bluuebunny/crossref_metadata_2025 huggingface dataset:
Photodissociation of Vinyl Bromide and the Heat of Formation of the Vinyl Radical
Authors: Wodtke Alec M. , Hintsa Eric J. , Somorjai John , Lee Yuan T.
Published: November 2013

We have performed measurements of the translational energy distributions and anisotropy parameters of the photodissociation products of vinyl bromide at 193 nm. Br‐atom and HBr elimination were observed with a branching ratio of 1.28 ± 0.05 between the two channels. Both processes occurred with a large release of translational energy, an average of about 2.0 eV for the Br‐atom channel and somewhat less for HBr elimination. The maximum release of translational energy for Br + C2H3 formation led to an upper limit of 77 ± 3 kcal/mol to D0(C2H3‐Br), from which an upper limit to the heat of formation of the vinyl radical, 71 ± 3 kcal/mol, was derived. This result was used to reexamine crossed molecular beams data relevant to the determination of the heat of formation of the vinyl radical. A metastable state of C2H3 observed in the photodissociation of vinyl bromide was interpreted as the formation of C2H3(Ã2A″).
High‐Resolution Laser Techniques Applied To Reaction Dynamics
Authors: Vetter Raymond J.
Published: November 2013

In this paper, using the particular case of the Cs(7P) + H2 → CsH + H reaction, it is shown how the application of high‐resolution continuous‐wave (CW) laser techniques to crossed‐beam experiments can yield information about the mechanism of reactive collisions. Firstly, CW laser techniques allow the preparation of Cs atoms in given fine and hyperfine structure levels; for this reaction, measurements of hyperfine cross‐sections vs. different orientations of the atomic orbital show how the molecular symmetry of the reagents controls the efficiency of the harpooning process involved. Secondly, the laser‐induced fluorescence (LIF) technique of detection allows the characterization of CsH products in given rotational levels. The high‐precision scanning of the CW laser frequency over absorption profiles of CsH molecules results in the determination of the Doppler shifts, which are associated with the recoil velocity of the products; two laser beam arrangements are used to provide the product velocity and angular scattering probability. A “forward” peaking is discovered, which depends on collision energy but not on product rotation. Finally, the technique of saturated absorption is applied for the first time to reactive experiments; it could lead to methods for selecting products which scatter in a given plane or in given directions.
Vibrational Breakup of Triatomic Collision Complexes: A Model Study for H+‐H2
Authors: Drolshagen Gerhard , Gianturco Francesco Antonio , Toennies J. Peter
Published: November 2013

The predissociative behavior of the H+3 system is studied from the point of view of purely vibrational coupling between the rotationally‐averaged motions of the neutral diatomic and the bound proton. By decoupling the effects of the usual rotational barriers from those induced by the potential strength as a function of relative separations, one is able to see that the vibrational interchannel coupling shows strong propensity and highly nonstatistical propagation among bound levels and that the inner chaotic motion often assumed for this system is mostly controlled by the presence of multiple angular momentum barriers. Tunneling through these multiple rotational momentum barriers appears to be responsible for the slow dissociation seen in spectroscopic experiments.
Quasi‐Classical Trajectory Studies of H + HX → H2 + X (X = Cl, Br, I) Reaction at High Collision Energy
Authors: Aker Pamela M. , Valentini James J.
Published: November 2013

Quasi‐classical trajectory calculations have been performed to investigate the dynamics of the H + HX → H2 + X reactions, where X = Cl, Br, and I. Calculations were performed for two collision energies, 16 and 36 kcal mol−1. The results show that the dynamics of the abstraction reactions at these high collision energies are rather unusual. The product energy and angular distributions can be accounted for by a mechanism that involves an isolated interaction between the two H atoms in the system to form H2 followed by hard‐sphere scattering of this newly formed H2 off the X atom.
Quantitating the Effect of Solvent Type on Neutral Macrocycle‐Mediated Cation Transport in Liquid Membranes
Authors: Izatt R.M. , Bruening R.L. , Bruening M.L. , Lamb J.D.
Published: November 2013

The recently confirmed diffusion‐limited transport model for neutral macrocyle‐mediated cation transport in membranes has been used to quantify the membrane solvent effects on the fundamental parameters in the model and their relative importance. The fundamental parameters involved are the extraction equilibrium constant, macrocycle partition coefficient, and diffusion coefficient. Experimental measurements and predictive correlation calculations have been made to quantify these fundamental parameters. Membrane stability has also been quantified by looking at solvent solubilities and solvent compatibility with various membrane types. It was found that membrane stability vs. rapid transport is the major choice to be made in choosing a membrane solvent. Retention of the macrocycle in the membrane and diffusion coefficients are greatest with short carbon chain solvents containing few chlorine atoms. On the other hand, high boiling points and low water solubility are obtained with long carbon chain solvents containing increasing numbers of chlorine atoms. Extraction equilibrium constants also decrease with increasing carbon chain lengths and increasing chlorination, but the decrease is not uniform like the changes in the other parameters. Little effect of solvent type on membrane selectivity was observed. The transport model and quantification of these fundamental parameters should allow for accurate predictions in the choice of solvent in designing stable membranes with optimal transport properties.
Field‐Flow Fractionation (FFF) and Related Techniques for the Separation of Particles, Colloids and Macromolecules
Authors: Levin Shulamit
Published: November 2013

Thin‐channel flow techniques can be used for the separation of samples ranging over 15 orders of magnitude in molecular weight, namely, macromolecules, colloids, and particles. The first part describes the group of field‐flow fractionation techniques. Flow system components are illustrated and described, as well as mode of operation and theory behind retention in the thin channel. An update of current topics under research is given, namely, normal retention mode of Brownian particles (under 1 μm in diameter) as compared to the steric retention mode of non‐Brownian particles (above 1 μm in diameter). Optimization of the separation by field programming is briefly discussed.

