SummaryThe I7O and 13C chemical shifts (6) of 14 a,p-unsaturated aldehydes and ketones and 33 acyl derivatives RXC= 0 (X= C1, OH, OMe, OEt, NH2 and R = H or alkyl) have been measured. In the unsaturated carbonyl series, a correlation exists between 6 I7O and the n electron density at thep-carbon atom. From this correlation, an6 1 7 0 of 530 ppm was extrapolated for the loss of one electron at the oxygen atom. In the acyl series, the 6 I7O were also sensitive to changes in the polarity of the carbon-oxygen bond. A partial correlation between "0-NMR. chemical shifts and the nuclear quadrupole coupling constants exists for aldehydes, ketones, esters and amides but not for acyl chlorides.Introduction. -The oxygen-17 chemical shifts (6 1 7 0 ) of saturated carbonyl compounds previously studied [ 11 are sensitive to modifications of the electronic density on the oxygen atom through changes in the carbon-oxygen n bond polarity. To verify the validity of this relation in a more diversified series of compounds, we now explore the

Chemical shifts and line broadenings induced by a series of lanthanide dipivaloylmethanates Ln(dmp), on the 0 NMR signal of methanol have been measured. The best high-field shift reagents are those containing the cations Tb+, and D Y +~; very large shifts have been observed for even quite low concentrations of reagent.The shift reagents are able to discriminate between different oxygen functions and can, in principle, be used for line assignments. 17Chelates of lanthanide ions have been commonly used in 'H NMR and I3C NMR as a helpful tool for spectral simplification and conformational For conformational elucidation it is very important to know with certainty the site of complexation and the various contributions to the chemical shift. Two distinct classes of shift are evident; those of nuclei such as 'H, 13C and 19F which are subject mainly to dipolar interactions and those of others, such as I4N, 15N, " 0 and ,'P, which carry lone pairs of electrons and are capable of direct bonding with the metal ion, thereby allowing direct interaction with the metal electron spin vector. Thus, the shift reagents which are most appropriate for 'H NMR are not necessarily the most suitable for other nuclei. The oxygen lone pairs are often sites of complexation, and I7O NMR could give a direct answer about the degree of complexation with oxygen atoms. Oxygen-17 has a large scale of chemical shift^,^ but in some cases the individual ranges corresponding to different functional groups overlap, causing ambiguity in line assignments. We have attempted first and foremost to determine the best shift reagent for oxygen through comparative studies of the effects of lanthanide dipivaloylmethanato chelates Ln(dpm),, and then to test their ability to discriminate between different possible sites of complexation. RESULTS AND DISCUSSIONThe experimental results are collected in Table 1 line broadening induced by Nd is prohibitive. The shielding reagents derived from Tb, Dy, Ho and Er show that the maximum effect is given by Dy (see Table 1 and Fig. 1).Dy(dpm), and Tb(dpm),, having the highest induced shift/line-broadening ratio, are by far the best shielding shift reagents for oxygen nuclei. We applied these two reagents to acetone oxygen as a donor group, and quite large induced chemical shifts were observed: -37ppm for Dy and -27ppm for T b (molar ratio [Ln(dpm)

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Two bond spin-spin coupling constants 2J(170iH) are determined for the oxygen in ether, aldehyde, acid, ester and amide groups by line shape analysis of the corresponding I7O NMR spectra.