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Substituent Effects (substituent + effects)
Selected AbstractsA Theoretical Investigation of Substituent Effects on the Absorption and Emission Properties of a Series of Terpyridylplatinum(II) Acetylide ComplexesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 10 2005Xiao-Juan Liu Abstract A comprehensive calculational investigation has been carried out on a series of complexes of the type [(terpyridyl-R1)Pt(C,C-R2)], where terpyridyl-R1 is a series of substituted 2,2':6',2"-terpyridyl ligands and C,C-R2 is a series of substituted acetylide ligands. In one series of complexes (I), the energy of the electronic excited state is varied by changing the substituents on the terpyridyl ligand (R1). In a second series of complexes (II), this electronic structure variation is obtained by changing the para substituents (R2) of the acetylide ligand. The effect of varying the substituents on the lowest-energy excited states of the complexes has been assessed by calculating their electronic structures and excitation energies. We anticipated that introduction of electron-withdrawing substituents on the terpyridyl ligand will benefit the LLCT (or MLCT) and prohibit the nonradiative pathways via d-d transitions in these complexes; introduction of electron-donating substituents on the acetylide ligand can also prohibit the nonradiative pathways by increasing the energy gaps between the HOMO,LUMO and d-d transitions. The results also reveal that the lowest-energy excitations of all complexes of series I and IIa,b complexes are dominated by a ,(C,C),,,,*(terp) (LLCT) transition mixed with some energetically d,(Pt),,,terpyridyl (MLCT) transition. However, for the complexes IIc,IId, in which phenyl rings are introduced on the acetylide ligand, the lowest-lying absorptions of IIc and IId are predominately LLCT in character, with less MLCT mixture, due to a lower contribution of the Pt(d) orbital to the HOMO, while for IIe, with a stronger donor on the acetylide, the lowest-lying absorption is completely LLCT in character. The absorption and emission calculations using the TDDFT method are based on the optimized geometries obtained at the B3LYP/LanL2DZ and CIS/LanL2DZ levels, respectively. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Theoretical Description of Substituent Effects in Electrophilic Aromatic Substitution ReactionsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 35 2008Tobias Schwabe Abstract The ability of the current Kohn,Sham density functional theory (DFT) to compute the change of the proton affinity (PA) of phenol derivatives due to substitution is investigated. These systems can be used as models to predict reactivities in electrophilic aromatic substitution reactions. The complexity of the problem is increased systematically by introducing successively up to four substituents in five typical cases (methyl, cyano, fluorine, chlorine, and bromine). Our investigation can be regarded as representative for an important class of problems consistently encountered in the DFT modeling of organic reactions. High-level theoretical reference data from CCSD(T) and SCS-MP2 wave-function calculations are presented, and the PAs are compared to those obtained by a series of density functionals (DFs). It is shown that not all DFs are capable of quantitatively reproducing the substituent effects. These can be simply linear in the number of substituents or show more complicated patterns. Especially for halogens, some DFs even fail completely. In these cases, linearly increasing errors with the number of groups are observed. Reliable results are obtained with hybrid DFs or the even more accurate double-hybrid DF approach. The errors are attributed to the common self-interaction (over-delocalization) error in part of the DFs. Comparison with Hartree,Fock results shows that a reliable account of electron correlation is necessary to compute the PA of unsaturated and highly substituted molecules with chemical accuracy.