Hammett Substituent Constants Research Articles

Kinetics and Mechanism of Thermal Decomposition of 1-(Dinitromethyl)-3-R-1H-1,2,4-triazoles

Kinetic regularities and the mechanism of thermal decomposition of 3-substituted 1-(dinitromethyl)1H-1,2,4-triazoles in the solid phase and in dibutyl phthalate solution have been elucidated. The activation parameters have been determined, and the correlation dependencies between the logarithm of the rate constant of the limiting stage and the Hammett constant of substituents as well as the acid pKa value have been revealed.

Reaction kinetics investigation of Malononitrile with substituted benzaldehydes in aqueous solutions of ethaline as deep eutectic solvent

AbstractDue to increasing demands for ecofriendly processes within the framework of green chemistry, and having shown substantial properties, especially in terms of toxicity, biodegradability, cost, and ease of preparation under ambient conditions, deep eutectic solvents (DESs) have become a suitable candidate as green solvents for reaction media in the past decade. In this work, condensation reaction of some para‐ and meta‐substituted benzaldehydes and Malononitrile was kinetically investigated in ethylene glycol‐choline chloride (ethaline) as a DES mixed with water, in the whole range of mole fractions, at room temperature. The correlation of second‐order rate coefficients based on Hammett's substituent constants yield a linear straight line with positive slope in different mole fractions of ethaline–water mixtures. The measured second‐order rate coefficients of the reaction showed a dramatic variation in ethaline–water mixtures with the increasing mole fraction of water.

The electrochemical properties and PIM1 kinase enzyme inhibition of some 2-(hydroxy phenyl amino) naphthalene-1,4-dione derivatives

Abstract In the present work, the electrochemical properties of some 2-(hydroxy phenyl amino) naphthalene-1,4-dione derivatives (HPAN-X), and the effects of π-stacking interactions on the mentioned properties were estimated using the quantum mechanical calculations. All substituents enhance the π-stacking interactions in the HPAN-X⋯C6H6 complexes, where changes are larger by electron-withdrawing groups. The π-stacking interactions often decrease the reduction potentials (E0red), the energy gap between natural frontier molecular orbitals LUMO and HOMO, and also decrease the electron affinities (EA), vertical (VEA), adiabatic (AEA), and zero point-corrected electron affinities (EAZPE) of compounds. The parameters of linear relationships between EA-AEA, EA-VEA, AEA-VEA, and EA-EAZPE pairs are also presented for compounds and complexes. Rational relationships are observed between the values of E0red and Hammett constants of substituents. The effects of π-stacking interactions on electrochemical properties are discussed on the base of results of the atoms in molecules (AIM) and natural bond orbital (NBO) analyses. In addition, the molecular docking were used to study the modes of interaction of the HPAN-X with PIM1 kinase active site. Using both softwares, the docking results present 2HPAN-NH2 as the best inhibitor, where the π- staking interaction with Phe187 amino acid residue substantially affects the ability of compound to inhibit the active site of PIM1 kinase.

Binding Constants of Substituted Benzoic Acids with Bovine Serum Albumin.

Experimental data on the affinity of various substances to albumin are essential for the development of empirical models to predict plasma binding of drug candidates. Binding of 24 substituted benzoic acid anions to bovine serum albumin was studied using spectrofluorimetric titration. The equilibrium constants of binding at 298 K were determined according to 1:1 complex formation model. The relationships between the ligand structure and albumin affinity are analyzed. The binding constant values for m- and p-monosubstituted acids show a good correlation with the Hammett constants of substituents. Two- and three-parameter quantitative structure–activity relationship (QSAR) models with theoretical molecular descriptors are able to satisfactorily describe the obtained values for the whole set of acids. It is shown that the electron-density distribution in the aromatic ring exerts crucial influence on the albumin affinity.

Charge transfer and nonlinear optical properties of anthraquinone D-π-A dyes in relation with the DFT based molecular descriptors and perturbational potential

The static and dynamic NLO properties of Anthraquinone based (D-π-A) dyes are determined by using the Density Functional Theory in ten different functionals. Five global hybrids and five long-range corrected hybrid functionals used based on Hartree-fork exchange contribution. The investigation of vertical excitation is also performed in the equivalent set of functionals. The elaborate study of molecular configuration i.e. intramolecular charge transfer character is analyzed by Frontier Molecular Orbital study, bond length alternation, bond order alternation, molecular electrostatic potential plot, quinoidal character (δr), and electrophilicity index (ω). Benchmarking studies are initiated with the standard deviation method and, the functionals BHHLYP and ωB97 resulted as the suitable functionals for determining the NLO properties of anthraquinone based dyes. The less studied reactivity descriptor hyper-hardness (Γ) and perturbation potential (Vo) is also explored and correlated with the NLO properties. The substitution effect on NLO properties is further confirmed by Hammett constant and excited state Hammett substituent constant.

