Carbon-13 nuclear magnetic resonance spectroscopy. Solvent effects on chemical shifts

Self-association of dipolar substances is studied by 13C magnetic resonance. The dilution shifts in carbon tetrachloride are analyzed in terms of the monomer–dimer equilibrium. Downfield 13C shifts of the nitrile and carbonyl groups are observed for the formation of the dimer of acetone, acetonitrile, and ethyl acetate. Upfield 13C shifts, on the other hand, are observed in the methyl and methylene groups of acetone, methylethyl ketone, ethyl acetate, acetonitrile, propionitrile, nitromethane, nitroethane, N,N-di-methylformamide, and N,N-dimethylacetamide. The equilibrium constants are 0.1–0.5 in units of mole fraction. The downfield 13C shifts of the nitrile and carbonyl groups are interpreted in terms of electric (reaction) field effects imposed by another molecule through dimer formation. The alternation of polarity by polarization of the X–Y group explains the upfield 13C shifts of methyl and methylene groups as shown in the following scheme: Hs+—C−—X+—Y−, where X—Y stands for C=O, C≡N, and NO2 groups which have been classified as –I− groups by Pople and Gordon.

Section: Introductionmentioning

confidence: 99%

¹³C Nuclear Magnetic Resonance Studies on Molecular Association. I. Self-association of Dipolar Molecules

Saitô¹,

Tanaka²,

Nagata³

et al. 1973

“…The next 'specific' interactions can probably be best described in terms of weak hydrogen bonding between the acid anion and the alkyl hydrogen atoms in the tetraalkylaminoniu~~~ ions. Hydrogen bonding to C-H bonds is known to cause low-field shifts for both 13C and lH nuclei (17). The net changes in the external shielding factors can be therefore represented for the aqueous solutions of an acid AH as:…”

Section: Resultsmentioning

confidence: 99%

“…Although the pmr chemical shifts differences are smaller in magnitude, and therefore less reliable, the observed trends can again be interpreted in terms of the hydrogen bonding to the medium and they support conclusions drawn from the results obtained for the carbon chemical shift data. Figure 3 shows model of specific acid anion hydrogen bonding since it has been shown that 13C chemical shifts in CHC13 are almost exactly two times as sensitive to hydrogen bonding interactions as the corresponding 'H chemical shifts (17).…”

Section: Resultsmentioning

confidence: 99%

Medium effects on tetraalkylammonium ions in aqueous acid solutions

Modro¹,

Reynolds²,

Yates³

1976

. Medium effects upon 1H and '3C chemical shifts have been determined for some tetraalkylammonium ions in 0-10 M aqueous solutions of H2S04, HCIO,, and H3P04. The observed effects are very weak in magnitude, thus justifying the choice of tetraalkylammonium ions as reference standards for studies of ionic species in aqueous solutions of strong acids. The trends in the observed low-field shifts are interpreted in terms of changes in cation hydration and in hydrogen bonding between alkyl hydrogens and acid anions.TOMASZ A. MODRO, WILLIAM F. REYNOLDS et KEITH YATES. Can. J. Chem. 54,1439Chem. 54, (1976. On a dCterminC les effets du milieu sur les dCplacements chimiques du ' H et du 13C pour quelques ions tCtraalkylammonium dans des solutions aqueuses de &I0 M e n H2S04, HCIO, et H3P04. Les effets observCs sont de trks faible amplitude ce qui justifie le choix des sels des ions tCtraalkylammonium comme produits de rCfCrence pour des Ctudes d'espkces ioniques dans des solutions aqueuses d'acides forts. On interprkte les tendances observCes dans les dCplacements B bas champs en termes de changements dans l'hydratation du cation et dans les liens hydrogknes entre les hydrogknes des groupes alkyles et les anions des acides.[Traduit par le journal]A knowledge of the behavior of activity coefficients of cationic species is of primary importance in studies of acid-base equilibria and acid-catalyzed reactions in nondilute solutions.' In the acidity function approach, the medium dependence of the activity coefficients of the indicator base (f,) and its conjugate acid CfBH+) provide a key to the understanding of the very individual behavior of different classes of organic bases in their protonation equilibria. For acid-catalyzed reactions it has been demonstrated (2) that it is possible to determine the acidity-activity coefficient dependence for the transition state of the rate-determining reorganization of the protonated substrate for some reactions in strongly acidic media. In this case, the activity coefficient under consideration, .fS, also involves an ionic system and its behavior can be directly coinpared with those of various stable cationic species of related structure. Since the activity coefficients of single ions are experimentally inaccessible, the generally accepted approach introduced by Boyd (3) involves the use of standard reference ions, and presents the ion activity coefficient relative to the reference ion, i.e.where X+ is the standard reference cation. The most commonly used reference cation is the tetraethylammonium ion (TEA+) which is expected to be relatively free of medium-dependent specific solute-solvent interactions? This rather intuitive choice can be, in principle, tested experimentally by examining medium effects on the TEA+ ion in aqueous solutions of strong acids and comparing them with similar effects on some structurally related ions. It is known (1) that the activity coefficients of some tetraalkylammonium ions, relative to that of TEAf, vary significantly with a change of acidity for aqueo...

“…(18). Although Roberts has suggested (20) that anisotropy effects may be negligible at the carbon nucleus of the solute, there is ample evidence of appreciable ring current effects on carbon chemical shifts (21 atoms may well be of this origin, with the small difference in relative shielding reflecting the asymmetric orientation of the solvent molecules necessitated by lone pair repulsions of the oximino oxygen.…”

Section: Methodsmentioning

confidence: 99%

¹³C Nuclear Magnetic Resonance of Oximes. I. Solvent and Isotope Effects on the ¹³C Chemical Shifts of Acetoxime

Gurudata¹

1972

The 13C n.m.r. spectrum of acetoxime has been obtained in five representative solvents and the chemical shifts of the three carbon atoms measured. The solvent effects on the chemical shifts are found to reflect specific solute-solvent interactions. The effect of deuteration of the u-protons on the chemical shift of the oximino carbon is also discussed.Le spectre r.m.n. de "C de I'acetoxime a ete obtenu dans cinq solvants judicieux, et les deplacements chimiques des trois atomes de carbone ont tte mesures. Les effets des solvants sur les deplacements chimiques rendent compte des interactions specifiques solute-solvant. I1 est egalement discute de I'influence d'une deuteration des protons en a sur le dtplacement chimique du carbone oximinique.