Field shifts

Comparison of the relevant data shows that an adjacent oxygen (Table 5) and especially a sulfur atom (Table 6) induce lower field shifts than either type of nitrogen atom. 

According to measurements on steroidal diazirines (49) there is also a high field shift of equatorial methyl groups, although their protons are more distant by one position from the diazirine ring. Reported values lie between 0.1 and 0.2 p.p.m., about 0.8 p.p.m. higher than in alkanes (65JA2665). 

Although [34]octaphyrin 80 fulfills Hiickel s rule, the II NMR spectrum indicates by the high-field shift of the methine protons that the system is nonaromatic. The X-ray structure analysis demonstrates clearly the reason for the lack of aromatic stabilization, namely the nonplanar loop conformation in which the whole macrocycle is twisted similarly to the [32]octaphyrin structure and which is also found for [36]octaphyrin and [40]decaphyrin structures (vide infra). 

Si4 revealed a distorted tetrahedral configuration, and the Sil-Si2 and Si2-Si3 bond distances of 2.240(2) and 2.244(2) A were intermediate between the Si=Si and Si-Si bond lengths of the precursor 19. This was explained by the delocalization of the positive charge over the Sil, Si2, and Si3 atoms, accompanied by the Sil-Si3 through-space orbital interaction, resulting in the overall homoaromaticity of 20. The hypothesis of homoaromaticity was further supported by the observation of an extremely low-field shifted signal of Si2, the central atom of the Sis homoaromatic system, at 315.7 ppm. 

Recently, a photoisomerization reaction of azoferrocene was found to proceed in polar solvents such as benzonitrile and DMSO through both a 7t it transition of the azo-group with a UV light (365 nm) and the MLCT transition with a green light (546 nm) (Fig. 6) (Scheme 1) (153). The quantum yields of the photo-isomerization reaction at 365 nm and 546 nm were estimated to be 0.002 and 0.03, respectively. The transformation into the cis form causes the higher field shift of Cp protons in the 1H-NMR spectrum and an appearance of u(N = N) at 1552 cm-1. The cis form is greatly stabilized in polar media, and dilution of the polar solution of cis-25 with less polar solvents resulted in a prompt recovery of the trans form. 

On going from I to II, one would expect a drastic low-field shift in the 29Si NMR spectrum. In the temperature region up to 100°C, we could not find any dramatic shift difference this indicates that the contribution of II can only be marginal [8]. On the other hand, the electrophilic character of 1 is indicated by several experiments [2]. Some new results will be described in the following. 

The bora-2,5-cyclohexadienes 25, 43, and 44 have been used in experimental studies. Photochemical reaction of 25 and 43 with Fe(CO)s produces the robust complexes 45 (29) and 46 (92) with 7)5-divinylborane structures (92), whereas thermally induced complex formation of 44 with Fe2(CO)9 is accompanied by ligand isomerization and affords complex 47 (67). We note in passing that S(UB) = 38.8 ppm for 47 is at rather low field. The only strictly comparable boraolefin known is l-methoxy-6-(trimethylsilyl)bora-2,4-cyclohexadiene [8(nB) = 47.1 ppm] (26). On this basis, the high field shift upon complexation (only 8.3 ppm) indicates weak Fe-B interaction. 

In monomethyl and mono n-propyl cyclopropenones, the down-field shifts of CH3 and ck-CH2 groups as well as the CH2 (a/ ) separation (0.94 ppm) are somewhat larger than in the dialkyl case, but only to about 50% of the relative displacement of vinylic hydrogens (distance vinyl-H (200) versus vinyl-H (210) taken as a reference (3.70 ppm)). 

When fewer than three hydrogen bonds exist to satisfy the oxygen valence of a siloxy group, as for example in some hydrous layer silicates mentioned above, then they need to be stronger (yielding a low-field shift in proton NMR) and/or additional interaction partners should be present. 

Another interesting result obtained from the NMR investigations was the strong low-field shift of the 15N signals in the complex la. The terminal N-atom Np shows with —16.7 ppm the... 

The electrochemical generation of the germyl anion has been the subject of a recent paper105. Evidence for its formation by SET reduction of Ph3GeH on Pt in DMF with tetrabutylammonium tetrafluoroborate is based on 13C NMR which shows a strong down-field shift of Ca 30 ppm for the ipso carbon of the anion. The anion tends to yield Ph3GeGePh3 above 20 °C, but is also trapped by reactions at —40°C with O2 and CH3I ... 

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