bluuebunny/crossref_metadata_2025_split huggingface dataset:
Quantum Topology: Theory of Molecular Structure and its Change
Authors: Bader Richard F. W. , Tal Yoram , Anderson Steven G. , Nguyen‐Dang T. Tung 
Published: November 2013

AbstractIn this paper we review and exemplify a new and rigorous approach to the problem of molecular structure and its morphogenesis: the theory of quantum topology. The basis for this approach is provided by the topology of the total charge density in a given molecular system. The essential observation is that the only local maxima of a ground state distribution occur at the positions of the nuclei. The nuclei are therefore identified as point attractors of the gradient vector field of the charge density. The associated basins partition the molecular system into atomic fragments. Each atom is a stable structural unit defined as the union of an attractor and its basin. The common boundary of two neighbouring atomic fragments, the interatomic surface, contains a particular critical point, which generates a pair of gradient paths linking the two neighbouring attractors. The union of this pair of gradient paths and their endpoints is called a bond path. The network of bond paths defines a molecular graph of the system.Having defined a unique molecular graph for any molecular geometry, the total configuration space is partitioned into a finite number of regions. Each region is associated with a particular structure defined as an equivalence class of molecular graphs. A chemical reaction in which chemical bonds are broken and/or formed is therefore a trajectory in configuration space which must cross one of the boundaries between two neighbouring structural regions. These boundaries form the catastrophe set of the system which, like a phase diagram in thermodynamics, denotes the points of “balance” between neighbouring structures. A general analysis of the structural changes in an ABC type system is given in detail together with specific examples of all possible structural elements in a molecular system.The properties of the topologically defined atoms and their temporal changes are identified within a general formulation of subspace quantum mechanics. It is shown that the quantum mechanical partitioning of a system into subsystems coincides with the topological partitioning: both are defined by the same set of “zero flux” surfaces. Consequently the total energy, or any other property, is partitioned into additive atomic contributions.We show that, in general, a definite structure can be assigned to a given molecular system. Quantum mechanically this structure is associated with an open neighbourhood of the most probable nuclear geometry. Finally we generalize the notion of molecular structure to non‐isolated molecules and, in contrast to recent work by Woolley, we conclude that molecular structure exists in spite of intermolecular interactions and not as a result of them.
Models for Active Sites of Metalloenzymes: Comparison of Zinc and Beryllium Containing Complexes
Authors: Osman Roman , Weinstein Harel 
Published: November 2013

AbstractMetal containing models of the active site of the metalloenzyme carboxypeptidase were studied by quantum chemical ab initio calculations. The comparative properties of [Zn(OH)(NH3)2]+ and [Be(OH)(NH3)2]+ indicate some of the characteristics of the role of transition metals in biological systems: the Zn complex, but not the Be complex, generates directionality constraints for an incoming ligand that are compatible with the geometrical features identified in the crystal structure of the active site — substrate complex. Moreover, an incoming ligand (H2O) is perturbed more strongly by the Zn complex than by the Be complex. This perturbation (a charge transfer and a characteristic polarization of the electronic density along the metal–ligand axis) is in good agreement with the proposed effect of the metal in the active site on the reactivity of a carbonyl carbon in a peptide bond. The perturbation caused by the incoming ligand in the electronic structure of the metal containing complex is also larger for the Zn complex than for the Be complex. These findings indicate that while the Be complex may simulate some of the purely electrostatic effects of the Zn containing model of the active site, it does not represent as well as the Zn complex some of the reactivity elements that have been implicated in the role of the metal in the catalytic mechanism of carboxypeptidase.
Observations on the Significance of the Electrostatic Potentials at the Nuclei of Atoms and Molecules
Authors: Politzer Peter 
Published: November 2013

AbstractThe first portion of this paper reviews the development of a series of relationships, both exact and approximate, between the energies of atomic and molecular systems and the total electrostatic potentials at the nuclei in these systems. Mathematical formulations are presented for such properties as the energy of the valence electronic charge of an atom, and the energies associated with the individual atoms in a molecule. An equation is obtained that relates the energy of a molecule to the changes in the electrostatic potentials at the nuclei of the free atoms when they combine to form the molecule. The second portion of the paper discusses some properties of the potentials at nuclei, including their remarkable insensitivity to the environments of the nuclei. It is pointed out that the potentials at nuclei serve as links between theory and various experimental techniques, including electron spectroscopy, nuclear magnetic resonance, and Mossbauer spectroscopy. A general theme of this paper is that the electrostatic potentials at nuclei are a fundamental and unifying property of atomic and molecular systems, which can help to clarify and generalize some very basic concepts.
Quantum‐Mechanical Calculations of Molecular Force Fields at the SCF Ab Initio Level Methyl Halides as Test Cases
Authors: Berthier Gaston 
Published: November 2013

