Through-bond shifts

Contact, or through-bond, shifts occur if there is a finite probability of finding the unpaired electron of the metal at the nucleus of interest. Predicting the direction and magnitude of contact shifts requires molecular orbital calculations. Contact shifts are significant in covalently bonded metal systems and are pronounced for paramagnetic transition metal complexes such as Ni(acac)2 and Co(acac)2. Contact shifts extend over 7T-electron systems, and are particularly large with Co(acac)2 since the Co(II) has an unpaired electron in a d-orbital that can jr-bond with certain substrates. ... 

The group moment always includes the C—X bond. When the group is attached to an aromatic system, the moment contains the contributions through resonance of those polar structures postulated as arising through charge shifts around the ring. 

There are basically three main types of 2D NMR experiments J-resolved, shift correlation through bonds (e.g., COSY), and shift correlations through space e.g., NOESY). These spectra may be of homonuclear or heteronuclear type involving interactions between similar nuclei (e.g., protons) or between different nuclear species (e.g., H with C). 

Allylic boranes such as 9-allyl-9-BBN react with aldehydes and ketones to give allylic carbinols. The reaction begins by Lewis acid-base coordination at the carbonyl oxygen, which both increases the electrophilicity of the carbonyl group and weakens the C-B bond to the allyl group. The dipolar adduct then reacts through a cyclic TS. Bond formation takes place at the 7-carbon of the allyl group and the double bond shifts.36 After the reaction is complete, the carbinol product is liberated from the borinate ester by displacement with ethanolamine. Yields for a series of aldehydes and ketones were usually above 90% for 9-allyl-9-BBN. 

If the spin-spin information was being transmitted by the normal through-bond mechanism the upfield three proton signal would be expected to occur as a doublet because these protons are the only ones which can assume the required planar zig-zag conformation 77>78h Preliminary results, using the change in chemical shift method 79>, indicates that the energy barrier to rotation is of the order of 20 k.cal.mole O. As expected the silicon compound (39) shows a nine proton doublet... 

With the low-valence iron pentacarbonyl, vinylcyclopropanes 22 are thermally transformed to the diene re-complexes, the (1,3-trans-pentadiene)iron carbonyl complexes 23, through bond fission, 1,2-hydrogen shift, and stereoselective coordination [15]. (Scheme 9)... 

CVs of [7] were recorded after addition of calculated equivalents of Na+, K+ and Mg2+ and equimolar mixtures of Na+/K+ and Na+/K+/Mg2+. The results obtained are presented in Table 2. One-wave metal cation-induced anodic shifts of the ferrocenyl redox couple are observed (mediated by a through-bond coupling pathway), and interestingly the magnitudes of these are... 

Interestingly, the sulfur-linked bis-crown ligand [8] shows an unprecedented cathodic potential shift upon addition of K+ cations to the electrochemical solution (Table 3). It is believed to be a conformational process that causes the anomalous shift of the ferrocene/ferrocenium redox couple and not a through-space or through-bond interaction, as these effects would produce the expected anodic potential shift of the ferrocene redox couple. The origin of the effect may be a redirection of the lone pairs of the sulfur donor atoms towards the iron centre upon complexation. This would increase the electron density... 

Using databases or tables of SCS to predict proton chemical shifts, only through-bond effects are effectively considered, and a typical r.m.s. difference between calculated and experimental shifts is 0.3 ppm. This is a lower value than for shifts, but this is a much higher proportion of the chemical shift range (3 vs. 0.75%). 

The unpaired electron(s) on the metal, Fe(III) in the case of the nitrophorins, act as beacons that illuminate the protons in the vicinity of the metal, by causing shifts (isotropic shifts) of the resonances from those observed in a diamagnetic protein, for example, the NO-bound forms of the nitrophorins. These shifts allow much to be learned about the intimate details of the electron configuration at the iron center. The two contributions to the isotropic shifts are the contact (through bonds) and dipolar or pseudocontact (through space) contributions, and these are discussed in considerable detail elsewhere 92-94). For the purposes of this chapter, a detailed physical treatment of these two contributions is not necessary or even desirable, but rather, the differences in the proton NMR spectra of the heme substituents, and what they tell us about the electronic environment at the iron(III) center, the effect of... 

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