NQR parameters of complexes and polarizability effect

The literature data on substituent influence on the nuclear quadrupole resonance frequencies (ν), quadrupole coupling constants (e²Qq ? h^{? 1}), and asymmetry parameters (η) for 36 series of the H‐complexes, charge–transfer complexes, transition metal complexes and other donor–acceptor complexes have been considered, using the correlation analysis. Generally the ν, e²Qq ? h^{? 1}, and η values were first established to depend on the inductive, resonance, polarizability, and steric effects of substituents. The presence or otherwise of certain effects as well as relation between their contributions are determined by the type of series. The polarizability effect owes its existence to the appearance of an excess charge on the indicator centre as a result of the complexation. The contribution of this effect ranges up to 75%. Copyright © 2012 John Wiley & Sons, Ltd.     

Spectral properties and absolute rate constants for beta-scission of ring-substituted cumyloxyl radicals. A laser flash photolysis study

A laser flash photolysis study of the spectral properties and beta-scission reactions of a series of ring-substituted cumyloxyl radicals has been carried out. All cumyloxyl radicals display a broad absorption band in the visible region of the spectrum, which decays on the microsecond time scale, leading to a strong increase in absorption in the UV region of the spectrum, which is attributed to the corresponding acetophenone formed after beta-scission of the cumyloxyl radicals. The position of the visible absorption band is red-shifted by the presence of electron-donating ring substituents, while a blue-shift is observed in the presence of electron-withdrawing ring substituents, suggesting that + R ring substituents promote charge separation in the excited cumyloxyl radical through stabilization of the partial positive charge on the aromatic ring of an incipient radical zwitterion. Along this line, an excellent Hammett-type correlation between the experimentally measured energies at the visible absorption maxima of the cumyloxyl radicals and sigma(+) substituent constants is obtained. A red-shift is also observed on going from MeCN to MeCN/H(2)O for all cumyloxyl radicals, pointing toward a specific effect of water. The ring substitution does not influence to a significant extent the rate constants for beta-scission of the cumyloxyl radicals, which varies between 7.1 x 10(5) and 1.1 x 10(6) s(-1), a result that suggests that cumyloxyl radical beta-scission is not governed by the stability of the resulting acetophenone. Finally, k(beta) increases on going from MeCN to the more polar MeCN/H(2)O 1:1 for all cumyloxyl radicals, an observation that reflects the increased stabilization of the transition state for beta-scission through increased solvation of the incipient acetophenone product.     

Rate constants for the beta-elimination of tosyl radical from a variety of substituted carbon-centered radicals

The rate constants for the beta-elimination of tosyl radical (Ts*) from a series of carbon-centered radicals have been determined by using the radical clock methodology. Depending on the substituents R in Ts-CH(2)-CH*R radicals, the rate constants at 293 K vary by more than 2 orders of magnitude in the range of 10(3)-10(6) s(-1). The lowest values were found for the 2-naphthyl and carbamoyl substituents, whereas the benzyl substituent is located at the other extremity. The effect of the substituent upon the stabilization of the starting radical exerts a predominant influence in this reaction in decreasing the rate of fragmentation.     

Infrared spectroscopic studies of transition metal complexes and polarizability effect

The literature data on substituent influence on the CS, CN, NC, NN, and NO stretching frequencies (ν) in the IR spectra and in specific cases on their respective stretching force constants (k) have been analyzed for 28 series of the transition metal complexes. The ν and k values were first established to depend not only on the inductive and resonance effects but on the polarizability of substituents as well. The contribution of the polarizability effect varies from 0 to 57% with the type of series.     

Transition metal NMR chemical shifts and polarizability effect in organometallic complexes

The literature data on substituent influence on the ⁵¹V, ⁵⁵Mn, ⁵⁷Fe, ⁵⁹Co, ⁶¹Ni, ⁹⁵Mo, ¹⁰³Rh, ¹⁸³W, ¹⁸⁷Os and ¹⁹⁵Pt NMR chemical shifts (δ) and on J (M, P; M = Mn, Fe, Mo, Rh, W, Os) coupling constants have been analyzed for 30 series of the organometallic complexes. It has been established for the first time that the δ and J values depend on the inductive, resonance and polarizability effects of substituents. The polarizability effect is caused by the partial charge on the central M atom. The contribution of this effect ranges from 3 to 86%. Copyright © 2009 John Wiley & Sons, Ltd.     