AbstractThe possibilities of the present quantum‐chemistry methods for calculating the vibrational properties of polyatomic molecules have been investigated, taking the CH3X series (X = F, Cl, Br, I) as bench tests. Finite‐difference computations carried out with various basis sets at the SCF ab initio level on methyl fluoride bring out the performance of the LCAO‐MO method as concerns the prediction of molecular force constants and integrated infrared intensities. Extension of these calculations to other halides is made possible using pseudo‐potentials for the interaction between the atomic cores and the valence‐shell electrons of the molecule. The atomic force field parameters defined in terms of Cartesian displacement coordinates suffer from a certain lack of transferability, apparently due to the variations of the chemical bonding effects when passing from fluorine to iodine.
An Improved Molecular Mechanics Force Field for Alcohols and Ethers
Authors: Allinger Norman L. , Chang Scott H.‐M. , Glaser Donna Hindman , Hönig Helmut 
Published: November 2013

AbstractThe MM2 force field has been extended to the title class compounds. Generally, the force field does a good job of calculating structures and energies. A number of interesting conformational problems are examined, including: conformational energies of methyl groups on oxane and 1,3‐dioxane rings, the syn‐axial methyl–hydroxyl interaction and a number of hydrogen–bonding problems, including the bifurcated hydrogen bond in axial‐5‐hydroxyl‐1,3‐dioxane.
Is the Effective Size of an Alkyl Group a Gauge of Dynamic Gearing?
Authors: Hounshell W. Douglas , Iroff Linda D. , Iverson D. J. , Wroczynski R. J. , Mislow Kurt 
Published: November 2013

AbstractThis paper presents a critical discussion of the perceived similarity between alkyl groups and mechanical gears. Specifically, we examine the notion that the availability of “free spaces” between the ligands of the central atom in an alkyl group leads to a reduction in the steric requirements of the group and, hence, to an effect on the rate of conformational interconversion. It is concluded that there is no compelling experimental evidence to support the operation of this type of dynamic gear effect.
The Configurational Stability of Triorganostannyl‐Transition Metal Compounds
Authors: Gielen Marcel , Eynde Ivan Vanden 
Published: November 2013

AbstractThe configurational stability of triorganostannyl–iron, –manganese and –cobalt complexes has been determined by studying the coalescence of the NMR signals of diastereotopic groups and by looking at the epimerization of diastereomeric mixtures of these compounds. Possible substitution reactions, transforming the studied derivatives into one another, have been checked.
Carbon‐13 Variable Temperature NMR Studies of Some Dimethyl‐1‐oxa‐3,4‐diazacyclohexanes1
Authors: Katritzky Alan R. , Patel Ranjan C. , Brito‐Palma Fernando M. S. , Riddell Frank G. , Turner Eric S. 
Published: November 2013

AbstractConformational equilibria are elucidated and rationalised for 3,4‐dimethyl‐1‐oxa‐3,4‐diazacyclohexane, and a trans‐bicyclic analogue. The major conformer has 3ax‐NMe, 4eq‐NMe: two minor conformations are also significantly populated. Kinetic and thermodynamic parameters are correlated with those for analogous heterocycles.
13C‐NMR Spectroscopic and Conformational Studies on Rotations in Substituted Ethanes
Authors: Freitag Wolfgang , Schneider Hans‐Jörg 
Published: November 2013

AbstractPossibilities and results of13C‐DMR line shape analysis are explored with 1X‐2,2‐dimethylpropanes (neopentanes,1), 2X‐3,3‐dimethylbutanes (2) and 2X‐2,3,3‐trimethylbutanes (3) with X = H, Hal, OR or CH3. The observed barriers ΔG* consistently increase in this sequence, e.g. for X = Cl from 6.5 (1) to 7.7 (2) to 10.4 (3) kcal/mol. Molecular mechanics force field calculations for1, although giving barriers too low by 0.5–1.5 kcal/mol, correctly reproduce the observed trends and explain the puzzling low barriers observed for X = CH3in comparison to X = Hal, as the result of the higher ground state energy of the methyl compounds. Conformational equilibria are investigated with the non‐topomeric 2X‐2,3‐dimethylbutanes. The ΔG0values obtained from low temperature signal integration agree with force field predictions only for X = H and F, which dominate, as X = OR does, in the all‐gauche conformation; in contrast, the predominance of the trans conformer for X = Cl, Br and J is not rationalized. The calculated ground state geometries show no significant geometry distortion upon change of the substituents X, which provides a basis for the13C shielding comparison to corresponding cyclohexanes of similar substitution pattern. The observed substituent effects on the13C shifts are in agreement with other models and mechanistic proposals for the screening constant variations.
1H‐NMR Conformational Study of 2‐Acyl‐3,5‐dialkylfurans Using Lanthanide Shift Reagents
Authors: Balaban Alexandru T. , Gheorghiu Mircea D. , Draghici Constantin 
Published: November 2013

