The Transmission of Electronic Effects through Carbon, Oxygen, and Sulfur Atoms. Proton Magnetic Resonance Chemical Shifts for Toluenes, Acetophenones, and Thioanisoles<sup>1a,b</sup>

. J. Chem. 55, 552 (1977).On the basis of the observed spin-spin coupling constants between the sulfhydryl and ring protons and a hindered rotor treatment of the twofold barrier to internal rotation in a series of para substituted benzenethiol derivatives, it is argued that V2 is essentially zero in p-aminobenzenethiol and is 2.5 + 0.2 kcal/mol in p-nitrobenzenethiol; having intermediate values for the methoxy, fluoro, methyl, and bromo derivatives in solution. The results are based on an assumed relationship between the four-bond and the fictitious six-bond couplings to the sulfhydryl proton. The conclusions are consistent with the observed magnitudes of the couplings oper six and seven bonds, respectively, between the sulfhydryl proton and the fluorine nucleus and the methyl protons in the appropriate derivatives; as well as with the coupling between the sulfhydryl and methyl protons in 4-bromo-3-methylbenzenethiol. The experimental barriers are compared with ab initio molecular orbital calculations of their substituent dependence.TED SCHAEFER et WILLIAM J. E. PARR. Can. J. Chem. 55, 552 (1977).En se basant sur les constantes de couplage spin-spin observees entre les protons sulfhydryles et les protons du cycle et en traitant comme un rotor empCchC la barriere binaire la rotation interne dans une serie de benzenethiols substituts enpam, on suggtre que V2 est pratiquement Cgal a zero dans lep-aminobenzenethiol et a 2.5 i 0.2 kcal/mol dans lep-nitrobenzenethiol; en solution, les valeurs sont intermediaires pour les derives methoxy-, fluoro-, methyl-et bromo-. Ces resultats sont bases sur une relation hypothktique entre les couplages a travers quatre liaisons et sur les couplages fictifs a travers six liaisons pour le proton sulfhydryle. Les conclusions sont en accord avec les ordres de g r a n d e~~r observes respectivement pour les couplages a travers six et sept liaisons entre le proton sulfhydryle et le noyau de fluor et les protons des methyles dans des derives appropries: ces conclusions sont aussi en accord avec le couplage entre les protons sulfhydryles et methyles du bromo-4 methyl-3 benzenethiol. On compare les barrieres experimentales avec les rtsultats de calculs ab inirio d'orbitales moltculaires predisant leur dependance sur les substituants.[Traduit par le journal]

Section: Resultsmentioning

confidence: 99%

The para substituent dependence of the internal rotational barrier in benzenethiol

Schaefer¹,

Parr²

1977

“…The compounds were prepared as dilute solutions (sec tabular data), (32,33), and is not sensitive to peculiar "ortho" or "heavy-atom" parameters (33). Figure 2 displays an illustrative plot2 of 4~y 3 versus SCH3 for 18 of the compounds in Table 2.…”

Section: J F C~i )mentioning

confidence: 99%

Some unique properties of in 2-fluorotoluene derivatives. Conformational dependence

Schaefer¹,

Sebastian²,

Veregin³

et al. 1983

, and REINO LAATIKAINEN. Can. J. Chem. 61, 29 (1983).A complete analysis of the 'H nmr spectrum of 2-fluorotolucne yields '~, 7 "~~, the spin-spin coupling constant between '"F and the methyl protons, as 1.99 Hz. Analysis of nmr spectra of 21 other derivatives of 2-fluorotoluenc shows that ' J~" "~ can vary between 1.69 and 2.55 Hz. This strong dependence on substitution contrasts with the near invariance of other long-range couplings such as 6 CI-'3 and SJ,;;'"~ J,, ' . The substituent dependence is discussed in terms of coupling mechanisms.INDO MO FPT calculations of ' j~' " "~ are inadequate. By means of appropriate model compounds, an adequate empirical conformational dependence is deduced for 4~~' C t " , which can be used to reproduce some observed couplings. INDO MO FPT computations, in which certain off-diagonal Fock matrix elements are suppressed, are used to show that spin polarization via interacton of the methyl hydrogen orbitals is a major source of the discrepancy bctween theory and experiment. Some S T 0 3G MO calculations are reported for 2-fluorotolucne conformations.TED SCHAEFEK, RUDY SEBASTIAN, RICHARD P. VEREGIN et REINO LAATIKAINEN. Can. J. Chem. 61, 29 (1983). Une analyse complktc du spectre de rmn du 'H du fluoro-2 tolukne fournit la constantc de couplage spin-spin entre le ' 9~ et les protons du methyle, 'J:"~' = 1,99 HZ. L'analyse des spectres de rmn de 21 autrcs dCrivCs du fluoro-2 tolukne montre que JJ,:'." '' peut varier cntre 1,69 et 2,55 Hz. Cettc forte influence dc la substitution contrastc avec la quasi invariance des autres dCduit une influence conformationnelle empiriquc adequate pour J,, q u~ pcut ktre utilisCe pour reproduire quelques-uns des couplagcs observCs. On a utilisC dcs calculs INDO MO FPT dans lcsqucls on a supprime certains ClCmcnts de la matrice de Fock hors diagonalc pour montrer que cettc polarisation de spin par I1intermCdiaire des orbitales des hydrogknes du mkthyle est la cause principale dc cettc diffirence entrc la thCorie et I'expCrience. On rapporte quclques calculs d'OM S T 0 3G sur les conformations du fluoro-2 toluknc.[Traduit par le journal]

“…If the substituent effect is maximal when the benzylic C-H bond is colinear with the adjacent p orbital and minimal when perpendicular to it then the results for this sulfoxide and for the decrease in p with increasing size of Zcan be accounted for. 4 Or, to rephrase the explanation we can say that a decrease in p represents a shift of conformational population from that shown in 2 to that shown in 3. Our previous results on the effect of substituents on geminal coupling constants in the tetrahydropyranyl ethers and sulfoxides led to the same conclusions (20).…”

mentioning

confidence: 97%

“…We wish to assume, as has been done for every Hammett correlation (4,19), that a parasubstituent affects only the n electron distribution of the benzene ring, and that no steric factors are involved. Then a change in p from series to series must be attributed to a change in some property or properties of Z.…”

mentioning

confidence: 99%

Effect of substituents on the chemical shift of benzylic protons

Fraser¹,

Gurudata²,

Renaud³

et al. 1969

The chemical shift of the benzylic protons in fifteen series of para-substituted toluenes bearing different groups in the alpha position are reported. In each series a plot of the chemical shift in tau units of the benzylic protons against the Hammett o value of thepara substituent is linear. The slopes of these plots vary from -0.20 to +0.01 p.p.m./o. The variation of slopes appears to be conformationally dependent, an observation at odds with current theory. In two rigid systems having the same geometry the slope has been found to vary from -0.12 to -0.18 indicating a dependence on other factors as well. The effect of solvent on the slopes has been determined, and shows no correlation with solvent dielectric properties.