bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal Isomerizations of Fluorinated Spiropentanes
Authors: Dolbier William R. Jr. , Sellers Simon F. , Al‐Sader Basil H. , Fielder Thomas H. Jr. , Elsheimer Seth , Smart Bruce E.
Published: November 2013

The syntheses and thermal rearrangements and/or fragmentations of 1,1‐difluorospiropentane, 6, 1,1,4,4‐ and 1,1,2,2‐tetrafluorospiropentanes, 7 and 8, 1,1,2,2,4,4‐hexafluorospiropentane, 9, and perfluorospiropentane, 10, are described., Spiropentanes 6 and 7 undergo only rearrangement to 2,2‐difluoromethylene‐cyclobutane and 1‐(difluoromethylene)cyclobutane and to 2,2,4,4‐tetrafluoromethylenecyclobutane, respectively. Spiropentanes 8 and 9 rearrange and extrude CF2: competitively, while perfluorospiropentane extrudes CF2: exclusively. Activation parameters are presented for all processes and the mechanisms of these thermal processes are discussed in detail.
The Cleavage of Three‐Membered Ring Compounds by Transition Metal Organometallic Complexes
Authors: Alper Howard
Published: November 2013

Transition metal organometallic complexes effect a wide range of useful and interesting cleavage reactions of three‐membered ring compounds. This review examines some recent developments in the organometallic chemistry of cyclopropanes, epoxides, episulfides and azirines. Novel synthetic approaches to heterocyclic [e.g., β γ and δ‐lactones, β‐lactams, isoxazoles, pyrazoles, indoles] and acyclic compounds [β‐diketones, vinyl isocyanates] have been achieved by means of these cleavage reactions.
Inhibition of the Primary Photochemical Events in Rhodospirillum Rubrum by Ubiquinone Analogues
Authors: Bering Charles L. , Bustamante Peggy L. , Loach Paul A.
Published: November 2013

Twenty‐one quinone compounds have been tested for their ability to inhibit the primary photochemical events when added to R. rubrum chromatophores. This was determined by measuring their effect on the light‐induced absorbance change at 605 nm and their ability to quench the variable portion of fluorescence (Fv). Benzoquinone structures with one or more bromine or iodine substituents and a bulky group (isopropyl, t‐butyl, n‐hexyl) were particularly effective inhibitors. The most potent of these were 2,3,5‐tribromo‐6‐n‐hexyl‐1,4‐benzoquinone and 2,3‐diiodo‐5‐t‐butyl‐1,4‐benzoquinone which caused 50% inhibition at less than μM quantities (or about 5 to 10 fold above the phototrap concentrations). From control experiments with egg yolk phosphatidylcholine liposomes containing bacteriochlorophyll, it was shown that although the effective inhibitors also quenched fluorescence when added to this model system, quenching depended on the viscosity of the bilayer and was not very sensitive to structural parameters. In the in vivo system, phototrap activity was inhibited at 77 K to the same extent as room temperature, indicating diffusion was not important. In the control experiments with liposomes, the concentration for which 50% quenching was observed by 2,5‐dibromo‐3,6‐dimethyl‐1,4‐benzoquinone and 2,3,5‐tribromo‐6‐n‐hexyl‐1,4‐benzoquinone only differed by two fold whereas in the chromatophore system, the difference was over 1000 fold; no effect was seen in vivo by the former compound and the latter was the most effective. It is suggested that most of the instantaneous fluorescence (Fo) is from a bacteriochlorophyll component(s) which does not communicate with the reaction center and a higher concentration of added quinone is required before this pigment fluorescence is quenched.
Biliproteins: Some Relationships Among Aggregation States, Spectra, and Excitation‐Energy Transfer
Authors: Maccoll Robert , Berns Donald S.
Published: November 2013

The key aggregation steps for C‐phycocyanin consist of denatured subunits (α and β polypeptide chains), monomers, trimers, hexamers and stacks. For allophycocyanin the steps are denatured subunits, monomers and trimers. Ultimately these assembly states lead to the formation of phycobilisomes in blue‐green and red algae. The spectroscopic changes that accompany each stage in the assembly can be monitored as a function of the ratio of visible to near‐UV absorption. For C‐phycocyanin, monomers and hexamers are the important levels in the modulation of tetrapyrrole conformation by protein‐chromophore interaction. The effect of protein aggregation on the protein‐chromophore interactions is the key to solar‐energy harvesting and excitation‐energy migration in photosynthesis.
Electronic Structure and Reactivity of 2,2‐ethano‐1‐methylene 1,2,3,4‐tetrahydronaphthalene and 1,1‐ethano‐2‐methylene 1,2,3,4‐tetrahydronaphthalene
Authors: Goldblum Amiram , Sarel Shalom
Published: November 2013

The electronic structures of the title compounds were calculated byab initioSTO‐3G minimal basis set following a MINDO/3 total geometry optimization of both. Their different reactivities towards electrophiles is suggested to result from different frontier (HOMO) orbital densities on the relevant atoms. Both MINDO/3 and STO‐3G calculations give similar results for the HOMO structures, where frontier density is comparable on exocyclic methylene and ethano bridge atoms in the 1,1‐ethano 2‐methylene analog, and much higher on the methylene than on the cyclopropyl atoms in the second molecule. Relative rates of reaction are also discussed.
Properties of the Triplet State of N,N,Nî,Nî‐Tetraethyloxonine
Authors: Kamat Prashant V. , Lichtin Norman N.
Published: November 2013

