Spectral assignment Experiment

In addition to sample rotation, a particular solid state NMR experiment is further characterized by the pulse sequence used. As in solution NMR, a multitude of such sequences exist for solids many exploit through-space dipolar couplings for either signal enhancement, spectral assignment, interauclear distance determination or full correlation of the spectra of different nuclei. The most commonly applied solid state NMR experiments are concerned with the measurement of spectra in which intensities relate to the numbers of spins in different environments and the resonance frequencies are dominated by isotropic chemical shifts, much like NMR spectra of solutions. Even so, there is considerable room for useful elaboration the observed signal may be obtained by direct excitation, cross polarization from other nuclei or other means, and irradiation may be applied during observation or in echo periods prior to... 

The 1H NMR spectrum of (DL)-penicillamine in D20 was obtained on a Bruker 500 MHz instrument, and the resulting spectrum is shown in Fig. 3. Confirmation of the spectral assignments was derived from a COSY experiment (see Fig. 4), and these assignments are summarized in Table 3. 

In the previous sections, only the basic, non-gradient ID TOCSY pulse sequence, its experimental aspects and applications were described. In the following, the more recent modifications and extensions of the basic pulse sequence and their applicability to spectral assignments and structural elucidation will be briefly reviewed. Some of these more sophisticated techniques may not be as readily implementable as the basic ID TOCSY experiments, and thus have not yet found wide applications in routine practice. 

The method proceeds as follows a series of spectroscopic results are first collected using appropriate pulse sequences for spectral assignments and control experiments. The spectral features are then assigned and critical... 

Evidence in support at the H and 13C spectral assignments was obtained from two-dimensional correlation spectroscopy (COSY) experiments. Proton-proton (H-H COSY) results are shown in Fig. 8.9. 

Complete NMR spectral assignments have been made for the trans,cis and transjrans spiroepoxides derived by epoxidation of (Z)-3-arylidenethioflavanones, their 1-oxides and 1,1-dioxides (Table 7). As a consequence of the similarity of the chemical shifts of the signals in the 13C NMR spectra of the isomer pairs, coupling constants data for 37n-2,c-8a (37hc 7.0-7.6 Hz sulfide Vhc 6.2-6.5 Hz sulfoxide Vhc 2.0-3.4 Hz sulfone) and NOESY experiments were used extensively together with ab initio MO calculations to elucidate the conformation of the various isomers <2001MRC251, 2003MRC193>. 

The spectral assignment process, complex as it is, is only a prelude to the use of NMR data to assist in the final determination of the three-dimensional structure of the protein. Here, the most important experiment is NOESY, often in conjunction with HMQC or other methods for spreading out the 2D NOESY peaks in three or four dimensions. For larger proteins 4D experiments are essential to spread the NOESY peaks according to both 13C and 15N chemical shifts. We... 

Figure 6-21 illustrates several advantages of the HETCOR experiment (1) The correlation between protons and carbons means that spectral assignment at one frequency... 

Further 2D methods are necessary if the structure is not learned in the process of making spectral assignments for hypothetical or expected structures. The COSY experiment lays the groundwork for identifying structure through connectivities based on 7 cou-... 

The absolute positions of the doublet states are overestimated by about 2500 cm-1. The spin-orbit coupled emitting doublet states is predicted to be the 2A(3/2) component of the 2Eg state in agreement with spectral assignment but in conflict with excited state EPR experiments. Despite the quality of the calculations the large zero-field splitting (ZFS) of the 2Eg (220-290 cm-1) has not been reproduced with theoretical values less than 100 cm-1. 

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