Steric Inhibition Of Resonance Research Articles

Estimate of Hyperconjugation Strength in Alkylaromatics and Unsaturated Hydrocarbons Derived from Refractometric Data

: A retrospective view of exaltation of refraction observed for many unsaturated and aromatic compounds demonstrates that this physical phenomenon is undeservedly considered only as a qualitative measure of conjugation. This mini-review discusses numerous papers by the author that have been published earlier in inaccessible periodicals and collections of scientific papers. Using a great number of illustrations, the author shows that this parameter can be successfully used for quantitative estimate of resonance effects in organic and polymer chemistry. The methods for derivation of strictly additive atomic and group refraction constants are described; these constants were subsequently used as a tool that allowed quantitative estimation of resonance effects in mono-, di-, tri- and polyalkylbenzenes, alkylnaphthalenes, some alkyl derivatives of unsaturated hydrocarbons. These effects cause strictly fixed increase in refraction of carbon atoms in different structural modifications (graphene, fullerene, diamond) and in polycyclic aromatic hydrocarbons. The relevant results regarding quantitative estimation of degree of steric inhibition of resonance in sterically hindered ortho-dialkylbenzenes, 1,2,3- trialkyl-, 1,2,3,4-tetraalkyl-, and 1,2,3,4,5-polyalkylbenzenes accumulated by the author are summarized.

Could X-Ray Analysis Explain for the Differing Antimicrobial and Antioxidant Activity of Two 2-Arylamino-3-Nitro-Coumarins?

Abstract X-Ray analyses of 4-(naphthalen-1-ylamino)-3-nitro-chromen-2-one and 3-nitro-4-phenylamino-chromen-2-oneshowedthatthementionedcompoundscrystallizeinthespace groups P1- (triclinic crystal system; unit cell parame-ters: a = 8.087(2) A˚, b = 9.241(3) A˚, c = 10.911(3) A˚,a = 93.77(3) , b = 102.51(3) , c = 106.44(2) ,V=756.4(4) A˚ 3 and Z = 2) and P2 1 2 1 2 1 (orthorhombic crystalsystem; unit cell parameters: a = 4.9274(9) A˚, b =14.725(3) A˚, c = 17.866(4) A˚, a = b = c = 90 ,V=1296.3(5)A˚ 3 andZ = 4),respectively.Theanalysesofcrystalstructures and gas phase conformations, inferred from singleX-ray crystallographic and molecular modeling experiments,respectively, showed that the changes in p delocalizationof the farmacoactive formal 3-amino-2-nitro-acrylic acidderivatives might explain the observed significant differenceof the antimicrobial and antioxidant activities and spectralproperties of two 4-arylamino-3-nitro-coumarin derivatives.Keywords 4-Arylamino-3-nitro-coumarins Synthesis X-ray analysis Conformation Antimicrobial activity Antioxidant capacityIntroductionIn the continuation of our investigation [1] of biologi-cally active coumarin derivatives we synthesized two2-arylamino-3-nitro-coumarins: 4-(naphthalen-1-ylamino)-3-nitro-chromen-2-one (1) and 3-nitro-4-phenylamino-chromen-2-one (2), Fig. 1. Previous research has shownthat 4-amino-3-nitrocoumarins, tested for pharmacologicalactivity in mice and rabbits, had neurotropic activity, gen-erally inhibiting spontaneous locomotor activity, decreasinghyperactivity induced by phenamine, and prolonging thio-pental sleep [2]. The same study [2] also pointed out to themost active of the derivatives containing a secondary aminogroup, compound 2, that inhibited phenamine hyperactivity2-5-fold and prolonged thiopental sleep 2-fold. Compoundspossessing a 4-amino-3-nitrocoumarin moiety were alsointeresting due to the previously noted steric inhibition ofresonance of the C-3 NO

Does 2-Methylacetophenone Comply with Steric Inhibition of Resonance? A Direct Experimental Proof of Its Nonplanar Conformation from a Joint Ab Initio/Electron Diffraction Analysis