Synthesis, Structural and Physicochemical Properties of Water-Soluble Mixed-Ligand Diruthenium Complexes Containing Anilinopyridinate Bridging Ligands.

A series of water-soluble Ru25+ complexes of the type Ru2(O2CCH3)3(L)Cl where L = 2,3,4,5,6-F5ap, 2,4,6-F3ap, 2-Fap, ap, 2-Meap, 2,6-Me2ap, or 2,4,6-Me3ap, where ap is the anilinopyridinate anion, have been characterized as to their structural and physicochemical properties in H2O and DMSO. Five of the newly synthesized complexes were structurally characterized, and the Ru-Cl bond lengths range from 2.477 to 2.544 Å while the Ru-Ru bond lengths range from 2.2838 to 2.2935 Å. The UV-vis spectra of each compound are characterized by three absorption bands in both H2O and DMSO, the intensity and position of which vary with both the type of bridging ligand and the solvent. The seven examined Ru25+ complexes exist as 1:1 electrolytes in water, and each undergoes a reversible one-electron reduction assigned to Ru25+/Ru24+ in both investigated solvents. A second irreversible reduction attributed to Ru24+/Ru23+ is also observed for each compound at more negative potentials in DMSO. A linear free energy relationship exists between the sum of the Hammett substituent constants (Σσ) on the ap-type bridging ligand and the wavenumber of an absorption band for the Ru25+ complexes. A linear relationship is also seen between Σσ and measured E1/2 values for the Ru25+/Ru24+ process in water containing 0.1 M KCl, but little to no effect is seen between the specific bridging ligand and the structural features of the investigated compounds.

Absorption and fluorescence spectral properties of azo dyes based on 3-amido-6-hydroxy-4-methyl-2-pyridone: Solvent and substituent effects

A series of nine 5-(4-substituted phenylazo)-3-amido-6-hydroxy-4-methyl-2-pyridones were synthesized and characterized by FT–IR, 1H and 13C NMR, UV–Vis, and PL spectroscopy. Photophysical properties of the dyes were examined in solvents of various polarities and at different pH values. The solvent effects on the absorbance and emission spectral shift were analyzed using Lippert–Mataga, Reichardt–Dimroth and Kamlet-Taft equations. Moreover, UV–Vis absorption and emission frequencies were correlated with Hammett substituent constants applying the linear free energy relationships. DFT calculations of the investigated dyes were accomplished to determine their structural and electronic properties.

In Ukrainian

Search for the “structure-reactivity” correlations of the thermally stimulated reactions of cinnamic acids on the surface of catalysts is important for the development of pyrolytic conversion methods of lignocellulosic biomass components into products with high added value, in particular, into styrene. Therefore, in this work, the kinetics of pyrolysis of the reaction series of para-substituted derivatives of trans-cinnamic acid (Н, -СН 3 , -С(СН 3 ) 3 , -ОСН 3 , -F) on the surface of nanosized silica by the method of thermoprogrammed desorption mass spectrometry (TPD MS) was investigated. The products of pyrolytic reactions on the surface are identified ‒ the corresponding, para-substituted vinylbenzenes, phenylacetylenes, and phenylketenes. The kinetic parameters of the decarboxylation, ketenization, and decarbonylation reactions were calculated. The correlations "structure-reactivity" between the kinetic parameters (activation energy) and thermodynamic parameters (Hammett substituents constants) have been obtained, which indicate that electron-donor substituents reduce the activation energy of these three reactions while the electron-acceptor substituents increase it. That is, in the transitional state of the rate-limiting step, a decrease in the electron density is observed at the reaction center. The calculated values of the reaction constants ρ show that the studied reactions dependent on the sensitivity to the effect of the substituents are placed in a sequence: decarbonylation> decarboxylation> ketenization. The formation of phenylacetylenes is the most sensitive to structural changes in the molecule and proceeds through the most polar transition state.

Electron Push-Pull Effects in 3,9-Bis(p-(R)-diphenylamino)perylene and Constraint on Emission Color Tuning.

