bluuebunny/crossref_metadata_2025 huggingface dataset:
Real and Ideal Relationships between Selectivity, Reactivity and Equilibrium
Authors: Argile A. , Carey A.R.E. , Fukata G. , Harcourt M. , More O'Ferrall R.A. , Murphy M.G.
Published: November 2013

Reactivity and selectivity are discussed in terms of quadratic rate—equilibrium relationships for reaction series sharing a common intrinsic barrier, notably Marcus' equation. R.D. Levine's emphasis on such reactions as forming a “Brønsted Series” is suggested as a basis for defining “ideal” reactivity behavior. Ideal relationships between selectivity and reactivity and selectivity and equilibrium are derived based on reaction schemes drawing a formal distinction between reagent and substrate. In a simple Brønsted relationship, the reagent varies and the substrate is held constant. In an extended relationship, in which both substrate and reagent vary, the Brønsted series becomes a selectivity “network” or matrix characterized by direct and cross quadratic coefficients reflecting curvature of a free energy relationship and the dependence of selectivity upon rate or equilibrium respectively. In the ideal case, the quadratic coefficients are equal but, in practice, they generally differ. Inspection of the relevant reaction schemes shows that in evaluating the coefficients, there is greater cancellation of experimental and chemical scatter arising from non‐systematic variations in reagent structure for the cross than the direct coefficient; this phenomenon accounts in part for the wider observation of reactivity—selectivity than non‐linear free energy relationships. New measurements are reported for reactions of carboxylic acids with enolate and imine substrates. The results are combined with literature data and plotted as reactivity—selectivity and extended Brønsted relationships. The correlations are satisfactorily considered in the context of departures of real from ideal behaviour.
On the Relation Between Reactivity and Selectivity
Authors: Levine R.D.
Published: November 2013

Can one relate in a systematic fashion changes in the nature of the reactants to changes in the magnitude of the reaction rate constant? It is suggested that such correlations can be given a precise meaning and that the inverse correlation high selectivity—low reactivity is indeed valid. What need not always be true is that a single selectivity index (e.g., location of the transition state) suffices to quantitatively describe the correlation. When a single selectivity index does suffice we prove the reactivity—selectivity principle including the Hammond postulate.
Brønsted βNuValues and Leaving Group Effects in SN2 Reactions. Tests of the Reactivity—Selectivity Postulate
Authors: Bordwell Frederick G. , Branca John C. , Cripe Thomas A.
Published: November 2013

The rate constants for reactions of a family of 19 carbanions, ArCHSO2Ph−(derived from benzyl phenyl sulfones) withn‐butyl chloride have been measured in Me2SO solution. A plot of logkvs. the pKaof the conjugate acids for 12 of these carbanions give a linear plot (R2= 0.999) with a Brønsted coefficient of βNu= 0.402. Points for para electron‐withdrawing substituents, SPh, SOPh, SO2Ph. COPh, CN and NO2, deviate substantially from the plot. The deviations are attributed to the enhanced solvation of these remote substituents in the anion which leads to rate retardation. A curved Brønsted plot can be drawn through all the points, which would be consistent with the predictions of the Hammond–Leffler postulate (HLP) and the reactivity–selectivity postulate (RSP), but this interpretation is rejected. Instead, it is suggested that the apparent curvature in Brønsted plots for acid–base reactions – upon which HLP and RSP are based – is also caused by deviations due to solvent effects, donor atom effects in the bases, mechanistic changes and/or the failure to keep electronic and steric effects constant. A reactivity–selectivity plot for reactions of 9 ArCHSO2Ph−ions withn‐butyl bromide andn‐butyl chloride indicated constant selectivity. A similar plot for 4 carbanions derived from α‐methylbenzyl phenyl sulfones reacting withn‐butyl bromide andn‐butyl chloride also showed constant selectivity. Based on these results and an examination of the literature, it is concluded that there is no firm experimental basis for HLP and RSP.
An Examination of the Relationship Between Transition State Geometry in Hydron Transfer Reactions and the Temperature Dependence of the Primary Kinetic Isotope Effect
Authors: Mclennan Duncan J. , Gill Peter M.W.
Published: November 2013

Several recent papers by Kwart feature hydron transfer reactions in which the primary kinetic isotope effect is temperature independent over wide temperature ranges. It has been asserted (without theoretical justification) that this phenomenon is associated with transition state X….H….Y bond angles which are significantly less than 180°, whereas normal temperature dependence may be associated with transition states in which the angle is close to 180°. This work employs model calculations of isotope effects, principally for [1,5] H‐shifts in 1,3‐pentadiene, to examine Kwarts's hypothesis. No model tested yielded a temperature‐independent isotope effect of substantial magnitude. The transition state (angle = 133°) was located on the MNDO potential energy surface and the isotope effect, calculated by a new and fast computational procedure, was again temperature dependent. Model calculations of secondary hydrogen isotope effects were carried out. When bending motions of the non‐reacting hydrogens were coupled to C….H stretching modes in the force constant matrix, it was found that kH/kD was greater than the equilibrium secondary isotope effect, kH/KD, and that kH/kD for hydrogen transfer exceeded kH/kD for deuteron transfer when simple tunnel corrections were incorporated. A novel interpretation to account for the observation of some temperature‐independent isotope effects is advanced.
The Application of 3‐Amino‐2H‐Azirines as Amino Acid Equivalents
Authors: Heimgartner Heinz
Published: November 2013