The structure and conformations of 2-methylacetophenone (1) have been investigated by ab initio calculations carried out at the MP2(full)/6-311++G** level and by gas electron diffraction (GED). According to both methods, 1 exists predominantly as a form with the C=O bond synclinal with respect to the C(ar)-C(O) bond (1B), with a torsional angle [C(6)-C(1)-C=O] of 32.7(24) degrees as determined by GED and 26.0 degrees from MP2(full)/6-311++G**. Calculations also predict the presence of a second conformer, the anticlinal structure (1C), with phi = 140.0 degrees, with an abundance of less than 6%, an amount hardly detectable by GED. Different DFT computational protocols both support a nonplanar form of the predominant conformer (B2PLYP) and are in contradiction (B3LYP, M052x, B98, B97-D) with this experimental finding. The GED results, supported by the calculations that involve long-range correlation, are in a good agreement with (13)C NMR spectroscopic investigations, UV spectra, and dipole moment studies. However, previous claims that assumed steric inhibition of resonance caused by a significantly nonplanar conformation with phi close to 90 degrees have been disproved. Steric crowding is evident from the geometrical parameters, particularly from the C(1)-C(2) bond length and from the C(1)-C(2)-C(H(3)) and C(2)-C(1)-C(O) bond angles. It is concluded that any explanation of reactivity by steric inhibition of resonance and by other steric factors must be supported by experimental and/or theoretical investigation of the actual molecular shape.

Acidity of ortho-substituted benzoic acids: an infrared and theoretical study of the intramolecular hydrogen bonds

The structures of ortho-substituted benzoic acids with substituents bearing hydrogen atoms (OH, NH2, COOH and SO2NH2) were investigated by means of IR spectroscopy and of density functional theory at the B3LYP/6-311 + G(d,p) level. All possible conformations, hydrogen bonds, tautomeric forms and zwitterions were taken into consideration and particular attention was given to intramolecular H-bonds and their effect on acidity. Strong H-bonds in the anions of all four acids, were revealed by calculations. In three cases they were confirmed by the IR spectra of the tetrabutylammonium salts in tetrachloromethane solution, while the salt of 1,2-benzenedicarboxylic acid was not sufficiently soluble. The H-bonds are of different strengths but in all cases they are the main cause of the strengthened acidity of these acids in the gas phase and also in solution, although their effect is opposed by weaker H-bonds present in the undissociated acid molecules. The substituent effect on the acidity was evaluated in terms of isodesmic reactions, separately in the acid molecules and in the anions. While the acidity of the 2-OH and 2-NH2 acids is determined essentially by the H-bonds, that of the 2-COOH and 2-SO2NH2 acids is strengthened by the polar effect operating in the undissociated molecule in addition to the H-bond in the anion. The steric inhibition of resonance (SIR), estimated from model conformations with fixed torsional angles, is of little importance. This analysis goes significantly beyond the classical explanation obtained from the acidities in solution but essentially conforms with it.

Steric inhibition of resonance: revision of the principle on the electronic spectra of methyl‐substituted acetophenones

AbstractThe principle of steric hindrance to resonance (SIR) was revised on the well‐known example of the electronic spectra of methyl‐substituted acetophenones. The spectra were reinvestigated on an extended series 1–14, and computer separation of bands was undertaken. The conformation of these molecules, total energy, rotational barrier, geometry of the transition state and electronic spectra were calculated within the framework of the density functional theory. The classical literature was correct that some molecules of the series are twisted from the planar arrangement by a variable angle ϕ. However, these angles do not agree with the values anticipated; the main difference is that all derivatives with only one ortho‐methyl group are planar (ϕ = 0). Our main objection is that the rotational barrier is in some cases very low; hence ϕ does not have the same importance with all derivatives. A better measure of steric hindrance is the calculated energy Δ4E of the isodesmic reaction Eqn (4), in which the molecule of the substituted acetophenone is formed from acetophenone and a methyl‐substituted benzene. In contrast to the angles ϕ, there was good agreement between calculation and experiment in electronic spectra, viz. between the extinction coefficients of the 1La band and the calculated oscillator strengths, at least in the relative values. Summarizing, SIR is valid as a qualitative principle but the reported angles ϕ based on several experimental quantities are often wrong. Each case must be examined separately as to the actual shape of the molecule. The classical paper by Braude et al. is a particularly bad example: the basic hypothesis of a planar excited state was not confirmed by our calculations. Copyright © 2004 John Wiley & Sons, Ltd.

The Dinitration of Dimethylacetanilides

The orientation of dinitration of the dimethylacetanilides has been examined by 1H NMR methods and a possible enhancement of the hyperconjugative influence of a methyl group has been considered as an additional factor augmenting the effect of the steric inhibition of resonance on the orientation of substitution.

