Proton heteronuclear broadband decoupling

In C NMR spectroscopy, three kinds of heteronuclear spin decoupling are used In proton broadband decoupling of C NMR spectra, decoupling is carried out unselectively across a frequency range which encompasses the whole range of the proton shifts. The speetrum then displays up to n singlet signals for the n non-equivalent C atoms of the moleeule. 

An alternative method for excitation of nuclei over a range of chemical shifts is by irradiation with a weak, noise-modulated radio-frequency, instead of with strong r.f. pulses. In one realization of this method, protons were irradiated with repetitive sequences of noise that was truly random,162 and, in another,163 fluorine nuclei were excited by pseudo-random noise generated by amplitude modulation of the r.f. with maximum-length sequences of pulses from a computer or shift register (a series of flip-flop devices connected by feedback loops). With the carrier wave suppressed, the latter process is equivalent to phase modulation of the r.f. by+7r/2 radians when the pulse is turned on, and by —ir/2 radians when it is turned off. This method is identical with that used in most broadband, heteronuclear, noise decouplers, except that greater power is required for decoupling. 

Figure 9.7. Heteronuclear broadband proton decoupling with a sequence of 180° proton pulses.

Fluorine-19 NMR data were acquired at a frequency of 188.22 MHz with a Varian XL-200 spectrometer. Typically, 100 transients were accumulated from a 5% polymer solution by volume in dimethylformamide-d7 placed in a 5 mm sample tube at 120 C with internal hexafluorobenzene as a reference ( = 163 ppm). A sweep width of 8000 Hz was used with 8 K computer locations (acquisition time 0.5s) and a 5.0 s delay between 90 pulses (9.0 s duration). Proton heteronuclear coupling was removed by broad-band irradiation centered at 200 MHz. A modified Bruker WH-90 spectrometer allowed carbon-13 NMR spectra to be obtained with simultaneous proton and fluorine-19 broadband decoupling (13). 

Broadband decoupling is a type of heteronuclear decoupling in which spin-spin splitting of C lines by H nuclei is avoided by irradiating the sample with a broadband RF signal that encompasses the entire proton S >ectral region, whereas the C spectrum is obtained in the usual way. Ordinarily, the proton signal is produced by a second coil located in the sample probe. 

NMR probes are designed with the X-coil closest to the sample for improved sensitivity of rare nuclei. Inverse detection NMR probes have the proton coil inside the X-coil to afford better proton sensitivity, with the X-coil largely relegated to the task of broadband X-nucleus decoupling. These proton optimized probes are often used for heteronuclear shift correlation experiments. 

Fig. 10.14. Gradient-enhanced HMQC pulse sequence described in 1991 by Hurd and John derived from the earlier non-gradient experiment of Bax and Subramanian. For 1H-13C heteronuclear shift correlation, the gradient ratio, G1 G2 G3 should be 2 2 1 or a comparable ratio. The pulses sequence creates heteronuclear multiple quantum of orders zero and two with the application of the 90° 13C pulse. The multiple quantum coherence evolves during the first half of ti. The 180° proton pulse midway through the evolution period decouples proton chemical shift evolution and interchanges the zero and double quantum coherence terms. Antiphase proton magnetization is created by the second 90° 13C pulse that is refocused during the interval A prior to detection and the application of broadband X-decoupling.

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