The triplet state of N,N,Nî,Nî‐tetraethyloxonine, TEO+generated by means of sensitization with 9,10‐dibromoanthracene, was investigated in ethanol solution by laser flash photolysis‐kinetic spectrometry employing the first harmonic of a Q‐switched ruby laser at 347.2 nm. Quenching of both the protonated triplet, TEOH2+(T1), τ = 47 μs, and the unprotonated triplet, TEO+(T1), τ = 285 μs, by TEO+(S0) proceeds with substantial net electron transfer. No net electron transfer is detectable in the quenching of TEOH2+(T1), and TEO+(T1) by O2or 1,3,5,7‐cyclooctatetraene, processes which apparently involve transfer of energy. Specific rates of quenching are reported and the implications of the quenching processes for stability of TEO+in devices for conversion or storage of solar energy are discussed.
A Novel Effect of a Polyelectrolyte on a Photochemically Induced Electron Transfer Process Involving a Zwitterion
Authors: Sassoon Richard E. , Rabani Joseph
Published: November 2013

A photoelectron transfer system containing the photosensitizer tris (2,2′‐bipyridyne) ruthenium II chloride, and the quencher N, Nî, di(4‐sulfonato toluyl) 4,4′‐bipyridine is described. Following a laser pulse the exited state of the photosensitizer is quenched by BSV giving a net yield of electron transfer of about 0.1. Addition of the polyelectrolyte polyvinyl sulfate (PVS) markedly enhances the rate of quenching. The decay of the electron transfer products Ru(bpy)3+3 and BSV+ is considerably retarded in the same system upon addition of PVS.
Photoinduced Electron Transfer Reactions as Photosynthetic Models: Reduction of Anthraquinones in SiO2 and ZrO2 Colloids
Authors: Willner Itamar , Degani Yinon
Published: November 2013

The photoinduced reduction of anthraquinone‐2‐sulfonate, AQS−, and anthraquinone‐2,6‐disulfonate AQS2‐2, in a negatively charged SiO2 colloid has been examined. Ru(bipy)2+3 was used as sensitizer and triethanolamine, TEOA, as an ultimate electron donor. The reduction of the quinones to the corresponding semiquinone radicals, AQS2‐ and AQS3‐2 proceeds only with propylviologen sulfonate, PVS0, (1), as a mediating electron acceptor. The primary electron acceptor, PVS0, functions as an efficient quencher of the excited species. The formation of AQS3‐2 is 4‐fold enhanced as compared to that of AQS2‐. This enhancement is attributed to the additional stabilization of AQS3‐2 from recombination with the oxidized species, Ru(bipy)3+3, due to electrostatic repulsions from the colloidal interface. Consequently, the subsequent oxidation of TEOA is facilitated. Similarly, a positively charged ZrO2 colloid has been used in controlling photosensitized electron transfer reactions. Anthraquinone‐2,6‐disulfonate, AQS2‐2, is photoreduced in the ZrO2 colloid to the respective AQHS2‐2 with the neutral sensitizer, Ru(bipy)2(CN)02, and cysteine as electron donor (Φ = 3 × 10−2). No photoreduction of the quinone is observed in a homogeneous aqueous phase. The success in driving the photoinduced reaction in the ZrO2 colloid is again attributed to the stabilization of the intermediate photoproducts from recombination by means of electrostatic interactions with the charged interface.
X‐ray Diffraction as a Tool for Studying Stoichiometry and Kinetics of Solid State Thermal Decomposition Reactions. Application to the Thermal Decomposition of Bischofite MgCl2 · 6H2O
Authors: Herbstein F. H. , Kapon M. , Weissman A.
Published: November 2013

High temperature in situ X‐ray diffractometry of polycrystalline samples is shown to be a very convenient method both for identification of the various stages of a complicated thermal decomposition reaction and for determination of the kinetics of each stage of the reaction. This method when used together with thermogravimetric and chemical analyses has enabled us to determine the stoichiometry and kinetics of the various stages in the thermal decomposition of bischofite (MgCl2 · 6H2O) to periclase (MgO) in atmospheres of dry nitrogen, moist air and dry steam. The overall reaction rate determined in the laboratory for the conversion of MgCl2 · 6H2O to MgO is similar to that found in industry.
Oxidation of Polycrystalline Niobium Nitride
Authors: Gallagher P. K. , Sinclair W. R.
Published: November 2013

Isothermal and dynamic TG were used to follow the oxidation of a sieved fraction, 177–250 μm, of niobium nitride powder in a flow of dry 0.1% O2 in Ar. The material underwent enormous expansion during the oxidation. The isothermal weight gain was measured in the temperature range from 817 to 897 K. It followed a pseudo‐first order reaction rate law best with an Arrhenius activation energy of 50 kcal mole−1. Dynamic studies at 1 K min−1 the pseudo‐first order rate law yielded similar values of 48 kcal mole−1 in 0.1% O2 in Ar and 55 kcal mole−1 in air. Because of its highly exothermic nature, the reaction will lead to a thermal runaway situation at heating rates approaching a temperature of 600 K at 20 K min−1, in air or at even lower rates of heating in pure O2, for material in this particle size range. Examination of the weight gain and X‐ray diffraction patterns of partially oxidized samples did not give any indication of intermediate species, such as oxynitrides or oxides of niobium, other than Nb2O5. The final product was a low temperature, pseudohexagonal polymorph of Nb2O5 but monoclinic γ'‐Nb2O5 appeared as an intermediate.
Thermal Evaluation of Bituminous Rocks and Pyroproducts
Authors: Aizenshtat Zeev
Published: November 2013

The stepwise pyrolytic method is compared with the “bulk flow” method on a CDS‐820 ZA automated pyro‐analyser. Bituminous rock samples as well as isolated kerogen were studied and evaluated as potential energy sources. The two approaches are correlated to the Fischer—Assay retorting method. Some samples of trapped pyro‐hydrocarbons are studied in detail. Applications of these methods are proposed and discussed.
Thermoanalytical Characterization of Flame Retardant Additive Action in Solids
Authors: Simon J. , Androsits B.
Published: November 2013

