bluuebunny/crossref_metadata_2025_split huggingface dataset:
The Ionization of Alkanes
Authors: Bouma Willem J. , Poppinger Dieter , Radom Leo 
Published: November 2013

AbstractAb initio molecular orbital theory has been used to study the ionization of methane, ethane, propane, n‐butane, isobutane, cyclopropane and cyclobutane. For methane radical cation, the preferred structure has C20 symmetry and a pair of long C—H bonds, resembling a complex of CH+2 with H2. This appears to be the only significant minimum on the CH+4 surface. For the remaining acyclic systems, the calculated structures of lowest energy (e.g., CH3‐CH2—CH+3, CH3‐CH2—CH2‐CH+3) are characterized by the extreme lengthening of a single C—C bond. However, alternative structures with two or more elongated C—C bonds have very low relative energies. The main structural conclusion for the alkane and cycloalkane radical cations is that the potential energy surface connecting various possible isomeric structures is generally very flat. Facile wide amplitude distortions and scrambling processes are therefore expected in these ions. The large structural changes which accompany ionization are manifested in large differences between vertical and adiabatic ionization potentials.
Theoretical Determination of Molecular Structure and Conformation. XI. The Puckering of Oxolanes
Authors: Cremer Dieter 
Published: November 2013

AbstractStructural, conformational and energetic properties of cyclopentane (1) and the seven oxolanes monoxolane (2), 1,3‐dioxolane (3), 1,2‐dioxolane (4), 1,2,4‐trioxolane (5), 1,2,3‐trioxolane (6), tetroxolane (7), and pentoxolane (8) are investigated employing the polarized 6‐31G* basis set at the Hartree‐Fock level of theory. Extensive geometry optimization is carried out within the model of the semirigid pseudorotor. The conformational potentials V of compounds 1–8 are evaluated as a function of the puckering amplitude q and the pseudorotation phase angle Φ. Ring molecules 1 and 8 are free pseudorotors, while pseudorotation is hindered by barriers ≤ 3.3 kcal/mol for oxolanes 2–7. Puckering and inversion barriers increase with the number of O‐O bonds but decrease with the number of ether bridges. Puckered C2‐symmetrical twist forms are the most stable conformations for compounds 2–7 but 6, where highest stability is found for the C2‐symmetrical envelope forms. At room temperature a multitude of conformers of 1–8 coexists either because of free pseudorotation (barriers < RT) or large amplitudes of pseudolibration. These results are rationalized in terms of the rotor potentials of appropriate reference compounds (C2H6, CH3OH, H2O2). A more elegant approach leads to a simple π electron count and an analysis of bonding and antibonding overlap in the π‐type HOMO's. In this way the effects of substituents can be predicted and conformational preferences of the furanose ring in nucleotides, nucleosides and carbohydrates explained. The relative stability of the oxolanes is analyzed by calculating O‐O bond energies, bond‐bond interactions and ring strain for each compound. The lability of the higher oxolanes is traced back to increased ring strain. A new method of the conformational analysis of ring compounds is outlined.
NMR Determinations of Molecular Geometries in Liquid Crystal Media
Authors: Long Robert C. , Goldstein J. H. 
Published: November 2013

AbstractThe application of NMR spectroscopy in oriented media to the determination of molecular geometries is illustrated by results for three distinctly different molecules: naphthalene, ethylene oxide and spiropentane. The need for systematic application of vibrational corrections is emphasized and some of the effects associated with the different choices of solvent media are considered.
The 1H and 13C Magnetic Shielding Tensors of Ethylene‐1,2–13C Determined by NMR of the Oriented Molecule
Authors: Diehl P. , Moia F. 
Published: November 2013

AbstractWe have determined the elements of the 1H– and 13C‐chemical shift tensors of ethylene‐1,2–13C dissolved in various liquid crystals with methane –13C as a reference substance. The 1H‐results have the same relative order of magnitude as the theoretical predictions. The 13C‐results agree perfectly with theory but differ slightly from solid state data. Deviations must be attributed to variations in local effect contributions to the shifts in the various solvents which cannot be eliminated.
Dynamics of Molecular Processes by NMR in Liquid Crystalline Solvents
Authors: Luz Z. 
Published: November 2013

