Wiley

The literature on ¹³C n.m.r. examinations of steroids has been reviewed and the shielding data for over 400 examples are tubulated. The assignments for each compound have been considered and, where necessary, revised in the light of more recent evidence and for consistency throughout each series. The methods available for assignments are reviewed and, in many cases, illustrated with specific examples. The major practical features concerning ¹³C studies of steroids are discussed as a guide to the use of the technique. From the main body of shielding data, an extensive set of substituent effects has been generated to aid the examination of new systems. The utility and the limitations of these effects are described.

The principles of the theoretical calculations of the carbon atom screening constant are reviewed. The Independent Electron, SCFMO and Valence Bond formulae are briefly discussed, and the relative importance of the different terms involved in an approximate decomposition of σ¹³C are pointed out. The variations of the excitation energy ΔE, the mean distance between the nucleus and 2p electrons, and the electronic charges and bond orders with the structure are presented. The nature of the steric effect is also explained. In a second part the various empirical correlations which make an evaluation of δ¹³C from structural and physical properties possible are shown. The actual substituent parameters of the main classes of compounds are listed and worked examples given to make the use of the tables easier. The correlations between δ¹³C and the electronic charge, electronegativity, Hammett‐Taft constants, electric field effects, geometrical parameters, spectroscopic data (electronic transitions, screening of other nuclei) and pH are also discussed with a view to appraising the carbon screening.

It is shown that in contrast to the use of coupling constants, chemical shift criteria may lead to unambiguous structural elucidations in pseudorotational frameworks (pentacycles, heptacycles, etc.).

The olefins Ph₂P(X)CHCHR [X=lone pair, O, S, Ch₃I; RCh₃, ph, P(X)ph₂] have been prepared and their ¹H, ¹³C and ³¹P NMR spectra measured. trans ³J[P(IV)C] (range 18.3–25.7 Hz) is greater than cis ³J[P(IV)C] (range 6.9–11 Hz) but this relationship does not hold for P(III) compounds. In the ³¹P spectra the E isomer absorbs to higher field than the Z isomer for P(III) and P(IV) compounds. The ¹H data are in accord with previous results; average substituent shielding coefficients for ph₂P(X) substituted alkenes are reported.

La structure de la phase basse température du radical nitroxyde tétraméthyl‐2,2,6,6 pipéridine oxyle‐1 (Tanane) a été déterminée. Ce radical libre est isomorphe du tétraméthyl‐2,2,6,6 pipéridinol‐4 oxyle‐1 (Tanol). Une étude en résonance paramagnétique électronique de monocristaux a permis de déterminer les valeurs principales et l'orientation des axes propres du tenseur ḡ pour ces deux radicaux. Pour le Tanane, la liaison NO est une direction propre du tenseur ḡ. Pour le Tanol, il existe un angle de 8° ± 2° entre cette direction propre et la liaison NO. Il est vraisemblable que, dans le cas du Tanol, la liaison hydrogène intermoléculaire est responsable de la déviation observée pour les axes propres situés dans le plan miroir , ainsi que des plus faibles valeurs mesurées pour g_xx, g_yy et g_zz.

The ¹³C n.m.r. spectrum of benzofuroxan at −15°C is assigned on the basis of selective decoupling experiments and by comparison with the ¹³C chemical shifts of model compounds. The ¹³C spectra were also measured in trifluoroacetic acid as a solvent. From the temperature dependence of the ¹³C spectrum of benzofuroxan in CDCl₃, a barrier of 14·0 ± 0·2 kcal mol⁻¹ is obtained for the degenerate tautomerism in this compound.

¹H and ¹³C NMR spectra of 8‐hydroxyquinoline (oxine) and its 5‐Me, 5‐F, 5‐Cl, 5‐Br and 5‐NO₂ derivatives have been studied in DMSO‐d₆ solution. The ¹H and ¹³C chemical shifts and proton–proton, proton–fluorine, carbon–proton and carbon–fluorine coupling constants have been determined. The ¹H and ¹³C chemical shifts have been correlated with the charge densities on the hydrogen and carbon atoms calculated by the CNDO/2 method. The correlation of the ¹H and ¹³C chemical shifts with the total charge densities on the carbon atoms is approximately linear (r_H² = 0.85, r_C² = 0.84). The proton in peri position to the nitro group in 5‐NO₂‐oxine is an exception.

The preferred solute conformation of ethyl 3α‐phenyltropane‐3β‐carboxylate hydrochloride, the tropane analogue of pethidine, is shown to be a piperidine chair with an axial 3‐phenyl substituent by analysis of its ¹H NMR characteristics and spectral comparisons with model compounds. Conformational studies of synthetic intermediates are also reported and favoured boat forms identified for 3α‐diphenylhydroxymethyl‐3β‐tropanol and 3α‐phenyl‐3β‐tropanyl phenyl ketone. In the hot‐plate test for analgesia performed on mice, ethyl 3α‐phenyltropane‐3β‐carboxylate is about 1·5 times as effective as pethidine.

Inverted files of spectral features should be employed when searching for identical or similar spectra. The efficiency of interescting the entries for chemical shifts with a small deviation is very high for larger molecules and incomplete spectra. A novel spectral code describing the spectral appearance in hierarchical order (SAHO) facilitates the search for complete spectra with a relatively high tolerance of the chemical shifts. A vailable information on the chemical composition of the molecule, as well as the incusion of SFOR‐multiplicities, will further narrow down the number of suitable reference compounds.

Using a topological parameter, λ_R, to characterize the effects of a variable mono or polyalkyl environment, R, on widely varied types of ¹³C sites gives rise to a homogeneous description of their chemical shifts over the entire range of δ¹³C variation (220 ppm/TMS). The proposed model, δ¹³C=ω_cλ_R+η_c expresses the chemical shift in terms of this parameter together with the sensitivity, ω_c, of the perturbed ¹³C site, bonded to a constant environment in a given family, and with η_c as the reference shift. In the case of polyalkyl environments, R=∑ R_i (n ⩽ 4); since the use of λ_R presupposes that groups R_i coexist in R without interacting, it is expressed as the sum of the intrinsic effects of each of these groups. Thus, this model demonstrates the general character of most of the common alkyl environment effects and the specific behaviour of the various types of ¹³C atoms. The homogeneity of this description relies on an assessment of the R_i contribution by the DARC/PELCO topological analysis of alkyl sites in a series of reference alkynes. The δ¹³C values calculated for 138 compounds, belonging to the alkane, alkene, allene, alcohol, amine, epoxide, ester and ketone families, are highly satisfactory (r̄=0.979, s̄=0.98).