Solvent effects in carbon-13 nuclear magnetic resonance. Electronic perturbation of aromatic systems

Abstract

13C chemical shifts of phenyl ring carbons in substituted benzenes can be used to monitor changes in charge distribution at those carbons. Strong solute-solvent interactions such as hydrogen bonding to basic substituents result in significant changes in ring carbon chemical shifts. The changes in 13C shifts are related to the electronic perturbation of the substituent and the ring system in a near quantitative manner.Studies of these solvation effects in relatively dilute solution are facilitated by the use of Fourier transform (FT) techniques. Dilution curves indicate that for groups such as -OCH3 or -COCH3 in CF3COOH, a 10-15 mol % solute concentration effectively simulates infinite dilution insofar as electronic perturbation of the solute is concerned. By use of para 13C resonances, estimates of + values can be obtained for most substituents in most media.n contrast to the extensive investigation of solvent effects on proton chemical shifts, much less attention has been paid to the effects of solvents on 13C chemical shifts. Solvent dependence of the 13C chemical shifts of methyl iodide,2 acetonitrile,2 and chloro-form3 has been reported. Extensive solvent studies have been made on 13C resonances of carbonyl carbons,4-6 which were more easily observable by the 13C instrumental methods of the early 1960's. The carbonyl carbon of acetone, for example, shows a 13C chemical shift range of some 40 ppm over a variety of solvents.4 Surprisingly little work, however, has been reported on the effect of solvents on 13C chemical shifts of substituted aromatic systems. Some work relative to the effect of solvents on the 13C chemical shifts of phenol,7 benzonitrile,8 ,/V,iV-dialkylanilines,9 and acetophenones10 has been reported. Other than

The charge state of trimethoprim bound to Lactobacillus casei dihydrofolate reductase

“…In addition, the 2-amino group probably forms a hydrogen-bond to Thr 116 (L. casei numbering) [ 11,15,1 6] and this will lead to a shift of the resonance of the directly bonded carbon. For example, in aniline the C a resonance is 2.1-2.7 ppm further downfield in dimethylsulphoxide or acetone solution than in carbon tetrachloride solution, presumably due, at least in part, to hydrogen-bonding of the amino group [22].…”

13C Fourier transform nuclear magnetic resonance. XII. Structure of the phytol–lecithin bilayer. Application of electric field effects

“…Changes in the electron density, 69, at a carbon atom as a result of the polarization of the bond by an electric field can be estimated bv where the sum is over all the bonds of an atom, El is the field resolved parallel to the bond, 1 is the length of the bond, bI1 is the longitudinal bond polarizability and e is the electronic charge. (19,20). Due to the large polarizability of a C-C bond relative to a C-C or C-H bond, the sum in [I] may be approximated by a single term.…”

Effet de solvant en résonance magnétique nucléaire de 13C. I. Cas d'un soluté apolaire: n-pentane

“…Par contre les corrilations avec g-e meme qu'en rmn du proton (26) incluent la phase eazeuse lnais 19Ccart-type est trks largement autres solvants, la proportionnalitt de U, B la fonction g2 suivant [7] est sensiblement vCrifiCe pour les trois sites magnttiques du rz-pentane et pour les deux sites mesurables du perfluoro-tzhexane (les pentes des corrtlations figurent dans le tableau 2), justifiant ainsi la validitt de la fonction g2. Mais [7] ne rend pas compte de la variation du facteur de solutt pour des sites magnktiques diffirents appartenant B une m@me molicule.…”

“…Mais [7] ne rend pas compte de la variation du facteur de solutt pour des sites magnktiques diffirents appartenant B une m@me molicule. Ce point sera discutk ci-dessous.…”

“…28, 31-36) (11,23,28,31,(34)(35)(36) Si on admet comme Rummens (26c) que le comportement macroscopique de CS2 est isotrope, I'icart champ fort relevC par rapport au terme en g2 pour les mithyles, reste inexplicable meme en adjoignant B [7] le facteur de site de mClange de deux solvants I est:…”