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Electronic Effects (electronic + effects)
Selected AbstractsFirst Propargyl Azides Bearing Strong Acceptor Substituents and Their Effective Conversion into Allenyl Azides: Influence of the Electronic Effects of Substituents on the Reactivity of Propargyl Azides,,EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 17 2005Joseph Rodolph Fotsing Abstract We have succeeded in the synthesis of propargyl azides containing 1- or 3-phenylthio functionalities. The selective oxidation of their sulfur atoms to sulfoxides and sulfones allows access to the first propargyl azides bearing acceptor substituents. Interestingly, the prototropic rearrangement of the latter propargyl azides leads to the formation of allenyl azides with relatively high stabilities and with moderate to good yields. Propargyl azides containing phenylthio functionalities react in the presence of nucleophiles to afford the expected N -unsubstituted 1,2,3-triazoles via short-lived allenyl azides. These results are entirely different from those of the corresponding sulfoxides and sulfones, which react under the analogous conditions either to produce the corresponding bis(triazolo)pyrazine derivatives or to yield newly substituted vinyl azides. The latter compounds can successfully be used as starting material providing access to azirines. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Steric and Electronic Effects on an Antibody-Catalyzed Diels,Alder ReactionHELVETICA CHIMICA ACTA, Issue 12 2002Yael Gozin A series of substituted thiophene dioxides was tested as diene substrates for the antibody 1E9, which was elicited with a hexachloronorbornene derivative and normally catalyzes the inverse electron-demand Diels,Alder reaction between 2,3,4,5-tetrachlorothiophene dioxide (TCTD) and N -ethylmaleimide (NEM). Previous structural and computational studies had suggested that the catalytic efficiency of this system derives in part from a very snug fit between the apolar active site and the transition state of this reaction. Nevertheless, replacing all the Cl-atoms in the hapten with Br-atoms leads to no loss in affinity (Kd=0.1,nM), indicating substantial conformational flexibility in the residues that line the binding pocket. Consistent with this observation, the 2,3,4,5-tetrabromothiophene dioxide is a good substrate for the antibody (kcat=1.8,min,1, KNEM=14,,M), despite being considerably larger than TCTD. In contrast, normal electron-demand Diels,Alder reactions between NEM and unsubstituted thiophene dioxide or 2,3,4,5-tetramethylthiophene dioxide, which are much smaller or nearly isosteric with TCTD, respectively, are not detectably accelerated. These results show that the electronic properties of the 1E9 active site are optimized to a remarkable degree for the inverse electron-demand Diels,Alder reaction for which it was designed. Indeed, they appear to play a more important role in catalysis than simple proximity effects. [source] Convenient General Asymmetric Synthesis of Roche Ester Derivatives through Catalytic Asymmetric Hydrogenation: Steric and Electronic Effects of LigandsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 16 2008Cyrielle Pautigny Abstract An efficient and concise asymmetric hydrogenation of acrylate esters promoted by the cationic ruthenium monohydride complex [Ru(H)(,6 -cot)SYNPHOS]+BF4, is reported. A full investigation of the effects of catalyst precursors, solvents, temperature, hydrogen pressure, substrates as well as steric and electronic properties of ligands was carried out. The corresponding valuable Roche ester derivatives were obtained in good to excellent isolated yields and high enantioselectivities under mild conditions. The robustness and practicability of this highly enantioselective hydrogenation was demonstrated by the synthesis of the 3-hydroxy-2-methylpropanoic acid tert -butyl ester on a multigram scale, resulting in excellent yield and ee up to 94%. [source] ChemInform Abstract: A Highly Regioselective Ring-Opening Metathesis , Cross Metathesis Process Modulated by the Electronic Effects of the Cross Metathesis Partner: An Entry to Quaternary Prolines.CHEMINFORM, Issue 27 2009Javier Carreras Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] ChemInform Abstract: Rhodium(II)-Catalyzed Transylidation of Aryliodonium Ylides: Electronic Effects of Aryl Groups Determine Their Thermodynamic Stabilities.CHEMINFORM, Issue 34 2008Masahito Ochiai Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] New