Steric Inhibition Of Resonance Research Articles

Influence of the 1,1,3,3‐tetramethylguanidinyl substituent on the charge distribution in chlorobenzenes, studied by 35Cl NQR

Abstract35Cl NQR measurements were carried out on a series of chlorobenzenes with the 1,1,3,3‐tetramethylguanidinyl substituent {TMG = –N=C[N(CH3)2]2}. The substituent constants σp, σm, σaI, and σR for the TMG group were estimated from the relationships between the NQR resonance frequencies, νQ, and substituent constant values, σ. The νQ value for 2‐(4‐chlorophenyl)‐1,1,3,3‐tetramethylguanidine is the lowest observed for all chlorobenzenes, and arises from the strong positive conjugative effect of the TMG group. This effect is attributed to the possible resonance stabilization of the positive charge. Steric inhibition of resonance is observed for the ortho‐substituted isomers.

Ring-size effects on the ionization potentials of N-substituted azacycloalkanes

The ionization potential of the lone-pair electrons of nitrogen has been determined for N-methyl- and N-tosyl-azacycloalkanes by means of gas-phase u.v. photoelectron spectroscopy. The cyclic compounds examined contain small, common, medium, and large-membered rings. The ionization potentials are affected by conformational features and, in particular, by the strain present in the ring. More precisely, the IPs reflect changes of hybridization of the nitrogen Ione pair upon varying the ring size, whereby a lower or higher energy is required for the ionization process of the cyclic compounds relative to a series of open-chain alkyl amines taken as reference. Steric inhibition of resonance by the N-tosyl substituent takes place in the case of the strained small rings.

17O NMR spectroscopy of heterocycles. Steric effects for N‐oxides

AbstractThe 17O chemical shift data for a series of 2‐ and 4‐alkyi‐ and ‐aryl‐substituted pyridine N‐oxides, as well as those for quinoline N‐oxide (13), benzo[f]quinoline N‐oxide (14) and benzo [h]quinoline N‐oxide (15) at natural abundance were measured in acctonitrile at 75°C. 2‐Alkyl groups cause a deshielding effect on the N‐oxide chemical shifts compared with those of the 4‐alkyl isomers. The largest effects are noted for 2‐tert‐butylpyridine N‐oxide (8) and 2,6‐dimethylpyridine N‐oxide (12). Similarly, the 17 O chemical shift of 15 is deshielded by 18 ppm compared with that of 13 and 14. 17O chemical shift data were obtained for 2‐and 4‐tert‐butylanisoles and compared with those of 8 and 4‐tert‐butylpyridine N‐oxide (9). The sensitivity of the N‐oxide 17O chemical shifts was substantially greater than that for the anisoles, an effect that was also observed in an analysis of the 13CNMR data of the tert‐butyl groups of 8 and 9. Steric inhibition of resonance is noted for 3‐methyl‐4‐nitropyridine N‐oxide; the N‐oxide 17O signal was shielded and the NO2 17O signal was deshielded relative to those for 4‐nitropyridine N‐oxide.

Basicity of the carbonyl group. Part 11. A re-examination of the electrophilic substituent constants for amino-substituents from thermodynamic measurements

Hammett–Brown correlations between σp+ and pKBH+ values or enthalpies of complexation, ΔH°, with boron trifluoride for three series of carbonyl compounds, 3-substituted 5,5-dimethylcyclohex-2-enones, 4-substituted acetophenones, and benzaldehydes, have enabled a re-evaluation of electrophilic substituent constants for various amino-groups to be made. A relationship is obtained between σp+(NR1R2) and Σσ*(R). The electron-releasing ability of the amino-groups is found to be larger, in a given series, than those expected from the Brown–Okamoto classical values and other more recent estimates. The discrepancies are attributed to charge delocalisation on the probe, steric inhibition of resonance, and to specific solvent associations with the amino-substituents. Owing to their large σp+ values, we suggest deletion of amino-group data when using a non-linear treatment of substituent effects.

Application of a quadratic free energy relationship to non-additive substituent effects

A quadratic free energy relationship is formulated in which the reaction constant dependes linearly upon substituent constant, e.g. in Hammett's formalism ρ=ρ0+ 2mσ. The relationship is applied to dual substitution at equivalent positions (X and Y) in the form log kXY/kHH=ρ0(σX+σY)+m(σX+σY)2, using data from the literature for the bromination of diphenylethylenes, solvolytic and equilibrium formation of diphenylmethyl carbonium ions, and the ionisation of 2,7-substituted fluorenes. Non-additivity of substituent effects is quantitatively described and quadratic co-efficients 2m/ρ02 can be derived without specifying substituent constants. Values of 2m/ρ02 measure the sensitivity of ρ and the selectivity of the reaction to changes in reactivity. Relative magnitudes are interpreted in terms of variations in transition state structure, resonance saturation, and steric inhibition of resonance. The relationship is compared with Miller's equation, log kXY/kHH=ρ0(σX+σY)+qσXσY, with respect to effectiveness of correlations, chemical interpretation, desirability of preserving linear relationships for monosubstitution, and practical application to multiple substitution at non-equivalent positions.

