Decoupler modulation frequency

In Section 2-4, we encountered a number of spectral acquisition parameters. The selection of these parameters is relatively direct and straightforward, with the exception of the flip angle a and the decoupler modulation frequency (for WALTZ decoupling). Neither a nor the decoupler modulation frequency is entered directly, and both parameters require that the spectrometer be calibrated. 

The decoupler modulation frequency employed in WALTZ decoupling is 4(752)-... 

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. 

The signals of a large number of coumarin derivatives [418] were assigned using the single-frequency off-resonance decoupling,. /-modulated spin echoe and INEPT techniques. 

The WURST decoupling sequence was implemented with the Pbox in Varian s VNMR software, using a constant adiabaticity modulated frequency, a 0.92 ms pulsewidth, and a 1.2 adiabaticity frictor (Q). The constant radiofiequency level yBaCrms) was 4.3 kHz. In order to compensate for residual imperfections in spin inversion, the five-step phase cycle (0 , 60°, 150°, 60°, 0°) of Tycko et al (3) was used. 

Step-scanning interferometers can enhance the photoacoustic technique, because the variable modulation frequency of the scanning interferometer is decoupled. Thus, a constant modulation frequency is applied to all wavelengths in the spectral range. One can modulate the IR beam via phase modulation in which an interferometer mirror dithers to oscillate the retardation about the set point of each interferometer step. With this approach, all of the frequencies are modulated synchronously, and lock-in modulation can be used to extract the phase-modulation [53]. When phase-... 

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... 

An alternative way of acquiring the data is to observe the signal. These experiments are referred to as reverse- or inverse-detected experiments, in particular the inverse HETCOR experiment is referred to as a heteronuclear multiple quantum coherence (HMQC) spectmm. The ampHtude of the H nuclei is modulated by the coupled frequencies of the C nuclei in the evolution time. The principal difficulty with this experiment is that the C nuclei must be decoupled from the H spectmm. Techniques used to do this are called GARP and WALTZ sequences. The information is the same as that of the standard HETCOR except that the F and F axes have been switched. The obvious advantage to this experiment is the significant increase in sensitivity that occurs by observing H rather than C. 

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)... 

Lyerla et al. measured Tic over a wide temperature range from room temperature down to 105 K [94], and concluded that Tic s of not only CH3 but also CH resonances depend on CH3 rotational motion, and that the broadening of the CH3 resonance below -100 °C is also due to modulation of CH3 rotational motion at the frequency of proton nutation in the presence of the decoupling field. Gomez et al. have also reported solid-state high-resolution 13C NMR spectra of isotactic polypropylenes [95]. They used samples characterized by X-ray crystallography and reconfirmed the results obtained by Bunn et al. 

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