Nucleus, energy absorption

Since the total number of nuclei N = N, + Np,itia evident from equation (1) that (N — N ), and hence the intensity of the n.m.r. signal, is proportional to N. Typically, the excess population in the -state is 1 10 . Application of the appropriate radiofrequency field induces both upward and downward transitions, but the former predominate. The observed signal indicating energy absorption would quickly be saturated (iVp = iV ) but for a non-radiative process, spin-lattice relaxation, by which the Boltzmann distribution can be continuously re-established. Spin-lattice relaxation times, that is the time required for a collection of nuclei to return to the Boltzmann distribution after perturbation, varies according to the type of nucleus (e.g., etc.) and, for a... 

The two averages in (19) have simple interpretations. If the nucleus with absorption cross section other nuclei, its absorption would be, per unit logarithmic energy range, Qaa. Hence, if we replace o-oby... 

The NMR technique is based on absorption of energy by the sample to be analyzed. The sample is placed in a strong magnetic field that will affect atoms in the sample. The nucleus can absorb energy from an additionally applied radio pulse when the frequency of the pulse matches that of the oscillating nucleus. The absorption is recorded by the instrument. 

The essential features of an NMR spectrometer shown m Figure 13 5 are not hard to understand They consist of a magnet to align the nuclear spins a radiofrequency (rf) transmitter as a source of energy to excite a nucleus from its lowest energy state to the next higher one a receiver to detect the absorption of rf radiation and a recorder to print out the spectrum... 

Very slow exchange. Slow exchange means that the lifetime ta = tb in each site is very long. Thus, a nucleus in site A precesses many times, at frequency (vq i a) in the rotating frame, before it leaves site A, and similarly for a nucleus in site B. Thus, there is time for absorption of energy from the radio-frequency field ffi, and resonance peaks appear at Va nd Vb in the laboratory frame. 

Figure 13.3 shows both the H and the l3C NMR spectra of methyl acetate, CH3CO2CH3. The horizontal axis shows the effective field strength felt by the nuclei, and the vertical axis indicates the intensity of absorption of rf energy. Each peak in the NMR spectrum corresponds to a chemically distinct 1H or 13C nucleus in the molecule. (Note that NMR spectra are formatted with the zero absorption line at the bottom, whereas IR spectra are formatted with the zero absorption line at the top Section 12.5.) Note also that 1H and 13C spectra can t be observed simultaneously on the same spectrometer because different amounts of energy are required to spin-flip the different kinds of nuclei. The two spectra must be recorded separately. 

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