CL-37/CL-35 ISOTOPE EFFECTS IN C-13-NMR SPECTROSCOPY OF CHLOROHYDROCARBONS

Isotope effects on the chemical shift of C-13 directly bonded to the isotopes Cl-35 and C-37 are reported for a wide range of chlorohydrocarbons. By recording C-13 NMR spectra under conditions of composite pulse H-1 decoupling, C-13 resonances can be obtained that are sufficiently narrow to allow the very small splittings of the order of a few ppb due to the Cl-35/Cl-37 isotope effect to be resolved at ambient temperature. Observation of this phenomenon allows the routine identification of C-13 resonances due to carbon atoms directly bonded to chlorine, since, for natural abundance samples, the isotope effect splits the C-13 signal into a set of lines with a characteristic intensity distribution. Various structural features influencing the magnitude of the isotope effect have been elucidated, and the temperature dependence of the isotope effect is also discussed. From low-temperature C-13 NMR spectra for chlorocyclohexane, isotope shifts for the carbon atom directly bonded to chlorine were measured separately for the axial and equatorial conformations. The measured values indicate that the isotope shift is larger for the axial conformation.