3‐amino‐2H‐azirines undergo a number of ring opening reactions. The cleavage of the azirine‐N(1), C(3) double bond leads thereby to synthons which can be considered as amino acid equivalents. The reaction of 3‐amino‐2H‐azirines with carboxylic acids yields N‐acyl amino acid amides, which can be converted by a selective amide cleavage to the corresponding N‐acyl amino acids. 2‐Oxazolin‐5‐ones are intermediates of this amide cleavage. This reaction sequence has been used for the extension of peptide chains as well as for a number of heterocycle syntheses. Likewise, the described synthesis of cyclic depsipeptides and lactones by direct amide cyclization proceeds via 2‐oxazolin‐5‐one intermediates. The selective amide cleavage was also applied to a novel method for the resolution of enantiomeric amino acid derivatives.
Reaction of Coumarins with Superoxide Anion Radical (O2): Facile Entry to o‐Coumarinic Acid Systems
Authors: Frimer Aryeh A. , Aljadeff Gladis , Gilinsky‐Sharon Pessia
Published: November 2013

Coumarins 1–5 were reacted with O2 in aprotic media. The course of the reaction was followed by using methyl iodide to “Cap” (trap) the oxy‐anions generated. Abstraction of the enolic hydrogen (where available) proved to be the most facile process followed by “saponification” of the lactone linkage. The cis‐stereochemistry of the original coumarin was maintained allowing easy access to a variety of o‐coumarinic acid systems.
Chiral Induction in Originally Racemic Amino Acids via 5‐Acyl and 5‐Acyloxyaminooxazoles
Authors: Lipshutz Bruce H. , Hungate Randall W. , Mccarthy Keith E.
Published: November 2013

A variety of chiral 2,4‐disubstituted‐5‐trifluoroacetamido‐,5‐carbobenzyloxyamino, and 5‐β,β,β,‐trichloroethoxycarbonylaminooxazoles have been prepared from diamide, dipeptide, or amide nitrile precursors. Conversion to their (poly)amide equivalents via a deacylation/hydration sequence is described. The extent of induction of chirality realized upon ring opening in each case is determined by HPLC.
Synthesis of Aminopyridazines from Aminocyclopropenylium Salts and Diazomethyl Compounds [1]
Authors: Heydt Heinrich , Eisenbarth Philipp , Feith Karin , Urgast Klaus , Maas Gerhard , Regitz Manfred
Published: November 2013

Amino‐substituted cyclopropenylium salts (11a,b) react with diazomethyl compounds (12a‐i) in dichloromethane or acetonitrile in the presence of a base (ethyldiisopropylamine, 1,5‐diazabicyclo[4.3.0] non‐5‐ene) to form 4‐aminopyridazines (13a‐i) specifically. The reaction is interpreted in terms of an initial electrophilic diazoalkane substitution to give diazomethylcyclopropenes (11 + 12 → 14) which undergo a [1.5]‐cyclisation to form the betaines 16 which, in turn, isomerise with opening of the bridging bond to give the aminopyridazines 13. In a similar manner, the tris[amino] cyclopropenylium salts 8a,b and 10a‐c are converted to the 3,4,5‐tris[amino]‐pyridazines (21a‐k) by treatment with diazomethyl compounds (12a, c‐f).
New Approaches to Solvent‐Mediated Molecular Interactions
Authors: Montgomery Pettitt B. , Rossky Peter J.
Published: November 2013

Recently developed integral equation methods for the evaluation of intermolecular and intramolecular interactions in a polar solvent medium are discussed and representative applications are presented. Examples include solvent modification of interionic interactions between atomic or molecular ions in water and liquid environment induced shifts in conformational equilibria for both non‐polar and polar flexible molecules in water. Emphasis is placed on the features of these phenomena which can be traced specifically to the molecularity of the solvent and hence cannot be adequately reproduced by a simple continuum dielectric solvent representation.
Homogeneous Catalyzed Generation of Formamides and Amines with Syngas as a Basic Building Block
Authors: Knifton John F. , Alexander David C.
Published: November 2013

N‐Alkylformamides, polymeric polyamines, and tertiary aliphatic and araliphatic amines, have been prepared using ruthenium homogeneous catalysis and synthesis gas as a basic building block. N,N‐dimethylformamide, for example, has been generated by a new synthesis route, directly from syngas plus ammonia, using the concept of ruthenium “melt” catalysis. Polymeric polyamine derivatives of 1,2‐polybutadienes have been prepared via the aminomethylation reaction using a two‐phase medium to facilitate product‐catalyst separation.
π‐π Interactions, Architecture, and Bandwidths in “Molecular Metals”. A Synthetic and Electronic Structure Study of Monomeric and Cofacially Joined Dimeric Germanium Phthalocyanines
Authors: Doris K. A. , Ciliberto E. , Fragala I. , Ratner M. A. , Marks T. J.
Published: November 2013

Small molecule spectroscopy on M(Pc)(OR)2 and ROM(Pc)OM(Pc)OR molecular fragments (M = Si, Ge; Pc = phthalocyaninato) of the conductive polymer [M(Pcp+)O]n provides a unique probe of the band structure‐forming π‐π interactions. We describe here the synthesis and characterization of the M = Ge monomer and dimer where R = (t‐Bu)Me2Si‐. The lowest energy He I photoelectron spectroscopic feature of the dimer is a doublet with a splitting of 0.19(3) eV. This splitting can be associated with the cofacial HOMO‐HOMO Pc‐Pc interaction and translates into a tight‐binding bandwidth in the polymer of 0.38(6) eV. This result is in good agreement with an experimental bandwidth of 0.48(5) eV obtained from a Drude analysis of the {[Ge(Pc)0]I1.1}n reflectance spectrum and a bandwidth of 0.50 eV derived from theoretical calculations.
Bisaxially Substituted 2,3‐Naphthalocyaninatoiron Compounds
Authors: Deger Sonja , Hanack Michael
Published: November 2013

