Olefinic protons chemical shifts

TABLE 12. Olefinic protons chemical shifts of 15 in the presence of LIS reagents (vs TMS)... 

A catalytically important system, methylalumoxane, was investigated by Zurek and Ziegler [88]. Computed 13C and proton chemical shifts of its Zr complexes were used to identify the active and dormant species of this black box activator of the dimethylcirconocene homogeneous olefin polymerization catalyst by comparison of the NMR parameters of the proposed species with experimental data (BP nonhybrid functional). 

Although proton chemical shifts are influenced significantly by factors other than electron density, they also reflect the polarization of the enamine framework and the degree of n,n interaction. Thus, the chemical shifts of the vinylic protons are modulated by the same factors discussed for the chemical shifts of the corresponding olefinic carbons, such as amine component, steric and electronic effects of the substituents and ring size effects. In particular, the chemical shift of the proton(s) at C(2) is lowered by increasing njt interaction, in parallel with what has been observed for < C(2). No general correlation exists between the chemical shifts of both nuclei probably as a consequence of their different sensitivity to steric, electronic and, particularly, anisotropic effects of the substituents. Nevertheless, for sets of structurally related compounds, reasonable linear correlations can be found between <5C(2) and <5H(2) (see below). Since the XH-NMR data available for enamines are more abundant than those for 13C and 15N, more complete structural information can be obtained for wider sets of compounds. 

The proton chemical shifts of representative acyclic and cyclic dienamines are collected in Tables 14-16. Examination of the Tables show that alkenyl substitution at C(l) or C(2) of an enamine produces also deshielding of the olefinic protons, as seen by comparing the <5H(2) values of compound 149 (3 3.75, 3.85 ppm Table 11) with those of the analogous cross-conjugated dienamine 105 ( 3.85, 4.05 ppm Table 14). The chemical shift of the vinylic protons of dienamines correlate only partially with the charge calculated for the corresponding olefinic carbons . In the linear conjugated... 

This weaker and stronger interaction of the olefinic bonds with the metal centre is reflected in the different chemical shifts of the olefinic carbon and the olefinic proton signals in the NMR spectra (Table 3). One of the olefinic carbon signals of the NBD ligand shifts more upfield than the other ca. 43 ppm). Similarly, one of the olefinic proton signals shifts more upfield than the other [ca. 1.3 ppm). The NMR and X-ray data suggest different lability of the two double bonds in this kind of complex. 

In Table II are recorded the chemical shifts for the protons in a number of methyl-substituted ethylenes and in the corresponding AgBF. 2-olefinates, The proton chemical shifts in the complexed olefin are in every instance downfield from those of the uncomplexed olefin but the spectral pattern is essentially unchanged (some small changes of coupling constants observed) indicating retention of the olefinic character as observed by POWELL and SHEPPARD (3). The downfield change of chemical shift is attributed to proton... 

By trapping PX at liquid nitrogen temperature and transferring it to THF at —80° C, the nmr spectmm could be observed (9). It consists of two sharp peaks of equal area at chemical shifts of 5.10 and 6.49 ppm downfield from tetramethylsilane (TMS). The fact that any sharp peaks are observed at all attests to the absence of any significant concentration of unpaired electron spins, such as those that would be contributed by the biradical (11). Furthermore, the chemical shift of the ring protons, 6.49 ppm, is well upheld from the typical aromatic range and more characteristic of an oletinic proton. Thus the olefin stmcture (1) for PX is also supported by nmr. 

The NOESY spectrum of buxatenone shows four cross-peaks, A-D. Cross-peak B represents the dipolar coupling between the most upfield C-19 cyclopropyl proton (8 0.68) with the most downfield olefinic proton (8 6.72). This could be possible only when the double bond is located either between C-1 and C-2 or between C-11 and C-12. The possibility of placing a double bond between C-11 and C-12 can be excluded on the basis of chemical shift considerations, since conjuga-... 

Figure 3. (a) The computer-simulated spectrum of the olefinic protons using chemical shifts of 6.3 and 6.1 ppm and a coupling constant (J) of 15 Hz. (b) The olefinic region of the 100 MHz H-NMR spectrum of the originally isolated xenognosin A. The marked resonances correspond exactly with the resonances of the simulated spectrum. 

The fact that we have three olefinic hydrogens means that our compound is a primary olefin, the fact that the other two carbons are both methylene carbons means that our substituent, bromine, is terminal. Thus the only possibility we have is that we are dealing with 4-bromo-1-butene (try to find another isomer that fits ). But this simple molecules has a highly complex proton spectrum, which can only be interpreted completely (exact chemical shift, coupling constants) by spectrum simulation. 