Steric effects and steric inhibition of resonance: structure and ionization of 2-tert-butylbenzoic acid

Download options Please wait... Article information DOI https://doi.org/10.1039/B005691J Article type Paper Submitted 14 Jul 2000 Accepted 05 Sep 2000 First published 02 Jan 2001 Download Citation New J. Chem., 2001,25, 250-254 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Steric effects and steric inhibition of resonance: structure and ionization of 2-tert-butylbenzoic acid S. Böhm and O. Exner, New J. Chem., 2001, 25, 250 DOI: 10.1039/B005691J To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Social activity Tweet Share Search articles by author Stanislav Böhm Otto Exner

Steric Inhibition of Resonance: A Revision and Quantitative Estimation on the Basis of Aromatic Carboxylic Acids

The classical concepts of steric inhibition of resonance (SIR) and primary steric effect (van der Waals interactions) were revised with the aid of methyl-substituted benzoic acids. The quantum chemical model was based on the energies of various conformations, calculated at the RHF/6–31+G(d,p) and B3LYP/6–311+G(3df,2pd)//RHF/6–31+G(d,p) levels. The molecule of 2-methylbenzoic acid is planar: no SIR is possible, and van der Waals interaction is practically equal in the acid molecule and in its anion. Therefore, the increased strength of this acid is not due to any steric effect but can be described in terms of electrostatic interaction pole/induced dipole, which lowers the energy of the anion. The molecule of 2,6-dimethylbenzoic acid is nonplanar. SIR is significant in the acid molecule but equal or even greater in the anion. The higher acidity cannot be connected with SIR and can be explained also by electrostatic interaction. In 2,3,5,6-tetramethylbenzoic acid, SIR is greater and may be responsible for one third of the acid-strengthening effect. The concept of SIR is to be applied with caution; even when the nonplanar conformation is proven, SIR need not be responsible for any observable quantity, and particularly not for the acidic and basic properties.

Steric inhibition of resonance: a revision and quantitative estimation on the basis of aromatic carboxylic acids

The classical concepts of steric inhibition of resonance (SIR) and primary steric effect (van der Waals interactions) were revised with the aid of methyl-substituted benzoic acids. The quantum chemical model was based on the energies of various conformations, calculated at the RHF/6-31 +G(d,p) and B3LYP/6-311 + G(3df,2pd)//RHF/6-31 +G(d,p) levels. The molecule of 2-methylbenzoic acid is planar: no SIR is possible, and van der Waals interaction is practically equal in the acid molecule and in its anion. Therefore, the increased strength of this acid is not due to any steric effect but can be described in terms of electrostatic interaction pole/induced dipole, which lowers the energy of the anion. The molecule of 2,6-dimethylbenzoic acid is nonplanar. SIR is significant in the acid molecule but equal or even greater in the anion. The higher acidity cannot be connected with SIR and can be explained also by electrostatic interaction. In 2,3,5,6-tetramethylbenzoic acid, SIR is greater and may be responsible for one third of the acid-strengthening effect. The concept of SIR is to be applied with caution; even when the nonplanar conformation is proven, SIR need not be responsible for any observable quantity, and particularly not for the acidic and basic properties.

Structure of Methyl 2-Methylbenzoate: Steric Effect of a Smallortho-Substituent

The structure and conformation of methyl 2-methylbenzoate (2) was investigated by ab initio calculations carried out at the MP2(full)/6-311++G** level of theory, and by gas electron diffraction (GED). Both methods gave the same result: there is an equilibrium between two conformations around the Car−C(O) bond, the sp (2B) and ap (2C) forms, found in approximately 4:1 abundance (energy difference 3.7 or 2.7 kJ/mol, respectively). This result confirms previous analyses of the steric effects in methyl-substituted benzoic acids and their esters. The ab initio calculations predicted that the minimum energy molecules are those with the planar sp and ap conformations, with torsional angles of 0° and 180°, respectively. On the other hand, the corresponding absolute values found by GED differed by 29° from the values of 0°and 180°, that is, the average absolute torsional angles were found to be 29° and 151°. The difference between the two approaches is discussed. From the other geometrical parameters, some expected deformations were observed, in particular lengthening of the C(1)−C(2) bond and widening of the adjoining angles. In terms of structural organic chemistry, the results imply that a small steric effect doesn’t necessarily distort planarity or result in steric inhibition of resonance, even when the steric effect demonstrably influence other geometrical parameters.

Steric Effects and Steric Inhibition of Resonance in Benzene Derivatives: 2,3-Dimethylbenzoic Acid

Structure of 2,3-dimethylbenzoic acid was determined both by single-crystal X-ray diffraction and by ab initio calculation at an RHF/6-31+G** level. Comparing with a previous X-ray analysis of another crystal modification of the same compound, it was possible to estimate the effect of crystal forces on the conformation. The isolated molecule is not planar as deduced previously, mainly from IR spectra, but the carboxyl group is twisted out of the ring plane by a torsion angle ϕ = 12°. In the crystal, the molecule is more flat and ϕ is reduced to 7 and 1° in the two modifications, respectively. Further significant differences between the modifications were not detected. The flattening in the crystal structure is accompanied by additional small changes in the geometry, connected with greater steric crowding in the less twisted molecule: the example shows well the limits in which molecular structure can be deduced from the solid-state structure. Previous division of methylated benzoic acids into two subgroups, planar and nonplanar, is to be formulated with more precision: the former group includes even acids with nearly planar conformation (ϕ up to 15°).

