有机化合物波谱解析课件 核磁共振部分习题及答案_2.pdfVIP

NMR problems 2nd part 1.Below are the 1H and 13C NMR spectra of 2-hexanone (CH COCH CH CH CH ). 3 2 2 2 3 Explain carefully how, using homonuclear and heteronuclear decoupling experiments, you could assign the each of the resonances in the 1H and 13C spectra to which nuclei give rise to them. 1H NMR spectra of 2-hexanone 13C NMR spectra of 2-hexanone Answer This is a commonly encountered problem. You obtain a 1H and a 13C NMR spectrum and need to assign each of the resonances in both 1H and 13C spectra. It is generally straight-forward to assign the 1H spectrum using a combination of 1 / 9 direct inspection (characteristic chemical shifts and multiplicities) in combination with homonuclear decoupling experiments. For 1H spectra, homonuclear decoupling gives you connectivity (which protons are on adjacent carbon atoms) because 1H is 100% abundant. Note that homonuclear decoupling cannot be used for 13C spectra because 13C is isotopically dilute there essentially no possibility that there will be 2 13C nuclei adjacent to each other in an organic molecule. For 2-hexanone, the 1H spectrum contains 5 resonances. You would expect to see one singlet (integral 3H) for the CH3 group attached to the ketone (ie at C1). The CH2 group adjacent to the ketone (ie at C3) would appear as a triplet (with coupling to the adjacent CH . The CH at C4 would appear as a multiplet (a triplet of triplets with 2 2 coupling to both the CH at C3 and the CH at C5). Likewise the CH at C5 would 2 2 2 appear a