Hammett Constants Σp Research Articles

Carbonate-induced enhancement of phenols degradation in CuS/peroxymonosulfate system: A clear correlation between this enhancement and electronic effects of phenols substituents

This study investigated the enhancement effects of dissolved carbonates on the peroxymonosulfate-based advanced oxidation process with CuS as a catalyst. It was found that the added CO32− increased both the catalytic activity and the stability of the catalyst. Under optimized reaction conditions in the presence of CO32−, the degradation removal of 4-methylphenol (4-MP) within 2 min reached 100%, and this was maintained in consecutive multi-cycle experiments. The degradation rate constant of 4-MP was 2.159 min−1, being 685% greater than that in the absence of CO32− (0.315 min−1). The comparison of dominated active species and 4-MP degradation pathways in both CO32−-free and CO32−-containing systems suggested that more CO3•−/1O2 was produced in the case of CO32−deducing an electron transfer medium, which tending to react with electron-rich moieties. Meanwhile, Characterization by X-ray photoelectron spectroscopic and cyclic voltammetry measurement verified CO32− enabled the effective reduction of Cu2+ to Cu+. By investigating the degradation of 11 phenolics with different substituents, the dependence of degradation kinetic rate constant of the phenolics on their chemical structures indicated that there was a good linear relationship between the Hammett constants σp of the aromatic phenolics and the logarithm of k in the CO32−-containing system. This work provides a new strategy for efficient removal of electron-rich moieties under the driving of carbonate being widely present in actual water bodies.

Predicting the Strength of Anion-π Interactions of Substituted Benzenes: the Development of Anion-π Binding Substituent Constants.

A computational study aimed at accurately predicting the strength of the anion-π binding of substituted benzenes is presented. The anion-π binding energies (Ebind) of 37 substituted benzenes and the parent benzene, with chloride or bromide were investigated at the MP2(full)/6-311++G** level of theory. In addition, energy decomposition analysis was performed on 27 selected chloride-arene complexes via symmetry adapted perturbation theory (SAPT), using the SAPT2+ approach. Initial efforts aimed to correlate the anion-π Ebind values with the sum of the Hammett constants σp (Σσp) or σm (Σσm), as done by others. This proved a decent approach for predicting the binding strength of aromatics with electron-withdrawing substituents. For the Cl--substituted benzene Ebind values, the correlation with the Σσp and Σσm values of aromatics with electron-withdrawing groups had r2 values of 0.89 and 0.87 respectively. For the Br--substituted benzene Ebind values, the correlation with the Σσp and Σσm values of aromatics with electron-withdrawing groups had r2 values of 0.90 and 0.87. However, adding aromatics with electron-donating substituents to the investigation caused the correlation to deteriorate. For the Cl--substituted benzene complexes the correlation between Ebind values and the Hammett constants had r2 = 0.81 for Σσp and r2 = 0.84 for Σσm. For the Br--substituted benzene complexes, the respective r2 values were 0.71 for Σσp and 0.79 for Σσm. The deterioration in correlation upon consideration of substituted benzenes with electron-donating substituents is due to the anion-π binding energies becoming more attractive regardless of what type of substituent is added to the aromatic. A similar trend has been reported for parallel face-to-face substituted benzene-benzene binding. This is certainly counter to what electrostatic arguments would predict for trends in anion-π binding energies, and this discrepancy is further highlighted by the SAPT2+ calculated electrostatic component energies (Eele). The Eele values for the Cl--substituted benzene anion-π complexes are all more binding than the Eele value for the Cl--benzene complex, with the exception of chloride-1,3,5-trimethylbenzene. Again, this is a similar trend to what has been reported for parallel face-to-face substituted benzene-benzene binding. A discussion on this surprising result is presented. In addition, an improved approach to predicting the relative anion-π binding strength of substituted benzene is developed using the results of the SAPT2+ calculations.

A Theoretical Study of the Relationship between the Electrophilicity ω Index and Hammett Constant σp in [3+2] Cycloaddition Reactions of Aryl Azide/Alkyne Derivatives.

The relationship between the electrophilicity ω index and the Hammett constant σp has been studied for the [2+3] cycloaddition reactions of a series of para-substituted phenyl azides towards para-substituted phenyl alkynes. The electrophilicity ω index—a reactivity density functional theory (DFT) descriptor evaluated at the ground state of the molecules—shows a good linear relationship with the Hammett substituent constants σp. The theoretical scale of reactivity correctly explains the electrophilic activation/deactivation effects promoted by electron-withdrawing and electron-releasing substituents in both azide and alkyne components.

