Chemical shift in ”C NMR

The C spectrum (15.04 MHz) of perimidine (13 R = H) (Table 1) is consistent with a symmetrical structure, reflecting rapid prototropic tautomerism between the annular nitrogen atoms as found for H NMR. Tautomerism is excluded in 1-alkyl derivatives as shown for (13 R = Me) in Table 1. The relative C chemical shifts parallel those of the corresponding protons in the H NMR spectrum. The relative order of the chemical shifts in C NMR of 1,8-naphthalenediamine is the same as in perimidine. All carbocyclic peaks other than C9b exhibit a pronounced upfield shift, in accord with the 7r-excessive character of the carbocyclic rings in the perimidine system. On the other hand, C2... 

Oxopyrrolo[l,2-c]imidazole (102) shows carbonyl stretching frequency in the IR region at 1750 cm" , abnormally high for an amide-type carbonyl, since the carbonyl chemical shift in C NMR... 

Chemical information obtained using and C NMR is usually obtained on samples in solution (liquid-state NMR) in order to improve resolution. However, C spectra can also be obtained on neat specimens, such as rubber. This is possible as long as there is sufficient molecular motion to average the orientation-dependent variation in chemical shift of chemically identical atoms (chemical shift anisotropy, CSA). Chemical shifts in C NMR spectra span a much wider range than in proton NMR, and therefore the former provides better spectral resolution. However, the Nuclear Overhauser effect (NOE) and other nuclear relaxation processes cause the C absorption intensities to deviate from direct proportionality to the number of carbon atoms. Thus, unless specific techniques are utilized, C NMR spectral intensities using standard liquid-state NMR acquisition methods are not quantitative. 

The calculation of p j i as a function of / provides an estimate of the distance that end effects penetrate into a long unperturbed chain. This calculation shows that end effects for polyethylene are confined to the first few bmids at the end of the chain [10]. The end effects can extend much further into the chain when the second-order interactirMis become more severe, as is frequently the case for the probability of a helical conformation, p/,, in a long homopolypeptide near the midpoint of its helix-coil transition [132]. In proton NMR, the values of p, , are helpful in understanding the values of the spin-spin coupling constants, using the Karplus relationship [111-114], and in understanding the y effect on the chemical shift in C NMR spectra [110,133]. The values of p, , have also been used to interpret the optical activity exhibited by chiral poly(a-olefins) [115] and other polymers [134,135]. 

Table 11.5 shows approximate chemical shifts in C-NMR spectroscopy. As with H-NMR, we can use the following rules of thumb to remember the chemical shifts of various types of carbons ... 

Table 2.2 shows the values of isotropic chemical shifts in c NMR spectroscopy (8c) for some typical chanical groups occurring in organic matter and in carbon materials. The values are given with respect to the single resonance peak in the H-decoupled NMR spectrum of tetramethylsilane (TMS), taken as the primary reference (0 ppm). The list is very concise more detailed information can be found in many tables and databases available in the literature and also on the Internet [5,9,10,32-36]. Furthermore, it is possible to predict isotropic chemical shifts for many ch ical compounds or groups by using first-principles quantum... 

Analyzing a NMR Spectrum Acquiring a C NMR Spectrum Chemical Shifts in C NMR Spectroscopy DEPT C NMR Spectroscopy... 

The It backbonding is characterized by a weakening of the C=0 bond that leads to a lowering of the infrared carbonyl absorption (1820-2150 cm" ) compared to free CO (2143 cm ) and to low-field chemical shifts in C NMR (around 200 ppm for neutral metal carbonyls). 

In an almost simultaneous publication by Bruch [53] concerning the same class of E/V copolymers, the chemical and steric sequence assignments were established by two-dimensional NMR methods. Also, the vieinal CH—OH proton coupling constants were found to be smaller in V units with a meso-V neighbor than with a racemic-V neighbor. This information could be utilized for characterizing some eonformational aspects of the polymer chain. Likewise the chemical shifts in C NMR spectra that are sensitive to the orientation of substituents in y position (the y-gauche effect) [7] could be considered for conformational characterization of these polymers. 

Chemical Shifts in C-NMR Spectroscopy Table 4.4. Chemical shifts for carbocation centers"... 

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