bluuebunny/crossref_metadata_2025 huggingface dataset:
Metalloporphyrins in Oxidative Catalysis. Oxygen Transfer Reactions of Oxochromium Porphyrins
Authors: Groves John T. , Kruper William J.
Published: November 2013

The oxidation of chloro‐5‐10,15,20‐tetramesitylporphyrinotoiron(III) with peroxyacids affords a reactive oxoiron(IV)‐porphyrin cation radical species 2. The characterization of 2 and its oxochromium analogs 3, 4 and 5 are reviewed. The nature of reactive oxochromium species derived from chromyl reagents is also reviewed. The oxidation of triphenylphosphine by CrOTPP (11), CrOTTP (13) and CrOTMP (14) is described. Variations in the rate constants indicate that steric factors affect the rate of oxygen atom transfer. Activation parameters for the oxidation of triphenylphosphine by 14 are ΔH≠ = 6.96 kcal/mol and ΔS≠ = −39 eu. The oxidation of t‐butylphenylcarbinol (18) by CrOTPP gave predominantly benzaldehyde via carbon—carbon bond cleavage while the chromium(III) porphyrin‐catalyzed oxidation of 18 by iodosylbenzene afforded t‐butylphenylketone.
Complexes of Hydrogen Peroxide with Dioxoactinide(VI) Species in Aqueous Carbonate and Bicarbonate Media. Formation of An(VI)–H2O2 Complexes
Authors: Thompson Mary E. , Nash Kenneth L. , Sullivan James C.
Published: November 2013

The spectra for 1:1 complexes formed between triscarbonatouranium(VI) + H2O2 and triscarbanatoneptunium(VI) + H2O2 are presented. The respective rates of formation (25°C, 0.05 M NA2CO3) are 565 ± 41 M−1 s−1 and (2.19 ± .01) X 103 M−1 s−1. The corresponding activation parameters are ΔH* = 67.8 ± 3.2 kJ/m, 43.6 ± 2.0 kJ/m, ΔS* = 30 ± 11 J/m °K and −36 ± 7 J/m °K, respectively. The U(VI) complex appears to be stable over a period of months while the Np(VI) complex is formed as a transient species that disappears via a complex process.
Pulse and Continuous Radiolyses of the Europium and Neodymium Decatungstate Anions in Aqueous Solution
Authors: Mulazzani Quinto G. , Venturi Margherita , Ballardini Roberto , Gandolfi Maria T. , Balzani Vincenzo
Published: November 2013

We have studied the behavior of aqueous solutions of the [M(3+)(W5‐O18(6‐))2]9− complexes (M = Eu or Nd) under pulse and continuous radiolysis conditions. Both complexes react with e−aq at a diffusion controlled rate. For the Nd3+ complex in solution containing 1% 2‐propanol, the decay of the e−aq absorption was accompanied by the formation of a broad absorption band in the visible (λmax 600–650 nm), with a further increase in absorption up to 2 ms. For the Eu3+ complex under the same conditions, the decay of the e−aq absorption was accompanied by the formation of an absorption maximum at ∼370 nm, which is characteristic of Eu2+ species. On a longer time scale, a relatively intense absorption arose in the visible (λmax 600–650 nm) according to a complicated kinetic behavior. The two complexes exhibited a quite similar behavior in continuous radiolysis experiments. Blue products (λmax 600–650 nm) were formed and, for the Eu3+ complex, the disappearance of the Eu3+ emission was observed. Even after high irradiation doses, the absorption (and, for Eu3+, emission) spectra of the solutions were quantitatively regenerated by air oxidation. The behavior of the Nd3+ complex is interpreted on the basis of the reduction of a W5 O186− ligand by e−aq and alcohol radicals, followed by protonation. For the Eu3+ complex, the reduction by e−aq causes mainly the formation of a Eu2+ complex. On a longer time scale, protonation and/or reaction with alcohol radicals drive back the electron from Eu2+ to the W5O186− ligands, with formation of stable blue products.
Small Rhodium Aggregates with Unusual Oxidation States. The Synthesis and Structure of [Rh3 ((Ph2PCH2)2PPh)2(CO)I4] [BPh4] · 0.77 Acetone
Authors: Balch Alan L. , Olmstead Marilyn M.
Published: November 2013

The oxidative behavior of rhodium(I) isocyanides and dimers bridged by diisocyanides and diphosphines, which leads to the formation of rhodium—rhodium bonded complexes in the +2, +5\3, +3\2 oxidation states, is reviewed. The results of an X‐ray crystallographic investigation of a new, related compound, [Rh3 ((Ph2PCH2)2‐PPh)2(CO)I4] [BPh4] · 0.77 C3H6O, are described.
Protection of 4‐Fluoroanisole from Aromatic Nucleophilic Photosubstitution by Cyclodextrins [1]
Authors: Liu Jerry H. , Weiss Richard G.
Published: November 2013

Aromatic nucleophilic photosubstitution reactions of 4‐fluoroanisole with cyanide and water have been investigated in aqueous α‐β‐, and γ‐cyclodextrin solutions. Although photoreactivity of the α‐ or β‐cyclodextrin‐complexed 4‐fluoroanisole toward nucleophilic substitution is suppressed almost completely, none of the cyclodextrins had a measurable influence on the rate of 4‐fluoroanisole fluorescence quenching by I−. In aqueous γ‐cyclodextrin, the efficiency of photohydroxylation was increased slightly but the efficiency of photocyanation was unaffected. From Benesi‐Hildebrand plots, the dissociation constants of the complexes of 4‐fluoroanisole with α‐ and β‐cyclodextrins were obtained. A good correlation between the relative quantum yields and the degree of complexation was found. Explanation for the lack of a marked effect on photosubstitution by γ‐cyclodextrin is presented.
The Effect of the Structural Elements on the Properties of Side Chain Mesomorphic Polymers
Authors: Cser F. , Horváth J. , Nyitrai K. , Hardy GY.
Published: November 2013

