Pulse angles

Most NMR experiments use combinations of two or more of the four pulse phases shown in Fig. 1.42. The phase of the pulse is represented by the subscript after the pulse angle. Thus, a 90f, pulse would be a pulse in which the pulse angle is 90° and -y is its phase (i.e., it is applied in the direction —y to +y), so it will cause the magnetization to rotate about the y-axis in the xz-plane. 

The pulse duration controls the extent to which the magnetization vectors are bent. A misalignment of the pulse would lead to various artifact signals. Including 180° spin-echo pulses can, to some extent, compensate for setting pulses incorrectly. But in certain experiments (e.g., inverse NMR experiments), it is extremely important for the success of the experiment that the proper pulse angles be determined and employed. 

Since the integration values form such an important element of structure determination, we need to set the spectrometer up properly before carrying out the NMR experiment. And one very important parameter which is often forgotten is the relaxation delay, the delay between the single NMR experiments which allows the nuclei to relax. Remember that relaxation is an exponential process, so that theory suggests that it is necessary for the best results to set this equal to at least five times (in our case more than 25 sec for the aromatic protons ). The other parameter we need to set correctly is of course the pulse angle, and the following set of experiments show how these are interrelated. 

We carried out two sets of experiments in which we set the pulse angle first at 90°, then at 30°. Using these two values we then varied the relaxation delay. Since the greatest difference in the relaxation times is that between the OH proton and the aromatic protons, we show in Fig. 11 the comparison between the integration values of the aromatic protons (set equal to 2.0) and of the OH proton for 90° pulses and for 30° pulses. The values approach each other with a relaxation delay of 10 sec and are virtually equal for a delay of 25 sec, but the 90° pulses give values which are completely wrong if a conventional delay of 1-2 sec is used On the other hand, the error is quite low if the delay is set at 2 sec and the pulse length is 30°. 

Re-evaluation of pulse delay times used to record fullerene 13C NMR spectra revealed that a 16 s pulse delay, twice the value for a standard detection, allowed the observation of a weak resonance in the sp3 region at 90.4 ppm in the 13C NMR spectrum of the unlabeled heterofullerene 114. Attempts were made to optimize the NMR experimental parameters for a long 7 i, i.e. the variation of delay times and pulse angles. Various conditions were tried on the labeled material without success. This is probably due to the mixture of the labeled and unlabeled 114 which give too low S/N for signal detection. Table 49 summarizes the NMR results obtained and illustrates a distinct pattern of the azafullerenes. 

Samples of about 50 mg are weighed and dissolved in 0.5 mL of (methyl sulfoxide)-d6 (DMSO-d6) and 10 pL of dichloromethane is added. A small portion of this solution is then withdrawn and diluted in an NMR tube with DMSO-d6- 1H NMR integration parameters are as follows 32K data points, recycle delay of 5 s, 30° pulse angle. Measurement is against the hydroxyl proton doublet (2 H) at 8 4.63. If the hydroxyl doublet is not satisfactorily resolved, additional dilution is performed. 

Irradiation of a sequence of very short resonant RF-pulses with a sum of pulse angles of 0° for on-resonant spins. Spin ensembles with long Tj show nearly full longitudinal magnetization after the pulse sequence. Spin ensembles with short T2 get severe transverse magnetization loss between and during the pulses and therefore is clearly reduced after application of the pulse sequence (e.g., Ref 44). 

Figure 4. Proton decoupled 19F-NMR spectrum of pABG5 /7-glucosidase inactivated with 2F/ GluF (conditions as described in text). This spectrum was recorded on a 270 MHz Bruker/Nicolet instrument using gated proton decoupling (decoupler on during acquisition only) and a 90° pulse angle with a repetition delay of 2s. A spectral width of 40,000 Hz was employed and signal accumulated over 10,000 transients.

To perform a quantitative imaging or spectroscopy experiment, the relaxation time characteristics of all species (and relevant physical states of those species) must be fully characterized so that the pulse angles and delays between pulses are optimized for that particular system. In particular, if successive repetitions of a pulse sequence are applied at time scales ( recycle delays ) of the order of or... 

As DEPT relies on the transfer of polarization from a directly bonded H atom to the carbon - resulting in the increased sensitivity of the carbon atoms - only C atoms that are attached to H atoms are detectable by this method, and so no quaternary carbon atoms are seen on DEPT spectra. Depending upon something called the pulse angle (which is expressed as a number after the acronym, but we do not need to know its significance), there are three different DEPT experiments that can be carried out on a particular sample. 

Fig. 2.5. (a) FID signal of hexadeuteriodimethyl sulfoxide neat liquid natural 13C abundance 22.63 MHz 30 C pulse width 10 qs for about 30 as pulse angle observation time 0.8 s 0.2 s of the decay are shown 2048 accumulated scans ... 

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