Deuterium multiple resonance

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. 

Figure 4.20 19F NMR spectrum of 4.44- (n-Bu)4N+F 0.5 1 after heating at 150 °C for 1 h in DMSO-c/6 followed by storage at 25 °C for 10 d. The resonances from left to right correspond to anion cryptates with respectively 0, 1, 2, 3, 4, 5 and 6 NH protons replaced by deuterium. The observed multiplicity follows the standard formula multiplicity = 2n/+ 1 where n is the number of H nuclei remaining and /is the nuclear spin quantum number of H, i.e. xh. (Reproduced with permission from [38] 2004 American Chemical Society).

Figures 1.9a and b demonstrate the effect of proton broadband decoupling in the 13C NMR spectrum of a mixture of ethanol and hexadeuterioethanol. The CH3 and CH2 signals of ethanol appear as intense singlets upon proton broadband decoupling while the CD3 and CD2 resonances of the deuteriated compound still display their septet and quintet fine structure deuterium nuclei are not affected by lH decoupling because their Larmor frequencies are far removed from those of protons further, the nuclear spin quantum number of deuterium is ID = 1 in keeping with the general multiplicity rule (2 nxh+ 1, Section 1.4), triplets, quintets and septets are observed for CD, CD2 and CD3 groups, respectively. The relative intensities in these multiplets do not follow Pascal 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 3.41. Residual protonated resonances of deuterated solvents (a) CHCI3 in CDCI3, (b) CHDCI2 in CD2CI2 and (c) CHD2COCD3 in (CD3)2C0. The multiplicity seen in (b) and (c) arises from two-bond (geminal) couplings to spin— 1 deuterium producing a 1 1 1 triplet and a 1 2 3 2 1 quintet respectively. The left-hand singlet in (b) is residual CH2CI2 in the solvent, the shift difference arises from the H-D isotope shift of 6 Hz.

Because deuterium is not a spin = nucleus, the n -l-1 Rule does not eorrectly predict the multiplicity of the carbon resonance. The n+l Rule works only for spin = nuclei and is a specialized case of a more general prediction formula ... 

The resonances in derivatives of aniline reveal that the carbon bearing a partially deuterated NH2 group appears as a multiple because of the deuterium isotope effect on the chemical shift, when the hydrogen exchange is low. This effect is larger for... 

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