Thermal analysis, in particular the derivatograph and its improved variants, were used to characterize flame retardation processes in solids. The techniques presented in this paper are applicable both to solid—liquid and to gas—liquid transitions. Naturally‐occurring and synthetic polymers and flame retardants of practical importance were chosen as model substances. The results confirm data obtained using other methods. The advantages and the limits, in the application of thermal analysis to the field of flame retardation, are discussed.
Energy Dependence of Diffraction Intensities: 4He + LiF (001)
Authors: Drolshagen Gerhard , Kaufhold Achim , Toennies J. Peter
Published: November 2013

The dependence of specular and first order diffraction probabilities on the atom beam energy has been calculated for the system He + LiF (001) in the energy range 18 to 58 meV. A realistic Lennard‐Jones—Devonshire atom surface potential with several values of the β‐parameter, determining the strength of the corrugated part, was used. A comparison with the predictions of the simple Rayleigh—Eikonal approximation shows agreement with respect to the overall trends, however the quantal calculations show some deviations and the additional effect of selective adsorption resonances. The results are compared with recent measurements for the (100) and (110) directions and good agreement is found for β10 = 0.086.
Rotationally Mediated Selective Adsorption as a Probe of Isotropic and Anisotropic Molecule—Surface Interaction Potentials: HD(J)/Ag(111)
Authors: Yu Chien‐Fan , Hogg Charles S. , Cowin James P. , Whaley K. Birgitta , Light John C. , Sibener Steven J.
Published: November 2013

Rotationally mediated selective adsorption scattering resonances are used to make an experimental and theoretical study of the laterally averaged interaction potential between HD and a weakly corrugated system, Ag(111). The experimentally observed resonances determine the vibrational levels of the HD/Ag(111) physisorption potential as a function of bound rotational state. These vibrational levels show J‐dependent shifts due to the orientational anisotropy of the potential. Exact quantum scattering calculations using a full laterally averaged potential of the form have been carried out to obtain rotationally inelastic transition probabilities. Experimental and theoretical resonance energies are compared for two forms of vo(z), a Morse and a variable exponent potential, as a function of β, and are found to be very close to the first‐order perturbed energies of a free rotor in bound states of vo(z). Both potential forms give equally good fits to the data, yielding an optimum value of the asymmetry parameter, β ≈ — 0.05. The determination of β is relatively insensitive to small changes in the vo(z) well depth.
Scattering of N2 from Ag(001)
Authors: Muhlhausen Carl W. , Serri John A. , Tully John C. , Becker Gordon E. , Cardillo Mark J.
Published: November 2013

Experimental and theoretical studies of the scattering of nitrogen molecules from the (001) face of silver are presented. Angular and time‐of‐flight distributions of the scattered molecules have been measured for incident N2 energies between 7 and 96 kJ/mol, and for surface temperatures between 205 and 600 K. Stochastic trajectory simulations have been carried out for the same conditions. Good agreement between experiment and theory has been obtained. The angular distributions are shown to be very sensitive to the extent of corrugation of the gas—surface interaction potential, but are relatively insensitive to rotational energy transfer. Scattered velocity distributions also show only a small dependence on rotational energy transfer.
An Electrodynamic Theory of the Non‐adiabatic Effects in Rate Processes for Molecules Adsorbed on Metals
Authors: Metiu Horia
Published: November 2013

We present a theory which computes the role of the electronic excitations in a metal substrate in dynamic processes involving adsorbed molecules. We use a model in which the molecular skeleton has effective charges whose magnitude and position varies when the nuclei of the molecule move. These moving charges couple to the electrons in the metal and excite them; as a result the molecule loses energy. The rate of such energy loss affects the vibrational line‐width and the fluorescence lifetime of the molecule, the van der Waals interaction between adsorbed molecules and the motion of the molecule at the surface. We show how these effects can be discussed in a unified way in terms of an electromagnetic theory. We emphasize that for obtaining reasonable numerical results we must replace phenomenological Maxwell equations with a microscopic theory that takes into account the non‐locality of the electron response to moving charges and the continuous nature of the interface.
Non‐Markoffian Theory of Activated Rate Processes II. Thermal Desorption
Authors: Nitzan Abraham , Carmeli Benny
Published: November 2013

A model for thermal desorption, described by a one dimensional classical generalized Langevin equation (GLE) for the motion of an adatom, is solved by reducing the GLE to a Fokker Planck equation in action space. The escape rate is obtained as the inverse mean first passage time for the particle to achieve a threshold energy (or action). A calculation using parameters corresponding to the desorption of Ar from W is compared with quantum mechanical results for the same model.
Ion Induced Surface Alterations Due to Electronic Charge Exchange and Chemical Reactions
Authors: Rabalais J. Wayne
Published: November 2013

A general discussion of the phenomena that can occur during ion—surface encounters is presented and specific examples of charge exchange processes on LiF surfaces and chemical reactions of N+2 on metals are shown. During rare gas ion bombardment of LiF, ejection of F+ ions and excited Li atoms and the energy distributions of secondary F+ ions can be correlated with Auger primary ion—surface charge exchange transitions determined by the potential energy of the primary ion vacancy and electron promotions resulting from close atomic encounters. Nitridation of Al, Cu, Mo, and Ni to produce very thin nitride surface films is induced by 0–50 eV mass selected N+2 beams. The efficiency of nitridation, monitored by the KL2L2 nitrogen Auger electron signal, exhibits a large kinetic energy Ek dependence below ca. 30 eV. The threshold impact energy for nitridation varies for the different metals; it is nominally zero for Al and Ni, ∼ 4 eV for Mo, probably > 0 for Cu, and does not occur at any energy below 200 eV for Ag. That the metal electronic structure is a critical parameter is evidenced by the facts that different ion doses are required to produce similar nitride signal levels on various metals and that plots of nitride intensity versus Ek exhibit structure or at least different shapes in the low Ek region. A simple quantum mechanical model is used to simulate the nitridation reaction by delineation into four elementary steps: (1) neutralization of incoming N+2, (2) decomposition of N2 into 2N, (3) deexcitation and thermalization of N, and (4) chemical reaction proper.
Dynamics of Surface Atoms and Molecules Under Irradiation
Authors: Shapira Yoram
Published: November 2013