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Substituent Effects on Crosslike Packing of 2,,7,-Diaryl- spiro(cyclopropane-1,9,-fluorene) Derivatives: Synthesis and Crystallographic, Optical, and Thermal Properties,ADVANCED FUNCTIONAL MATERIALS, Issue 2 2007Z. Wang Abstract A series of 2,,7,-diarylspiro(cyclopropane-1,9,-fluorene) derivatives are efficiently synthesized and characterized to determine the reason for the "green-light" emission of these compounds. These compounds exhibit bright-violet to blue photoluminescence (PL) (,PLmax,=,353,419,nm) with excellent PL quantum efficiencies (,PL,=,83,100,%) in solution and show high thermal stabilities (Td,=,267,474,°C). The variation of the optical properties of these molecules in the solid state depends on the different stacking modes of these compounds containing different substituents, which are revealed by crystallographic analysis. CH,, hydrogen bonds instead of intermolecular ,,, interactions act as the driving force between adjacent fluorenes, even though a very small dialkyl group (cyclopropane) is introduced at the C-9 position of fluorene. The crosslike molecular stacking efficiently reduces the energy transfer between the herring-like aggregates and therefore results in the absence of a "green-light" emission tail. In order to determine the cause of the "green-light" emission tails, the fluorescence spectra of the films annealed in N2 or in air are recorded. Broad green-light emission tails were observed for the films annealed in air, which might be caused by fluorenone defects generated during processing or during the course of the photophysical analysis by reaction with residual oxygen. [source] Synthesis of Well-Defined Hyperbranched Polyamides by Condensation Polymerization of AB2 Monomer through Changed Substituent Effects,ANGEWANDTE CHEMIE, Issue 32 2009Yoshihiro Ohta Der Wechsel des Substituenteneffekts beim Übergang vom Monomer zum Polymer kann genutzt werden, um die selektive Kondensationspolymerisation eines AB2 -Monomers an einem Kernmolekül zu steuern (siehe Schema). Man erhält hochverzweigte Polyamide mit sehr niedriger Polydispersität und einem über das Monomer/Kern-Verhältnis kontrollierbaren Molekulargewicht. [source] ChemInform Abstract: Substituent Effects in the Preparation of Naphthacenes by the Coupling Reaction of Diyne-Derived Zirconacyclopentadienes with Tetraiodobenzene.CHEMINFORM, Issue 27 2008Tomomi Seri 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] Substituent Effects on Competitive Release of Phenols and 1,3-Rearrangement in ,-Keto Amide Photochemistry.CHEMINFORM, Issue 25 2004Chicheng Ma Abstract For Abstract see ChemInform Abstract in Full Text. [source] Coping with Substituent Effects in Divinylcyclopropyl Diazene Rearrangements.CHEMINFORM, Issue 20 2003Georgia Law Carroll No abstract is available for this article. [source] Pentiptycene-Derived Light-Driven Molecular Brakes: Substituent Effects of the Brake ComponentCHEMISTRY - A EUROPEAN JOURNAL, Issue 38 2010Wei-Ting Sun Abstract Five pentiptycene-derived stilbene systems (1,R; R=H, OM, NO, Pr, and Bu) have been prepared and investigated as light-driven molecular brakes that have different-sized brake components (1,H<1,OM<1,NO<1,Pr<1,Bu). At room temperature (298,K), rotation of the pentiptycene rotor is fast (krot=108,109,s,1) with little interaction with the brake component in the trans form ((E)- 1,R), which corresponds to the brake-off state. When the brake is turned on by photoisomerization to the cis form ((Z)- 1,R), the pentiptycene rotation can be arrested on the NMR spectroscopic timescale at temperatures that depend on the brake component. In the cases of (Z)- 1,NO, (Z)- 1,Pr, and (Z)- 1,Bu, the rotation is nearly blocked (krot=2,6,s,1) at 298,K. It is also demonstrated that the rotation is slower in [D6]DMSO than in CD2Cl2. A linear relationship between the free energies of the rotational barrier and the steric parameter A values is present only for (Z)- 1,H, (Z)- 1,OM, and (Z)- 1,NO, and it levels off on going from (Z)- 1,NO to (Z)- 1,Pr and (Z)- 1,Bu. DFT calculations provide insights into the substituent effects in the rotational ground and transition states. The molar reversibility of the E,Z photoswitching is up to 46,%, and both the E and Z isomers are stable under the irradiation conditions. [source] Theoretical Study of Remote Substituent Effects on X,H(X=CH2, NH, O) Bond Dissociation Energies of AzuleneCHINESE JOURNAL OF CHEMISTRY, Issue 7 2007Yi-Yun Yu Abstract In the study, the X,H (XCH2, NH, O) bond dissociation energies (BDE) of para -substituted azulene (Y-C10H8X-H) were predicted theoretically for the first time using Density Functronal Theory (DFT) methods at UB3LYP/6-311++g(2df,2p)// UB3LYP/6-31+g(d) level. It was found that the substituents exerted similar effects on the X,H BDE of azulene as those on benzene, except for 6-substituted 2-methylazulene. Owing to the substituent-dipole interaction, the reaction constants (,+) of 2- and 6-Y-C10H8X-H (XNH and O only) varied violently. The origin of the substituent effects on the X,H BDE of azulene was found, by both GE/RE and SIE theory, to