Air-persistent monomeric (amino)(carboxy) radicals derived from cyclic (alkyl)(amino) carbenes.

A series of monomeric (amino)(carboxy) radicals featuring carbonyl substituents with increasing electron-withdrawing properties (3a, phenyl; 3b, 3,5-bis(trifluoromethyl)phenyl; 3c, perfluorophenyl; 3d, heptafluoropropyl; 3e, 2H-pyrroliumyl) were synthesized in two or three steps from stable cyclic (alky)(amino)carbenes (CAACs). Although (amino)(carboxy) radicals had been previously considered as highly air-sensitive, some of these compounds feature half-lives of hours (3d), and even days (3c and 3e) in well-aerated solutions. DFT calculations show that (amino)(carboxy) radicals evolve from C-centered radical to ambidentate C,O-radicals when increasing the electron-withdrawing properties of the carbonyl substituent. This is paralleled with a destabilization of the peroxide resulting from the addition of dioxygen to the radical. This latter reaction is even predicted to be endothermic for substituents with Hammett constant σp > 0.2.

Cationic isonitrile complexes of the CpFe(CO)2 fragment

[CpFe(CO)2(THF)]+BF4− undergoes exchange of the labile THF ligand against isonitriles resulting in a shift of the NC-stretching vibration to higher energies compared to the free isonitriles. Quantum chemical calculations show that this shift is not a real indicator for the electronic character and thus the electronic influence of the substituents at the para-functionalized phenylisonitriles. In contrast, other calculated parameters such as reactions enthalpies, Fe–C distances, charges and CO-stretching frequencies clearly correlate with the Hammett constant σp.

Redox properties of (RC2R')Co2(CO)6-nLn and reactivity of (RC2R')Co2(CO)6-nLn towards PPh3[LPPh3, P(OEt)3

Cyclic voltammetric studies of clusters (C5H5-C2C6 H4-R-p)Co2(CO)6-n Ln[n=0,2; L=PPh3, P(OEt)3] and (RCH2C)2Co2(CO4) (PPh3)2 on Pt electrode are described. The primary reduction (0 / −1) and oxidation (+ 1 / 0) steps are considered as a mono-electron process for all clusters. For the clusters (C5H5C2C6H4-R-p)Co2(CO)6, a good linear relation between reduction potential Epred and Hammett constant σp of R in the clusters is found. For the clusters (RC2R')Co2(CO)4L2, their radical anions are extremely unstable at room temperature and fragment into a series of mononuclear species, one of which is (RC2R')Co(CO)2PPh3. The reaction of radical anions of (RC2R')Co2(CO)6–n (PPh3)n(n=0,2) with PPh3 also produces mononuclear species (RC2R')Co(CO)2PPh3 which has been detected by means of cyclic voltammetry and ESR. The influence of R on redox properties of clusters is discussed.

Insight into Markovnikov regioselectivity rule via molecular face and ABEEM-σπ theory

Electrophilic additions of HCl to a series of asymmetric alkenes in the gas phase are investigated by the Molecular Face (MF) theory and ABEEM-σπ model. The interesting features of regioselectivity for these reactions are characterized by the electron density (ED) encoded on the MF of alkenes and charge distribution of alkenes obtained via the ABEEM-σπ model, respectively. It is then demonstrated that for a series of alkenes, the Hammett constant σp (substituent constant) has a good linear correlation with KED, where KED is character of the ED at the π region in the initial state of alkenes. Comparison between investigations using MF, ABEEM-σπ, molecular electrostatic potential, and DFT theories, in essence, give similar conclusions for explaining the regioselectivity of the electrophilic additions to alkenes, although from different points of view. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Influence of the Change in Charges on the Atoms of the CNO2Fragment of Nitrobenzene Derivatives Caused by Substituents on the Frequency of the Antisymmetric Stretching Vibration of the Nitro Group

The dependence of the frequency of the antisymmetric stretching vibration of the nitro group of the para-derivatives of nitrobenzene on the charges on the atoms of the CNO2 group calculated by the PM3 method has been investigated. It is shown that a change in the position of the absorption band of the antisymmetric stretching vibration of the nitro group of para-derivatives of nitrobenzene, as well as Hammett's constants σ p , can be used as a measure of the electronic effect of substituents, that is, of a change in the charges on the atoms of the nitro group.

