Hammett Constants Research Articles

MIL-101(Cr)-NO2 as efficient catalyst for the aerobic oxidation of thiophenols and the oxidative desulfurization of dibenzothiophenes

A series of MIL-101(Cr)-X functionalized with electron withdrawing (NO2, SO3H or Cl) or electron donor (NH2 or CH3) groups has been tested for the solvent-free oxidative coupling of thiophenol to disulfides. No byproducts were observed. A relationship between the catalytic activity of these MOFs with the substituent meta Hammet constant on the terephthalate ligand and with their redox potential was found, MIL-101(Cr)-NO2 being the most active catalyst. NO2-substituted MIL-101 is also more efficient than the parent MIL-101(Cr) to promote the aerobic desulfurization of dibenzothiophenes in n-dodecane or commercial Diesel as solvent. No byproduct formation was observed. Mechanistic studies reveal that MIL-101(Cr)-NO2 is acting as heterogeneous catalyst in thiophenol oxidation and as radical initiator for the aerobic desulfurization. For both reactions, the catalyst can be reused without deactivation, maintaining its crystallinity and with negligible metal leaching.

Substituent Effect on the Acidity Strength of para-C6H4XB(OH)2 Boronic Acid: A Theoretical Investigation

The para-substituent effect on the acidity strength of boronic acid (para-C6H4XB(OH)2; X = NMe2, NH2, OMe, H, F, Cl, CHO, COOH, NO2) is explored at the B3LYP*/6-31+G(d, p) level of theory in an aqueous solution. The conductor-like polarized continuum model (CPCM) is used for the calculation in the aqueous phase. The CPCM calculations are performed with the SMD-Coulomb atomic radii. Then, the pKa values corresponding to these molecules are calculated in an aqueous solution. Atomic charges of the acidic proton are calculated with QTAIM, APT, and Mulliken methods. A linear correlation between the calculated pKa values with the Hammett constant, deprotonating energy, and atomic charges of acidic proton are analyzed.

Heterogeneous Oxidation of Phenolic Compounds with Photosensitizing Catalysts Incorporated into Chitosan

The increasing amount of hazardous micropollutants in the aqueous environment has recently become a concern, especially because they are not usually included in environmental monitoring programs. There is also limited knowledge regarding their behavior in the environment and their toxicity. This paper presents results regarding the heterogeneous photosensitized oxidation of 10 phenolic compounds under visible light. All of the selected compounds are classified as pollutants of emerging concern. For the first time, the application of photosensitizing catalysts incorporated into a chitosan carrier was investigated from several points of view, namely, structure characterization, singlet oxygen generation potential, photodegradation ability, biodegradability, and toxicity assessment. It was found that compounds of different origins were degraded with high effectivity. Photoactive chitosan was stable and could be reused for at least 12 cycles without losing its photocatalytic activity. The Hammett constants for all of the degraded compounds were determined. Improved biodegradability after the treatment was achieved for almost all compounds, apart from 4-hydroxybenzoic acid, and only slightly for 2-phenylphenol. The acute toxicity was assessed using bioluminescent Vibrio fischeri bacteria, indicating lower toxicity than the parent compounds.

Donor Strengths Determination of Pnictogen and Chalcogen Ligands by the Huynh Electronic Parameter and Its Correlation to Sigma Hammett Constants.

The suitability and accuracy of the Huynh electronic parameter (HEP) was further tested to reveal remote substituent effects in pyridines, which are located five or six bonds away from the reporter probe. These values show an excellent correlation to Hammett σ‐constants of the respective substituents with coefficients of R 2=0.9856 (σ m) and R 2=0.9857 (σ p). Based on this observation, a methodology for the re‐evaluation of certain Hammett constants with larger uncertainties has been proposed and demonstrated. Moreover, the scope of HEP was extended to various neutral pnictogen and chalcogen donors during which “transphobia effects” were revealed for mixed NHC complexes containing phosphites, arsine and stibine for the first time.

