13C, coupling

C coupling has very little significance in everyday proton NMR interpretation, though it has been used in the past to crack specific problems by means of selective enrichment of a specific carbon during synthesis, with a greater than normal percentage of 13C isotope, which makes detection easy. 

INADEQUATE Incredible natural abundance double quantum transfer experiment. Two-dimensional technique showing 13C-13C coupling. It should be the holy grail of NMR methods but is in fact of very limited use due to extreme insensitivity. 

The 13C NMR sensitivity can sometimes be a problem, but for the kind of samples studied here the effective concentration of monomer units is several molar which does not place excessive demands on present Fourier transform NMR spectrometers. In addition to the sensitivity of the chemical shift to structure (9), the relaxation of protonated carbons is dominated by dipole-dipole interaction with the attached proton (9). The dependence of the relaxation parameters T, or spin-lattice, and Tor spin-spin, on isotropic motional correlation time for a C-H unit is shown schematically in Figure 1. The T1 can be determined by standard pulse techniques (9), while the linewidth at half-height is often related to the T2. Another parameter which is related to the correlation time is the nuclear Overhauser enhancement factor, q. The value of this factor for 13C coupled to protons, varies from about 2 at short correlation times to 0.1 at long correlation... 

The signs of the geminal, 19F-13C coupling-constants both for glyco-pyranosyl and glycofuranosyl fluorides were found to be positive,278 whereas, in another investigation, the spectra were considered to be too complex for determination of the signs, but, by comparison with noncarbohydrate systems, they were assumed to be positive.279... 

A more sensitive alternative is to use HCCH-TOCSY experiments [63-65], which transfer magnetization through the much larger 13C-13C coupling constants. In addition, these coupling constants are almost independent of the secondary structure and very uniform... 

D 1H-13C CT-HSQC/HMQC identifies proton-carbon spin pairs in both sugar and base. The constant time evolution removes the peak splitting due to 13C-13C coupling. The proton and carbon resonances are subsequently connected by... 

Fig. 17.5 Use of an adiabatic inversion pulse in a 13C filter. An almost linear relationship exists between a 13C chemical shift and the corresponding Yh,13C coupling constant (left panel). With an adiabatic 180° pulse on 13C, sweeping from high...

V. NMR DETERMINATIONS OF STRUCTURE IN SOLUTION A. Substituent Effects on One-bond 15N-13C Couplings... 

In aniline derivatives, ring substituents are known to have to profound influence on the directly bonded 15N-1 II and 15N-13C spin coupling interactions67-69. Binsch and coworkers70 have successfully related the one-bond 15N-13C coupling constant to the atomic hybrizations by the empirical equation 4... 

The most significant results for 31P-13C couplings are presented in Table XXXII. It is evident that these couplings are very sensitive to ... 

H,13C coupling constants through one bond are positive. The major structural influence is the carbon hybridization state, and its -character (s) can be estimated using the empirical correlation ... 

C,13C coupling constants through one bond are governed by the hybridization states (. -character) of both carbon atoms involved142 14,3 ... 

Vicinal 7H,13C couplings show essentially the same structural influences as VH,H, i.e., a Karplus-type dependence of the torsional angle 6 (see 1-4), and a dependence on the length of the central carbon carbon bond and the electronegativity of adjacent substituents272"274. 

Another tool for trisubstituted olefins is the measurement of geminal 1H,13C coupling constants (2/h C), since an additivity rule has been developed in which the substituent increments are not only dependent on the nature of the substituents, but also on their stereochemical position154, i75,276 (for representative examples see Table 1). 

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