An unlikely coincidence in coupled ¹³C nuclear magnetic resonance spectra. ²J(C,H) in 2,6-dichlorotoluene and related molecules

Although as many as 13 peaks are observable in each half of the 13C nuclear magnetic resonance spectrum of C-4 in 2,6-dichlorotoluene at 22.6 MHz, the iterative analysis yields an incorrect value of 2J(C,H). Theoretical spectra for two values of this coupling constant are essentially identical under normal linewidth conditions. At 75.5 MHz, the 13C spectrum displays most of the 48 peaks expected for the X part of an AB2XR3 spectrum, yielding the correct choice for 2J(C,H). The theoretical spectrum at 125 MHz is similar to that at 22.6 MHz but only for the correct 2J(C,H). Correct values of 2J(C-4,H) are given for the 2,6-dichloro derivatives of toluene, ethylbenzene, benzyl alcohol, benzyl bromide, and chloride; these were given incorrectly previously.