The thermal properties of mesomorphic polymers depend on the relative amounts of the different structural elements (hard core, flexible chains, main chain) of the polymer. Literature data are compared with the conclusions obtained from the three‐component thermodynamic model of side chain mesomorphic polymers. The effects of the different soft elements (main chain, spacer and p‐alkyl or alkoxy chain) depend first of all on the length of the spacer and its interaction with the main chain. The thermal properties of the polymer can be well regulated by varying the different structural elements of the homo‐ and co‐polymers. The glass transition temperature (Tg) of the polymer can be reduced by building O and N atoms into the main chain and/or by binding the side chains on 3rd, 4th, etc. atoms of the main chain. The Tg can be further reduced by increasing the length of the spacer. If the spacers are long enough, the layer type structures are favored, with p‐alkoxy chains behaving also as a plasticizer of the main chain. The clearing point can be influenced by copolymerization of monomers with different hard cores. The three‐component thermodynamic model of side‐chain mesomorphic polymers well explains the effect of different structural elements on the structure and properties of these polymers.
Influence of Dye Aggregation on Photosensitized Hole Injection into Anthracene Crystals from Langmuir‐Blodgett Monolayers Containing Cyanine Dyes
Authors: van Der Auweraer M. , Willig F.
Published: November 2013

Monolayers containing cyanine dyes can be transferred to anthracene crystals and are able to inject holes into the crystal upon excitation. At low dye coverages only the dye monomer injects holes while at higher coverages energy transfer to aggregates is the most important decay process of an excited dye monomer. These aggregates are also able to inject holes but with a more than ten times smaller efficiency. The occurrence of dye aggregates is confirmed by the absorption spectra of the monolayers. In this system non‐geminate recombination remains an unimportant pathway even at high dye coverages.
The Synthesis, Crystal and Molecular Structure, and Oxidation State of Iron Complex from Pyridoxal Isonicotinoyl Hydrazone and Ferrous Sulphate
Authors: Avramovici‐Grisaru Schely , Sarel Shalom , Cohen Shmuel , Bauminger Rivka E.
Published: November 2013

The reaction of isonicotinoyl hydrazone of pyridoxal (PIH), a biologically active iron‐carrier, with FeSO4‐7H20 at pH ∼ 6 generates the delta, lamda species of the N,N‐trans‐O,O‐cis‐cis coordination isomer of an iron(III) complex with iron‐to‐ligand ratio of 1:2. The dark red‐brown crystals are monoclinic, space group C2/c, with unit‐cell dimensions a = 14.487(2), b = 18.586(2), c = 27.508(4) Å, β = 102.76(3)°, and Z = 8. The coordination around the metal is distorted octahedral and involves the protonated organic ligands, which are chelated through the phenolic oxygen [Fe‐O1 1.941(6), Fe‐O1′ 1.938(6)], an enolic form of the carbonyl oxygen [Fe‐O3 2.017(6), Fe‐O3′ 2.018(6)] and the azomethinic nitrogen [Fe‐N2 2.133(8), Fe‐N2′ 2.133(8)]. Packing is determined by systems of N‐H….O and O‐H….O hydrogen bonds involving the protonated pyridoxal nitrogens, the pyridoxal hydroxymethyl group, and the [SO4]2− group. The Mössbauer spectra at different temperatures (300 K, 88 K and 4.1 K) clearly prove that the iron atom in the complex is in a high‐spin trivalent state.
The Effect of Temperature on Competing Photochemical Hydrogen Abstraction Reactions in the Solid State
Authors: Greenhough Trevor J. , Scheffer John R. , Secco Anthony S. , Trotter James , Walsh Leueen
Published: November 2013

Irradiation of 4aα,5,8,8aα‐tetrahydronaphthoquin‐1‐on‐4β‐o1 (1a) and its 6,7‐dimethyl derivative 1b in the solid state leads to two products. The first is formed via intramolecular hydrogen abstraction by carbonyl oxygen followed by radical coupling, and the second is produced through initial hydrogen abstraction by enone carbon and subsequent radical coupling. X‐ray crystallography reveals that both processes are topochemically allowed, and the geometric parameters for the hydrogen transfers and the radical coupling reactions are tabulated. In the case of 1b, the photoproduct ratio was found to be temperature dependent with hydrogen atom abstraction by carbon predominating at lower temperatures. From the slope of the linear plot of the natural logarithm of the photoproduct ratio versus the reciprocal of the temperature, an activation energy difference of approximately 4 kcal/mole is calculated. The results are interpreted as being due to competing reactions from (n,π*)3 and (π*)3 excited states, the latter (hydrogen abstraction by carbon) having a lower activation energy than the former (hydrogen abstraction by oxygen).
Effect of Molecular Symmetry and Intermolecular Halogen‐Halogen Interactions on the Crystal Structures of Halogen‐Substituted Benzoic Acids. X‐ray Crystal Structure of m‐Iodobenzoic Acid
Authors: Patil Anjali A. , Curtin David Y. , Paul Iain C.
Published: November 2013

The crystal structure of m‐iodobenzoic acid is reported. The crystals are monoclinic, P21/c, with a = 6.220(3) Å, b = 4.689(2) Å, c = 26.67(1) Å, β = 101.80(3)°, Z = 4, C7H5IO2. The structure has been determined by direct methods and refined to R 0.026 for 1288 reflections recorded with an automatic single crystal diffractometer. The structure is composed of essentially planar hydrogen‐bonded dimer units with significant intermolecular iodine‐iodine contacts. The configuration of the dimer unit is transoid. Since this is the penultimate member of the family of mono‐halogen‐substituted benzoic acids its structure is compared with those previously reported. In particular, the effects of differences in molecular geometry of the o‐, m‐, and p‐isomers and the strengths of the intermolecular halogen‐halogen interactions are surveyed. The geometry of the carboxyl group of m‐iodobenzoic acid in the solid state and the carbonyl absorption in the Ftir spectrum of the crystalline acid at room temperature suggest the presence of dynamic proton isomerism as has been recently found by other investigators in crystalline benzoic acid and some of its derivatives.
Mechanistic Aspects of Chiral Discrimination by Copper Complexes of N,N‐Dialkyl‐α‐Amino Acids
Authors: Weinstein Shulamith , Leiserowitz Leslie
Published: November 2013