Chiral 1,3-Diphosphine Ligands for Rh-Catalyzed Enantioselective Hydrogenation: A Search for Electronic Effects.CHEMINFORM, Issue 17 2006Natalia V. Dubrovina Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] One-Dimensional Collective Electronic Effects in the Helically Stacked Cs2[Ni(CN)4]×H2O and Cs2[Pt(CN)4]×H2O: X-Ray Structure, Polarized Specular Reflectance, and ZINDO Calculations.CHEMINFORM, Issue 31 2003Jeffrey B. Cornelius Abstract For Abstract see ChemInform Abstract in Full Text. [source] Ligand Electronic Effects in Enantioselective Diethylzinc Additions.CHEMINFORM, Issue 15 2003Mike Casey Abstract For Abstract see ChemInform Abstract in Full Text. [source] ChemInform Abstract: Electronic Effects in the Acid-Promoted Deprotection of N-2,4-Dimethoxybenzyl Maleimides.CHEMINFORM, Issue 22 2001Daniel J. Watson Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] How Can Rotaxanes Be Modified by Varying Functional Groups at the Axle?,A Combined Theoretical and Experimental Analysis of Thermochemistry and Electronic EffectsCHEMISTRY - A EUROPEAN JOURNAL, Issue 7 2010Christian Spickermann Dipl.-Chem No abstract is available for this article. [source] Effective Manipulation of the Electronic Effects and Its Influence on the Emission of 5-Substituted Tris(8-quinolinolate) Aluminum(III) ComplexesCHEMISTRY - A EUROPEAN JOURNAL, Issue 17 2006Victor A. Montes Abstract The unique electron-transport and emissive properties of tris(8-quinolinolate) aluminum(III) (Alq3) have resulted in extensive use of this material for small molecular organic light-emitting diode (OLED) fabrication. So far, efforts to prepare stable and easy-to-process red/green/blue (RGB)-emitting Alq3 derivatives have met with only a limited success. In this paper, we describe how the electronic nature of various substituents, projected via an arylethynyl or aryl spacer to the position of the highest HOMO density (C5), may be used for effective emission tuning to obtain blue-, green-, and red-emitting materials. The synthetic strategy consists of four different pathways for the attachment of electron-donating and electron-withdrawing aryl or arylethynyl substituents to the 5-position of the quinolinolate ring. Successful tuning of the emission color covering the whole visible spectrum (,=450,800 nm) was achieved. In addition, the photophysical properties of the luminophores were found to correlate with the Hammett constant of the respective substituents, providing a powerful strategy with which to predict the optical properties of new materials. We also demonstrate that the electronic nature of the substituent affects the emission properties of the resulting complex through effective modification of the HOMO levels of the quinolinolate ligand. [source] Current trends in QSAR on NO donors and inhibitors of nitric oxide synthase (NOS),,MEDICINAL RESEARCH REVIEWS, Issue 4 2002Christos A. Kontogiorgis Abstract This article evaluates the quantitative structure-activity relationships (QSAR) of nitric oxide (NO) radical donors and nitric oxide synthases (NOS) inhibitors, using the C-QSAR program of Biobyte. Furoxans, triazines, amidoximes, tetrazoles, imidazoles and N,,2-nitroarylamino acid analogues were included in this survey. In nine out of seventeen cases, the clog P plays a significant part in the QSAR of the NO radical donors and of the NOS inhibition. Many of the compounds must be interacting with a hydrophobic space in a non-specific way. In some cases molecular refractivity CMR/MR as well as sterimol parameters (B1 and L) are important. Electronic effects, with the exception of the Hammett's constant , and the Swain,Lupton parameter F, are not found to govern the biological activity. Stereochemical and electronic features are also found to be important. Indicator variables were used after the best model was found to account for the usual structural features. © 2002 Wiley Periodicals, Inc. Med Res Rev, 22, No. 4, 385,418, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/med.10012 [source] Application of entropy calculations to the determination of transition temperature in zirconiumPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 8 2005Pornjuk Srepusharawoot Abstract The Helmholtz free energy is determined in order to calculate the bcc,hcp phase transition temperature in zirconium. The molecular dynamics method is employed to calculate the vibrational entropy which comprises both the harmonic and anharmonic vibrational entropy. Electronic effects are estimated and taken into account. The anharmonic vibrational entropy is determined from the anharmonic free energy and the electronic entropy is calculated via the electronic density of states. By considering the Helmholtz free energy, the bcc,hcp transition temperature from our simulations is 1047 ± 210 K compared to 1135 K from experimental results. