Chemical shift, carbon proton

NMR chemical shifts of protonated carbonic acid 109 and 110 were investigated experimentally and computationally using IGLO/II chemical shift calculations for MP2/6-31G(d) optimized geometries.140... 

However, if side-chain carbon assignments are wanted, C(CC)(CO)NH experiments [33] that start directly with carbon magnetization and transfer it further to the amide proton for detection are available. If protonated substituents, for example methyl groups, have been introduced into the otherwise perdeuterated protein, the usual HC(C)(CO)NH-TOCSY pulse sequence can be used to obtain the proton chemical shifts. These protons can provide a small number of NOEs that, together with residual dipolar couplings and the secondary structure identification from chemical shifts, make the determination of the global fold of large proteins possible. 

The first attempt to correlate proton chemical shifts of methyl groups in carbonium ions with charge on the adjacent carbon atom seems to have been made by Mac Lean and Mack or (1961). They examined the pnmr spectra of a number of cyclohexadienyl cations (benzenium ions) [6] where R was CH3, and found that chemical shifts for protons directly attached to positive carbons were linearly... 

The really important aspect to all of this is that nuclei in similar chemical environments exhibit similar chemical shifts. Thus protons ( H nuclei) attached to a carbon atom bonded to oxygen, H-C-O, show a characteristic chemical shift (3.5-A.5 ppm), while protons attached to a carbon atom bonded to nitrogen, H-C-N, have a different chemical shift range (2.5-3.5 ppm) and, since the carbon is attached to the less electronegative N atom, resonate at lower frequency. We can therefore use chemical shifts to our great advantage when interpreting NMR spectra. 

Consider first the chemical shifts of protons attached to an sp3 carbon, —C—H. 

Table A3.6 Chemical shifts of protons attached to atoms other than carbon 1427...

Table A3.6 Chemical shift of protons attached to atoms other than carbon (all signals are sensitive to solvent, concentration and temperature and are removed by deuteration)...

Thus, it is seen that the effect of a single fluorine substituent upon the chemical shifts of protons attached to the same carbon adheres to... 

Lastly, as can be seen from the data at the bottom of Scheme 2.8, the inductive effect of a trifluoromethyl group affects chemical shifts of protons as distant as three carbons away. 

Correlation spectroscopy COSY Connectivity information of protons on adjacent carbons. Cross-peaks connect the chemical shifts of protons that are coupled they are symmetrically placed around a central diagonal (see Figure 15.1). Specifically a H-H 2-3J couphng through bonds. Can provide useful information. Flowever, TOCSY tends to be more sensitive than COSY for NOM. With a short mixing time ( 30ms), COSY-type correlations can be observed using TOCSY. 

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