A model for the separation of enantiomers of several classes of compounds by copper complexes of N,N‐dialkyl‐α‐amino acids, based on chromatographic results and supported by crystal growth and dissolution studies, is described.
Complex of Tertiary Acetylenic Alcohol and Brucine or Sparteine. X‐Ray Crystal Structural Study and Application to Optical Resolution
Authors: Toda Fumio , Tanaka Koichi , Ueda Hideo , Ōshima Tokio
Published: November 2013

Some tertiary acetylenic alcohols were optically resolved efficiently by complexation with brucine or sparteine. In the latter case, mutual optical resolution of sparteine and the tertiary acetylenic alcohol is possible. For an efficient resolution, the tertiary acetylenic alcohol should have at least one aromatic ring such as phenyl, naphthyl, thienyl as a substituent. The crystal structure of the 1:1 brucine complex and of the 1:1 (‐)‐sparteine complex of (‐)‐l‐(o‐bromophenyl)‐1‐phenyl‐2‐propynol (1a) are reported. In both cases, two hydrogen bonds,

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are important to fix the two components at a close position where mutual chiral recognition is efficient.
Stereochemical Discrimination at Organic Crystal Surfaces 2: The Effect of the Molecular Structure and Conformation of Dipeptides Additives on the Morphology of Glyclyglycine Crystals
Authors: Weissbuch I. , Berkovitch‐Yellin Z. , Leiserowitz L. , Lahav M.
Published: November 2013

The centrosymmetric monoclinic crystals of glycyl‐glycine were grown in the presence of three dipeptides (racemic and resolved Gly‐Ala, Ala‐Gly and Gly‐Leu); the presence of each yielded a different morphology. It was found by a combined theoretical and experimental study that the conformation of the guest molecule is determined by the highly stereospecific intermolecular interactions between adsorbed additive and the molecules of particular surface layers of the growing crystals. Theoretical calculations predicted the preferred conformation of the leucine side chain of Gly‐Leu consistent with the change in morphology. Occluded racemic Gly‐Leu segregates along the b axis of the single crystals of Gly‐Gly.
Thermal Rearrangements, XIII [1]. Thermal Isomerization of Exocyclic Allenes
Authors: Hopf Henning , Gottschild Dietmar , Lenk Winfried
Published: November 2013

The thermal isomerization of several exocyclic mono‐ and bis‐allenes in a high‐temperature flow system has been investigated. At 600°C vinylidenecyclobutane (10) rearranges cleanly to the allene dimer 1,2‐bismethylenecyclobutane (12) by an allene‐diene isomerization. The methylallene 16 behaves analogously (600°C), although the primary product 18 does not survive under the reaction conditions: the isolated cross‐conjugated triene 20 is a product of a 1,5‐hydrogen shift followed by an electrocylic ring‐opening reaction. Whereas vinylidenecyclopentane (27) yields aromatic products only (800°C, toluene (36), benzene (38)), the five‐ring 1,3‐bisallene 30 isomerizes to the cyclopentene derivative 43 at 650°C. Vinylidenecyclohexane (46) affords only aromatic products again (760°C, o‐xylene (57), 36, 38) and 1,3‐bisvinylidenecyclohexane (49) isomerizes to the dienynes 60 and 61 by a series of hydrogen shifts at 550°C. On the other hand, the allene‐diene isomerization is observed again, when a C–C bond is doubly activated by an allene and methylene group as in 52 or by two allene moieties as in 53: at 500°C the triene 63 and the exocyclic tetraene 65, respectively, are obtained in good yield.
1,3‐Dipolar Cycloadditions of 1,1‐Difluoroallene, Nitrones, Nitrile‐Oxides and Carbonyl Ylides
Authors: Dolbier William R. , Burkholder Conrad R.
Published: November 2013

The 1,3‐dipolar cycloadditions of C,N‐diphenylnitrone, C‐phenyl‐N‐methylnitrone, triphenylnitrone, mesitylnitrile oxide and tetracyanocarbonyl ylide with 1,1‐difluoroallene (DFA) are reported and discussed with respect to regiochemistry. Each of the dipoles adds exclusively to the C2—C3, non‐fluorine‐substituted π‐bond of DFA. The nitrones all undergo regiospecific reactions while mesitylnitrile oxide does not. The results are discussed with respect to FMO theory and other factors which can give rise to regiochemical variation.
Allene Synthesis by Organo‐Metallic Reactions
Authors: Crabbe Pierre , Schlemper Elmer O. , Fair Kay , Tran P.T. , Searles Scott
Published: November 2013

Several original allene syntheses are reviewed. Two novel organometallic approaches are discussed in some detail. Recent observations made in the case of the one‐step synthesis of allenes by homologation of alkynes are described. The reaction mechanism which is proposed is supported by X‐ray crystallographic data on a CuBr complex which appears to be a reaction intermediate.
Cyclophilic Reactions of Allene‐1,3‐dicarboxylic Esters. Part 4. Pyrazolone Formation and a Novel Indole Synthesis from the Nucleophilic Addition of 1,2‐Diphenylhydrazine to Dimethyl Allene‐1,3‐dicarboxylate
Authors: Onwuyali E. Ify , Scheinmann Feodor
Published: November 2013

