bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

AbstractThe 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

AbstractThe 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

AbstractFive 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

AbstractThe 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

AbstractWe 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

AbstractVarious 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

AbstractDiffusion 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

AbstractThe 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

AbstractOptical 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

AbstractThe 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

AbstractCoherent 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.
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

AbstractThe 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

AbstractThe 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

AbstractWe 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

AbstractThree 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

AbstractThe 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

AbstractExcitation 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

AbstractThe 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.
Identification of Short Turn Motifs in Proteins Using Sequence and Structure Fingerprints
Authors: Wintjens René T. , Rooman Marianne J. , Wodak Shoshana J. 
Published: November 2013

AbstractFamilies of 20‐residue turn motifs are identified in a dataset of known protein structures using a fully automatic classification procedure that relies on dihedral angle values and atomic root mean square deviations of the polypeptide backbone. Four of the identified motifs, a novel αα connection, and three well‐known αα, βα, and ββ motifs, are used to investigate the possibility of identifying sequences that adopt these motifs in a library of 20‐residue sequence segments from the protein dataset. To this end, several types of fingerprints are derived for individual members of each turn motif family, and for each family as a whole. These fingerprints represent the sequence conservation among family members, or different reduced descriptions of the protein 3D structure that consider the backbone conformation or the tertiary interactions in the context of the parent proteins. All sequence segments in the library are successively mounted onto the fingerprints, without allowing gaps, and the energy of each mount is evaluated using effective potentials derived from known protein structures. The results show that the ability to recognize native sequences associated with a turn motif is improved when different types of fingerprints are combined, and that it fluctuates significantly according to the specific turn family considered. Overall, however, this ability remains rather limited to a level which is much below the native recognition performance generally achieved for full proteins. The fingerprints and their associated potentials are nevertheless found to be quite effective in generating subsets of solutions that are significantly enriched for the correct sequence‐structure combinations. This may have very useful applications in protein folding simulations and in homology modeling.