37Cl/35Cl isotope effects on the carbon and proton shielding in CH3Cl: measurement and theory

✍ Scribed by William T. Raynes; Martin Grayson; Nickolai M. Sergeyev; Natalia D. Sergeyeva

• Publisher: Elsevier Science
• Year: 1994
• Tongue: English
• Weight: 637 KB
• Volume: 226
• Category: Article
• ISSN: 0009-2614
• DOI: 10.1016/0009-2614(94)00781-0

✦ Synopsis

A model is proposed to explain the small isotope shifts in the 'H resonance ( = 0.2 ppb) and in the "C resonance ( GZ 6 ppb) of CH3Cl when 35C1 is replaced by "Cl. 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 and cubic force field are Ar( C-Cl) = -0.000041 A, Ar( C-H) =O.OOOOOl A and AB(HCC1) =0.00063" at 300 K when "Cl replaces 35Cl. The derivatives are obtained from a large basis set calculation at the SCF level. Excellent agreement between calculated and experimental results is obtained. In the case of the r3C 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.