Pulse INADEQUATE

Typical 13C rectangular pulses with y Blmax = 20 kHz (12.5 and 25 /is for 90° and 180° pulses) generate large off-resonance effects for chemical shift range of 200 ppm required for 13C. These effects can lead to a significant attenuation or complete loss of signal in multi-pulse INADEQUATE pulse sequences. Practical implementations of INADEQUATE pulse sequences must, therefore, include some sort of compensation of off-resonance effects. Several approaches have been reported to date. 

In many instances, the design of suction and discharge pulsation dampening drums (or bottles) for reciprocating compressors is based on piston displacement and volumetric efficiency, and this design normally will suffice to reduce peak pulsation to approximately 5% of the line pressure. In special or other cases, experience has shown that operational difficulties (vibrations, meter pulsations, etc.) may indicate that the peak pulse pressure of 5% line pressure is inadequate. Thus, the pressure in pulsation-reduction design selection is... 

A two-dimensional INADEQUATE experiment therefore requires a pulse sequence capable of producing the following four changes in the spin and energy states ... 

SELINQUATE (Berger, 1988) is the selective ID counterpart of the 2D INADEQUATE experiment (Bax et al., 1980). The pulse sequence is shown in Fig. 7.4. Double-quantum coherences (DQC) are first excited in the usual manner, and then a selective pulse is applied to only one nucleus. This converts the DQC related to this nucleus into antiphase magnetization, which is refocused during the detection period. The experiment has not been used widely because of its low sensitivity, but it can be employed to solve a specific problem from the connectivity information. 

Figure 7.4 Pulse sequence for the SELINQUATE experiment, which is a selective INADEQUATE experiment. (Reprinted from Mag. Reson. Chem. 29, H. Kessler et at, 527, copyright (1991), with permission from John Wiley and Sons Limited, Baffins Lane, Chichester, Sussex P019 lUD, England.)...

All disilanes and trisilanes were synthesized with methods described in the literature [6]. 29Si NMR spectra were recorded with a BRUKER MSL 300 spectrometer, using solutions of the silanes in C6D6 (app. 50 %). 29Si29Si coupling constants were measured with the standard INADEQUATE pulse sequence. 

Figure 2 (A) Pulse sequence used to acquire the 1,1- and INADEQUATE experiments... 

Pulsed FT-NMR has facilitated the study of nuclei other than H where the sensitivity obtainable from a CW instrument is totally inadequate. In particular, 13C NMR, the sensitivity of which is nearly 10-4 less than that of the proton (Table 9.9), is now a well-established technique that yields information on the skeletal structure of complex molecules. The pulsed technique also enables proton spectra to be obtained from samples as small as a few micrograms. 

The principle of multiple selective excitation has been incorporated into a few ID and 2D experiments, the schemes of which are shown below (fig. 1). Depending on the experiment, either a DANTE pulse train (ID TOCSY [2]), frequency selective 180° pulses (ID NOE [3], ID INADEQUATE [4], ID C/H COSY [5] and 2D TOCSY-COSY [6]) or frequency selective 90° pulses (2D HMBC [11]) are applied to selectively perturb and uniquely label selected spins. Besides the DANTE pulse , composed itself of a series of non-selective rectangular pulses, Gaussian-shaped 180° and... 

Fig. 1. Pulse sequences modified for multiple selective excitation. I ID TOCSY, II het-eronuclear ID NOE, III ID INADEQUATE, IVa heteronuclear ID COSY (optimized to detect Jch), IVb heteronuclear ID COSY (optimized to detect "Jch), V 2D TOCSY-COSY, Via 2D HMBC (designed to detect heteronuclear long-range couplings "Jch only), VIh 2D HMBC (extended pulse sequence to detect both heteronuclear long-range "Jch and...

TOPHAT-shaped 90° pulses are used in other cases as the best compromise with respect to the excitation profile, the phase homogeneity and length. Depending on the type of the detected spin-spin interaction - being either scalar or dipolar coupling - each selected spin is initially perturbed only once (ID TOCSY, ID INADEQUATE, ID C/H COSY, 2D TOCSY-COSY and 2D HMBC), or for several times (ID NOE). With each of the selected spins initially perturbed only once the inherently smaller transient NOEs would be detected in the latter case, whereas with the multiple excitation of a selected spin within the NOE build-up period the stronger steady-state NOEs are more or less approximated. 

Fig. 2.48 illustrates one variant of the INADEQUATE sequence [58]. Carbon-13 transverse magnetization generated by the initial 90° pulse (a) results from a strong component S0 for the 13C—12C pairs ( 99%) and a very weak one S2 for the less abundant 13C, 3C pairs ( 1 %). When proton decoupling is applied, the strong component S0... 

Fig. 2.48. INADEQUATE pulse sequence (a I) and pulse-driven motion (a-f) of carbon-13 magnetizations S0 for UC —12C singlet and S2 for 13C — 13C doublet signals in the rotating frame. Proton broadband decoupling is applied throughout the experiment.

As shown in Fig. 2.48 (e), the vectors of C13 — C13 doublet magnetizations are aligned in opposite directions when irradiated by the 90 pulse. Thus, in the INADEQUATE spectrum the C13 —C13 doublet signals will appear with the corresponding antiphase relationship, as shown in Fig. 2.49, which also demonstrates the effective suppression of the strong 13C — 12C signals of piperidine. Analysis of carbon-carbon coupling constants can be performed easily in this simple case. 

The antiphase relationship of the C13 —C13 doublet signals in the INADEQUATE spectrum can be eliminated by an additional spin-echo sequence (— 1/4Jcc — 180° — 1 /4 Jcc —) before the 90 monitor pulse [58]. The sensitivity of the experiment may be improved by the application of stronger magnetic fields or by using proton polarization transfer techniques [59]. 

A constant evolution period t, is the new feature of. /-modulated spin-echo, DEPT, and INADEQUATE sequences. During this time period, a 7-modulation or a polarization transfer may evolve. Such pulse sequences provide FID signals S(t2) which are still functions of one variable time t2. The Fourier transforms, however, are NMR spectra with specific information, depending on the constant evolution period ti. One simple example is the generation of the quaternary carbon-13 subspectrum by means of a. /-modulated spin-echo experiment with an evolution time of tj2 = x = as in... 

The AB and AX systems of all 13C —13C bonds appear in one spectrum when the INADEQUATE pulse sequence (Fig. 2.48) is applied. Complete interpretation usually becomes difficult in practice due to signal overlapping, isotope shifts and AB effects (Section 2.9.4). A separation of the individual 13C— 13C two-spin systems by means of a second dimension would be desirable. It is the frequency of the double quantum transfer (d e) in Fig. 2.48 which introduces a second dimension to the INADEQUATE experiment. This double quantum frequency vDQ characterizes each 13CA — I3CX bond, as it depends on the sum of the individual carbon shieldings vA and vx in addition to the frequency v0 of the transmitter pulse located in the center of the spectrum if quadrature detection is applied [69c, 71] ... 

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