Illustrated Glossary of Organic Chemistry

Non-first order coupling (second order coupling): In NMR spectroscopy, spin-spin coupling in which the energy differences between the nuclear spin states are not equal. The n+1 rule is not obeyed; complex splitting patterns (non-first-order splitting) often result.

First order coupling J_ab = J_ac		Non-first-order coupling J_ab ≠ J_ac

First-order splitting results The n+1 rule is obeyed A triplet is observed		Non-first-order splitting results The n+1 rule is not obeyed A doublet of doublets is observed