Broad-band proton decoupling

The basic INEPT spectrum cannot be recorded with broad-band proton decoupling, since the components of multiplets have antiphase disposition. With an appropriate increase in delay time, the antiphase components of the multiplets appear in phase. In the refocussed INEPT experiment, a suitable refocusing delay is therefore introduced that allows the C spin multiplet components to get back into phase. The pulse sequences and the resulting spectra of podophyllotoxin (Problem 2.21) from the two experiments are given below ... 

Compound 1 contains one phosphorus atom, so that the broad-band proton decoupled spectrum is extremely simple it consists of only one line at 8.5 ppm. This spectrum is shown in Fig. 22, together with the proton-coupled spectrum. 

This is the simple heteronuclear -X COSY (or HETCOR) pulse sequence adapted to long-range correlations simply by adjustment of the Ai and A2 delays. Small coupling constants rule out the use of broad-band proton decoupling during evolution time t by BIRD, and thus the correlation cross-peaks display the structure of proton multiplets, which reduces further the sensitivity of the experiment. 

Under normal measurement conditions with broad-band proton decoupling and short measurement cycles (pulse repetition rates typically of the order of 1 sec), 13C signals cannot be integrated because of the widely dif-... 

In general, broad-band proton-decoupled 13C-NMR spectra of unenriched samples consist of single resonances for each type of carbon... 

Tautomerism involving hemiacetal bridges has been explored by C n.m.r. for aldosterone (16), 18-hydroxy-11-deoxycorticosterone (19), and 18-hydroxy-progesterone (18)." Aldosterone and its 21-acetate (17) each showed 32 lines in their broad-band proton-decoupled spectra, indicating the presence of both the... 

As 13C has a natural abundance of only 1.1%, the signals are around 1/6000 times as strong as those for II. However, spectra can be routinely recorded by FT-NMR using short pulses of radio frequency radiation. Broad-band proton decoupling is usually employed, which removes all 13C- H coupling, so that singlet peaks are observed for each different kinds of carbon. (As most 13C are surrounded by 12C, 13C-13C coupling is minimal.)... 

The Waterloo NMR spectrum was recorded using a 9% solution in DMSO-dg at 62.9 MHz and 50 C with broad-band proton decoupling. 

Figure 9.25 Polymorphic phases available to hydrated liquid-crystalline phospholipids and the corresponding P-NMR spectra. The bilayer spectrum was obtained from aqueous dispersions of egg-yolk phosphatidylcholine the hexagonal spectrum corresponds to soybean phosphatidyl-ethanolamine the isotropic motion spectrum was obtained from a mixture 15 mol% egg-yolk phosphatidylcholine. The spectra were obtained at 30 °C in the presence of broad-band proton decoupling (from Cullis and deKruijff, 1979).

Compound Qhas the molecular formula CyHs- The broad-band proton decoupled spectrum of Qhas signals at 8 50 (CH), 85 (CHs). and 144 (CH). On catalytic hydrogenation Qis converted to R (CyHig)- Propose structures for Qand R... 

Compound T (CsHgO) has a strong IR absorption band at 1745 cm . The broad-band proton decoupled sp>ectrum of T shows... 

All NMR experiments were performed on a Varian XL-200 spectrometer at 50.31 MHZ. Relevant instrument settings include 90 degree pulse angle, 1.0 second acquisition time, 0.5 second pulse delay, 238.5 ppm spectral width, and broad band proton decoupling. About 40,000 transients were collected for each spectrum. Temperature was maintained at 40 C. Spin-lattice relaxation time (Tl) and Nuclear Overhauser Enhancement (NOE) values for all C-13 NMR resonances were carefully measured to determine the optimum NMR experimental conditions. The spectral intensity data thus obtained were assured of having quantitative validity. 

Fig. 2.33. Carbon-13 spectra of suvanine (25) in DMSO-dg. a Broad-band proton decoupling b DEPT spectra (0 = 135) with broad-band proton decoupling c selective INEPT sequence with single frequency irradiation of 6H singlet at = 2.94 (-NMe2) (251). (L. V. Manes, P. Crews, MR. Kernan, D.J. Faulkner, F.R. Fronczek, R.D. Candour 1988 J. Org. Chem. S3, 570)...

The best method for observing the deuterium isotopic shift in the carbon spectrum is the differential shift technique using coaxial NMR tubes (142). This technique utilizes an inner and outer tube of equal volume. One tube contains the sample dissolved in the deuterated solvent, such as D2O or methanol-d4, whereas the other contains the corresponding protio-solvent. Those carbons experiencing isotopic shifts appear as double resonances in the broad-band proton-decoupled spectrum, one signal originating from the protio-sample and the isotopically shifted resonance from the deuterium-exchanged sample. 

Can the three isomeric pentanes be distinguished unambiguously from their broad-band proton-decoupled NMR spectra alone Can the five isomeric hexanes be distinguished in this way ... 

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