Another group of thin‐channel techniques for small‐scale processing, the split‐flow thin (SPLITT) cells, in which fractionation is based on mass transport across the channel thickness, is presented in the second part. Principles of separation in this technology as well as possible applications are illustrated and discussed.
The Natural Bite Angle of Chelating Diphosphines
Authors: Casey Charles P. , Whiteker Gregory T.
Published: November 2013

The natural bite angles of chelating diphosphine ligands have been determined by molecular mechanics calculations using the MACROMODEL program with a modified AMBER force field. The natural bite angle (βn) is defined as the preferred chelation angle determined only by ligand backbone constraints and not by metal valence angles. Potential energy diagrams for diphosphine chelates were constructed to estimate chelate flexibility. Molecular mechanics calculations have been used to select diphosphine ligands with natural bite angles of 120° for diequatorial chelation in trigonal bipyramidal metal complexes.
Stereochemical Control in Palladium‐catalyzed Allylic Etherification
Authors: Keinan Ehud , Roth Zeev
Published: November 2013

Two alternative approaches for increasing the stereospecificity of Pd(0)‐catalyzed allylic etherification are presented, both of which also suppress product isomerization and elimination. One of them involves addition of chlorotrimethylsilane to a benzene reaction mixture. In the model reaction studied, it increases the ratio of the two stereoisomeric products, arising from either retention or inversion of configuration at the allylic carbon, from 3:1 to 9:1, respectively. The second approach involves the employment of both donor‐ and acceptor‐chelating ligands. Essentially 100% stereospecificity (retention of configuration) was achieved when the reaction was carried out in benzene at 60°C in the presence of Pd2(dba)3 and bis‐1,2‐diphenylphosphinoethane.
Reversible Carbon‐Carbon Bond Cleavage of a 3‐Vinyl‐1‐Cyclopropene by Rh(I). Molecular Structures of Two Sterically Crowded 1,2,3,5‐η‐Pentadienediyl Complexes of Rh(III)
Authors: Donovan Bernadette T. , Egan James W. , Hughes Russell P. , Spara Paul P. , Trujillo Hernando A. , Rheingold Arnold L.
Published: November 2013

1,2,3‐Tri‐tert‐butyl‐3‐vinyl‐1‐cyclopropene 6a reacts with [RhCl(C2H4)]2 to give the dimeric 1,2,3,5‐η‐pentadienediyl complex 7a. The cyclopentadienyl derivative of this complex, 8a, is obtained by reaction of 7a with T1(C5H5). Crystal structures of both 7a and 8a were determined: 7a; orthorhombic, Pbcn, a = 28.136 (4) Å, b = 10.637 (2) Å, c = 12.154 (2) Å, V = 3637.6 (1.2) Å3, and Z = 4: 8a; triclinic, P 1 ‐bar, a = 9.906 (2) Å, b = 9.736 (2) Å, c = 12.126 (2) Å, α = 76.07 (2)°, β = 78.21 (2)°, γ = 65.34 (2)°, V = 1024.6 (3) Å3, and Z = 2. Treatment of 7a with one equivalent of PMe3 per Rh center results in regeneration of the vinylcyclopropene, demonstrating the reversibility of the ring opening reaction. Deuterium labelling studies show that both the ring opening and closing reactions proceed with retention of configuration at the vinyl olefin. Thus the trans‐deuterated vinylcyclopropene 6b reacts with [RhCl(C2H4)2]2 to give 7b in which deuterium is located exclusively in the syn position. Treatment of 7b with T1(C5H5) yields 8b. Treatment of 7b with one equivalent of PMe3 per Rh center results in formation of 6b as the sole organic product. Unlike their triphenyl relative 2, the tri‐Bu complexes 7a, 7b, 8a, and 8b are stereochemically rigid in solution at room temperature, and do not undergo an η3 → η1 → η3 isomerization on the NMR time scale. However, after prolonged heating, syn‐anti site exchange of H and D slowly takes place with 8b.
Stereoselective Alkylations of Enolates Derived from Ligands Attached to the Indenyl Iron Chiral Auxiliary [(η5‐C9H7)Fe(CO)(PPh3)]: X‐ray Crystal Structure and Conformational Analysis of [(η5‐C9H7)Fe(CO)(PPh3)COCH3]
Authors: Davies Stephen G. , Sarah Holland K. , Sutton Kevin H. , Mcnally John P.
Published: November 2013

The preparation, X‐ray crystal structure, and conformational analysis of the acetyl complex [(η5 ‐ C9H7)Fe(CO)(PPh3)COCH3] (5) are described. Deprotonation and alkylation of complex 5 generates the corresponding propanoyl (6) and butanoyl (7) complexes. Deprotonation and alkylation of complexes 6 and 7 to generate complementary diastereoisomers of the corresponding 2‐methylbutanoyl complex occur with high stereoselectivities. The aldol reaction of the diethylaluminium enolate derived from 5 with acetaldehyde also shows a high stereoselectivity (95:5). The reactions of the indenyl complex 5 clearly parallel those for the cyclopentadienyl analogue and this is rationalised in terms of the conformational analysis.
Unsaturated Bis(phosphido)‐bridged Heterobimetallic Polyhydrides via Dihydrogen Activation
Authors: Thomas Baker R. , Glassman Timothy E. , Ovenall Derick W. , Calabrese Joseph C.
Published: November 2013

