Protonenresonanz‐Untersuchungen zur inneren Rotation, VIII. Konformationsanalyse α.α.<i>o</i>.<i>o</i>′‐tetrasubstituierter Toluole mittels magnetischer Kernresonanz und halbempirischer Energieberechnung

. 56,36(1978). On the basis of the long-range couplings over six bonds between the protons on the side chain and the ring protons in benzal fluoride, chloride, and bromide and some of their derivatives, a hindered rotor model ( J method) yields limits to the barriers to internal rotation. These are 0.0 to 1.8,0.7 to 2.4, and 1.7 to 3.8 kcal/mol for the fluoride, chloride, and bromide, respectively. Refinement of the model suggests barriers of 1.1 f 0.2, 2.2 f 0.3, and 3.5 rt 0.6 kcall mol, respectively. The barrier values in the chloride and bromide are considerably less sensitive to the model parameters than is the barrier in the fluoride. It would be of interest to have gas phase values for these barriers. It is also shown that a o-x mechanism holds for the six-bond proton-proton coupling constant in benzal fluoride but that the analogous coupling between 19F on the side chain and the para ring proton carries a non o-x contribution of +0.3 Hz.TED SCHAEFER, WERNER DANCHURA et WALTER NIEMCZURA. Can. J. Chem. 56.36(1978). [Traduit par le journal]

Section: A Ibsupporting

confidence: 57%

Limits to barriers to internal rotation in benzal fluoride, chloride, and bromide solution by the J method

Schaefer¹,

Danchura²,

Niemczura³

1978

“…The chloro and methyl substituents are often (19) methyl groups cause (8) to be nearer 90" than do the two ortho chlorine atoms. There are some experiments, for example, on ortho disubstituted isopropyl benzene derivatives (19,20), that agree that a methyl has larger steric requirements than a chlorine substituent. Yet, a methyl group is a better n electron donor than a chlorine substituent, therefore possibly causing a larger 8 for the SCH3 group (compare 'J(C,C) for 4-methyl and 4-chlorothioanisole in Table 1).…”

Section: Resultsmentioning

confidence: 99%

Mechanisms of long-range ¹³C, ¹³C spin–spin coupling in thioanisole and its derivatives. Conformational applications

Schaefer¹,

Penner²

1988

. Can. J. Chem. 66, 1229Chem. 66, (1988.The I3c nuclear magnetic resonance chemical shifts and the long-range I3C,l3c spin-spin coupling constants are reported for 23 thioanisole derivatives enriched in I3C at the methyl position. Forpara and metn substituted thioanisole derivatives, "J(C,C) (n being the formal number of bonds intervening between the coupled nuclei) can be related to functions of the angle by which the thiomethyl group twists out of the aromatic plane. For n = 3 , 4 , 5 , the ensuing relationships yield estimates of the twofold barriers to rotation about the C( l ) -S bond. The barrier is lower in ethyl phenyl sulfide than in thioanisole derivatives. Complications arise for ortho substituted thioanisole derivatives but estimates of the torsional motion about the C( 1) -S bond can be obtained from the observed "J(C,C). Among the complications is the expected fact that 3~( C , C ) , which is shown to be larger in the cis than the trans orientation of the intervening bonds ("anti-Karplus" behaviour), is perturbed by the substituent attached to the coupled nucleus. It is confirmed that in 2-hydroxythioanisole the thiomethyl group is oriented effectively perpendicular to the benzene plane, attributable to a stereospecific hydrogen bond between the hydroxyl group and the 3p lone-pair on the sulfur atom. In acetone-d6 solution, an equilibrium exists between this conformation and one in which an intermolecular hydrogen bond exists with solvent molecules. In the latter, the thiomethyl group prefers a coplanar orientation. In 2-aminothioanisole, the thiomethyl group twists out of the plane by about 60" so as to optimize the N-H. . .3p interaction. This twist angle is changed very little in acetone-d6 solution because the second N-H bond can hydrogen bond to the solvent molecules without disrupting the intramolecular N-H. . .3p interaction. It is also shown that the chemical shift of the I3C nucleus in the methyl group is a good conformational indicator in rneta and para substituted thioanisoles. Therefore it can be used as such for molecules in which "J(C,C) is difficult to find, in 1,4-dithiomethylbenzene, for example.On a mesurk les dkplacements chimiques en rLsonance magnktique nuclCaire du I3C ainsi que les constantes de couplage spin-spin A longue distance I3c,l3C pour 23 dkrivCs du thioanisole enrichis en I3C dans la position du mCthyle. Pour les dkrivks du thioanisole substitutts en positionspara ainsi que me'ta, les constantes 'J(C,C), pour lesquelles n correspond au nombre de liaisons formelles intervenant entre les noyaux qui sont couplks, peuvent Ctre relikes aux fonctions de l'angle de torsion du groupement thiomkthyle par rapport au plan aromatique. Pour n = 3 , 4 ou 5, les relations qui en dkrivent permettent d'kvaluer les banikres binaires A la rotation autour de la liaison C( 1) -S.La barrikre est plus faible dans le cas du sulfure d'ithyle et de phknyle que dans les cas des dkrivks du thioanisole. On fait face i des complications avec les dCrivks du thioanisole substituks en ortho; tout...