Functional analogue reaction systems of the DMSO reductase isoenzyme family: probable mechanism of S-oxide reduction in oxo transfer reactions mediated by bis(dithiolene)-tungsten(IV,VI) complexes.

The recent development of structural and functional analogues of the DMSO reductase family of isoenzymes allows mechanistic examination of the minimal oxygen atom transfer paradigm M(IV) + QO M(VI) O + Q with the biological metals M = Mo and W. Systematic variation of the electronic environment at the WIV center of desoxo bis(dithiolene) complexes is enabled by introduction of para-substituted phenyl groups in the equatorial (eq) dithiolene ligand and the axial (ax) phenolate ligand. The compounds [W(CO)2(S2C2(C6H4-p-X)2)2] (54-60%) have been prepared by ligand transfer from [Ni(S2C2(C6H4-p-X)2)2] to [W(CO)3(MeCN)3]. A series of 25 complexes [W(IV)(OC6H4-p-X')(S2C2(C6H4-p-X)2)2]1- ([X4,X'], X = Br, F, H, Me, OMe; X' = CN, Br, H, Me, NH2; 41-53%) has been obtained by ligand substitution of five dicarbonyl complexes with five phenolate ligands. Linear free energy relationships between E1/2 and Hammett constant p for the electron-transfer series [Ni(S2C2(C6H4-p-X)2)2]0,1-,2- and [W(CO)2(S2C2(C6H4-p-X)2)2]0,1-,2- demonstrate a substituent influence on electron density distribution at the metal center. The reactions [WIV(OC6H4-p-X')(S2C2(C6H4-p-X)2)2]1- + (CH2)4SO [W(VI)O(OC6H4-p-X')(S2C2(C6H4-p-X)2)2]1- + (CH2)4S with constant substrate are second order with large negative activation entropies indicative of an associative transition state. Rate constants at 298 K adhere to the Hammett equations log(k([X4,X']/k[X4,H]) = rho(ax)sigma(p) and log(k[X4,X']/k([H4,X']) = 4rho(eq)sigma(p). Electron-withdrawing groups (EWG) and electron-donating groups (EDG) have opposite effects on the rate such that k(EWG) > k(EDG). The effects of X' on reactivity are found to be approximately 5 times greater than that of X (rho(ax) = 2.1, rho(eq) = 0.44) in the Hammett equation. Using these and other findings, a stepwise oxo transfer reaction pathway is proposed in which an early transition state, of primary W(IV)-O(substrate) bond-making character, is rate-limiting. This is followed by a six-coordinate substrate complex and a second transition state proposed to involve atom and electron transfer leading to the development of the W(VI)=O group. This work is the most detailed mechanistic investigation of oxo transfer mediated by a biological metal.

13C chemical shifts in the aryl complexes PdX(p-RC6H4)(TMEDA). Is the PdX(TMEDA) fragment a substantial π-donor?

A series of aryl complexes of the form PdX(p-RC6H4)(TMEDA)(1, R=OMe, Me, H, CO2Me, CN, NO2, TMEDA=tetramethylethylenediamine, X=Br, I) were prepared, together with the derivatives PdI(C6F5)(TMEDA) and PdI(2-Me-C6H4)(TMEDA). Their 13C NMR spectra were recorded and showed that (i) the ipso carbon, C-1, with δ=107.6–162.0, is always shifted to higher frequencies, relative to the model organic compound HC6H4R, (ii) C-1 is markedly affected by the nature of the p-R substituent, with electron-withdrawing groups producing high-frequency shifts, (iii) there is a linear correlation between δC-1 and the Hammett constant σp+ with a correlation coefficient R=0.998 and (iv) there is no compelling evidence for a large π−donor effect from palladium into the aryl ring. Screening constants for C-1 in a series of phenyllithium compounds, p-RC6H4Li, were calculated. There is an excellent correlation (R=0.972) of ΔδC-1 for 1 with ΔσC-1 for p-RC6H4Li. Copyright © 1999 John Wiley & Sons, Ltd.