Hydrogenation (1 atm) of (PCy2)2Re(μ‐PCy2)2M(1,5‐COD) (M = Rh, Ir; Cy = cyclohexyl; COD = cyclooctadiene), (PCy2)2Re(μ‐PCy2)2Rh(DMPE) (DMPE = [Me2PCH2]2), [(PCy2)2ReH(μ‐PCy2)2Rh(DMPE)]BF4 and (PCy2)2ReH(μ‐PCy2)2Pd‐(PPh3) proceeds stepwise with initial additions of H2 across the Re = P multiple bonds to give ReH(PCy2H) moieties. Further addition of H2 occurs only for M = Rh. The DMPE complex gives the tetrahydride (PCy2H)2ReH3(μ‐PCy2)2RhH(DMPE), which loses PCy2H at 80 °C to give (PCy2H)ReH4(μ‐PCy2)2Rh(DMPE). Hydrogenation of the COD complex affords the stable hexahydride (PCy2H)ReH5(μ‐PCy2)2RhH(PCy2H) via the cyclooctenyl complex. (PCy2)2ReH(μ‐PCy2)2Rh(η3‐C8H13). While the hexahydride is an active homogeneous catalyst precursor for the hydrogenation of alkenes, dienes, and alkynes, attempts to activate the Re center for C‐H bond activation have not yet been successful. 31P and 1H DNMR spectroscopy have been used to investigate the solution dynamics of the heterobimetallic polyhydrides, and several X‐ray diffraction studies serve to define the solid state structures.
Percolation Model of Nuclear Magnetic Relaxation in Porous Media: Slow Diffusion
Authors: Mendelson Kenneth S.
Published: November 2013

Nuclear magnetic relaxation in a porous medium is simulated by a random walk on the open sites of a percolation lattice. This model is used to study relaxation in the case that diffusion to the pore surface is slow compared to relaxation at the surface. The computed decay curves can be fit by sums of exponentials. For slow diffusion the principal relaxation time is independent of the surface relaxation rate. It is inversely proportional to the diffusion coefficient and directly proportional to the mean square displacement of the walkers. It appears that a measurement of nuclear magnetic relaxation in the slow diffusion range will yield only an average pore size. This pore size is given by the root mean square displacement to the surface for a random walker starting with equal probability from any point in the pore space.
Reaction Kinetics in Restricted Spaces
Authors: Kopelman Raoul , Koo Yong‐Eun
Published: November 2013

Reactions in restricted spaces rarely get stirred vigorously by convection and are thus controlled by diffusion. Furthermore, the compactness of the Brownian motion leads to both anomalous diffusion and anomalous reaction kinetics. Elementary binary reactions of the type A + A → Products, A + B → Products, and A + C → C + Products are discussed theoretically for both batch and steady‐state conditions. The anomalous reaction orders and time exponents (for the rate coefficients) are discussed for various situations. Global and local rate laws are related to particle distribution functions. Only Poissonian distributions guarantee the classical rate laws. Reactant self‐organization leads to interesting new phenomena. These are demonstrated by theory, simulations, and experiments. The correlation length of reactant production affects the self‐ordering length scale. These effects are demonstrated experimentally, including the stability of reactant segregation observed in chemical reactions in one‐dimensional spaces, e.g., capillaries and microcapillaries. The gap between the reactant A (cation) and B (anion) actually increases in time and extends over millimeters. Excellent agreement is found among theory, simulation, and experiment for the various scaling exponents.
Influence of the Ionization Potential and of the Interfacial Electric Field on Photoinduced Hole Transfer Processes in Langmuir‐Blodgett Films
Authors: Itaya Akira , van der Auweraer Mark , Verschuere Bart , De Schryver Frans C.
Published: November 2013

The presence of a monolayer of ω‐(2‐anthryl)‐alkanoic acid on an anthracene or perylene single crystal modifies the kinetics of photosensitized hole injection. An increase in the chain length of the ω‐(2‐anthryl)‐alkanoic acid leads to a decrease in the quantum yield of the sensitized hole current (SHC). This effect becomes more important when the ionization potential of the dye is decreased. This argument supports the hypothesis that electron transfer is assisted by an interaction between the HOMO of the dye and that of the alkyl chain. The permanent electric field existing at the interface between the monolayer of ω‐(2‐anthryl)‐alkanoic acid and the monolayer containing the dye is also expected to influence the free energy gain associated with this electron transfer process. When the dye is diluted by tripalmitine instead of arachidic acid, the change in the electrical potential difference at the interface with the monolayer of ω‐(2‐anthryl)‐alkanoic acid leads to a tenfold decrease in the quantum yield of the SHC.
Fractal Nature of Fluorescence Quenching and its Relation to Percolative Conduction in Water‐in‐Oil Microemulsions
Authors: Lianos Panagiotis , Malliaris Angelos
Published: November 2013

Results of fluorescence quenching and electrical conductivity measurements in water‐in‐oil microemulsions were analyzed in terms of fractal models. It was found that for SDS/water/pentanol/alkane microemulsions there exists a connection between the fractal nature of the reaction domain and the magnitude of electrical conductivity. Conductivity values increase as the domain becomes less fractal.
Asymmetric Intramolecular Ene Reaction Catalyzed by a Chiral Titanium Reagent and Synthesis of (—)‐ε‐Cadinene
Authors: Narasaka Koichi , Hayashi Yujiro , Shimada Satoru , Yamada Jun
Published: November 2013

A highly diastereo‐ and enantio‐selective intramolecular ene reaction was developed by employing a chiral titanium reagent generated in situ from TiCl2(OPri)2 and a tartrate‐derived chiral 1,4‐diol. This reaction afforded masked cyclohexanone and cyclopentanone derivatives with high optical purities which were synthetically useful chiral building blocks. From the intramolecular ene product (—)‐ε‐cadinene was synthesized by the use of the newly developed intramolecular Michael reaction.
Mixed Crystals as Host‐Guest Systems for Probing Molecular Interactions
Authors: Vaida M. , Weissbuch I. , Lahav M. , Leiserowttz L.
Published: November 2013