AbstractThe effect of dynamic processes on the lineshapes of NMR spectra in liquid crystalline solutions is discussed. The discussion includes dynamic 1H spectra of s‐trioxane, cyclooctatetraene and bullvalene undergoing, respectively, ring inversion, bond shift and bond rearrangement. Also discussed are dynamic deuteron spectra using the examples of cyclohexane‐d12 and p‐dioxane‐d8 undergoing ring inversion. Finally the formalism required to simulate dynamic NMR spectra in liquid crystalline solution is reviewed and the possible factorization of the problem by symmetry considerations is discussed.
Infrared Photochemistry with Shaped Laser Pulses
Authors: Ashfold M. N. R. , Atkins C. G. , Hancock G. , Macrobert A. J. 
Published: November 2013

AbstractA relatively simple design for a hybrid CO2 laser and electrooptic crystal switch is described, which can be used to produce pulses with fast (< 10 ns) rise and fall times with constant power over their duration (50–1000 ns). Gaussian beam profiles allow separate and quantitative determination of both fluence (J cm−2) and intensity (W cm−2) dependences of processes induced by IR multiple photon absorption (MPA). Visible luminescence accompanying MPA of OsO4 is shown to depend on intensity, and the dependences of MPA cross sections in SF6 on fluence and intensity are derived from optoacoustic and long path absorption measurements.
IR Multiphoton Excitation of Small Asymmetric Top Molecules: Rotational and Vibrational Inversions for Laser Action in Ozone
Authors: Quack M. , Sutcliffe E. 
Published: November 2013

AbstractDetailed quantum‐dynamical calculations for the time‐evolution matrix of the IR multiphoton excitation of a small asymmetric molecule, including vibrational and rotational couplings, are presented. It is shown that rotationally and vibrationally specific multiphoton excitation is possible and gives inversions that are potentially useful for lasers in the IR and FIR.
Infrared Multiple Photon Photochemistry of Gas Phase Ions
Authors: Tumas William , Foster Robert F. , Brauman John I. 
Published: November 2013

AbstractAn ion cyclotron resonance (ICR) spectrometer can be used to generate, trap, and detect gas phase ions which are irradiated with an infrared laser. Infrared multiple photon (IRMP) photochemistry can be conveniently studied under these conditions. Results of several recent studies are discussed.
The Structure of Valinomycin Cesium Picrate
Authors: Steinrauf L. K. , Folting Kirsten 
Published: November 2013

AbstractThe structure by X‐ray crystallography of the valinomycin–cesium picrate complex has been determined by the heavy atom method. The crystal class was trigonal, space group P31, with unit cell dimensions a = b = 14.028(4) â, 31.888(10) Ǎ, Z = 3, filtered Mo radiation; the final value of R was 0.111 for 4088 reflections measured at − 170 °C. The enclosed cation was found to be too large for the cavity, causing the valinomycin to be incompletely closed and forming a weak bond with the picrate at the D‐valyl end, which was the same end as had been previously found to be involved in the sodium complex. This result is in agreement with the proposal that the cation enters and exits by the D‐valyl end.
The Redox Chemistry of Manganese(III) and ‐(IV) Complexes
Authors: Yamaguchi Kenneth S. , Sawyer Donald T. 
Published: November 2013

AbstractThe oxidation‐reduction thermodynamics for the manganese(III), ‐(IV), and ‐(II) ions, and their various complexes, are reviewed for both aqueous and aprotic media. In aqueous solutions the reduction potential for the manganese(III)/(II) couple has values that range from +1.51 V vs. NHE (hydrate at pH 0) to −0.95 V (glucarate complex at pH 13.5). The Mn(IV)/(III) couple has values that range from +1.0 V (solid MnIVO3 at pH 0) to −0.04 V (tris gluconate complex at pH 13.5). With anhydrous media the propensity for the Mn(III) ion to disproportionate to solid MnIVO2 and Mn(II) ion is avoided. For aprotic systems the range of redox potentials for various manganese complexes is from +2.01 V and +1.30 for the Mn(IV)/(III) and Mn(III)/(II) couples (bis terpyridyl tri‐N‐oxide complex in MeCN), respectively, to −0.96 V for the Mn(IV)/(III) couple (tris 3,5,‐di‐tert‐butylcatecholate complex in Me2SO). The redox reactions between manganese complexes and dioxygen species (O2, O2, and H2O2) also are reviewed.
Aggregate Structures of Carboxylic Acids and Amides
Authors: Etter Margaret C. 
Published: November 2013