2,3‐Naphthalocyaninatoiron(II) (2,3‐NcFe) (1) is reacted with L = 1,4‐diisocyanobenzene (dib), tert‐butylisocyanide (t‐buNC), cyclohexylisocyanide (c‐hxNC), benzylisocyanide (bzNC), pyrazine (pyz), 4,4′‐bipyridine (bpy) and pyridine (py) to form the corresponding bis‐axially coordinated 2,3‐naphthalocyaninatoiron(II) compounds, 2,3‐NcFeL2 3, 4, 5, 6, 7, 8, 9. All 2,3‐NcFeL2‐complexes are characterized by IR‐, and if soluble enough by 1H‐NMR‐ and UV/VIS‐spectroscopy, TG/DTA‐measurements and elemental analyses. The dc‐room temperature conductivities of the 2,3‐NcFeL2‐complexes are measured and found to be several orders of magnitude higher than the corresponding phthalocyaninato‐iron‐complexes PcFeL2.
Artificial Allosteric System 5. Allosteric Gain, Total Cooperativity and Local Cooperativity as Appropriate Indices for Allosteric Effect
Authors: Tabushi Iwao , Nishiya Takako
Published: November 2013

Magnitude of cooperativity of a hypothetical m‐subunit allosteric system of hemoglobin type is discussed, based on computation of the amount of O2 to be transferred to the corresponding monomeric nonallosteric system of myoglobin type. For quantitative discussion, “allosteric gain” is defined as the difference between the amount of O2 to be transferred from the allosteric to the nonallosteric system and that from the hypothetical system of average O2 affinity to the nonallosteric system. These two quantities, allosteric gain and amount of O2 transfer, are quite sensitive to equilibrium constant ratio, especially Km/K1, (Ki+1/Ki)max, and i. Hill's coefficient appropriately describes neither the amount of O2 transferred nor allosteric gain. Allosteric gain or the amount of O2 transfer which is best characterized by the total cooperativity index (Km/K1) and the local cooperativity indices (Ki+1/Ki) seem to be appropriate measures of the magnitude of cooperativity.
A Thermal Expansion Model for the Special Pair of the Bacterial Reaction Center
Authors: Won Youngdo , Friesner Richard A.
Published: November 2013

The temperature dependence of the longwavelength absorption band of the Rhodopseudomonas viridis reaction center is investigated with a thermal expansion model of the special pair. The validity of the model is demonstrated through the reproduction of optical lineshapes measured at various temperatures and through the consistency of the energetic parameters obtained in this fashion. The connection with the temperature dependence of the primary electron transfer process is briefly discussed.
Directed Mutations Affecting the Putative Bacteriochlorophyll‐Binding Sites in the Light‐Harvesting I Antenna of Rhodobacter capsulatus
Authors: Bylina Edward J. , Robles Steven J. , Youvan Douglas C.
Published: November 2013

Oligonucleotide‐mediated mutagenesis of the light‐harvesting I (LHI; B880 complex) antenna genes from Rhodobacter capsulatus has been used to study the bacteriochlorophyll‐peptide interactions in this photosynthetic membrane complex. A system of deletion strains and complementing plasmids has been used to assay the effects of changing single amino acid residues in the a structural polypeptide of LHI. Fourteen mutations have been introduced in the putative bacteriochlorophyll‐binding site: Ala‐X‐X‐X‐His. Residues with side chains smaller than valine can substitute for alanine, but the LHI complex is lost from the membrane if alanine is replaced by residues larger than valine, or if histidine is replaced by arginine, asparagine, aspartic acid, glutamine, proline, or threonine. The block in biogenesis is not transcriptional, since the associated reaction center, whose genes are known to be downstream from the LHI genes, continues to be inserted into the membrane. The membrane‐bound reaction center is photochemically active in these LHI‐depleted strains. Such strains may be useful for directly observing electron transfer properties of the reaction center in the membrane. This system is a model for studying the assembly of integral membrane proteins, since easily assigned optical transitions of the prosthetic groups signal proper assembly.
A Comparison of the Manganese Center Responsible for Photosynthetic Water Oxidation in O2‐Evolving Core Particles and Photosystem II Enriched Membranes: EPR of the S2 State
Authors: Sivaraia M. , Charles Dismukes G.
Published: November 2013

We have characterized the electron donors to Photosystem II (PS II) in an O2‐evolving reaction center core preparation from spinach (Ghanotakis, D.F.; Yocum, C.F. FEBS Lett., 1986, 197: 244–248) using EPR spectroscopy of the manganese center involved in water oxidation and of the tyrosine donor responsible for signal II. Both the 16‐line and the 19‐line form of the S2 multiline EPR signal can be observed in the core particles under conditions similar to those which produce these signals in PS II membranes. Consequently, the structure of the coupled cluster of three or four Mn ions remains intact in the core particles. The Kok parameters which characterize the number of reaction centers capable of advancing by 0, 1, or 2 equivalents in response to a short laser pulse are found to be the same in PS II membranes and in the core particles. Also, there is no evidence for partially inactivated centers which reach the S2 or S3 states, but which do not advance to O2 release. Thus the new class of highly resolved O2‐evolving particles appears to be well suited for biophysical studies directed at the mechanism of water oxidation.
Magnetic Resonance and Molecular Orbital Studies of the Primary Donor Cation Radical P+.960 in the Photosynthetic Bacterium Rhodopseudomonas viridis
Authors: Plato Martin , Lubitz Wolfgang , Lendzian Friedhelm , Möbius Klaus
Published: November 2013

The light‐induced radical cation of the primary electron donor, P+.960, in photosynthetic reaction centers (RCs) from Rhodopseudomonas viridis has been investigated by various magnetic resonance techniques (ESR, ENDOR, TRIPLE). Comparison of the measured hyperfine couplings with those of the cation radical of monomeric bacteriochlorophyll b (BChl b) and with RHF‐INDO/SP molecular orbital calculations, performed on P+.960 using coordinates of an X‐ray structure analysis, consistently show an asymmetric distribution of the unpaired electron over the two BChl b molecules which constitute P960. The derived spin density ratio is 2.8 in favor of the dimer half bound to the L protein subunit. The possible relevance of this result for the primary electron transfer step in the RC is briefly discussed.
Solvent Polarity Dependent Photophysics of a Fixed‐Distance, Symmetric Chlorophyll Dimer. A Model of the Special Pair in Photosynthetic Reaction Centers
Authors: Johnson Douglas G. , Svec Walter A. , Wasielewski Michael R.
Published: November 2013