Thermal Isomerization of mer-[RhH(SAr)(SiHAr′2)(PMe3)3] to fac-[RhH2{SiAr′2(SAr)}(PMe3)3] Involving Thiolato Group Transfer from Rh to Si

Abstract Heating of a benzene or toluene solution of mer-[RhH(SAr)(SiHAr′2)(PMe3)3] (Ar = C6H5, C6H4Me-p, C6H4OMe-p; Ar′ = C6H5, C6H4Me-p, C6H4F-p, C6H4CF3-p) at 30—50 °C gives fac-[RhH2{SiAr′2(SAr)}(PMe3)3] quantitatively through thiolato group transfer from Rh to Si. X-Ray structural analysis of fac-[RhH2{SiPh2(SPh)}(PMe3)3] reveals octahedral coordination with three PMe3 ligands at facial coordination sites, while the 1H NMR spectra show the presence of two equivalent hydrido ligands. The other Rh complexes are isolated or characterized in situ by means of NMR spectroscopy. The reaction of mer-[RhH(SC6H4Me-p)(SiHPh2)(PMe3)3] to form the Rh complex obeys first-order kinetics with the kinetic parameters: ΔG‡ = 98.6 kJ mol−1, ΔH‡ = 94.9 kJ mol−1, and ΔS‡ = −12 J mol−1 deg−1 at 298 K. The rate constants of the reaction of mer-[RhH(SC6H4X-p)(SiHPh2)(PMe3)3] (X = H, Me, OMe) increase with increasing Hammett constant σp of substituent X. The substituent of the diarylsilyl ligand of mer-[RhH(SC6H4X-p){SiH(C6H4Y-p)2}(PMe3)3] (X = Me or OMe; Y = H, Me, F, CF3) also influences the reaction rate constant, while the degree of enhancement is not directly related to the electron donating or withdrawing ability of substituent Y. Possible reaction mechanisms are discussed based on these results.

Three-dimensional delocalization in tris(1,2-dithiolenes)

Tungsten tris(1,2-dithiolene) complexes [W(S 2 C 2 R 1 R 6 )(S 2 C 2 R 2 R 5 )(S 2 C 2 R 3 R 4 )] (R 4,5,6 = H or Ph; R 1,2,3 = para-substituted Ph), have been found to exhibit properties ( 1 H NMR, UV/VIS, IR spectral) closely related to each other through Hammett constant σ p + , indicating extensive delocalization and conjugation. A comparative study of complexes containing differently substituted dithiolenic rings showed that the electron delocalization is not confined within each dithiolenic ring separately, but involves all of them, i.e. it is three dimensional. This is further supported by a NMR and/or UV/VIS study of the effect of water and acid on the dimethylamine derivative. The oxidation number was assigned by recording the NMR spectra of the metal nuclei; the metals have effectively been stripped of their valence electrons; which are placed on the ligands. The non-planar ‘aromaticity' of the dithiolenes differs from the familiar flat organic aromaticity since it involves mixing of σ and π bonding.

FT-IR and 13C-NMR Correlations for Some N-Substituted Azoles and Benzazoles

The FT-IR (in carbon tetrachloride) and 13C-NMR (in deuterochloroform) spectra of six p-substituted aniline derivatives, as reference compounds, and nine 1- p-aminophenyl-azoles and benzazoles were recorded. The measured spectral parameters from FT-IR were symmetric, vs(NH2), and antisymmetric, vas(NH2), stretching fundamentals and their band intensity ratio, A( vas)/ A( vs), while the corresponding 13C-NMR parameters were C-1 to C-4 chemical shifts of 1- p-aminophenyl carbons. Different IR and 13C-NMR spectral parameters and Hammett constants σp (or σp-) were correlated, and statistic analysis of the results permitted an estimation of σp values of the nine (azol-1-yl) substituents. An empirical equation to calculate the Hammett constants of new azoles and benzazoles is proposed.

Substituent effects on the tautomer ratios between the hydra‐zone imine and diazenyl enamine forms in 3‐(arylhydra‐zono)methyl‐2‐oxo‐1,2‐dihydroquinoxalines. Correlation of the hammett constants σp with the tautomeric equilibrium constants KT

AbstractThe 3‐(arylhydrazono)methyl‐2‐oxo‐1,2‐dihydroquinoxalines 1a‐e and 2a‐i showed tautomeric equilibria between the hydrazone imine A and diazenyl enamine B forms in dimethyl sulfoxide media. The sub‐stituent effects on the tautomer ratios of A to B in compounds 1a‐e and 2a‐i were studied by the nmr spec‐troscopy. The electron‐donating or electron‐withdrawing p‐substituents R1 in compounds 2a‐i represented a tendency to increase the ratios of the tautomer A or the tautomer B, respectively, exhibiting the linear correlation of the Hammett constants σp (‐0.17 to +0.78) with the tautomer ratios of A to B or the tautomeric equilibrium constants KT. However, the presence of the ester group R2 in compounds 1a‐e induced the exclusive existence of the tautomer A regardless of the nature of the p‐substituents R1. In the tautomeric thermodynamic study, the elevating temperature increased the ratios of the hydrazone imine tautomer A in compounds 2a‐i. The tautomeric thermodynamic parameters ΔG°, ΔH° and ΔS° were derived from the van't Hoff plots for compounds 2a,b,h,i, wherein the entropy term dominated the free‐energy difference between the A and B tautomers.

