Overhauser shift

EPR but provides greatly enhanced resolution. Double resonance techniques (e.g. electron nuclear double resonance (ENDOR) and Overhauser shift measurements) combine the sensitivity of EPR with the resolution of NMR. Many such measurements on thin films are performed by combining optical detection with ENDOR, greatly enhancing the resolution of ODMR and taking advantage of its superior sensitivity. 

While the Overhauser shift is due to delocalised electrons, the ENDOR experiments require more localised electrons. Both groups were able to observe ENDOR on the effective mass donor resonance but not on the deep donor signal. An illustrative ENDOR spectrum is shown in FIGURE 4, in which the 69 71Ga ENDOR lines are observed. The 69Ga line is clearly split by quadrupole interactions just as in the Oveihauser shift measurements, while the quadrupolar interaction is much less resolved in the 71 Ga line, due to its smaller quadrupole moment. In the three samples investigated by the two groups, the linewidths were all too broad to resolve any hyperfine interaction and so no definitive identification of the residual donor was possible. 

Koschnick et al [59] report an EFG of 6.50 x 1020 V/m2 for their thin film sample (similar to the results of NMR and the Overhauser shift measurements) while Glaser et al [60] report EFGs of 5.91 x 1020 V/m2 and 5.32 x 1020 V/m2 for two different thin film samples. The disparity in these values is more than can be accounted for by experimental errors. Glaser and co-workers [61] have compared their results with the in-plane compressive strain ( 2 - 3 x 10 3) measured by X-ray diffraction and assumed the EFG obtained from the Overhauser shift measurements represents an unstrained GaN crystal. The reduction of the EFG with compressive strain indicates that the atoms in the fihns are in a somewhat more tetrahedral environment than those in the bulk, suggesting that the c-direction Ga-N braid is somewhat shorter than the three others in both the films and the bulk. This is consistent with the ratio of lattice constants, c/a, being less that the ideal value of (8/3)1/2... 

Kaiser et al.119 have studied 19F NMR in (Pyrene)i2(SbFg)7 cation salt. The rotational motion of those SbFg anions of this salt can be best discriminated by the analysis of the temporal evolution of the 1H Overhauser shift of the conduction electron ESR line. Temperature dependence of the Overhauser shift detected proton-SLR rate recorded at 9.46 GHz electron spin and 14.4 MHz proton NMR frequency. It is important to note that the proton relaxation reflects only the low temperature BPP-peak of SbFg anion rotation, in addition to conduction electron contribution. This salt undergoes a 3D-ordered Peierls transition at 113 K, which is due to the freezing of the anion motion. 

Fig. 9.23 The Overhauser shift ABqy of the ESR line from a (Fa)2PF6 crystal at room temperature, a the ESR line at a fixed microwave frequency vesr- the change in the ESR signal with an additional fixed applied magnetic field Bq = and irradiation with a variable radio-frequency field of frequency...

Azz is the component of the hyperfine tensor in the Bq direction. Avesr is called the Overhauser shift. It is analogous to the Knight shift Avnmr of tho nuclear-spin resonance frequency v mr in the presence of conduction electrons with a polarisation P. ... 

The Overhauser shift is very small the contribution to Avesr/ esr of the protons is of order 10" and that of less abundant nuclear spins, e.g. is of order 10" [34]. Their measurement is accomplished by a double resonance technique. The shift A Bov of the resonance field for ESR is measured at a constant ESR-microwave frequency vesr (e.g. 9.4 GHz), while at the same time a strong radio-frequency field with the variable frequency Vjfis applied. When Vjf= v mr the nuclear-spin resonance, e.g. the nuclear-spin resonance of the protons at Vrf=Vp, will be saturated. This produces equal populations of the two nuclear-spin Zeeman levels, causing the nuclear-spin polarisation P to vanish. This leads according to Eq. (9.27) to a shift of the ESR resonance field by an amount... 

Nuclear Overhauser enhancement (NOE) spectroscopy has been used to measure the through-space interaction between protons at and the protons associated with the substituents at N (20). The method is also useful for distinguishing between isomers with different groups at and C. Reference 21 contains the chemical shifts and coupling constants of a considerable number of pyrazoles with substituents at N and C. NOE difference spectroscopy ( H) has been employed to differentiate between the two regioisomers [153076 5-0] (14) and [153076 6-1] (15) (22). N-nmr spectroscopy also has some utility in the field of pyrazoles and derivatives. 

The measurement of correlation times in molten salts and ionic liquids has recently been reviewed [11] (for more recent references refer to Carper et al. [12]). We have measured the spin-lattice relaxation rates l/Tj and nuclear Overhauser factors p in temperature ranges in and outside the extreme narrowing region for the neat ionic liquid [BMIM][PFg], in order to observe the temperature dependence of the spectral density. Subsequently, the models for the description of the reorientation-al dynamics introduced in the theoretical section (Section 4.5.3) were fitted to the experimental relaxation data. The nuclei of the aliphatic chains can be assumed to relax only through the dipolar mechanism. This is in contrast to the aromatic nuclei, which can also relax to some extent through the chemical-shift anisotropy mechanism. The latter mechanism has to be taken into account to fit the models to the experimental relaxation data (cf [1] or [3] for more details). Preliminary results are shown in Figures 4.5-1 and 4.5-2, together with the curves for the fitted functions. 