Critical Hammett Electron-Donating Ability of Substituent Groups for Efficient Water Oxidation Catalysis by Mononuclear Ruthenium Aquo Complexes.

[Ru(Rtpy)(bpy)(H2O)]2+ (1R; bpy = 2,2'-bipyridine, and Rtpy = 2,2':6',2″-terpyridine derivatives) complexes with a variety of 4'-substituent groups on Rtpy were synthesized and characterized to reveal the effects of substituents on their structures, physicochemical properties, and catalytic activities for water oxidation. The geometric structures of 1R are not considerably influenced by the electron-donating ability of the 4'-substituent groups on Rtpy. Similar multistep proton-coupled electron transfer reactions were observed for 1R, and the redox potentials for each oxidation step tended to decrease with an increase in the electron-donating ability of the substituent, which is explained by the increased electron density on the Ru center by electron-donating groups, stabilizing the positive charge that builds up upon oxidation. This is consistent with the red-shift of the absorption bands around 480 nm assigned to the metal-to-ligand charge transfer transition for 1R due to the increased d orbital energy level of the Ru center. The turnover frequency (kO2) of 1R for water oxidation catalysis, however, depended greatly on the Rtpy ligands, varying from 0.05 × 10-2 to 44 × 10-2 s-1 (as the highest kO2 was observed for R = ethoxy) by a factor of 880. A critical electron-donating ability of the 4'-substituent groups with a narrow range of Hammett constants (σp = -0.27 to -0.24) found for the highest kO2 values is valuable for understanding the great difficulty in the search for efficient water oxidation catalysts. On another front, the kO2 values increased with a decrease in the redox potentials of RuIV═O/RuV═O for 1R, indicating that the potential of formation of RuV═O species for 1R is crucial for water oxidation catalysis under the employed conditions.

Synthesis and photophysical properties of isocoumarin-based D-π-A systems

We prepared a small library of polarity-sensitive fluorescent dyes characterized by an isocoumarin core properly functionalized with a conjugated push-pull system. The key step of the synthesis is based on a regio-selective silver(I)/p-TSA co-catalyzed cyclization of 2-alkynylbenzoates recently optimized in our laboratory. The photophysical properties of isocoumarin-based D-π-A systems have been investigated and a rationale was proposed based on their dipole moments and Hammett constants of the ED and EW groups involved.

Computational study of substituent effect on the electronic properties of ferrocylidene acetophenones complexes

In this study, the substituent effect on the electronic, spectroscopic properties and thermodynamic parameters of neutral and oxidized states of ferrocylidene acetophenone complexes was investigated by adopting the hybrid meta exchange-correlation functional of M06-2X. The frontier orbitals and the highest occupied molecular orbitals–lowest unoccupied molecular orbitals gaps of the substituted compounds were determined. Ionization potential (IP), electron affinity, and reorganization energy values of these molecules were estimated. The thermodynamic parameters (free energy and enthalpy) of the oxidation reaction of the studied complexes were calculated. Also, a variation on the wavenumber of carbonyl group in both states was revealed. Correlations between the evaluated properties and Hammett's constant were explored.

Multipolar triphenylamines: Effect of spectator donor-acceptor pair on intramolecular charge transfer interactions

Donor (D) - Acceptor (A) compounds based on triphenylamine (TPA) substituted with cyano, nitro, and formyl functional groups were synthesized in dipolar (DA), quadrupolar (DA2), and octupolar (DA3) configurations and characterized using different spectral techniques. Bond-length alternation (BLA) values of the phenyl ring were found to be in the range between 0.012 and 0.022 Å, suggest the charge transfer occurs in the ground state itself, which in fact differs distinctly with respect to number of acceptor groups in the TPA core. Steady-state and time-resolved fluorescence studies indicates the existence of dipolar charge transfer interactions in all the molecules by the virtue of reducing their molecular symmetry. Interestingly, for dipolar molecule, the efficiency of ICT interactions is elucidated by the slope of the Lippert-Mataga plot which exhibits the linear relationship with the Hammett (σ) constant. Controlling the rotation of N–C bond of amino and phenylene moiety by lowering the surrounding temperature drastically affect the fluorescence and reveals that the structural changes at the excited state plays a crucial role in ICT processes. Notably, the mono-nitro substituted triphenylamine exhibits specific solvent-solute interactions exclusively with the chlorinated solvents at the excited state with the emission in NIR region and is tailing over 800 nm.