Molecular structure and crystal packing of five 4-aminophenyl (4-substituted phenyl) sulfones. Correlations between structural distortions, spectroscopic parameters and electronic substituent effects

The crystal structures of five 4-aminophenyl 4-X-phenyl sulfones (X = NO2, CN, F, OCH3, NH2) are reported. The molecules present a relevant degree of charge redistribution where electronic charge transfer mainly occurs from the amino to the sulfonyl group with consequent shortening of C–N and C–S distances and increase of the quinoid character of the phenylene group. The p-aminophenylsulfonyl moiety is characterized by a substantial geometrical constancy in spite of the fact that the p-X-phenyl group is able to modulate the properties of the whole molecule as shown by the strict intercorrelation between the Hammett constant σp of the substituent X on one side and the proton NMR chemical shift of the p-amino group on the other side. The electronic effects of X are transmitted without increasing the quinoid character of the attached phenylene, and change only its S–C distance. This suggests an inductive charge transfer, without resonance components, from X to the SO2 sulfur, whose partial charge can modulate the properties of the terminal aminic group through the almost constant π-delocalization of the p-aminophenylsulfonyl moiety. A detailed analysis of the hydrogen bonds present in the packing of the five crystals has been carried out. It is shown that the hydrogen bond scheme is mainly controlled by p-aminophenylsulfonyl moieties forming linear chains or bidimensional networks of π-resonant hydrogen bonds linking the molecules by means of N–H ⋯ OS interactions. The N ⋯ O distances and the IR frequencies show that these hydrogen bonds are rather weak and similar to those found in p-nitroaniline derivatives.

The Effect of ω-Substituted Acceptors in 4-Dimethylamino-trans-Styrenes on the Polarity in the Ground and Excited Singlet State

ABSTRACT The electric dipole moment in the ground (μg) and in the first excited singlet state (μe) of co-substituted acceptors in 4-dimethylamino-trans-styrenes (Z = P(S)Ph2 , P(O)Ph2 , SO2CH3 ) were determined by solvatochromic and thermochromic methods. The obtained values of μe and μg and the values for Z = CN and Z = NO 2 known from the literature [2] fulfill the linear relation between the dipole moments (μe or μg and the Hammett constants σp of the substituents. On pincreasing the electron-acceptor power of Z, μe grows faster than μg

Hammett‐Beziehungen zwischen 1H‐NMR‐ bzw. 31P‐NMR‐chemischen Verschiebungen und Substituenten‐konstanten in ω‐Diphenylphosphinyl‐und ω‐Diphenylthiophosphinyl‐trans‐styrenen

Hammett Relationships between 1H‐NMR and 31P‐NMR Chemical Shifts, Respectively, and Substituent Constants in ω‐Diphenylphosphinyl and ω‐Diphenylthiophosphinyl trans‐StyrenesA well linear correlation between the chemical shifts of the ethylenic protons HB and the 31P nuclei, respectively, and the Hammett constants σp of substituents R in 4‐position for diphenylphosphinyl trans‐styrenes 1 and the analogous diphenylthiophosphinyl trans‐styrenes 2 has been found. A similar relationship for the proton HA has been observed in the case of 1 only, but the substituent‐induced shifts are not so sensitive. Donor substituents R in 1 and 2 effect an increase in the shielding of HB and a deshielding of the P atom being connected with shifts of the i. r. PO absorption bands in 1 in the direction of lower wavenumbers. A mesomeric interaction between R and PY substituents (Y = O, S) is concluded to produce polymethin‐like π‐electron density distribution.

A re-evaluation of the hammett ? p values for the hydroxymethyl and formyl groups

The Hammett constant σp for the hydroxymethyl group is estimated as –0.04 ± 0.03, and that for the formyl group is re-evaluated as +0.57 ± 0.07.