oeneahtogatsltesignalofaresonating 6ogahn e68oe 器8 oeteteeceemagoRa25n2nmeetgneea s-lsing of the 7 7 One neighbor splits the signal of a resonating nucleus into a doublet. Consider two protons, Ha and Hb. The population of each of these protons is very close to 50% α and 50% β in the external magnetic field, H0. This means that in 50% of the molecules, Ha protons will have Hb protons in the α state and in 50% of the molecules, Ha protons will have Hb protons in the β state. The total field seen by 50% of the Ha protons will therefore be slightly greater than H0 and slightly less than H0 for the other 50 % of the Ha protons. What would have been a singlet NMR peak is now split into a doublet of peaks, symmetrically displaced from the original peak. The chemical shift of the Ha nucleus is reported as the center of the doublet. The amount of mutual splitting is equal. The distance between the individual peaks making up the doublet is called the “coupling constant” (J). Here J is 7 Hz. Coupling constants are independent of the field strength of the NMR spectrometer being used. Spin-spin splitting is usually observed only between hydrogen atoms bound to the same carbon (geminal coupling) or to adjacent carbons (vicinal coupling). Hydrogen nuclei separated by more than two carbon atoms (1,3 coupling) is usually negligible. Finally, equivalent nuclei do not exhibit mutual spin-spin splitting. Ethane exhibits only a single line at δ = 0.85 ppm. Splitting is observed only between nuclei with different chemical shifts. Local-field contributions from more than one hydrogen are additive. Consider the triplet above. It corresponds to the methyl protons being split by the methylene protons. The methylene proton spins will statistically orient in the external magnetic field as αα, αβ, βα and ββ. Each methyl proton will see an increased field 25% of the time (αα), no change 50% of the time (αβ and βα), and a decreased field 25% of the time (ββ)