A method is described for probing interactions between molecules at the surfaces of growing crystals. For this purpose, mixed crystals have been treated as unique host‐guest systems where the guest molecules are occluded into the host crystal through stereospecific interactions occurring at the different growing surfaces of the host. Owing to the process of molecular recognition at such surfaces, the adsorption and occlusion of the guest molecules takes place at specific sites, resulting in a reduction in symmetry of the mixed crystal compared to the pure host. The concept of reduction in symmetry is general, and has been instrumental for probing subtle molecular interactions at the surfaces of growing crystals, for the transformation of centrosymmetric single crystals into crystals with polar arrangement, and for “absolute” asymmetric synthesis inside centrosymmetric crystals.
Bifunctional Binding and Catalysis in Host‐Guest Chemistry
Authors: Breslow Ronald
Published: November 2013

Some dimers of cyclodextrin have been prepared that can bind appropriate substrates in water with binding constants as strong as those of strong antigen/antibody complexes. Doubly linking two cyclodextrins with connections at neighboring glucose units produces a hinged molecule that resembles a clamshell. Another isomer is also produced, in the so‐called loveseat configuration, in which the two cyclodextrin units cannot cooperate in binding a single substrate. When the two links are of different length, the clamshell dimer preferentially binds a non‐linear substrate with an affinity exceeding 1011 M−1, at least 103 better than the binding of a linear substrate isomer. Binding of substrates occurs by induced fit, in which the host closes around and engulfs the substrate. Mechanistic studies on models for the enzyme ribonuclease have led to a proposed mechanism different from that normally suggested for the enzyme. This mechanism has guided the synthesis of a new enzyme mimic with improved properties. Isotope studies show that this enzyme mimic uses a simultaneous bifunctional mechanism, as does the enzyme. The double binding of the transition state by the two imidazole units of the enzyme mimic occurs because of their linkage. The geometric preference for this binding reflects anon‐linear transition state.
Enantioselective Hosts for Neutral Derivatives of Amino Acids
Authors: Clark Still W. , Kilburn Jeremy D. , Sanderson Philip E.J. , Liu Ruiping , Wiley Michael R. , Hollinger Frank P. , Hawley Ronald C. , Nakajima Makoto , Bernardi Anna , Hong Jong‐In , Namgoong Sung Keon
Published: November 2013

A series of macropolycyclic receptors for hydrogen bond donor/acceptor guests were prepared and studied. Molecular modeling and X‐ray crystallography provided structural information, while NMR titration studies allowed determination of free energies of association. Some of die receptors prepared showed enantioselectivity as high as 3 kcal/mol in binding derivatives of simple amino acids in organic solvents. These studies show that conformational homogeneity is one of the most important characteristics of a selective host and that molecular modeling can play an important role in the design of such substances.
Supramolecular Photochemistry and Photophysics. Adducts of Metal Complexes with the Natural Ionophore Lasalocid A Anion
Authors: Ballardini Roberto , Gandolfi Maria Teresa , Moya Maria Luisa , Prodi Luca , Balzani Vincenzo
Published: November 2013

The formation of adducts of Cr(bpy)33+, Cr (phen)33+, Ru(bpy)32+, and Pt(bpy)(NH3)22+ (bpy = 2, 2′‐bipyridine; phen = 1, 10‐phenanthroline) with the lasalocid A anion (LAS‐) in CH2Cl2 solution has been investigated by means of electronic absorption and emission spectroscopy. Cr(bpy)33+ forms a 1:1 adduct with LAS−. In this adduct, the fluorescence of LAS− is quenched, whereas the phosphorescence intensity and lifetime of Cr(bpy)33+ increase. In the case of Cr(phen)33+, only a dynamic quenching of the Cr(phen)33+ phosphorescence by LAS− is observed. For Ru(bpy)32+, either no adduct is formed or the interaction is too weak to cause changes in the absorption and emission properties. For Pt(bpy)(NH3)22+, evidence has been obtained for formation of a 1:2 Pt(bpy)(NH3)22+ / LAS− adduct, where the LAS− fluorescence is statically quenched.
Effects of Electrostatic and π—π Interactions on the Stabilities of Xanthene Dye—4,4′‐Bipyridinium Complexes: Structural Design of a Geared Supramolecular Machine
Authors: Willner Itamar , Eichen Yoav , Doron Amihood , Marx Sharon
Published: November 2013

Xanthene dyes and substituted 4,4′‐bipyridinium salts form well defined 1:1 complexes in solution and in the crystalline state. Electrostatic, charge‐transfer, and π‐π interactions are the driving forces for the formation of these complexes. Electrostatic interactions contribute 1.1–1.3 kcal/mol per ion pair, and interring interactions, i.e., charge transfer and π‐π interactions, contribute 6.2–6.3 kcal mol−1 to the complexes' stabilization. Application of a bis‐bipyridinium host results in enhanced binding of eosin Y, 1, due to increased π‐π and donor‐acceptor interactions and complementary electrostatic attractions. The binding properties of the bipyridinium moiety to eosin Y is controlled by its functionalization with a photochromic azobenzene unit. The association constants of 8‐trans and 8‐cis to eosin Y, 1, are 3000 M−1 and 38000 M−1, respectively. This difference in the association constants of the diad isomers to 1 allows the photo‐controlled reversibly‐geared association and dissociation of the diad to and from eosin Y in the supramolecular assembly.
The Mechanisms of Making Molecules to Order
Authors: Brown Christopher L. , Philp Douglas , Spencer Neil , Fraser Stoddart J.
Published: November 2013

