Substituent Constants Σp Research Articles

Correlation between Absorption and Substitution of Photochromic 1,2‐Bis(thienyl)ethenes (BTEs) Using Modified Spectroscopic Hammett Equations

AbstractSeries of photochromic 1,2‐bis(thienyl)ethenes possessing perfluorocyclopentene backbones, either hydrogen or methyl groups at the β‐positions of the thiophenes, and a variety of substituents in their α'‐positions were prepared, which cover the range from electron‐donating to electron‐withdrawing (Me, −CH2OH, −OTBS, −TMS, −Br, 1,3‐dioxan‐2‐yl, pyridin‐4‐yl, −CH2OH, −COOH). As a linear free energy relationship the spectroscopic Hammett equation gives fair to excellent fits to the excitation energy of the absorption maxima of the ring‐opened as well as the ring‐closed forms of the BTEs, when Hammett substituent constants σp were replaced by Brown's modified substituent constants σp+ and σp−. Vice versa, hitherto unknown Hammett‐Brown substituent constants can be estimated from the UV spectra. Furthermore, we compared the experimentally measured absorption maxima with values which we calculated by three different methods (DFT STEOM‐DLPNO‐CCSD/def2‐TZVPP, TD‐DFT ωB97X‐D3/6‐31G*, TD‐DFT ωB97X‐D3/6‐311++G**).

A G4 approach to computing the Hammett substituent constants σp, σm, σ−, σ+, and σ+m

AbstractBy performing appropriate empirical scaling of G4 energy differences, it is possible to calculate Hammett substituent constants σp, σm, σ−, σ+, and σ+m with a typical accuracy (mean absolute error) of ±0.1. The procedure is straightforward and easily carried out by non‐experts using the Gaussian software package. Values are provided for 41 substituents of widely varying electronic character, including some reactive species for which experimental values are unavailable. Additionally, comparison of calculated and experimental values suggests that some previously published numbers might be significantly in error, particularly for some ionic, acidic, and/or basic substituents. Values are also provided for a newly defined substituent constant, σ−m.

Arylamination via ortho-fusion on an azo-appended pyridine carboxamide complex of copper(II)

A square-pyramidal complex [CuII(L)(O2CMe)(H2O)] 1 (HL = 2–[N–(2–phenylazo)carbamoyl]pyridine) has been synthesized using a tridentate deprotonated arylazo-appended pyridine-2-carboxamide ligand. Aerobic reactions of 1 with para-substituted anilines in neat condition at 120 °C afforded isolation of its ortho-arylaminated products [CuII(L−N−C6H4−p−X)] (X = H 2, Me 3, OMe 4, F 5 and Cl 6). The starting brown complex 1 and arylaminated violet complexes 2−6 have been structurally characterized. The electronic spectral feature and redox properties of 1−6 have been examined. Along with CuII/CuI redox, azo-centered cathodic responses were also observed. For 2–6, a characteristic oxidative response due to Cu(II)-coordinated amine oxidation has been observed, modulated by ligand substituents. A linear correlation between the redox potential values of the arylaminated products 2–6 and the Hammett substituent constant σp for para-substituted anilines has been obtained, to rationalize the proposed mechanistic pathways of the observed ortho-fusion reaction. Density functional theory (DFT) at the B3LYP level and time-dependent (TD)-DFT calculations have allowed to rationalize the experimental results and to interpret the observed electronic transitions, respectively.

3-Substituted Benzo[ e][1,2,4]triazines: Synthesis and Electronic Effects of the C(3) Substituent.

