Predicting non-first-order behaviour in fully coupled ¹³C-¹H spectra: a general rule

Abstract

The proton-coupled carbon-13 n.m.r. spectra in ABCDX, ABCX and ABX spin systems are shown to be non-first order when the condition ¹J_CH/2 = Δδ is met, where Δδ is the proton chemical shift separation between two closely coupled protons associated with adjacent carbon atoms. Solvent and temperature effects are shown to be useful in removing the spectral complexity and thus assist in complete analysis. Examples in heteroaromatic and hydrocarbon compounds are presented. For a chosen solvent system the non-first-order behaviour may be predicted from the ¹H n.m.r. spectrum and used as an aid in ¹³C n.m.r. spectral assignment.