C NMR and Peak Intensities

Acetylenes are anomalous in as in NMR. 577-Hybridized carbons are less shielded than 577 -hybridized ones, but more shielded than 5/7 -hybridized ones. 

Electronegativity and hybridization effects combine to make the carbon of a carbonyl group especially deshielded. Normally, the carbon of C=0 is the least shielded one in a NMR spectrum. 

Which would you expect to be more shielded, the carbonyl carbon of an aldehyde or a ketone Why  

We will have more to say about chemical shifts in later chapters when various families of compounds, especially those that contain carbonyl groups, are discussed in more detail. 

Two features that are fundamental to NMR spectroscopy—integrated areas and splitting patterns—are not very important in NMR. 

The NMR spectrum of l-bromo-3-chloropropane contains peaks at 8 30, 8 35, and 8 43. Assign these signals to the appropriate carbons. 

Hybridization Effects. Here again, the effects are similar to those seen in NMR. As illustrated by 4-phenyl-l-butene, -hybridized carbons are more shielded than sp -hybridized ones. 

Of the 5p -hybridized carbons, C-1 is the most shielded because it is bonded to only one other carbon. The least shielded carbon is the ring carbon to which the side chain is attached. It is the only 5/7 -hybridized carbon connected to three other carbons. 

Consider carbons x, y, and z in p-methylanisole. One has a chemical shift of 8 20, another has 8 55, and the third 8 157. Match the chemical shifts with the appropriate carbons. 

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C NMR Spectroscopy 565 C Chemical Shifts 567 C NMR and Peak Intensities 569 C—Coupling 570... 

In the research described in the preceding problem, Randall was able to assign the five peaks associated with tetrads in the C-NMR spectrum on the basis of their relative intensities, assuming zero-order Markov (or Bernoulli) statistics with Pm = 0.575. The five tetrad intensities and their chemical shifts from TMS are as follows ... 

Further support for this pathway was provided by competition feeding studies. If 104 were not a true biosynthetic intermediate and were incorporated due to some flexibility in the biosynthetic enzymes, then its incorporation would be expected to be reduced by an equivalent concentration of the true substrate 102 or 103. If it were a real intermediate, then it would be expected that incorporation of earlier intermediates would be reduced by an equivalent concentration of 104. Precursor 104, labeled with deuterium at the O-methyl group, was co-fed with an approximately equal concentration of either 102 or 103 labeled with deuterium at the C-5 methyl group, and relative incorporation levels were compared by measurement of the CD3 peak intensities in the 2H NMR spectrum of the labeled azinomycin B. In each case, there was approximately twice as much deuterium labeling at the O-methyl group as at the other methyl group, consistently with 104 being a true biosynthetic intermediate. 

Otsuka et al. (107) describe [Ni(CNBu )2], as a reddish brown microcrystalline substance, which is extremely air-sensitive. It can be recrystallized from ether at —78°C, and is soluble in benzene in the latter solution the infrared spectrum (2020s, br, 1603m, 1210m) and proton NMR (three peaks of equal intensity at t8.17, 8.81, and 8.94) were obtained. Neither analytical data nor molecular weight is available on this complex. The metal-ligand stoichiometry is presumably established by virtue of the molar ratio of reactants and by the stoichiometries of various reaction products. 

The only S9Co NMR study is on Co4(CO)i2 and the apparent discrepancy between the results obtained by two independent groups in 1967 104,173 has recently been resolved66. The s9Co NMR spectrum of a solution of Co (CO)12 in hexane at 30 °C shows two peaks at 8,400 and 9,670 p.p.m. to high field of [Co(NH3)6]C13 with relative intensities 1 3 respectively66. This is clearly consistent with the solid state structure (Fig. 4) but is inconsistent with the 13C NMR results66 92). 

Figure 30 Variable-temperature solid-state NMR spectra (relative intensities distorted due to different relaxation rates) (a) 29Si (referenced to Q8M8 (Bruker) (b) 2H (isotropic peak truncated to show hem and crystalline fractions).285 Reprinted with permission from Mueller, C. Schmidt, C. Frey, H. Macromolecules 1996, 29, 3320-3322. 1996 American Chemical Society.

Polymer Structure. The reaction studied here is summarized in Equation 21. As shown in the experimental section, it is possible to prepare these polymers at various degrees of substitution. As the degree of substitution increases, the ratios of the infrared C=0/0H absorption peaks and the phenyl/aliphatic C-H absorption peaks increase in a linear manner (Table I). (It would be possible to determine the degree of substitution from such calibrated curves.) At the same time, the intensity of the OH band in the NMR spectra diminishes while a strong set of peaks due to the phenyl group forms. Elemental nitrogen analysis values for the modified polymers agree closely with the calculated values. In addition, the infrared spectra show the necessary carbamate N-H bands. These factors enable us to have confidence that the polymer structure is as shown in Equation 21. 

In comparison, no structural modification of model B was seen before 120 h of aging (80 °C). However, after 120 h two small doublets appeared in the NMR spectrum and several additional peaks became noticeable in the NMR spectrum. It was determined by NMR and IR spectroscopy that the hydrolysis products were an imide/carboxylic acid and an imide/anhydride. Model B was then aged for 1200 h at 80 °C to quantitatively determine the amount of hydrolysis products as a function of time. The relative intensity of the peaks due to carboxylic acid is constant after some time. The authors suggest that an equilibrium occurs between model B and the products formed during hydrolysis, and therefore, the conversion to hydrolysis products is limited to about 12%. This critical fraction is probably enough to cause some degradation of polymeric materials, but research on six-membered polyimides has remained active. 

In i C NMR spectroscopy the i C signal due to the carbon in CDCI3 appears as a triplet centred at 5 77.3 with peaks intensities in the ratio 1 1 1 (due to spin-spin coupling between i C and H). This resonance serves as a convenient reference for the chemical shifts of i C NMR spectra recorded in this solvent. 

A 100 MHz NMR spectrum of a mixture of benzene (CeHe) 5 128.7 (CH), ethyl acetate (CH3CH2OCOCH3) 5 170.4 (C=0), 5 60.1 (CH2), 6 20.1 (CH3), 5 14.3 (CH3) and dioxane (C4H8O2) 5 66.3 (CH2) in CDCI3 solution is given below. The spectrum was recorded with a long relaxation delay (300 seconds) between acquisitions and with the NOE suppressed. Estimate the relative proportions (mole %) of the 3 components from the peak intensities in the spectrum. 

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