Π-acceptor Substituents Research Articles

Butterfly Methanes: Designing a Novel Class of anti-van't Hoff Carbons.

Among different possible non-classical structures, the stabilization of half-planar tetracoordinate carbon conformation is believed to be the most difficult one. Herein, we designed three types of half-planar tetracoordinate carbon compounds computationally by employing hybrid stabilization effects of substituents. The axial hydrogens of unstable half-planar methane are substituted with π-acceptor and σ-donor substituents such as -BH2 , -Li and the equatorial substituents selected are a combination of electropositive atoms (σ-donors)/electronegative atoms (σ-acceptors and π-donors). To establish the stabilization factors, we conducted a detailed study on vibrational frequency analysis, molecular orbital analysis (including Natural Bond Orbitals) and electrostatic potential (ESP) analysis of optimized molecular geometries using density functional theory.

Effect of Substituents on the Stability of Sulfur-Centered Radicals.

High-level ab initio calculations have been used to calculate the standard and inherent radical stabilities (RSEs) of a test set of 41 sulfur-centered radicals, chosen for their relevance in fields as diverse as combustion, atmospheric chemistry, polymer chemistry, and biochemistry. Radical stability was shown to be profoundly affected by substituents, varying over a 30 kcal mol(-1) range for the test set studied. Like carbon-centered radicals, substituent effects on sulfur-centered radical stabilities result from the competition between the stabilizing effect of electron delocalization by lone pair donation and π-acceptance, and the destabilizing effect of σ withdrawal. However, in contrast to carbon-centered radicals, the heavier thiyl radicals are better able to undergo resonance and lone-pair donor interactions with heavier substituents. In particular, sulfur-containing lone pair donor and π-acceptor substituents have the greatest stabilizing effect, whereas σ-withdrawing substituents such as carbonyls and pyridines are the least stabilizing. The stabilities predicted using the standard definition and Zavitsas's inherent RSEZ scheme are shown to be in surprisingly good agreement with one another for most species tested. The RSEZ values have also been shown to be capable of making chemically accurate estimates of bond energies by comparing our calculated values with 34 currently available experimental ones.

Mechanism and Thermodynamics of Reductive Cleavage of Carbon-Halogen Bonds in the Polybrominated Aliphatic Electrophiles.

Quantum-mechanical computations revealed that, despite the presence of electron-withdrawing and/or π-acceptor substituents, the lowest unoccupied molecular orbitals (LUMO) of the polybromosubstituted aliphatic molecules R-Br (R-Br = C3Br2F6, CBr3NO2, CBr3CN, CBr3CONH2, CBr3CO2H, CHBr3, CFBr3, CBr4, CBr3COCBr3) are delocalized mostly over their bromine-containing fragments. The singly occupied molecular orbitals in the corresponding vertically excited anion radicals (R-Br(•-))* are characterized by essentially the same shapes and show nodes in the middle of the C-Br bonds. An injection of an electron into the antibonding LUMO results in the barrierless dissociation of the anion-radical species and the concerted reductive cleavages of C-Br bonds leading to the formation of the loosely bonded {R(•)···Br(-)} associates. The interaction energies between the fragments of these ion-radical pairs vary from ∼10 to 20 kcal mol(-1) in the gas phase and from 1 to 3 kcal mol(-1) in acetonitrile. In accord with the concerted mechanism of reductive cleavage, all R-Br molecules showed completely irreversible reduction waves in the voltammograms in the whole range of the scan rates employed (from 0.05 to 5 V s(-1)). Also, the transfer coefficients α, established from the width of these waves and dependence of reduction peak potentials Ep on the scan rates, were significantly lower than 0.5. The standard reduction potentials of the R-Br electrophiles, E(o)R-Br/R·+X(-), and the corresponding R(•) radicals, E(o)R(•)/R(-), were calculated in acetonitrile using the appropriate thermodynamic cycles. In agreement with these calculations, which indicated that the R(•) radicals resulting from the reductive cleavage of the R-Br molecules are stronger oxidants than their parents, the reduction peaks' currents in cyclic voltammograms were consistent with the two-electron transfer processes.