Experiments are described for the addition of 1,2‐diphenylhydrazine to dimethyl allene‐1,3‐dicarboxylate and the subsequent conversion of dimethyl 3‐(1,2‐diphenylhydrazino)pent‐2‐enedioate (3) to either the pyrazolone (4) or a mixture of the indole (5) and the pyrazolone (4).
The Effect of the Counter Anion on the Complexation of Alkali Metal Salts to Polyethylene Glycol
Authors: Neumann R. , Dermeik S. , Sasson Y.
Published: November 2013

The complexation of alkali metal salts by polyethylene glycol is strongly dependent on the counter anion of the salt. In general, potassium salts are more easily complexed. In fact, sodium salts at low free energy of formation are barely complexed at all. By conductometric and index of refraction measurements it was shown that anions capable of hydrogen bonding such as OH−, F−, HSO−4 and HCO−3 were ion paired to the complexed potassium cation and existed as relatively “naked” anions. Other anions such as Cl−, Br−, I−, SCN−, NO−3, NO−2 AND HF−2 existed as disassociated anions with significant solvation shells. The use of polyethylene glycol as a phase transfer catalyst considering the results found is discussed.
Anomalous Selectivities in Methyl Transfers to Water: An Explanation Using Free Energy Surfaces which Model the Effects of Non‐Equilibrium Solvation
Authors: Kurz Joseph L. , Kurz Linda C.
Published: November 2013

Many experimental observations imply that transition states for methyl‐transfers to water have little or no water—methyl covalent bonding and that the rate‐determining process is predominantly or entirely a fluctuation in solvent structure. Currently accepted descriptions of methyl‐transfer mechanisms cannot easily account for such a transition state structure or for the anomalous selectivities observed in these reactions. This paper derives a model free energy surface, which includes solvent polarization as one of the coordinates, and shows how that surface can account for the observed behavior of these methyl‐transfers. The transition state on that surface has a structure in which the solvent polarization is not in equilibrium with the internal charge distribution.
Limitations of the Reactivity—Selectivity Principle. Application to Free Radical Addition to Alkenes and Nucleophilic Aliphatic Substitution
Authors: Pross Addy
Published: November 2013

The limitations of the reactivity—selectivity principle (RSP) are discussed in terms of the qualitative configuration mixing (CM) model. It is concluded that the RSP is expected to break down for cases in which the reaction transition state takes on character largely absent in reactants or products, though not all breakdowns of the RSP are attributed to this cause. This conclusion is illustrated with two model reactions that do not follow the RSP: (a) radical addition to alkenes, and (b) nucleophilic aliphatic substitution. Analysis leads us to agree with previous statements by Jencks and Arnett that the RSP should be abandoned as a predictive tool.
Application of Aziridine to the Facile Synthesis of (±)‐Supinidine and (±)‐Trachelanthamidine
Authors: Kametani Tetsuji , Higashiyama Kimio , Otomasu Hirotaka , Honda Toshio
Published: November 2013

A facile synthesis of the pyrrolizidine alkaloids, (±)‐supinidine (14) and (±)‐trachelanthamidine (8) was achieved by employing aziridine as the reactive species.
Monte Carlo Studies of the Aqueous Hydration of cis‐ and trans‐ N‐ Methyl‐Acetamide: Sensitivity of Results to Convergence and to Choice of Intermolecular Potential Functions
Authors: Mezei M. , Harrison S. W. , Ravishanker G. , Beveridge D. L.
Published: November 2013

A series of liquid state Monte Carlo computer simulations on the N‐methyl acetamide in water (NMA molecule) are described, considering both the cis and trans isomeric forms of NMA. Two independent sets of calculations were performed, one set based on the potential functions developed by Clementi and co‐workers (CPF) used in conjunction with the Matsuoka et al. (MCY) water‐water potential, and the other set using the OPLS potential of Jorgensen and coworkers for solute‐water interactions and the TIP4P potential for the water‐water interactions. The relative hydration energies of the cis and trans isomers of NMA and the transfer energies of NMA from free space to water are discussed with respect to the convergence level of the simulations and the sensitivity of results to the choice of the intermolecular potential functions. The structural chemistry of the aqueous hydration for NMA is developed from the simulation result based on Proximity Analysis. The sensitivity of the structural and energetic indices of analysis to choice of potential is described. Preliminary results of the calculation of the hydration contribution to the free energy are also presented.
Linear Chain Ferromagnetic Charge Transfer Compounds
Authors: Miller Joel S. , Epstein Arthur J. , Reiff William M.
Published: November 2013

Charge transfer complexes possessing a … DADA … structure with both the donor, D, and acceptor, A, being S = 1/2 radicals may exhibit cooperative magnetic phenomena. The complex [Fe(C5Me5)2]+·[TCNQ]−· exhibits metamagnetic behavior. The similarly structured [TCNE]−· and [C4(CN)6]−· complexes are ferromagnets, whereas the [DDQ]−· salt is a paramagnet. The high temperature magnetic susceptibility obeys the Curie‐Weiss expression with θ = + 30, + 30, and + 3 for the [TCNE]−·, [C4(CN)6]−·, and [TCNQ]−· salts, respectively. The ferromagnetic [TCNE]−· salt exhibits zero field Zeeman split 57Fe Mossbauer spectra with an internal field of 425.6 kOe at 4.23 K. After reviewing the current papers discussing ferromagnetism in molecular (organic) compounds, a qualitative model consistent with the necessary bulk spin alignment required for a ferromagnet is presented.
Photochemical, Electrochemical, and Hydrogen‐Driven Enzymatic Reductions in Reversed Micelles
Authors: Laane Colja , Verhaert Raymond
Published: November 2013

