Methylene protons chemical shifts

Proton chemical shift data from nuclear magnetic resonance has historically not been very informative because the methylene groups in the hydrocarbon chain are not easily differentiated. However, this can be turned to advantage if a polar group is present on the side chain causing the shift of adjacent hydrogens downfteld. High resolution C-nmr has been able to determine position and stereochemistry of double bonds in the fatty acid chain (62). Broad band nmr has also been shown useful for determination of soHd fat content. 

Again, collections of H shifts exist which can be searched manually e.g., methine and methylenes. More usefully, proton chemical shifts have also been the subject of SCS or additivity rules. Prestsch and co-workers devised... 

With more H NMR specroscopic data available for these complexes, further trends between the proton chemical shifts and structural types are becoming apparent. For example, in tautomerically related complexes, the methylene protons of rj -bound allenyl ligands (type VII) appear downfield of propargylic counterparts (1 < A8 2.5 ppm), allowing a... 

Figure 3-9 Proton chemical-shift ranges for common structural units. The symbol CH represents methyl, methylene, or methine, and R represents a saturated alkyl group. The range for —CO2H and other strongly hydrogen-bonded protons is off scale to the left. The indicated ranges are for common examples actual ranges can be larger.

The first measurements of the dependence of proton chemical shifts 8 on temperature were performed using ethanol, and observing the shift of the OH proton with respect to the methyl and methylene protons. The first extensive study" of hydrogen-bonding mediated chemical shifts showed that, in the gas phase at very low pressures, the chemical shift measured at a particular temperature is essentially liquid phase, two factors alter the chemical shift from the value measured in the gas phase. The first factor is the change in bulk susceptibility caused by the change in density. The second factor is the association shift , which is the difference between the calculated and measured chemical shifts. The calculated value is given by ... 

Table 13.1 collects chemical-shift information for protons of various types. Within each type, methyl (CH3) protons are more shielded than methylene (CH2) protons, and methylene protons are more shielded than methine (CH) protons. These differences are small—only about 0.7 ppm separates a methyl proton from a methine proton of the same type. Overall, proton chemical shifts among common organic compounds encompass a range of about 12 ppm. The protons in alkanes are the most shielded, and O—H protons of carboxylic acids are the least shielded. 

Figure 8. Proton chemical shifts of iso-HMTT LiBr in mixed benzene-methylene chloride solution. The upfield methylene shift ACH is defined as zero in pure methylene chloride (see Table II). Resuhs are shown for two concentrations of chelated salt.

Figure 3.37 The spectra of butylamine. The amine protons give rise to the sharp singlet at 1.1 ppm. The adjacent methylene protons are shifted to about 2.7 ppm by the electronegative nitrogen, while the other alkyl methylene and methyl protons appear at their usual chemical shift positions. (Reprinted with permission of Aldrich Chemical Co., Inc., modified by addition of the structure and peak identification.)...

Proton NMR chemical shifts of methylene groups in phenolic resins Methylene group Chemical shift (ppm)... 

As the proton chemical shifts of the hydrogen atoms in the methylene group of cumulenes are linearly correlated to the C chemical shifts of the methylene carbons (10), the proton chemical shifts of allene (11), ketene imine (49) (10), and ketene (43) (10) also vary systematically with the nuclear charges (Fig. 13). 

Fig. 11 AOT proton chemical shifts as a function of cosurfactant (acrylamide) concentration, for H5, H6, and H7 methylene protons HIO methyl protons H8 methyl protons and HIO methyl protons...

The final product was characterized using 500 MHz NMR spectroscopy. The individual methylene resonances of the cyclohexane ring as well as the remaining resonances were identified by two-dimensional proton-proton chemical shift correlation spectroscopy (COSY), which provided a map of coupling networks between protons in the molecule. The coupling pathway was determined by correlating the chemical shifts of coupled spins in a stepwise manner. 

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