取代基效应定量研究新进展

综述了最近20多年有机化学取代基效应定量研究所取得的诸多新进展.主要内容包括：（1）电负性均衡原理得到普遍认同并用于计算分子中电荷分布和基团的诱导效应,分子电负性计算方法在几何平均法、调和平均法、加权平均法的基础上又提出了价电子均衡方法,进一步扩展了电负性均衡原理的应用范围.（2）试剂亲电、亲核能力的实验测定,提出了以二苯甲基正离子和醌甲基化合物等作为参考化合物的标定方法,其研究范围被扩展到气相条件、有机金属反应和自由基体系,得到一系列试剂的亲核参数N和亲电参数E.理论上提出亲电性指数和亲核性指数的概念及其定量计算公式,并对实验测定参数和理论计算指数之间的关系进行了深入研究.（3）极化效应参数从最初的经验方法,进而采用量子化学方法计算,再用统计方法提出了烷基极化效应指数PEI和基团极化效应参数PEIX,被广泛用于解释和估算有机化合物的气相酸碱性、电离能、生成焓、键能、反应速率、水溶解性和色谱保留值.（4）直接由取代苯的紫外光谱吸收能量计算得到的取代基激发态参数exCC,不同于取代基在分子处于基态的极性参数和自由基状态下的自旋离域效应参数,该参数用于对位二取代苯、取代二苯乙烯、二取代氮苄叉苯胺等多类有机化合物紫外光谱的定量相关,结果良好.（5）立体屏蔽效应的提出区分了传统的三种立体效应.在羰基与亲核试剂加成反应立体选择性定量表示,提出了立体选择性指数Ci;在表示基团对反应中心表面积屏蔽的定量提出了屏蔽参数SR,进一步基于反应中心体积被屏蔽的角度提出了拓扑立体效应指数TSEI,这些参数在分子内二面角、反应的立体选择性、烯烃和烷基苯的生成焓、咪唑离子液体的酸性以及烷烃与羟基自由基的反应速率等方面得到良好的应用.此外,论文还对有机化学取代基效应定量研究中值得进一步深入探索的领域和问题提出了建议和展望.     

Transition state for alkyl group hydrogenation on Pt(111)

Substituent effects have been used to probe the characteristics of the transition state to hydrogenation of alkyl groups on the Pt(111) surface. Eight different alkyl and fluoroalkyl groups have been formed on the Pt(111) surface by dissociative adsorption of their respective alkyl and fluoroalkyl iodides. Coadsorption of hydrogen and alkyl groups, followed by heating of the surface, results in hydrogenation of the alkyl groups to form alkanes, which then desorb into the gas phase. Temperature-programmed reaction spectroscopy was used to measure the barriers to hydrogenation, DeltaE(H)(double dagger), which are dependent on the size of the alkyl group (polarizability) and the degree of fluorination (field effect). This example is one of only two surface reactions for which the influence of the substituents on DeltaE(H)(double dagger) has been correlated with both the field and the polarizability substituent constants of the alkyl groups in the form of a linear free energy relationship. Increasing both the field and the polarizability constants of the alkyl groups increases the value of DeltaE(H)(double dagger). The substituent effects are quantified by a field reaction constant of rho(F) = 27 +/- 4 kJ/mol and a polarizability reaction constant of rho(alpha) = 19 +/- 3 kJ/mol. These suggest that the transition state for hydrogenation is slightly cationic with respect to the alkyl group on the Pt(111) surface, RC + H <--> {RC(delta+)...H}(double dagger).     

Fluorescent pH sensors with negligible sensitivity to ionic strength

Optical pH determination has the fundamental disadvantage of measuring a signal that depends on the ionic strength of the sample. The problem originates from the complex relationship between the proton activity and the concentration of the pH-sensitive dye. The effect of ionic strength on the signal depends on the charge of the indicator and its environment, e.g. the immobilisation matrix. We present novel lipophilic fluorescein esters carrying one negative charge. They are embedded in an uncharged, highly proton-permeable hydrogel to give optical pH sensors that show a negligible cross-sensitivity towards ionic strength. The fluorescent dyes differ in their substituents. This variation of substituents results in dissociation constants between 5.5 and 8.5. The indicators were made lipophilic by esterification of the carboxy group with a C(18) alkyl chain. Since their spectral properties are quite similar, two indicators may be used in one sensor. This results in an optical pH sensor with a dynamic range that extends from pH 4.5 to 8.     