A series of perylene-based donor-acceptor-donor (D-A-D) compounds, 3,9-bis(p-(R)-diphenylamino)perylene (R: CN (2a), F (2b), H (2c), Me (2d), and OMe (2e)), was synthesized using 3,9-dibromoperylene with p-(R)-diphenylamine, and the intramolecular charge transfer (ICT) on the D-A-D system with regard to the electron push-pull substituent effect was investigated. By introducing various p-(R)-diphenylamine derivatives with electron-donating or electron-withdrawing R groups, the energy band gaps of the D-A-D compounds were systematically controlled and the emission colors were efficiently tuned from green to red. As expected, the steady state emission spectra of all D-A-D compounds were observed, as well as the emission color controlled, depending on the Hammett substituent constants (σp). In the Lippert-Mataga plots, a different charge-transfer character was observed depending on the electron push-pull substitution, which showed gradually increased ICT characters from the electron-withdrawing to donating substitution. However, exceptionally, the strong electron-withdrawing group of CN-substituted 2a did not correlate with the other R group compounds. From the experimental data and density functional theory calculations, we assume that there is a constraint on emission color tuning to generate higher energy of blue emission in the D-A-D molecular system, due to the reverse charge-transfer property caused by the strong electron-withdrawing group.

Substitution induced tunable emission of an airplane-like pyrene-based fluorophore: First-principles study

The substitution effect on the optical emission properties of an airplane-like pyrene-based fluorophore is systematically investigated based on first-principles calculations. It is found that functional modification at the para position to the donor fragment allows for modulation of both the emission color and quantum efficiency. As the electron negativity of the substituent is increased, the luminescence becomes blue-shifted and brighter. The correlation between the Hammett substituent constant and emission properties has been well established, which provides a predictive strategy for the rational design of efficient and full-color pyrene-based fluorescent emitters.

Structure–property relationship of phenoxyimine ligands/metal chloride initiating systems for controlled cationic polymerizations of alkyl vinyl ethers

ABSTRACTThe catalyst structure–property relationships of the phenoxyimine complexes in controlled cationic polymerization of vinyl ethers were investigated based on the Hammett correlation. The correlation analyses of a series of experiments using the phenoxyimine ligands/TiCl4 initiating systems indicated that the substituents on the N‐aryl phenoxyimine ligands affected the polymerization rate and stereoselectivity. Importantly, a linear correlation was observed between the Hammett substituent constants and the polymerization rates, which indicates that the Lewis acidity of the complex is affected by the electron‐withdrawing and ‐donating effects of the substituents. The tacticity of product polymers correlated to the Hammett substituent constants. Unlike the relationship with the polymerization rates, the σ– values, which account for the enhanced resonance effects, were more appropriate for the relationship with the tacticity than the normal σ values. In contrast, the polymerization behavior using o‐substituted ligands exhibited a trend different from those using p‐ or m‐substituted ligands. The structural change, which was caused by the rotation of the CN bonding, most likely triggered the acceleration effect in the case of the o‐substituents. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 2021–2029

From phenols to quinones: Thermodynamics of radical scavenging activity of para-substituted phenols

Radical scavenging activity and subsequent oxidation resulting in quinone products represent one of the important features of phenols occurring in plants and other biological systems. However, corresponding thermochemistry data can be still considered scarce. For phenol and 25 para-substituted phenols, we investigate the thermodynamics of the individual reaction steps, including three subsequent hydrogen atom transfers, as well as hydroxyl HO radical addition, leading to final ortho-quinone formation. The substituent and solvent effect of water on corresponding reactions enthalpies is elucidated. Solvent enhances substituent induced changes in the investigated reaction enthalpies. The reliability of employed computational methods for the thermodynamics of hydrogen atom donating ability of studied phenols and catechols is assessed, too. Obtained linear equations enable estimation of studied reaction enthalpies from Hammett constants of substituents.

Toward triplet disilavinylidenes: A Hammett electronic survey for substituent effects on singlet‐triplet energy gaps of silylenes by DFT

