Decoupling of deuterium

Because axial and equatorial protons are different, when the flexing of the cyclohexane chair has been frozen out, the geminal protons (i.e., protons on the same carbon) will couple with each other. Furthermore, axial and equatorial protons can also couple with equatorial and axial protons (respectively) on adjacent carbons. Thus, in the experiment described above, all but one proton was replaced by deuterium and the decoupling of deuterium (/ = 1) from protium effected. 

Figures 1.9a and b demonstrate the effeet of proton broadband deeoupling in the C NMR speetrum of a mixture of ethanol and hexadeuterioethanol. The C//j and CH2 signals of ethanol appear as intense singlets upon proton broadband deeoupling while the CD3 and CD2 resonanees of the deuteriated eompound still display their septet and quintet fine strueture deuterium nuelei are not affected by H decoupling because their Larmor frequeneies are far removed from those of protons further, the nuelear spin quantum number of deuterium is 7/3=/ in keeping with the general multiplieity rule (2nx Ix+ 1, Seetion 1.4), triplets, quintets and septets are observed for CD, CD2 and CD3 groups, respeetively. The relative intensities in these multiplets do not follow Paseal s triangle (1 1 1 triplet for CD 1 3 4 3 1 quintet for CD2 1 3 6 7 6 3 1 septet for CD3).

Figure 1.9. NMR spectra of a mixture of ethanol and hexadeuterioethanol [27 75 v/v, 25 °C, 20 MHz], (a) H broadband decoupled (b) without decoupling. The deuterium isotope effect Sch - d on chemical shifts is 1.1 and 0.85 ppm for methyl and methylene carbon nuclei, respectively...

An important first step in interpreting the C-13 spectra is to distinguish a-carbons from 3-carbons, i.e. methine from methylene. Observation of multiplicity when the proton decoupler is off is one way, but this is not always easy if the lines are broadened by chemical shift multiplicity. Measurement of has been used for this purpose since the 3-carbon with two bonded protons relaxes about twice as fast as the a-carbon with only one. A very positive way is by deuterium labelling. In Fig. 3 is shown the main-chain 25 MHz carbon spectrum of two styrene-S02 copolymers containing 58 mol% styrene, or a ratio of styrene to SO2 of 1.38 (7 ). In the bottom one, 3,3-d2-styrene has been used, cind all the 3-carbon resonances are distinguishable from the a-carbon resonances since the presence of deuterium has eliminated their nuclear Overhauser effect because of this eind the deuterium J coupling ( 20 Hz), they are markedly smaller eind broader than the a-carbon resonances. 

The thermal stereomutations of deuterium-labeled phenylcyclopropanes (Scheme 3) were studied in a progressive manner. First, the racemic and both achiral isomers were synthesized to provide material for kinetic work and to verify analytical methods 62. The isomerizations among these three isomers at 309.3 °C were followed using either 2H decoupled H NMR spectroscopy or Raman spectroscopy the two kinetic parameters (k, + k22) = 0.36 x 10 5 s 1 and (k2 + k]2) - 1.07 x 10"5 s 1 at 309.3 °C were measured. Published spectra of both sorts for authentic samples of syn, anti and trans isomers, and of thermal reaction mixtures, provided... 

Double Resonance Spin Decoupling and Deuterium Exchange Studies of Cimetidine... 

Substitution of D for H on a carbon results in a dramatic diminution of the height of the 13C signal in a broadband-decoupled spectrum for the following reasons. Since deuterium has a spin number of 1 and a magnetic moment 30% that of H, it will split the 13C absorption into three lines (ratio 1 1 1) with a J value equal to 0.30 X 7CH. Furthermore, 7, for 13C—D is longer than that for 13C—3H because of decreased dipole-dipole relaxation. Finally, the NOE is lost, since there is no irradiation of deuterium. A separate peak may also be seen for any residual 13C— H since the isotope effect usually results in a slight shift to lower frequency of the l3C—D absorption (—0.2 ppm per D atom). The isotope... 

Deuterium labelling combined with deuterium decoupling is well known as a method in spectral analysis and in stereochemical and conformational studies. The deuterium-decoupled proton spectra of XCHD.CHDY compounds produced during mechanistic studies are useful for determining the stereochemical course of reactions such as the methoxymercuration of ethylene, (478) the conversion of tyramine into tyrosol, (479) the non-catalytic addition of deuterium molecules to cyclopentadiene, (480) and alkyl transfer and olefin elimination in 2-phenyl-l,2-dideuterioethyl transition metal compounds. (481) The proton spectra of [ H ]- and [ Hg]-t-butylcyclohexanes were... 

The HX experiment can be automated by following two distinct experimental workflows. In one paradigm, the experimental design decouples automated digestion and LC-MS analysis from the HX sample preparation step (i.e., decoupled HX-MS). To enable the LC-MS analysis to be decoupled from the HX expraiment, samples are flash frozen in liquid nitrogen afta- the quench and stored at -80°C until required. Samples can be stored for days/weeks at -80°C with minimal loss of deuterium label. When the sample is ready for LC-MS analysis, it is thawed immediately prior to analysis with LC-MS by the autosampler. 

Accurate n.m.r. parameters have been obtained for a series of specifically deuteriated methyl- and t-butyl-substituted cyclohexanes with the aid of deuterium decoupling and computer simulation and iteration of the spectra. In [3,3,5,5- H J-methylcyclohexane (40) no change in the value of — 3 6 Hz was noted with... 

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