8.3: Proton (1H) NMR: Chemical Shift
Organic molecules primarily contain carbon and hydrogen atoms. While all the hydrogen isotopes are NMR-active, protium or hydrogen-1 is the most abundant. It has a significant energy separation between its nuclear spin states due to its large gyromagnetic ratio. As per Boltzmann's distribution, an increase in the energy separation implies a greater excess population of nuclei available for excitation, resulting in a strong NMR absorption signal.
Absorption signals of all the protium nuclei in a sample are recorded as their chemical shifts in a hydrogen-1 or proton NMR spectrum. For example, the proton NMR spectrum of methyl acetate shows two signals apart from the TMS proton signal at δ 0, corresponding to the chemical shifts of the two types of protons in the compound. The peak at 2.1 ppm corresponds to the protons of the methyl groups adjacent to the carbonyl. In contrast, the downfield signal at 3.7 ppm corresponds to the relatively deshielded protons of the methoxy group. Most proton chemical shifts are obtained in the narrow range of 0–12 ppm downfield from the TMS signal.