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Synthetic Scope of Alcohol Transfer Dehydrogenation Catalyzed by Cu/Al2O3: A New Metallic Catalyst with Unusual SelectivityCHEMISTRY - A EUROPEAN JOURNAL, Issue 24 2006Federica Zaccheria Dr. Abstract A method for the anaerobic oxidation of a wide series of alcohols including cyclohexanols and steroidal alcohols, has been set up. It relies on a transfer dehydrogenation reaction from the substrate alcohol to styrene catalyzed by a heterogeneous, reusable copper catalyst under very mild liquid-phase experimental conditions (90,°C, N2) and shows unusual selectivity. Thus, the method is selective for the oxidation of secondary and allylic alcohols even in the presence of unprotected primary and benzylic alcohols. Electronic effects and the choice of the hydrogen acceptor account for the selectivity observed. [source] Magnetic, 57Fe Mössbauer, and IR Monitoring of the Thermal Spin Transition in a New Family of Iron(II) Spin-Transition ComplexesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 12 2003Arno F. Stassen Abstract A new family of iron(II) tetrafluoroborate and perchlorate spin-crossover compounds has been synthesised and is discussed. The iron(II) ion is surrounded by six 1-ethyltetrazole ligands, which are halogen-substituted on the C2 atom of the ethyl group. The spin-crossover temperatures T1/2 are high compared to the unsubstituted (1-alkyltetrazole)iron(II) complexes. The shape of the spin-transition curve (i.e. ,HS vs. T) varies largely over the different complexes and appears to be influenced neither by the crystal packing, nor by the electronic effects. The temperature-dependent spin-transition behaviour has been studied by magnetic susceptibility and by 57Fe-Mössbauer spectroscopy. These data have been supported by temperature-dependent mid-range infrared spectroscopy: the thermally induced spin transition has been observed by direct monitoring of the aromatic C,H stretching frequency. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Enol Forms of 1,3-Indanedione, Their Stabilization by Strong Hydrogen Bonding, and Zwitterion-Assisted InterconversionEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 14 2010Mark Sigalov Abstract By analyzing NMR spectroscopic data, and supported by IR, UV/Vis, Raman, dielectrometry, and DFT techniques, a comprehensive study of the 1:2 adducts of picolinaldehyde and 1,3-indanediones is presented. The parent indanedione derivative 5 exists in an equilibrium between all-keto and enol forms, the latter being stabilized by an intramolecularO,H···N hydrogen bond. Only the all-keto form was observed in the 5,6-dimethoxy compound 6, whereas solely the enol tautomer was observed with its 5,6-dichloro analogue 7. Polar solvents and low temperatures shift the equilibrium towards the enol tautomer in 5. The structure of adduct 8, formed with isonicotinaldehyde, prevents the formation of intramolecular O,H···N hydrogen bonds and thus it exists in the all-keto form in low polar solvents. However, in DMSO solutions it adopts a zwitterionic form with a strong anionic O,···H···O hydrogen bond. Thus, the enol form in indanedione adducts was unequivocally characterized in solution and the factors that determine the keto,enol tautomerism, namely electronic effects, solvent, temperature, and intramolecular hydrogen bonds, have been methodically studied by spectroscopic and quantum mechanical methods. [source] Efficient Diphosphane-Based Catalyst for the Palladium-Catalyzed Suzuki Cross-Coupling Reaction of 3-Pyridylboronic AcidsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 13 2009Xing-Li Fu Abstract A highly active catalyst system derived from PdCl2 and2,2,,6,6,-tetramethoxy-4,4,-bis(diphenylphosphanyl)-3,3,-bipyridine (P-Phos) has been developed for the Suzuki cross-coupling reaction of pyridylboronic acid with a variety of aryl halides in good to excellent yields, even in the presence of hindered and functional groups. In addition, P-Phos also exhibited high activity in the palladium-catalyzed Suzuki reaction of 2,6-dimethoxypyridylboronic acid in excellent yields with a fast rate. The steric and electronic effects of the P-Phos,palladium complex to this cross-coupling reaction were also discussed.