Substituent Constants Research Articles

Strategy for Molecular Design of Photochromic Diarylethenes Having Thermal Functionality.

Thermal reactivities of photochromic diarylethene closed-ring isomers can be controlled by the introduction of substituents at the reactive positions. Diarylethenes having bulky alkyl groups undergo thermal cycloreversion reactions. When bulky alkoxy groups are introduced, the diarylethenes have both thermal cycloreversion reactivities and low photocycloreversion quantum yields. Such photochromic compounds can be applied to thermally reusable photoresponsive-image recordings. The thermal cycloreversion reactivity of the closed-ring isomers can be evaluated using specific steric substituent constants and be correlated with the parameters. By introduction of trimethylsilyl or methoxymethyl groups at the reactive positions, the diarylethene closed-ring isomers undergo thermal irreversible reactions to produce by-products at high temperatures. These diarylethenes may be useful for secret-image recordings. Furthermore, thiophene-S,S-dioxidized diarylethenes having secondary alkyl groups at the reactive positions undergo thermal by-product formation reactions, in addition to the photostability of the colored closed-ring isomers. Such materials may be used for light-starting thermosensors. The thermal by-product formation reactivity can be evaluated by the specific substituent constants and theoretical calculations of quantum chemistry. These results supply the strategy for the molecular design of the photochromic diarylethenes having thermal functionality.

Single‐Electron Transfer in σ‐Complexation Reactions of 2,6‐Dimethoxy‐3,5‐dinitropyridine with Para‐X‐phenoxide Anions in Aqueous Solution

ABSTRACTSecond‐order rate constants have been measured spectrophotometrically for reactions of 2,6‐dimethoxy‐3,5‐dinitropyridine 1 with 4‐X‐substituted phenoxide anions (X = OMe, Me, H, Cl, and CN) 2a–e in aqueous solution at various temperatures. The effect of phenoxide substituents on the reaction rate was examined quantitatively on the basis of kinetic measurements, leading to nonlinear correlations of Δ≠H and Δ≠S with Hammett's substituent constants (σ). Each Hammett plots exhibits two intersecting straight lines for the reactions of 1 with the phenoxide anions 2a–e, whereas the Yukawa–Tsuno plots for the same reactions are linear. The large negative ρ values (−4.03 to −3.80) obtained for the reactions of 1 with the phenoxide anions possessing an electron‐donating group supports the proposal that the reactions proceed through a single‐electron transfer mechanism.

Quantitative structure-activity relationship analysis of substituted arylazo pyridone dyes in photocatalytic system: Experimental and theoretical study

A series of arylazo pyridone dyes was synthesized by changing the type of the substituent group in the diazo moiety, ranging from strong electron-donating to strong electron-withdrawing groups. The structural and electronic properties of the investigated dyes was calculated at the M062X/6-31+G(d,p) level of theory. The observed good linear correlations between atomic charges and Hammett σp constants provided a basis to discuss the transmission of electronic substituent effects through a dye framework. The reactivity of synthesized dyes was tested through their decolorization efficiency in TiO2 photocatalytic system (Degussa P-25). Quantitative structure-activity relationship analysis revealed a strong correlation between reactivity of investigated dyes and Hammett substituent constants. The reaction was facilitated by electron-withdrawing groups, and retarded by electron-donating ones. Quantum mechanical calculations was used in order to describe the mechanism of the photocatalytic oxidation reactions of investigated dyes and interpret their reactivities within the framework of the Density Functional Theory (DFT). According to DFT based reactivity descriptors, i.e. Fukui functions and local softness, the active site moves from azo nitrogen atom linked to benzene ring to pyridone carbon atom linked to azo bond, going from dyes with electron-donating groups to dyes with electron-withdrawing groups.

Effects of single bond-ion and single bond-diradical form on the stretching vibration of C[dbnd]N bridging bond in 4,4′-disubstituted benzylidene anilines

Fifty-seven samples of model compounds, 4,4′-disubstituted benzylidene anilines, p-X-ArCH=NAr-p-Y were synthesized. Their infrared absorption spectra were recorded, and the stretching vibration frequencies νCN of the CN bridging bond were determined. New stretching vibration mode was proposed by means of the analysis of the factors affecting νCN, that is there are mainly three modes in the stretching vibration of CN bond: (I) polar double bond form CN, (II) single bond-ion form C+–N− and (III) single bond-diradical form C–N. The contributions of the forms (I) and (II) to the change of νCN can be quantified by using Hammett substituent constant (including substituent cross-interaction effects between X and Y groups), whereas the contribution of the form (III) can be quantified by employing the excited-state substituent constant. The most contribution of these three forms is the form (III), the next is the form (II), whose contribution difference was discussed with the viewpoint of energy requirements in vibration with the form (III) and form (II).