The solvatochromic comparison method. 7. Solvent polarity and hydrogen bonding effects on steric inhibition of resonance

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTThe solvatochromic comparison method. 7. Solvent polarity and hydrogen bonding effects on steric inhibition of resonanceMary Elizabeth Jones, R. W. Taft, and Mortimer J. KamletCite this: J. Am. Chem. Soc. 1977, 99, 26, 8452–8453Publication Date (Print):December 1, 1977Publication History Published online1 May 2002Published inissue 1 December 1977https://doi.org/10.1021/ja00468a012RIGHTS & PERMISSIONSArticle Views276Altmetric-Citations9LEARN 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 (250 KB) Get e-Alerts Get e-Alerts

Application of NMR spectroscopy to the study of coupling effects

1. Study has been made of the NMR13C spectra of various alkylthio- and alkoxybenzenes. 2. Data on the benzene-ring13C chemical shift indicate that the shift results from p-π coupling of the hetero atom with the unsaturated fragment, a factor which is more significant in the alkoxybenzenes than in the alkylthiobenzene. The degree of p-π coupling is determined by steric inhibition of resonance. 3. The effect of the alkyl substituent on the coupled hetero atom-unsaturated fragment system is the same in the alkyl phenyl and alkyl vinyl ethers and sulfides.

The kinetics and mechanisms of aromatic halogen substitution. Part XXXIII. Kinetics and products of chlorination of some aryl acetates

The rates and products of reaction of 4-methylphenyl acetate, 3,4- and 2,6-dimethylphenyl acetate, and 2,3,5,6-tetramethylphenyl acetate with molecular chlorine in acetic acid have been studied. ‘Abnormal’ reaction paths play a large part in the determination of the products of the chlorination of the first two of these compounds, probably through electrophilic attack on an activated ‘ipso’-position. The observed rates of reaction are compared with those calculated on the basis of additivity of substituent effects; they allow an estimate to be made of the effect of steric inhibition of resonance on the directing power of the acetoxy-group. The product of chlorination of the corresponding phenols are recorded also.

Steric Inhibition of Resonance in Ortho- substituted Diphenylsulphides Studied by Photo- electron (He I) Spectroscopy

The analysis of the four lowest ionization energy values related to π MO's in diphenylsulphide and its o-o'-dimethyl and o-o-o'-o'-tetramethyl derivatives indicates that the conformation of the unsubstituted compound largely deviates from planarity, that this deviation increases with ortho-substitution and that the two phenyl rings in the tetramethyl derivative are nearly orthogonal to each other

NMR-13C spectra and effects of conjugation in alkoxy- and alkylthioethylenes

1. The NMR-13C spectra of a number of vinyl alkyl sulfides was studied. An analysis of the chemical shifts of the carbons of the multiple bond gives unambiguous evidence that weak p-π conjugation of the heteroatom with the multiple bond, the intensity of which is three times lower than in vinyl alkyl ethers, operates in these compounds. 2. The chemical shifts of13Cβ in the series of vinyl alkyl sulfides and ethers show that in both cases the degree of p-π conjugation is determined by the steric inhibition of resonance, which is more pronounced in the series of sulfides.

Studies of 13CH coupling constants: III—the conformational dependence of 1J(13CH) for the formyl proton in aromatic aldehydes

Abstract1J(13CH) and 2J(13CH) involving the aldehydic proton in ortho‐alkylbenzaldehydes have been examined for evidence of a hyperconjugative contribution from the aromatic π‐electrons. The expected hyperconjugative effect is obscured by changes in the coupling constants resulting from steric inhibition of resonance.

Steric inhibition of resonance in nitroanilines by ESCA

Steric inhibition of resonance in nitroanilines by ESCA

Hexamethylazoxybenzene. Structure, Basicity, and Rearrangement

2,2′,4,4′,6,6′-Hexamethylazoxybenzene (1) has been prepared and characterized. The u.v. spectrum is indicative of large distortion from coplanarity. 1 is only slightly more basic than azoxybenzene despite expectations due to electronic effect of the methyl substituents. Two factors are considered responsible for this: steric inhibition of resonance in the unprotonated species and steric hindrance to hydration in the conjugate acid.Rearrangement of 1 in moderately concentrated sulfuric acid gives rise to the alcohol 4-hydroxymethyl-2,2′,4′,6,6′-pentamethylazobenzene (6). A Bamberger type phenolic product which would result from methyl migration is not observed. Possible reaction mechanisms are given.

Transmission of electronic effects in organosilanols

The effectiveness of the silicon atom in transmitting electronic effects in a series of substituted diphenylsilanediols, dialkylsilanediols, and poly(diorganosiloxane)-αω-diols has been shown by an i.r. spectroscopy study to be high. The acidities of the diorganosilanediols give an approximately linear correlation with Σσ* values, while the protondonating properties of the substituted diphenylsilanediols fall in the order m-CF3 > m-Cl > p-Cl > p-F > o-Me > H > m-Me > p-Me > p-MeO > p-Me2N. In accord with alkylbenzoic acids, the ortho-isomer of ditolysilanediol is the most acidic and this has likewise been attributed to steric inhibition of resonance. The acidities of the halogenodiphenylsilanediols have been interpreted in terms of contributions from resonance structures and related to thermal condensation data. Enthalpy values for the interaction of phenol with the poly(diorganosiloxane)-αω-diols have been calculated and the changes in basicity have been interpreted in terms of multicentre π-bonding in the siloxane chain. A number of the para-substituted diphenylsilanediols appear to be dimorphous.

