Vinyl chemical shifts

Chemical shift nonequivalence can occur when two environments are stereochem ically different The two vinyl protons of 2 bromopropene have different chemical shifts... 

Substituents on both sides of the double bond are considered separately. Additional vinyl carbons are treated as if they were alkyl carbons. The method is applicable to alicyclic alkenes in small rings carbons are counted twice, i.e., from both sides of the double bond where applicable. The constant in the equation is the chemical shift for ethylene. The effect of other substituent groups is tabulated below. 

The increase in the proportion of the tetrasubstituted isomer in the cases of the morpholine and piperidine enamines of 2-methylcyelohexanone has been ascribed to both steric and electronic factors. The authors propose that the overlap of the electron pair on the nitrogen atom and the v electrons of the double bond is much more important in the case of the pyrrolidine enamines and much less with the others. Support for this postulate was provided by the NMR spectra of these enamines, wherein the chemical shifts of the vinylic protons of the pyrrolidine enamines were at a higher field than those of the corresponding morpholine and piperidine enamines by 20-27 Hz. The greater amount of overlap or electron delocalization, in the case of pyrrolidine enamine, is in accord with the postulate of Brown et al. (7- ) that the double bond exo to the five-membered ring is more favored than the double bond exo to the six-membered ring. 

The chemical shifts for the vinylic protons of some enamines are given n Table 4. 

Much of the difference in chemical shift between aromatic protons (6.5-8.0 8) and vinylic protons (4.5-6.5 5) is clue to a property of aromatic... 

Since it is clear that the presence of an unshared pair of electrons on the sulfur of the sulfoxide group leads to no special instability in the case of the known thiirene oxides (i.e., 18a, 28a,b and the first alkyl-substituted thiirene oxide 30 recently synthesized60), the reduced antiaromatic properties of the thiirene oxides relative to that of thiirenes have been manifested experimentally. As far as the possibility of electron-attracting conjugative stabilization involving the sulfur atom in thiirene oxides is concerned, the experimental evidence accumulated so far is not decisive. Thus, the chemical shift of the vinylic carbon of... 

Three sets of nmr chemical shifts for the ethynyl proton have been correlated with eq. (2). Of these sets, two gave significant correlations with eq. (2). Nevertheless, as the most extensive collection of substituents is included in the set which did not give significant correlation, it seems likely that chemical shifts of ethynyl protons are not correlated by the extended Hammett equation. This behavior contrasts with that of chemical shifts for trans- and c/s-vinyl protons and is in agreement with the behavior of geminal vinyl protons. 

The SECSY spectrum of the coumarin presents cross-peaks for various coupled nuclei. These cross-peaks appear on diagonal lines that are parallel to one another. By reading the chemical shifts at such connected cross-peaks we arrive at the chemical shifts of the coupled nuclei. For instance, cross-peaks A and A exhibit connectivity between the vinylic C-4 and C-3 protons resonating at 8 7.8 and 6.2, respectively. The C-4 methine appears downfield due to its )3-disposition to the lactone carbonyl. Similarly, cross-peaks B and B show vicinal coupling between the C-5 and C-6 methine protons (8 7.6 and 7.1, respectively) of the aromatic moiety. The signals C and C represent the correlation between the oxygen-bearing C-11 (85.4) andC-12 (84.6) methine protons in the side chain. These interactions are presented around the structure. 

From these results it is reasonable to conclude that no inversion in the order of the chemical shifts of the tertiary carbon atoms belonging to the different triads occurs from the starting PV0CC1 to the poly(vinyl phenyl carbonate). Moreover the chemical modification of PV0CC1 by phenol does not induce any degradation of the polymer. 

Table 3. Chemical shift of C (10) vinyl proton in different B -derivatives (131-133)...

Single vinylic fluorine substituents absorb over quite a wide range of chemical shifts, with fluoroallene at the high field end (-169 ppm) and P-fluoroacrylate derivatives at the low field end (-75 ppm) (Scheme 3.36). 

The chemical shifts of terminal (1 °) vinylic fluorines are not affected significantly by conjugation of the fluorine-bearing double bond with either another C=C double bond or a benzene ring (Scheme 3.40). In this case, however, the fluorines of the (Z)-isomers are slightly deshielded relative to those of the (F)-isomers. 

Whether the SF5 group is aliphatic-, vinylic-, or aromatic bound does not seem to have much influence upon the fluorine chemical shifts observed. However, an SF5 group that is proximate to a functional group can have its chemical shifts, particularly those of the equatorial fluorines, influenced somewhat by that functional group. 

Table 10. Vinylic C-H proton and a-C-H chemical shift in monosubstituted cyclopropenones and triafulvenes...

Very powerful tools for the study of dienes and, to some extent, polyenes (in particular annular polyenes) are both H and 13 C NMR spectroscopies, which will be discussed in a separate section. As previously mentioned 1,3-butadiene is more stable in the s-trans conformation and in the H NMR spectrum both butadiene (1) and 2,3,6,7-tetramethyl-2,4,6-octatriene (3) display the vinyl proton at a low chemical shift value. In these simple examples the S value can be predicted theoretically. The 111 NMR spectrum of a C25-branched isoprenoid was examined as part of the structural determination for biomarkers and is shown in Figure l6. The other spectral and structure assignments are described later in this review. 

Despite the successful prediction of chemical shifts for a great structural variety of carbocations some difficulties have been encountered for vinyl cations.47 The effect of electron correlation, basis sets and geometry on calculated NMR spectra of vinyl cations has been studied in some detail also for the parent vinyl cation in its linear form.48 Comparative experimental and computational NMR studies, however, have... 

The allyl-resonance stabilized E- and Z-pent-l,3-dienyl-2-cations (22 and 23) are the smallest member of vinyl cations observed as persistent species in superacid solution 49 These are difficult to generate experimentally50 but structures with only five heavy atoms are suitable candidates for coupled cluster model calculations. A challenging task of quantum chemistry was to assign the 13C NMR spectrum of the mixture of isomers (Fig. 3), which exhibits pairs of signals of 22 and 23 which differ only by a few ppm, to the chemical shifts for the specific carbon atoms of the E- and Z-isomers, respectively. 

C NMR chemical shifts of a series of higher substituted a-vinyl substituted vinyl cations 24-27 were calculated to explore the sensitivity of the predicted isotropic shifts to electron correlation, basis set and geometry effects in differently substituted l,3-dienyl-2-cations.51... 

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