A few examples of (multi)enzymatic reactions in reversed micellar media are described and discussed. The key enzymes in these examples are 20β‐hydroxy‐steroid dehydrogenase, enoate reductase, and an NAD+‐independent hydrogenase, catalyzing the reactions of interest, viz., the site‐ and stereo‐specific reduction of 20‐ketosteroids to their corresponding 20β‐hydroxy form, the chiral hydrogenation of α,β‐unsaturated carboxylates, and the production of hydrogen, respectively. These cofactor‐requiring enzymes are coupled to various in situ cofactor‐regenerating systems, powered either by light, electricity, or hydrogen. The performance of all these systems is shown.
Single Crystal Electron Spin Resonance Studies of the Photochemical Reaction Center from Rhodobacter sphaeroides Wild Type Strain 2.4.1
Authors: Budil David E. , Taremi Shahriar S. , Gast Peter , Norris James R. , Frank Harry A.
Published: November 2013

Single crystals were obtained from the photochemical reaction center protein of the photosynthetic bacterium Rhodobacter sphaeroides wild type strain 2.4.1. At low temperatures, the crystals display pronounced anisotropy in their triplet state electron spin resonance (ESR) spectra. An analysis of the angle dependence of the resonance field positions revealed two to four magnetically nonequivalent primary donor molecules per unit cell. This is consistent with the symmetry assignment of P deduced from previous X‐ray diffraction studies (Frank, H.A.; Taremi, S.S.; Knox, J.R. J. Mol. Biol., 1987, 198: 139–141). However, the ESR data could not be fit assuming that the unit cell axes coincide with the crystal morphological axes as was the case previously for Rb. sphaeroides R26 (Gast, P.; Norris, J.R. FEBS Lett., 1984, 177: 277–280). In order to explain the present data, it was necessary to assume a rhombic crystal cross‐section with the unit cell axes lying along the diagonals of the rhombus. The unit cell axes b and c were found to make angles of 60 ± 2° and 30 ± 2°, respectively, with one of the faces of the crystal. The orientation of the triplet state dipolar tensor axes relative to the unit cell axes differs slightly from that in Rb. sphaeroides R26 crystals, suggesting that the Rb. sphaeroides 2.4.1 reaction center is rotated by approximately 7° relative to the orientation found in Rb. sphaeroides R26.
Subpicosecond Spectroscopy of Charge Separation in Rhodobacter capsulatus Reaction Centers
Authors: Kirmaier Christine , Holten Dewey
Published: November 2013

The time‐evolution of the near‐infrared absorption changes accompanying the conversion of the excited primary electron donor, P*, to the radical pair state, P+BPhL−, has been examined in detail in Rhodobacter capsulatus reaction centers. In a series of spectra spanning the time interval from about 600 fs to 15 ps, two isosbestic points occurring at 765 run and 798 nm are maintained throughout. The finding of an isosbestic point at 798 nm in this series of spectra, which encompass the time during which P* decays and BPhL− forms, places severe constraints on the possible reduction of the 800‐nm‐absorbing monomeric BChlL in the initial stage of the charge‐separation process. The near‐infrared P* spectrum, which is revealed clearly only when P is excited directly in its long‐wavelength absorption band, contains bleaching only of the 855‐nm band and a broad featureless transient absorption below 810 nm. This spectrum shows that P does not contribute any significant oscillator strength to the 800‐nm ground state absorption band.
Photoinitiated Electron Transfer in Carotenoporphyrin‐Quinone Triads: Enhanced Quantum Yields via Control of Reaction Exergonicity
Authors: Moore Thomas A. , Gust Devens , Hatlevig Susan , Moore Ana L. , Makings Lewis R. , Pesski Peter J. , De Schryver F.C. , van Der Auweraer M. , Lexa Doris , Bensasson René V. , Rougée Michel
Published: November 2013

Carotenoporphyrin‐quinone triad molecules have previously been found to successfully mimic many aspects of photosynthetic electron and energy transfer. These molecules generate long‐lived, energetic charge‐separated states via a biomimetic multistep electron transfer scheme. In many of these molecules, the overall quantum yield for charge separation has been limited by an unfavorable partitioning of an intermediate charge‐separated state between charge recombination and further reaction to yield the desired species. One strategy for overcoming this limitation is based on the fact that, in general, electron transfer rate constants do not depend linearly on reaction free energy change. In the triads, raising the energy of the intermediate charge‐separated state would be expected to speed up the desired electron transfer (“normal” Marcus behavior), but slow down charge recombination (which formally lies in the “inverted” region). Laser flash photolysis and transient fluorescence measurements on two new triad molecules reveal enhanced quantum yields of long‐lived charge‐separated states which are consistent with this expectation.
Formation Dynamics, Decay Kinetics, and Singlet–Triplet Splitting of the (Bacteriochlorophyll Dimer)+ (Bacteriopheophytin)− Radical Pair in Bacterial Photosynthesis
Authors: Bixon M. , Michel‐Beyerle M.E. , Jortner Joshua
Published: November 2013