be directly associated with variation of the radical effects, although the ground effects also played a modest role in determining the net substituent effects. [source] , -Substituent Effects on Si,H, P,H and S,H Bond Dissociation EnergiesCHINESE JOURNAL OF CHEMISTRY, Issue 3 2006Yao Fu Abstract CBS-Q and G3 methods were used to generate a large number of reliable Si,H, P,H and S,H bond dissociation energies (BDEs) for the first time. It was found that the Si,H BDE displayed dramatically different substituent effects compared with the C,H BDE. On the other hand, the P,H and S,H BDE exhibited patterns of substituent effects similar to those of the N,H and O,H BDE. Further analysis indicated that increasing the positive charge on Si of XSiH3 would strengthen the Si,H bond whereas increasing the positive charge on P and S of XPH2 and XSH would weaken the P,H and S,H bonds. Meanwhile, increasing the positive charge on Si of XSiH2· stabilized the silyl radical whereas increasing the positive charge on P and S in XPH· and XS· destabilized P- and S-centered radicals. These behaviors could be reasonalized by the fact that Si is less electronegative than H while P and S are not. Finally, it was demonstrated that the spin-delocalization effect was valid for the Si-, P- and S-centered radicals. [source] Systematic Studies on Photoluminescence of Oligo(arylene-ethynylene)s: Tunability of Excited States and Derivatization as Luminescent Labeling Probes for ProteinsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 14 2006Yong-Gang Zhi Abstract Functionalized oligo(phenylene-ethynylene)s (OPEs) with different conjugation lengths, p -X(C6H4C,C)nSiMe3 (n = 1,4; X = NH2, NMe2, H) were synthesized by Sonogashira coupling of (phenylene-ethynylene)s and 1-iodo-4-(trimethylsilylethynyl)benzene, followed by desilylation of the p -substituted (trimethylsilylethynyl)benzenes with potassium hydroxide. The photoluminescent properties for the OPE series with different chain lengths and their solvatochromic responses were examined. The absorption maxima were red-shifted with increasing numbers of ,(C6H4C,C), units (n), and a linear plot of the absorption energy maxima vs. 1/n was obtained for each series. The emission spectra in dichloromethane showed a broad and structureless band, the energies of which (in wavenumbers) also fit linearly with 1/n. Both the absorption and emission wavelength maxima of the NH2 - and NMe2 -substituted OPEs exhibited significant solvent dependence, whereas the parent OPEs (X = H) showed only minor shifts of the ,max values in different solvents. Substituent effects upon the photoluminescent characteristics of the OPEs and the tunability of the excited states were examined with the p -X(C6H4C,C)nSiMe3 (n = 2, 3; X = NH2, NMe2, H, SMe, OMe, OH, and F) series. The H- and F-substituted counterparts exhibited high-energy vibronically structured emissions attributed to the 3(,,*) excited states of the (arylene-ethynylene) backbone. For compounds bearing NH2 and NMe2 groups, a broad red-shifted emission with a remarkable Stokes shift from the respective absorption maximum was observed, which can be assigned to an n , ,* transition. The n , ,* assignment was supported by MO calculations on the model compounds p -X(C6H4C,C)2SiH3 (X = NH2, H). Functionalization of the oligo(arylene-ethynylene)s with the N -hydroxysuccinimidyl (NHS) moiety enabled covalent attachment of the fluorophore to HSA protein molecules. A series of fluorescent labels, namely p -X(C6H4C,C)nC6H4NHS, (n = 1, X = NH2, NMe2, SMe, OMe, OH, F; n = 2, X = NH2, NMe2) and p -Me2NC6H4C,C(C4H2S)C,CC6H4NHS were synthesized, and their conjugates with HSA (human serum albumin) were characterized by MALDI-TOF mass spectrometry, UV/Vis absorption spectroscopy, and gel electrophoresis. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] The Tris(trimethylsilyl)silane/Thiol Reducing System: A Tool for Measuring Rate Constants for Reactions of Carbon-Centered Radicals with ThiolsHELVETICA CHIMICA ACTA, Issue 10 2006Chryssostomos Chatgilialoglu Abstract An extension of the well-known ,free-radical-clock' methodology is described that allows one to determine the rate constants of carbon-centered radicals with a variety of thiols by using the tris(trimethylsilyl)silane/thiol couple as a reducing system. A total of 20 rate constants for the hydrogen abstraction from a variety of alkyl-, silyl-, and aryl-substituted thiols by the primary-alkyl radical 2 in toluene at 80° were determined with the aid of the 5- exo-trig cyclization as a timing device. Further, seven rate