Synthesis, assessment of substituent effect and antimicrobial activities of (4E)-4-(benzylideneamino)-1,2-dihydro-2,3-dimethyl-1-phenylpyrazol-5-one compounds

A series of substituted (4E)-4-(benzylideneamino)-1,2-dihydro-2,3-dimethyl-1-phenylpyrazol-5-one compounds has been synthesized from 4-aminoantipyrine and substituted benzaldehydes. Their structures have been confirmed by their physical constants, UV, IR and NMR spectral data. The observed UV absorption maximum λmax(nm), IR frequencies νCN(cm−1), NMR δ(ppm) of C–H & CN chemical shift values have been correlated with Hammett substituent constants and F and R parameters using single and multi-linear regression analyses. From the results of statistical analysis, the effect of substituents on the spectral data has been studied. The antimicrobial activities of all the Schiff bases synthesized have been studied using Bauer–Kirby method.

A high‐level theoretical study into the atmospheric phase hydration, bond dissociation enthalpies, and acidity of aldehydes

CBS‐Q//B3, G4(MP2), and G4 composite method calculations were used to estimate atmospheric phase standard state (298.15 K, 1 atm) free energies of hydration (ΔhydrG°(g)), hydration equilibrium constants (log Khydr,(g)), bond dissociation enthalpies (BDEs), and enthalpies (ΔdH°(g)) and free energies (ΔdG°(g)) of aldehydic proton acid dissociation for various substituted aldehydes with electron withdrawing and electron releasing groups. Good quality log Khydr,(g) correlations with the Swain–Lupton resonance effect parameters R and R+ were found, allowing extension of the model to predict log Khydr,(g) values for 487 substituted aldehydes having available R‐values and 108 substituted aldehydes having available R+ values. Good correlations were also found between experimental aqueous phase hydration equilibrium constants (log Khydr,(aq)) and summative R/R+ values for peripheral substituents on a range of carbonyl derivatives (aldehydes, ketones, esters, and amides), suggesting that the structure–reactivity modeling approach can be extended to include all possible combinations of R1C(O)R2 carbonyl substitution in both gas and aqueous systems. Computationally derived BDEs and ΔdH°(g)/ΔdG°(g) were in good agreement with the limited experimental and theoretical datasets. BDEs did not generally correlate with any of the Hammett substituent constants or Swain–Lupton parameters considered. Gas phase acidities exhibited high correlation coefficients with Hammett inductive substituent constants (σI) and field effect parameters (F), allowing these to be employed as surrogates for estimating the gas phase aldehydic proton acidities of a larger potential compound range. The resulting models will be of use in predicting the environmental behavior for a broad range of environmentally relevant compounds containing carbonyl functionalities. Copyright © 2016 John Wiley & Sons, Ltd.

Quantitative structure–hydrophobicity relationships of molecular fragments and beyond

Quantitative structure-property relationship (QSPR) models were firstly established for the hydrophobic substituent constant (πX) using the theoretical descriptors derived solely from electrostatic potentials (EPSs) at the substituent atoms. The descriptors introduced are found to be related to hydrogen-bond basicity, hydrogen-bond acidity, cavity, or dipolarity/polarizability terms in linear solvation energy relationship, which endows the models good interpretability. The predictive capabilities of the models constructed were also verified by rigorous Monte Carlo cross-validation. Then, eight groups of meta- or para-disubstituted benzenes and one group of substituted pyridines were investigated. QSPR models for individual systems were achieved with the ESP-derived descriptors. Additionally, two QSPR models were also established for Rekker's fragment constants (foct), which is a secondary-treatment quantity and reflects average contribution of the fragment to logP. It has been demonstrated that the descriptors derived from ESPs at the fragments, can be well used to quantitatively express the relationship between fragment structures and their hydrophobic properties, regardless of the attached parent structure or the valence state. Finally, the relations of Hammett σ constant and ESP quantities were explored. It implies that σ and π, which are essential in classic QSAR and represent different type of contributions to biological activities, are also complementary in interaction site.

Quantum chemical clarification of the alkyl chain length threshold of nonionic surfactants for monolayer formation at the air/water interface.