Various classes of interactions between surface species and irradiation are discussed. Electron induced decomposition of halides, photon‐ and electron‐stimulated desorption and photodesorption are emphasized. Experimental work, involving a wide range of techniques, has been carried out on different materials whose surfaces can be subject to those interactions, enhancing our insight into the corresponding processes. The data are described in terms of existing and new mechanisms, while the similarities and differences between the various interactions are pointed out.
Electronic Structures and Reactivities of Pyramidal Alkenes and Carbonyls
Authors: Houk K. N. , Rondan Nelson G. , Brown Frank K.
Published: November 2013

Theoretical predictions that alkenes and carbonyls pyramidalize in asymmetric environments were made before any conclusive experimental evidence for this phenomenon had been noted. In the last two years, several such pyramidalizations have been discovered. This review summarizes the predictions, experimental discoveries, and theoretical explanations of pyramidalizations of alkene and carbonyl carbons and describes the evidence indicating that both pyramidalization and stereoselectivity in reactions of such molecules arise from torsional (closed‐shell repulsion) effects.
Structure‐Resonance Valence Bond Theory for Ferrocenes and Ferrocenylalkyl Cations
Authors: Herndon William C. , Agranat Israel
Published: November 2013

Structure‐resonance theory can be used to derive charge distributions and bond orders in ferrocenes, open ferrocenes and α‐ferrocenylalkyl cations. These species are of interest because they are unusually accessible and stable and understanding the electronic origin of this stabilization would aid in developing bonding models for organometallic systems. The methods for carrying out the calculations will be outlined. They involve shortcuts and algorithms for counting structures. The choice of structure basis, limited to conventional principal resonance structures, will be discussed. Charges and bond orders will be compared with NMR chemical shift and X‐ray data. The unusual stabilities of the ions and the role of the metal in stabilizing positive charge will be understandable within the conventional ideas of structural organic chemistry and resonance theory. These metal‐organic systems are highly delocalized and consequently resonance stabilized to a significant degree. They can be characterized as three‐dimensional aromatic compounds.
Structures, Energetics and Vibrational Frequencies of Cyclopropyne
Authors: Fitzgerald George , Schaefer Iii Henry F. ,
Published: November 2013

The electronic structure of the lowest singlet and triplet states of cyclopropyne have been investigated using ab initio molecular quantum mechanics. Both double zeta (DZ) and double zeta plus carbon d function (DZ + d) basis sets have been used in conjunction with self‐consistent‐field (SCF), multiconfiguration (MC) SCF, and configuration interaction (CI) methods. Singlet cyclopropyne is predicted to be a transition state for the degenerate rearrangement of propadienylidene (H2C=C=C:). However, triplet cyclopropyne is a minimum on the C3H2 potential energy hypersurface and vibrational frequencies are predicted at two levels of theory. Structures and vibrational frequencies are compared with those of the relatively stable cyclopropene molecule.
The Origin of Alkyl Substituent Effects on the Reactivity and Thermodynamic Stability of Alkenes. A Theoretical Study on Alkene Radical Cations
Authors: Bach Robert D. , Wolber Gregory J. , Pross Addy
Published: November 2013

Fully optimized STO‐3G SCF MO calculations were conducted on a series of methyl substituted ethylene radical cations. The stabilizing effect of a methyl group is found to be 14 kcal/mol using ethylene as a reference. Analysis of the orbital make‐up of the radical cations and their calculated geometry suggests that hyperconjugative 1‐, 2‐, and 3‐electron interactions between the alkyl groups and the π‐system is largely responsible for the stabilization. Analysis of hyperconjugation in propene suggests that 2‐electron stabilization outweighs 4‐electron destabilization energetically but that effects of the 2‐electron stabilization are manifested in a low‐lying MO (Φ1, Fig. 6).
A MO Study of β‐Substituted Ethyl Radicals
Authors: Hoz Tova , Sprecher Milon , Basch Harold
Published: November 2013

The preferred ground state conformation of β‐substituted ethyl radicals, with the Cβ heteroatom substituent group being PH2, SH and CI1 have been determined by ab initio spin‐unrestricted self‐consistent field gradient optimization techniques. All the ground state geometries are found to have the Cβ‐heteroatom bond coplanar with the Cα‐centered radical orbital. No tendency towards heteroatom group bridging between Cα and Cβ is found in the systems studied here. The radical centers are calculated to be non‐planar, with the Cα hydrogen atoms tilting towards the Cβ substituent group. Calculated rotation energy profiles about the Cα‐Cβ and Cβ‐heteroatoms bonds, as well as the atomic orbital spin population distributions, are interpreted in terms of possible interactions between the Cα‐centered radical orbital and the Cβ‐heteroatom σ bond orbitals (hyperconjugation, type I) or the heteroatom 3d (type II) or lone pair (type III) atomic orbitals. Type I is the only significant stabilizing interaction found in these systems, whereas the type III (two orbital, three electron) interaction is actually found to be destabilizing.
The Insertion of Singlet Silylene into Ethylene
Authors: Anwari Freidun , Gordon Mark S.
Published: November 2013

Ab initio calculations at the minimal, split valence and extended basis set levels, both with and without electron correlation corrections, were used to study the insertion of singlet silylene into ethylene to form silacyclopropane. The least motion C2v path is symmetry forbidden and an artificial barrier of about 36 kcal/mol is found within C2v symmetry. Relaxation of symmetry to C, eliminates the symmetry forbiddenness of the reaction. The 3‐21G and 6‐31G* SCF calculations still predict a small barrier along this symmetry allowed path; however, the addition of second order correlation corrections results in the prediction of an insertion process with no barrier. The overall ΔE for the reaction is predicted by MP2/6‐31G*//3‐21G to be 47.9 kcal/mol.
Synthetic Methods in Bile Pigment Chemistry
Authors: Gossauer Albert
Published: November 2013