AbstractA configurational analysis of hydrogen‐bonded aggregates of carboxylic acids and amides has been developed using a notation derived from graph theory. This concept of aggregate structure, independent of crystal structure, is useful for making correlations between intermolecular interactions in different phases, and for comparing physicochemical properties of isostructural aggregates. The analysis also provides a means for assessing the role of intermolecular bonds on the course of reactions or phase transformations which take place in aggregate structures.
Phase Transfer Catalyzed Bromide—Chloride Exchange: Dependence of Equilibrium Position and Selectivity Constant on Nature and Composition of Aqueous Phase
Authors: Yonovich‐Weiss Minda , Sasson Yoel 
Published: November 2013

AbstractThe position of the equilibrium in the PTC exchange reaction of alkyl chloride with metal bromide salts RCl + MBr ⇌ RBr + MCI under various condtions is utilized for calculation of the selectivity constant KselCl/Br. This constant is found to be dependent on the nature of the metal cation and the inorganic phase composition. KselCl/Br can be defined for any point on the phase diagram MCl—MBr—H2O at a given temperature, solvent and quaternary cation.
Computer Simulation of Enzymatic Reactions
Authors: Warshel A. , Russell S. , Sussman F. 
Published: November 2013

AbstractOur strategy in modeling ezymatic reactions is demonstrated by considering two complimentary approaches, which are based on the powerful EVB method. The first approach uses the X‐ray coordinates at their face value and estimates the reaction potential surface by using a simplified and efficient microscopic dipolar model for the solvent. The second approach (which became practical only upon the emergence of supercomputers) uses a surface constrained all atom solvent model, molecular dynamics (MD) and the umbrella sampling method to evaluate the actual free energy barrier. Both approaches indicate that the correct evaluation of the “solvation energy” of the ionic resonance structures is the key for understanding enzyme catalysis. The MD version of the EVB method is used in a preliminary study of the validity of linear free‐energy relationships in enzymatic reactions. This study supports the ad hoc assumptions introduced in the simplified method.
1,3‐Diketones as Active Ligands in the Nickel‐Catalyzed Linear Oligomerization of Olefins
Authors: Behr A. , Falbe V. , Freudenberg U. , Keim W. 
Published: November 2013

AbstractThe novel complex (4‐cyclooctene‐l‐yl)‐ bis(trifluoroacetamido) nickel oligomerizes ethene to olefins of about 85% linearity. Unusual high activities of 11500 mol ethene/mol nickel were obtained. Also higher α‐olefins such as propene, butene or octene could be oligomerized yielding 70‐80% linear dimers. By using (4‐cyclooctene‐1‐yl)(1,1,1,5,5,5‐hexafluoro‐2,4‐pentanedionato) nickel activated by BF3 the bulky olefin neohexene was converted selectively into dimers, which are linear, considering the C‐C‐linkage.
Reduction of n‐C5H11N3 Catalyzed by [Mo2Fe6S8(OMe)3(SC6H4‐p‐n‐C8H17)6]3– in an Aqueous Micellar Solution
Authors: Moriya Koji Tanaka Makoto , Tanaka Toshio 
Published: November 2013

AbstractReduction of n‐C5H11N3 by Na2S2O4 was performed in the presence of (n‐Bu4N)3 [Mo2Fe6S8(OMe)3(SC6H4‐p‐n‐C8H17)6] ((n‐Bu4N)3 [Mo‐Fe]) as a catalyst in aqueous Triton X‐100 micellar solutions. The rate of the reduction is enhanced efficiently by the addition of methyl viologen (MV2+). The methyl viologen radical cation (MV+) produced by the reaction of MV2+ with Na2S2O4 undergoes a disproportionation reaction to afford MV2+ and MV° in the micellar solution. The resultant MV° formed in the micelle transfers two electrons to [Mo‐Fe]3− effectively to give [Mo‐Fe]5−, which reduces n‐C5H11N3 with two electrons to produce n‐C5H11NH2 and N2.
EPR Characteristics of the Electron Acceptors A0, A1, and (Fe‐S)X in Digitonin and Triton X‐100 Solubilized Pea Photosystem I
Authors: Mansfield Royston W. , Evans Michael C.W. 
Published: November 2013