A theoretical basis is provided for the experimental fact that for various surfactant classes the alkyl chain length threshold varies for the formation of condensed monolayers. The existence of the alkyl chain length threshold for a surfactant enabling the formation of monolayers is determined by the entropy increment to the Gibbs' energy, assessed by using the quantum chemical semiempiric method PM3. The value of the clusterization threshold is not stipulated by the surfactant solubility in water, rather by the electron-donor and electron-seeking properties of the head groups. These properties in turn impact the value of the solubility threshold for surfactants. The value of the clusterization threshold depends quadratically on the substituent constants, i.e. it is independent of whether the functional group is a donor or an acceptor of electrons. Rather it depends only on the donor or the acceptor 'force' of the substituent. The square-law dependence of the surface clusterization threshold of the amphiphile on the solubility threshold is evidenced.

Estimation of activation energies for carbene/alkene additions

Activation energies for the additions to alkenes of carbenes CXY can be estimated from the carbene selectivity indices, mCXY, which are readily derived from the substituent constants (σI and σR+) of X and Y.

Solvent effects on the structural, electronic properties and intramolecular N–H O hydrogen bond strength of 5-aminomethylene-pyrimidine-2,4,6 trion with DFT calculations

Density functional theory (DFT) calculations at the B3LYP/6-311++G** level of theory were realized to analyze the effect of various solvents on the hydrogen bond (HB) strength of 5-aminomethylene-pyrimidine-2, 4, 6 trion and its 22 derivatives in R position. The influence of the solvent was considered using the Tomasi's Polarized Continuum Model (PCM). The geometry optimization, π electron delocalization, vibrational frequencies and several well-established indices of aromaticity have been evaluated at the same level of theory. The excited-state properties of intramolecular hydrogen bonding in substituted systems have been investigated theoretically using the time-dependent density functional theory method. The results show that the HB strength of 5-aminomethylene-pyrimidine-2, 4, 6 trion and its derivatives in non-polar solvent is higher than polar solutions. Its HB strength in polar solvent with respect to water as natural solvent is close to each other. A set of mostly common substituents having different properties in resonance effect according to values of substituent constants were chosen to simulate the influence of substitution in R position of title molecule on the quasi-delocalization and H-bonding. The substituent effect is also analyzed and it is found that the strongest/weakest hydrogen bonds exist for CN/CHO substitutions. The intramolecular hydrogen bonding (IMHB) interaction has been explored by calculation of electron density ρ(r) and Laplacian ∇2ρ(r) at the Bond Critical Point (BCP) using Atoms-in-Molecule (AIM) theory. Natural bond orbital analysis (NBO) was performed for better understanding of the nature of intramolecular interactions. The aromaticity is measured using several well-established indices of aromaticity such as NICS, HOMA, PDI, FLU, and FLUπ. The calculated chemical shifts of the chelated proton for all molecules, using GIAO method, are well correlated with the calculated geometrical parameters results.

Synthesis, structural elucidation, solvatochromism and spectroscopic properties of some azo dyes derived from 6-chloro-4-hydroxyquinoline-2(1H)-one

Malondianilide (I) derived from p-chloroaniline was cyclized to 6-chloro-4-hydroxyquinoline-2(1H)-one (II) in moderately good yield using polyphosphoric acid as catalyst. This compound was then coupled with some diazotized aromatic amines to give the corresponding azo disperse dyes 1–12. A systematic study of the effect of solvent, acid, base and pH upon the electronic absorption spectra of the dyes 1–12 was carried out. In DMSO, DMF, CH3CN, CHCl3, EtOH and acidic media (CH3COOH, acidified EtOH) these dyes that theoretically may be involved in azo-hydrazone tautomerism have been detected only as hydrazone tautomers T1 and T2. The acidic dissociation constants of the dyes were measured in 80 vol% ethanol-water solution at room temperature and ionic strength of 0.1. The results were correlated by the Hammett-type equation using the substituent constants σx.

The influence of the excited-state substituent effect on the reduction potentials of Schiff bases

The reduction potentials (ERed) of 30 N-(phenyl-ethylene)-anilines (XArC(Me)=NArY) were determined, and the effect of substituents on ERed was investigated. During the reduction process, a molecule obtains an electron to form a radical anion; the electron distribution is somewhat similar to that of the excited state. Therefore, the substituent effect cannot be described thoroughly only with Hammett parameter, which is a classic descriptor for electronic effect of substituent in ground state. Excited-state substituent constant was employed as a structural descriptor characterizing the stability of radical anion. The performances of quantitative structure-reduction potential relationship model can be largely improved by introducing excited-state substituent constant to the model, which has been confirmed by the investigations of reduction potentials of disubstituted N-benzylidenebenzenamines, aryl-substituted acetophenone azines, and 1-substituted naphthalenes. It is indicated that the excited-state substituent effect is very important to the correlation of reduction potentials of Schiff bases. Copyright © 2015 John Wiley & Sons, Ltd.