The methodology of bile pigment synthesis including oxidative ring cleavage of porphin derivatives, total syntheses from pyrrole precursors and mutual transformation of bile pigments has been critically reviewed.
NMR Spectroscopy of Bilirubin and its Derivatives
Authors: Kaplan Doron , Navon Gil
Published: November 2013

Literature NMR data of bilirubin and related compounds and their interpretation in terms of structure, solution conformation and dynamic properties have been reviewed. 1H‐spectrum assignments as well as solvent and temperature dependencies of the chemical shifts are described. Proton exchange rates, longitudinal relaxation times and nuclear Overhauser enhancements are surveyed. NMR is shown to provide knowledge on the basic structure, conformation, tautomerism, hydrogen bonding, acidity constants, segmental motion of side‐chains, interconversion between enantiomers and aggregation—disaggregation of these compounds in solution.
A Kinetic Study of the Interaction Between Bilirubin and Thermally Produced Singlet Oxygen
Authors: Galliani G. , Manitto P. , Monti D.
Published: November 2013

Thermal decomposition of 3‐(4‐methyl‐1‐naphthyl)‐propionic acid‐1,4‐endoperoxide at 34,7°C has been used to produce singlet oxygen in non‐photochemical conditions. Thus the reaction of bilirubin with singlet oxygen has been studied. A kinetic scheme is proposed taking into account physical and chemical quenching of singlet oxygen by bilirubin and quenching by the endoperoxide. From the lifetime, τ = 100 μs, of 102 in methylene chloride the following rate constants have been determined: for bilirubin: kphys quenching = 5.0 × 109 M−1s−1,kchem quenching = 5.0 × 108 M−1s−1 and for the endoperoxide: k = 8.5 × 107 M−1s−1. These same constants in the same order as above but in water (τ = 3μs) are: 4.9 × 109 M−1s−1, 1.0 × 109 M−1 s−1 and 9.0 × 108 M−1's−1 Moreover it has been shown that bilirubin is scarcely protected by albumin from singlet oxygenation. The protection in photochemical systems seems to be due to the lower ability of bilirubin to behave as a photosensitizer.
Determination of Deuterium Quadrupole Coupling Constants in the Liquid Crystalline Phase — Values for Chloroform and Dichloromethane
Authors: Voigt J. , Bjorholm T. , Jacobsen J. P.
Published: November 2013

The deuterium quadrupole coupling constants of chloroform and methylenechloride have been determined as 174.7 ± 0.5 kHz and 167.8 ± 1.3 kHz, respectively. The values were determined from the quadrupole splitting and the 13C, 2H dipole–dipole couplings obtained from the natural abundance 13C‐satellites in the 2H‐NMR spectrum of the molecules in nematic phase. The dipole‐dipole coupling has been corrected for vibrational motion.
Nitrogen‐14 NMR of Solutes in Liquid Crystals: NH4NO3 and (CH3)4N NO3 in the DSCG–Water System
Authors: Gilboa Haggai , Loewenstein Aharon
Published: November 2013

Measurements of the nitrogen‐14 and deuterium quadrupolar splittings in NH4NO3 and (CH3)4 N NO3 dissolved in the disodiumcromoglycate–water liquid crystalline system are presented. Nitrogen‐14 T1 values in aqueous solutions for these salts are also given. The results are discussed and compared with similar studies on solutes containing 14N in ordered media.
Modern Spin Relaxation Techniques for Liquid Crystals: Nuclear Spin Probes in Thermotropic Mesophases
Authors: Vold Robert L. , Vold Regitze R.
Published: November 2013

The existence of well‐resolved dipolar and/or quadrupolar splittings in the NMR spectra of solutes in ordered phases greatly facilitates analysis of nulcear spin relaxation pathways in terms of individual spectral density parameters evaluated at well‐defined frequencies. Both auto‐ and cross‐correlation terms are experimentally accessible via an appropriate combination of modern pulse techniques, and this detailed information is of use in assessing the validity of various models for molecular motion in ordered systems. Spectral density data for small, rigid solutes in nematic mesophases can be rationalized in terms of rotational diffusion, including effects of static order, together with order director fluctuations. The techniques developed for the simpler nematics can readily be used for solutes in the smectic phase as well as for studies of the liquid crystalline solvent molecules themselves.
NMR Relaxation in Thermotropic Liquid Crystals
Authors: Dong Ronald Y.
Published: November 2013

In this review some essential features of the spin relaxation theory in liquid crystals are outlined, with particular emphasis on orientational order director fluctuations and molecular reorientations in anisotropic environments. Experimental relaxation studies using 1H, 2H and 13C nuclei during the past five years are surveyed and compared with existing theories. While the molecular dynamics are relatively well understood in the nematic and smectic phases, more work is needed to further understand cholesteric and discotic mesogens.
Substituent, Temperature, and Solvent Effects in the Singlet Oxygenation of 7‐Alkyl‐ and 7‐Aryl‐substituted 1,3,5‐Cycloheptatrienes
Authors: Adam Waldemar , Rebollo Hector
Published: November 2013

