The 15.1 Mc/s I3C n.1n.r. spectra of 34 a,p-unsat~~rated carbonyl compounds have been obtained and the chemical shifts of the olefnic and carbonyl carbon nuclei are reported. Esamples were drawn from the acyclic and five-and six-membered ring alicyclic series. A few aldehydes are included although most of the con~pounds are ketones. In the case of compounds ass~~rned to be planar, variations of the olefinic shieldings with alkyl substitution parallel those observed previously for olefinic hydrocarbons. The key factor governing the chemical shifts of the carbo~lyl-carbon nuclei appears to be the extent of conjugation in a given system. Conseq~~ently, the 13C results prov~de evidence for steric interference to coplanarity in the hi~ldered systems. This approach is compared with other methods of estimating the extent of steric inhibition of co~ljugation and its merits are assessed.

No abstract

The 15.1 Mc/s 13C nuclear magnetic resonance spectra of 15 aliphatic and alicyclic ketones having a three-membered ring alpha to the carbonyl group have been obtained and the shieldings of their carbonyl carbon nuclei are reported. A number of derivatives of bicyclo-[4.l.O]heptan-2-one, bicyclo[3.l.0]hexan-2-one, spiro[2.5]octan-4-one, and spiro[2.4]heptan-4-one have been included. The primary objective was to investigate the question of conjugation of the cyclopropyl ring with a carbonyl grouping, since we had shown previously that 13C shieldings of carbonyl carbons are significantly affected by conjugative interactions. The 1SC data for several related compounds were obtained to aid in our assessment of the cyclopropyl results. Some evidence of cyclopropyl conjugation is presented.

PROTON SPECTROSCOPY has been employed as a tool for the estimation of potential barriers to rotation about a-bonds to carbonyl groups in several systems in which this bond possesses some n-character.' Amides and closely related derivatives have received most attention, but estimates for some benzaldehydes have been presented.2 We report results for the p-NN-dimethylamino-(A) and p-methoxy-(B) derivatives of benzaldehyde (I) and acetophenone (11), obtained by examination

The carboxyl substituent effects for branched carboxylic acids have been determined by carbon-13 Fourier transform nuclear magnetic resonance of eighteen measured and reported branched acids. For a-branched systems the substituent effects are 01 = 15.3 ppm, p = 2.6 ppm, y = -1.9 ppm, S = 0.9 ppm and E = 0.5 ppm. For other branched acids, the determined carboxyl substituent effects are a = 19.2 ppm, p = 2.1 ppm, y = -1.4 ppm, S = 0.6 ppm, and CE = 0.6 ppm.