“…The J method suffers from an oppoWhen X = CH,, the classical DNMR method and site disadvantage. As the bamer increases, the molecular mechanics calculations agree that the observed 6J becomes less sensitive to V,, particubamer is twofold (2) and much larger than in 1 (X = larly above about 3 kcallmol. H).…”

Section: Introductionmentioning

confidence: 56%

Internal barriers to rotation in 2,6-difluoroisopropyl and 2,6-difluoroethylbenzenes. Overlap of dynamic nuclear magnetic resonance (DNMR) and the J method

Schaefer¹,

Veregin²,

Laatikainen³

et al. 1982

The temperature-dependent I9F nrnr spectra of 2,6-difluoroisopropylbenzene yield AGh5, AH*, and AS* as 6.93 (5) kcal/mol, 6.1(1) kcal/mol, and -5.0(8)cal/mol K, respectively, for the internalrotation ofthe isopropylgroupaboutthespz-sp3 carbon-carbon bond. The long-range spin-spin coupling constant over six bonds, 6JpH-CH, combined with the J method gives a twofold internal potential barrier of 5.0 f 1.6 kcal/mol at 305 K. Although in this barrier range the J method suffers from large errors, the two methods yield comparable values for the banier height. The lineshape method is inapplicable to 2,6-difluoroethylbenzene. The J method finds the preferred conformation and a twofold barrier of 6.0 + 2 kcal/mol, again in a barrier region where this method is inaccurate.Relative to hydrogen, the fluorine substituents cause substantial increases in the barriers to internal rotation. Signs of the stereospecific couplings, 4J0F,CH, are determined. La mesure de spectres de rmn du I9F en fonction de la temperature du difluoro-2.6 isopropylbenzene perrnet d'obtenir les valeurs des ACTs5, AH* et AS* qui sont respectivement de 6,93 (5) kcal/mol, 6,1(1) kcal/mol et -5,08(8) cal/mol K pour la rotation interne du groupe isopropyle autour de la liaison carbone-carbonesp2-sp3. Laconstantede couplage alongue distance a travers six liaisons 6JpH,CH, combinCe ? I la mkthode J donne une baniere de potentiel interne binaire de 5,O + 1,6 kcal/rnol 305 K. Bien que dans cet ordre de grandeur, pour la baniere, la mCthode J cornporte degrandes erreurs, les deux methodes donnent des valeurs comparables pour la hauteur de la baniere. La methode de la forrne des lignes ne peut s'appliquer dans le cas du difluoro-2,6 ethylbenzene. La mCthode J perrnet d'etablir la conformation prkintkgrke et une barriere binaire de 6,O f 2 kcal/rnol; il s'agit encore d'une rkgion de la baniere ou cette methode n'est pas precise. Par rapport a I'hydrogtne, les substituants fluor provoquent une augmentation substantielle des banieres internes la rotation. On a dktermine des signes de couplages stkrCospkcifiques 4J0F,CH.[Traduit par le journal]