Red and Near-Infrared Photoluminescence of D-π-A-Type Compounds Based on a 1,4-Diaryl-1-thio-1,3-butadiene Conjugated System in a Dibenzobarrelene Skeleton

1,4-Diaryl-1-thio-1,3-butadiene derivatives having a π-donor dimethylamino group and several π-acceptor substituents at both terminals of the conjugated system were synthesized by intramolecular [4 + 2] cycloaddition of 1-thio-enynes and subsequent chemical transformations. They displayed largely red-shifted absorption and emission spectra in solution, the powder state, and in polymer films. The derivatives with a 2,2-dicyanoethenyl group as the π-acceptor exhibited inverted solvatochromism in both optical absorption and fluorescence spectroscopies.

Crystal structures of azine derivatives of (η5-acylcyclopentadienyl)dicarbonylnitrosylchromium and unequivocal assignments of C(2,5) and C(3,4) on the cyclopentadienyl ring of azine derivatives of (η5-acylcyclopentadienyl)dicarbonylnitrosylchromium, (η5-acylcyclopentadienyl)tricarbonylmethylmolybdenum, and (η5-acylcyclopentadienyl)tricarbonylmethyltungsten in 13C NMR spectra

Azine derivatives—(4), (5), (12), (13)—of (η5-C5H4CHO)Cr(CO)2(NO) (2), (η5-C5H4COCH3)Cr(CO)2(NO) (3), (η5-C5H4CHO)W(CO)3(CH3) (10), and (η5-C5H4COCH3)W(CO)3(CH3) (11) were obtained from condensation reaction of hydrazine with each corresponding carbonyl group. The structures of 4 and 5 were determined by X-ray diffraction studies. The chemical shifts of the C(2)–C(5) carbon atoms of 4, 5, 12, and 13 have been assigned using two-dimensional HETCOR NMR spectroscopy. We observed that C(2,5) resonate at a lower field than C(3,4) in [(CO)2(NO)Cr(η5-C5H4CRN–)]2 (R=H, 4; R=CH3, 5), whereas C(3,4) resonate at a lower field than C(2,5) in [(CO)3(CH3)W(η5-C5H4CRN–)]2 (R=H, 12; R=CH3, 13). The correlation between the magnitudes of nonplanarity of Cp-exocyclic carbon to π-acceptor substituents and the twist angle of NO to Cp-exocyclic carbon are discussed. The electron density distribution in the cyclopentadienyl ring is discussed on the basis of 13C NMR data and those of 4 and 5 are compared with calculations using the density functional B3LYP exchange–correlation method.

Significant Substituent Effects on Pyridinolysis of Aryl Ethyl Chlorophosphates in Acetonitrile

The substituent effects on the pyridinolysis (XC5H4N) of Y-aryl ethyl chlorophosphates are investigated in acetonitrile at 35.0 C. The two strong π-acceptor substituents, X = 4-Ac and 4-CN in the X-pyridines, exhibit large positive deviations from the Hammett plots but little positive deviations from the Bronsted plots. The substituent Y effects on the rates are really significant and the Hammett plots for substituent Y variations in the substrates invariably change from biphasic concave downwards via isokinetic at X = H to biphasic concave upwards with a break point at Y = 3-Me as the pyridine becomes less basic. These are interpreted to indicate a mechanistic change at the break point from a stepwise mechanism with a rate-limiting bond formation (ρXY = –6.26) for Y = (4-MeO, 4-Me, 3-Me) to with a rate-limiting leaving group expulsion from the intermediate (ρXY = +5.47) for Y = (4-Me, H, 3-MeO). The exceptionally large magnitudes of ρXY values imply frontside nucleophilic attack transition state.

