Magnetic non-equivalence

IR spectra of these complexes suggest that the ligands are coordinated by the oxygen atom. The magnetic non-equivalence of the methylene groups due to flanking by the sulphur is influenced seriously by the oxygen coordination to the tin atom. 

The H-NMR spectrum of 2 in CDCI3 (Figure 1) exhibits broad unresolved resonances in the aromatic region similar to those found in the monomer. Broad signals with lack of resolution are consistent with magnetic non-equivalence of the methyl group protons resulting from a mixture of triad tacticities. 

The experiment took 11 min, and the spectrum shows quite clearly the correlation signals for the acetal 5 cross peaks between the methyl and methylene signals from the ethyl group and between the magnetically non-equivalent protons of the ethylene bridge. CH correlation experiments can easily also be carried out, even though in the case of the two acetals 4 and 5 they require between two and three hours ... 

APT, you will perhaps remember, stands for Attached Proton Test, meaning that this spectrum tells you the multiphcity of the signals (Me, CH2, CH or quaternary C). These two spectra tell you how many magnetically non-equivalent types of carbon are present in the molecule, but (for the reasons we discussed earlier) we do not use integration to try to find out relative numbers. We shall present APT spectra as follows CH, CH3 in negative phase (down), CH2 and quaternary C in positive phase (up). 

Our standard molecule is however not ideally suited for certain experiments (e.g. magnetic non-equivalence, NOE, HPLC-NMR coupling). In such cases other simple compounds of the same type, compounds 2-7, will be used ... 

In terms of chemical equivalence, (or more accurately, chemical shift equivalence) clearly, Ha is equivalent to Ha. But it is not magnetically equivalent to Ha because if it was, then the coupling between Ha and Hb would be the same as the coupling between Ha and Hb. Clearly, this cannot be the case since Ha is ortho to Hb but Ha is para to it. Such spin systems are referred to as AA BB systems (pronounced A-A dashed B-B dashed). The dashes are used to denote magnetic non-equivalence of the otherwise chemically equivalent protons. What this means in practise is that molecules of this type display a highly characteristic splitting pattern which would be described as a pair of doublets with a number of minor extra lines and some broadening at the base of the peaks (Spectrum 5.6). 

RULE 4 Interaction is normally observable between close groups of magnetically non-equivalent... 

The chirality at the transition metal is evidenced by the magnetic non-equivalence of the benzylic protons in the benzyl derivative ... 

The n.m.r. spectrum (220 MHz) of N-deuteriated [Pt(en)3] shows small chemical shift differences which were attributed to incomplete conformational averaging. The possibility that these differences may arise from inherent magnetic non-equivalence was excluded on the basis of observed differences in Pt—H coupling constants. Comparison of the aqueous spectrum of... 

Strong evidence for the non-planarity at sulfur in thiophene 1-oxides is provided by the magnetic non-equivalence at -10 °C of the side-chain methylene protons in (144b). The... 

Nuclei may be chemically equivalent but magnetically non-equivalent. To be magnetically equivalent nuclei must couple in exactly the same way to all other nuclei in the system. Thus the two protons in 1,1-difluoroethylene are magnetic-... 

NMR provides a powerful tool for the investigation of paramagnetic metal complexes paramagnetism allows the resolution of magnetically non equivalent environments that cannot be resolved in a comparable diamagnetic system. Furthermore the large expansion of the chemical shift range of the observed resonances (the isotropic shift) permits the characterization of fast dynamic processes. 

The magnetic non-equivalence of the 13C nuclei of the tropane ring in protonated scopolamine [260] and in protonated atropine [261] indicates the tropic acid group to be near C(6) and C(7) in [260] but to be in a position affecting all the carbon nuclei on one side of the tropane ring in [261], (170) The absorption of the equatorial and axial A-methyl 13C nuclei in [261] and [260] respectively may be compared to the corresponding absorptions in methylatropine [262],... 

Because the amide C-N bond has partial double bond character, there is restricted rotation around this bond. Therefore, in addition to the three signals expected for the thiophene protons in the NMR spectrum, there are two broad singulets for the two amide protons since these are both chemically and magnetically non-equivalent. The amide hydrogen atoms are diastereotopic and therefore can be distinguished by the descriptors pro-Z and pro-E. 

The 80 MHz H n.m.r spectrum of 7V-(cylcopropylmethyl)scopolaminium bromide in D20 has been analysed and the conclusions have been compared with those from X-ray crystal-structure studies.19 They were consistent with the equatorial position of the cyclopropylmethyl group, as shown in (17). All protons in symmetrical positions (1 and 5, 6 and 7, 2 and 4) of the tropane ring were found to be magnetically non-equivalent, as in scopolamine hydrobromide and in atropine. This was attributed to the different shielding effects that arise from the aromatic substituent which is attached to the asymmetric centre of the tropic acid moiety. The carboxyl group of the tropate residue was found to be opposite to H-3. 

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