The synthesis and properties of a fixed‐distance symmetric bis‐pyrochlorophyllide‐a molecule are described. The molecule, 1, consists of two methyl pyrochlorophyllide‐a (MePChlide‐a) moieties which share a common vinyl group at the 2‐position of each macrocycle. The two chlorophylls are trans to one another across the vinyl linkage. The dihedral angle between the plane of each macrocycle and the vinyl linkage is about 50°. The Qy absorption band of this dimer occurs at 689 nm and is essentially independent of solvent. The band is red‐shifted by about 27 run relative to that of MePChlide‐a. The fluorescence maximum of 1 occurs at 722 nm. This is a red‐shift of 49 nm relative to methyl MePChlide‐a. The circular dichroism spectrum of 1 shows strong, nonconservative bands in the red region. The fluorescence quantum yield, fluorescence lifetimes, and picosecond transient absorbance kinetics all exhibit a very strong dependence on solvent polarity. The fluorescence quantum yields of 1 in toluene, butyronitrile, and N, N‐dimethyl formamide (DMF) are 0.18, 0.008, and 0.002, respectively. The bleach of the 689 nm band (following excitation with a 1.5‐ps, 610‐nm laser pulse) recovers with single exponential time constants of 2.51 ns, 100 ps, and 64 ps in toluene, butyronitrile, and DMF, respectively. Possible mechanisms for the polarity‐dependent increase of the nonradiative decay rate of 1 are considered.
Electron‐Nuclear Multiple Resonance on Stable and Transient Radicals
Authors: Möbius Klaus , Plato Martin , Lubitz Wolfgang , Lendzian Friedhelm
Published: November 2013

The physical principles underlying steady‐state electron‐nuclear double resonance (ENDOR) and electron‐nuclear‐nuclear triple resonance (TRIPLE) experiments on radicals in solution are discussed. The gain in spectral resolution achievable by these techniques, as compared with standard ESR, allows hyperfine coupling constants to be measured even for large and low‐symmetry radicals. ENDOR/TRIPLE is applied to cation radicals related to bacterial photosynthesis. The measured hyperfine couplings are interpreted by means of the all‐valence electron MO method RHF‐INDO/SP. Information about the spatial structure of chromophores in the reaction centers of photosynthetic bacteria, Rb. sphaeroides and Rps. viridis, is obtained from an analysis of the spin density distribution and from minimization of the total energy. The results are compared with recently available structural data of the primary donor obtained from X‐ray diffraction experiments. The spin density distribution in the primary donor cation radical P+960, which consists of two bacteriochlorophyll b molecules, is discussed in terms of symmetry and perturbations from the protein environment.
Two‐Dimensional Deuterium NMR Exchange Spectroscopy in Liquid Crystalline Solutions
Authors: Boeffel C. , Luz Z. , Poupko R. , Vega A.J.
Published: November 2013

A theoretical analysis is given of deuterium 2D exchange spectroscopy using Zeeman‐ and quadrupole‐order experiments in liquid crystalline solutions. For the analysis, a phenomenological vector model for spin I = 1 nuclei under the effect of the quadrupole Hamiltonian and rf pulses is employed, which makes it easy to follow the evolution of the various coherences during the pulse sequences. In both experiments several types of cross‐peaks are obtained; however, only part of them are relevant to the exchange process. It is shown that proper combination of the signals from the Zeeman‐ and quadrupole‐order experiments can lead to 2D exchange spectra with “pure” exchange cross‐peaks as the only off‐diagonal signals. These exchange cross‐peaks can provide estimates for the exchange rates and indicate the relative signs of the quadrupole interactions in the exchanging sites. The method is applied to two compounds which undergo ring inversion, i.e., cyclohex‐ ane‐d12 and 1,4‐dioxane‐d8, dissolved in liquid crystalline solvents. In the Appendices procedures for phase cycling are discussed, and peak intensities for a general two‐site 2D exchange spectrum are derived.
Strategies for Obtaining Biochemical and Clinically Diagnostic Information from Human Subjects Using 31P Magnetic Resonance Spectroscopy
Authors: Styles Peter , Radda George K.
Published: November 2013

The examination of human subjects by 31P magnetic spectroscopy with a view to obtaining biochemical or diagnostic information requires that some well defined strategy be employed, both in performing the investigation and interpreting the results. The various approaches that can be utilised are reviewed in the light of experience gained in the Oxford laboratory. It is shown how resting metabolite concentration may be interpreted in terms of known enzymatic and physiological parameters, and how stress tests can be used to offer additional insight into metabolic response and control. Illustrations of the applications of these techniques include studies of human muscle, heart, liver and brain.
Hyperfine and Structural Isotope Effects in Muonated Cyclohexadienyl and Cyclopentyl Radicals
Authors: Roduner Emil , Reid Ivan D.
Published: November 2013

Incorporation of muonium, the ultra‐light isotope of hydrogen with a mass of only one‐ninth the mass of H, leads to pronounced isotope effects in hyperfine coupling constants of radicals. They reflect the mass dependence of the internal dynamics. A detailed analysis is presented for the cyclohexadienyl and the cyclopentyl radicals. It is concluded that the isotope dependent bond length in an anharmonic potential accounts quantitatively for the observed effects in rigid species.
An ENDOR Study of Nitrosyl Myoglobin Single Crystals
Authors: Kappl R. , Hüttermann J.
Published: November 2013