Crystal structure of (CO)2(NO)Cr{η5-C5H4C(CH3) = NNH[2,4-(NO2)2C6H3]} and unequivocal assignments of C(2,5) and C(3,4) on the cyclopentadienyl ring of cynichrodene, tricarbonyl(η5-cyclopentadienyl)methylmolybdenum and tricarbonyl(η5-cyclopentadienyl)methyltungsten bearing a 2,4-dinitrophenyl hydrazinyl substituent in 13C NMR spectra

The Schiff’s base complexes (CO)2(NO)Cr{η5-C5H4C(R)=NNH[2,4-(NO2)2C6H3]} (4 R=H, 5 R=CH3,6 R=C6H5)) and (CO)3(CH3)M{η5-C5H4C(R)=NNH[2,4-(NO2)2C6H3]} (13, M=Mo, R=H; 14, M=Mo, R=CH3; 17 M=W, R=H; 18 M=W, R=CH3) were obtained from condensation reactions. The structure of 5 was determined by X-ray diffraction studies. The chemical shifts of the C(2)–C(5) carbon atoms of 4–6, 13, 14, 17 and 18 have been assigned using two-dimensional HetCOR NMR spectroscopy. We observed that C(2,5) resonates at a lower field than C(3,4) in 4 and 6, whereas C(3,4) resonates at a lower field than C(2,5) in 5. The correlation between the magnitude of the non-planarity of the Cp-exocyclic carbon to the π-acceptor substituents and the extent of deshielding on C(3,4) of the Cp ring is discussed. The electron density distribution in the cyclopentadienyl ring is discussed on the basis of the 13C NMR data and those of 5 and 14 were compared with calculations using the density functional B3LYP exchange–correlation method.

Crystal structure of (CO)3(CH3)Mo[(η5-C5H4)C(CH3)[dbnd]N–N[dbnd]C(CH3)(η5-C5H4)]Mo(CO)3(CH3) and unequivocal assignments of C(2,5) and C(3,4) on the cyclopentadienyl ring of cynichrodene, tricarbonyl(η5-cyclopentadienyl)methylmolybdenum, tricarbonyl(η5-cyclopentadienyl)methyltungsten, ferrocene, and dicarbonyl(η5-cyclopentadienyl)cobalt derivatives bearing an electron-withdrawing carbonyl substituent in 13C NMR spectra

Schiff's bases (η5-formylcyclopentadienyl)tricarbonylmethylmolybdenum 2,4-dinitrophenylhydrazone (6), (η5-acetylcyclopentadienyl)tricarbonylmethylmolybdenum 2,4-dinitrophenylhydrazone (7), and (η5-acetylcyclopentadienyl)tricarbonylmethylmolybdenum azine (8) were obtained from condensation reaction of (CO)3(CH3)Mo(C5H4CHO) (3) or (CO)3(CH3)Mo(C5H4COCH3) (4) with each corresponding primary amine. The structures of 7 and 8 were determined by X-ray diffraction studies.The chemical shifts of C(2)–C(5) carbon atoms of the series of substituted-cyclopentadienyl Cp(M) derivatives—(CO)2(NO)M[η5-(C5H4COOCH3)] (1 M = Cr, 2 M = Mo), (CO)3(CH3)Mo[η5-(C5H4-sub)] (3–5, 6, 7, 8)—have been assigned using two-dimensional HetCOR NMR spectroscopy. The assigned chemical shifts were compared with the NMR data of other organometallic Cp(M) analogs, (CO)2(NO)CrCp, (CO)3(CH3)WCp, Cp2Fe, and (CO)2CoCp. We observed that C(3,4) resonate at a lower field than C(2,5) in (CO)3(CH3)Mo[η5-(C5H4CHO)] 3 and 4, whereas C(2,5) resonate at a lower field than C(3,4) in (CO)3(CH3)Mo[η5-(C5H4COOCH3)] 5. The correlation between the magnitudes of nonplanarity of Cp-exocyclic carbon to π-acceptor substituents and the extent of deshielding on the C(3,4) of the Cp ring was discussed. The electron density distribution in the cyclopentadienyl ring was discussed on the basis of 13C NMR data and those of 4, 7, and 8 were compared with calculations using the density functional B3LYP exchange-correlation method.