constants for the hydrogen abstraction from a variety of alkyl- and silyl-substituted thiols by the acyl radical 9 in benzene at 80° were measured using the decarbonylation process as a timing device. The rate constants varied over two orders of magnitude from 106 to 108,M,1 s,1. Substituent effects were rationalized. The radical-trapping abilities of these reducing systems and those of other common hydrogen donors were compared. [source] Substituent effects on O,H and S,H bond dissociation enthalpies of disubstituted phenols and thiophenolsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4 2008Daniel J. V. A. dos Santos Abstract The O,H and S,H homolytic bond dissociation enthalpies of a set of disubstituted phenols and thiophenols (NH2, OH, CH3, Cl, CF3, and NO2) have been computed by a density functional theory procedure with the 6-311++G(d,p) basis set. A very good agreement between our results and available experimental ones is observed. The effect of substituents on structure, charges and BDEs are investigated and their correlation with Hammett parameters is studied. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] Substituent effects on the physical properties and pKa of phenolINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4-5 2001Kevin C. Gross Abstract Substituent effects on the physical properties and pKa of phenol were studied using density functional theory [B3LYP/6-311G(d,p)] calculations. Substituents alter the physical properties of phenol such as the hydroxyl-group CO and OH bond lengths, the C OH bond angle, and the energy barrier to rotation about the C O bond, and also influence the hydroxyl-group pKa. Except for the rotational barrier, Hammett , constants showed strong correlation with these property changes. Several quantum chemical parameters, including the natural charge on the phenolic hydrogen Qn(H) and the natural charge on the phenoxide oxygen Qn(O,), the HF/6-311G(d,p) HOMO energy Ehomo, and the proton-transfer energy ,Eprot, outperformed the empirical Hammett constants in modeling changes in the pKa. All of these latter parameters yielded correlation coefficients ,r,>0.94 for the pKa. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001 [source] Substituent effects on conformational preference in , -substituted , -fluorophenylacetic acid methyl ester model systems for chiral derivatizing agentsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 10 2009Riadh Sahnoun Abstract In connection with study of chiral derivatizing agents (CDAs) for NMR determination of absolute configuration of organic compounds, factors controlling the conformational preference between syn - and anti -forms in , -substituted , -fluorophenylacetic acid methyl ester (FC(X)(Ph)COOMe) model systems were theoretically investigated. Substituents X at the stereogenic carbon atom were X,=,H, CCH and CH3, the electronic and steric properties of which were significantly different from each other. The model system with X,=,CCH and that with X,=,CH3 were found to be possible candidates for fluorine-containing CDAs. The syn conformation is stable compared with the anti one by 0.7,kcal,mol,1 for the ester with X,=,CCH. On the other hand, the anti conformation is stable compared with the syn one by 0.5,kcal,mol,1 for the ester with X,=,CH3. Both natural bond orbital (NBO) analysis and deletion of selected orbitals based on the donor,acceptor NBO scheme were adopted for semi-quantitative estimation of factors responsible for the conformational preference as well as a qualitative inspection of occupied canonical molecular orbitals (MOs). It was shown that [,,(,*,+,,*)(CO)] and [,,,*(Ph) and ,(Ph),,*] hyperconjugations are the main factors controlling the conformational preferences between the syn and anti conformations. Other types of effects such as electrostatic effects were also investigated. The role of the fluorine atom was also clarified. Copyright © 2009 John Wiley & Sons, Ltd. [source] Substituent effects in the Baeyer,Villiger reaction of acetophenones: a theoretical studyJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 7 2009Lino Reyes Abstract This paper reports the first complete theoretical study of substituent effects on the mechanism of the Baeyer, Villiger (BV) reaction in non-polar solvents taking into account the lowest-energy mechanism that has been proposed for this rearrangement which is non-ionic and fully concerted. The BV reaction of p -substituted acetophenones, p -XC6H4COCH3 (X,=,NO2, CN, H, CH3, OCH3), with performic (PFA) and trifluoroperacetic (TFPAA) acids, catalyzed by formic (FA) and trifluoroacetic (TFAA) acids, respectively, using the MPWB1K functional and the 6-311G(d,p) and 6-311++G(d,p) basis sets, are studied. Solvent effects are taken into account by means of the PCM continuum model using dichloromethane as solvent. Electron-donating substituents on the aryl group have a relatively small activation effect on the first step, but a pronounced activation effect on the second to the point of being able to change the rate-determining step (RDS) of the reaction, as observed in the case of p -methoxyacetophenone with TFPAA acids. After analyzing the changes in Gibbs free energy of activation, geometrical parameters, and charge distributions of the transition states (TSs), explanations are provided for the two distinct effects that substituents on the ketone have on the kinetics of the addition and migration steps of the BV oxidation. The effect of the acid/peracid pair used is also discussed. Copyright © 2008 John Wiley & Sons, Ltd. [source] Substituent effects of phthalimide-based nucleoside analogs on binding a CG Watson,Crick base pairJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 10 2007Z. Xiao Abstract Five differently substituted phthalimide nucleosides were studied by NMR spectroscopic techniques for their ability to recognize and bind a cytosine,guanosine (CG) Watson,Crick base pair in CD2Cl2. Whereas only rather weak binding was observed for analogs with an amino, acetamido, or benzamido substituent, strong binding was observed with the analogs carrying an ureido and n -butyl ureido residue. 2D NOE measurements at low temperatures confirm the proposed binding mode for the high-affinity ligands but indicate binding interactions for the weakly bound analogs different from the expected geometry. Copyright © 2007 John Wiley & Sons, Ltd. [source] Substituent effects on ion complexation of para - tert -butylcalix[4]arene esters,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 11 2006Márcio Lazzarotto Abstract Phenoxy-carboxy-methoxy- p-tert -butylcalix[4]arene esters were synthesized in order to evaluate the role of electronic parameters on the complexation of alkaline metal cations. Extraction constants of metal picrates to organic phase were determined. Plots of log (KR/KH) against Hammett , and , gave good linear correlations. The best correlations with , were obtained for K+ and Rb+, while the best correlations with , were obtained for Li+ and Na+. All Hammett plots gave a straight descending line, which is consistent with a dependence of the electronic density on the CO. Treatment of data using the Yukawa,Tsuno equation revealed a variation in the contribution of resonance in the complexation of alkaline metal ions, which is maximum for Na+ and minimum for Rb+. Electronic parameters were calculated for a related acyclic model structure and only the HOMO energy showed a good correlation with log (KR/KH). Copyright © 2007 John Wiley & Sons, Ltd. [source] Inductive effect of uncharged groups: dependence on electronegativityJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 7 2006Otto Exner Abstract Substituent effects in rigid non-conjugated systems were followed on the series of 3-substituted 1-fluoro-bicyclo[1.1.1]pentanes and 2-substituted 1-fluoroethanes in the fixed ap conformation. Their energies were calculated within the framework of the density functional theory at the B3LYP/6-311++G(3df,3pd)//B3LYP/6-311++G(3df,3pd) level and the substituent effects were expressed in terms of isodesmic homodesmotic reactions. The results were confirmed by the energies of 1,4-disubstituted bicyclo[2.2.2]octanes reported in the literature and calculated at a lower level. Interaction of two common substituents of low or medium polarity cannot be described as the classical inductive effect by one term, proportional in all series, but an additional parameter is necessary, which depends only on the first atom of the substituent and may be identified with its electronegativity. The second term decreases with the distance more steeply than the first term and is always much less important. Nevertheless its statistical significance was proved by several sensitive tests at the highest level used in statistics. When one of the substituents is charged (or at least strongly polar as NO2 or CN), the first term is much increased and the second becomes less significant or insignificant. Therefore, the standard definition of the inductive effect with a uniform, universally valid constant can be retained as far as one treats only the ionization equilibria, both in solution and in the gas