14N, 15N and 1H ENDOR spectra were taken from single crystals of 15NO ligated myoglobin at about 5 K in order to probe the spatial arrangement of the protons and nitrogens from the environment interacting with the Fe‐N‐O complex and to analyze its bonding configuration. The 14N hyperfine (20.4 MHz, 15.2 MHz and 16.8 MHz) and quadrupolar tensors (+1.67 MHz, −0.86 MHz, −0.98 MHz) of NE2 of the proximal histidine expand the previously reported picture from ESR analysis showing a distorted NO bond configuration. 15N data of the N(NO) ligand indicate, however, an orientation of the NO ligand differing from previous ESR reports. The single crystal spectra are applied to an interpretation of the more complex powder‐type spectra. Nine different 1H interaction tensors have been analyzed and assigned on the basis of theoretical simulation of the single crystal gamut of line positions. For the distal NE2 bound histidine proton, a small position distribution, combined with a strong anisotropy of interaction, is shown to broaden the ENDOR patterns beyond detection. The consequences of this finding with respect to the tetrameric molecule hemoglobin are discussed.
ENDOR Study of Biradicals Randomly Oriented in a Rigid Matrix
Authors: van Willigen Hans , Kirste Burkhard
Published: November 2013

An ENDOR study has been made of the structure of ketyl dimers with triplet ground state randomly oriented in a rigid matrix. The measurements give signs and magnitudes of hyperfine coupling constants and also give information on 23Na quadrupole couplings. The ENDOR results can be accounted for using the structural model previously proposed for these dimers.
Stereochemistry of Electrophilic Reactions of 4‐t‐Butyl‐1‐phenylcyclohexyllithium, ‐sodium, ‐potassium and ‐cesium
Authors: Keys Boyd A. , Eliel Ernest L. , Juaristi Eusebio
Published: November 2013

The first three title compounds were prepared by treating the diastereomeric 4‐t‐butyl‐1‐phenyl‐1‐phenylthiocyclohexanes 8 or 9 (Scheme 3) with lithium, sodium, or potassium naphthalenide, respectively. The potassium derivative was also obtained by treatment of 9 with sodium/potassium alloy and the cesium derivative was similarly obtained by reaction with cesium/potassium/sodium alloy. Quenching of any of these salts with D2O gave cis‐ and trans‐4‐t‐butyl‐1‐phenylcyclohexane‐1‐d(cis‐ and trans‐7‐d) in an approximately 1:1 ratio. The reaction was stereoconvergent and the product ratio independent of the metal; the result is explained in terms of a diffusion controlled reaction of a planar or near‐planar carbanion. In contrast, methylation with methyl iodide gave the product of axial methylation 10 in an approximately 2:1 predominance over diastereomer 11. This result is explained in terms of Curtin‐Hammett kinetics involving a stereoelectronically preferred axial approach to the electrophile. 13C NMR spectral studies of the lithium, potassium, and cesium salts indicate extensive delocalization of the anionic charge into the ring—somewhat more so with the K and Cs than with the Li salt. This finding supports the hypothesis that at least the K and Cs salts involve a planar carbanion moiety, whereas the Li salt may be slightly nonplanar. Both spectrally and stereochemically, the title compounds differ substantially from the earlier studied 2‐phenyl‐1,3‐dithianes.
2,2′,7,7′‐Tetrahydroxy‐1,1′‐Binaphthyl: A Versatile Chiral Spacer for Monotopic and Ditopic Cyclophane Hosts with Apolar Binding Sites
Authors: Hester Matthew R. , Uyeki Mike A. , Diederich François
Published: November 2013

Derivatives of 2,2′,7,7′‐tetrahydroxy‐1,1′‐binaphthyl (5) are synthetically readily available chiral shapes with a major and a minor groove. Previous work by Cram et al. has shown that efficient chiral crown ether binding sites can be constructed at the binaphthyl minor groove. The major groove is almost twice as wide and possesses suitable dimensions to shape flat apolar binding sites in chiral cyclophane hosts. The eight‐step syntheses of the two novel cyclophanes 6 and 7 are described. The apolar cavities of these hosts are formed by bridging with two linear four‐carbon chains the 7,7′‐oxygens at the major groove of 5 to the two phenolic oxygens of a 4,4‐bis(4‐hydroxy‐3,5‐dimethylphenyl)piperidine unit. 1H NMR complexation studies show that 1:1 complexes of 6 and 7 with 6‐methoxy‐2‐naphthonitrile and other naphthalene derivatives in aqueous methanol are as stable as the previously reported complexes of bis(diphenylmethane)‐hosts. The different solvation characteristics of cation and apolar binding are demonstrated with the ditopic host 7. In addition to the apolar binding site at the major groove, 7 also possesses a crown ether binding site at the minor groove of the binaphthyl spacer. By changing the methanol content of a water‐methanol mixture, this host can be switched from an efficient binder of neutral naphthalene guests to a good host for potassium cations. The activities of the two binding sites in 7 are almost entirely independent of each other; consequently, the binding of a naphthalene guest in the 1:1:1 (host‐naphthalene‐K+) complex is nearly as strong as in the 1:1 complex. The methyl ester of (S)‐naproxen forms stable complexes with the racemic hosts 6 and 7. The 1H NMR spectra of these complexes show considerably different shifts for the aromatic resonances of the host enantiomers. These differences indicate the formation of diastereomeric complexes of different geometries or stabilities.
Design and Synthesis of Functionalized Oxaziridines as Topological Analogs of Penicillins
Authors: Marchand‐Brynaert Jacqueline , Bounkhala‐Khrouz Zoubida , van Keulen Berend Jan , Vanlierde Huguette , Ghosez LÉOn
Published: November 2013

Oxaziridines were examined as potential alkylating inhibitors of bacterial serine D,D‐peptidases. Structures were designed to mimic known β‐lactam antibiotics taking into account stereoelectronic requirements for a nucleophilic attack on a three‐membered ring. Novel oxaziridines equipped with functionalized side chains were prepared. They showed poor antibacterial activities in vitro, perhaps as a result of their low chemical stability.
Quantum Effects in Three‐Dimensional H + D2 Reaction at High Energies
Authors: Last Isidore , Ron Shlomo , Baer Michael
Published: November 2013