AbstractPyrylium salts undergo oxidative ring contraction to 2‐acyl‐3,5‐dialkylfurans II. A systematic 1H‐NMR room‐temperature study was made for compounds II a‐e having methyl and/or ethyl substituents, using Eu(fod), as lanthanide shift reagent. The conformation always is trans (IIB), minimizing steric interactions. Calculations also indicate a preferred planar conformation IIC for the 5‐ethyl group. The corresponding oximes III were also studied similarly, but the effects are too complex to be understood at the present stage.
Peptide Conformation, IX Conformational Studies on Cyclotripeptides Containing N‐Benzyl‐Glycine
Authors: Kessler Horst , Krämer Peter , Krack Gerhard 
Published: November 2013

AbstractSynthesis and NMR spectra of the cyclic tripeptides of the general structure cyclo[ProxBzl.Gly3‐x] (x = 0–2; Bzl.Gly. = N‐Benzylglycyl) are reported. Two slowly interconverting conformations of these tripeptides containing only cis peptide bonds have been observed in solution by NMR spectroscopy. Structural proof and assignments to a flexible boat conformation and a rigid crown conformation, respectively, have been achieved by interpretation of the NMR parameters and the correlation with the crystal structure of one compound. The conformational behaviour of cyclic tripeptides is discussed.
Understanding Benzannulenes
Authors: Mitchell Reginald H. 
Published: November 2013

AbstractThe effect on NMR data when annulenes are benzannelated is discussed for forty‐three compounds. It is shown that benzannelation reduces the diatropicity or paratropicity of the macrocyclic system and the reasons for this in terms of increased bond localisations in the macrocylic ring are indicated. Examples are discussed where no decrease in diatropicity is found to occur on dibenz (naphth)annelation, and these are shown to be consistent with no change in bond orders.
Exocyclic Charge Flight from Cyclopropenide Systems as a Criterion of Antiaromaticity
Authors: Mitchell R. David , Bauld Nathan L. 
Published: November 2013

AbstractThe unusually extensive exocyclic charge flight (from cyclopropenyl to organo moiety) proposed for the cyanocyclopropenide ion is affirmed by MINDO/3 calculation. Nevertheless, the MINDO/3 wave function for this ion is shown not to be absolutely antiaromatic. The basis for exocyclic charge flight is shown, using the “de‐perturbation” approach, to be the small (cyclopropenide) HOMO/ (cyano) LUMO gap.
Evaluation of Strain Effects on the Reactivity of Small Rings
Authors: Stirling Charles J. M. 
Published: November 2013

AbstractThe qualitative effect of ring strain on the reactivity of small ring systems is briefly reviewed. It is pointed out that nucleofugalities of groups cannot be measured for nucleophilic substitution but that such measurements are possible for activated eliminations. Application of the techniques for determination of nucleofugality to eliminative fission of small strained rings shows that nucleofugality is very greatly enhanced. The extent of enhancement cannot be determined for epoxides, but has been determined for cyclopropanes, the results showing that about 60% of the ring strain energy is released in the transition state of ring fission. Catalysis of ring fission requires very specific placement of proton donor groups; gem‐dimethyl substitution raises the enthalpy of the transition state for eliminative fission of cyclopropanes.
The Nature of Sulfur Monoxide from Thiirane Oxide: A Stereochemical Test
Authors: Aalbersberg W. G. L. , Vollhardt K. P. C. 
Published: November 2013

AbstractIn order to study the stereochemistry of thiirane oxide decomposition, syn‐ and anti‐2,3‐dideuterothiirane‐1‐oxides and trans‐2,3‐dideutero‐thiirane‐1‐oxide have been synthesized in three steps from cis‐ and trans‐dideuteroethylene via stereospecific formation of the corresponding oxiranes and thiiranes. The presence of the deuterium label was used so as not to perturb the course of the decomposition from that of the parent thiirane oxide. Pyrolysis of the labeled thiirane oxides in both gas and solution phase proceeded with greater than 90% retention of stereochemistry. Analyses were performed by comparison of the infrared spectra of the product mixtures with standard calibrated samples. Attempts to trap possible biradical intermediates with di(p‐anisyl)thioketone gave a low yield of products, possibly derived from addition to thiirane oxide. These results suggest a possible partial contribution of a biradical mechanism to the thiirane oxide decomposition. If bond rotation in the biradical is slow, this mechanism could be the sole mode of sulfur monoxide extrusion.
Strained Mono‐ and Dications. On the Structures of [(CCH2)7H]2⊕ and [(CCH3)7CH3]2⊕ Dications
Authors: Carnadi H. , Giordano C. , Heldeweg R. F. , Hogeveen H. , van Kruchten E. M. G. A. 
Published: November 2013