The chemical shifts of the olefinic protons from the centrosymmetric (all-/ ) zeaxanthin are very similar to the chemical shifts of (all-//) lutein except for proton 7. The resonances of protons 11/11 (6.65 ppm) and of protons 15/15 (6.62 ppm) show a multiplet with an integration value of four. The... 

In all recorded spectra the 3Jee coupling constants between the olefinic protons are on the order of 11-12 Hz, proving the all-E configuration of the investigated carotenoids. Minor differences between the reported chemical shifts and literature data are due to the effect of different solvent compositions. 

Signals of two olefinic protons on a C=C bond, which are isolated from the other double bond and the functional group in a long chain, appear at almost the same chemical shift, and their coupling constant is not clear because of f ailing the first-order approximation. Instead of that, the chemical shift values... 

The advancement of >400 MHz NMR instruments with spin decoupling and Fourier transform software now allows identification of individual olefinic protons of nanogram carotenoids53. We have shown two examples (lycopene and capsantin) for which the chemical shifts have been employed in the assignment of relative configuration49. As for review of the 13C NMR of carotenoids, Englert in 198154 gave information especially on the position of the cis double bonds in a polyene chain. 

In the case of 1,3-butadiene, the chemical shifts of inner (H2, H3) protons and outer (HI, H4) is large, while in the case of cycloalkadienes (e.g. 1,3-cyclopentadiene and 1,3-cyclohexadiene), the difference is very small. It is interesting to note that in 1,3,5-cycloheptatriene, the chemical shifts of three kinds of olefinic protons are very diverse. The effect of the ring size and in the chemical shifts of radialenes was also included. 

TABLE 22. H NMR chemical shift differences (ppm) of olefinic protons of (all-/i)-... 

The H NMR spectram (CDClj, 500 MHz) of 12 showed two singlets (8 0.83 and 8 0.95), each integrating for three protons due to the C-18 and C-19 methyl protons. Three 3H doublets at 8 0.78 (J= 6.5 Hz), 8 0.79 (J= 6.5 Hz) and 8 0.85 (J = 7.0 Hz) were due to the secondary C-26, C-27 and C-21 methyl protons, respechvely. The C-3 methine proton resonated as a one-proton double doublet at 8 3.63 (JJ= 10.5 Hz and J2= 3.5 Hz) and its downfield chemical shift value was indicative of the presence of a geminal hydroxyl funchonality. A one-proton mulhplet at 8 5.21 was ascribed to the C-6 olefinic proton. The C-28 exocyclic methylene protons appeared as two broad singlets at 8 5.40 and 5.58. The C-NMR spectram (CDCl, 125 MHz) showed the resonance of all 28 carbon atoms. The combination of H and C-NMR data suggested that compound 12 has a sterol like structure as most of the H and C-NMR chemical shift values of 12 were similar to those of sterols reported in the literature [19, 20]. The H and C-NMR chemical shift values were assigned with the aid of COSY-45 , HSQC and HMBC spectral data. Compound 12 was found to have modest inhibitory activity against C. xerosis and S. aureus with minimal inhibitory concentration values of 82.35 and 146 pg/ml, respectively. 

Tables 5.2 and 5.3 give characteristic shifts for nuclei in some representative organic compounds. Table 5.4 gives characteristic chemical shifts for protons in common alkyl derivatives. Table 5.5 gives characteristic chemical shifts for the olefinic protons in common substituted alkenes. To a first approximation, the shifts induced by substituents attached an alkene are additive. So, for example, an olefinic proton which is trans to a -CN group and has a geminal alkyl group will have a chemical shift of approximately 6.25 ppm [5.25 + 0.55(tra .s-CN) + 0.45(gew-alkyl)].

Table 5.5 Approximate H Chemical Shifts (5) for Olefinic Protons C=C-H...

Protons attached to the C atoms of the 1,2,4-trioxolane moiety of FOZs have chemical shifts at distinctly lower field than alcohols, ethers or esters. For example, the chemical shifts of the ozonide product in equation 100 (Section Vin.C.b.a) are S (CDCI3) 5.7 ppm for the H atoms of the trioxolane partial structure, and 4.1 ppm for the protons at the heads of the other ether bridge . Measurement of the rate of disappearance of these signals can be applied in kinetic studies of modifications in the ozonide structure. The course of ozonization of the methyl esters of the fatty acids of sunflower oil can be followed by observing in H and C NMR spectra the gradual disappearance of the olefinic peaks and the appearance of the 3,5-dialkyl-1,2,4-trioxolane peaks. Formation of a small amount of aldehyde, which at the end of the process turns into carboxylic acid, is also observed . 

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