phase, or kinetics with a strongly polar transition state. In contrast to the firm statistical proofs, the physical meaning of the electronegativity term was not established. Any relation to various group electronegativities does not exist, similarity to the 13C NMR shifts is merely qualitative. Copyright © 2006 John Wiley & Sons, Ltd. [source] Substituent effects in reductions of heteroaromatic cationsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 10 2002David Heyes Abstract A set of 11 each of 2,4,6-triphenylpyrylium, -thiopyrylium and - N -methylpyridinium tetrafluoroborates carrying a range of substituents in the phenyl rings were prepared. First and second wave reduction potentials were determined. For the thiopyrylium series there are linear correlations between scaled potentials (E°/0.05915) and summed Hammett constants for substituents in the pendant phenyl groups (,,=,2.29 and 3.38 for first and second waves respectively). For the pyrylium series, a good linear relationship (,,=,2.79) is obtained for all substituent patterns for the first wave reduction potentials, but for the second wave there are separate correlations for salts carrying substituents in the 4-phenyl and for those carrying substituents in 2- and 6-phenyls. For the pyridinium series, the first wave potentials show separate correlations for salts carrying substituents in the 4-phenyl and for those carrying substituents in 2-and 6-phenyls, but a single linear relationship for the second wave potentials. These are related to particular structural features in the cations, radicals and anions in each series. Rates and products were determined for reductions of the pyrylium and thiopyrylium cations by sodium cyanoborohydride and of all cations by sodium borohydride in acetonitrile solution. Reactions are first order in reducing agent and cation. Primary kinetic isotope effects were determined for borohydride reduction of the least reactive of each of the series of cations. Plots of logarithms of second-order rate constants against summed Hammett constants for substituents in the pendant phenyl groups are linear for all combinations of reagent and cation with 0.91 < , < 1.50 across all substituent patterns. For parent pyrylium and thiopyryliums, kBH4/kCNBH3,=,8.4,×,104 and 1.5,×,104, respectively, and for reductions by borohydride the reactivities of the pyrylium, thiopyrylium and pyridinium, series decrease in the order 1.4,×,105:8.8,×,103:1. Constant selectivities are not observed. Comparison of the correlations for electrochemical reduction and for hydride addition leads to the conclusion that charge neutralization in the hydride addition transition states runs ahead of bonding changes at the originating B,H bond. Copyright © 2002 John Wiley & Sons, Ltd. [source] Substituent effects in solvolysis of 1,1-diphenylethyl p -nitrobenzoates.JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 8 2002Symmetrically disubstituted, monosubstituted systems Abstract The rates of solvolysis of 1,1-diarylethyl p -nitrobenzoates and chlorides were determined conductimetrically at 25,°C in 80% (v/v) aqueous acetone. Applying the Yukawa,Tsuno (Y,T) equation, the symmetrical (X,=,Y) subseries gave a precise additivity relationship for the whole substituent range with a ,sym value of ,3.78 and an rsym value of 0.77. While any Y subsets gave statistically less reliable Y,T correlations, the apparent , value changed significantly depending on the fixed Y substituents; the , value decreases with the more electron-donating fixed substituents Y, which is compatible with the Hammond shift of the transition state coordinate. Nevertheless, the concave correlations of the More O'Ferrall non-linearity relationship for any Y subsets are not in line with what is expected from the reactivity,selectivity relationship suggesting an anti-Hammond shift of transition state. However, we found a precise extended Brønsted relationship between the pK values of 1,1-diarylethylenes and solvolysis rate process with a constant slope of ,,=,1.03,±,0.03. This is direct evidence indicating that there is no significant shift of the transition-state coordinate over the whole range of substituent change. Copyright © 2002 John Wiley & Sons, Ltd. [source] Substituent effects on benzyne electronic structuresJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 9 2001William T. G. Johnson Abstract The effect of inductive strength and ,-conjugating tendency on the structure and energetics of singly substituted benzynes is studied for different substituents. For amino and cyano substituents, all