AbstractTo reach for novel triplet silylenes, para‐ and meta‐R‐1,1‐diphenyldisilavinylidenes with various electron donating and withdrawing groups (EDGs and EWGs, respectively) are compared and contrasted using the B3LYP/6‐311++g** method, where para silylenes consist of 1H, , 3OH, , 5F, 6Cl, , 8CHO, , and , whereas their meta isomers are , 3′OH, , 5′F, 6′Cl, , 8′CHO, , and . The overall trend for triplet tendency of the latter group based on their singlet‐triplet energy gap (ΔEs‐t) is 3′OH > > 1H > > > 8′CHO > 5′F > 6′Cl > > while that for the former, para series is > > 3OH > > 5F > 6Cl > 1H > > 8CHO > . Plotting disilavinylidenes ΔEs‐t, against their corresponding Hammett substituent constants (σp or σm) shows rather significant ρ factors of −2.07 and −1.42 with good correlations of R2 = 0.98 and 0.63, respectively. The negative sign of the ρ indicates that the more EDGs have higher impacts on stabilizing or reaching for the triplet ground states, displaying and as two silylenes with triplet ground states. The EWGs increase electrophilicity (ω) making (7.62 eV) the most electrophilic. In contrast, EDGs increase nucleophilicity (N) showing (5.03 eV) as the most nucleophilic silylene. Moreover, shows the highest proton affinity (PA = 388.13 kcal/mol), chemical potential (μ = −3.67 eV), energy of HOMO (EHOMO = −4.46 eV), dipole moment (5.44 debye), and the lowest band gap (ΔEH‐L = −1.59 eV), and electrophilicity (ω = 4.24 eV), with the lowest NBO charge on divalent Si atom (+0.129) among the scrutinized species.

Substituent Effect Transmission Power of Alkyl, Alkenyl, Alkynyl, Phenyl, Thiophenyl, and Polyacene Spacers.

The transmission of substituent effect through a variety of spacers, that is to say, alkyl, alkenyl, alkynyl, phenyl, thiophenyl, and polyacene has been studied by modeling Y-G-X type molecular systems (Y: reaction center; G: spacer moiety; X: substituent) using B3LYP/6-31G(d,p) density functional theory calculations. The reaction center is always kept as a C=C double bond and the molecular electrostatic potential (MESP) minimum (Vmin ) observed for this bond showed subtle variation with respect to the changes in the spacer unit and the nature of substituent. Strong linear correlations are observed between Hammett substituent constants (σI and σp ) and Vmin , which recommend the aptness of Vmin as an electronic descriptor to quantify the substituent effect. Since Vmin offers an alternative measure of substituent effect, the correlation between Vmin and σp has been used for assessing the transmission of substituent effect through a variety of spacer moieties. The highest transmission coefficient (γ) is always observed for smaller spacer length. Among all the spacers, alkenyl showed the highest and alkyl showed the lowest transmission power. The study recommends the use of short chains of C=C double, C≡C triple or a combination of both as spacers for the effective transmission of substituent effect to the reaction center.

Substituent Effect on the Properties of pH Fluorescence Probes Containing Pyridine Group

AbstractA series of 4–styrylpyridine (SYP) pH fluorescence probes with different substituents (–NH2, –OH, –H, –Br, –NO2) on para–benzene ring were developed. These compounds exhibited good pH–dependent performance that their spectra red–shifted upon protonation of pyridine N–atom and blue–shifted upon deprotonation. These sensors utilized the substituent on the para–benzene ring as electron donor and worked by intramolecular charge transfer (ICT) under acidic pH conditions. The pH detection range and pKa value of them were tuned by the electron–donor ability of substituents. The pKa had a significant linear correlation with the Hammett constant (σp) of substituent, the pKa was larger when the Hammett constant of substituent was larger.

Molecular structure and electronic properties of substituted tetrabenzocoronenes: DFT and TD‐DFT investigations

Abstract1,2:3,4:7,8:9,10‐Tetrabenzocoronene (TBC) is a highly π‐conjugated polycyclic aromatic hydrocarbon (PAH) with an intriguing nonplanar, twisted molecular shape featuring two [4]helicene subunits on the opposite sides of the molecule. In this study, we examined the electronic substituent effects on the molecular properties of a series of TBC derivatives by density functional theory (DFT) and time‐dependent DFT (TD‐DFT) computational analysis at the B3LYP/6‐31G(d) and TD‐B3LYP/6‐311 + G(d,p) levels. The equilibrium geometries for the stable conformers of the TBC derivatives in the ground singlet state (So) and related transition states were optimized to obtain the energy barriers associated with the interconversion among these conformers. Variations of aromatic character for these compounds were evaluated by nucleus‐independent chemical shift (NICS) calculations, while the frontier molecular orbital (FMO) energies and the vertical electronic absorption and emission energies were calculated. The calculated absorption and emission spectra of these TBC derivatives show good correlations with the Hammett substituent constant (σ). Overall, our study has demonstrated that the attachment of electron‐donating/withdrawing substituents to the edges of TBC can exert significant effects on the structural, electronic, and photophysical properties. Understanding of the substitution effects on TBC provides valuable insight for the design and development of novel PAH‐based organic electronic materials and devices.

