Cl-37/Cl-35 and C-13/C-12 isotope effects on the fluorine shielding in several two-carbon chlorofluorocarbons

Isotope effects on the chemical shift (IECS) of F-19 bonded through two or three bonds to the isotopes Cl-35 and Cl-37 are reported for a wide range o f two-carbon chlorofluorocarbons (and, for a few cases, with the isotopes B r-79 and Br-81, for two-carbon bromofluorocarbons). These IECS through two bonds were found to range, for one chlorine atom, from -5.0 to -7.6 ppb for aliphatic compounds and from -4.5 to -6.4 ppb for olefinic compounds. The IECS through three bonds were found to be less than -1.15 ppb. No deviation from additivity was detected for the IECS when several equivalent chlorine atoms are present. The broadening of the F-19 lines, due to scalar relaxat ion arising from F-19-Cl-37/35 spin-spin interactions, ranges from 0.32 to 2.50 Hz in our series and are related to the (2)J(Cl,F-19)T-1(Cl) term. IEC S of F-19 bonded through one or two bonds to the isotopes C-13 and C-12 are also reported for the same series. Several correlations, related to struct ural information, can be made and discussed between these IECS [C-13/12 IEC S through two bonds as a function of (1)J(C-13, F-19) spin-spin coupling co nstants and C-13/12 IECS as a function of Cl-37/35 IECS]. Cl-37/35 IECS can also be observed on C-13 satellites of F-19 spectra. For non-symmetrical c ompounds, the dependence of these signals on the Cl-37/35 IECS is the same as that for the main F-19 signal; however, for the centrosymmetrical compou nds, the main F-19 signal appears to be sensitive to all the chlorine atoms , whereas the C-13 satellites seem to be sensitive only to the directly bon ded chlorine. Copyright (C) 2001 John Wiley & Sons, Ltd.