The mechanisms of formation of a [2]catenane and one of its molecular components have been investigated. While the synthesis of the tetracationic cyclophane, [BBIPYBIXYCY]4+ from bipyridine (BP) and 1,4‐bis(bromomethyl)benzene (BBB), directed by the template, 1,5–bis[2(2–hydroxyethoxy)ethoxy]naphthalene(1/5BHEEN) becomes less efficient (23 to 5%) under ultra‐high pressure reaction conditions (12 kbars), the self‐assembly of {[2]‐[BPP34C10]‐[BBIPYBIXYCY] catenane}4+ from BP and BBB in the presence of BPP34C10 can be achieved with increased efficiency (18 to 42%) at 12 kbars. This difference in the trends of the yields can be ascribed to the enhanced templating action of BPP34C10 relative to that of 1/5BHEEN when two moles of BP and two moles of BBB are being employed to construct the tetracationic cyclophane. The self‐assembly of the [2]catenane from BP, BBB, and BPP34C10 has been followed by 1H NMR spectroscopy in D7‐DMF solution. On the basis of this spectroscopic evidence and supporting chemical data, the formation of {[2]‐[BPP34C10]‐[BBIPYBIXYCY] catenane}4+ from two moles of BP, two moles of BBB, and one mole of BPP34C10 is believed to proceed via the monoquatemary intermediate, [MBXYBIPY]+, which has not been isolated, and the dicationic species, [BBIPYXY]2+, which has been isolated and shown to be an intermediate in the self‐assembly process leading to the [2]catenane, presumably via the [BXYBBIPYXY]3+ trication — the final intermediate which again has not been isolated.
Selective Complexation and Membrane Transport of Guanidinium Salts by Calix[6]arene Amides
Authors: Casnati Alessandro , Minari Patrizia , Pochini Andrea , Ungaro Rocco , Nijenhuis Wilma F. , De Jong Feike , Reinhoudt David N.
Published: November 2013

The p‐tert‐butylcalix[6]arene hexamide 2 and syn‐1,3,5–trimethoxy‐2,4,6‐triamide 4 were synthesized by reaction of α‐chloro‐N,N‐diethylacetamide and the corresponding calix[6]arenes 1 and 3, respectively. 1H NMR spectroscopy shows that 2 is a mixture of different conformations whereas 4 is fixed in a cone conformation. Extraction experiments, (S‐L) and (H2O–CDCl3) with picrate salts, indicate that 2 complexes both alkali metal (1:2 complex) and guanidinium salts (1:1 complex) but 4 complexes only guanidinium (1:1 complex). Incorporated in supported liquid membranes (Accurel® / o‐nitrophenyl n‐octyl ether) the calixarenes 2 and 4 transport guanidinium salts. The hexamide 2 is a more efficient carrier of guanidinium but the syn‐1,3,5‐trimethoxy‐2,4,6‐triamide 4 is much more selective. Both 2 and 4 are sufficiently lipophilic to give membranes that are stable over longer periods (weeks).
Molecular Recognition of Phosphate Esters: A Balance of Hydrogen Bonding and Proton Transfer Interactions
Authors: Hirst Simon C. , Tecilla Paolo , Geib Steven J. , Fan Erkang , Hamilton Andrew D.
Published: November 2013

The interaction of different phosphate esters with a series of hydrogen bonding receptors has been studied using UV‐visible and NMR spectroscopies and X‐ray crystallography. Critical for binding is a combination of an acidic proton and the potential for bidentate hydrogen bonding either between charged or uncharged components. Phosphotriesters show no binding to the receptors. Phosphodiesters bind to both bis‐(2, 6–diacylaminopyridine) and mono‐(2, 6–diacylaminopyridine) receptors in chlorocarbon solvent via proton transfer to form the pyridinium phosphate ion pair and bidentate hydrogen bonding between the anion and the cation. Titration experiments as well as Job's analyses show that for cyclic and acyclic bis‐(2, 6–diacylaminopyridine) receptors 2: 1 complexes can be formed. X‐ray crystal structures demonstrate that in the solid state two different binding arrangements are present; either a direct bidentate interaction or intramolecular hydrogen bonding with self‐assembly of an oligomeric structure. Phosphomonoesters bind to mono‐(2, 6–diacylaminopyridines) in a similar way to the diesters, via proton transfer and bidentate hydrogen bonding. In this, as in the diester case, only a single acid‐base interaction is possible and proton transfer is preferred. However, in the interaction of phosphomonoesters with bis‐(2, 6–diacylaminopyridine) derivatives two acid‐base interactions are possible between the two pyridines and two POH groups, and little proton transfer is seen. Strong binding (Ka = 1.0 × 105M−1) occurs via tetrahydrogen bond formation. Thus, there appears to be a balance between the occurrence of proton transfer and the number of hydrogen bonds formed between receptor and substrate.
Selective Ion Permeation Across Membranes Prepared from Steroidal Crown Ethers
Authors: Shinkai Seiji , Nishi Tadahiko , Shimamoto Katsuhiro , Manabe Osamu
Published: November 2013

Five crown ethers covalently‐linked to the cholesterol skeleton were synthesized. Thermal analysis established that four of them have a cholesteric liquid‐crystal phase. In particular, the monobenzo‐13–crown‐4 covalently‐linked through an ethylene spacer to the cholesterol skeleton shows a stable cholesteric mesophase even at room temperature. The stable cholesteric mesophase at room temperature was also obtained by mixing two steroidal crown compounds. These mixed systems were embedded in a polymer matrix and the polymer/liquid‐crystal composite membranes were subjected to ion‐permeation experiments. It was found that the permeation selectivity is basically explained by the hole‐size selectivity in the crown‐metal interaction. In particular, composite membranes containing a monobenzo‐13–crown‐4 unit showed the remarkably high Li+ selectivity. It was concluded on the basis of examination of activation parameters that ion permeation occurs across channel‐like stacks of crown ether rings.
Rotor Synchronized MAS Two‐Dimensional Exchange NMR in Solids. Principles and Applications
Authors: Luz Zeev , Spiess Hans W. , Titman Jeremy J.
Published: November 2013