Theoretical Study of First Singlet Excited State of Para-Substituted Platinabenzene Complexes

The structure, electronic properties, and aromaticity of the para-substituted platinabenzenes were illustrated by applying the hybrid density functional MPW1PW91 theory. The electron donor groups (EDG) and electron withdraw groups (EWG) effects on geometry, frontier orbital energies, reactivity indices and aromaticity in the first singlet excited state of platinabenzene were investigated and compared to ground state. The contribution of the fragments of the studied complexes in the frontier orbitals were calculated both in terms of the ground state and the first singlet excited state. Linear correlations between the studied parameters with Hammett's constants (sp) were given in the two studied states.

Electrochemical treatment of phenol-containing wastewater by facet-tailored TiO2: Efficiency, characteristics and mechanisms

Electrochemical oxidation is widely used for water and wastewater treatment. Anodic material is crucial and the shape-tailored {001}-exposed TiO2 has been proven to be an ideal electrode material for pollutant oxidation. In this work, the electrochemical treatment of wastewater containing typical p-substituted phenols by facet-tailored TiO2 is studied in terms of efficiency, characteristics and mechanisms. Experimental results demonstrate that the anodic oxidation of p-substituted phenols becomes more difficult with the increasing Hammett's constant (σ) of phenols, while their degradation rates (k) increase continuously with the initial surface concentration (Γ). Phenols are degraded mainly by surface-bound ·OH and direct electron transfer on the TiO2/Ti electrode, rather than by bulk-free ·OH suspended in the aqueous phase. Theoretical calculations reveal that the surface-bound ·OH-mediated oxidation mechanism is attributed mainly to the strong surface bond strength between shape-tailored TiO2 and water molecule as well as the reactive ·OH. Such strong interactions are associated with the higher density of atomic steps, edges and kinks of low-coordinate surface atoms with a large number dangling bonds on the high-energy {001} polar facet. For practical treatment of real wastewater with different matrixes, the facet-tailored TiO2/Ti electrode exhibits both a high efficiency and a fast kinetics. Our findings provide a new chance to degrade phenolic pollutants in wastewater and offer atomic-scale insights into the preparation, modification and application of TiO2-based anodic materials for electrochemical water treatment.

Theoretical Study of Substituent Effect on the pKa Values of Cr(CO)3(para-XC6H4COOH) Complexes

In this investigation substituent effect on the pKa values of the Cr(CO)3(para-XC6H4COOH) complexes (X = NH2, OH, H, F, Cl, CN, NO2) was demonstrated at the wB97XD/6-311G(d,p) level of theory through aqueous phase calculation. The conductor-like polarized continuum model (CPCM) was used for calculation in solution phase. The CPCM calculations were accompanied with SMD-Coulomb atomic radii. The linear correlation relationships that can be established between the calculated pKa values with Hammett constants and deprotonating energy were analyzed. Also, the atomic charges of the acidic proton were calculated through QTAIM and NBO methods and their correlations with the obtained pKa values were studied.

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.

Intramolecular hydrogen bonding, π-π stacking interactions, and substituent effects of 8-hydroxyquinoline derivative supermolecular structures: a theoretical study

Quantum chemical calculations have been performed to gauge intramolecular hydrogen bonding and π-π stacking interactions of 8-hydroxyquinoline (8-HQ) derivatives (substituents X = OH, CH3, H, F, Cl, CF3, CN, NO2) supermolecular structures. The effects of substituents on 8-HQ derivatives have also been studied with the M06-2X method and 6-311++G** basis set. The basis set superposition error (BSSE)-corrected binding energies (ΔE), hydrogen bonding distances (rH···N), and intermolecular distance (d┻) were calculated with corresponding Hammett electronic parameters (σ). The topological parameters (ρ, ∇ρ2) and energy of intramolecular hydrogen bonding interaction (EH···N) of these π-π stacking 8-HQ derivatives have been studied by using atoms in molecules (AIM) theory. The electrostatic potential (ESP) analysis has been applied to gain the most negative and the most positive electrostatic potential V(r) values (Vs,min and Vs,max). It can be found that there are good relationships between the Hammett constants and the Vs,min values.

