NMR dynamic

NMR spectra can even be obtained from a number of the common transition metal nuclei, but this is not yet a routine procedure. 

Many organometallic species give fewer NMR resonances than would be predicted from their static structures. This is usually because the molecules are nonrigid, and the nuclei concerned are exchanging places at a rate faster 

As we continue to warm the sample, the broadening increases until the two peaks coalesce. The exchange rate required to do this depends on how far apart the two peaks were initially the appropriate equation is shown as Eq. 10.4, where Av is the separation of the two resonances of the static structure. 

On further warming, the now single peak gets narrower according to Eq. 10.5, and we finally reach a point at which the signal is sharp once more. 

FIGURE 10.5 The changes in the H NMR spectrum of a two-site system on warming as the Ha and Hb protons begin to exchange at rates comparable with the NMR timescale. 

Dynamic NMR is a very powerful method for obtaining kinetic information about processes that occur at a suitable rate, typically ones having a barrier in the 12-18 kcal/mol range. 

FIGURE 10.6 Ruxionality of CpFe CO)2(ij -Cp), showing ihe faster collapse of flic Hb resonance, indicating the operation of a 1,2, rather than a 1,3 shift. Only the resonances for the Cp group is shown, for greater simplicity. 

Representative examples are depicted in Fig. 5. The cholestane sfan probes (CSL), matching the dimension of most mesogenic units, are expected to reflect the orientational order and dynamics of the LCs. Likewise, deuterons, attached to a phenyl ring or alkyl chain segment, are employed to report on the molecular properties of these groups. It should be noted, however, that the technique is easily extended to other spin probes, propriate for dynamic NMR or ESR investigations. 

Generally speaking, dynamic NMR is a time domain technique [45]. The spin system is subject to a sequence of non-selective radio frequency pulses and the response 

Typical pulse sequences, commonly employed in relaxation studies of 1 = 1 spin systems are shown in Fig. 6. The quadrupole echo (QE) sequence (top) [51] provides the spin-spin relaxation time Tjg [52]. Since Tjg is most sensitive to motions with correlation times equal to the inverse quadrupolar coupling constant, quadrupole echo sequences of deuterons ( H) offer a means to study molecular dynamics in the 

Schematic representation of various pidse sequences, employed in dynamic NMR of I = I spin systems Quadrupole echo sequence (QE), inversion recovery sequence (// ), saturation recovoy sequence (SR) and Jeener-Broekaert sequence (JB) 

Dependence of calculated NMR powder spectra on type and timescale of various motions a) Two-site jumps, 6k = 60°, b) two-site jumps, 6k = 109°, c) tteee-site jumps, = 109°, d) planar rotational (fusion, 6k = 109°, e) tetrahedral jumps, I) isotropic spherical diffusion. 6r = angle between rotation axis and C-D bond direction 

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Bain A D and Duns G J 1996 A unified approach to dynamic NMR based on a physical interpretation of the transition probability Can. J. Chem. 74 819-24... 

Limbach H H 1991 Dynamic NMR spectroscopy in the presence of kinetic hydrogen/deuterium isotope effects NMR Basic Principles and Progress vol 23, ed P Diehl, E Fluck, H Gunther, R Kosfeld and J Seelig (Berlin ... 

A collection of reviews, covering essentially all of dynamic NMR np to abont 1974. 

Temperature-dependent (dynamic) NMR studies are suited to the study of processes with rate constants between 10 and 10 s Some applications are shown in Table 2.13 and in problems 13 and 14. 

J. Sandstrom, Dynamic NMR Spectroscopy, Academic Press, New York, 1982. 

G. Binsch, Top. Stereochem. 3 97 (1968) F. G. Riddell, Nucl. Magn. Reson. 12 246 (1983) J. Sandstrom, Dynamic NMR Spectroscopy Academic Press, New %rk, 1982 J. L. Marshall, Nuclear Magnetic Resonance, Veilag Chemie, Deerfield Beach, Florida, 1983 M. Oki, Applications of Dynamic NMR to Organic Chemistry, VCH Publishers, Deerfield Beach, Florida, 1985 Y. Takeuchi and A. P. Marchand, eds.. Applications of NMR Spectroscopy in Stereochemistry and Corformationol Analysis, VCH Publishers, Deerfield Beach, Florida, 1986. 

M. Old, Applications of Dynamic NMR Spectroscopy to Organic Chemistry, VCH Publishers, Deerfield Beach, Florida, 1983. 

It must be emphasized that all time-dependent chemical phenomena, such as tautomerism, are sensitive to temperature changes. In this section, we treat classic DNMR (dynamic NMR) studies and cases of changes in the NMR spectra with temperature together. In Table XI we have classified these studies according to the physical state of the sample and to the nuclei... 

Accounting for this effect, it was possible to apply dynamic NMR spectroscopy to measure energy barriers to the prototropic rearrangements of pyrazoles. Temperature-variable spectra of a series of 4-substituted pyra-zoles 5 and 6 have been studied in methanol-d4 solutions and the free energy barriers of the degenerate type 2a 2b tautomerization reported (93CJC1443). 

The conformational preference of 1,3,5-trithiane-l-oxide has been determined in solution and in the solid state . and dynamic NMR studies indicated that the S=0 bond is equatorial (182) in solution, as did molecular mechanics calculations. Surprisingly, the axial conformation (183) is preferred in the crystalline state. 

Although the drug delivery to the lipid bilayer membrane is just the first step for bioactivities and phopholipid vesicles are rather simple in view of the composite structure of biomembranes, the unambiguous specification of the preferential location of the drug is essential the successive processes of the action are expected to be induced via the delivery site in membranes. We expect more advances in the dynamic NMR study, so that we can get insight into the mechanism of DD in membranes. 

Y. Xia, P.T. Callaghan 1991, (Study of shear thinning in high polymer solution using dynamic NMR microscopy), Macro. Mol. 24 (17), 4777 1786. 

Hyperpolarized 129Xe NMR Spectroscopy, MRI and Dynamic NMR Microscopy for the In Situ Monitoring of Gas Dynamics in Opaque Media Including Combustion Processes... 

Fig. 5.3.4 (A) Stimulated echo dynamic NMR microscopy pulse sequence. The first field gradient pulse (g,) of duration 8 serves to encode spatial positions of spins and the second field gradient pulse has a refocusing effect.

Figure 5.3.5 displays dynamic NMR microscopy of xenon gas phase Poiseuille flow with an average velocity of 25 mm s-1 and self-diffusion coefficient of 4.5 mm2 s-1 at 130 kPa xenon gas pressure with numerical simulation (A) and experimental flow profiles (B-D) of xenon gas. 

The previous two sections described the development of hp-xenon techniques for the investigation of gas dynamics in porous or opaque media. The usefulness of continuous flow hp-xenon as a tool for dynamic NMR imaging experiments was... 

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