Broadband decoupling INEPT

Both experiments are based on polarization transfer from sensitive nuclei to insensitive nuclei, and therefore the mjyor portions of their pulse sequences are common. The INEPT experiment, without refocusing and decoupling, however, yields spectra with distorted" multiplets. For instance, the two lines of a doublet appear in antiphase with respect one another. Similarly, the central line of a triplet may be too small to be visible, while the outer two lines of the triplet will be antiphase to one another. Introducing a variable refocusing delay A and broadband decoupling in the INEPT sequence can convert this experiment into a more useful one. 

To appreciate the sensitivity gains from the application of the INEPT sequence, one should compare the results with those obtained from the usual direct observation of the low-y species. This invariably means the spectrum obtained in the presence of proton broadband decoupling for which signal enhancement will occur by virtue of the H-X NOE (Chapter 8). Thus, to make a true comparison, we need to consider the signal arising from polarisation transfer versus that from observation with the NOE, which for an XH pair is given by ... 

DEPT is usually performed with broadband H decoupling. It is relatively insensitive to the precise matching of delays with coupling constants, and so is much easier to use than the closely related INEPT or the JMOD (APT) (see section 3.3.2.3) sequence. DEPT, on the other hand, is more sensitive to pulse imperfections than INEPT or JMOD. 

JMOD (APT) is usually performed with broadband H decoupling and is relatively sensitive to the precise matching of the delay D2 to the J,., coupling constant, and so is less easier to use than the polarization techniques DEPT and INEPT. On the other hand, only one single experiment is necessary to measure the signals of all carbon multiplicities. 

Figure 1. Pulse sequences used to monitor the heteronuclear NOE (bottom panel) and the spin lattice relaxation (top panel). The NOE experiment is a simple extension of the basic pulse sequence introduced by Kay et al. (1989) and utilizes continuous broadband H decoupling during the preparation period to generate the NOE. Two dimensional spectra with and without H decoupling (lightly shaded region) define the NOE. The T, relaxation experiment is a simple extension of the basic pulse sequence introduced by Sklenar et al. (1987). The NOE via H decoupling rather than coherent polarization transfer is used to polarize the carbons. For both the NOE and T, measurement, the proton pulse 0 (or the delay of the corresponding reverse INEPT) is set to the magic angle as described by Palmer et al. (1991). The constant time period, A, is set to minimize cos(n [27i J + 27t Jo,]). When x is set to l/2 Jc then 2A = - 1/2 J( ...

NMR Spectrometry. Liquid phase and NMR spectra were recorded on a Varian XL300 NMR spectrometer at carbon and nitrogen resonant fi equencies of 75.4 and 30.4 MHz, respectively, using a 10 mm broadband probe. Quantitative NMR spectra of the unreacted fulvic and humic acid samples were recorded in DMSO-d6, 99.9 atom % as previously described (23). INEPT (24) and ACOUSTIC (25) N NMR spectra were recorded on the aniline-reacted fulvic and humic acids. Refocussed INEPT (proton decoupled) spectra were recorded as previously described (9). ACOUSTIC spectra, with the exception of the bimessite catalyzed sample, were recorded with the use of paramagnetic relaxation reagent (100-200 mg chromium (III) acetylacetonate). Acquisition parameters included an 18,656.7 Hz spectral window (613.7 ppm), 0.5-s acquisition time, 45° pulse angle, 2.0-s pulse delay, and t delay of 0.1 ms. Neat formamide in a 5 mm NMR tube, assumed to be 112.4 ppm, was used as an external reference standard for all spectra. N NMR chemical shifts are reported in ppm downfield of ammonia, taken as 0.0 ppm. 

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