Conformation and geometry of cyclopropane rings having π-acceptor substituents: a theoretical and database study

The 3e' orbitals of cyclopropane have the correct symmetry to interact with low-lying unoccupied orbitals of π-acceptor substituents and maximum overlap occurs when the two orbital systems are parallel, i.e. when the π-acceptor bisects the ring in projection down the substituent bond. Since the cyclopropyl group is a common component of active pharmaceutical and agrochemical ingredients, it is important that these strong conjugative interactions are well modelled by computational techniques, and clearly represented in experimental crystal structures. Here we show that torsion angle distributions derived from crystal structure data in the Cambridge Structural Database are in excellent correspondence with torsional energy profiles computed using density functional theory (DFT) for a range of substituents: -COOR, -CONR(2), -NO(2), vinyl and phenyl. We also show that crystal structure information is invaluable in modelling conformations of compounds that contain multiply substituted rings, where steric interactions require some substituents to adopt energetically disfavoured conformations. Further, conjugative interactions with π-acceptors lead to significant asymmetry in the cyclopropane ring bond lengths and again the experimental and computational results are in excellent agreement. Such asymmetry effects are additive, and this explains bond-length variations in cyclopropane rings bearing two or more π-acceptor substituents.

Kinetics and mechanism of the pyridinolysis of N -aryl-P ,P -diphenyl phosphinic amides in dimethyl sulfoxide

Kinetic studies for the reactions of Z-N-aryl-P,P-diphenyl phosphinic amides with X-pyridines have been carried out in dimethyl sulfoxide at 85.0 °C. The two strong π-acceptor substituents, X = 4-Ac and 4-CN in the X-pyridine, exhibit positive deviations from both the Hammett and Brönsted plots. The Hammett plots for substituent Z variations are anomalously biphasic concave upwards with a break point at Z = H, and the sign of ρZ changes from negative for electron-donating to positive for electron-withdrawing substituents Z in the leaving group. The negative sign of the cross-interaction constants (ρXZ) for both electron-donating (large magnitude of ρXZ = −1.54) and -withdrawing (ρXZ = −0.27) substituents Z indicates that the reaction proceeds through a concerted mechanism. These results are indicative of frontside and backside nucleophilic attacks at the P reaction center for electron-donating and -withdrawing substituents Z, respectively. The poor leaving group mobility of NHPhZ yields the change of the nucleophilic attacking direction, resulting in biphasic concave upwards Hammett plots for substituent Z variations. Copyright © 2010 John Wiley & Sons, Ltd.

Enhancement of molecular polarizabilities by the push–pull mechanism: A DFT study of substituted hexatriene

Abstract The static polarizabilities α for a series of substituted hexatriene molecules of the NO 2 –(CH CH) 3 -D and NO 2 –(CH CH) 3 -A types (D, A = N(Me) 2 , Br, OCH 3 ,CH 3 , NH 2 , Cl, OH, F, COCN, C 2 H, COF, NO, CHO, CN, CF 3 ) have been computed using DFT method at B3LYP/6-311 G(d,p) level of theory. Our results allowed to sort out the considered Π-donor and Π-acceptor substituents by decreasing static isotropic α value. An excellent agreement between the DFT and PM6 results, they give a correlation coefficient of 0.97. Miller QSAR-quality polarizability calculations give a correlation coefficient of 0.99 when compared with B3LYP/6-311G(d,p) values. Empirical models based on molecular volumes give unrealistic values for α > but these values correlate well 0.97 with B3LYP/6-311G(d,p) results.

Substituent Effects on the Stability of Sulfenes

The geometries and energies of substituted sulfenes RCH═SO2 and alkenes RCH═CH2 were calculated at the HF/6-311+G(2d,p) level. An isodesmic reaction was designed to study substituent effects on the stability of sulfenes. The correlation between substituent group electronegativity χBE and the stability of sulfenes was reasonably good. Electropositive and π-acceptor substituents stabilized sulfenes while electronegative substituents destabilized them.

Conformations, Chemical Reactivities and Spectroscopic Characteristics of Some Di-substituted Ketenes: An ab initio Study