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] The Effects of Fluorine and Chlorine Substituents across the Fjords of Bifluorenylidenes: Overcrowding and StereochemistryEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 22 2006Sergey Pogodin Abstract The bistricyclic aromatic enes (BAEs) (E)- and (Z)-1,1,-difluorobifluorenylidene, 1,8,1,,8,-tetrafluorobifluorenylidene, (E)- and (Z)-3,3,-difluorobifluorenylidene, 3,6,3,,6,-tetrafluorobifluorenylidene, and their chlorinated analogues were subjected to a DFT study of overcrowding in their fjord regions. The B3LYP hybrid functional was employed to calculate energies and geometries of the twisted conformations of these BAEs. The diastereomers E11,F2 and Z11,F2 have identical twist angles (, = 37.1°) and similar degrees of overcrowding, but differ in the degree and mode of pyramidalization, ,. In E11,F2, ,(C9) = +,(C9,) = 7.0° (syn -pyramidalization), while in Z11,F2, ,(C9) = ,,(C9,) = 1.0° (anti -pyramidalization). By contrast, in E11,Cl2 and Z11,Cl2, , = 40.6° and 42.7°, respectively. Introducing four halogen substituents results in higher twist angles: , = 40.3° in 181,8,F4 and 52.6° in 181,8,Cl4. Surprisingly, Z11,F2 is more stable than E11,F2 (,H298 = ,1.9 kJ/mol), whereas Z11,Cl2 is less stable than E11,Cl2 (,H298 = 2.2 kJ/mol). Both results are consistent with the experimental relative stabilities of these diastereomers. The unexpected stability of Z11,F2 is explained by a combination of steric and electronic effects. Calculations of Coulomb energies for point charge systems of atoms C, F, and H in the fjord regions shows stabilization of the (Z) diastereomer by ,45.5 kJ/mol. The dipole,dipole interactions in the fjord region destabilize Z11,F2 by 6.4 kJ/mol relative to E11,F2. Careful examination of the NMR spectra of E11,F2 and Z11,F2 shows, in the latter, evidence of long-range fluorine,fluorine coupling over seven bonds (11.4 Hz) and carbon,fluorine coupling over six bonds (4.8 Hz).(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Diastereoselective Reactions of the Tiglic Acid Functionality Mediated by Oxazolidine Chiral Auxiliaries: A Mechanistic Comparison of DMD andm -CPBA Epoxidations versus Singlet Oxygen and PTAD Ene ReactionsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 14 2005Aurelia Pastor Abstract 2,2-Dimethyloxazolidines have been utilized as chiral auxiliaries for the diastereoselective functionalization of the optically active tiglic acid derivatives (S)- 1 by means of epoxidation with DMD or m -CPBA and ene reactions with 1O2 or PTAD. In the DMD and m -CPBA epoxidations, high diastereoselectivities but opposite senses of diastereomer selection were observed. In contrast, the stereochemistry of the 1O2 and PTAD ene reactions depended on the size of the attacking enophile: whereas essentially perfect diastereoselectivity was obtained with PTAD, much lower stereoselection was observed with 1O2. The stereochemical results for the DMD and m -CPBA epoxidations and the PTAD ene reaction are explained in terms of the energy differences for the corresponding diastereomeric transition states, dictated by steric and electronic effects. The PTAD ene reaction for these tiglic acids (S)- 1 provides, after removal of the chiral auxiliaries, an attractive synthetic route for optically active ,-amino acid derivatives.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Microbial Baeyer,Villiger Oxidation of Prochiral Polysubstituted Cyclohexanones by Recombinant Whole-Cells Expressing Two Bacterial MonooxygenasesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 5 2005Marko D. Mihovilovic Abstract The microbial Baeyer,Villiger oxidation of prochiral 3,5-dimethylcyclohexanones bearing various functionalities with recombinant E. coli cells overexpressing cyclohexanone monooxygenase from Acinetobacter sp. NCIMB 9871 and cyclopentanone monooxygenase from Comamonas sp. NCIMB 9872 has been investigated. A distinct difference in substrate specificity and stereoselectivity of the two enzymes was observed, and enantiocomplementary products were obtained in some cases. The biocatalytic systems enabled access to chiral lactones as valuable intermediates for the total synthesis of various natural compounds. Substituents with varying lipophilicity and hybridization have been prepared by a diastereoselective synthetic route and subsequently bio-transformed for the investigation of conformational and electronic effects on the biooxidation,. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Quenching of Singlet Oxygen by Tertiary Aliphatic Amines.HELVETICA CHIMICA ACTA, Issue 10 2006Products, Structural Effects on Rates Abstract A kinetic and product study of the reaction of a series of , -methyl-substituted N -methylpiperidines with thermally generated 1O2 in MeCN was carried out. It was found that as the number of , -methyl groups (Me in , -position relative to the N-atom) increases, the rate of 1O2 quenching (physical plus chemical) slightly decreases. This finding shows that, with respect to the reaction rate, steric effects are much more important than electronic effects as the latter should have produced the opposite result. The opposite outcome was instead found for the chemical quenching that leads to the N -demethylation products and N -formyl derivatives. The same trend was observed for the ratio between N -demethylation and formation of the N -formyl derivatives (NH/NCHO ratio). All these results are consistent with the mechanism reported in Scheme,1 where an exciplex is first formed that by a H-atom transfer process produces an , -amino-substituted C-radical. The latter forms the product of N -demethylation by one electron oxidation, or affords the N -formyl derivative by radical coupling (Scheme,1). Similar results were obtained with N,N -dimethylcyclohexanamine. However, this ,acyclic' amine exhibited behaviors quite distinct from those of the N -methylpiperidines series, with respect to reaction rate, extent of chemical quenching, and NH/NCHO ratio. [source] Kinetic investigation on the reactions of p -toluenesulfonyl chloride with p -substituted benzoic acid(s) in the presence of triethylamine in aprotic solventsINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 5 2009Subbiah Ananthalakshmi Second-order rate constants of the reactions of p -toluenesulfonyl chloride with p -substituted benzoic acids in the presence of triethylamine in acetonitrile/acetone under equimolar and pseudo-first-order conditions have been determined by the conductometric method using the Guggenheim principle at 25, 30, 35, and 40°C. The reactions follow second order with respect to the whole and first order with respect to each of the reactants. The order of reactivity of the substituents in benzoic acid is rationalized. Activation parameters are obtained by applying the usual methods. The Hammett plot has been found nonlinear, whereas the Bronsted plot shows good correlation. This may be explained on the basis of electronic effects of substituents on the reaction center. Kinetic data and the product analyses indicate that the reaction proceeds through direct nucleophilic attack on the sulfur center. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 303,308, 2009 [source] Comparative studies on the pyrolysis of N -arylideneaminoamides: Kinetic and mechanistic studiesINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 2 2007Nouria A. Al-Awadi Rates of thermal decomposition of title compounds have been measured using a static reaction system. They undergo a unimolecular first-order elimination to give arylnitrile and the corresponding substituted amides. The decomposition parallels that of N -arylidenamino cyclic amide. The relative elimination rates at 600 K were calculated. The kinetic data reveal that the electronic effects of substituents, such as methyl, phenyl, benzyl, and allyl groups, are associated with the opposing directions in which the lone pair of electrons on the nitrogen atom of the arylidene moiety is being delocalized. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 39: 59,66, 2007 [source] Palladium catalyzed cross-methylation of bromoheterocycles with intramolecularly stabilized dimethyl indium reagentsJOURNAL OF HETEROCYCLIC CHEMISTRY, Issue 4 2003Nimer Jaber Although the intramolecularly stabilized [(3-dimethylamino)propyl]dimethylaluminum (1a) fails to undergo palladium-catalyzed cross-coupling with bromopyridines and with bromofuran derivatives, the analogous gallium and indium reagent 1b and 1c smoothly cross-methylate these and other bromoheterocycles. The cross-coupling can be performed also with the dimeric indium and gallium compounds bis(,-[2-(dimethylamino)ethanolato- N,O:O]tetramethyldigallium and -diindium (2b and 2c, respectively). The indium reagent is fairly air-stable and the reactions can be carried out under ambient conditions. The yields are generally