possible isomers are examined. For silyl and ammonio substituents, meta isomers are examined. Copyright © 2001 John Wiley & Sons, Ltd. [source] Substituent effects on the chain-transfer behavior of 7-methylene-2-methyl-1,5-dithiacyclooctane in the presence of disulfides and thiolsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 24 2002Simon Harrisson Abstract The chain-transfer behavior of 7-methylene-2-methyl-1,5-dithiacyclooctane was investigated in the presence of four chain-transfer agents: thiophenol (PhSH), thiobenzoic acid (BzSH), diphenyl disulfide (PhSSPh), and dibenzoyl disulfide (BzSSBz). The chain-transfer constants for these compounds at 60 °C were 0.38 (PhSH), 0.76 (BzSH), 0.24 (PhSSPh), and 0.05 (BzSSBz). The variations in the thiol chain-transfer constants could be explained in terms of the stability of the resulting radicals. The chain transfer to the disulfides, however, appeared to be determined by the electronic character of the disulfide bond, and this suggests that the transfer took place via an addition,fragmentation mechanism. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4421,4425, 2002 [source] Substituent effects of the N,N -dimethyl- sulfamoyl group on the 1H and 13C NMR spectra of positional isomers of quinolines,MAGNETIC RESONANCE IN CHEMISTRY, Issue 2 2008Andrzej Ma, lankiewicz Abstract The complete 1H and 13C NMR spectral assignments of seven positional isomers of N,N -dimethylsulfamoylquinolines 2,8 and quinoline have been made using 1D and 2D NMR techniques, including COSY, HMQC and HMBC experiments. ,,H and ,,C substituent effects induced by the sulfamoyl group were determined. The sulfamoyl substituent affects proton and carbon chemical shifts both in the parent and in the fused (pyridine or benzene) ring. Copyright © 2007 John Wiley & Sons, Ltd. [source] Substituent effects on the photofading of disperse azo dyes on poly(ethylene terephthalate) substrateCOLORATION TECHNOLOGY, Issue 3 2010Yasuyo Okada The relationships between the chemical structures and oxidative fading of the disperse azo dyes, p -nitrophenylazo- and benzothiazoleazo-anilines, on poly(ethylene terephthalate) substrate are discussed in terms of the parameters k0,i (rate constants of reaction towards 1O2) and fi (photosensitivity), the molecular parameters of molecular orbital theory and substituents in the diazo and coupling components, on the assumption that the initial rates of oxidative fading are proportional to the product of k0,i and fi. 2-Methoxy-5-acetylamino- N -substituted aniline couplers exhibited large fi values. 2-Chloro and 4-nitro substituents of aniline diazo components exhibited small fi values or high quantum yields of internal conversion, while 4-nitro substituent did not. A close correlation between N -substituents and light fastness, proposed by Müller and supplemented by Dawson, demonstrates the applicability of frontier orbital theory, through the highest occupied molecular orbital (HOMO) energy of the dyes, to the analysis of oxidative fading. Dyes with N -2-cyanoethyl substituents, which gave a lower HOMO energy, also exhibited superior light fastness compared with N -2-hydroxyethyl substituents. [source] Theoretical Description of Substituent Effects in Electrophilic Aromatic Substitution ReactionsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 35 2008Tobias Schwabe Abstract The ability of the current Kohn,Sham density functional theory (DFT) to compute the change of the proton affinity (PA) of phenol derivatives due to substitution is investigated. These systems can be used as models to predict reactivities in electrophilic aromatic substitution reactions. The complexity of the problem is increased systematically by introducing successively up to four substituents in five typical cases (methyl, cyano, fluorine, chlorine, and bromine). Our investigation can be regarded as representative for an important class of problems consistently encountered in the DFT modeling of organic reactions. High-level theoretical reference data from CCSD(T) and SCS-MP2 wave-function calculations are presented, and the PAs are compared to those obtained by a series of density functionals (DFs). It is shown that not all DFs are capable of quantitatively reproducing the substituent effects. These can be simply linear in the number of substituents or show more complicated patterns. Especially for halogens, some DFs even fail completely. In these cases, linearly increasing errors with the number of groups are observed. Reliable results are obtained with hybrid DFs or the even more accurate double-hybrid DF approach. The errors are attributed to the common self-interaction (over-delocalization) error in part of the DFs. Comparison with Hartree,Fock results shows that a reliable account of electron correlation is necessary to compute the PA of unsaturated and highly substituted molecules with chemical accuracy.