Singlet oxygen is a sufficiently reactive dienophile to intervene in the tropilidene‐norcaradiene valence isomerization equilibria of 7‐aryl‐ and 7‐alkyl‐1,3,5‐cycloheptatrienes, leading to both the norcaradiene and tropilidene endoperoxides 2N and 2T, respectively, besides the thermally rather labile 1,2‐dioxetanes 3 and the substituted benzaldehydes 4, derived from the latter on thermal decomposition. Quantitative product studies by 1H‐NMR reveal that the proportion of 2N and 2T endoperoxides is a delicate function of (a) the electronic nature of the 7‐aryl substituent, i.e. the 2N/2T ratio increases in the order p‐Cl > H > p‐OMe, (b) the steric demand of the 7‐alkyl substituent, i.e. the 2N/2T ratio increases in the order tBu > iPr > Et > Me, (c) the temperature of singlet oxygenation, i.e. the 2N/2T ratio increases with increasing temperature, and (d) the polarity of the solvent, i.e. the 2N/2T ratio decreases in the order CCl4 > C6H6 > CH2Cl2 > CH3CN. A mechanism is proposed in which substituent and temperature effects are interpreted in terms of appropriate shifts in the 2T ⇌ 2N equilibria. The solvent effect is rationalized in terms of reversible exciplex formation between the tropilidene isomer and singlet oxygen (electronically excited), leading to the 1,4‐dipolar intermediate 7.
Heterolytic and Homolytic Oxygen Transfer Reactions from d0 Metal Peroxides to Hydrocarbons
Authors: Mimoun Hubert
Published: November 2013

An attempt is made to correlate the catalytic activity of d0 metal complexes in the oxidation of hydrocarbons by H2O2 and ROOH with the oxidizing properties of their peroxidic derivatives. The triangular structure of bound peroxo and alkylperoxidic groups is a common characteristic of d0 metal peroxides. The heterolytic or homolytic nature of the catalytic oxidation is governed by the heterolytic or homolytic cleavage of the triangular peroxidic moiety. The mechanism of selective heterolytic epoxidation involves the peroxymetalation of the coordinated olefin. Homolytic oxidations are less selective and involve the non‐stereospecific epoxidation of olefins and the hydroxylation of alkanes and arenes by biradical peroxidic species.
Radiolytic Studies of the Cumyloxyl Radical in Aqueous Solutions
Authors: Neta P. , Dizdaroglu Miral , Simic Michael G.
Published: November 2013

Formation and reactions of the cumyloxyl radical in aqueous solutions were studied by steady‐state and pulse radiolytic techniques. Cumene hydroperoxide reacts with e−aq (k = 4.4 × 109 M−1s−1) to yield the cumyloxyl radical. The spectrum recorded after the pulse indicates formation of a species absorbing at 250 nm. This product was identified as acetophenone, which is formed by the fragmentation of the cumyloxyl radical. By comparison of the pseudo‐first‐order rates of e−aq decay at 600 nm with the rate of production of acetophenone at 245 nm at increasing concentrations of cumene hydroperoxide, it was possible to derive a rate constant of 1.0 × 107 s−1 for the cleavage of cumyloxyl to acetophenone and methyl radical. This value is higher than that measured previously in organic solvents (1 × 106 s−1), as expected. HPLC analysis of the radiation products acetophenone and cumyl alcohol permitted determination of rate constants for hydrogen abstraction by the cumyloxyl radical, in competition with the fragmentation. The rate constants for H abstraction from i‐PrOH, EtOH, and MeOH by CmO were found to be 9.9 × 106, 3.8 × 106, and 8.5 × 105 M−1 s−1, respectively.
Measurement of Absolute Propagation and Termination Rate Constants for Alkylperoxyls in Solution by the Hydroperoxide Method
Authors: Howard J. A.
Published: November 2013

Kinetic and product studies of liquid phase autoxidations in the presence of high concentrations of alkyl hydroperoxides have enabled absolute propagation and termination rate constants for alkylperoxyls to be measured. A large number of rate constants and activation parameters have been accumulated over the past few years by this technique and the highlights of this work are presented in this review.
Organoboranes. 36. A Simple, General Synthesis of Alkynyl Ketones1 and [E]‐1,3‐Alkenynes via Thexylchloroborane‐Dimethyl Sulfide
Authors: Brown Herbert C. , Bhat N. G. , Basavaiah D.
Published: November 2013

Thexylchloroborane‐dimethyl sulfide (1) hydroborates representative 1‐bromo‐1‐alkynes in the presence of 10% boron tribromide as a catalyst to afford thexyl‐(cis‐1‐bromo‐1‐alkenyl)‐chloroboranes (2). Treatment of these intermediates with isopropyl alcohol provides the corresponding isopropyl thexyl(cis‐1‐bromo‐1‐alkenyl)borinates (3B). Subsequent treatment with alkynyllithium gives the corresponding “ate” complexes (4) which undergo intramolecular migration of the alkynyl group from boron to the adjacent carbon, displacing the bromine, producing the corresponding cis‐vinylboranes (5). These intermediates (5) provide the corresponding alkynyl ketones (6) on oxidation and the corresponding stereodefined conjugated enynes (7) of high isomeric purity on protonolysis. Yields in the range of 60–65% were realized.
Cyclic Carboalumination of Alkynylsilanes Forming Exocyclic Alkenes
Authors: Miller Joseph A. , Negishi Ei‐Ichi
Published: November 2013

The reaction of 6‐trimethylsilylhex‐1‐en‐5‐yne with i‐Bu3Al (1.1 equivalents) and a catalytic amount of Cl2ZrCp2 in 1,2‐dichloroethane or CH2Cl2 at room temperature produces, after protonolysis, trimethylsilylmethylenecyclopentane in 85% yield. Upon iodinolysis of the reaction mixture, the corresponding iodide is obtained in 68% yield. Under the same reaction conditions, 7‐trimethylsilylhept‐1‐en‐6‐yne is converted into trimethylsilylmethylenecyclohexane in 72% yield upon protonolysis of the reaction mixture. Attempts to produce four‐ or seven‐membered ring compounds have not been successful. The i‐Bu3Al–Cl2ZrCp2 reaction of 6‐trimethylsilyl‐4‐methylhex‐1‐en‐5‐yne is nonstereoselective. However, the use of I2ZrCp2 in place of Cl2ZrCp2 improves the E/Z ratio to ca 80/20. The reaction of 1‐trimethylsilylhex‐5‐en‐1‐yn‐3‐ol with i‐Bu3Al–Cl2ZrCp2 gives the ca 90% pure Z isomer under “kinetic” conditions and the ca 97% E isomer under thermally equilibrating conditions in excellent yields.
Enantiocontrolled Synthesis of (R)‐3‐Substituted Cyclohexanones Using (S)‐2‐(p‐Tolylsulfinyl)‐2‐cyclohexenone and Various Organometallic Reagents in 2,5‐Dimethyltetrahydrofuran
Authors: Posner Gary H. , Frye Leah L.
Published: November 2013

