Multiple Resonance Techniques

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Multiple pulse experiments were carried out in the early days of NMR, but remained largely in the domain of physics until the 1970s. Since then a number have come into use. We now describe some important types. 

Two-Dimensional NMR. There are excellent reviews of two-dimensional (2D) NMR spectroscopy. The basis for NMR spectroscopy in two frequency dimensions is spin memory. Once the nuclear spins in a sample are made to precess, they continue to do so for a time of the order of the spin-spin relaxation time Tj. The behavior of the nuclei during the detection period 2 can be related to the behavior of 

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Methods of disturbing the Boltzmann distribution of nuclear spin states were known long before the phenomenon of CIDNP was recognized. All of these involve multiple resonance techniques (e.g. INDOR, the Nuclear Overhauser Effect) and all depend on spin-lattice relaxation processes for the development of polarization. The effect is referred to as dynamic nuclear polarization (DNP) (for a review, see Hausser and Stehlik, 1968). The observed changes in the intensity of lines in the n.m.r. spectrum are small, however, reflecting the small changes induced in the Boltzmann distribution. 

B. Historical Development of Microwave-Optical Multiple-Resonance Techniques... 

Multidimensional NMR eombined with multiple resonance techniques can be used to remove most of the resonanees from the speetrum and to seleetively detect unique struetures sueh as those found at ehain-ends. For example, the unique oeeurrence of... 

Nevertheless, the presence of multiple stable conformations in a gas-phase sample comphcates spectral analysis and may prevent the assignment of a structure by comparison with theory. One would thus like to be able to study individual corrformations. Two basic approaches can be used to remove the conformational heterogeneity of a spectmm physically separate the conformers before making spectroscopic measurements on a sample or use multiple-resonance techniques to assign each line in a spectrum to a particular conformer. Both t)q)es of techniques, which are independent and can be used in tandem, are discussed below. 

The use of single resonance experiments for sign determination depends on the presence of second-order features which in favorable cases incidentally lead to relative sign information. This has been widely used in magnetically inequivalent systems such as AA XX. While the early relative sign data were obtained in this fashion, multiple resonance techniques are the preferred direct method. 

The ENDOR techniques, of course, are not confined to studies of transition metal complexes. A fast growing interest on electron nuclear double and multiple resonance experiments is also noticed in other fields of natural sciences, such as radical, radiation and polymer chemistry, solid state physics, biophysics and mineralogy. 

M. A. El-Sayed, Optical pumping of the lowest triplet state and multiple resonance optical techniques in zero field, J. Chem. Phys. 54, 680-691 (1971). 

The development of simple, multiple-frequency solvent suppression techniques has greatly improved the quality of data that can be obtained using LC-NMR. One of the most useful methods for multiple resonance solvent suppression in LC-NMR is... 

Lynn F. Gladden, M.D. Mantle and A.J. Sederman, Quantifying Physics and Chemistry at Multiple Length-Scales using Magnetic Resonance Techniques... 

The potential signal enhancement available from these double-resonance techniques depends on the available polarization ( H or 14N), the detection frequency, and the polarization transfer efficiency. The transfer efficiency depends not only on the coupling between the isotopes and the time spent with the frequencies matched but also on the number of 14N transitions matched and the order in which the matching conditions occur [91]. Irradiating multiple transitions, as described in Section 4.2.1., will also affect the choice of 14N transitions to polarize [94,95],... 

QUANTIFYING PHYSICS AND CHEMISTRY AT MULTIPLE LENGTH-SCALES USING MAGNETIC RESONANCE TECHNIQUES... 

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