very high but are influenced by steric and electronic effects. [source] Synthesis and nuclear magnetic resonance spectroscopic studies of 1-arylpyrrolesJOURNAL OF HETEROCYCLIC CHEMISTRY, Issue 1 2000Chang Kiu Lee A series of m - and p -substituted 1-phenyl, 1-benzyl, 1-benzoyl, and 1-(2-phenylethyl)pyrroles was prepared and their 1H and 13C nmr spectroscopic characteristics were examined. In general, good correlations were observed between the chemical shift values of the ,H and the ,C of pyrroles [except 1-(2-phenylethyl)pyrroles] and the Hammettt ,. The observation may be explained in terms of the electronic effects of the substituents which are transmitted through bonds and through space by interaction of the p orbitals between ,Cs of the pyrrole ring and m - and pCs of the phenyl ring. Substituent constants of 1-pyrrolyl, 1-pyrrolylmethyl, and 1-pyrroloyl groups for the 1H and 13C chemical shifts of phenyl ring are also presented. [source] linear free energy relationships;JOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 11 2007UB3LYP/6-31G(d The substituent effect on the reactivity of the CN bond of molecular ions of 4-substituted N -(2-furylmethyl)anilines toward two dissociation pathways was studied. With this aim, six of these compounds were analyzed by mass spectrometry using electron ionization with energies between 7.8 and 69.9 eV. Also, the UB3LYP/6-31G (d,p) and UHF/6-31G (d, p) levels of theory were used to calculate the critical energies (reaction enthalpies at 0 K) of the processes that lead to the complementary ions [C5H5O]+ and [M , C5H5O]+, assuming structures that result from the heterolytic and homolytic CN bond cleavages of the molecular ions, respectively. A kinetic approach proposed in the 1960s was applied to the mass spectral data to obtain the relative rate coefficients for both dissociation channels from ratios of the peak intensities of these ions. Linear relationships were obtained between the logarithms of the relative rate coefficients and the calculated critical energies and other thermochemical properties, whose slopes showed to be conditioned by the energy provided to the compounds within the ion source. Moreover, it was found that the dissociation that leads to [C5H5O]+ is a process strongly dependent upon the electron withdrawing or donating properties of the substituent, favored by those factors that destabilize the molecular ion. On the contrary, the dissociation that leads to [M , C5H5O]+ is indifferent to the polar electronic effects of the substituent. The abundance of both products was governed by the rule of Stevenson,Audier, according to which the major ion is the one of less negative electronic affinity. Copyright © 2007 John Wiley & Sons, Ltd. [source] Tunnelling corrections in hydrogen abstractions by excited-state ketonesJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 7 2010Monica Barroso Abstract Hydrogen abstraction from 1-phenylethanol by triplet acetophenone occurs from both CH and OH bonds. The reaction path of the Interacting-State Model (ISM) is used with the Transition-State Theory (TST) and the semiclassical correction for tunnelling (ISM/scTST) to help rationalizing the experimental kinetic results and elucidate the mechanisms of these reactions. The weak exothermicity of the abstraction from the strong OH bond is compensated by electronic effects, hydrogen bonding and tunnelling, and is competitive with the more exothermic abstraction from the ,-CH bond of 1-phenylethanol. The alkoxy radical formed upon abstraction from OH reacts within the solvent cage and the primary product of this reaction is 1-phenylethenol. The corresponding kinetic isotope effect is ca. 3 and is entirely consistent with a tunnelling correction ca. 9 for H abstraction. We therefore demonstrate that the tunnelling correction is the major contributor to the kinetic isotope effect. Copyright © 2010 John Wiley & Sons, Ltd. [source] Density functional study on the structural conformations and intramolecular charge transfer from the vibrational spectra of the anticancer drug combretastatin-A2JOURNAL OF RAMAN SPECTROSCOPY, Issue 4 2009L. Padmaja Abstract Combretastatin-A2 (CA2), a potential anticancer drug in advanced preclinical development, is extracted from the medicinal plant Combretum caffrum. The NIR-FT Raman and FT-IR spectral studies of the molecule were carried out and ab initio calculations performed at the B3LYP/6-31G(d) level to derive the equilibrium geometry as well as the vibrational