The H + D2 reactive system was studied in the energy range 0.84 < Etr < 2.70 eV employing the ordinary Quasi‐Classical Trajectory Approach (QCTA), the Classical and the Quantum Reactive Infinite Order Sudden Approximations (CRIOSA and QRIOSA), and the Angle‐Oriented Transition State Theory (AOTST). From a detailed analysis of the various integral cross‐sections and opacity functions, it was concluded that pronounced quantum effects ( > 10%) are expected in the high‐energy region (≤ 2 eV).
Negligible Nuclear Spin Relaxation Concurrent with Extensive Rotational Cooling in Jet Expansions of Methyl Halides, from Analysis of Hexapole Focusing
Authors: Choi Seung E. , Bernstein Richard B.
Published: November 2013

Experimental data of the UCLA group (S.R. Gandhi et al. Chem. Phys. Lett., 1986, 132: 6; J. Phys. Chem., 1987, 91: 5437) on focusing and rotational state selection of supersonic, seeded beams of methyl halide molecules via the electrostatic hexapole technique are analyzed to ascertain the extent of spin conversion (i.e., proton spin orientational relaxation) accompanying rotational cooling. Just as for the ortho‐para hydrogen system, the rate of spin conversion (here, between the two different symmetry species, A and E, of the methyl halide molecules) is found to be negligible compared to the extensive collision‐induced rotational cooling (down to rotational temperatures as low as 2 K).
Dynamics and Potential Surfaces of Small Clusters
Authors: Berry R. Stephen , Braier Paul , Hinde R.J. , Cheng Hai‐Ping
Published: November 2013

Some of the tantalizing puzzles regarding clusters are described. Some specific results concerning the dynamical behavior of clusters of particular sizes and having various potentials are described, to give some insight into the relationship between the form of a potential and the kind of behavior it yields. Solid‐liquid, solid‐soft solid, surface melting, and a remarkable single “subsurface layer mobility” all can be found in particular instances.
Correlation of Zeno (Z = 1) Line for Supercritical Fluids with Vapor‐Liquid Rectilinear Diameters
Authors: Ben‐Amotz Dor , Herschbach Dudley R.
Published: November 2013

For a wide range of substances, extending well beyond the regime of corresponding states behavior, the contour in the temperature‐density plane along which the compressibility factor Z = P/ρkT is the same as for an ideal gas is nearly linear. This Z = 1 contour, termed the Zeno line, begins deep in the liquid region and ascends as the density decreases to the Boyle point of the supercritical fluid, specified by the temperature TB for which (dZ/dρ)T = 0 as ρ → 0; equivalently, at TB the second virial coefficient vanishes. The slope of the Z = 1 line is — B3/(dB2/dT), in terms of the third virial coefficient and the derivative of the second, evaluated at TB. Previous work has examined the Zeno line as a means to extend corresponding states and to enhance other practical approximations. Here we call attention to another striking aspect, a strong correlation with the line of rectilinear diameters defined by the average of the subcritical vapor and liquid densities. This correlation is obeyed well by empirical data for many substances and computer simulations for a Lennard‐Jones potential; the ratios of the intercepts and slopes for the Zeno and rectilinear diameter lines are remarkably close to those predicted by the van der Waals equation, 8/9 and 16/9, respectively. Properties of the slightly imperfect fluid far above the critical point thus implicitly determine the diameter of the vapor‐liquid coexistence curve below the critical point.
Foam Fractionation and Ion Flotation of Simple and Complex Anions with Cationic Surfactants
Authors: Grieves Robert B.
Published: November 2013

Experimental data are presented and interpreted on the foam fractionation of an extensive series of simple and complex anions from dilute (of the order 10−6−10−4 molar) aqueous solutions, utilizing a quaternary ammonium surfactant with which the anions form soluble ion pairs in competition with the surfactant's counterion. Selectivity coefficients, based on a bubble‐interface, ion exchange model, are established in a single‐equilibrium‐stage, continuous‐flow, foam fractionation unit for each of a series of 13 anions and oxyanions versus the surfactant's bromide counterion. Three additional series of batch experiments with multi‐metal solutions establish the separation and concentration in the foam of the oxyanions of Re(VII), Mo(VI), Cr(VI), W(VI), and V(V); of the cyanide complex anions of Zn(II), Cd(II), Hg(II), and Au(III); and of the chloride complex anions of Zn(II), Cd(II), Hg(II), and Au(III). The metal oxyanions and metal cyanide and chloride complex anions can be separated from each other and from competing chloride, cyanide, or nitrate.

Ion flotation of anionic species with a cationic surfactant involves an entirely different mechanism, in which a precipitation reaction occurs and particle flotation follows. Hexavalent chromium (primarily HCrO−4) is ion‐floated with each of a series of variable chain length quaternary ammonium surfactants, elucidating the effects of temperature, the surfactant/Cr(VI) feed ratio, mixing time, and surfactant chain length in terms of the roles of the surfactant as precipitant, dispersant, collector, and frother.
Metal Induced Decarboxylation of Aliphatic Free Radicals. I. Kinetics of the Reactions of Copper(I) and Copper(II) Ions with the 2‐Methyl‐2‐Carboxylicacid‐Propyl Free Radical in Aqueous Solutions. A Pulse Radiolysis Study
Authors: Masarwa Mohamed , Cohen Haim , Saar Jacob , Meyerstein Dan
Published: November 2013

CH2C(CH3)2CO2H free radicals react with Cu2+aq to form CuIII‐CH2C(CH3)2CO2H2+aq, which probably is rapidly transformed into intermediate I. This intermediate decomposes into Cu+aq + CO2 + CH2=C(CH3)2. This is the first system in which a metal ion induced β‐carboxyl elimination reaction was observed. ·CH2C(CH3)2CO2H free radicals react with CuIII‐CH2C(CH3)2CO2H2+aq to form Cu2+aq and a mixture of (‐CH2C(CH3)2CO2H)2, (CH3)3CCO2H, and HOCH2C(CH3)2CO2H. CH2C(CH3)2CO2H reacts with Cu+aq to form CuII‐CH2C(CH3)2CO2H+aq. The latter intermediate decomposes both via homolysis and by a reaction with Cu2+aq to form 2Cu2+aq + (CH3)3CCO2H. The spectra of the intermediates and the kinetics of reaction are reported.
Dinuclear Rhodium Complexes Containing the Diyne 1,4‐C6H4(C≡CH)2 and the Isomeric Bisvinylidene 1,4‐C6H4(CH=C:)2 as Bridging Units
Authors: Werner Helmut , Rappert Thomas , Wolf Justin
Published: November 2013