We explore the interrelationship between (i) the kinetic data for the primary electron transfer (ET) from the excited singlet state (1P*) of the bacteriochlorophyll dimer (P) to bacteriopheophytin (H) in the reaction centers (RCs) of purple photosynthetic bacteria and (ii) the energetics and dynamics of the primary radical pair, P+H, i.e., its singlet‐triplet splitting rate, J, and triplet recombination rate, kT. We present a critical scrutiny of the ET mechanisms with regard to the role of the accessory bacteriochlorophyll (B). On the basis of the very weak temperature dependence of J, which sets a lower limit to the energy of (hypothetic) transient short‐lived ionic intermediate states, we argue against the applicability of sequential ET mechanisms involving P+B− H or P+BH−. The nonadiabatic/adiabatic ET mechanism is fraught with difficulties pertaining to a very low medium reorganization energy for the nonadiabatic crossing and an exceedingly high rate of the adiabatic crossing, together with a predicted order‐of‐magnitude enhancement of the low‐temperature (77 K) fluorescence quantum yield in high electric fields (5–10 mV/Å) which seem not to be borne out by experiment We propose that primary ET occurs via the unistep superexchange 1P*BH r̊ P+H−, which is mediated by electronic coupling with P+B−H. The analysis of the competition between superexchange and thermally activated ET resulted in large values of the intermolecular electronic couplings at the equilibrium nuclear configuration of 1P*BH. We suggest that the structural relaxation of one of the prosthetic groups (H) accompanying the primary ET results in the decrease of the intermolecular electronic coupling at the equilibrium configuration of P+BH−. It was established that the superexchange‐predicted kinetic data are consistent with the magnetic data for the radical pair. The small value of J for P+BH− originates from the cumulative effect of: (i) an essential, model‐independent cancellation between the triplet contribution, which is related to kT by a self‐consistency relationship, and the singlet contribution; (ii) the effect of configurational relaxation on the singlet contribution. Our analysis supports the superexchange mechanism for primary ET in bacterial photosynthetic RCs.
The Cytochrome‐c Binding Surface of Reaction Centers from Rhodobacter sphaeroides
Authors: Tiede David M. , Chang Chong‐Hwan
Published: November 2013

In this paper, we describe the water‐exposed, periplasmic surface of the bacterial photosynthetic reaction center from Rhodobacter sphaeroides R‐26. This surface contains binding sites for c‐type cytochromes. The approximate two‐fold structural symmetry is evident in the folding of the L and M subunits of the reaction center on this surface. In spite of this, the distribution of charged residues shows a marked deviation from symmetry. One group of charged residues is distributed symmetrically. For this group, equivalently charged residues are located at symmetry‐related positions on both L and M subunits. However, charged residues are also found at positions which do not follow the two‐fold symmetry. Seven charged residues appear on the L and M subunits at positions which do not have a charged residue at the corresponding position on the other subunit. These symmetry‐breaking charged residues appear in a single region, or “patch”, on the surface of the reaction center. We propose that this patch may function as a site for cytochrome c docking. A model of the Rb. sphaeroides cytochrome c2‐reaction center electron transfer complex has been built. Preliminary energy calculations show that this configuration is at least a local energy minimum.
Quantitative Analysis of Magnetic Resonance Signals in the Time Domain
Authors: De Beer R. , van Ormondt D. , Pijnappel W.W.F. , van der Veen J.W.C.
Published: November 2013

This paper is concerned with quantification of magnetic resonance signals in terms of physically significant parameters such as amplitudes, frequencies, damping factors (line widths) and phases. The approach considered here is to fit an appropriate model function, in the same domain as that in which the data are collected. Important advantages of this approach are 1) absence or rejection of data points need not impair the quantification and 2) non‐uniform sample distributions aimed at resolution enhancement can be accommodated in a natural fashion. Aspects given attention are singular value decomposition (SVD) of the data matrix, SNR enhancement, linear prediction (LPSVD), state space formalism (HSVD), variable projection method (VARPRO) in various forms, exploitation of prior knowledge, precision of the fitted parameters, experimental design and 2‐D time domain model function fitting.
Rotary Resonance Recoupling in Heteronuclear Spin Pair Systems
Authors: Levitt Malcolm H. , Oas Terrence G. , Griffin Robert G.
Published: November 2013

In solid state magic angle spinning experiments on a polycrystalline sample containing dilute heteronuclear spin pairs, the usual spectrum is insensitive to small heteronuclear couplings between the spins, and to the relative orientations of the various interaction tensors. Applying a radio frequency of an intensity chosen to satisfy a rotary resonance condition can restore the heteronuclear splittings in the spectrum. Theory and numerical simulations of this effect are presented. The sensitivity of the spectra to the intensity of the radio frequency field and to the relative orientations of tensors is explored. To fully explain the experimental results, it is necessary to postulate a small phase modulation induced by the motion of the sample through a radio frequency field of non‐uniform direction.
Applications of Two‐Dimensional 1H NMR Methods to Photo‐Chemically Induced Dynamic Nuclear Polarisation Spectroscopy
Authors: Stob Sijtze , Scheek Ruud M. , Boelens Rolf , Dukstra Klaas , Kaptein Robert
Published: November 2013

Photo‐CIDNP is a useful technique to identify surface residues in proteins and to assign resonances in their 1H NMR spectra by the spectral simplification that is obtained. In spite of this simplification, conventional photo‐CIDNP spectra can be quite complex due to spectral overlap. For this reason two‐dimensional (2D) methods are welcome. We present combinations of photo‐CIDNP with 2D J‐correlated spectroscopy (CIDNP‐COSY) and with 2D NOE spectroscopy (CIDNP‐NOESY). In these experiments we insert a saturation pulse sequence and a short laser‐irradiation period at the beginning of the preparation period to generate CIDNP, leaving the remaining of the pulse scheme essentially unaltered. The CIDNP‐COSY and CIDNP‐NOESY spectra of a hen egg‐white (HEW) lysozyme are presented. The CIDNP‐NOESY spectrum of lysozyme is compared with the results from 1D cross‐polarisation experiments on the same protein. CIDNP‐NOESY spectra of lysozyme with and without inhibitor NAG are compared.
Enhancement of Contrast in Magnetic Resonance Images by Paramagnetic Agents: Possibilities and Problems
Authors: Koenig Seymour H.
Published: November 2013