Steric Effects and Steric Inhibition of Resonance in Isopropylbenzoic Acids in the Gas Phase and in Solution

The substituent effects of the isopropyl group in 2-, 3- and 4-isopropylbenzoic acids were investigated on the enthalpies of formation, gas-phase acidities, acidities in methanol and in dimethyl sulfoxide, and on the IR spectra in tetrachloromethane. Particular attention was given to the influence of variable conformation on the observed steric effect. In contrast to 2-tert-butylbenzoic acid and similarly to 2-methylbenzoic acid, 2-isopropylbenzoic acid exists in two planar conformations in equilibrium. Due to this conformational freedom, the steric effects of the isopropyl group on the conformation of the carboxyl group or on the gas-phase acidity are relatively small, rather close to that of a methyl group. The gas-phase acidity can be qualitatively described in terms of pole/induced dipole interaction in the anion. The latter effect is observable with a lower intensity even in 3-isopropylbenzoic acid: in this action from a more remote position, the isopropyl group resembles more the tert-butyl than the methyl group. In methanol and dimethyl sulfoxide solutions, the steric effects and particularly the pole/induced dipole interaction are attenuated and effect of isopropyl acquires intermediate values between methyl and tert-butyl. Solvent effects are thus deciding for the position of the isopropyl substituent on the scale of steric substituent constants.

ChemInform Abstract: Hydroxide‐Promoted Redox Reactions in Water of α‐Phenyl‐4‐ nitrobenzenemethanol, α‐(p‐Nitrophenyl)‐4‐pyridinemethanol, and . alpha.‐(p‐Nitrophenyl)‐4‐pyridinemethanol N‐Oxide. Steric Inhibition of Resonance.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Structures of Some Dialkoxyphenylmethylium Ions; Steric Inhibition of Resonance

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTStructures of Some Dialkoxyphenylmethylium Ions; Steric Inhibition of ResonanceRonald F. Childs, Christopher S. Frampton, Guogong J. Kang, and Teresa A. WarkCite this: J. Am. Chem. Soc. 1994, 116, 19, 8499–8505Publication Date (Print):September 1, 1994Publication History Published online1 May 2002Published inissue 1 September 1994https://doi.org/10.1021/ja00098a010RIGHTS & PERMISSIONSArticle Views97Altmetric-Citations10LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (772 KB) Get e-AlertsSupporting Info (3)»Supporting Information Supporting Information Get e-Alerts

Chemistry of the Fullerenes: The Manifestation of Strain in a Class of Continuous Aromatic Molecules

Within the wr-orbital axis vector theory, the total rehybridization required for closure of the fullerenes is approximately conserved. This result allows the development of a structure-based index of strain in the fullerenes, and it is estimated that about 80 percent of the heat of formation of the carbon atoms in C60 may be attributed to a combination of v strain and steric inhibition of resonance. Application of this analysis to the geometries of structurally characterized organometallic derivatives of C60 and C70 shows that the reactivity exhibited by the fullerenes may be attributed to the relief of a combination of local and global strain energy. C60 is of ambiguous aromatic character with anomalous magnetic properties but with the reactivity of a continuous aromatic molecule, moderated only by the tremendous strain inherent in the spheroidal structure.

Spin-charge exchange in allodial radical ions, a novel intramolecular single electron transfer equilibrium

The intramolecular single-electron transfer in radical anions ane radical cations, constituted by two identical triphenylmethyl moieties, has been studied. The preparation of the radical ions has been effected by mixing the corresponding stable diions (tetra-n-butylammonium or hexachloroantimonate salts) and chemically inert diradicals: by partial oxidation of the dianion salts with iodine (radical anions) and by partial oxidation of the diradicals with SbCl 5 (radical cations). The ESR 13 C hyperfine coupling constants and linewidths and UV-vis absorptivity spectra afford compelling evidence for a rapid spin-charge exchange equilibrium: (C 6 Cl 5 ) 2 C-C 6 Cl 4 -Sp-C 6 Cl 4 -C(C 6 Cl 5 ) 2 ⇔(C 6 Cl 5 ) 2 C-C 6 Cl 4 -Sp-C 6 Cl 4 -C(C 6 Cl 5 ) 2 (Sp=none, CH 2 CH 2 , C≡C. *=−,+). Relevant structural aspects, such as steric inhibition of resonance, electron paths, and counterion involvement, are discussed. The synthesis and isolation of related disalts from the corresponding diradicals are also described