One disadvantage of the APT experiment is that it does not readily allow us to disdnguish between carbon signals with the same phases, i.e., between CH3 and CH carbons or between CH2 and quaternary carbons, although the chemical shifts may provide some discriminatory information. The signal strengths also provide some useful information, since CH3 carbons tend to be more intense than CH carbons, and the CH2 carbons are usually more intense than quaternary carbons due to the greater nuclear Overhauser enhancements on account of the attached protons. 

Figure 3.1 The various time periods in a two-dimensional NMR experiment. Nuclei are allowed to approach a state of thermal equilibrium during the preparation period before the first pulse is applied. This pulse disturbs the equilibrium ptolariza-tion state established during the preparation period, and during the subsequent evolution period the nuclei may be subjected to the influence of other, neighboring spins. If the amplitudes of the nuclei are modulated by the chemical shifts of the nuclei to which they are coupled, 2D-shift-correlated spectra are obtained. On the other hand, if their amplitudes are modulated by the coupling frequencies, then 2D /-resolved spectra result. The evolution period may be followed by a mixing period A, as in Nuclear Overhauser Enhancement Spectroscopy (NOESY) or 2D exchange spectra. The mixing period is followed by the second evolution (detection) period) ij.

Total assignment of the H and 13C NMR chemical shifts as well as the relative configuration of the Diels-Alder adducts 33-35 was accomplished with the help of 2D (111-111 COSY, H-111 NOESY (NOESY = nuclear Overhauser enhancement spectroscopy), H- C XHCORR (XHCORR = nucleus X-hydrogen correlation), H-13C COLOC) and NOE difference spectroscopy <1996JHC697>. 

The 13C NMR sensitivity can sometimes be a problem, but for the kind of samples studied here the effective concentration of monomer units is several molar which does not place excessive demands on present Fourier transform NMR spectrometers. In addition to the sensitivity of the chemical shift to structure (9), the relaxation of protonated carbons is dominated by dipole-dipole interaction with the attached proton (9). The dependence of the relaxation parameters T, or spin-lattice, and Tor spin-spin, on isotropic motional correlation time for a C-H unit is shown schematically in Figure 1. The T1 can be determined by standard pulse techniques (9), while the linewidth at half-height is often related to the T2. Another parameter which is related to the correlation time is the nuclear Overhauser enhancement factor, q. The value of this factor for 13C coupled to protons, varies from about 2 at short correlation times to 0.1 at long correlation... 

Little difference was noted when peak heights were used. The error in the T data is less than + 10%. Nuclear Overhauser enhancement factors (q) were obtained by measuring the integrated intensity of peaks in a difference spectrum from one with enhancement minus one with no enhancement and dividing that value by the integral from the one with no enhancement i.e. n ( nOe no nOe / (I nOe" Accuracy should be 10% or better. Linewidtns were measured at half heights, and chemical shifts are relative to TMS. 

LC-NMR plays a central role in the on-line identification of the constituents of crude plant extracts (Wolfender and others 2003). This technique alone, however, will not provide sufficient spectroscopic information for a complete identification of natural products, and other hyphenated methods, such as LC-UV-DAD and LC-MS/MS, are needed for providing complementary information. Added to this, LC-NMR experiments are time-consuming and have to be performed on the LC peak of interest, identified by prescreening with LC-UV-MS. NMR applied to phenolic compounds includes H NMR,13 C NMR, correlation spectroscopy (COSY), heteronuclear chemical shift correlation NMR (C-H HECTOR), nuclear Overhauser effect in the... 

Convincing evidence was found that the majority of acyclic aldo-nitrones exist in the Z-form, by investigating the ASIS-effect (aromatic solvent induced shift effect) (399). However, in some cases, specified by structural factors and solvent, the presence of both isomers has been revealed. Thus, in C -acyl-nitrones the existence of Z -and -isomers was detected. Their ratio appears to be heavily dependant on the solvent polar solvents stabilize Z-isomers and nonpolar, E-isomers (399). A similar situation was observed in a- methoxy-A-tert-butylnitrones. In acetone, the more polar Z-isomer was observed, whereas in chloroform, the less polar E-isomer prevailed. The isomer assignments were made on the basis of the Nuclear Overhauser Effect (NOE) (398). /Z-Isomerization of acylnitrones can occur upon treatment with Lewis acids, such as, MgBr2 (397). Another reason for isomerization is free rotation with respect to the C-N bond in adduct (218) resulting from the reversible addition of MeOH to the C=N bond (Scheme 2.74). The increase of the electron acceptor character of the substituent contributes to the process (135). 

Assignment of the isotropically shifted signals observed for the CuNiSOD example discussed in the previous paragraph has been achieved by means of anion titrations (not discussed here) and nuclear Overhauser enhancement spectroscopy (NOESY), to be discussed next. In Figure 3.24B the CuNiSOD active site is depicted with histidine nitrogens and protons identified for the discussion of the NOESY results. The copper(II) ion is coordinated to the N ligand atoms of his46... 

Nuclear Overhauser Effect, Spin-spin coupling and Chemical Shift... 

---