DFT Study on the Oxidative Addition of 4-Substituted Iodobenzenes on Pd(0)-Phosphine Complexes

The oxidative addition of 4-substituted iodobenzenes on Pd(0)-PMe3 complexes has been studied at the BP86 level of theory including dispersion correction and solvation effect, with tetrahydrofuran as solvent. The bisphosphine pathway was found to be barrierless, whereas the monophosphine route is hampered by the high dissociation energy of trimethylphosphine. The reaction free energy of this step shows linear correlation with the Hammett constants of the para substituents with the most electron withdrawing groups being the most exergonic.

Conformational stability of 5-substituted orotic acid derivatives analyzed by measuring 13C NMR chemical shifts and applying linear free energy relationships

Conformational stability of various 5-substituted orotic acid derivatives was studied by applying linear free energy relationships (LFER) to the 13CNMR chemical shifts. The correlation analysis for the substituent-induced chemical shifts (SCS) with inductive (σI), and various resonance (σR) parameters were carried out through Single Substituent Parameter (SSP) and Dual Substituent Parameter (DSP) methods, and multiple regression analysis. Good Hammett correlations for all carbons were obtained, while electrophilic substituent constants better fitted for C2 carbon with electron-donor substituents. Conformational analysis of various derivatives using RB3LYP/6-311++G (3df,3dp) DFT method, together with 13CNMR data suggests that most of the substituted orotic acid derivatives exist in planar conformation, except nitro and alkyl substituted derivatives. Internal rotation of carboxylic group showed significant impact on the extent of conjugative interaction making syn conformation more stable in all the derivatives studied. Further, of all 5-substituted orotic acid derivatives, diketo form proved to be the most stable form compared to zwitterionic and enol tautomeric forms. Optimized geometries and transmission effects of particular substituent through well-defined π-resonance units suggest that these units behave as isolated as well as conjugated fragments, depending on the type of substituent.

Structure–reactivity landscape of N-hydroxyphthalimides with ionic-pair substituents as organocatalysts in aerobic oxidation

The N-hydroxyphthalimides with ionic-pair substituents were studied to design possibly recoverable and highly reactive hydroxyimide organocatalysts. The reactivities of catalysts could be enhanced by increasing the charge (QO) and spin density (SDO) or decreasing the SDN and isotropic Fermi contact couplings (IFCCN) in their NO parts, or reducing the anionic charge (QAnion); their correlations follow QO≈IFCCN>SDN>SDO>QAnion. The reactivities of catalysts with ionic-pair substituents could be largely improved compared with those of conventional counterparts; most of them could be comparable to or higher than those of known efficient organocatalysts. A linear correlation between the Hammett constants of ionic-pair substituents and the reactivities of catalysts was established. Changing the anion, rather than changing the cation, should be a more powerful way to increase the reactivity, and the multiple ionic-pair substituent effects would be superior to single ones. The appropriate NOH bond strengths of these catalysts are responsible for their good catalytic performances.

Synthesis, Hammett Spectral Correlations and Antimicrobial Activities of (<i>E</i>)-1-Benzylidene-2-(4-Chlorophenyl)Hydrazines

In the present study, a series of nine number of (E)-1-benzylidene-2-(4-chlorophenyl) hydrazine compounds have been synthesized by condensation reaction of meta and para substituted benzaldehydes with 4-chlorophenylhydrazine using acetic acid catalyst. They are characterized by their physical constants, UV (200-450 nm), Infra-Red (KBr, 4000-400 cm−1) and NMR (1H and 13C) spectral data. The observed UV, IR and NMR spectral data have been correlated with Hammett substituent constants and Swain-Lupton’s F and R parameters using single and multi-linear regression analysis. From the results of statistical analysis the effects of substituents on the spectral data have been discussed. The antimicrobial activities of all the synthesized (E)-1-benzylidene-2-(4-chlorophenyl) hydrazine compounds have been studied using Bauer–Kirby method.