Four different (R)‐3‐substituted cyclohexanones of 65–96% enantiomeric purity have been prepared in 58–84% yields via organometallic conjugate additions to (S)‐2‐(p‐tolylsulfinyl)cyclohexenone in 2,5‐dimethyltetrahydrofuran (DMTHF) as a solvent which allows more effective metal ion chelation by the bidentate β‐ketosulfoxide (and therefore higher asymmetric induction) than does THF. Several recently‐reported organotitanium reagents have been used effectively for the first time as Michael donors.
Tricarbonyl(η6‐Cycloheptatriene)manganese Hexafluorophosphate: Reactions with Enolate Nucleophiles and Decomplexation of the Product Dienyl Complexes
Authors: Pearson Anthony J. , Bruhn Paul R. , Richards Ian C.
Published: November 2013

Reaction of tricarbonylcycloheptatrienemanganese hexafluorophosphate with a wide range of enolate nucleophiles derived from malonic esters, phenylsulfonylesters, β‐ketoesters and cyclohexanones proceeds cleanly to give 6‐substituted tricarbonyl(1–5‐η‐cycloheptadienyl)manganese complexes 3. It was found that demetallation with concomitant stereospecific hydroxylation, to give substituted cycloheptadienes, could be accomplished by direct treatment of the dienyl complexes with ceric ammonium nitrate in buffered wet acetone. Complexes 3a–3c gave mixtures of hydroxy esters and gamma lactones 6 and 7 in proportions which depended on the nature of the substituent present. A novel and potentially useful demetallative cyclofunctionalization was found to occur on treatment of hydroxy ester 3m with excess iodine in acetonitrile, to give the cyclic ether 10 as the only observable product.
Vinyl Ether–Iron Complexes as Vinyl Cation Equivalents. Preparation and Use of an α‐Acrylic Ester Cation Equivalent
Authors: Chang T. C. T. , Rosenblum M.
Published: November 2013

The organoiron complex 4 is readily prepared from α‐bromopyruvic ester diethyl acetal, and has been shown to serve as an effective α‐acrylic ester cation equivalent, in the synthesis of α‐methylene‐γ‐lactones. Cyclohexanone lithium enolate is transformed to either the cis or trans fused lactones 12c, t, and ethyl acetoacetate lithium enolate can be converted to the lactone 21t. The use of methyl 3‐oxohexadecanoate as starting material provides a stereospecific synthesis of protolichesterinic acid methyl ester.
Synthesis of Homoallyl Selenides and Sulfides by Allylation of α‐Seleno‐ and α ‐Thioalkyllithiums in the Presence of Copper(I) Salts
Authors: Clarembeau M. , Bertrand J. L. , Krief A.
Published: November 2013

New species, detected by 77Se NMR, are formed on mixing α‐selenoalkyllithium compounds and CuI/SMe2. These species decompose at −40° to olefins 8 but can be allylated in high yield at −100°. Similar reactions are observed with thio‐analogues.
General Synthetic Methods for the Preparation of 1,4‐ and 1,5‐Dicarbonyl Compounds. New Annulation Methods Based on the Palladium‐Catalyzed Oxidation of Terminal Olefins to Methyl Ketones
Authors: Tsuji Jiro , Shimizu Isao , Kobayashi Yuichi
Published: November 2013

New synthetic methods for the preparation of 1,4‐diketones, 4‐oxopentanals, and 5‐oxohexanals were developed based on the oxidation of terminal olefins to methyl ketones with PdCl2‐CuCl‐O2 as a key reaction. They constitute useful annulation methods.
The Status of the Photochemistry and Photophysics of Tritium‐From‐Deuterium Isotope Separation by Infrared Laser Multiple‐Photon Dissociation of Chloroform
Authors: Herman Irving P. , Magnotta Frank , Aldridge Frederick T.
Published: November 2013

Several experiments that examine the multiple‐photon absorption and dissociation and the post‐photolysis chemistry of tritium‐from‐deuterium isotope separation by 12 μ laser photolysis of chloroform are described. Results of recent experimental measurements of T/D selectivity in absorption and dissociation are reviewed. The fluence dependence of CTCl3 IR photolysis is obtained from experiment and an energy‐averaged master equation model. Preliminary measurements of two‐frequency dissociation of CTCl3 are presented. The pathways of the post‐photolysis chemistry are deduced from product formation in the 11 μ dissociation of neat CDCl3, and of the carbon‐isotope labelled reactants 12CDCl3/13CHCl3.
Infrared Multiple Photon Absorption of Silicon Tetra‐Fluoride and Anisole
Authors: Haas Y. , Citri O. , Yahav G.
Published: November 2013

The infrared multiple absorption probabilities of the title molecules are measured at two frequency ranges: one for which the single photon absorption (SPA) cross section is large (range I), and another for which it is much smaller (range II). For SiF4, the high power absorption probability in range I is much smaller than the SPA cross section. Collisions significantly increase the average number of photons absorbed. In range II, the absorption cross section increases upon increasing the laser fluence. For anisole, the same fluence effect is qualitatively observed, but it is less pronounced in range I. For anisole pressure exceeding 0.1 torr collisions with argon do not change the average number of photons absorbed per molecule. The 10.2 μ absorption in SiF4 is suggested to be initiated by a single photon hot band absorption, followed by absorption in the quasi‐continuum (QC). QC absorption cross sections are in the range of (5–20) × 10−21 cm2 for both SiF4 and anisole.
A Quantitative Test of the Statistical Limit for Multiphoton Ionization‐Fragmentation
Authors: Ohmichi N. , Silberstein J. , Levine R. D.
Published: November 2013

