The Origin of the Difference Between the13C and17O Shift Behaviour of Carbonyl Compounds RCOX:Ab initio Calculation of the Shielding Tensors

Abstract

In carbonyl compounds RCOX (X=H, Me, SiR3, SR, Cl, F, OMe, OH, NH2, O‐; R=H, Me), the 17O shift values of the carbonyl group depend on the electron donor–acceptor properties of X, whereas the 13C shift values are determined by other factors too. By IGLO ab initio calculations, the difference between 13C and 17O has been traced to differences in the relative importance of the shielding tensor component in the direction of the C—O bond (σzz). The deshielding contribution of this component is mainly determined by the energy of the n–π* excitation; donor–acceptor‐type interactions influence the level of both orbitals. As the n orbital of this transition is essentially localized on the O atom, the excitation acts less on 13C and is thus mainly responsible for the difference in substituent sensitivity between 13C and 17O shifts. An analogous difference of substituent sensitivities between 13C and 17O shifts exists in aroyl compounds p‐YC6H4COX towards changes in the para substituent Y; it is explained on the same basis as the effect of X upon RCOX, without the necessity of referring to a ‘reverse’ substituent effect. Finally, the surprising absence of substituent effects upon the 17O shift in aroyl cations p‐YC6H4CO+ can be explained by the fact that, for symmetry reasons, the n–π*‐type excitation is absent in linear compounds. © 1997 by John Wiley & Sons, Ltd.

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

“…The deshielding contribution to the r zz component was found to be mainly due to the energy of the n ?p * excitation. Since the n-orbital of this transition is essentially localized on the O atom, this excitation has a minor effect on 13 C and is, thus, mainly responsible for the difference in substituent sensitivity between d( 13 C) and d( 17 O) [353].…”

Influence of ortho substituents on 17O NMR chemical shifts in phenyl esters of substituted benzoic acids

“…[13,14] It was found that in methyl benzoates reaction series an introduction of electron-attracting substituents causes deshielding and electron-donating substituents shielding of the carbonyl and methoxy O atoms. [14][15][16] It has been shown [26][27][28][29] that in substituted carbonyl derivatives, Y-C 6 H 4 CO-X, RCO-X, the shielding of the carbonyl oxygen is increased and the sensitivity of the Y substituent is diminished with an increase in the electron-donating power of the X group.…”

“…This is consistent with the increased double bond character of the carbonyl group produced by an electron-withdrawing groups and an increase in single bond character resulting from the electron-donating substituents. [26,36,63,65] An increase in the negative charge on the carbonyl oxygen indirectly causes the oxygen lone pair to be less tightly bound. Earlier [8,9] the increased double bond character of the carbonyl group produced by an electron-withdrawing groups in phenyl esters of substituted benzoic acids was proved by the carbonyl carbon 13 C NMR chemical upfield shifts, δ CO , and the increased infrared stretching frequencies of the carbonyl group, ν CO .…”

“…[36] In phenyl esters of ortho-substituted benzoic acids the calculated sensitivities of the δ( 17 O) values towards the substituent inductive effect, (ρ I ) ortho , was more than three times stronger ((ρ I ) ortho ≈ 17, Table 2) as compared to the same sensitivities, (ρ I ) ortho , in the case of the carbonyl carbon 13 C NMR chemical shifts, δ CO , ((ρ I ) ortho = 5.00 [8] ). In the substituentinduced chemical shifts of carbonyl carbon for phenyl esters of ortho-substituted benzoic [26] have reported that the (de)shielding effect of the carbonyl oxygen atom is mainly determined by the energy of the n−π * excitation; donor-acceptor type interactions influence the level of both orbitals. As the n orbital of this transition is nearly exclusively located on the O atom, the extinction acts less on 13 C and thus mainly responsible for the difference in the substituent sensitivity between 13 C and 17 O shifts.…”

17O NMR study of oxo metalloporphyrin complexes: Correlation with electronic structure of M O moiety

“…Recently, 17 O NMR spectroscopy is quite attractive tool to investigate structure, electronic state, and reactivity. For example, the 17 O NMR signal of carbonyl or carboxyl oxygen has been used to estimate its nucleophilicity [51][52][53]. 17 O NMR spectroscopy has been also utilized to detect the hydrogen bonding interaction in amides and peptides [54].…”