37Cl/35Cl-Induced 13C Isotope Shifts in Chlorinated Methanes

Proton-coupled 13C 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 13C lineshapes at different temperatures: 37Cl/35Cl isotope effects on the 13C shielding constants and line broadenings due to scalar relaxation arising from 13C-37Cl (or 35Cl) 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, 37Cl/35Cl-induced isotope effects on 13C chemical shifts were found ranging from -3.0 to -6.0 ppb per one 37Cl/35Cl replacement decreasing in magnitude with the number of chlorines. 37Cl/35Cl isotope shifts were measured for CH3Cl, CH2Cl2, and CHCl3, at different temperatures, revealing in all cases a temperature dependence of about -0.015 ppb/°C. Additional broadenings of the 13C NMR lines due to 13C-37Cl/35Cl 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 J2CClT1Cl. They increase with temperature due to increases of T1Cl and they are of approximately 11% less in isotopomers containing 35Cl. The 37Cl/35Cl isotope shifts are attributed to the very slight shortening of CCl bonds by about 4 × 10-5 Å at room temperature on going from C-35Cl to C-37Cl. Additional broadenings in the 13C NMR spectra of chlorinated methanes allow one to estimate JCCl coupling constants ranging from approximately 11 Hz in CH3Cl to approximately 38 Hz in CCl4. © 1995 Academic Press, Inc.