Carbon chemical shifts electron donation

Bisalkyne d4 monomers, with N = 3 by symmetry, exhibit proton and carbon chemical shifts at higher fields than those of monoalkynes with N = 4. The proton chemical shift of 10.45 ppm for Mo(PhC=CH)2-(S2CNEt2)2 (52) falls nicely between the four-electron donor Mo(CO)-(PhC=CH)(S2CNEt2)2 case (12.6 ppm) and the two-electron donor (7r-C5H5)2Mo(HC=CH) case [7.68 ppm (Table II)]. Additional data for bisalkyne complexes, including pyrrole-N-carbodithioate derivatives, support a correlation of H chemical shifts with alkyne ttj donation, with three-electron donors typically near 10.0 0.5 ppm. Similar H values are found for cyclopentadienyl bisalkyne complexes with terminal alkyne ligands. Chemical shifts between 8.5 and 10.5 ppm characterize all the neutral and cationic bisalkynes listed in Table V except for [CpMo-(RC=CH)2(MeCN)]+ where one isomer has S near 11 ppm for the acetylenic proton (72). 

The data in Table 21 indicate that C(2) is shielded, and C(3) is deshielded, in the Z-isomer relative to the Eav-isomer. It has been found113 that in compounds of general formula XCH=CHY, where X and Y are, respectively, an electron-withdrawing and an electron-donating group, the olefinic carbon chemical shifts of isomeric E,Z pairs are simply related by an equation bz = bE + A. For enamino ketones 343 (and other structurally related compounds113), the relationship for C(2) is... 

Lippmaa (204) has used the para carbon chemical shifts of substituted phenyl and benzyl silanes to demonstrate the electron accepting properties of SiR3 and the electron donating ability of CH2SiR3 (Table VI). 

The chemical shift of the ester carbon of coordinated P(OCH3)3 is likewise sensitive to electron-donation by the trans R ligand (5). A plot of the pyridine y-C chemical shifts vs. the trimethyl phosphite ester carbon chemical shifts... 

The NMR chemical shifts of the C=N carbon in the open tautomeric forms of 2-aryl-substituted 3,4-dihydro-277-1,3-benzoxazines 5 and l,4-dihydro-277-3,l-benzoxazines 6 exhibited a reverse dependence on the benzylidenic substituents X. Electron-withdrawing substituents caused shielding (the shift was reduced), while electron-donating ones caused deshielding <2003JOC2151>. 

The unsubstituted phenonium ion, as well as other phenonium ions substituted with electron-donating groups, have been recently observed as stable ions in superacid medium.34 That the structure is actually 18 and not an unsym-metrically bridged ion (19) nor a nonclassical ion (20) (see Section 6.2) in which there are three-center bonds was shown by the nmr evidence. The ring carbon that is bonded to the aliphatic carbons was established by 13C shifts to be tetrahedral in nature and 13C and proton chemical shifts in the ring were similar to those of cations shown to have Structure 21. 

When the 13 C NMR chemical shift of the Cy carbon in dienyl cations 349, 350, 352, 353, 355 and 356 is used as a probe for the ability of /J-substituents to hyperconjugatively donate electrons to the formally empty 2p(7t) orbital of the vinyl cation C , the order in Scheme 35 is obtained ... 

Solvent effects on nuclear magnetic resonance (NMR) spectra have been studied extensively, and they are described mainly in terms of the observed chemical shifts, 8, corrected for the solvent bulk magnetic susceptibility (Table 3.5). The shifts depend on the nucleus studied and the compound of which it is a constituent, and some nuclei/compounds show particularly large shifts. These can then be employed as probes for certain properties of the solvents. Examples are the chemical shifts of 31P in triethylphosphine oxide, the 13C shifts in the 2-or 3-positions, relative to the 4-position in pyridine N-oxide, and the 13C shifts in N-dimethyl or N-diethyl-benzamide, for the carbonyl carbon relative to those in positions 2 (or 6), 3 (or 5) and 4 in the aromatic ring (Chapter 4) (Marcus 1993). These shifts are particularly sensitive to the hydrogen bond donation abilities a (Lewis acidity) of the solvents. In all cases there is, again, a trade off between non-specific dipole-dipole and dipole-induced dipole effects and those ascribable to specific electron pair donation of the solvent to the solute or vice versa to form solvates. 

The global utility of this H-NMR alkyne probe is decreased by the scarcity of terminal alkyne adducts relative to the abundance of internal alkyne adducts. Diphenylacetylene and dimethylacetylenedicarboxylate (DMAC) are two particularly popular alkyne ligands which have no acetylenic proton to monitor. An empirical correlation between alkyne irx donation and, 3C chemical shift for the bound alkyne carbons has been recognized (155) which partially fills this spectroscopic need. A plot of alkyne 13C chemical shifts, which span over 100 ppm (Tables II and III), versus N, the number of electrons donated per alkyne to fulfill the effective atomic number guideline, reveals both the advantages and the limitations... 

Fig. 22. Alkyne carbon l3C chemical shift (ppm) versus formal alkyne electron donation number, N.

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