVZPE(MP 2(full)/6-31 G*) revealed that a classical “benzyldiazonium ion” does not exist. The interaction of N2 with benzyl cation M-1 results in an electrostatically bound complex 2C with a long C—N distance (2.935 Å) as the most stable structure. A covalently bound planar benzyldiazonium ion 2A with a “normal” C—N bond length (1.514 Å) is the transition-state structure for automerization of the electrostatic complex with concomitant rotation about the exocyclic C—C bond. The potential energy surface characteristics result from the highly efficient π-dative Ph → CH2 bonding in M-1; this is clearly demonstrated in its structure and that of its transition state for rotational automerization TS-1 by the very high activation barrier for rotation (47.6 kcalmol-1!) and by the gradient vector fields of the total electron densities of conformers of 1 and 2. The rotational barrier for 1 is reduced to 27.9 kcalmol-1 in the N2 complex 2, and the potential energy surface characteristics of benzyldiazonium ion essentially facilitate the N2-catalyzed rotational isomerization of benzyl cation. The benzyldiazonium ion system shows for the first time that the interaction of a donor molecule with a carbenium ion with a valence LUMO can lead to the formation of an electrostatic complex as opposed to dative bond formation. Dative σ-bond formation between N2 and the CH2 carbon of 1 is energetically not competitive with dative Ph → CH2 π;-bond formation.
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  • 17
    ISSN: 0947-6539
    Keywords: ab initio calculations ; copper compounds ; organometallic compounds ; tetrahydroborato ligand ; theoretical chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: All-electron ab initio calculations (MP4/MP2 level) have been performed on [Cu(BH4)(PH3)n] (n = 1, 2, 3) complexes. Full-geometry optimizations were carried out in each case, and the stationary points were characterized by the diagonalization of the analytically calculated Hessian matrix. The η2 coordination mode, with a tetrahedral arrangement around the copper atom, is the most stable structure for n = 2, while for n = 3 a strongly nonlinear η1 coordination mode is preferred. These results are in agreement with the experimental data available on related complexes. For n = 1, for which there is no experimental data, the η3 structure turns out to be the most stable. The energy differences associated with some changes in the coordination mode (η1 → η2 for n = 3 and η3 → η2 for n = 1) are small. Finally, a mechanism for the exchange between terminal and bridging hydrogen atoms is proposed for each complex under study.
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  • 18
    ISSN: 0947-6539
    Keywords: ab initio calculations ; benzene ; fulvene ; isomerization ; photochemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Results of semiempirical MN-DOC-CI and ab initio CASSCF calculations reveal that fulvene is a primary product of the photolysis of benzene. The photochemical step is assumed to lead to prefulvene, and both a synchronous conversion and a two-step process via bicyclo[3.1.0]hexa-1,3-diene (isofulvene) could be ruled out as possible pathways from prefulvene to benzene. The most probable mechanism for the photochemical isomerization of benzene to fulvene involves the intermediate structures prefulvene and 1,3-cyclopentadienylcarbene and has to compete with the almost barrierless formation of benzvalene and rearomatization to benzene. The short-lived intermediates are rather flexible structures with negligible barriers to inversion at the radical center in the threemembered ring of prefulvene and prebenzvalene and to rotation around the exocyclic single bond in 1,3-cyclopentadienylcarbene.
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  • 19
    ISSN: 0947-6539
    Keywords: ab initio calculations ; chalcogen compounds ; copper clusters ; ligand stabilisation ; thermodynamics ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The results of ab initio calculations for the sulfur-bridged copper clusters [Cu2nSn(PR3)m] (n = 1-4, 6; m = 0, 2, 4, 6, 8; R=H, CH3) were compared with those of theoretical investigations of the selenium-containing analogues that have been recently reported. The theoretical work was carried out in order to find a possible explanation for the experimentally observed-different-properties of both cluster types. The structural principles turned out to be independent of the chalcogen, except the necessary shortening of the copper-chalcogen atomic distance that results from the formal substitution of sulfur for selenium. Comparison of the energetics of the sulfur- and selenium-containing compounds showed that it is necessary to discuss the influence of the tertiary phosphine ligands that protect the existing clusters from reaction to give the solids Cu2S and Cu2Se. However the results of the calculations suggest that the different thermodynamic data of the cluster core and the Cu—P bonds-at least if PH3 or PMe3 are taken into account-cannot be the only cause for the differences in the experimental behaviour. The responsibility rests rather with kinetic effects, such as the steric demand of the actual ligands or the activation energy of decomposition of the ligand shell during the cluster-forming reaction.
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  • 20
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 89-98 
    ISSN: 0947-6539
    Keywords: ab initio calculations ; Cambridge structural database ; fluorine compounds ; hydrogen bonds ; protein data bank ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Statistical analysis of structural data and detailed inspection of individual crystal structures culled from the Cambridge Structural Database and the Brookhaven Protein Data Bank show that covalently bound fluorine (in contrast to anionic fluoride) hardly ever acts as a hydrogen-bond acceptor. The weakness of covalently bound fluorine as hydrogen-bond acceptor is backed by results of new molecular orbital calculations on model systems using ab initio intermolecular perturbation theory (IMPT), and is in accord with results of other physicochemical studies and with the physical properties of fluorinated organic compounds. Factors influencing the strength of hydrogen bonding in extended systems are discussed.
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