wavenumbers and intensities of the spectral bands. The vibrational analysis showed that the molecule has a similar geometry as that of cis-stilbene, and has undergone steric repulsion resulting in twisting of the phenyl ring with respect to the ethylenic plane. Vibrational analysis was used to investigate the lowering of the stretching modes, and enhancement of infrared band intensities of the C,H stretching modes of Me2 may be attributed to the electronic effects caused by back-donation and induction from the oxygen atom. Analysis of phenyl ring modes shows that the CA2 stretching mode 8 and the aromatic C,H in-plane bending mode are equally active as strong bands in both IR and Raman spectra, which can be interpreted as the evidence of intramolecular charge transfer (ICT) between the OH and OCH3groups via conjugated ring path and is responsible for bioactivity of the molecule. Copyright © 2008 John Wiley & Sons, Ltd. [source] Vibrational spectral studies and the non-linear optical properties of a novel NLO material L -prolinium tartrateJOURNAL OF RAMAN SPECTROSCOPY, Issue 12 2006L. Padmaja Abstract Vibrational spectral analysis of the novel non-linear optical (NLO) material, L -prolinium tartrate (LPT) was carried out using NIR-FT-Raman and FT-IR spectroscopy. The density functional theoretical (DFT) computations have been performed at B3LYP/6,31G (d) level to derive equilibrium geometry, vibrational wavenumbers, intensities and first hyperpolarizability. The reasonable NLO efficiency, predicted for the first time in this novel compound, has been confirmed by Kurtz,Perry powder second-harmonic generation (SHG) experiments. The charge-transfer interaction between the pyrrolidine ring and the carbonyl group of the tartrate anion through the intramolecular ionic hydrogen bonds is confirmed by the simultaneous activation of ring modes in IR and Raman spectra. The splitting of the ring-breathing mode, pseudo-rotational ring puckering modes and the NH2 modes of the pyrrolidine ring lead to the conclusion that the pyrrolidine ring adopts a conformation intermediate between the envelope (bent) form and the half-chair (twisted) form, resulting in the lowering of symmetry from C2 to Cs. The lowering of the methylenic stretching wavenumbers and the enhancement of the stretching intensities suggest the existence of the electronic effects of back-donation in LPT. The positional disorder of the pyrrolidine ring, the presence of blue-shifting H-bonds as well as other non-bonded interactions in LPT, low frequency H-bond vibrations and the role of intramolecular charge transfer and the hydrogen bonds in making the molecule NLO active have been analysed on the basis of the vibrational spectral features. Copyright © 2006 John Wiley & Sons, Ltd. [source] Additive interactions in the stabilization of film grade high-density polyethylene.JOURNAL OF VINYL & ADDITIVE TECHNOLOGY, Issue 2 2002Part I: Stabilization, influence of zinc stearate during melt processing The melt stabilization activity of some of the most commercially significant phenolic antioxidants and phosphites (alone and in combination), without and with zinc stearate, was studied in high-density polyethylene (HDPE) produced by Phillips catalyst technology. Multiple pass extrusion experiments were used to degrade the polymer melt progressively. The effect of stabilizers was assessed via melt flow rate (MFR) and yellowness index (YI) measurements conducted as a function of the number of passes. The level of the phenolic antioxidant remaining after each extrusion was determined by high-performance liquid chromatography (HPLC). Phenolic antioxidants and phosphites both improved the melt stability of the polymer in terms of elt viscosity retention; the influence of zinc stearate was found to be almost insignificant. However, phosphites and zinc stearate decreased the discoloration caused by the phenolic antioxidants. A correlation was found between the melt stabilization performance of phosphites and their hydroperoxide decomposition efficiency determind via a model hydroperoxide compound. Steric and electronic effects associated with the phosphorus atom influenced the reactivity towards hydroperoxides. Furthermore, high hydrolytic stability did not automatically result in lower efficiency. Besides phosphite molecular structure, stabilization activity was also influenced by the structure of the primary phenolic antioxidant and the presence of zinc stearate. [source] | ||||||||||||||||||||||||||||||||||