Noise-modulated decoupling

Figure 10. Selective irradiation of linear PE (2 X 10 mol wt, 1 — K 0.5). Spectral details are 35°C 67.9 MHz sweep width 5 KHz (quadrature detection) line broadening 9.7 Hz pulse width 35 jisec (90°C = 48 /jsec) delay = 1.0 sec, 4K data points 1024 scans accumulated 10-mm sample tube. Decoupling 7W (forward), 0.4W (reflected), broad band noise modulated decoupling.

The nucleus is present naturally only in 1.1% abundance and, even if it were 100% abundant, its effective sensitivity would only be ca. 1% that of an equivalent number of protons. Thus,tEere is an effective decrease in signal-to-noise of approximately 10 compared with the proton. Fortunately, the pulse F.t. method, together with other important instrumental developments such as noise-modulated decoupling, is equal to this task. 

It is also usually possible to remove all the couplings from a particular isotope, e.g. H, provided that one only wishes to observe the spectrum from another isotope, e.g. Either the decoupling frequency is noise-modulated to cover the relevant range of chemical shifts, or else the same decoupling is achieved more efficiently, and with less heating of the sample, by using a carefiilly designed, continuous sequence of... 

Figure 4.7 Pulse schemes representing separation of decoupling effects from the nOe during X nucleus acquisition. The decoupler is programmed to produce noise-modulated irradiation or composite pulse decoupling at two power levels. Suitable setting of the decoupler may produce either (a) nOe only, (b) proton decoupling only, or (c) both nOe and proton decoupling.

The noise-modulated broadband decoupled 13C NMR spectrum of (/)/.)-penicilla-mine is shown in Fig. 5. Two methyl carbons were found to resonate at chemical shifts of 27.7 and 30.2 ppm, in addition to a CH carbon at 64.6 ppm. Two quaternary... 

For off-resonance decoupling of 13C NMR spectra, frequency offsets of about 0.5 1 kHz are used. In order to avoid complete collapsing of multiplets by large, noise-modulated frequency bands, non-modulated decoupling fields are usually applied. The decoupling frequency offset can be adjusted until the multiplets are so narrow that no or only slight overlapping occurs. 

C spin-lattice relaxation times of individual nuclei can also be measured by PFT 13C H experiments using a 90°, r, 90", r,... pulse train and noise modulation of the proton decoupling frequency. This method is known as progressive saturation [43] and is based on the following concept. 

The spin-locking FT method is illustrated in Fig. 2.33 for o-dichlorobenzene in a 13C 1H experiment avoiding noise modulation of the decoupling frequency during application of the CW field [47]. The results are compared in Fig. 2.33 with Tx measurements in order to demonstrate the relation between Tt and T2 in liquids T[ S T2. 

Using single-frequency and noise-modulated resonance and off-resonance proton decoupling, 7] relaxation time measurements, relaxation reagents like Gd (fod)3 and specifically deuterated compounds, all the carbons in retinal isomers, the model compounds a-and /i-ionone, and vitamin A and its isomers [165, 555-557] were assigned. The olefinic ring carbons (C-5 and C-6) could be identified on the assumption that the 13C relaxation times are dominated by intramolecular dipole-dipole interactions with neighboring protons and that the same rotational correlation time characterizes the interactions for both carbons. Consequently the ratio of T/s for C-5 and C-6 can be estimated from eq. (5.1)... 

UC NMR spectrum A noise-modulated broad-band decoupled 13C NMR spectrum (Fig. 7.6) showed six resonance bands, which is in accordance with what would be anticipated for vigabatrin. A DEPT experiment (Fig. 7.7) permitted the identification of two methines at 53.9 and 132.9 ppm in addition to three methylenes at 28.9, 33.3, and 121.0 ppm. One carbonyl absorption was assigned at 181.3 ppm. These assignments were all conformed through the performance of HSQC experiment (Fig. 7.8). 

The noise modulated broad band decoupled 13C-NMR spectrum of EDTA in D2O (Figure 8) showed two methylene carbon atoms resonating at... 

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. 

FIG. 13. (a) Aromatic carbon region in the convolution-difference natural abundance C spectra of myoglobins recorded at 15-18 MHz, under conditions of noise-modulated off-resonance decoupling and 32768 accumulations per spectrum. See ref. 683 for line assignments, (b) The eight titratable carbons of the histidine residues (plus C of Tyr-103, peak 27) of horse cyanoferrimyoglobin at 38°C. 

One effect of using noise modulation in the decoupling of one nucleus from another is to make available an additional mechanism of transverse relaxation for the observed nucleus. That is, there is another contribution to T, and in experiments,for example, it is not... 

Resonance spectra, employing noise-modulated proton decoupling, have permitted identification of signals due to each carbon atom in the steroid skeleton (up to C28) at the natural abundance of this isotope. Chemical shifts covering a span of 200 p.p.m. are found. Data are reported for a range of sterols and steroid hormones. 

In the last few years, the impact of the use of 13C n.m.r. spectroscopy in natural product chemistry has been substantial, and in this regard alkaloids have not escaped attention. The natural-abundance 13C n.m.r. spectrum of nicotine has been determined at 15.8 MHz using both the noise-modulated total proton decoupling and the specific and off-resonance, single-frequency proton decoupling... 

Medium effects (corrected for the bulk-susceptibility effect) have been reported for four commonly used solvent-internal reference systems in a paper by Stephen and his co-workers, who point out that the difference in chemical shifts between internal and external reference signals may be as great as 0.54 p.p.m. A novel type of noise-modulated, heteronuclear decoupling has been used by Evelyn and Hall to simplify the n.m.r. spectra of sugar derivatives containing the — CHgF group. By suitable adjustment of the modulation bandwidth and the radio-frequency intensity, the resonances of the coupled nuclei are broadened sufficiently... 

The way in which strong rf fields (which may or may not be modulated by noise or otherwise) can achieve spin decoupling is well... 

NMR spectroscopy as a routine tool. The double-irradiation field B2 was traditionally centered at about 8 5 of the H range. To cover all the H frequencies, B2 was modulated with white noise, so the technique often was called noise decoupling. 

Within the realm of heteronuclear decoupling, we have a choice of irradiating all of the lines in the proton spectrum (in the proton-carbon system we are still considering as an example) or just some of them. In order to irradiate all of the proton lines, we need a scheme for distributing the irradiation over the appropriate chemical shift range. Most commonly, noise decoupling is used wherein the decoupler output is modulated in a random way to make its spectrum... 

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