Proton decoupling gated

Figure 1.10. NMR spectra of 2,4,6-trichloropyrimidine [CeDe, 75% v/v 25 °C, 20 MHz], (a) NMR spectrum without proton decoupling (b) NOE enhanced coupled NMR spectrum (gated decoupling)...

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.

Fig. 2.46. 13CNMR spectra of ( — )-menlhol (lOOmg/mL deuteriochloroform 100.6 MHz) (a) proton broadband-decoupled spectrum (b, c) DEPT spectra with 0y = 90" for CH selection with and without proton decoupling (d, e) DEPT experiments with 0y = 135" for positive CH and CH3 but negative CH2 signals with and without proton decoupling (f) gated-decoupled spectrum for reference (a-e) 16 scans (f) 256 scans. 

Fig. 2.51.. /-Modulated spin-echo sequence with gated proton decoupling for acquisition of -/-resolved two-dimensional 13C NMR spectra, and the CH magnetization vectors in the x y plane controlled by pulses and. /-modulation. During the preparation period between successive experiments, nuclear Overhauser enhancement of 13C magnetization is retained by minimum proton decoupling.

Since JCH coupling is only in operation during one t,/2 period in the J-modulated spin-echo sequence with gated proton decoupling, as drawn in Fig. 2.51, the second Fourier transformation provides only one half of the actual Jal magnitudes. Therefore, resolution of smaller couplings will be poor. 

Keto-enol tautomerism of fi-keto esters can be analyzed by carbon-13 NMR as illustrated by an inverse gated proton-decoupled 13C NMR spectrum of ethyl acetoacetate in Fig. 4.6 [73 i]. 

The increments of Table 4.78 permit a prediction of carbon shifts in substituted alkanes. Thereby, they may provide an assignment aid which is particularly useful when proton decoupling experiments (off-resonance, gated, selective) and the application of modern pulse sequences fail because of equal multiplicities (( —CH2 —)n chains) or signal overcrowding in the i3C and iH NMR spectra. The practical use of the increments is illustrated for 1,4-dibromopentane and lysine. 

Fig. 5.1. C NMR spectra of 5a-cholcstan-3-onc in dcutcriochloroform (50 mg/0.5 mL) (a) proton broadband-decoupled, 400 scans (b). /-modulated spin-echo experiment for quaternary carbon selection, 1000 scans (c-e) CH, Cl I2, and CH3 subspectra generated from linear combination of three DEPT experiments (see Section 2.9.3.2), 200 scans per experiment (f) gated proton-decoupled experiment for comparison.

FIGURE 4.4 (a) Inversion recovery pulse sequence with inverse gated proton decoupling for 7, measurement. 

FIGURE 4.5 Gated proton decoupling pulse sequence. Rd is relaxation delay, 6 is a variable pulse angle, t2 is the acquisition time. 

Figure 5.2b is the pulse sequence for a 13C experiment. The sequence in the 13C channel is exactly the same as the sequence in the H channel in Figure 5.2a. The protons are decoupled from the 13C nuclei by irradiating the protons during the experiment that is, the proton decoupler is turned on during the entire experiment. In other experiments, the decoupler for a given nucleus can be turned on and off to coincide with pulses and delays in another channel (i.e., for another nucleus). This process is termed gated decoupling. (See Sections 4.2.5 and 4.4)...

H and 19F NMR spectra are recorded with a normal one-pulse sequence or, alternatively, the XH spectra are recorded with a sequence that allows simultaneous solvent suppression with presaturation (31) or a sequence that includes some other method of suppression 13C 1H and 1P 1H spectra are recorded with proton broadband (composite pulse) decoupling (32), and 31P spectra with gated proton decoupling (33). 

As an alternative to the above method for eliminating the NOE an instrumental technique is available. This depends upon the realization (247) that the time-dependent behaviour of the NOE and of spin decoupling are different. Thus the NOE takes a time comparable for Tj to build up or to decay after application or removal of a rf field, whereas spin decoupling effects appear or disappear almost instantaneously. Consequently if the proton decoupler is gated off immediately prior to... 

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