Double-resonance Experiments

1 Double-resonance Experiments. - TROSY-type experiments have been traditionally based on the cross-correlation between dipolar and chemical shift anisotropy relaxation mechanisms. Tugarinov et al. extended the application of the relaxation compensation principles to cancellation of the intra-methyl H- H and dipole-dipole interactions. The analysis of the relaxation of the 

By using double-resonance experiments, one can greatly simplify the spectrum. Coupling between different types of nuclei is confirmed by both the disappearance of the peak for the saturated nuclei and the collapse of the fine structure of the coupled nuclei. Nuclei of the same type can be decoupled, as in the proton-proton example given earlier this is called homonuclear decoupling. It is of course possible to decouple unlike nuclei, such as decoupling this is 

Multipulse sequence used to distinguish even and odd numbers of protons coupled to C through one bond. Even numbers of bound protons give positive peaks odd numbers give negative peaks. 

Multipulse conditions ate chosen so that only nuclei with the same number of bound protons have enhanced resonances. Four experiments must be performed, but DEPT is more definitive than APT. 

Multipulse allows obsenration of natural abundance coupling. Only 

1 carbon atom in 10 carbon atoms is a K bonded to another C, hence the name incredible.  

For better elucidation of the measured MAS spectra, several techniques well known to the polymer science community are often added to the classical MAS measurements. 

One well-known and often used technique employed in conjunction with MAS is proton decoupling, where the influence of protons surrounding the measured species, usually carbon or lithium, is inactivated or saturated by a 90° pulse train at the proton frequency, a so-called broad band decoupling. The resulting spectral line is then free from proton dipolar interactions and is considerably narrowed. Another advantage of this technique is that the heteronucleus directly bonded to a proton will be affected much more than other sites that are not, therefore facilitating spectral assignments. 

13 Deconvolution of Li proton-decoupled MAS NMR spectra at 223 K for AP2/PC 50 50 wt% doped with (a) 0.25 mmol LiCI04 g , (b) 0.5 mmol UCIO4 g (c) 1.0 mmol UCIO4 g , and (d) 2.0 mmol UCIO4 g polymer. From Y-H. Liang, C-C. Wang, C-Y. Chen, Journal of Power Sources 2008, 176, 340-6.  

14 CP/MAS spectra of EDS-UCIO4 complexes doped with various salt concentrations (0/Li ). The ether carbon (ca. 70 ppm) and methyl carbon (18 ppm) regions are enlarged as shown in the inset. From W-J. Liang, Y-P. Chen, C-P. Wu, P-L. Kuo, J. Phys. Chem., 2005,109, 

A similar experiment was used to characterize interactions between polymer and salt in LiC104-PEO-poly(acrylonitrile) (PAN) hybrid polymer electrolyte. A broadening of the corresponding band in the observed spectra upon PAN addition was determined to indicate that a more complicated distribution of the PEO segment environments or a reduction of the segmental motion of the PEO chains results. 

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An exception to this rule arises in the ESR spectra of radicals with small hyperfine parameters in solids. In that case the interplay between the Zeeman and anisotropic hyperfine interaction may give rise to satellite peaks for some radical orientations (S. M. Blinder, J. Chem. Phys., 1960, 33, 748 H. Sternlicht,./. Chem. Phys., 1960, 33, 1128). Such effects have been observed in organic free radicals (H. M. McConnell, C. Heller, T. Cole and R. W. Fessenden, J. Am. Chem. Soc., 1959, 82, 766) but are assumed to be negligible for the analysis of powder spectra (see Chapter 4) where A is often large or the resolution is insufficient to reveal subtle spectral features. The nuclear Zeeman interaction does, however, play a central role in electron-nuclear double resonance experiments and related methods [Appendix 2 and Section 2.6 (Chapter 2)]. 

Try to imagine what you can expect to get out of a double-resonance experiment (look for atoms with nuclear spins in relevant structures) and preevaluate its biological relevance. 

The double resonance experiment can be used to simplify a spectrum as discussed above, or to probe correlations between different nuclei. Two types of double resonance experiments are described. In the homonuclear double resonance experiment the nuclei irradiated are the same isotope as those observed Shorthand... 

The already known geissoschizol (7, C19H24N20, MP 224-226°C, [a]D -70°) 184) and its 10-hydroxy derivative (8, C19H24N202, MP 264°C) (184), were isolated from the roots of T. bufalina (Ervatamia hainanensis) (53). The detailed analysis of the H-NMR spectra of 7 and 8 diacetate were reported (Table III) and the assignment of all of the protons was made by application of consecutive double-resonance experiments. 

Unfortunately, such NMR experiments are very insensitive because of the small population differences of nuclear energy levels and the very broad lines encountered in paramagnetic species. Therefore, a double resonance experiment must be performed... 

Fig. 11. Optical pumping of molecules and scheme for double resonance experiments on laser-pumped molecules in case of Zeeman splitting of rotational levels of the electronic ground state...

Fig. 11. Single-spin double-resonance experiment for the calibration of irradiation power. The proton line in chloroform is irradiated with the homonuclear decoupler. The strong central feature is at the irradiation frequency, and the separation of the two satellites is...

The low-temperature EPR experiments used to determine the DNA ion radical distribution make it very clear that electron and hole transfer occurs after the initial random ionization. What then determines the final trapping sites of the initial ionization events To determine the final trapping sites, one must determine the protonation states of the radicals. This cannot be done in an ordinary EPR experiment since the small hyperfine couplings of the radicals only contribute to the EPR linewidth. However, detailed low-temperature EPR/ENDOR (electron nuclear double resonance) experiments can be used to determine the protonation states of the low-temperature products [17]. These proto-nation/deprotonation reactions are readily observed in irradiated single crystals of the DNA base constituents. The results of these experiments are that the positively charged radical cations tend to deprotonate and the negatively charged radical anions tend to protonate. 

Also, in most cases, double-resonance experiments provide the forward, kf, and reverse, k, rate constants defined in Eq. (2). 

Figure 9.22—Spin decoupling experiment on butanone. Spectral modification a) by irradiating the CHj group at 2.47 ppm b) by irradiating the CH3 group (of ethyl) at 1.07 ppm, compared to the spectrum shown in Fig. 9.1. For such a simple compound, these experiments are only used to illustrate the principle. On the other hand, a double resonance experiment would be useful to precisely determine the coupling in aspirin (shown in Fig. 9.20).

The cross-polarization (CP), i.e. the transfer of I-spin polarization to the dilute spins (S), is a double resonance experiment in which the I and S spins are coupled by a certain interaction, determined by the cross relaxation time tb. From the dynamics of the CP process, usually described with the spin temperature concept, the following equation for the time dependence of S-spin polarization could be derived ... 

J. P. Vigier, New theoretical implications of Neutron interferometric double resonance experiments, Int. Workshop on Matter Wave Interferometry in the Light of Schrodinger s Wave Mechanics (Vienna, Austria, Sept. 14-16, 1987) Physica B, C 151(1-2), 386-392 (1988), ISSN 0378-4363 (Conf. sponsor Hitachi Erwin Schrodinger Gesellschaft Siemens et al.). 

Swept double resonance experiments were performed to elucidate ionic precursors in gas phase reactions. In some cases, the observed anions were found to arise from reactions with trace impurities that are present in the vacuum system or the Cl reagent gas, such as water and oxygen. 

Figure 4. Swept double resonance experiment on perdeutero-fluorene using H2180 as a reagent gas. Mass 174 was monitored as masses from 15 to 21 amu were sequentially ejected at 0.2 amu intervals.

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