Proton decoupling broadband

Fig. 7. Nmr spectra of quinine [103-95-0] C2QH24N2O2, acquired on a Bruker 300AMX spectrometer using a Bmker broadband CP/MAS probe, (a) Proton-decoupled spectmm of quinine in CDCl (b) the corresponding spectmm of solid quinine under CP/MAS conditions using high power dipolar decoupling (c) soHd-state spectmm using only MAS and dipolar decoupling, but without cross-polarization and (d) soHd quinine mn using the...

Broadband proton-decoupled pulse Fourier transform C n.m.r. were recorded in deuterochloroform at 20 MHz using a Varian CFT-20 spectrometer. 

Broadband XH decoupling, in which the entire proton spectral window is irradiated, collapses all of the 13C multiplets to singlets, vastly simplifying the 13C spectrum. An added benefit of broadband proton decoupling is NOE enhancement of protonated 13C signals by as much as a factor of three. 

Of the multitude of ID 13C NMR experiments that can be performed, the two most common experiments are a simple broadband proton-decoupled 13C reference spectrum, and a distortionless enhancement polarization transfer (DEPT) sequence of experiments [29]. The latter, through addition and subtraction of data subsets, allows the presentation of the data as a series of edited experiments containing only methine, methylene and methyl resonances as separate subspectra. Quaternary carbons are excluded in the DEPT experiment and can only be observed in the 13C reference spectrum or by using another editing sequence such as APT [30]. The individual DEPT subspectra for CH, CH2 and CH3 resonances of santonin (4) are presented in Fig. 10.9. 

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. 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.

In the commonly used CPD or broadband proton-decoupled 13C spectrum (see Section 4.2.1), the peaks... 

We described the nuclear Overhauser effect (NOE) among protons in Section 3.16 we now discuss the het-eronuclear NOE, which results from broadband proton decoupling in 13C NMR spectra (see Figure 4.1b). The net effect of NOE on 13C spectra is the enhancement of peaks whose carbon atoms have attached protons. This enhancement is due to the reversal of spin populations from the predicted Boltzmann distribution. The total amount of enhancement depends on the theoretical maximum and the mode of relaxation. The maximum possible enhancement is equal to one-half the ratio of the nuclei s magnetogyric ratios (y s) while the... 

S, H) spin-spin coupling in the S spectra of dimethyl sulphone, sulpholane 10, and butadiene sulphone 11 was reported for the first time by Hinton.75 Afterward, the values of /( S, H) in dimethyl sulphone and sulpholane 10 have been estimated by comparing the line width value of the 33S resonance lines in 1H-coupled spectra and in spectra obtained under broadband proton decoupling. 

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). 

Nuclear magnetic resonance spectroscopy has emerged as the most powerful tool for elucidating the molecular structures of cyclophos-phazene derivatives in solution. Proton NMR spectroscopy has been widely used because of its easy accessibility. The recent development of sophisticated instrumental facilities and the application of broadband proton decoupling have greatly improved the quality and usefulness of the 31P spectra (252) of cyclophosphazenes, and it is likely that this technique will become increasingly popular in the future. Fluorine NMR studies are useful for deducing the structures of fluorocyclophosphazenes, and the potential of this technique has been demonstrated in recent years (209, 210, 213, 307, 308, 343). 

Spectra are usually recorded with complete decoupling of protons by broadband noise decoupling. The general scheme of the sequence for a routine spectrum is illustrated in Fig. 5.4.5a and a set of recording values are listed in Table 5.4.2. 

With the advent of FT NMR and subsequently multinuclear spectrometers broadband proton decoupling has become widespread and is usually accompanied by a NOE which is often substantial [equation (16)] and can provide a useful gain in sensitivity. However, the dependence upon the competition between dipole-dipole and other relaxation mechanisms means that the NOE can vary from one site to another which militates against reliable intensity measurements. In these circumstances it is desirable to be able to quench the NOE, and this may also be necessary for nuclei with a negative magnetogyric ratio for which a really unfortunate combination of relaxation times can lead to zero signal intensity. 

NMR Spectrometry. Liquid phase and NMR spectra were recorded on a Varian XL300 NMR spectrometer at carbon and nitrogen resonant fi equencies of 75.4 and 30.4 MHz, respectively, using a 10 mm broadband probe. Quantitative NMR spectra of the unreacted fulvic and humic acid samples were recorded in DMSO-d6, 99.9 atom % as previously described (23). INEPT (24) and ACOUSTIC (25) N NMR spectra were recorded on the aniline-reacted fulvic and humic acids. Refocussed INEPT (proton decoupled) spectra were recorded as previously described (9). ACOUSTIC spectra, with the exception of the bimessite catalyzed sample, were recorded with the use of paramagnetic relaxation reagent (100-200 mg chromium (III) acetylacetonate). Acquisition parameters included an 18,656.7 Hz spectral window (613.7 ppm), 0.5-s acquisition time, 45° pulse angle, 2.0-s pulse delay, and t delay of 0.1 ms. Neat formamide in a 5 mm NMR tube, assumed to be 112.4 ppm, was used as an external reference standard for all spectra. N NMR chemical shifts are reported in ppm downfield of ammonia, taken as 0.0 ppm. 

---