The principles of the rotor synchronized magic angle spinning (MAS) two‐dimensional exchange NMR, first proposed by Veeman and coworkers are reviewed, with particular emphasis on situations where chemical exchange in solids proceeds in concert with molecular reorientation. Calculations of cross peak intensities as function of the ratio between the chemical shift anisotropy and the spinning rate are presented for several cases. These calculations emphasize the advantage of using slow spinning rates (ωR < ωLΔσ) in such experiments when detailed information about mechanistic pathways in solids is sought. Three applications of the method to solid systems using carbon‐13 NMR are described. These include: (a) Trimethylsulfoxonium iodide, in which the molecules undergo 120°‐jumps about the molecular C3 symmetry axis; (b) Tropolone, where the tautomeric hydrogen shift is found to be a consequence of the self diffusion within the crystal lattice, and in general accompanied by molecular reorientation. Here the two‐dimensional pattern is used to obtain information about the various mechanisms of the diffusion process; (c) Bullvalene, where a quantitative analysis of the cross peak intensities as function of the mixing time provides kinetic information on two independent processes, viz. symmetric threefold jumps and a concerted Cope rearrangement‐molecular reorientation reaction.
NMR Studies of Simple Molecules Adsorbed on Metal Catalysts: Acetylene Decomposition on Pt
Authors: Klug C.A. , Slichter C.P. , Sinfelt J.H.
Published: November 2013

We present the results of an NMR investigation to study the decomposition of acetylene adsorbed on small Pt clusters similar to industrial catalysts. We begin by reviewing some of our earlier results where it was shown that acetylene converts primarily to vinylidene (CCH2) upon room temperature adsorption. It was proposed that the vinylidene gradually dehydrogenates between 450 and 650 K leaving C2 surface species whose carbon‐carbon bond breaks at about 670 K. In this work we have shown that ethylidyne (CCH3) plays an important role in providing an alternate decomposition mechanism. In addition, although there is some evidence that CCH surface species are formed upon room temperature adsorption, no C2 surface species were observed in our experiments. Therefore, although dehydrogenation prior to carbon‐carbon bond scission may occur, further experiments where the ethylidyne mechanism is eliminated are needed to completely unravel the decomposition mechanism.
NMR Study of the Dynamic Stereochemistry of Substituted Bicyclo[4.2.2]decanes and Bicyclo[5.2.2]undecane
Authors: Kost Daniel , Strul Hava , Bittner Shmuel
Published: November 2013

Various NMR methods have been applied to investigate the ring‐reversal process in the title compounds. In order to measure barriers an exact assignment of proton‐exchange connectivities was required. This assignment was made using phase‐sensitive NOESY and difference NOE and saturation transfer spectroscopies. The barriers for ring reversal were found to differ substantially between compounds 1–3 on one hand (9–10.5 kcal mol−1), and compound 4 on the other (20.2 kcal mol−1). This was rationalized in terms of the different geometries of the ring systems.
Diffusion Barriers in Pulsed‐Gradient Spin‐Echo NMR Microscopy
Authors: Zawodzinski Thomas A. , Springer Thomas E. , Neeman Michal , Sillerud Laurel O.
Published: November 2013

Diffusion barrier effects on diffusion and flow mapping by NMR microscopy were simulated for the general case of an interface between two compartments with different relaxation properties and diffusion coefficients. The extreme case of a completely impermeable barrier (i.e. a glass wall) shows substantial changes in signal intensity and phase at distances of a few micrometers from the surface. Permeable boundaries show varying degrees of intensity and phase changes that can be used for deducing permeability properties of barriers that would otherwise be below the true spatial resolution of the image. The simulations were done for the case of a solid, water‐imbibing polymer immersed in water and for water diffusion between intra‐ and extracellular compartments across a biological membrane. These specific boundary effects have substantial implications for the determination of image resolution and for the separation and measurement of coherent flow and random diffusion.
Coherent Raman Beat Detection of Nuclear Coherences: An Alternative to Electron Spin Echo Envelope Modulation
Authors: Bowman Michael K.
Published: November 2013

The use of coherent Raman beats for the detection of nuclear coherences in solids from nuclei with hyperfine couplings to unpaired electron spins is discussed. The coherent Raman beats do not require refocusing of the electron spins and therefore are not strongly affected by spectral diffusion of the electron spins. Several methods for generating nuclear coherences using a single microwave pulse, two microwave pulses, or a combination of microwave and radiofrequency pulses are discussed. One‐dimensional and two‐dimensional spectroscopies for the measurement of hyperfine interactions in solids are proposed.
Enhanced Redox Chemistry in Quantized Semiconductor Colloids
Authors: Micic Olga I. , Rajh Tuana , Nedeljkovic Jovan M. , Comor Mirjana I.
Published: November 2013

