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Showing papers in "Journal of the American Chemical Society in 1994"





Journal ArticleDOI
TL;DR: In this paper, the authors combine high-level ab initio quantum chemical calculations with a continuum description of the solvent to obtain accurate solvation free energies of organic solutes in water.
Abstract: In this paper, we combine high-level ab initio quantum chemical calculations with a continuum description of the solvent to obtain accurate solvation free energies of organic solutes in water. By using correlated wave functions at the generalized valence bondperfect pairing (GVB-PP) level, we are able to efficiently produce accurate gas-phase charge distributions. These are then used to obtain solvation energies in a self-consistent formalism which cycles through quantum chemical calculations in the solvent reaction field and continuum electrostatic calculations utilizing polarized solute charges. An average error of 0.6 kcal/mol for solvation energies is obtained for 29 molecules. A systematic discrepancy between theory and experiment is obtained for the difference in solvation free energy between several methylated and unmethylated primary amines and amides. This poses a major puzzle in theoretical modeling of solvation effects.

1,022 citations


Journal ArticleDOI
TL;DR: In this article, the authors classified O-H-O hydrogen bonds into three fundamental classes: negative charge assisted hydrogen bonding (CAHB), positive charge assisted HO bonding (PHA), and resonance-assisted HO bonding.
Abstract: All cases of strong (2.50≤d(O--O)≤2.65 A) and very strong (d(O--O) <2.50 A) O-H---O hydrogen bonds whose geometries are known from accurate neutron or X-ray diffraction studies are reviewed and classified in chemical classes belonging to three fundamental types: (A) -O-H---O-, or negative charge assisted hydrogen bonding, (-)CAHB; (B)=O---H + --O=, or positive charge assisted hydrogen bonding, (+)CAHB; and (C) -O-H---O=, where the two oxygens are interconnected by a system of π-conjugated double bonds, or resonance-assisted hydrogen bonding, RAHB

990 citations


Journal ArticleDOI
TL;DR: In this paper, an all-atom CFF93 force field for polycarbonates based on ab initio calculations is reported, and the force field parameters are derived by fitting to quantum mechanical total energies, first and second derivatives of total energies and electrostatic potentials, all generated from AB initio quantum mechanical calculations on model compounds at HF/6-31GS level of theory.
Abstract: An all-atom CFF93 force field for polycarbonates based on ab initio calculations is reported. Force field parameters are derived by fitting to quantum mechanical total energies, first and second derivatives of total energies, and electrostatic potentials, all generated from ab initio quantum mechanical calculations on model compounds at HF/6-31GS level of theory. Valence parameters and ab initiocharges are then scaled to correct for differences between experiment and the Hartree-Fock approximation. The van der Waals parameters and the scaling factors for atomic partial charges are determined from crystal structures. Based on the force field, molecular mechanics calculations are performed for several model compounds, and the results are compared with experimental values and with the results of the ab initio calculations.

933 citations






Journal ArticleDOI
TL;DR: In this article, the structural and chemical properties of a series of electron-deficient cationic zircocene alkyl and hydrido complexes with high olefin polymerization activities were studied.
Abstract: We have isolated and systematically studied the structural and chemical properties of a series of electron-deficient cationic zircocene alkyl and hydrido complexes with high olefin polymerization activities. The single crystal X-ray diffraction-derived solid state and NMR-derived solution structures of these complexes correlate well with each other and afford considerable insight into the nature of these species. Through the use of the novel organoborane, we have, for the first time, unambiguously demonstrated the role of the key metallocene-cocatalyst interaction that has been proposed for MAO, alkyl aluminum halides, and dehydroxylated alumina. 56 refs., 10 figs., 19 tabs.