Structural and Energetic Impact of Non-natural 7-Deaza-8-azaguanine, 7-Deaza-8-azaisoguanine, and Their 7-Substituted Derivatives on Hydrogen-Bond Pairing with Cytosine and Isocytosine.

The impact of 7-deaza-8-azaguanine (DAG) and 7-deaza-8-azaisoguanine (DAiG) modifications on the geometry and stability of the G:C Watson-Crick (cWW) base pair and the G:iC and iG:C reverse Watson-Crick (tWW) base pairs has been characterized theoretically. In addition, the effect on the same base pairs of seven C7-substituted DAG and DAiG derivatives, some of which have been previously experimentally characterized, has been investigated. Calculations indicate that all of these modifications have a negligible impact on the geometry of the above base pairs, and that modification of the heterocycle skeleton has a small impact on the base-pair interaction energies. Instead, base-pair interaction energies are dependent on the nature of the C7 substituent. For the 7-substituted DAG-C cWW systems, a linear correlation between the base-pair interaction energy and the Hammett constant of the 7-substituent is found, with higher interaction energies corresponding to more electron-withdrawing substituents. Therefore, the explored modifications are expected to be accommodated in both parallel and antiparallel nucleic acid duplexes without perturbing their geometry, while the strength of a base pair (and duplex) featuring a DAG modification can, in principle, be tuned by incorporating different substituents at the C7 position.

Solvothermal synthesis of novel phenylpyrazole Schiff base fluorescent insecticides fused extended conjugate units for enhancing bioactivities, photophysical and electrochemical properties

In order to find a new type of fluorescent insecticides, a series of phenylpyrazole Schiff base derivatives have been synthesized by a facile one-pot solvothermal method based on the amino group of fipronil addition-elimination condensation reaction with the corresponding benzaldehyde derivatives in high yields, which were fully characterized by 1H NMR, IR, MS and elemental analysis. Crystal structure determination indicated that 1-(2,6-dichloro-4-(trifluoromethyl)phenyl)-5-(4-fluoro-benzylidene-amino)-4-(trifluoromethylsulfinyl)-1H-pyrazole-3-carbonitrile (7) crystallized into zigzag chains by π···π packing interactions between phenyl rings of neighbouring molecules. The investigation for insecticidal bioactivities of these pesticides against 3rd instar larvae of Plutella xylostella exhibited better activities than that of fipronil, which can be elucidated by enhanced calcium ions' coordination abilities with CN double bonds and chloride ions, extended conjugated parent structure, steric hindrance of the stereoscopic structure and electronegativity of substituted groups. The electronic absorption, fluorescent and electrochemical properties of phenylpyrazole Schiff base derivatives have been systematically investigated for the first time, which all emitted blue-green light in both solution and solid state at room temperature. The relationships between extended conjugated moieties of parent structure, positions of substituent group and the kinds of substitutes at 4-position on the phenyl ring and photophysical properties have been discussed, which was confirmed by electrochemical analysis. The alteration of absorption and emission wavelengths can be rationalized by substitutes' electronegativity and Substituents’ Hammett constants. The fluorescent quantum yields of phenylpyrazole Schiff base insecticides were almost 4 times that of Fipronil, which laid a foundation for their natural degradation and fluorescence detection of pesticide residues. This approach proposes a unique insight to provide a great number of novel phenylpyrazole Schiff base fluorescent insecticides by a general green method.