A systematic study of the structure, energetics and spectral characteristics of substituted aminoketenes R(NH 2 )C=C=O (R = H, CH 3 , NH 2 , OH, OCH 3 , CH=CH 2 , C≡CH, CN, CHO, NO, NO 2 ) which are highly reactive and transient intermediates in synthesis has been conducted by ab initio calculations at the MP2/6-31G*//MP2/6-31G* level. Twenty four stable isomers of the eleven substituted aminoketenes having dihedral angles Φ NH2 ∼120° and 60° have been identified and their optimized geometries and energies obtained. Electrostatic and steric effects on the molecular geometries have been analyzed. While the π-acceptor groups lead to planar conformations, the electron-donor groups give rise to non-planar conformations. Isodesmic substituent stabilization energies relative to alkenes have been calculated and correlation with group electronegativities established. Role of induction effect by the substituent groups and resonance effects in charge distribution in the molecules has been analyzed. An analysis of the asymmetric stretching frequencies and intensities of the C=C=Ο group shows that affect of non-π acceptor substituents on the frequency is determined by the field effect (F) and resonance effect (R) parameters, the calculated intensities I (km/mol.) are correlated to group electronegativities Χ of the substituents by the relationship I = 640.2-100.1 Χ(r = 0.92). The π-acceptor substituents increase the intensity which may be explained in terms of their delocalizing effect on the negative charge at the C β atom.

Regularities of the trans-Influence of Ligand L on the Energy Characteristics of the Metastable Isomers of trans-[RuX4(NO)L]q (L = H2O, NH3, Pyrazine, Cl−, OH−, CN−, NO 2 − ; X = Cl−, NH3)

The geometric structures of the ground state and metastable isomers of the nitroso complexes trans-[RuCl4(NO)L]q (L = H2O, NH3, pyrazine (Pz), q = −1; Cl−, OH−, CN−, NO 2 − , q = −2) and cis[RuCl4(NO)L]q (L = Pz, q = −1) were optimized in terms of the density functional theory. The variation of the trans-ligand L influences the relative energy of the metastable isomer with a side NO coordination. The presence of π-acceptor substituents in the trans-ligand L decreases the energy.

Photo- and Thermochromic Spirans. New Metal Chelates Based on Azomethines and Hydrazones Containing a Spiropyran Fragment

Imine- and hydrazone-based metal chelates containing a spiropyran fragment derived from 3-methyl-4-oxo[2H]-1,3-benzoxazinone were prepared. Attempts are made to monitor the photochromic activity of spiropyran using nontraditional π-acceptor substituents (products of condensation of the formyl group in 6"-dimethyl-8"-formylspiro-(4-oxo-3,4-dihydro-2H-1,3-benzoxazine-2,2"-[2H]chromene) with o- aminophenol and aromatic acid hydrazides) and using transition metal ions.

Bond Dissociation Energies and Radical Stabilization Energies Associated with Substituted Methyl Radicals

Bond dissociation energies (BDEs) and radical stabilization energies (RSEs) associated with a series of 22 monosubstituted methyl radicals (•CH2X) have been determined at a variety of levels including, CBS-RAD, G3(MP2)-RAD, RMP2, UB3-LYP and RB3-LYP. In addition, W1‘ values were obtained for a subset of 13 of the radicals. The W1‘ BDEs and RSEs are generally close to experimental values and lead to the suggestion that a small number of the experimental estimates warrant reexamination. Of the other methods, CBS-RAD and G3(MP2)-RAD produce good BDEs. A cancellation of errors leads to reasonable RSEs being produced from all the methods examined. CBS-RAD, W1‘ and G3(MP2)-RAD perform best, while UB3-LYP performs worst. The substituents (X) examined include lone-pair-donors (X = NH2, OH, OCH3, F, PH2, SH, Cl, Br and OCOCH3), π-acceptors (X = BH2, CHCH2, C⋮CH, C6H5, CHO, COOH, COOCH3, CN and NO2) and hyperconjugating groups (CH3, CH2CH3, CF3 and CF2CF3). All substituents other than CF3 and CF2CF3 result in radical stabilization, with the vinyl (CHCH2), ethynyl (C⋮CH) and phenyl (C6H5) groups predicted to give the largest stabilizations of the π-acceptor substituents examined and the NH2 group calculated to provide the greatest stabilization of the lone-pair-donor groups. The substituents investigated in this work stabilize methyl radical centers in three general ways that delocalize the odd electron: π-acceptor groups (unsaturated substituents) delocalize the unpaired electron into the π-system of the substituent, lone-pair-donor groups (heteroatomic substituents) bring about stabilization through a three-electron interaction between a lone pair on the substituent and the unpaired electron at the radical center, while alkyl groups stabilize radicals via a hyperconjugative mechanism. Polyfluoroalkyl substituents are predicted to slightly destabilize a methyl radical center by inductively withdrawing electron density from the electron-deficient radical center.