AbstractPhotosystem I (PS I) particles prepared from pea chloroplasts using either digitonin or Triton X‐100 as the solubilizing detergent were used to investigate the EPR characteristics of the electron acceptors A0, A1, and (Fe‐S)X. The signal from component A1 appears asymmetric in both digitonin and Triton X‐100 solubilized samples. However, the difference spectrum which represents the isolated signal from component A0 is symmetric in Triton X‐100 solution, but not in digitonin. Upon deuteration of digitonin solubilized PS I, the signal from A1 became partially narrowed to give a spectrum comparable to that of deuterated Triton X‐100 solubilized particles. However, deuteration did not produce new spectral features in the digitonin solubilized preparation, but emphasized those already present Upon storage of either type of particles at 77 K, it was found that the signal from component A1, but not that from A0, decayed. The difference spectrum of A1 obtained with g = 2.0058 and a peak‐to‐peak linewidth of 0.95 mT is compatible with the identification of A1 as an anionic semiquinone. In digitonin solubilized particles, component A0 and not A1 affected the EPR signal of the iron‐sulphur center X, which suggests a magnetic interaction between A0 and X. These results suggest that component A0 is closer to X than is A1, and that A1 may not be in a linear electron transport pathway between A0 and the iron‐sulphur centers. They also show that Triton X‐100 perturbs the structure of the reaction center.
Fourier Transform‐EPR Spectroscopy of Electron Transfer from Excited State of Chlorophyll and Porphyrin to Duroquinone
Authors: Bowman M.K. , Toporowicz M. , Norms J.R. , Michalski T.J. , Angerhofer A. , Levanon H. 
Published: November 2013

AbstractElectron transfer reactions between the photoexcited triplets of several porphyrin and chlorophyll precursors, 3P (electron donors), and neutral duroquinone, DQ (electron acceptor), were studied by selective laser excitation‐Fourier transform (FT)‐EPR spectroscopy. Four different P, which are related to the photosynthetic apparatus, were examined in ethanol solutions at −243 K: pyrochlorophyll a (pChla), zinc‐pyrochlorophyll a (ZnpChla), zinc‐tetraphenylporphyrin (ZnTPP), and magnesium‐tetraphenylporphyrin (MgTPP). This highly time‐resolved EPR spectroscopy (∼10 ns delay time, ẗd, between the laser excitation and the detection pulse) permits differentiation between the various CIDEP mechanisms, i.e., triplet mechanism (TM) and radical‐pair mechanism (RPM), associated with the electron transfer process. The spectra of the DQ.− generated by the different triplet precursors strongly depend on two inherent parameters characteristic of the molecular system: (1) the zero‐field splitting (ZFS) and the selective singlet‐triplet (Ti l̊ S1) population rates, Ai (i = x, y, z); (2) the difference in g‐values and hyperfine components between the donor and acceptor radicals. Since the differences in g‐values and hyperfines are very nearly the same in all four P.+‐DQ.− systems examined, the different results, particularly in the time range 10 ns < ẗ<d < 300 ns, are attributed to triplet spin memory transfer via TM. A quantitative spectral analysis is presented to describe the time evolution FT‐EPR spectra. This method is used to determine the following dynamic processes: (1) spin lattice relaxation rates of the triplet precursors in solution; (2) donor‐acceptor electron transfer reaction rates; (3) spin lattice relaxation rates of the acceptor radical.
Techniques for the Measurement of Small Anisotropies in Magic Angle Spinning NMR Spectra
Authors: Raleigh D.P. , Kolbert A.C. , Levitt M.H. , Griffin R.G. 
Published: November 2013

AbstractTwo novel methods for enhancing the effect of small anisotropies on magic angle spinning (MAS) NMR spectra are described. The first method is a simple two‐dimensional (2D) spin echo experiment, which generates rotational sidebands at a fraction of the spinning frequency in the ω1 dimension, while maintaining the effective spinning speed in ω2. It is shown that the rotational sidebands in the 2D spectrum are much larger than in a normal one‐dimensional MAS spectrum obtained at the same spinning speed, and the relative intensities of the two‐dimensional sideband array can be analyzed to yield the anisotropy and asymmetry parameter of the coupling tensor. The second method is designed to measure homonuclear dipolar couplings. We show that the effect of a dipolar coupling between members of an isolated spin pair can be enhanced by adjusting the spinning speed such that nωr = ωΔiso, where n is an integer, ωr is the spinning speed, and ωΔiso is the difference between isotropic shifts. When this condition, termed rotational resonance, is satisfied, a broadening or a splitting of the normally sharp resonance lines is observed. In addition, rapid exchange of Zeeman order between the dipolar coupled spins is seen. This method holds promise for the measurement of through space dipolar couplings and thus internuclear distances in polycrystalline solids.
Hyperfine Interactions in the Lowest Triplet State of Pyridine
Authors: Groenen E.J.J. , Buma W.J. , Schmidt J. 
Published: November 2013

