Pulse technique, sensitivity enhancement

Pulse techniques, coupled with the observation of the decay of enhancement (Atkins et al., 1970a, b Glarum and Marshall, 1970 Smaller etal., 1971) constitute the most sensitive procedure for detecting CIDEP. Both net and multiplet polarization have been described. As with CIDNP, the former is believed to arise essentially from the Zeeman interaction and the latter from the hyperfine term. Qualitative rules analogous to Kaptein s rules should be capable of development. 

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... 

The most economical and efficient method of sensitivity enhancement in 13C NMR of organic molecules is the pulse Fourier transform technique (PFT) in combination with decoupling methods such as proton broad band decoupling and polarization transfer. These methods will be described in the following sections. 

The projection-reconstruction approach is a technique unrelated to covariance processing which can provide data typically inaccessible to the natural product chemist. For example, 13C-15N correlation spectra were obtained for vitamin B12 at natural abundance.104 Compared with a conventional three-dimensional 13C-15N correlation experiment, the projection-reconstruction method provides a sensitivity enhancement of two orders of magnitude. The final 13C-15N spectrum was reconstructed from data obtained from ll l5N and H- C correlations acquired using a time-shared evolution pulse sequence that allowed all the information to be obtained in one experiment.104 Martin and co-workers also demonstrated the ability to generate 13C-15N correlation spectra using unsymmetrical indirect covariance NMR with vinblastine as an example.105 In the latter case, 13C-15N correlation spectra were obtained from - C HSQC data and H-1sN HMBC data that were acquired separately. Both methods provide access to correlations that would be inaccessible for most natural products at natural abundance. 

An alternative pulse sequence that provides the same multiplicities as INEPT but with intensity ratios that follow the binomial theorem is DEPT (distortionless enhancement by polarization transfer).The pulse sequence, depicted in Fig. 12.3a, can be used, like refocused INEPT, for sensitivity enhancement but is usually employed as an editing technique. The three evolution periods T are chosen to approximate 1/2/, but the length of the pulse labeled 0 can be varied. As we show in the following, CH has maximum intensity at 0 = 90° CH2 has zero... 

Because the application of proton broad band decoupling in solution N NMR causes quantification problems, the inverse-gated decoupling technique often is used. With this technique no enhancement is observed, and the spectra can be quantified. However, a problem that arises in NMR experiments with nuclei with low sensitivity such as N is acoustic ringing. Ringing occurs because the rf pulse causes brief vibrations in the probe which masquerade as a signal. This signal overlaps the entire spectra which results in a distortion of the baselines. Quantification of these distorted spectra is impossible and in many cases the spectra become useless. 

A fundamentally different approach to signal excitation is present in polarization transfer methods. These rely on the existence of a resolvable J coupling between two nuclei, one of which (normally the proton) serves as a polarization source for the other. The earliest of these type of experiments were the SPI (Selective Population Inversion) type (19>) in which low-power selective pulses are applied to a specific X-satellite in the proton spectrum for an X-H system. The resultant population inversion produces an enhanced multiplet in the X spectrum if detection follows the inversion. A basic improvement which removes the need for selective positioning of the proton frequency was the introduction of the INEPT (Insensitive Nucleus Excitation by Polarization Transfer) technique by Morris and Freeman (20). This technique uses strong non-selective pulses and gives general sensitivity enhancement. 

For the vast majority of polymeric materials, only exists in high concentration and it has been the subject of the majority of applications of NMR to date. However, recent advances in instrumentation and computer capabilities, coupled with pulse techniques and Fourier transformation, have greatly enhanced the sensitivity of NMR spectroscopy. Thus, NMR spectra may be obtained from solutions of very low concentration, nuclei may be... 

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