The reaction of [RhCl(PiPr3)2] (1) with 1,4‐C6H4(C≡CH)2 at 0°C leads almost quantitatively to the formation of the bis(alkyne) complex [(PiPr3)2ClRh‐(HC≡C6H4‐C≡CH)RhCl(PiPr3)2] (2). At elevated temperatures (THF, 60°C) it rearranges to give the isomeric bis(vinylidene) complex [(PiPr3)2ClRh‐(=C=CH‐C6H4‐CH=C=)RhCl(PiPr3)2] (3). A one‐pot synthesis of 3 is also described. Treatment of either 2 or 3 with pyridine affords the bis(alkynyl)dihydrido compound [(PiPr3)2(py)Cl(H)Rh(‐C≡C‐C6H4‐C≡C‐)Rh(H)Cl(py)(PiPr3)2] (4) in which both metal centers are octahedrally coordinated. Whereas the reaction of 2 with NaC5H5 produces the complex CsH5(PiPr3)Rh(HC≡C‐C6H4‐C≡CH)Rh(PiPr3)C5H5 (7), the bis(vinyl‐idene) isomer C5H5(PiPr3)Rh(=C=CH‐C6H4‐CH=C=)Rh(PiPr3)C5Hs (8) is obtained from 4 and NaC5H5. Electrophiles preferably attack the Rh=C bonds of 8 and thus on protonation with CF3CO2H the bis(vinyl) complex C5H5(PiPr3)(CF3CO2)‐Rh(Z,Z‐CH=CH‐C6H4‐CH=CH)Rh(O2CCF3)(PiPr3)C5Hs (Z‐9) is formed. In acetone solution, it rearranges to give the E isomer. Reaction of 8 with sulfur affords the bis(thioketene) complex C5H5(PiPr3)Rh(≡2‐C,S; η2‐C,S‐S=C=CH‐C6H4‐CH=C=S)‐Rh(PiPr3)C5H5 (12), for which only one diastereomer is observed. All attempts to prepare mononuclear rhodium compounds containing the diyne HC≡C‐C6H4‐G≡CH or the isomeric vinylidene: C=CH‐C6H4‐G≡CH as ligand failed.
Hydrogen‐Atom Transfer Reactions of Transition‐Metal Hydrides
Authors: Eisenberg David C. , Norton Jack R.
Published: November 2013

Many reactions of transition‐metal hydrides involve H* transfer. With olefins such transfer gives a radical cage, from which escape and collapse lead to product formation. Inverse isotope effects, second‐order kinetics independent of ligand concentration, and CIDNP are diagnostic for this mechanism. Many other reactions of transition‐metal hydrides occur by radical chain mechanisms, in which H* is abstracted by carbon‐centered radicals or by metal radicals.

Reasonably accurate values are now available for the M‐H bond strengths of most of the common hydrides, and these values help rationalize the known H* transfer reactions of these hydrides. While the rates of certain H* transfer reactions have been measured by radical clock methods, the measurement of H* transfer rates to a substituted trityl radical has provided the first general comparison of the H* donor abilities of the various hydrides. These relative H* transfer rates are significantly affected by steric factors.
Interface Structure in a Polymer Blend in the Presence of Selective Solvents
Authors: Winnik Mitchell A. , Pekcan ÖNder , Disanayaka Bimsara , Croucher Melvin D.
Published: November 2013

Energy transfer experiments were carried out on a material composed of poly(methyl methacrylate) (PMMA, 98.5 monomer mol%) and polyisobutylene (PIB, 1.5 monomer mol%), with PMMA labeled with phenanthrene, the donor, and PIB labeled with pyrene, the acceptor. In this material, prepared by nonaqueous dispersion polymerization, all of the PIB is present as a graft copolymer with PMMA. Here we demonstrate that the extent of energy transfer increases enormously when the particles are dispersed in aliphatic hydrocarbon liquids. These swell the PIB phase leading to formation of a diffuse interface where substantial mixing of the PMMA and PIB components occurs. The amount of energy transfer, and hence the amount of swelling, varies significantly among the various solvents.
A New Method for the Stereoselective Synthesis of Nucleosides by Means of Sodium Borohydride Mediated Reductive C‐C or C‐N Bond Cleavage Reaction
Authors: Kaneko Chikara , Katagiri Nobuya , Nomura Masahiro , Sato Hiroshi
Published: November 2013

New dienophiles, dimethyl acetoxymethylenemalonate and its analogues, have been synthesized. These dienophiles reacted with cyclopentadiene to give Diels‐Alder adducts, which served as versatile synthetic precursors for carbocyclic C‐nucleosides. The method has the following three characteristics: (1) the C‐C bond in the adduct originated from the dienophile can be cleaved stereoselectively by the reductive retrograde aldol (RRA) reaction, (2) the CC bond in the adduct serves as a latent functionality of 2′‐ and 3′‐ positions of the nucleoside, and (3) the malonate unit can be transformed to a variety of heterocycles. The high pressure mediated Diels‐Alder reaction of furan with the same dienophiles also gave the cycloadducts, which served as synthons for C‐nucleosides. Use of chiral dienophiles, such as di‐1‐menthyl acetoxymethylenemalonate, for the above Diels‐Alder reactions has offered a new route to the enantioselective synthesis of C‐nucleosides and their carbocyclic analogues. Essentially the same methodology has also been applied to the synthesis of carbocyclic nucleosides from 2‐azabicyclo[2.2.1]hept‐5‐en‐3‐one, in which a reductive amido bond cleavage reaction similar to (1) is involved. Synthesis of carbocyclic oxetanocin and its analogues is also described.
Chiral Molecular Clefts For Dicarboxylic Acid Complexation
Authors: Alcázar Victoria , Morán joaquín R. , Diederich François
Published: November 2013