A series of 19 structurally diverse C(3)-substituted derivatives of benzo[ e][1,2,4]triazine were synthesized from 3-chloro- (1c) and 3-iodobenzo[ e][1,2,4]triazine (1d) obtained in three steps from 2-nitroaniline in 37-55% yields. Nucleophilic aromatic substitution and metal-catalyzed (Pd, Cu) reactions led to functional derivatives that include alkyl (C5H11), (het)aryl (Ph, 2-thienyl, ferrocenyl), ArC≡C, amine (NHPh and morpholine), PO(OEt)2, sulfanyl (SBu- t), alkoxide (OEt, OMe), and CN. The synthesis of C(3)-CF3 derivative 1g via the Ruppert reaction with 1d and its 1-oxide analogue 2d led to the substitution followed by formal addition of HCF3 to the C═N bond. Pd-catalyzed carbonylation reactions of 1d and 2d did not give the corresponding C(3)-carboxylic acids. Therefore, acid 1f was obtained through hydrolysis of the CN. The substituent effect on the electronic structure of the benzo[ e][1,2,4]triazine ring was investigated by spectroscopic methods (UV-vis and NMR) augmented with density functional theory calculations. Results show significant effect of the C(3) substituent on the π-π*(1) transition energy and good correlation of the 1H NMR chemical shift with the substituent constant σp. Molecular and crystal structures of six derivatives were established with the single-crystal X-ray diffraction method, and the substituent impact on the molecular geometry was investigated.

Trans-kinetic effects in ligand substitution processes of ruthenium polypyridyl complexes

A concerted mechanism is disclosed when studying trans-kinetic effects in substitution processes of complexes of formula [Ru(trpy)(4,4′-(X)2-bpy)(H2O)]2+, with trpy = 2,2′:6′,2″-terpyridine, bpy = 2,2′-bipyridine and X = H, CH3, OCH3, NH2 and N(CH3)2. The second order rate constants k2 for substitution of water by acetonitrile increase with increasing donor ability of the X group. The correlation between ln k2 and Hammett substituent constants σp of X gives a slope of near 1, indicating high sensitivity of the kinetic parameters for water replacement to the donor ability of the para-substituent attached to a bipyridine trans- to the water ligand and considerably separated from it. Besides, the activation enthalpies decrease with increasing pKa values of the 4,4′-(X)2-bpy ligands and the activation entropies have negative values almost one order of magnitude higher than those reported before in similar processes. These data indicate a concerted ligand interchange mechanism, infrequently found in substitution reactions of octahedral complexes which can be explained by the fact that H-bonds between the leaving ligand (H2O) and the entering ligand (CH3CN) will favor an intermediate transition state of increased order respect to the initial state. These results are pertinent for finding the best candidates in the quantitative detection of CH3CN, an important contaminant in radiopharmaceuticals used in PET (positron emission tomography) studies.

Synthesis and properties of π-conjugated donor–acceptor macrocycles derived from phenanthrylene building blocks

Phenanthrylene-ethynylidene macrocycles combining electron donor and electron acceptor subunits in their shape-persistent fully conjugated core were synthesized. The donor subunits consisted of two 9,10-dialkoxyphenanthrenes linked either with 1,2-ethynylidene or 2,5-thienylene bridge. The acceptors were 9,10-phenanthroquinone and dibenzoquinoxaline and dibenzophenazines derived from it. Solvatochromic photoluminescence from intramolecular-charge-transfer (ICT) excited state was observed mainly in non-polar solvents. In more polar solvents, the excited states favor non-radiative relaxation. DFT calculated HOMO/LUMO energies of the macrocycles correlate well with spectroscopic and electrochemical data. In the series of substituted dibenzophenazine acceptors a good correlation with Hammett substituent constants σp– was found.

In-silico analysis of substituent effect on the static first order hyperpolarizability of electron donating mono substituted Chalcone derivatives.

Noncentrosymmetric π conjugated systems with suitable electron donor acceptor groups play a crucial role in material NLO activity. The influence of an electron donating mono substituent at the para position of the phenylene ring of chalcone was investigated as a resource for second harmonic generation. The geometrical optimization of 11 electron donating group substituted chalcones were performed using density functional theory at the B3LYP/6-311G(d,p) level and compared with experimental geometrical parameters of five reported chalcones. All the derivatives are transparent to visible radiation as shown by the electronic absorption spectra investigated by the TDDFT-CAM B3LYP/6-311G(d,p) method, and the maximum absorption wavelength was due to the πPhB → π* transition. The first order hyperpolarizability βtot, calculated using the CAM B3LYP/6-311G(d,p) method, increases with the electron donating ability of the substituent, and the largest βtot was observed for dimethylamino substituent. The Hammett substituent constant (σp) shows good linear correlation with β, λmax, and Egap in the ground state. The Brown constant (σp+) was better correlated indicating the polarization of carbonyl group in the excited state. Frontier molecular orbitals also reveal the valence electron excitation. Correlation of σp with various parameters was analyzed to assess the property interrelationship with electronic reorganization in the molecule. The electronic structures of molecular fragments were described in terms of natural bond orbital analysis, which shows intramolecular interactions.