The effect of geometric isomerism on the carbon-13 chemical shift of diiodoethylene and related compounds

The origin of the large difference between the 13C chemical shifts of cis and trans diiodoethylene and related compounds was investigated. The involvement of significant medium effects was ruled out by results with dilute solutions of 13C-enriched materials. Experiments with the analogous diethyl maleate and fumarate in a variety of solvents including hydrogen-bonding acids and data on some other symmetrical disubstituted ethylenes cast doubt on an earlier interpretation based on the steric inhibition of resonance in cis isomers. An application of the Pople-Karplus formalism in the manner previously used for unsymmetrical ethylenes does not lead to a satisfactory interpretation if the mean ΔE is assumed to be independent of geometrical isomerism.

Tri‐o‐tolylphosphine: A rapid photopolymerization initiator

AbstractThe rate of tri‐o‐tolyphosphine‐initiated photopolymerization of methyl methacrylate was found to be significantly higher than that for the triphenylphosphine initiated polymerization. The mechanism was shown to be dependent upon the concentration of the initiator. At low concentrations (10−4 mole/l.), an activated complex is the initiating species, while at higher concentrations (above 10−2 mole/l.) homolytic cleavage of the phosphorus–carbon bond is the radical source. Steric inhibition of resonance is advanced as an explanation for the enhanced activity of tri‐o‐tolylphosphine over triphenylphosphine.

Substituent effects of pyrrolidino and piperidino groups in benzoic acids and nitrobenzenes

AbstractBenzoic acids and nitrobenzenes, substituted in the para position with a pyrrolidino or a piperidino group are compared with dialkylaminobenzoic acids and dialkylaminonitrobenzenes.The acidities of the substituted benzoic acids, the polarographic halfwave potentials of the nitrobenzenes and the rate of reduction of the nitrobenzenes with disodium disulfide, give the following order of electron releasing effect of the substituents:piperidino < dimethylamino < pyrrolidino < diethylamino < di‐isopropylamino.No steric inhibition or promotion of resonance is found with the pyrrolidino group, but steric inhibition of resonance seems important in the piperidinobenzenes. The basicities of the dialkylaminobenzoic acids show a greatly different sequence which is attributed to the changing configuration around the basic site.

Steric Inhibition of Resonance in the Two Diastereoisomeric 4-t-Butyl-N-methyl-N-phenylcyclohexylamines

Steric Inhibition of Resonance in the Two Diastereoisomeric 4-t-Butyl-N-methyl-N-phenylcyclohexylamines

Studies in the indole series. 3. Steric inhibition of resonance in cycloakan[b]indolones.

The ultraviolet, infrared and nuclear magnetic resonance spectra of cycloalkan [b] indolones have been investigated. Steric inhibition of resonance is accompanied by a bathochromic and hypochromic shift in the ultraviolet spectra. It was found that the carbonyl stretching frequency of cycloalkan [b] indolones increased as increasing ring size. The nuclear magnetic resonance spectra also showed ring size effect.

Liquid scintillators. XIII. steric inhibition of resonance in liquid scintillators

Abstract2″,3′ ‐ Dimethyl ‐ p ‐ quaterphenyl (I), 2″, 3′, 5′, 6″ ‐tetramethyl ‐ p ‐ quaterphenyl (II), 6,7 ‐ dihydro ‐ 6 ‐ methyl ‐ 3, 9 ‐ diphenyl ‐ 5H ‐ dibenz[c,e]azepine (III), 5,7 ‐ dihydro ‐ 3, 9 ‐ diphenyl ‐ dibenzo[c,e]thiepin (IV), 5,7 ‐ dihydro ‐ 3,9 ‐ diphenyldibenzo[c,e]selenepin (V), and 6, 6 ‐ dicarbethoxy ‐ 6,7 ‐ dihydro ‐ 3,9 ‐ diphenyl ‐ 5H ‐ dibenzo[a,c]cycloheptene (VI) have been synthesized and screened as potential primary liquid scintillation solutes. The scintil‐ lation properties of I, II, III, IV, V, and VI have been compared with 2,7‐diphenyl‐ fluorene (VII) (4), 9,10‐dihydro‐2,7‐diphenylphenanthrene (VIII) (5), and 5,7‐dihydro‐ 3, 9‐diphenyldibenz[c,e]oxepin (IX) (5). The relative pulse‐heights of I, VI, VII, VIII, and IX at 3 mM in toluene gave a linear relationship when plotted against the cos2 of the respective estimated angles of torsion about the 1″. 4′‐bond in the p‐quaterphenyl system. The azepine III, thiepin IV, selenepin V, and tetramethyl‐p‐quaterphenyl II were very poor scintillators in toluene solution.