Three efficient cleft‐type receptors, 3,5, and 6 have been prepared by attachment of two amidopyridine units as hydrogen bonding centers to either the 2,2′‐positions of 9, 9′‐spirobifluorene or the 6,6′‐positions of 1,1′‐binaphthyl spacers. The easy availability of these compounds in short synthetic routes make them attractive complexing agents for aliphatic and aromatic dicarboxylic acids which undergo bidentate binding in chloroform solution. 1H NMR binding studies show that substrates of different size can be accommodated into the clefts and form 1:1 complexes that are predominantly stabilized by multiple host‐guesthydrogen bonds. The flexible aliphatic substrates diethylmalonic, 2,2‐diphenylsuccinic, glutaric, and pimelic acid form complexes with association constants Ka ranging from 103 to 104 L mol−1. Significantly more stable complexes (Ka > 105 L mol−1) are obtained with the more rigid, preorganized substrate 5‐dodecyloxyisophthalic acid.
Calixarenes 30. Calixquinones
Authors: Reddy P. Amruta , Kashyap Ram P. , Watson William H. , Gutsche C. David
Published: November 2013

Chlorine dioxide and thallium trifluoroacetate are shown to be useful reagents for the preparation of calixquinones. p‐H‐Calix[4]arene(1a),p‐H‐calix[5]arene (1b), and p‐H‐calix[6]arene (1c) are oxidized in modest yields by ClO2 to the fully quinonoid compounds calix[4]tetraquinone (2a), calix[5]pentaquinone (2b), and calix[6]hexaquinone (2c), respectively. Although Tl(OCOF3)3 is less effective for the oxidation of 1a‐c, it proves to be the reagent of choice for converting partially etherified or esterified calixarenes carrying p‐tert‐butyl groups directly to partially quinonoid calixarenes. Thus, monosubstituted calix[4]arenes yield triquinones; disubstituted calix[4]arenes yield diquinones; trisubstituted calix[4]arenes yield monquinones; and tetrasubstituted calix[6]arenes yield diquinones. The structures of the calixquinones have been established by elemental analysis, 1H NMR spectroscopy, mass spectroscopy, and in the case of 2c by X‐ray crystallography. Since the starting materials are readily accessible, the calixquinones become easily available compounds for further study.
Correlation of Isotropic and Anisotropic Chemical Shifts in Solids by Two‐Dimensional Variable‐Angle‐Spinning NMR
Authors: Frydman Lucio , Chingas Gerard C. , Lee Young K. , Grandinetti Philip J. , Eastman Margaret A. , Barrall Geoffrey A. , Pines Alexander
Published: November 2013

We describe a new solid‐state nuclear magnetic resonance (NMR) technique for correlating anisotropic and isotropic chemical shifts in powdered samples. Two‐dimensional (2D) NMR spectra are obtained by processing signals acquired in independent experiments for different angles between the sample spinning axis and the Zeeman magnetic field. This 2D NMR approach can therefore resolve individual static anisotropic lineshapes according to their isotropic chemical shift frequencies, without use of sudden mechanical motions or multiple‐pulse irradiation schemes. Applications of the technique are illustrated with an analysis of the chemical shift anisotropy for the eight distinct 13C sites in tyrosine.
A Simple Theory of 13C Nuclear Spin Diffusion in Organic Solids
Authors: Tycko R. , Dabbagh G.
Published: November 2013

We derive an expression for the rate of spin diffusion between 13C nuclei based on a simple model appropriate for organic solids. In this model, couplings between carbons and protons and among protons are included only as a source of fluctuating local magnetic fields at the carbon nuclei. Our results are essentially equivalent to the results of previous calculations in which the surrounding protons are treated quantum mechanically using memory function formalisms.
63Cu NQR Nutation Spectroscopy and Spin Counting in Copper Oxides
Authors: Vega Alexander J.
Published: November 2013

Nutation spectroscopy of 63Cu nuclei at zero static magnetic field is explored as a method for the determination of the quadrupole asymmetry parameters η of Cu sites in copper oxides, with special emphasis on high‐Tc superconductors. Since the nutation spectra reveal the actual radio frequency (rf) intensities experienced by the 63Cu nuclei, the method also provides a verification of a quantitative Cu spin counting procedure based on integration of nuclear quadrupole resonance (NQR) echo‐signal intensities over wide frequency ranges. The paper presents numerical simulations of spectra and signal intensities that were obtained with a density‐matrix formalism whereby the spin dynamics is described as precessions in a 3D operator space. The quantitative effects of the echo refocusing pulse, rf amplitude fluctuations, and variations in η are examined. Examples of 63Cu NQR nutation spectra are shown for randomly oriented powders of Cu2O and YBa2Cu3O7.
The Transition Amplitudes of Centerband and Sidebands in NMR Spectra of Rotating Solids
Authors: Schmidt Asher , Vega Shimon
Published: November 2013

Expressions for the transition amplitudes of the centerband and sidebands of magic angle spinning (MAS) NMR spectra are derived by applying the Floquet theory. Signal amplitudes are defined in terms of transition amplitude operators, which are linear combinations of the Floquet operators. Two examples of the utilization of the Floquet approach for the evaluation of MAS signals are presented. In the first, the REDOR experiment on heteronuclear spin systems is discussed. The effects of finite pulse lengths on the REDOR signals are also derived. The second example deals with the MAS spectrum of a spin coupled to a heteronuclear spin that is irradiated by an rf field. Recoupled spectra are examined with the help of the Floquet theory and decoupled spectra are evaluated. In all cases the methodology of the Floquet approach is emphasized.