Effect of substituent groups (R= [sbnd]CH3, [sbnd]Br and [sbnd]CF3) on the structure, stability and redox property of [Cr(R-pic)2(H2O)2]NO3·H2O complexes

Complexes [Cr(3-CH3-pic)2(H2O)2]NO3·H2O (1), [Cr(5-Br-pic)2(H2O)2]NO3·H2O (2) and [Cr(5-CF3-pic)2(H2O)2]NO3·H2O (3) were synthesized (pic = pyridine-2-carboxylic acid) and characterized by X-ray crystal diffraction. Crystal structure indicates that two bidentate ligands occupy equatorial position and two H2O occupy axial positions in trans-configuration. (i) Decomposition of complexes 1, 2 and 3 in different medium (phosphate buffered saline (PBS), apo-ovotransferrin (apootf) and EDTA) indicates that decomposition rate constants of these complexes follow the sequence of 1 < 2 < 3. (ii) The redox potential of Cr(III)/Cr(II) by cyclic voltammetry follows the sequence of 1 (−1.20 V) > 3 (−1.29 V) > 2 (−1.31 V). (iii) In addition, ·OH-generation of the new synthesized complexes was determined by Fenton-like reaction in comparison with Cr(pic)3, and it may be related to the reduction potential of the complexes. (iv) Moreover, Hammett substituent constants σp (inductive) and σm (resonance) (R = 3-CH3, 5-Br, 5-CF3) were introduced to evaluate the impact of substituent groups on the bond length and decomposition kinetics. The substituent group on the ligand has great effect on the properties of the complexes.

Pybox-Iron(II) Spin-Crossover Complexes with Substituent Effects from the 4-Position of the Pyridine Ring (Pybox = 2,6-Bis(oxazolin-2-yl)pyridine)

Spin-crossover (SCO) behavior of a series of [Fe(X-pybox)2](ClO4)2 was investigated, where X-pybox stands for 4-X-substituted 2,6-bis(oxazolin-2-yl)pyridine with X = H, Cl, Ph, CH3O, and CH3S. We confirmed that the mother compound [Fe(H-pybox)2](ClO4)2 underwent SCO above room temperature. After X was introduced, the SCO temperatures (T1/2) were modulated as 310, 230, and 330 K for X = Cl, Ph, and CH3S, respectively. The CH3O derivative possessed the high-spin state down to 2 K. Crystallographic analysis for X = H, Cl, CH3O, and CH3S was successful, being consistent with the results of the magnetic study. Distorted coordination structures stabilize the HS (high-spin) state, and the highest degree of the coordination structure distortion is found in the CH3O derivative. A plot of T1/2 against the Hammett substituent constant σp showed a positive relation. Solution susceptometry was also performed to remove intermolecular interaction and rigid crystal lattice effects, and the T1/2’s were determined as 260, 270, 240, 170, and 210 K for X = H, Cl, Ph, CH3O, and CH3S, respectively, in acetone. The substituent effect on T1/2 became very distinct, and it is clarified that electron-donating groups stabilize the HS state.

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.

Substituent effects on selectivity of coupled oxidation of iron tetraphenylporphyrins

Coupled oxidation of iron tetraphenylporphyrins bearing either a OMe , Me , F , Cl , COOMe , CF 3 or CN group in the para-position of the phenyl groups gave tetraarylbiladien-ab-1-one and triarylbilindione. The ratios of the yields of the former to those of the latter were linearly correlated with the Hammett substituent constants σp+, with a positive slope (Δρ = 0.64). The Hammett plot of the oxidation rate vs. the substituent constant σp also showed a positive slope (ρ = 0.30). These substituent effects suggest that a nucleophilic step is included in the formation of bilindione. Coupled oxidation of an A3B type tetraarylporphyrin having an electron withdrawing nitro group in one of the phenyl groups indicated that the oxidation leading to biladienone occurred rather statistically in any of the meso-positions, while the oxidation leading to bilindione occurred preferentially in the meso-position bearing the 4-nitrophenyl group.

