CL-37/CL-35-INDUCED C-13 ISOTOPE SHIFTS IN CHLORINATED METHANES

Proton-coupled C-13 NMR spectra of the chlorinated methanes CH3Cl, CH2Cl2, CHCl3, and CCl4 have been measured at 125 MHz resonance frequency in the temperature range from 303 K down to 193 K. Two factors were found to determine the C-13 lineshapes at different temperatures: Cl-37/Cl-35 isotope effects on the C-13 Shielding constants and line broadenings due to scalar relaxation arising from C-13-Cl-37 (or Cl-35) spin-spin interactions. With instrumental resolution not worse than 0.1 Hz at all temperatures used and with digital resolution better than 0.01 Hz, Cl-37/Cl-35-induced isotope effects on C-13 chemical shifts were found ranging from -3.0 to -6.0 ppb per one Cl-37/Cl-35 replacement decreasing in magnitude with the number of chlorines. Cl-37/Cl-35 isotope shifts were measured for CH3Cl, CH2Cl2, and CHCl3, at different temperatures, revealing in all cases a temperature dependence of about -0.015 ppb/degrees C. Additional broadenings of the C-13 NMR lines due to C-13-Cl-37/Cl-35 spin-spin interactions range from less than 0.1 Hz in CH3Cl at 193 K to about 2 Hz in CCl4 at ambient temperatures. These broadenings are proportional to J(CCl)(2)T(1Cl). They increase with temperature due to increases of T-1Cl and they are of approximately 11% less in isotopomers containing Cl-35. The Cl-37/Cl-35 isotope shifts are attributed to the very slight shortening of C-Cl bonds by about 4 X 10(-5) Angstrom at room temperature on going from C-Cl-35 to C-Cl-37. Additional broadenings in the C-13 NMR spectra of chlorinated methanes allow one to estimate J(CCl) coupling constants ranging from approximately 11 Hz in CH3Cl to approximately 38 Hz in CCl4. (C) 1995 Academic Press, Inc.