Dependence of tautomerism on substituent type in o-hydroxy Schiff bases

Quantum computational methods were used to elucidate the structures of the o-hydroxy Schiff bases with different substituents. It is possible for a Schiff base to have different tautomeric structures depending on intramolecular proton transfer from the phenolic oxygen atom to the nitrogen atom. Proton transfer results in two tautomeric structures known as the phenol-imine and keto-amine forms. To explain the substituent effect on the proton transfer process in five o-hydroxy-Schiff bases, possible geometric structures in gas phase were optimized using density functional theory (DFT) at the B3LYP/6-311G(d,p) level. To describe tautomerism including intramolecular proton transfer, potential energy surface (PES) scans were performed starting from the optimized geometry of the phenol-imine form. HOMA indices were calculated in order to estimate p-electron delocalization. In addition, the substituent effect on the tautomerization rate was examined using Hammett substituent constants and calculating the activation energies.

Substituent effect study on the experimental 13C NMR chemical shifts of 3-(substituted phenyl)-3a,4,8,8a-tetrahydro-1,3-dioxepino[5,6-d] [1,2] isoxazoles

Novel heterocyclic derivatives containing isoxazole ring were synthesized by the 1,3-dipolar cycloaddition reaction of substituted nitrile oxides with cis-4,7-dihydro-1,3-dioxepin in the present study. These 3-(substituted phenyl)-3a,4,8,8a-tetrahydro-1,3-dioxepino[5,6-d] [1,2] isoxazole derivatives were characterized by their physical constants and IR, 1H NMR, 13C NMR and HRMS data. 13C NMR spectra of studied molecules were measured in Deuterochloroform (CDCl3). The correlation analysis for the substituent-induced experimental 13C NMR chemical shifts (δ, ppm) (SSC) of C=N, C4, C5, C7, C9 and C10 carbons of these isoxazole derivatives have been correlated with various Hammett substituent constants, and Swain-Lupton parameters using single (SSP) and multi-linear (DSP) regression analysis. Negative ρ values were found for correlations of C=N, C4 and C5 carbons. The other carbons were found to have positive ρ values for the electronic effect of substituent on 13C NMR chemical shifts.

Deep eutectic solvent as an operative media on structure‐reactivity relationships

AbstractDeep eutectic solvents seem to be environmentally friendly solvents, particularly because they are prepared easily and have very low‐vapor pressures under ambient conditions. They are suitable candidates as green solvents for reaction media with special properties. To present this behavior, substitution reactions of some para‐ and meta‐substituted anilines with 1‐fluoro‐2,4‐dinitrobenzene have been spectrophotometrically investigated in varying mole fractions of ethaline as a deep eutectic solvent in dimethyl sulfoxide (DMSO). The measured rate coefficients of the reaction demonstrated a noticeable variation with the increasing mole fraction of ethaline in ethaline‐DMSO mixtures. The linear free energy relationship (LFER) of second‐order rate coefficients based on Hammett's substituent constants demonstrates a reasonably linear straight line with a negative slope in different mole fractions of ethaline‐DMSO mixtures. Another LFER investigation based on the polarity parameters of the media showed a good agreement with hydrogen bond donor and acceptor abilities of the solvent. Non‐LFER assay according to the preferential solvation model confirmed differences between the microsphere solvation of the solute molecules and the bulk composition of the solvents.

Syntheses of Chalcone-Type Chlorophyll Derivatives Possessing a Bacteriochlorin, Chlorin or Porphyrin π-System and Their Optical Properties.

C3-(Trans-2-arylethenyl)carbonylated chlorophyll derivatives possessing a bacteriochlorin or chlorin π-system were synthesized by cross-aldol (Claisen-Schmidt) condensation of methyl pyrobacteriopheophorbide-a or 3-acetyl-3-devinyl-pyropheophorbide-a bearing the C3-acetyl group with p-(un)substituted benzaldehydes under basic conditions. The corresponding porphyrin-type chlorophyll derivatives were prepared by the oxidation (17,18-didehydrogenation) of the chlorin-type. Their Qy absorption and fluorescence emission maxima in dichloromethane correlated well with Hammett substituent constants of the p-substituents. Several electron-withdrawing p-substituents suppressed the emission due to photoinduced electron transfer quenching in a molecule. The substitution sensitivities for their maxima and fluorescence quantum yields decreased in the order of bacteriochlorin-, chlorin- and porphyrin-type derivatives.