AbstractResults are reported of nitrogen and deuterium hyperfine studies of the triplet state of pyridine‐d5 as a guest in a single crystal of benzene at 1.2 K. Through the use of electron spin echo (ESE) techniques complete hyperfine tensors have been obtained: for nitrogen from the ESE detected EPR spectra and for the para‐deuterium atom from the modulations of the ESE envelope. These tensors unambiguously show that the molecule is no longer planar in the lowest triplet state T0. Pyridine's structure considerably changes upon excitation, but the mirror plane perpendicular to the molecule is preserved. Experimental and theoretical evidence is presented for the fact that this deformation is an intrinsic property of T0 and is not induced by the crystal field. About fourty percent of the triplet electron spin density is found to be localised in the π‐orbital of nitrogen, about ten percent in the lone pair orbital of nitrogen and somewhat more than thirty percent in the π‐orbital of the para‐carbon atom.
Steric Modulation of the Porphycene System by Alkyl Substituents: 9,10,19,20‐Tetraalkylporphycenes
Authors: Vogel Emanuel , Köcher Matthias , Lex Johann , Ermer Otto 
Published: November 2013

AbstractPorphycene (2), a planar structural isomer of porphyrin (1) recognized and synthesized only recently, exhibits short N … N distances allowing exceptionally strong N‐H … N hydrogen bonds to be formed. Although the N4‐coordination hole of 2 is smaller than that of 1, the new tetrapyrrolic macrocycle is capable of complexation with a broad spectrum of metal ions to give the respective metalloporphycenes. In accord with model considerations, both N‐H … N hydrogen bonding and complexation of 2 may be modulated by peripheral alkyl groups through changes in the geometry of the porphycene skeleton arising from non‐bonded repulsions between the substituents. In this publication are delineated some of the consequences of structural variation of 2 by introduction of alkyl groups at the 9,10,19,20‐positions.
Photodynamics of Small Aliphatic Ketones in a Supersonic Jet Expansion
Authors: Haas Yehuda , Zuckermann Hanna 
Published: November 2013

AbstractThe photodynamics of acetone and some other simple aliphatic ketones in the nπ* state are discussed in the light of recent results obtained in seeded supersonic beams. The Born‐Oppenheimer approximation is used to describe the sequence of events beginning with light absorption and ending in molecular fragmentation. Intersystem crossing is shown to be an important process even for the isolated molecule. Its rate increases with the internal energy and may be related to the density of states in the triplet state. For all molecules studied so far, the results can be rationalized by assuming that dissociation takes place on the triplet surface, the S1 surface being stable in the energy range pertinent to these experiments (up to 35,000 cm−1 above the ground state). Fluorescence spectroscopy may be used also to detect small clusters. In the case of acetone, their presence is indicated by the decrease in the intensity of some spectral lines as a function of cooling efficiency. The results are used to estimate an upper limit for the van der Waals binding energies.
Light Scattering Probes of the H + H2 Reaction
Authors: Krause Jeffrey L. , Shapiro Moshe 
Published: November 2013

AbstractWe present calculated Raman and CARS (coherent anti‐Stokes Raman scattering) spectra of the collinear H + H2 reaction. These spectra, obtained by detuning the laser frequencies from all H2 absorptions, show strong enhancements as the collision energy on the ground state potential energy surface is scanned through a reactive scattering resonance. We also demonstrate that the Rayleigh scattering and degenerate four‐wave mixing spectra show enhancements greater by orders of magnitude than the corresponding Raman and CARS peaks. The experiments suggested in this paper offer a unique, direct method for obtaining information about the vibrational dynamics of the transition state of chemical reactions.
Superexchange‐Assisted Through‐Bridge Electron Transfer: Electronic and Dynamical Aspects
Authors: Kosloff Ronnie , Ratner Mark A. 
Published: November 2013

AbstractThe effects of electronic structure on bridge‐assisted electron transfer are considered. Using static perturbation theory, time‐dependent perturbation theory, and direct time‐dependent dynamics within generalized tight binding models, we examine the role of energy gaps, relative energetics of donor and acceptor orbitals with hole‐type and electron‐type superexchange sites, damping and dephasing, and overall energetics in electron transfer. We find that a generalization of the simple McConnell relationship to nonresonant transfer can in fact be made, but that no simple formula describes all limits; this is important in applications to a number of systems, including photosynthetic reaction centers, in which such superexchange occurs. The dynamical studies indicate some important and unexpected phenomena: these include quantum interferences between different pathways, recurrences and oscillations, and competitive effects of hole‐type and electron‐type superexchange. We suggest that direct dynamical study, as has begun to appear from several laboratories, provides an excellent way to visualize the intermediate state contributions to intramolecular electron transfer processes.