cdecouples hydroxy 0t- tsspin-spin couping to as5eieaegCebiesRheiorYatmeachange 10-9 Carbon-13 Nuclear Magnetic Resonance (an(ove NMR as 。 的然3 10 10 In the case of 1-bromopropane, the hydrogens on C2 are also coupled to two non-equivalent sets of neighbors. A theoretical analysis of this resonance would predict as many as 12 lines (a quartet of triplets). Because the coupling constants are very similar, however, many of the lines overlap, thus simplifying the pattern. Fast proton exchange decouples hydroxy hydrogens. In the spectra of 2,2- dimethyl-a-propanol, the OH absorption appears as a single peak and is not split by the CH2 protons. In addition, the CH2 protons are not split by the OH. The OH proton is weakly acidic and is both between alcohol molecules and traces of water on the NMR time scale at room temperature. This type of decoupling is called “fast proton exchange.” It may be slowed or removed by removal of traces of water or acid or by cooling. Rapid magnetic exchange self-decouples chlorine, bromine, and iodine nuclei. Fluorine is the only halogen that exhibits spin-spin coupling to 1H in a proton NMR spectra. Chlorine, bromine, and iodine exhibit a fast internal magnetic equilibration on the NMR time scale which precludes an adjacent proton from recognizing them as having different alignments in the external field. This is termed “self-decoupling,” in contrast to exchange￾decoupling, as exhibited by the hydroxy protons. 10-9 Carbon-13 Nuclear Magnetic Resonance The NMR spectroscopy of 13C is of greater potential utility than that of 1H NMR. The 13C spectra of an organic compound is much simpler than the 1H spectra because spin-spin coupling between adjacent carbon atoms and between carbon and hydrogen atoms can be avoided. Carbon NMR utilizes an isotope in low natural abundance: 13C. Carbon occurs as a mixture of two principle isotopes, 12C (98.89%) and 13C (1.11%). Of these, only 13C is active in NMR. Because of the low abundance of 13C and its weaker magnetic resonance (1/6000 as strong as 1H), FT NMR is usually used for 13C spectroscopy because multiple pulsing and signal averaging allows the accumulation of strong signals than would otherwise be possible. Carbon-carbon coupling is absent in 13C spectra due to the very low probability of two 13C nuclei being adjacent to each other in a single molecule (.0111 x .0111 ~ .0001). 13C-1H coupling is present, however, the chemical shift range of 13C is much greater than the splittings due to 1H, which precludes the overlapping of adjacent multiplets. The 13C chemical shifts are reported relative to an internal standard, usually (CH3)4Si