Journal ArticleDOI
TL;DR: A covalently closed molecular complex whose double-helical edges have the connectivity of a truncated octahedron has been assembled from DNA on a solid support as discussed by the authors.
Abstract: A covalently closed molecular complex whose double-helical edges have the connectivity of a truncated octahedron has been assembled from DNA on a solid support. This three-connected Archimedean solid contains six squares and eight hexagons, formed from 36 edges arranged about 24 vertices. The vertices are the branch points of four-arm DNA junctions, so each vertex has an extra exocyclic arm associated with it. The construct contains six single-stranded cyclic DNA molecules that form the squares and the extra arms; in addition, there are eight cyclic strands that correspond to the eight hexagons. The molecule contains 1440 nucleotides in the edges and 1110 in the extra arms; the estimated molecular weight for the 2550 nucleotides in the construct is 790 kDa



Journal ArticleDOI
TL;DR: In this paper, the aziridination of olefins was evaluated and found to be both catalyst and substrate dependent, and it was concluded that the olefin selectivity profile for the reaction is independent of the oxidation state of the copper catalyst employed.
Abstract: Soluble Cu(1) and Cu(I1) triflate and perchlorate salts are efficient catalysts for the aziridination of olefins employing (N-@-tolylsulfonyl)imino)phenyliodinane, PhI=NTs, as the nitrene precursor. Electron-rich as well as electron-deficient olefins undergo aziridination with this reagent in 55-958 yields, at temperatures ranging from -20 OC to +25 OC. The catalyzed nitrogen atom-transfer reaction to enol silanes and silylketene acetals has also been developed to provide facile syntheses of a-amino ketones. Other metal complexes were found to be less effective at catalyzing the reaction, while PhI=NTs proved to be superior to other imido group donors as the nitrene precursor. Reaction rates and yields are enhanced in polar aprotic solvents such as MeCN and MeN02. Reaction stereospecificity in the aziridination of cis and trans disubstituted olefins was evaluated and found to be both catalyst and substrate dependent. Intermolecular competition experiments between pairs of monoand disubstituted olefins indicate that the olefin selectivity profile for the reaction is independent of the oxidation state of the copper catalyst employed. It is concluded that these reactions are proceeding through the 2+ catalyst oxidation state under the conditions employed





Journal ArticleDOI
TL;DR: In this article, an appropriate synthetic array of pigments could function as a molecular photonic wire and a boron-dipyrromethene dye provided an optical input at one end, a linear array of three zinc porphyrins is employed as a signal transmission element, and a free base porphrin provides an optical output at the other end, combining modular building blocks in a stepwise manner using coupling reactions similar to those in their synthesis of multi-porphyrin light harvesting arrays.
Abstract: We felt that an appropriate synthetic array of pigments could function as a molecular photonic wire The structure of the molecular photonic wire is shown A boron-dipyrromethene dye provides an optical input at one end, a linear array of three zinc porphyrins is employed as a signal transmission element, and a free base porphyrin provides an optical output at the other end The synthesis combines modular building blocks in a stepwise manner using coupling reactions similar to those in our synthesis of multi-porphyrin light-harvesting arrays 14 refs, 2 figs




Journal ArticleDOI
TL;DR: Bis(cycIopentadienyl)titanium(III) chloride reacts with epoxides by initial C-0 homolysis and the regiochemistry of the opening is determined by the relative stabilities of the radicals as discussed by the authors.
Abstract: Bis(cycIopentadienyl)titanium(III) chloride reacts with epoxides by initial C-0 homolysis. The regiochemistry of the opening is determined by the relative stabilities of the radicals. Depending on the reaction partners, these radicals undergo intramolecular (hex-5-enyl cyclization) or intermolecular additions to olefins. The resultant radicals are efficiently scavenged by a second equivalent of Ti(II1) to afford the corresponding Ti(1V) derivative. Treatment of this intermediate with electrophiles such as H+ or halogens provides a route to functionalized cyclopentanes and other useful products. The radical initially formed from an epoxide can also be trapped by H-atom donors such as 1,4- cyclohexadiene or tert-butyl thiol, resulting in an overall reduction of the epoxide. In the absence of a H-atom donor or an olefin, this radical is trapped by Ti(", resulting in a fl-oxido-Ti organometallic species which undergoes facile elimination to give an olefin. The reaction conditions are remarkably mild and are applicable to very sensitive substrates. The considerable utility of epoxides as building blocks for organic synthesis reflects both their ready availability and their ability to undergo selective nucleophilic substitution reactions (eq la) with predictable stereochemistry.' In contrast, the two-