Substituent effects on stability of oxiranes, oxirenes, and dioxiranes

Homodesmotic reactions were designed to study substituent effects on stability of oxiranes, oxirenes, and dioxiranes. Good or fair correlation between their homodesmotic stabilization energies and Taft's dual-substituent-parameters has been found. Oxiranes are stabilized by σ-donating and π-donating substituents but destabilized by σ-accepting and π-accepting substituents. The π-effects on oxiranes are comparable to σ-effects for π-acceptor substituents and are much stronger than σ-effects for π-donor substituents. Oxirenes are stabilized by σ-donating, π-donating, and π-accepting substituents but destabilized by σ-accepting substituents. The π-effects on oxirenes are stronger than σ-effects. Oxirenes with strong π-donor substituents such as F, OH, and NH2, or strong σ-donor substituents such as Li and Na are neither real molecules nor transition states. Dioxiranes are stabilized by σ-donating and π-donating substituents but destabilized by σ-accepting and π-accepting substituents. The π-effects on dioxiranes are stronger than σ-effects for π-donor substituents, but weaker than σ-effects for π-acceptor substituentsKey words: oxirane, oxirene, dioxirane, homodesmotic reaction, substituent effect.

Deprotonating Molecules and Free Radicals to Form Carbon-Centered Anions: A G2 ab Initio Study of Molecular and Free Radical Acidity

Molecules CH3X and free radicals •CH2X can be deprotonated to form carbon-centered anions CH2X- and radical anions HCX•-, respectively. We have studied the geometric and thermochemical changes that accompany such deprotonation processes for a variety of substituents X including the π-donor groups NH2, OH, OCH3, PH2, SH, F, Cl, and Br and the π-acceptor groups BH2, AlH2, CHO, NO2, CN, and NC. Thermochemical properties calculated and discussed include the gas-phase acidities of the molecules and free radicals, the electron affinities of the •CH2X free radicals, various dissociation energies, and the heats of formation of all species. The acidities of •CH2X radicals are predicted to be greater than those of CH3X for π-donor substituents but less for π-acceptor substituents (except CN and NC). The changes that are predicted to occur upon deprotonation in C−X bond lengths, C−X homolytic and heterolytic bond dissociation enthalpies, C−H homolytic bond dissociation enthalpies, and radical stabilization energies may be understood by examining the orbital interactions that take place in each species.

The Effects of Protonation on the Structure, Stability, and Thermochemistry of Carbon-Centered Organic Radicals

The effects of protonation on the geometries and stabilization energies of prototypical •CH2X radicals (X = NH2, OH, OCH3, PH2, SH, F, Cl, Br, CN, CHO, and NO2) have been studied with the use of ab initio molecular orbital calculations at the G2 level. The proton affinities at X of the •CH2X radicals and the analogous substituted methanes, CH3X, are compared and the corresponding heats of formation calculated. For π-donor substituents (X = NH2, OH, OCH3, PH2, SH, F, Cl and Br), protonation at X leads to considerable +C :XH character for both CH3XH+ and •CH2XH+, resulting in substantially lower heterolytic bond dissociation enthalpies and longer C−X bonds. Protonation also strengthens the C−H bonds in CH3XH+ and, in combination with the reduced interaction of the lone pair on X with the singly-occupied orbital at the radical center in the radicals, results in negative radical stabilization energies for •CH2XH+. For the π-acceptor substituents (CN, CHO, and NO2), protonation enhances hyperconjugative electr...

Poly( p-phenylene vinylene) as active layer in light-emitting diodes: a theoretical investigation of the effects of derivatization

In this work, we investigate the way the geometric, electronic and optical properties of para-phenylene vinylene oligomers are affected upon derivatization, either by introducing π-donor and/or π-acceptor substituents along the chain or by incorporating nitrogen atoms within the conjugated backbone. We show that the results of our quantum-chemical calculations are relevant in the context of the use of the corresponding polymers in light-emitting diodes.