SECSY spectra

Zero-quantum coherence has also been recently used as an alternative to double-quantum coherence for establishing carbon-carbon connectivities. This is shown for the C-NMR spectrum of n-butanol in Figure 5.67C. The diagonal lines establish the connectivities between coupled carbons. An advantage of the zero-quantum coherence spectra, which resemble the SECSY spectra in appearance, is that in the latter, one usually encounters a large ridge of unresolved peaks near Fj = 0, but this is largely suppressed in zero-quantum spectra. 

Figure 539 The structure and SECSY spectrum of majidine. Cross-peaks represent...

The SECSY spectrum of an isoprenyl coumarin along with the H-NMR chemical shifts are shown. Determine the homonuclear shift correlations between various protons based on the SECSY spectrum. 

SECSY (spin-echo correlated spectroscopy) is a modified form of the COSY experiment. The difference in the pulse sequence of the SECSY experiment is that the acquisition is delayed by time mixing pulse, while the mixing pulse in the SECSY sequence is placed in the middle of the period. The information content of the resulting SECSY spectrum is essentially the same as that in COSY, but the mode... 

The SECSY spectrum of buxapentalactone displays a number of off-diagonal peaks. Pairs of interconnected cross-peaks can be identified... 

The SECSY spectrum of the coumarin presents cross-peaks for various coupled nuclei. These cross-peaks appear on diagonal lines that are parallel to one another. By reading the chemical shifts at such connected cross-peaks we arrive at the chemical shifts of the coupled nuclei. For instance, cross-peaks A and A exhibit connectivity between the vinylic C-4 and C-3 protons resonating at 8 7.8 and 6.2, respectively. The C-4 methine appears downfield due to its )3-disposition to the lactone carbonyl. Similarly, cross-peaks B and B show vicinal coupling between the C-5 and C-6 methine protons (8 7.6 and 7.1, respectively) of the aromatic moiety. The signals C and C represent the correlation between the oxygen-bearing C-11 (85.4) andC-12 (84.6) methine protons in the side chain. These interactions are presented around the structure. 

The recent development of a gradient high-resolution MAS probe will extend the utility of 2D experiments by removing artifacts that generally accompany MAS 2D NMR data on resin samples (46). The lack of artifacts is illustrated by the high quality SECSY spectrum shown in Fig. 16. SECSY data contain the same information as a COSY spectrum, but the appearance of the spectrum is different. The diagonal lies along the FI =0 and the off-... 

Figure 5.39 The structure and SECSY spectrum of majidine. Cros peaks represent coupling interactions between various protons. These interactions are also presented around the structure. ...

FOCSY Spectra. Another modification of the Jeener 2D-shift correlated spectroscopy which finds limited application in some special cases is known as Foldover Corrected Correlated Spectroscopy (FOCSY). The COSY spectrum is modified by software amendments to correct folding errors along the axis, and the chemical shifts of the normal spectrum appear on a horizontal line at coj = 0, so that the 2D contour plot resembles the SECSY spectrum. Again no reduction in data matrix size is achieved unless the largest chemical shift difference between coupled nuclei is smaller than half of the spectral width. 

The pulse methods rely on selective irradiation of a particular resonance line with a radio frequency (rf) and observation of the resulting effects in the rest of the spectrum. Among commonly employed methods are 2D correlated spectroscopy (COSY), 2D spin-echo correlated spectroscopy (SECSY), 2D nuclear Overhauser and exchange spectroscopy (NOESY), 2D J-resolved spectroscopy (2D-J), and relayed coherence-transfer spectroscopy (RELAYED-COSY) (Wutrich, 1986). 

Figure 15 (a) Spin echo (b) projection of the SECSY data MAS H NMR spectrum for DMF-d7-swollen Fmoc-isoleucine on Wang resin 1 and (c) solution NMR spectrum for Fmoc-isoleucine methyl ester. 

Figure 16 Gradient 2D SECSY NMR spectrum for Alloc-Asp-derivatized oxazoli-dinone on SCAL-linked aminomethylpolystyrene swollen with DMF-d7obtained with a 4-mm high-resolution gradient MAS probe.

Within Molecular Sophe there are a number of standard (default) experiments that have been optimized and are stored in /usr/mosophe/examples. Over time the number of examples will increase. Shown in the figures above and below are some of die results. Proton and nitrogen HYSCORE simulated spectra are shown in Figure 50. A pulsed MIMS ENDOR spectrum is shown in Figure 51, and a SECSY experiment in Figure 52. 

Figure 5.61. Pulse sequences for homonuclear 2D-shift correlated NMR spectroscopy (A) COS 7-90 pulse sequence (B) SECSY pulse sequence. The second 90° mixing pulse causes an exchange of magnetization between the coupled nuclei. If the second pulse in (A) is a 45° pulse, then a simplified COS 7-45 spectrum is obtained with reduced intensities of cross peaks near the diagonal.

Figure 5.68. Homonuclear spin-echo correlated 2D spectrum (SECSY) of 9, 11-didehydro [18] annulene derivative.

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