77Se chemical shift

On the other hand, the situation is more complex for the 3-substituted derivatives. In such compounds the substituent can be considered electronically to be located either meta or para to the selenium ring atom (cf. resonance formulas 8 and 9 for a -I -M substituted derivative). However, the substituent-caused shifts in the 3-substituted derivatives indicate that the heteroatom and the substituents are para and not meta related. 77Se chemical shifts are more sensitive than the 13C shifts to changes in electron... 

Fig. 12 Left-. 77Se (spectra a-d) and 67Zn (spectra e-h) NMR of ZnSe nanoparticles with three different sizes/morphologies. Right. DFT calculations of 77Se chemical shifts vs distance from the surface. Reprinted with permission from [140], Copyright 2009 by the American Physical Society...

Se and 13C NMR spectra of some bicyclic 1,2,3-selenadiazoles were studied and it was shown that the 77Se chemical shifts are a suitable tool for chiral discrimination in diastereomeric compounds <1995MRC490>. The diastereomeric mixtures of menthyl ester-substituted 1,2,3-selenadiazoles 64 and 65 show different splitting in their 77Se signals. 

Se chemical shifts Compound Solvent ppm Reference... 

There are many analogies in the structural influences on S, P and Se NMR chemical shifts. Wasylishen et al.36 have compared the relative 33S and 77Se chemical shift scales and found that the correspondence is noticeable. For example, the resonances of the hydrides H2S and PH3 lie at the extreme upheld end of the total spectral range. Substitution of a hydrogen atom with a methyl group provokes downfield shifts of 44.7 and 64.5 ppm, respectively. Further substitution of a second hydrogen atom with a methyl causes downfield shifts by 31 and 64.9 ppm, respectively. 

It is also interesting that a parallel trend is observed for 33S, 31P and 77Se chemical shifts when lone pairs are used to form hypervalent species. When one lone pair is used to coordinate one oxygen atom, S, P and Se chemical shifts increase significantly, which means that the nuclei are more deshielded (as expected, since the electron density around the nucleus decreases). However, in coordinating another oxygen atom, the loss of the second lone pair produces a shielding effect. 

There has been an ab initio calculation of 77Se chemical shifts for SxSe4 x2+ (where x = 0-3).834 The 77Se NMR spectra of PhSeCH = C(SePh)R (R =... 

Table 6 77Se NMR chemical shift of tetraphosphorylselenophene and related compounds...

This basic behavior has been invoked to rationalize observed changes in the chemical shifts to low frequency on going from the bulk semiconductor to a QD, in the case of 77Se shifts in CdSe [329, 332] and of 31P shifts in InP [333]. However, there are additional factors to consider, as discussed below. 

Another study of the surface structure of CdSe NCs of 3.7 nm size used 31P MAS-NMR and 31P/77Se rotational-echo double-resonance (REDOR) to identify overlapping broad peaks from two surface species trioctylphosphine oxide (TOPO) at 29.3 ppm and trioctylphosphine selenide (TOPSe) at 22.2 ppm [343]. Both the isotropic chemical shift and CSA of the surface-bound TOPO were substantially different from those of the free ligand. Spin-echo experiments on 31P were stated to indicate an average P-P distance of 8-10 A at the surface, consistent with capping at alternate atomic sites (all Cd but not Se). 

The effects of 2- and 3-substituents on the 77Se NMR chemical shifts in selenophene and benzolb]selenophene have been investigated in detail (75CS(8)8, 8lOMR(l6)14). The 125Te shifts in 2-substituted tellurophenes exactly parallel the 77Se shifts in similarly substituted selenophenes (76CS(10)139>. 

Table 25 Chemical shift 8 (ppm) and coupling constants J (Hz) for 77Se NMR spectra of compounds 44, 45, 78, and 47a,b ...

Only for three selenete complexes 12a, 12b, and 18 have 77Se NMR data been reported <1995CC2461>. The chemical shifts of 77Se NMR spectra in the selenetes are compiled in Table 4. 

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