Measurements of 13C spin-lattice relaxation times (TO and nuclear Overhauser effects at 25 MHz are reported for a number of aromatic compounds and are discussed in terms of four relaxation mechanisms: dipoledipole (DD), chemical shift anisotropy (CSA), spin rotation (SR), and scalar (SC). Examples are given of compounds with carbons dominantly relaxed by all of these mechanisms. Protonated ring carbons are largely relaxed by the DD mechanism; nonprotonated ring carbons are relaxed by both the DD and SR mechanisms, with the exception of 79Br-bonded carbons, which can relax entirely by the SC mechanism. For bromine-bonded carbons, the relaxation is nonexponential since the TVs are different for the two bromine isotopes. The CSA mechanism is negligible in these compounds but is the dominant relaxation mechanism for the central acetylenic carbons in diphenyldiacetylene, as shown by experiments at 25 and 63 MHz. The large contributions of DD and SR relaxation and nearly insignificant CSA contribution for the nonprotonated carbon of toluene were approximately determined from 25-and 63-MHz experiments. Dipole-dipole relaxation of protonated aromatic ring carbons in substituted benzenes is strongly affected by ring substitution. Large or polar substituents reduce molecular tumbling, lengthening the molecular correlation time, r" thereby shortening observed TVs. Anisotropic motion has an easily observable effect on the DD contribution to 7) and can form the basis for spectral assignments, as in 3-bromobiphenyl. With phenol and aniline, strong solvent effects owing to molecular association or protonation are found and affect not only the absolute values of 7), but also the ratios of 7>n;/7>. 3089 (1972).

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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

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6238was determined as a function of concentration [(0.94 -25.16) X MI relative to the chloroform signal at 7 2.73 (727 Hz downfield from TMS) (Figure 4). The spectra of the acetates (I and 11, coupling constants were determined either from first-order analyses or from appropriate ABX approximation^.^^ R = Ac) also were obtained in carbon tetrachloride and benzene-& (0.50-0.93 M ) . In general, all spectra were recorded under identical conditions and were reproducible. Chemical shifts and 1959, Chapter 6.Abstract: Basicity constants determined by proton nmr are reported for several types of ketones. Carbonyl substituent chemical shifts are plotted against Hammett acidity (Ho) yielding half-protonation values that reproduce spectrophotometric basicity constants. The nmr method facilitates basicity measurements for extremely weak ketone bases in superacid media. Basicity constants for several a-halogen substituted acetones are reported. Fluoro, chloro, and bromo substituents all reduce ketone basicity substantially. The three halogens are approximately equal in their effect. Carbonyl compounds that are not stable enough for spectrophotometric basicity studies may be evaluated by nmr. The nmr basicity constant for acetaldehyde (pK = -10.2) indicates that the aldehyde is far less basic than expected. Several ketones are used as a set of indicators for rapid evaluation of (Hammett) medium acidity by nmr. Applicability of the nmr method for acidity evaluation over the entire Hammett scale (currently Ho = 0 to -17.5) allows facile measurements in both superacid and conventional acid systems.he quantitative study of ketones as weak organic (b) D. G. Lee, Cart.