Investigation of substitution effect on fluorescence properties of Zn2+-selective ratiometric fluorescent compounds: 2-(2′-Hydroxyphenyl)benzimidazole derivatives

2-(2′-Hydroxyphenyl)benzimidazole derivatives (X-HBIs), modified by various substituents X (X=H, CH3, OH, OCH3, NO2, NHCOCH3, NH2, N(CH3)2), were synthesized and their fluorescent behaviors and equilibriums in aqueous solution were studied. Strong fluorescence attributed to the tautomer emission was observed in aqueous solution at pH 7.4. The fluorescence intensities of the X-HBIs were enhanced selectively by addition of Zn2+ but not by addition of Na+, K+, Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, and Cu2+. Additionally, the effective ratiometric fluorescence response to Zn2+ addition was observed in 5-NH2-HBI and 5-N(Me)2-HBI. The pH-titration and speciation studies proved that the X-HBIs have two or three protonation equilibriums and one complexation equilibrium corresponding to the formation of the [Zn(X-HBI)]+ complex. Further structural studies using extended X-ray absorption fine structure analyses and density functional theory calculations identified the dominant Zn2+ species as the [Zn(HBI)(H2O)3]+ complex in aqueous solution. Based on the substituent effect on the fluorescence properties of X-HBIs and their Zn2+ complexes in aqueous solution, the maximum fluorescence excitation and fluorescence wavelengths of both the tautomeric form and the Zn2+ complexes were dependent on the Hammett substituent constants of X, which was attributed to the change of the π–π* energy gap of HBI by introduction of the substituent.

Synthesis, Hammett Spectral Correlations and Antimicrobial Activities of (<i>E</i>)-1-Benzylidene-2-(4-Chlorophenyl)Hydrazines

In the present study, a series of nine number of (E)-1-benzylidene-2-(4-chlorophenyl) hydrazine compounds have been synthesized by condensation reaction of meta and para substituted benzaldehydes with 4-chlorophenylhydrazine using acetic acid catalyst. They are characterized by their physical constants, UV (200-450 nm), Infra-Red (KBr, 4000-400 cm−1) and NMR (1H and 13C) spectral data. The observed UV, IR and NMR spectral data have been correlated with Hammett substituent constants and Swain-Lupton’s F and R parameters using single and multi-linear regression analysis. From the results of statistical analysis the effects of substituents on the spectral data have been discussed. The antimicrobial activities of all the synthesized (E)-1-benzylidene-2-(4-chlorophenyl) hydrazine compounds have been studied using Bauer–Kirby method.

Highly Selective Liquid-Phase Benzylation of Anisole with Solid-Acid Zeolite Catalysts

Zeolites were evaluated as solid acid catalysts for the liquid-phase benzylation of anisole with benzyl alcohol, benzyl bromide, and benzyl chloride at 80 °C. Among the examined zeolites, H-mordenite-10 (H-MOR-10) demonstrated particular high activity (>99 %) and excellent selectivity (>96 %) to the monobenzylated products, ortho/para benzyl anisole, using benzyl alcohol as reagent (anisole/alcohol molar ratio = 28). Analogous reactions with benzyl halides resulted in lower yields of the desired monobenzylated products. The conversion of benzyl alcohol was confirmed to be proportional to the amount of added H-MOR-10, and the linear free-energy relationship relating the conversion of substituted benzyl alcohol and substituent constants as well as the orientation effect of substituents of anisole on the distribution of ortho/para/meta isomers was evaluated.

Simple colorimetric chemodosimeters for selective sensing of cyanide ion in aqueous solution via termination of ICT transition by Michael addition

Three simple ferrocene based chemodosimeters for cyanide ion in aqueous solution have been prepared by one step condensation of commercially available starting materials and characterized spectroscopically. The receptors were designed in such a way that the intra molecular charge transfer (ICT) transition existing between the donor (ferrocene) and acceptor (cyano olefine) moieties imparts an intense pink color to the receptors (logɛ=2.92–3.06). The results of UV–vis, fluorescence and 1H and 13C NMR spectral studies indicated that the sensing mechanism involves Michael addition of CN− to the electropositively polarized C-atom of the CC bond. Such an addition reaction leads to “turn-off” of ICT transition and thus leading to a striking color change from pink to colorless. The observed variation in the electrochemical reduction potential (Epc) for the reduction of ferrocene moiety and the formation constants of the 1:1 receptor–CN− ion complexes could be explained using the Taft's polar substituent constants (σ*) of the substituents attached to the acceptor moiety. Moreover, the test strips fabricated using these receptors could instantaneously sense cyanide ion present in aqueous NaCN solution.