Nuclear multiplet structure

All nuclear multiplet structures due to coupling of nonequivalent nuclei are, as noted earlier, subject to effects on line shapes by chemical or positional exchange. For those multiplet structures arising from coupling of nuclei, one of which has a nonzero nuclear quadrupole moment, effects of quadrupole relaxation must be considered. For example, if a proton or fluorine atom is bonded to a nitrogen nucleus (I = 1), a triplet resonance will be expected in the proton or fluorine spectrum. For observation of this fine structure it is necessary that the lifetimes of the nuclear spin states of nitrogen (m = 1, 0, —1) be greater than the inverse frequency separation between multiplet components, i.e., t > l/ANx (106). The lifetimes of N14 spin states can become comparable to or less than 1 /A as a result of quadrupole relaxation. When the N14 spin-state lifetimes are comparable... 

Multiplets in NMR. The second effect yields a spin-spin coupling constant / (usually quoted in hertz), which it generates a multiplet structure that is due to nuclear spin-nuclear spin interactions between equivalent or inequivalent protons (in H1 NMR). The spin interaction is actually a tensor quantity due to... 

Of course, as is well known, the indirect nuclear interaction is tire cause of the multiplet structure observed in most NMR spectra. 

ER Andrew. The narrowing of NMR spectra of solids by high-speed specimen rotation and the resolution of chemical shift and spin multiplet structures for solids. Prog Nuclear Magn Reson Spectrosc 8 1-39, 1972. 

In conclusion, although it has been demonstrated that a three-spin effect exists, it is usually unimportant unless the radical concentration is low. This is readily understandable, since the magnetic moment of the electron is much larger than that of any nucleus so that nuclear-electron interactions are the dominant relaxation terms, except at low concentrations where nuclear—nuclear interactions become important. The presence of a three-spin effect can be revealed most easily either by observation of the transient relaxation behaviour of the nuclear resonance or by triple irradiation experiments. In the latter case, account must be taken of the collapse of any multiplet structure in the interpretation of the results. 

Bo = 11.7T (v ( H) 500MHz) cover the frequency ranges of 7.5, 19, 8.2, and 1.1 kHz, respectively. In addition, a hyperfine spHtting and multiplet structure of resonance signals can be observed as a result of electron-mediated nuclear spin-spin scalar interactions. Both chemical shifts and scalar interactions reflect the chemical environment and provide a wealth of structure-related data. Since specific information about the individual atoms is available, NMR data can be successfully used to calculate a three-dimensional structure of RNA and DNA molecules. Fuelled by methodological developments during the past 10 years, NMR became an indispensable tool for structure elucidation of nucleic acids. At present (April 2003), almost 600 NMR structures of nucleic acids and/or protein/nucleic acid complexes can be found in the PDB database. 

Figures 1.9a and b demonstrate the effect of proton broadband decoupling in the 13C NMR spectrum of a mixture of ethanol and hexadeuterioethanol. The CH3 and CH2 signals of ethanol appear as intense singlets upon proton broadband decoupling while the CD3 and CD2 resonances of the deuteriated compound still display their septet and quintet fine structure deuterium nuclei are not affected by lH decoupling because their Larmor frequencies are far removed from those of protons further, the nuclear spin quantum number of deuterium is ID = 1 in keeping with the general multiplicity rule (2 nxh+ 1, Section 1.4), triplets, quintets and septets are observed for CD, CD2 and CD3 groups, respectively. The relative intensities in these multiplets do not follow Pascal s triangle (1 1 1 triplet for CD 1 3 4 3 1 quintet for CD2 1 3 6 7 6 3 1 septet for CD3).

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