Homolytic C-O cleavage in phosphates and sulfonates.

The C-O homolytic bond dissociation enthalpies(BDEs) were calculated by high-level ab initio including G4, G3B3, G3, CBS-QB3 and a series of density function theory (DFT) methods. It is found that the wB97 method gave the most reliable C-O BDEs and the root-mean-square deviation (RMSD) is 7.6 kJ/mol. Therefore, the C(sp(2))-O BDE predictions and the substituent effects of alkenyl phosphates/sulfonates and aryl phosphates/sulfonates were investigated in detail by using the wB97 method. Interestingly, there exist different substituent effects in α- and β-substituted alkenyl phosphates/sulfonates. Excellent linear relationships between the C-O BDEs of β-substituted alkenyl phosphates/sulfonates with substituent constant σp(+) were found. In addition, the NBO analysis further disclosed the essence of the substituent effects on C-O BDEs.

NMR characterization of substituent effects in cation–π interactions

Substituent effects in C6H5X…M+ (M+=Li+, Na+, K+, and NH4+) systems have been characterized using isotropic nuclear magnetic shielding constants (σ). Good linear relationship between σ and the Hammett substituent constant σp is established which suggests that the substituent effects can be quantified by NMR experiments. Irrespective of the electronic nature of X, meta carbons, and all the hydrogens are always deshielded and this property is useful for detecting cation–π interaction. The σ-scan plots revealed that the deshielding effect of the cation on the aromatic ring is significant even at large cation–π distance (∼4.5Å).

Substituent Effect on the C–NO2 and N–NO2 Bond Dissociation Energies of Nitroaromatic Molecules

AbstractSix density function theory methods (B3LYP, B3P86, MPWB1K1, MPWPW91, PBEPBE, TPSS1KCIS3) were used to calculate bond dissociation enthalpies of nitro compounds, where the B3P86 method was found to give the most accurate predictions. Using the B3P86 method meta‐ and para‐substituted nitroaromatics were systematically studied for the first time. The remote substituent effects, Hammett relationships, and the origin of the substituent effects were discussed on the basis of the calculated results. Both meta‐ and para‐substituted nitromethyl‐benzenes showed significant substituent effects and a fair correlation against substituent constants σp+ The ground state effects were found to play the major role in determining the overall substituent effects. Meanwhile, nitroamino‐ benzenes showed irregular substituent effects and a poorer Hammett correlation, where both ground and radical state effects contributed to the overall substituent effects.

SUBSTITUENT EFFECTS IN DEHALOGENATION OF ARYL BROMIDES WITH Naalh2(OCH2CH2OCH3)2

Kinetics of dehalogenation of the bromobenzenes 4-RC6H4Br (R=F, Cl, H, CH3, CH3O and N(CH3)2) with the title hydride (SDMA) and with SDMA in the presence of Co(acac)2 have been examined. The rate constants of the debromination were correlated with Hammett substituent constants σp and E1/2 reduction potentials of the bromobenzenes. ρ values (+4.3 for SDMA reduction and +1.9 for the Co-catalyzed debromination) imply a negative charge concentration effect in transition states of both debrominations. Differences in the course of these reactions are discussed.

Electronic Substituent Effects of Furoxan and Furazan Systems

Heterocyclic systems 3-furoxanyl, 4-furoxanyl and 3-furazanyl have been characterised as substituents by their substituent constants σm, σp, σI and σR, determined from pK and 19F NMR shifts: they are strongly electron attracting but weakly conjugated.