Substituent Effects on the C-H Bond Dissociation Energy of Toluene. A Density Functional Study

The bond dissociation energies of the benzylic C-H bond of a series of 16 para-substituted toluene compounds (p-X-C(6)H(4)CH(3)) have been calculated with the density functional method (BLYP/6-31G). The calculated substituent effects correlate well with experimental rates of dimerization of para-substituted alpha,beta,beta-trifluorostyrenes and rearrangement of methylenearylcyclopropanes. Both electron-donating and electron-withdrawing groups reduce the bond dissociation energy (BDE) of the benzylic C-H bond because both groups cause spin delocalization from the benzylic radical center. The calculated spin density variations at the benzylic radical centers correlate well with both the ESR hyperfine coupling constants determined by Arnold et al. and the calculated radical effects of the substituents. The relative radical stabilities are mainly determined by the spin delocalization effect of the substituents, and polar effect of the substituents are not important in the current situation. The ground state effect is also found to influence the C-H BDE.     

Charge density and NMR parameters. Aliphatic derivatives

The charge distribution in several substituted aliphatic derivatives was determined by a parametric MOLCAO method. The charges were successfully employed to interpret NMR parameters, namely proton and carbon chemical shift and vicinal proton-proton coupling constants in ethyl derivatives and carbon-proton coupling constants. The linear correlations found between NMR parameters and charge densities are restricted to substituents of the same row of the periodic system. The decay along the aliphatic chain of the perturbation induced by substituents on proton chemical shift is also reproduced by charge distribution. Some results also seem to indicate that the substituents affect the hybridization of the attached carbon atom.     

Computational prediction of alpha/beta selectivities in the pyrolysis of oxygen-substituted phenethyl phenyl ethers

Phenethyl phenyl ether (PPE; PhCH 2CH 2OPh) is the simplest model for the most common beta-O-4 linkage in lignin. Previously, we developed a computational scheme to calculate the alpha/beta product selectivity in the pyrolysis of PPE by systematically exploiting error cancellation in the computation of relative rate constants. The alpha/beta selectivity is defined as the selectivity between the competitive hydrogen abstraction reaction paths on the alpha- and beta-carbons of PPE. We use density functional theory and employ transition state theory where we include diagonal anharmonic correction in the vibrational partition functions for low frequency modes for which a semiclassical expression is used. In this work we investigate the effect of oxygen substituents (hydroxy, methoxy) in the para position on the phenethyl ring of PPE on the alpha/beta selectivities. The total alpha/beta selectivity increases when substituents are introduced and is larger for the methoxy than the hydroxy substituent. The strongest effect of the substituents is observed for the alpha-pathway of the hydrogen abstraction by the phenoxyl chain carrying radical for which the rate increases. For the beta pathway and the abstraction by the R-benzyl radical (R = OH,OCH 3) the rate decreases with the introduction of the substituents. These findings are compared with results from recent experimental studies.     

Effect of substituents on the first ionization potentials of aromatic and heteroaromatic compounds. The {ie2532-1} resonance parameters of Si-, Ge-, Sn-, and Pb-containing fragmentsresonance parameters of Si-, Ge-, Sn-, and Pb-containing fragments

The first vertical ionization potentials (IP) of monosubstituted benzenes, hexafluorobenzenes, naphthalenes, anthracenes, furans, thiophenes, and selenophenes and the secondIP of monosubstituted tellurophenes are related to the inductive, resonance, and polarization parameters of substituents by dependences of the type {ie2532-2}. The contribution of polarizability effect, {ie2532-3}, to theIP value is determined by the degree of delocalization of the positive charge in radical cations formed upon photoionization of the compounds studied. The {ie2532-4} resonance parameters of nineteen Si-, Ge-, Sn-, and Pb-containing substitutents in naphthalene, furan, and thiophene series were calculated. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2434–2438, December, 1998.     

Influence of substituent effects on the coordination ability of salicylaldehyde Schiff bases

Abstract

The logK₁ was quantified by four parameters, metal ion radii R, metal ion charge Q, metal atom polarizability P, and Hammett constants σ of substituent, where logK₁ was the stability constant of complexes (179) involving 10 Schiff bases Sal-A-X (N-(5-chloro-4-6-dimethyl-benzylidene)-anilines) reacting with 18 metal ions. The effect of substituents on the coordination ability of Sal-A-X was systematically investigated, and an expression with good correlation was obtained. The result shows that the electronic effect of substituent X on the aniline ring has an important influence on the stability of the complex; its electron-donating effect enhances the stability of the complex, whereas its electron-withdrawing effect lowers the stability of the complex. The theoretical results provide a potential value for the design of coordination compounds with unique function and reactivity. Moreover, their coordination selectivity to specific metal-ions can also be evaluated according to Equation (2).     

