Nuclei proton signals

The NMR signal arises from a quantum mechanical property of nuclei called spin . In the text here, we will use the example of the hydrogen nucleus (proton) as this is the nucleus that we will be dealing with mostly. Protons have a spin quantum number of V . In this case, when they are placed in a magnetic field, there are two possible spin states that the nucleus can adopt and there is an energy difference between them (Figure 1.1). 

The INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) experiment [6, 7] was the first broadband pulsed experiment for polarization transfer between heteronuclei, and has been extensively used for sensitivity enhancement and for spectral editing. For spectral editing purposes in carbon-13 NMR, more recent experiments such as DEPT, SEMUT [8] and their various enhancements [9] are usually preferable, but because of its brevity and simplicity INEPT remains the method of choice for many applications in sensitivity enhancement, and as a building block in complex pulse sequences with multiple polarization transfer steps. The potential utility of INEPT in inverse mode experiments, in which polarization is transferred from a low magnetogyric ratio nucleus to protons, was recognized quite early [10]. The principal advantage of polarization transfer over methods such as heteronuclear spin echo difference spectroscopy is the scope it offers for presaturation of the unwanted proton signals, which allows clean spec-... 

Figure 11.3 shows the downfield portion of the spectrum of sucrose (gl doublet, /HH = 3.8 Hz) with normal vertical scaling and with the vertical scale increased by a factor of 100 to show the 13C satellites. The satellites show the same doublet 7rh coupling observed in the 12C-bound proton signal (7=3.8 Hz), with an additional 169.6 Hz coupling to the 13C nucleus (Vch)- The peak height is roughly half (actually half of 1% because the vertical scale is 100 x) of the central peak because they are part of a doublet with concentration about 1% of the concentration of the 12C species. If you look closely you will see that the center of the double doublet is not exactly the same chemical shift... 

NMR Spectrum in Deuterochloroform at Room Temperature. The spectrum in Figure 1 shows a methyl triplet at 1.5 p.p.m. and a second methyl triplet at 0.38 p.p.m. which we attribute to the cis-enol form. The shift to higher field is attributed to the shielding effect of the benzene nucleus. The Stuart model shows that the methyl group of the cis-enol form can lie over the middle of one of the benzene nuclei. We assume from the integral ratio of the methyl proton signals that approximately... 

The fluorine ( F) nucleus has magnetic properties of the same kind as the proton. It gives rise to nmr spectra, although at a quite different frequency-field strength combination than the proton. Fluorine nuclei can be coupled not only with each other, but also with protons. Absorption by fluorine does not appear in the proton nmr spectrum— it is far off the scale—but the splitting by fluorine of proton signals can be seen. The signal for the two protons of l,2-dichloro-l,l-difluoroethane, for example. 

The first successful nuclear magnetic resonance (NMR) measurements were made by E. M. Purcell et al. at Harvard, and F. Bloch et al at Stanford, independently in 1946. They observed the proton ( H) signal of the hydrogen nucleus of samples of paraffin wax (solid) and water (liquid), respectively. Their success in observing the proton signal was based on using samples in which the relaxation time was short. 

Most recently, it has been reported that there is a large difference in solid-state chemical shifts between the a-helix and j8-sheet for polypeptides as shown in Fig. 22.3 [7j. In (Ala) , the chemical shift of the C H proton for the a-helix form appears at lower frequency by 1.2-1.4 ppm than for the j8-sheet form. The NH proton signal is very broad due to the quadrupolar interaction with the nucleus. In (Leu) and (Glu(OBzl)) , it was found that similar behavior exists. 

The presence of dialkylated aromatics is apparent from an examination of the aromatic protons by NMR. The extent of dialkylation on the aromatic nucleus was estimated by comparing the relative intensities of the proton signals above and below 7.0 ppm. Nuclear dialkylation increases from 18 to 100% within the series benzene, toluene, ethylbenzene, and cumene. This is exactly the order expected on the basis that substitution of a benzylic carbon atom decreases its relative acidity (14) so that substitution upon the nucleus becomes more prominent. 

---