The Catalytic Activities of Carbocyclic Fused Pyridineimine Nickel Complexes Analogues in Ethylene Polymerization by Modeling Study

In this work, two carbocyclic fused pyridineimine nickel analogue systems (Ni1 and Ni2) with different fused member rings were investigated to reveal the relationship between catalyst structure and reaction activity. Multiple linear regression analysis was performed by means of five electronic descriptors and two steric descriptors, including the Hammett constant (F), effective net charge (Qeff), energy difference (ΔE), HOMO–LUMO energy gap (Δε1, Δε2), open cone angle (θ), and bite angle (β). Very good values of correlation coefficient (R2) over 0.938 were obtained by using a combination of effective net charge (Qeff) and open cone angle (θ) for both individual analysis and comparisons between analogue systems. By analyzing the contribution of descriptors, it indicates that the dominant descriptor is effective net charge (Qeff) in the Ni1 system and open cone angle (θ) in Ni2 systems, respectively. This may explain the different variation trends of catalytic activities in two Ni complexes systems as a function of substituents.

Effects of Polar Substituents and Media on the Performance of N,N′‐Di‐tert‐Butoxycarbonyl‐Indigos as Photoswitches

AbstractThe N,N′‐di‐tert‐butoxycarbonylindigo parent compound and four symmetrically substituted derivatives alternatively containing methoxy and nitro substituents in positions 5 and 6 were synthesized. The photochromic behavior was studied in solvents ranging in polarity from toluene to acetonitrile/water mixtures and characterized in terms of trans and cis isomer spectra, conversion quantum yields and thermal half‐lives of the metastable cis isomers. The half‐lives at 21 °C varied from a few seconds (5‐nitro in polar solvents) to several days (6‐methoxy in apolar media) and can be described by the substituents’ Hammett constants. Spectroscopic and photochemical parameters, in contrast, are governed largely by the substituents’ resonance effects according to a scheme well known from thioindigos and these are termed here favorable and unfavorable resonance effects (FRE and URE, respectively). The title photoswitches show little propensity to quenching by protons and may be operated in aqueous media.

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.

One pot solvent-free solid state synthesis, photophysical properties and crystal structure of substituted azole derivatives

A solvent-free solid state method has been developed to synthesize three series of substituted azole derivatives (including 1,3,4-oxadiazole, 1,3,4-thiadiazole and 1,2,4-triazole) in high yields by condensation cyclization of N,N′-bishydrazide derivatives with corresponding cyclization agents, respectively. This general method avoids the use of organic solvents, saves cost and resources, simplifies post-processing and increases reaction rates. These compounds have been fully characterized by 1H NMR, 13C NMR, elemental analysis and MS. The structure of 2,5-di (4-tertbutylphenyl)-1,3,4-oxadiazole has been determined by single crystal X-ray diffraction analysis. The cell parameters are as follows: monoclinic crystal system, Cc space group, a = 18.735 (2) Å, b = 6.3375 (8) Å, c = 16.824 (2) Å and α = γ = 90°; β = 98.292 (2). The electronic absorption and fluorescent properties of these compounds have been systematically investigated for the first time. The relationships between heterocyclic structures and photophysical properties have been discussed. The alteration of emission and absorption wavelengths can be elucidated by hybrid atoms' electronegativity and Substituents' Hammett constants. This approach proposes a novel insight to provide a great number of novel substituted azole derivatives with good photophysical properties by a general green method.

Assessment of substituent effects on the parameters of 35Cl nuclear quadrupole resonance in para‐substituted benzene‐sulphenyl chloride via quantum chemical calculations

In this research, substituent effects on the parameters of 35Cl nuclear quadrupole resonance (NQR) in para‐substituted benzene‐sulphenyl chloride were studied at M062X/6‐311G(d,p) theory level. The 35Cl NQR parameters of the quadrupole coupling constant (QCC) and electric‐field gradient (EFG) tensor, as well as an asymmetric parameter, were shown to be correlated with Hammett constant following their calculations. The frontier orbital energy levels, HOMO‐LUMO gaps, hardness, electrophilicity, and chemical potential values of these molecules were calculated as well. natural bond orbital (NBO) analysis was applied for calculating natural populations at chlorine atoms.