Formation and decay of localized contact radical ion pairs in DNA hairpins

The dynamics of charge separation and charge recombination in synthetic DNA hairpins possessing diphenylacetylene-4,4'-dicarboxamide linkers have been investigated by means of femtosecond time-resolved transient absorption spectroscopy. The lowest excited singlet state of the linker is capable of oxidizing nearest neighbor adenine as well as guanine. A large wavelength shift in the transient absorption spectrum accompanies the conversion of the singlet linker to its anion radical, facilitating the investigation of electron-transfer dynamics. The rate constants for charge separation are dependent upon the oxidation potentials of the neighboring nucleobase donors but not upon the identity of nonnearest neighbors. Thus, the charge separation processes yield a contact radical ion pair in which the positive charge is localized on the neighboring nucleobase. Rate constants for charge recombination are dependent upon the identity of the first and second nearest-neighbor nucleobases but not more remote bases. This dependence is attributed to stabilization of the contact radical ion pair by interaction with its nearest neighbor. The absence of charge migration to form a base-pair separated radical ion pair is a consequence of Coulombic attraction in the contact radical ion pair and the low effective dielectric constant (epsilon < 7) experienced by the contact radical ion pair. Photoinduced charge injection to form a base-pair separated radical ion pair is necessary in order to observe charge migration.     

Determination of Absolute Rate Constants for Radical Polymerization and Copolymerization of Ethyl α-Chloroacrylate: Effects of Substituents on Reaction Rates of Monomer and Polymer Radical

The propagation and termination rate constants (k_p and k_t) for the radical polymerization of ethyl a-chloroacrylate (ECA) were determined by the rotating sector method k_p = 1660 and k_t = 3.33 × 10⁸ L/mol?s at 30° C. The absolute rate constants for cross-propagations in copolymerization were evaluated from the k_p determined for ECA or those for common monomers and the monomer reactivity ratios. The reactivities of ECA and poly-(ECA) radicals estimated as the rate constants of cross-propagations were accounted for by using equations relating these rate constants to the polar and resonance effects of the substituents. ECA was highly reactive toward various polymer radicals as expected from the resonance effects of the carbethoxy and chloro substituents. The poly(ECA) radical was found to be more reactive than common polymer radicals. The reactivity of a polymer radical in cross-propagation seemed to increase with increasing electron-accepting power by facilitating electron transfer from a monomer required for the new C-C bond formation.     

Recent progress in quantifying substituent effects

This paper summarizes significant progress in quantifying organic substituent effects in the last 20 years. The main content is as follows: (1) The principle of electronegativity equalization has gained wide acceptance, and has been used to calculate the intramolecular charge distribution and inductive effect of groups. A valence electrons equalization method was proposed to compute the molecular electronegativity on the basis of geometric mean method, harmonic mean method, and weighted mean method. This new calculation method further extended the application of the principle of electronegativity equalization. (2) A scale method was established for experimentally determining the electrophilic and nucleophilic ability of reagents, in which benzhydryliumions and quinone methides were taken as the reference compounds, and the research field was extended to the gas phase conditions, organometallic reaction and radicals system. Moreover, the nucleophilicity parameters N and electrophilicity parameters E for a series of reagents were obtained. The definition and quantitative expression of electrophilicity index and nucleophilicity index were proposed theoretically, and the correlation between the parameters from experimental determination and the indexes from theoretical calculation was also investigated. (3) The polarizability effect parameter was initially calculated by empirical method and further developed by quantum chemistry method. Recently, the polarizability effect index of alkyl (PEI) and groups (PEI X ) were proposed by statistical method, and got wide applications in explaining and estimating gas-phase acidity and basicity, ionization energy, enthalpy of formation, bond energy, reaction rate, water solubility and chromatographic retention for organic compounds. (4) The excited-state substituent constant ex CC obtained directly from the UV absorption energy data of substituted benzenes, is different from the polar constants in molecular ground state and the radical spin-delocalization effects constants in molecular radical state. The proposed constant ex CC correlated well with the UV absorption energy of many kinds of organic compounds, such as 1,4-disubstituted benzenes, substituted stilbenes, and disubstituted N-benzylidenebenzenamine. (5) The establishment of the steric shielding effect distinguished the three traditional steric effects. The stereoselectivity index C i was proposed to quantify the stereoselectivity of the addition reaction of carbonyl with nucleophilic reagent. The shielding parameter S R was defined to quantitatively express the specific surface of the reaction center screened by a group. Further, the Topological Steric Effect Index (TSEI) of a group was proposed on the basis of the relative specific volume of reaction center screened by the atoms of substituents. These parameters can be applied in estimating the intramolecular dihedral angles, stereoselectivity of reaction, enthalpies of formation of alkenes and alkylbenzene, acidity of substitutedimidazolium ionic liquid, and the reaction rate of alkane and hydroxyl radical. In addition, some suggestions and prospects for further studies on quantifying the organic substituent effects were presented in this paper.     