Structure-activity relationships for chemical and glutathione S-transferase-catalysed glutathione conjugation reactions of a series of 2-substituted 1-chloro-4-nitrobenzenes

Glutathione S-transferases (GSTs) constitute an important class of phase II (de)toxifying enzymes, catalysing the conjugation of glutathione (GSH) with electrophilic compounds. In the present study, Km, kcat and kcat/Km values for the rat GST 1-1-, 3-3-, 4-4- and 7-7-catalysed conjugation reactions between GSH and a series of 10 different 2-substituted 1-chloro-4-nitrobenzenes, and the second-order rate constants (ks) of the corresponding base-catalysed reactions, were correlated with nine classical physicochemical parameters (electronic, steric and lipophilic) of the substituents and with 16 computer-calculated molecular parameters of the substrates and of the corresponding Meisenheimer complexes with MeS- as a model nucleophile for GS- (charge distributions and several energy values), giving structure-activity relationships. On the basis of an identical dependence of the base-catalysed as well as the GST 1-1- and GST 7-7-catalysed reactions on electronic parameters (among others, Hammett substituent constant sigma p and charge on p-nitro substituents), and the finding that the corresponding reactions catalysed by GSTs 3-3 and 4-4 depend to a significantly lesser extent on these parameters, it was concluded that the Mu-class GST isoenzymes have a rate-determining transition state in the conjugation reaction between 2-substituted 1-chloro-4-nitrobenzenes and GSH which is different from that of the other two GSTs. Several alternative rate-limiting transition states for GST 3-3 and 4-4 are discussed. Furthermore, based on the obtained structure-activity relationships, it was possible to predict the kcat/Km values of the four GST isoenzymes and the ks of the base-catalysed GSH conjugation of 1-chloro-4-nitrobenzene.

Chemometric Analysis of Substituent Effects. II. Relation Between Hammett Substituent Constants σm and σp and a New Model for Quantitative Description of Substituent Effects

The paper presents values of 25 substituent constants σi obtained by optimizing 46 data series of dissociation constants of substituted benzoic acids in various media. The constants σi fulfil the general relation between the substituent constants of the Hammett type in meta and para positions enabling the description of substituent effects from both positions at the same time by a single constant. The Hammett substituent constants are interpreted by means of the σi constants with an accuracy better than 0.03 units. In addition to it, the validity of general relationship between σp and σm was verified on a set of 56 substituents with the prediction accuracy of 0.06 units for σp, and after excluding the probably incorrectly parametrized substituents NHCOC6H5, CH3S, and F the accuracy has improved to 0.05 units (98% of interpreted variability). The given relationship has served as a basis for suggesting a new model of transfer of substituent effects to a reaction centre: the model involves both the Hammett equation and the Yukawa-Tsuno equation and explains their background. The suggested model uses generalized transmission coefficients to separately describe the transformation of a single primary substituent effect - depending on its structure - into one inductive and two resonance effects which are transmitted through two independent channels to the reaction centre and here transformed into the resulting observable effect. From the model it follows that the substituent constant σp is not a substituent constant in the true sense of the word since it involves the characteristics of skeleton and of reaction centre.

Medium Effect on the Base‐Catalysed Hydrolysis of Substituted Triphenylsilanes in Acetonitrile

AbstractThe hydrolysis of some p‐substituted triphenylsilanes in the presence of tetrabutyl‐ammonium hydroxide has been investigated in various water‐acetonitrile media with the aim of comparing the results with those already published for triphenylsilane. It appears that the initial rate is first order with respect to both silane and base whereas the order during the time is not constant for some silanes, depending on the water composition. The second order rate constant varies in agreement with the Hammett substituent constant σp. From the calculations of the transfer Gibbs functions for the activated state and for OH− from a reference medium (water mole fraction = 0.225), it is concluded that the solvent effect is not fundamentally altered by the p‐substituents and that the medium influence on the rate can be essentially attributed to solvation modifications around OH−.

Determination of the substituent constant σp- for the thioamide group -CSNH2 from the ionization of anilines and phenols

Thermodynamic ionization constants pKa 2.62 ± 0.01 for the primary aromatic amino group of 4-aminothiobenzamide and pKa 8.28 ± 0.01 for the phenolic hydroxy group of 4-hydroxythiobenzamide have been determined by ultra-violet spectrophotometry at 25°C. By using these pKa values and the Hammett equations (log Ka vs σ) for the ionization of fourteen 3- or 4-substituted phenols and thirteen 3- or 4-substituted anilines the substituent constant σp- = 0.68 (from the ionization of anilines) or σp- = 0.73 (from the ionization of phenols) of the thioamide group -CSNH2 was calculated.