Optical properties of extremely small metal chalcogenide particles that exhibit quantization effects have been examined. For extremely small semiconductor clusters with low effective carrier mass, optical absorption edges are shifted by several volts as a result of quantization effects that increase the effective bandgap of particles. Injection of electrons or holes into quantized semiconductor colloidal particles was studied by the pulse radiolysis technique. The reducing potential of photogenerated carriers was found to be greatly enhanced in quantized semiconductor particles, compared to the corresponding bulk material. Excess electrons trapped on the surface lead to a blue‐shift in the absorption edge of colloids. The appearance of this shift depends critically on the method of colloid preparation. The presence of OH− ions in PbS, CdS, and CdTe colloidal solutions blocks the sites where electrons are trapped and shortens the lifetime of bleaching, whereas in a slightly acidic solution long‐lived bleaching was observed. Different spectroscopic properties were found for hole excess and OH adducts of hydrous titanium oxides as a consequence of the existence of different environmental conditions of Ti(IV) atoms at the surface. The position where hole injection occurs lies in the mid‐gap level of 25‐Å TiO2.
Femtosecond Laser Spectroscopy and Computer Simulations of Barrierless Isomerization in Solution
Authors: Åberg Ulf , Åkesson Eva , Fedchenia Igor , Sundström Villy
Published: November 2013

The barrierless isomerization of 1,1‐diethyl‐4,4‐cyanine in alcohol solutions has been studied with 75‐fs laser pulses in transient absorption experiments. The measured dynamics are compared with Monte‐Carlo simulations based on the BFO‐theory (Bagchi, B.; Fleming, G.R.; Oxtoby, D.W.J. Chem. Phys. 1983. 78: 7375) for condensed‐phase barrierless isomerizations, and the viscosity dependence of the long‐time exponential decay is well reproduced with a coherent set of parameters. In the most viscous solvents, early‐time (≲ 1 ps) kinetics are observed which are characteristic of damped wavepacket dynamics in the reaction coordinate.
Picosecond Resonance Coherent Anti‐Stokes Raman Spectroscopy of Light‐ and Dark‐Adapted Bacteriorhodopsin
Authors: Ujj L. , Atkinson G.H.
Published: November 2013

Coherent anti‐Stokes Raman scattering (CARS) from the C=C stretching band region of retinal, recorded with low‐power, 7‐ps (fwhm) laser excitation selected to be in electronic resonance with the retinal chromophore, is presented for light‐ and dark‐adapted bacteriorhodopsin (BR) samples. By minimizing photochemistry with low‐power excitation, the picosecond resonance CARS (PR/CARS) spectrum of BR‐570 (13‐trans, 15‐anti‐retinal) is obtained directly from the light‐adapted BR sample. The PR/CARS spectrum of BR‐548 (13‐cis, 15‐syn retinal) is derived from a quantitative analysis of PR/CARS data from dark‐adapted BR which contains comparable amounts of BR‐570 and BR‐548. Band origin positions, lineshapes, relative intensities, and phase factors describing the electronic resonances are obtained from a quantitative fit of the PR/CARS spectra to third‐order susceptibility (χ(3)) relationships. These PR/CARS data, the first reported for a molecular system as large as BR, demonstrate the experimental viability of CARS for recording the vibrational spectrum of a chromophore within a trans‐membrane protein. In addition, the χ(3) analysis shows that a CARS spectrum of a mixture of isomeric chromophores can be quantitatively separated into CARS data assignable to its individual components. The significance of these results with respect to analyzing picosecond time‐resolved CARS data from intermediates in the BR photocycle is discussed.
Some Perspectives on Quantum Calculations
Authors: Davidson Ernest R.
Published: November 2013

An interpretation of the first‐order wave function is presented in terms of correlation factors in the two‐body distribution function. An argument is given for using N(N‐1)/2 instead of N/2 as the number of pairs in approximate corrections to Single and Double Configuration Interaction (SDCI).
Does Hydride Ion Transfer from Silanes to Carbenium Ions Proceed Via a Rate‐Determining Formation of a Silicenium Ion or Via a Rate‐Determining Electron Transfer? An Ab Initio Quantum Mechanical Study and a Curve‐Crossing Analysis
Authors: Apeloig Yitzhak , Merin‐Aharoni Osnat , Danovich David , Ioffe Alexander , Shaik Sason
Published: November 2013

The hydride transfer reactions from simple silanes to carbenium ions are studied by ab initio calculations. The simplest reaction, H4Si + CH3+ → H3Si+ + CH4, is also studied with inclusion of the solvent effect (with the SCRF method) in the ab initio scheme. Under all conditions the preferred mechanism is the synchronous hydride transfer (SHT), which is barrierless in the gas phase but possesses small barriers in solution. The mechanistic alternative involving a rate‐determining single electron transfer (SET) step followed by H‐atom abstraction is found to be of very high energy. Modelling of the primary isotope effect for the SHT process of H3SiH(D) + CH3* → H3Si+ + H3CH(D) shows that the primary isotope effect is small, between ca. 1.1 and 2.7, for the entire relevant range of Si—H(D) distances (1.5–2.3 Å). Furthermore, the pattern of the computed primary isotope effect shows it to be an insensitive probe of the SHT mechanism.

The curve‐crossing method is used to model the mechanistic dichotomy. It is shown that the reaction profiles for both SHT and SET arise from an avoided crossing between the ground state and a charge transfer state of the R3SiH//R′3C+ reactant pair. Thus, in the SHT mechanism a single electron switches sites in synchronicity with bond reorganization, while in SET the electron switch precedes the bond coupling. This avoided bond coupling is the foremost disadvantage of the SET mechanism. The common origin of the avoided crossing elucidates the reason why SHT exhibits characteristics of an electron transfer process without actually being a SET process.
Energy Trapping in Dications
Authors: Basch Harold , Hoz Shmaryahu , Goldberg Moshe
Published: November 2013

The energy interaction curves of a number of diatomic and polyatomic dication systems were calculated in order to study their energy‐trapping properties. Generally, the ab initio complete active space multiconfiguration self‐consistent field method was used in an extended valence + polarization basis set, with compact effective potentials replacing the core electrons. The diatomic dications include all ten possible binary combinations of oxygen, sulphur, selenium, and tellurium. O22+ shows the largest exothermicity, measured from equilibrium to the monocation combination asymptote, and highest barrier to dissociation. The calculated equilibrium bond length and harmonic vibrational frequency agree very well with experiment. The O22+, SO2+, SeO2+, and TeO2+ series show progressively decreasing exothermicities but similar barrier heights. The non‐oxides, in contrast, show similar exothermicities but decreasing barriers with increasing size of the atom constituents. These trends are interpreted in terms of both valence bond curve‐crossing and molecular orbital bonding models.