AbstractThe reactions of alcohol 1 with HF‐SbF5 and of alcohol 8 with FSO3H‐SbF5 at low temperatures afforded dications [(CCH3)7H]2⊕ and [(CCH3)7CH3]2⊕, respectively. Both NMR (1H and 13C) data and analysis of the products, obtained upon quenching of the cations, were used to discuss the possible structures of these cations. Both ions showed a remarkable NMR behaviour, viz. an equivalency of all methylgroups on the one hand and all skeleton carbon atoms on the other hand, indicating a high symmetry or a rapid (on the NMR time scale) process. Using model compounds the weighted average 13C‐NMR chemical shifts of reasonable structures for the ions were calculated and compared with the experimentally obtained values. The conclusion is drawn that the former reaction presumably leads to dications 34 and 36 and the latter reaction to dication 51.
Electron Spin Polarization in Photosynthetic Models: Chlorophyll a Mediated Photoreduction of 1,4‐Benzoquinone
Authors: Warden Joseph T. 
Published: November 2013

AbstractThe observation of Chemically Induced Dynamic Electron Spin Polarization (CIDEP) is reported for the chlorophyll a cation radical and 1,4‐benzosemiquinone anion generated by laser flash photolysis of ethanolic solutions of chlorophyll a and benzoquinone. The signs of the polarizations exhibited by the radicals are consistent with the predominant operation of the Triplet Mechanism for CIDEP. Chemical decay for the photoinduced radicals occurs by recombination via reverse electron transfer (Kt ∼ 2.9×0.8×109 M−1s−1).
Energetics of Light‐Induced Charge Separation Across Membranes
Authors: Warshel Arieh 
Published: November 2013

AbstractThe efficiency of light induced electron transfer across membranes is examined on a molecular level. The activation energies that determine this efficiency are evaluated by pure electrostatic considerations in terms of the membrane dielectric and the relaxation of the dielectric at the sites of the electron donors and acceptors. These considerations allow one, for the first time, to assess the efficiency of photosynthetic systems in terms of their molecular components. It is demonstrated that a conduction chain of stacked acceptors immersed in a low dielectric membrane cannot be expected to provide an efficient photosynthetic system. On the other hand, efficient photosynthetic systems can be obtained when the acceptors are placed in polar active sites of proteins and arranged in order of increasing redox potentials.
Photolysis of Saturated Four‐Membered Rings with Two Exocyclic Double Bonds: A Photoextrusion Route to (Hetero)Methylenecyclopropanes
Authors: Quast Helmut , Fuss Andreas 
Published: November 2013

AbstractThe photochemistry of cyclobutanes and saturated four‐membered heterocycles with two exocyclic double bonds (except bisalkylidenecyclobutanes) is reviewed. Emphasis is laid on photoproducts rather than on mechanism, with special regard for the formation of (hetero)methylenecyclopropanes via photoextrusion of small stable molecules.
Alternative Modes of Addition of Electrophilic Olefins to the Vinylcyclopropane Systems in 2,2‐Ethano‐1‐Methylene‐1,2,3,4‐Tetrahydronaphthalene and 1,1‐Ethano‐2‐Methylene‐1,2,3,4‐Tetrahydronaphthalene
Authors: Sarel Shalom , Felzenstein Abraham‐Meir , Weisz Moshe 
Published: November 2013