Despite the beauty and excellent spatial resolution of magnetic resonance images of native tissue, the improvement in contrast achievable by the introduction of exogenous paramagnetic agents often more than offsets the toxicity risks, and several such agents are currently in clinical trials. The efficacy of any agent relates to the changes it induces in tissue relaxation rates per unit concentration of agent (its relaxivity), which in turn — as is well known—depends both on magnetic field strength and on the physical and chemical environment of the agent. The latter depends on in vivo biochemistry, and may vary from one tissue to another. In addition, current imaging (including animal studies) is done in the range 0.2–200 MHz proton Larmor frequency and, at the highest fields, water and lipid protons can be distinguished and imaged separately. Thus there is much to consider, several classes of problems and related paramagnetic agents exist: (1) Agents that remain in the intact circulation until excreted through the kidney. These include small chelate complexes of Fe3+ and Gd3+ as well as conjugates, both covalent and hydrophobic, with plasma proteins, generally albumin or immunoglobulins. (2) Agents that are scavenged by the liver, ultimately to be excreted, via the bile and gall bladder, into the intestine. There is a search now for safe agents of this class, which is complicated by liver biochemistry that can release the metal ions and increase toxicity. (3) It has been discovered that some non S‐state ions, usually regarded as shifters rather than relaxers, can have high relaxivities at high fields, in part because of directed orbitals. Mn3+‐porphyrin is an example. (4) Aggregates and particulates of magnetic material are becoming popular, in part because they influence 1/T2 preferentially at imaging fields, and in part because their fate in vivo is rather benign. (5) Lipophilic agents, such as nitroxide free radicals, may enhance the relaxation rates of adipose tissue preferentially. It has recently been shown that neat triolein is a good model for such tissue, providing a sizable area for new investigations. (6) Nuclei other than protons may be important, perhaps the most interesting being fluorine because of its presence in synthetic blood substitutes. We present an overview of our recent collaborative work in these many areas, results now under the rubric of “relaxometry”. This work is made possible because of continued advances in the specialized instrumentation (“field‐cycling relaxometers”) required for such investigations.
Cation Radicals of Covalently Linked Porphyrin Dimers and their Monomeric Constituents: An ESR and ENDOR Study
Authors: Huber Martina , Galili Tamar , Möbius Klaus , Levanon Haim
Published: November 2013

The spin density distributions in the cation radicals of various covalently linked porphyrin dimers have been studied by liquid phase ESR and ENDOR methods to find out whether these systems show intramolecular electron delocalization. Such a delocalization is known to occur in the bacteriochlorophyll dimer (“special pair”) in the photosynthetic reaction center. The dimers that were studied in this work were derived from zinc mesotetratolylporphyrin (ZnTTP) and linked at the ortho or para positions of one phenyl ring with varying bridge lengths. 1H and I4N hyperfine coupling constants could be measured for the dimer cation radicals and compared with those of monomeric ZnTTP as well as ZnTTP derivatives that carry alkoxy or hydroxy substituents to mimic the bridges of the dimers. By comparing the hyperfine data of the monomer and dimer cation radicals it is concluded that in the present dimers the unpaired electron is localized on one porphyrin unit.
Recent ENDOR and Pulsed Electron Paramagnetic Resonance Studies of Cytochrome c Peroxidase ‐ Compound I and Its Site‐Directed Mutants
Authors: Scholes Charles P. , Liu Yajun , Fishel Laurence A. , Farnum Martin F. , Mauro J. Matthew , Kraut Joseph
Published: November 2013

EPR, ENDOR and pulsed EPR were applied to the radical signals of yeast cytochrome c peroxidase‐compound I from this protein expressed in E. coli and two variants prepared after site‐directed mutagenesis. The compound I complexes studied were first, that of the parent peroxidase expressed in E. coli, and next, those of the following amino acid mutants as specific probes for the radical in compound I: 1. Tryptophan‐51 → Phenylalanine; 2. Tryptophan‐191 → Phenylalanine. Tryptophan‐51 lies near the heme on the distal (H2O2‐binding) side of cytochrome c peroxidase, while Tryptophan‐191 lies in contact with the proximal heme ligand, Histidine‐175. EPR above 20 K and saturation recovery T1 measurements at liquid He temperatures showed the unusual temperature‐dependent relaxation of compound I's axially symmetric signal (g__ = 2.035, g⟂ = 2.00), which has comprised the major radical species in bakers' yeast wildtype compound I and here in all but the Tryptophan‐191 → Phenylalanine mutant. The ENDOR of the major radical species was characterized by the substantial hyperfine couplings previously proposed to be at a sulfur‐based radical. The mutation Tryptophan‐51 → Phenylalanine does not perturb these ENDOR features. Pulse field‐sweep EPR resolved for the first time distinct proton hyperfine couplings due to the features observed by ENDOR. A narrow, more typical organic free radical signal at g = 2.001 with resolved hyperfine structure was the only species observed from the Tryptophan‐191 → Phenylalanine mutant and was a minor feature in all samples. The Tryptophan‐191 → Phenylalanine mutation does perturb or destabilize the majority radical species of compound I. Thus, Tryptophan‐191 is either intimately involved with the kinetics of radical formation or participates intimately in the majority radical site, despite not being a sulfur‐containing amino acid.
Propellanes 100. The Nature of the Stereoelectronic Effect in the Highly Stereospecific Hydroxyl Elimination from syn, anti‐[4.3.3]Propellane‐8,11‐diol upon Isobutane Chemical Ionization
Authors: Ashkenazi P. , Domon B. , Gutman A.L. , Mandelbaum A. , Müller D. , Richter W.J. , Ginsburg D.
Published: November 2013

A stereospecific 18O‐labelling study shows that the syn‐hydroxy group is preferentially lost from the syn, anti‐title diol upon i‐C4H10 chemical ionization, indicating occurrence of a stereoelectronic effect in this process. A collision‐induced dissociation study of deuterium‐ and 18O‐labelled analogs shows that a symmetrical ether structure is not formed in the above process by an SNi mechanism. This result leads to the conclusion that the stereoelectronic assistance of the anti‐OH group in the loss of the syn‐hydroxyl is expressed either in a weak transition state interaction or by other processes such as β‐elimination via hydrogen transfer with concomitant double bond formation.
Chiral Synthesis via Organoboranes 23. Enantioselective Ring Opening of meso‐Epoxides with B‐Halodiisopinocampheylboranes. The First General Synthesis of Optically Active 1,2‐Halohydrins
Authors: Srebnik M. , Joshi N.N. , Brown Herbert C.
Published: November 2013