Thermolabile Kohlenwasserstoffe, 30 Thermische Stabilität und Bildungsenthalpie von 1,1,2,2‐ und 1,1,1,2‐Tetraphenylethan. – Die Stabilisierungsenergien von Benzhydryl‐ und Triphenylmethyl‐Radikalen

Thermolabile Hydrocarbons, 30. ‐ Thermal Stability and Heat of Formation of 1,1,2,2‐ and 1.1,1,2‐Tetraphenylethane. ‐Energy of Stabilisation of Benzhydryl and Triphenylmethyl RadicalsHeats of combustion of symmetrical (3) and unsymmetrical tetraphenylethane (4) were determined by combustion calorimetry: ΔH(c)(3) = ‐ 3249.5 ± 0.8;(4) = ‐ 3252.0 ± 0.6 kcal/mol. Enthalpies of sublimation of 3 (ΔH = 32.7 kcal/mol) and 4 (31.7 kcal/mol) were obtained from vapour‐pressure‐temperature relations, which were determined by a gassaturation method. The resulting heats of formation ΔH(g) (3) = 85.4 ± 1.3 and ΔH(g)(4) = 87.3 ± 0.8 kcal/mol correspond to low strain energies: H, (3) = 1.5 and Hs (4) = 4.6 kcal/mol. The thermolysis reactions above 200°C in solution with thiophenol as a scavenger are clean first order processes. The following activation parameters are obtained from kinetic measurements: ΔH# (3) = 47.3 ± 0.3, (4) = 45.0 ± 0.3 kcal/mol, ΔS# (3) = 12.8 ± 0.5 and (4) = 14.0 ± 0.6 e.u. From the presented thermodynamic and kinetic data the stabilisation energies of the benzhydryl (1) and the trityl radical (2) have been derived by comparison with the corresponding data for the alkanes 7 and 9. As a result 1 is stabilysed by 12.9 ± 0.9 and 2 by 20.0 ± 1.7 kcal/mol, compared with a secondary or tertiary alkyl radical, respectively. Thus the substituent effect of two (or three) phenyl rings in 1 (or 2) is smaller than twice (or thrice) the value for one phenyl group14): 7.8 ± 2.0 for a secondary and 8.4 ± 1.2 kcal/mol for a teriary benzyl radical. This can be explained by steric repulsion between the phenyl rings and steric inhibition of resonance.

15N‐13C spin coupling as a probe of electronic structure in benzenediazonium ions: INDO—SOS—MO calculations and steric inhibition of resonance

AbstractThe magnitude of 1J(15N13C) in para‐substituted benzenediazonium ions has been used to assess the importance of the several resonance forms contributing to the ground state of the ion. From a comparison of values observed in 4‐nitro‐ and 4‐dimethylamino‐benzenediazonium ions and their respective 3,5‐dimethyl derivatives, it is concluded that diazo‐type structures are important only with strong electron‐releasing substituents in the aromatic ring. INDO–SOS–MO calculations indicate that the large and negative Fermi contact term dominates this interaction.

ChemInform Abstract: Structure and Acidity of 4‐(2′,4′,6′‐Trinitrophenyl)phenol: Steric Inhibition of Resonance in Unsymmetrical Biphenyls.

ChemInform Abstract: Structure and Acidity of 4‐(2′,4′,6′‐Trinitrophenyl)phenol: Steric Inhibition of Resonance in Unsymmetrical Biphenyls.

Specific effect of pentafluorophenyl fragment on electronic state of carbonyl group in ketones, amides, and hydrazides containing a pentafluorophenyl group

1. The effect of the C6F5 group on the ΝC=0 frequency of C6F5COR compounds depends on the structural features of the R fragment; for ketones, amides, and hydrazides there is steric inhibition of resonance in the C6F5CO group and an increase in the role of the inductive effect from C6F5. 2. In N-dimethylpentafluorobenzamide the decrease in resonance is compensated by p,π-conjugation in the CON group, and the inductive effect of the C6F5 ring appears to be weaker than in pentafluoroacetophenone. 3. The presence of the second nitrogen atom and the change in relative orientation of the unshared electron pair when affected by substituents at N1 and N2 in pentafluorobenzahydrazide molecules leads to an increase in the torsion angle in C6F5-CO in comparison to N-dimethylpentafluorobenzamide, which increases the -I effect of C6F5 on ΝC=0 in pentafluorobenzhydrazides.