AbstractThe title compounds 2,2‐ethano‐1‐methylene and 1,1‐ethano‐2‐methylene‐1,2,3,4‐tetrahydronaphthalene (13 and 14 respectively) were synthesized by the Wittig reaction from the corresponding ethano‐tetralones (16) and (19) which were, in turn, prepared from the respective α‐ and β‐tetralones (15) and (18). It is shown that the reaction of (13) with TCNE at 0° yields the corresponding [2 + 2] cycloaddition adduct (20), isomerizing thermally to the formal [2 + 5] cycloaddition adduct (21). Compound (13) reacts with N‐tosylisocyanate (TIC) and with N‐phenylsulfonylisocyanate (PSI) to a 4:1‐isomeric mixture of (Z)‐ and (E)‐acryloamides (24) and (26), respectively. The reaction of (14) with TCNE at 0° leads to the [2 + 5] cycloadduct (30), while with (TIC) and (PSI) the corresponding tricyclic adducts (28a) and (28b) result. Bromination of (16) with NBS yielded the respective 4‐bromo‐2,2‐ethano‐1‐tetralone (31) which gave, upon dehydrobromination with the aid of DBU, the naphtho‐dihydrofurane derivative (34). The latter was formed most likely via facile cyclopropylketone upon dihydrofurane rearrangement of an initially formed 2,2‐ethano‐1‐oxo‐1,2‐dihydronaphthalene (32) which was desired for the formation of (33). Structures of all new compounds were assigned on basis of analytical and spectroscopic data (IR, UV, mass spectra, 1H‐NMR).
The Cycloaddition of Vinylaziridines with Unsaturated Substrates
Authors: Hassner Alfred , Chau William , D'Costa Rosario 
Published: November 2013

AbstractVinylaziridines undergo ring expansion reactions which in case of divinylaziridines lead to dihydroazepines. Addition of 2‐phenyl‐2‐vinyl‐aziridine 10a to dimethyl acetylene dicarboxylate provides a facile method of in situ generation of a divinylaziridine and leads to isolation of an azepine derivative. Likewise we show that cycloaddition of 10a proceeds even with olefinic nitriles, sulfones and esters with formation of tetrahydroazepines. However, when the substrates are unsaturated nitro compounds or ketones, a novel rearrangement to open chain enamine derivatives takes place. Deuterium labelling in the β‐nitrostyrene substrate and in the vinylaziridine provide evidence for a retro‐ene reaction mechanism leading to the rearrangement. These data provide a framework for assessing the factors that guide the reactions either towards formation of seven‐membered ring azepine derivatives or toward open‐chain rearranged products.
New Photosensitizers of the Ruthenium‐Polypyridine Family for the Water Splitting Reaction
Authors: Juris Alberto , Barigelletti Francesco , Balzani Vincenzo , Belser Peter , Von Zelewsky Alex 
Published: November 2013

AbstractThe photophysical and electron transfer properties of the following Ru(II) complexes have been characterized: Ru(bpy)3‐n (biq)2+n, Ru(bpy)2(Cl2‐bpy)2+, Ru(Cl2‐bpy)2+3, Ru(bpy)2(NO2‐bpy)2+, Ru(NO2‐bpy)2+3, Ru(bpy)2(opdi)2+, Ru(bpy)2(phedi)2+, where n = 1 to 3, bpy = 2,2′‐bipyridine, biq = 2,2′‐biquinoline, and opdi and phedi are o‐quinodiimine ligands. The properties of these and other Ru‐polypyridine type photosensitizers are discussed on the basis of the requirements needed to split water with solar energy in an efficient way. It is shown that the Ru(bpy)2(LL)2+ complexes, where LL is a polypyridine‐type ligand which is easier to reduce than bpy, are expected to be more efficient photosensitizers than Ru(bpy)2+3.
The Potentiality of Chromium(III)‐Polypyridyl Complexes Toward Use in Photochemical Solar Energy Conversion
Authors: Hoffman Morton Z. , Serpone Nick 
Published: November 2013

AbstractThe properties of polypyridyl complexes of Cr(III), Cr(NN)3+3, are examined with regard to the photochemical determinants of efficient solar energy utilization and compared with those of corresponding complexes of Ru(II), Fe(II), Os(II), Rh(III), and Ir(III). The following characteristics are discussed: absorption spectra, nature and lifetimes of excited states, intersystem crossing efficiency between lowest spin‐allowed and spin‐forbidden excited states, energetics and kinetics of excited‐state redox quenching, and the properties of redox‐generated energy‐rich charge‐carriers. Recent results dealing with the effect of solution medium on intersystem crossing efficiency and excited‐state lifetimes are presented. Cr(NN)3+3 complexes have been applied to solar energy conversion schemes and a brief review and critique are offered. The advantages and disadvantages of Cr(NN)3+3 as solar energy harvesters and electron‐transfer mediators are discussed in terms of future research directions.
Photogeneration of Hydrogen from Polymeric Viologen Systems
Authors: Lee P. C. , Matheson M. S. , Meisel D. 
Published: November 2013

AbstractPolymers containing pendant viologen groups were prepared and studied for their quenching efficiencies of the Ru(bpy)2+3excited state, their back reactions following the quenching, their viologen radical yields, and finally for their effectiveness in hydrogen production in the presence of colloidal platinum. Results are compared with those from methyl viologen containing systems to evaluate the performance of the polymeric viologen systems.