Selection of shift reagents for ¹⁷O NMR. Application to line assignments

Abstract

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)

Quantification of Aromaticity in Oxocarbons: The Problem of the Fictitious “Nonaromatic” Reference System

“…This fact can be related with the high stability of cyclopropenones, [37] despite their high ring strain [38] and in contrast to cyclopropanones and cyclopropenes, attributed to resonance stabilization by its polar formula which represents a pseudoaromatic system. Such a high shielding is only observed for the exocyclic oxygen atom of sydnone (d 232) [40] and related mesoionic compounds which are currently formulated with À O À in the place of O. Such a high shielding is only observed for the exocyclic oxygen atom of sydnone (d 232) [40] and related mesoionic compounds which are currently formulated with À O À in the place of O.…”

Oxygen-17 NMR spectroscopy: Basic principles and applications (Part I)

“…Dy(dpm) 3 and Tb(dpm) 3 (dpm: 2,2,6,6-tetramethylheptane-3,5-dionate) and Eu(fod) 3 (fod: 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate) have been shown to be the most effective shift reagents in 17 O NMR [167][168][169][170]; the mechanism is essentially due to the contact and pseudo-contact shift (see Section 8.5). The shift effect for carbonyl oxygen, extrapolated to [Dy(dpm) 3 ]:[substrate] = 1, is several thousand ppm to low frequency.…”

“…Oxygen-17 shieldings are significantly influenced by lanthanide shift reagents due to complexation which can be utilized for resolution enhancement and assignment purposes [167]. Dy(dpm) 3 and Tb(dpm) 3 (dpm: 2,2,6,6-tetramethylheptane-3,5-dionate) and Eu(fod) 3 (fod: 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate) have been shown to be the most effective shift reagents in 17 O NMR [167][168][169][170]; the mechanism is essentially due to the contact and pseudo-contact shift (see Section 8.5).…”

Solvent effects on the nitrogen NMR shieldings of 3‐methylsydnone and ab initio calculations of the shieldings including related oxazole and oxadiazole systems

“…However, more recent experiments with effects on 1 of lanthanide shift reagents revealed that it is the resonance of the more shielded oxygen nucleus which is essentially affected, and the latter was tentatively assigned to the exocyclic oxygen. 17 This has finally been proven by the 17 O spectra of specifically labelled 1 and arguments based on the unusually high magnetic shieldings of 17 O in cyclopropanones. 18 A comparison of the computed and experimental 17 O shieldings (Table 4 and Fig.…”