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Heteroatoms and substituent effects: The importance of heteroatom hyperconjugationHETEROATOM CHEMISTRY, Issue 5 2002James F. King We have found that the specific rate of ,-sulfonyl carbanion formation in a ,-substituted sulfone shows a sizable dependence on the HC,C,X torsion angle. Defining kN = (kexch)X/(kexch)model (where the model has X = H or an alkyl group) we observed for a collection of ,-alkoxy sulfones (X = OR) acceptable agreement with the expression log kN = a + b cos2 , (where a = 1.70 and b = 2.62). Extension to other ,-substituents (X = RS, R2N, and R3N+) yields the same pattern, with the last showing very large dependence of kN on the torsion angle (b = 6.3). These observations are ascribed to the presence (in addition to the inductive and field effects) of negative hyperconjugation responsible for accelerations of 1000-fold and more, deriving from donation of the incipient negative charge on carbon into the ,*CX orbital in the transition state. These observations reflect, and at the same time underline, the importance of the low-lying antibonding orbitals present in heteroatomic molecules. © 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:397,405, 2002; Published online in Wiley Interscience (www.interscience.wiley.com). DOI 10.1002/hc.10067 [source] Solvent and substituent effects on the reaction of 2- and 4-chloro-3,5-dinitrobenzotrifluorides with substituted anilinesINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 12 2009Basim H. M. Asghar The solvent effect on a nucleophilic substitution reaction of 2- and 4-chloro-3,5-dinitrobenzotrifluoride with substituted anilines was studied in methanol, acetonitrile, and toluene at 25°C. This reaction is of second order, except 2-chloro-3,5-dinitrobenzotrifluoride in toluene shows third order. The kA values are found to be dependent on the substituent in aniline and give good Hammett correlations. The obtained , values are ,4.07 and ,4.62, for the reaction of anilines with 2-chloro-3,5-dinitrobenzotrifluoride in methanol and acetonitrile, respectively. The , values for the reaction of the anilines with 4-chloro-3,5-dinitrobenzotrifluoride are ,3.38, ,4.11, and ,4.34 in methanol, acetonitrile, and toluene, respectively. The reaction of the former compound with anilines in toluene shows a second order in aniline. The dependence of the reaction on the external base such as DABCO suggests a proton transfer controlling step. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 777,786, 2009 [source] Pyrazole formation: Examination of kinetics, substituent effects, and mechanistic pathways,INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 7 2008Joseph C. Sloop Reaction kinetics for the condensation of 1,3-diketones 1a,o with selected arylhydrazines (aryl = Ph, 4-NO2Ph, 4-CH3OPh, and 2,4-diNO2Ph) was studied using 19F NMR spectroscopy. Product regioselectivity is modulated by reactant ratios, substituents, and acidity. Reaction rates were found to be influenced by substituents on the diketones and on phenylhydrazines as well as by acidity of the reaction medium with rates varying as much as 1000-fold. Hammett , values for these cyclizations were determined. The reaction was found to be first order in both the diketone and arylhydrazine. The rate-determining step for pyrazole formation shifts as a function of pH. Mechanistic details and reaction pathways supporting these findings are proposed. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 370,383, 2008 [source] Combinatorial Design of Copper-Based Mixed Nanoclusters: New Catalysts for Suzuki Cross-CouplingADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 8 2003Mehul Abstract Quantum dots (2,5,nm) of copper and copper/palladium mixtures are found to be good catalysts for Suzuki cross-coupling. The catalysts are applicable to a wide range of iodo- and bromoaryl substrates, and give moderate yields using chloroaryl substrates. Cluster activity and stability is found to depend strongly on the preparation method and the reaction conditions. The mechanism of cluster deactivation and the sensitivity of the cluster-catalysed reaction to substituent effects are studied and discussed. [source] | |||||||||||||||||||||||||||||||