The n+1 Rule

The number of sub-peaks in a split ¹H NMR peak is given by the n+1 rule:number of sub-peaks = n + 1where n is the number of protons on the adjacent carbon atom(s). Do not count the protons in the environment itself, or O-H protons, which usually do not cause splitting.• n = 0: singlet
• n = 1: doublet
• n = 2: triplet
• n = 3: quartet
• n ≥ 4: multipletThe rule also works backwards: a quartet tells you there are 3 protons on the adjacent carbon (a CH₃ group), and a triplet tells you there are 2 (a CH₂ group).Common patterns to recognise:
• an ethyl group, CH₃CH₂-, gives a triplet (3H) and a quartet (2H)
• an isopropyl group, (CH₃)₂CH-, gives a doublet (6H) and a peak with 7 sub-peaks (1H), usually just called a multiplet. For example, in 2-bromopropane the CH₃ protons have 1 neighbour (doublet) and the CH proton has 6 neighbours (7 sub-peaks).Worked example: ethyl ethanoate, CH₃COOCH₂CH₃.
• CH₃ bonded to C=O: the adjacent carbon has no H, n = 0, singlet (about 2.0 ppm)
• O-CH₂: adjacent CH₃, n = 3, quartet (about 4.1 ppm)
• CH₃ of the ethyl group: adjacent CH₂, n = 2, triplet (about 1.3 ppm)