B‐Halodiisopinocampheylboranes, Ipc2BX, where X is Cl, Br, or I, cleave cyclohexene oxide at low temperatures to give the 1,2‐halohydrins, in 22, 84, and 91% enantiomeric excess (ee), respectively. 1,2‐Epoxycyclohex‐4‐ene is converted to the bromohydrin and iodohydrin in 84 and 91% ee. cis‐2,3‐Epoxybutane, cis‐3,4‐epoxyhexane, and cyclopentene oxide were also studied with all three halides. In general, optical induction increases in the order I > Br > Cl for any given epoxide. Thus cis‐2,3‐epoxybutane furnishes the corresponding chlorohydrin in 35% ee, the bromohydrin in 69% ee, and the iodohydrin in 78% ee. In certain cases recrystallization provides essentially optically pure material, e.g., (1R, 2R)‐2‐bromocyclohexanol, (1R, 2R)‐2‐iodocyclohexanol and (1R, 2R)‐2‐iodocyclohex‐4‐en‐1‐ol. In all cases examined dIpc2BX (derived from (+)‐α‐pinene) provide (1R, 2R) halohydrins, in which the enantiotopic S C—O bond is cleaved. Ring cleavage occurs in an anti‐periplanar manner, consistent with an SN2 type reaction pathway. Modified B‐chloromonoisopinocampheylboranes, IpcBCl(OR), cleave cyclohexene oxide at a slower rate but furnish the chlorohydrin in up to 35% ee (R = Bnz). This study is not only a novel application of chiral haloboranes, but constitutes the first general synthesis of symmetrical optically active 1,2‐halohydrins.
[4.4.1]Propellatetraene and Related Propellanes
Authors: Frauenrath Herbert , Kapon Moshe , Rubin Mordecai B.
Published: November 2013

Reaction of dichlorovinylene carbonate with 1,6‐methano[10]annulene (2) followed by hydrolysis of the intermediate adduct afforded α‐diketone 3. Photolysis of 3 at liquid nitrogen temperature yielded the title compound, 1. The activation parameters for the thermal isomerization 1 → 2 were ΔH+ = 5.1 kcal/mole and ΔS−= −19 eu (100 K). The structure of 3 was established by X‐ray analysis. This analysis and MM2 calculations on a number of related compounds provide additional examples of the so‐called “Klammer” effect in propellanes.
The Stereochemistry of the Cope Rearrangement: Qualitative Theory (OCAMS) and Computation (AM1)
Authors: Halevi E. Amitai , Rom Rami
Published: November 2013

Contrary to common preconceptions, orbital correlation diagrams can be useful for deducing the mechanism and stereochemistry of degenerate sigmatropic rearrangements. The correlation lines have to be drawn between the reactant and the postulated transition state, rather than between the homomeric reactant and product. In the case of the Cope rearrangment, this is done most economically with Orbital Correspondence Analysis in Maximum Symmetry (OCAMS), by drawing correspondence lines between molecular orbitals of the reactant and those of a formal superposition (STS) of alternative transition structures (TS) in a symmetry point group that includes those of both as subgroups. The alternative boat (C2v) and chair (C2h) transition structures postulated for the degenerate Cope rearrangement of 1,5‐hexadiene are formally combined to an STS of D2h symmetry. OCAMS then selects its C2h component, indicating that the pathway via the chair‐like transition state is preferred, in agreement with experimental and computational results for both the Cope rearrangement and the isoelectronic Claisen rearrangement. Rearrangement through the less favored boat pathway is shown to arise from a changed electronic configuration in the transition state. Computational evidence is presented showing that the unexpectedly facile rearrangement of polycyclic molecules like semibullvalene and barbaralane, which are constrained to react via the boat pathway, is primarily due to steric and stereoelectronic factors that are explicable in molecular orbital terms, rather than to the incursion of a biradicaloid transition state.
Rotational Distributions in the Photodetachment of IHI− and in the I + HI Reaction: The Influence of IHI Transition State Resonances
Authors: Schatz George C.
Published: November 2013

Franck‐Condon factors which determine IHI− photodetachment spectra have been calculated using a three‐dimensional coupled‐channel reactive scattering program. These Franck‐Condon factors show peaks as a function of energy that are in reasonable agreement with measured spectra due to Neumark and co‐workers. Some of these peaks are due to IHI transition state resonances and others are due to direct scattering reactive threshold effects. An analysis of the rotational distributions associated with the Franck‐Condon factors indicates that when direct scattering is dominant, the distributions peak at the rotational state which is nearest to its effective reaction threshold. At energies where IHI transition state resonances are important, the rotational distribution changes, thus providing a characteristic signature of resonance formation. The I + HI bimolecular rotational distributions are also considered, and they also show important differences between direct and resonant energies.
Quantitative Determination of HD Internal State Distributions via (2+1) REMPI
Authors: Rinnen Klaus‐Dieter , Kliner Dahv A.V. , Zare Richard N. , Huo Winifred M.
Published: November 2013

The relationship between quantum state populations and ion signals in (2+1) resonance‐enhanced multiphoton ionization (REMPI) detection of HD is investigated both experimentally, by measurements of hot‐nozzle population distributions, and theoretically, by calculations of the two‐photon E,F 1Σg+—X1Σg+ transition moments. The experimental and theoretical relative rotational cross‐sections are in good agreement for v″ = 0 and v″ = 1, but differ for v″ = 2 and for the relative vibrational cross‐sections. Arguments are presented that saturation of the E,F‐X transition is the main cause of these discrepancies. In spite of the presence of saturation, this calibration gives confidence in determining the relative internal state populations of HD from its (2+1) REMPI spectrum.