CL-37 CL-35 ISOTOPE EFFECTS ON THE CARBON AND PROTON SHIELDING IN CH3CL - MEASUREMENT AND THEORY

A model is proposed to explain the small isotope shifts in the H-1 resonance (almost-equal-to 0.2 ppb) and in the C-13 resonance (almost-equal-to 6 ppb) of CH3Cl when Cl-35 is replaced by Cl-37. These shifts are related to the slight changes in the C-Cl and C-H bond lengths and the HCCl interbond angles brought about by the isotopic substitution and to the derivatives of the shielding with respect to these parameters. The changes in geometry calculated from a quadratic cubic force field are DELTAr(C-CL) = - 0.00041 angstrom, DELTAr(C-H) = 0.000001 angstrom and DELTAtheta(HCCl) = 0.0063-degrees at 300 K when Cl-37 replaces Cl-35. The derivatives are obtained from a large basis set calculation at the SCR level. Excellent agreement between calculated and experimental results is obtained. In the case of the C-13 isotope effect, changes in the C-Cl bond length and the mean square of the C-Cl bond lengths are sufficient to account for the shift. For the proton shift the contributions from the slight changes in the C-H bond length and HCCl angles, though they tend to cancel, are individually larger than experimental error and cannot be neglected even though the changes in the C-Cl bond length are again dominant.