2D exchange spectroscopy

Figure 1.45 Coherence transfer pathways in 2D NMR experiments. (A) Pathways in homonuclear 2D correlation spectroscopy. The first 90° pulse excites singlequantum coherence of order p= . The second mixing pulse of angle /3 converts the coherence into detectable magnetization (p= —1). (Bra) Coherence transfer pathways in NOESY/2D exchange spectroscopy (B b) relayed COSY (B c) doublequantum spectroscopy (B d) 2D COSY with double-quantum filter (t = 0). The pathways shown in (B a,b, and d) involve a fixed mixing interval (t ). (Reprinted from G. Bodenhausen et al, J. Magn. Resonance, 58, 370, copyright 1984, Rights and Permission Department, Academic Press Inc., 6277 Sea Harbor Drive, Orlando, Florida 32887.)...

The 2 2 complex formed between /i-CD and reduced tetracyanoquinodimethane shows separate signals for the free and bound CD.203 2D exchange spectroscopy gave an exchange rate of 0.9 s 1 at 30 °C for the exchange between the free guest and the 2 2 complex. The exchange may occur via numerous steps, but no resolution of the intermediate steps could be achieved from the data treatment. 

The chemical exchange, in NMR sense, reflects all processes of intermolec-ular and intramolecular rearrangements that occur while the observed spins change their magnetic environments [12, 13]. However, for 2D exchange spectroscopy, only the slow processes in which the observed spins change their resonance frequencies are observable. Here, slow refers to an exchange rate kij between sites i and j that is smaller than the difference between... 

Cyclo(Pro-Gly) (fig. 3) is a convenient model for demonstration of various aspects of 2D exchange spectroscopy. It is small rigid molecule with 10 protons, of which 8 are spectroscopically well resolved. It is well dissolved in dimethyl sulfoxide (DMSO)Zwater mixtures and stable at a broad range of temperatures. We used a 10 mM solution of cyclo(Pro-Gly) in 70/30 volume/volume mixture of DMSO/water. This solvent mixture is suitable for the cross-relaxation studies because it is rather viscous even at room temperature and does not freeze down to 223 K [29, 30]. Thus, molecules dissolved in this mixture can be studied at a broad range of temperatures (correlation times). 

Modern NMR techniques such as quantitative analysis of multisite exchange using either ID magnetization transfer experiments (JOS) or the 2D exchange spectroscopy (EXSY) method (104,105) promise to be of great help in unraveling the complex stereochemical exchange networks involved in cluster fluxionality. The usefulness of EXSY in the context of this article is illustrated by the phase-sensitive 13C 1H EXSY spectrum (255 K, tm = 0.5 sec)... 

As a result of the short relaxation times of most vanadate species, 51V 2D exchange spectroscopy is limited to dynamic processes that occur within a few tens of milliseconds. This timescale is conveniently lengthened to 1 sec or longer in cases where proton (or other) NMR spectroscopy can be employed, for instance, in ligand exchange reactions. 

Fig. 6 The RFDR-sequence with two 7x/2-pulses sandwiching the t evolution period. The mixing period is comprised of rotor-synchronized n-pulses. If these pulses are absent, the sequence is that for 2D-exchange spectroscopy. The third 7i/2-pulse produces transverse magnetisation...

NMR is ideally suited to explore molecular motions in the polymer. Different types of motion can be discriminated on behalf of their timescale and geometry of exchange. One-dimensional quadrupole echo lineshapes (see Section 6.2.7.1) are particularly sensitive to segmental dynamics [1-6, 9-12], when there is either fast exchange between discrete geometries (with Tc <1/Avq) or when the motion occurs on the intermediate timescale (tc= 1/Aj q). Dynamic processes in the intermediate to slow motional limit (tc > l/Ar Q) are addressed by 2D exchange spectroscopy (see Section... 

A very helpful and often unaccounted method is 2D exchange spectroscopy (EXSY). Cross peaks indicate the chemical exchange between two states. Unequal intensities of cross peaks give a hint at the ligand exchange processes [14] (Section 13.4.1.3). 

In 2002, Goward et al. carried out the first study of proton conductivity using the high-resolution solid-state MAS NMR [25], They characterized the ethylene oxide tethered imidazole heterocycles by variable temperature experiments, as well as 2D homonuclear DQ NMR and 2D exchange spectroscopy (EXSY). The MAS NMR identified three types of hydrogen-bonded N—resonances, and DQ NMR identified hydrogen-bonded protons which are mobile on the time scale of the experiment and participate in proton transport. The local mobifity of protons was characterized by spin—spin relaxation times as a function of temperature,... 

G.S. Harbison, D.P. Raleigh, J. Herzfeld, R.G. Griffin, High-field 2D exchange spectroscopy in rotating solids, J. Magn. Reson. 64 (1985) 284. 

The effect of exchange processes can be observed in two-dimensional spectrum and be analyzed in a very similar way. The 2D exchange spectroscopy (EXSY) is in principle, identical to the NOESY experiment. Cross peaks in 2D EXSY experiments arise from noncoherent magnetization transfer between sites with different resonances. Noncoherent magnetization transfer takes place either by exchange of nuclei between different sites or by cross-relaxation (NOE). However, the mixing time in EXSY is usually chosen to be shorter,... 

In low molar mass liquid crystals, conformational dynamics is typically too fast to be studied by 2D exchange NMR or line shape analysis. Dynamical studies of small molecules belong therefore almost exclusively to the realm of relaxation experiments. There are, however, cases of slow motions even in low molar mass systems, which can be investigated by 2D exchange spectroscopy. A recently investigated example is a discotic tetrabenzocyclophane derivative, which shows very slow motions in the columnar phase compared to other discotic compounds [97]. As illustrated in Fig. 6, deute-... 

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