The electro-optic properties of fluorinated polydialkoxyphosphazenes with different lengths of side substituents

Polydialkoxyphosphazenes with partially fluorinated side substituents of different lengths in dioxane and ethyl acetate are studied via the methods of the equilibrium electro-optic Kerr effect, isothermal translational diffusion, and viscosimetry. It is experimentally found that the magnitude and sign of the Kerr constant depend on the length of the side substituents. The observed dependence is explained by the changes in the optical polarizability tensor and the angle between the direction of the permanent dipole moment and the primary axis of optical polarizability of the monomer unit of these macromolecules with an increase in the number of carbon atoms in the chains of alkoxy substituents attached to phosphorus. It is shown that fluorinated polydialkoxyphosphazenes are comblike polymers with high equilibrium skeletal rigidity and non-coincident principal directions of the optical polarizability and dielectric polarizability of the monomer unit.     

Hydrogen-bonding effects on the properties of phenoxyl radicals. An EPR,kinetic, and computational study

The effect of 1,1,1,3,3,3-hexafluoropropan-2-ol (HFP) on the properties of phenoxyl radicals has been investigated. HFP produces large variations of the phenoxyl hyperfine splitting constants indicative of a large redistribution of electron spin density, which can be accounted for by the increased importance of the mesomeric structures with electric charge separation. The conformational rigidity of phenoxyl radicals with electron-releasing substituents is also greatly enhanced in the presence of HFP, as demonstrated by the 2 kcal/mol increase in the activation energy for the internal rotation of the p-OMe group in the p-methoxyphenoxyl radical. By using the EPR equilibration technique, we have found that in phenols the O-H bond dissociation enthalpy (BDE) is lowered in the presence of HFP because it preferentially stabilizes the phenoxyl radical. In phenols containing groups such as OR that are acceptors of H-bonds, the interaction between HFP and the substituent is stronger in the phenol than in the corresponding phenoxyl radical because the radical oxygen behaves as an electron-withdrawing group, which decreases the complexating ability of the substituent. In phenols containing OH or NH(2) groups, EPR experiments performed in H-bond accepting solvents showed that the interaction between the solvent and the substituent is much stronger in the phenoxyl radical than in the parent phenol because of the electron-withdrawing effect of the radical oxygen, which makes more acidic, and therefore more available to give H-bonds, the OH or NH(2) groups. These experimental results have been confirmed by DFT calculations. The effect of HFP solvent on the reactivity of phenols toward alkyl radicals has also been investigated. The results indicated that the decrease of BDE observed in the presence of HFP is not accompanied by a larger reactivity. The origin of this unexpected behavior has been shown by DFT computations. Finally, a remarkable increase in the persistency of the alpha-tocopheroxyl radical has been observed in the presence of HFP.     

Radical cations of branched alkanes in solutions: time-resolved magnetic field effect and quantum chemical studies

Radical cations of heptane and octane isomers, as well as several longer branched alkanes, were detected in irradiated n-hexane solutions at room temperature by the method of time-resolved magnetic field effect (TR MFE). To identify radical cations, the hyperfine coupling constants as determined by simulation of the TR MFE curves were compared to the constants calculated using the density functional theory (DFT) approach. The g-values of the observed radical cations were close to that of the 2,2,3,3-tetramethylbutane radical cation studied earlier by optically detected electron spin resonance (ESR) and TR MFE techniques. No evidence of the decay of the radical cations of branched alkanes to produce olefin radical cations was found, which was further supported by the observation of positive charge transfer from the observed radical cations to cycloalkane molecules. The lifetimes of the radical cations of the branched alkanes were found to be longer than tens of nanoseconds.