The ozone dication, O32+, is found to have a number of low‐lying singlet and triplet stationary state structures spanning near‐linear to D3h2+ symmetries. Although the calculated exothermicity is even larger than for O22+, the barrier to O2+ + O+ dissociation is predicted to be low in each case.

O22+ surrounded by six argon atoms to model an isolating environment shows increased equilibrium O–O bond length, decreased exothermicity, and increased barrier to dissociation, relative to the bare dication. O22+ flanked at each end by a perpendicularly oriented H2 molecule in a staggered conformation is obstructed from direct conversion to the water dimer dication by a high barrier. However, [(H2O)2]2+ dissociates smoothly from equilibrium to two water monocations with a large exothermicity but a small barrier.
A Continuum Solvation Model for the AM1 Semi‐Empirical Method
Authors: Dixon Richard W. , Leonard Joseph M. , Hehre Warren J.
Published: November 2013

We outline a continuum solvation model for aqueous solution similar to the recent SMx models of Cramer and Truhlar.1–3 As in those models, the solvation energy is given as a sum of polarization and cavity‐dispersion contributions. The numerical integration required for calculation of the polarization energy in the SMx models has been replaced by a scaled two‐center Coulomb integral. The cavity‐dispersion energy is based on molecular volume and includes a term involving the molecular dipole moment. The present method is more accurate than the SMx models and is considerably less time‐consuming.
Dynamics of the Inversion Reaction
Authors: Bersohn Richard , Kawasaki Masahiro
Published: November 2013

Three examples are presented of homolytic bimolecular substitutions, F + CH3I, H + CD4, and H + SiD4. All are shown by laser techniques to proceed by inversion. For the latter two reactions, the proof is based on the demonstration that the entering H atom and the leaving D atom have approximately parallel velocities. The isotopic substitution reactions are almost vibrationally adiabatic in the sense that the departing D atom carries away about 80% of the kinetic energy of the incoming H atom.
Doppler‐Resolved Laser Probing of Photon‐Initiated Bimolecular Collisions O(1D) + CH4 → OH(v,N) + CH3
Authors: Brouard M. , Duxon S.P. , Simons J.P.
Published: November 2013

The stereodynamics of the reaction O(1D) + CH4 → OH(v ≤ 4,N) + CH3 at collision energies of ˜ 40 kJ mol−1, has been probed via Doppler‐resolved, polarized laser‐induced fluorescence spectroscopy. Velocity‐aligned, reagent O(1D) atoms were generated under bulb conditions via polarized laser photodissociation of N2O. Analysis of the Doppler profiles of nascent OH(v,N) fragments has allowed both scalar and vectorial product correlations to be determined, including differential scattering cross sections and rotational alignments. These imply the operation of a ‘delayed’ collision mechanism, dominated by the deeply attractive potential energy surface associated with the insertion to form CH3OH; the intermediacy of a long‐lived complex is excluded, however. Rotational angular momentum in the recoiling OH fragments is preferentially aligned perpendicular to, and azimuthally distributed about, their recoil velocity. The rotational excitation is thereby attributed to bending motion in the C···O···H plane during the reagent collisions. The scalar product pair correlations establish a near‐zero translational exoergicity and anticorrelated internal energy distributions in the two reaction products.
Two‐Photon Photoassisted Reactions of Collision Pairs Consisting of Xe with Halogen‐Containing Polyatomic Molecules
Authors: Huang Feng , Setser D.W. , Cheong B.S.
Published: November 2013

Excitation spectra have been acquired for the two‐photon, laser‐assisted reactions between collision pairs of Xe with CCl4, CCl1Br, CH3I, CH2I2, CH2ICl, and SOCl2. In each case the product of the reaction is XeX(B,C); X = halogen. The excitation spectra obtained at 300 K in 100 Torr of Xe and 1.3 Torr of reagent have thresholds near 255 nm and increase toward shorter wavelength. Unexpected resonance‐like features have been found in the excitation spectra of Xe/CH2ICl and Xe/CCl3Br. A set of relative two‐photon excitation rates was obtained at 238 and 227 nm for all pairs by comparison to CCl4, which was selected as a standard. The excitation spectra for the polyatomic molecular pairs can be compared to previously published excitation spectra of diatomic halogen molecules with Xe. The two‐photon, laser‐assisted reaction of the Xe·F2 van der Waals molecule prepared by free‐jet expansion also is reported.
Molecular Modeling of Heme Peptides: Molecular Dynamics Simulation of Microperoxidase
Authors: Ranghino Graziella , Antonini Giovanni , Fantucci Piercarlo
Published: November 2013

The simulation of the dynamic behavior and of the aggregation of a peptide containing a heme group (MP11) is performed. The force field terms describing the transition metal come from a newly fitted potential which gives the correct heme structure and flexibility. The model of an aggregated state is the dimer of MP11; a possible sixth ligand of the originally pentacoordinated iron of one heme could be either a water molecule or a lysine side chain of the second peptide, but the distances to the iron are longer than expected in both cases.