Coupling interactions are strongest between NMR-active nuclei bonded to each other, where spin information can be transmitted directly through the pair of bonding electrons. While nuclei polarize their electrons to the opposite spins, the bonding electron pair has opposite spins. Configurations with antiparallel nuclear spins are expected to be lower in energy. When coupling makes antiparallel states more favorable, J is considered to have a positive value. The one-bond coupling constant, 1J, is usually positive with a large magnitude. While the coupling magnitude is evident from the fine structure in NMR spectra, the sign of the coupling constant cannot be.
Spin information is transmitted more effectively with increasing s character of the involved orbitals. This explains the increasing 1JC–H values for ethane, ethene, and ethyne (125, 156, and 249 Hz). Similarly, the increase in the s character of exocyclic bonds in strained cyclic systems is responsible for the large C–H coupling constants. As a result, cyclohexane which is not a strained system, has a 1JC–H of 125 Hz, similar to ethane, while cyclopentane, cyclobutane, and cyclopropane have 1JC–H values of 129, 134, and 160 Hz, respectively.
1JC–H values also increase when the coupled carbon has an electronegative substituent as seen in the chlorinated methanes (125, 150, 178, and 209 Hz, respectively, for CH4, CH3Cl, CH2Cl2, and CHCl3). Despite the strength of one-bond coupling, carbon–hydrogen coupling is rarely observed in proton NMR spectra because of the low abundance of carbon-13.