Observed ionic fragmentation patterns are compared against the extreme statistical limit. A quantitative measure of closeness of fit is employed to assess the need for refinements in the theory and in the noise level of the experiment. It is concluded that for such molecules as benzene or toluene there is no compelling reason to go beyond the extreme statistical limit in order to account for the observed fragmentation patterns. There is some indication for meaningful deviances in MPI‐D of aliphatic compounds.
On the Dynamics of Molecular Two‐Photon Excitation through Real Intermediate States
Authors: Bigio Laurence , Grant Edward R.
Published: November 2013

New results from our laboratory on the two‐photon absorption spectroscopy and photodissociation dynamics of NO2 are examined in the light of a simple nonperturbative theory for the time evolution of the intermediate state in a three‐level system. Experiments on NO2 show effects distinct to two‐photon excitation, evident spectroscopically in intense bent‐to‐linear origins and dynamically in unique patterns of energy disposal in two‐photon photodissociation. We trace these effects to intramolecular relaxation which can be described by the evolution of an intermediate wave packet composed of a superposition of vibronic states prepared by the absorption of the first photon. Our theoretical treatment addresses the timescale for this evolution. Its results suggest that this timescale will depnd on the intensity of the driving field, and so offer the intriguing possibility that the spectroscopy and dynamics of two‐photon excitation through a system of real states will depend on laser intensity in a manner that tracks the time dependence of the intermediate wave packet.
Multiphoton Ionization Photoelectron Spectroscopy
Authors: Reilly James P.
Published: November 2013

Recent research involving the monitoring of photoelectrons generated in the course of molecular multiphoton ionization is reviewed. Information about the order of the multiphoton ionization process, the identity and ionization potential of the species ionized, and spectroscopic data about excited states of neutral molecules and the ground states of ions can all be extracted using this new technique.
Coherent Vacuum Ultraviolet in Chemical Physics
Authors: Hepburn J. W.
Published: November 2013

Methods of generating coherent vacuum ultraviolet radiation by frequency mixing techniques are reviewed. The theory and methodology of four‐wave mixing in gas mixtures is discussed and some representative methods for generating coherent light in the range from 900 Å to 2000 Å are described. In the last section some recent applications of these VUV sources to chemical physics will be outlined.
On the Absorption Maxima of Protonated Retinal Schiff Bases. An Interaction with External Charges
Authors: Baasov Timor , Sheves Mordechai
Published: November 2013

The influence of external positive charges on the absorption maximum of protonated retinal Schiff base is markedly enhanced by weakening the interaction between the external positive charge and its counter‐ion.
Synthesis and Structures of Substrates for Investigation of Intramolecular Electronic Energy Transfer
Authors: Rubin Mordecai B. , Migdal Shmuel , Speiser Shammai , Kaftory Menahem
Published: November 2013

A series of cyclic α‐diketones incorporating a benzene ring (ortho, meta or para substituted) have been synthesized for studies of intramolecular energy transfer (intra‐ET). The synthetic method involved acyloin cyclization of the corresponding dicarboxylic acid diesters followed by oxidation with DMSO‐acetic anhydride reagent. All compounds were sufficiently flexible so that, even at temperatures in the range of −120°, only broadened lines were observed in their NMR spectra, consistent with observations on temperature dependence of the intra‐ET process. The structure of one compound (P‐4,4) was established by X‐ray crystallographic analysis and of the remaining compounds by a combination of spectroscopic data and molecular mechanics calculations. The calculated structure of P‐4,4 is essentially identical to the X‐ray structure. In all compounds the methylene chains assume staggered conformations, the dicarbonyl torsion angle aproached 180°, and the molecule folds in such a way that, with one exception, the dicarbonyl moiety lies above (or below) the plane of the benzene ring. Distances between chromophores range from about 3.3 to 5.8 Å.
The Chemistry of Platinum in the +3 Oxidation State
Authors: O'Halloran Thomas V. , Lippard Stephen J.
Published: November 2013

Platinum(III) is no longer an uncommon oxidation state. Numerous binuclear platinum(III) complexes have been prepared and structurally characterized over the past eight years. These include sulfate‐bridged dimers of D4h symmetry, [Pt2(SO4)4L2]2−, L = H2O, DMSO; phosphate‐bridged complexes [Pt2(HPO4)4‐(H2O)2]2− and [Pt2(H2PO4)(HPO4)3(py)2]−; POP (H2 P2 O2−5)‐bridged ions [Pt2‐(POP)4X2]2−, X = halide; an extensive series of α‐pyridonate (C5H4NO−)‐bridged head‐to‐head and head‐to‐tail complexes, [Pt2(NH3)4 (C5H4NO)2XY]n−, X, Y = NO3, NO2, H2O, Cl, Br; n = 2, 3; and organometallic derivatives such as [Pt2(CH3)4‐(CF3CO2)2(4‐Mepy)2]. In all cases there is a Pt–Pt single bond of length 2.47–2.7 Å, pseudo‐octahedral geometry about platinum, and two or more bridging ligands. The complexes are stable in solution and some undergo quasi‐reversible two‐electron redox reactions. Mononuclear platinum(III) complexes are less well characterized structurally, but have been stabilized in diamagnetic host lattices in the solid state and by macrobicyclic cage ligands in solution following pulse radiolytic or γ‐irradiation of precursor platinum(II) complexes. The first unequivocal, crystallographically characterized mononuclear platinum(III) complex, [Pt(C6Cl5)4]−, has just been reported.