In a high resolution 1H NMR spectrum, many peaks are split into a group of closely spaced lines. This is called spin-spin splitting (or spin-spin coupling).It happens because each hydrogen nucleus is a tiny magnet. Hydrogens on adjacent carbon atoms can each be aligned with or against the external field, which slightly increases or decreases the field felt by the hydrogen being observed. Each different arrangement of the neighbouring spins gives a slightly different resonance frequency, so the signal splits into several lines.
For example, one neighbouring H can be aligned with or against the field: two equally likely arrangements, so the peak splits into two lines of equal height, a doublet. Two neighbouring H have three possible arrangements (both with, one each way, both against), and the middle one can happen in two ways, giving three lines in the ratio 1:2:1, a triplet.Names of splitting patterns: singlet (1 line), doublet (2), triplet (3), quartet (4) and multiplet (many).Important points:
• splitting is caused by non-equivalent H on neighbouring carbons; H further away usually have no effect
• equivalent H do not split each other, so the three H in a CH3 group do not split one another
• the H of an OH group usually appears as a singlet, because it exchanges rapidly between molecules
• coupling is mutual: if group A splits group B, then B also splits AThe rule that links the number of lines to the number of neighbouring H is the n+1 rule.
For example, one neighbouring H can be aligned with or against the field: two equally likely arrangements, so the peak splits into two lines of equal height, a doublet. Two neighbouring H have three possible arrangements (both with, one each way, both against), and the middle one can happen in two ways, giving three lines in the ratio 1:2:1, a triplet.Names of splitting patterns: singlet (1 line), doublet (2), triplet (3), quartet (4) and multiplet (many).Important points:• splitting is caused by non-equivalent H on neighbouring carbons; H further away usually have no effect
• equivalent H do not split each other, so the three H in a CH3 group do not split one another
• the H of an OH group usually appears as a singlet, because it exchanges rapidly between molecules
• coupling is mutual: if group A splits group B, then B also splits AThe rule that links the number of lines to the number of neighbouring H is the n+1 rule.