AbstractAccording to the theory of long‐range proton spin coupling by pi electrons, |JHH| in acetone should decrease with increasing polarity of the carbonyl group. Experimentally the opposite has been found. To explain the observed trend a “through‐space” mechanism is proposed.JHHis now interpreted as the result of a negative coupling by the pi electrons and a positive “through‐space” contribution. On this hypothesis a better fit numerically can als