Temperature dependent carbon-13 and proton isotope shifts of some isotopomers of methane

Abstract

The carbon-13 and proton isotope shifts of 13CH 3D, 13CHD 3 and 13CD 4 with respect to 13CH 4 have been measured over the temperature range 200–370 K. The isotope shifts increase with temperature in all cases, with the increase being more marked for the more heavily deuterated isotopomers. The variations are attributable entirely to intramolecular effects and the data have been fitted by weighted least-squares regression analyses to defined carbon-13 and proton shielding surfaces to obtain derivatives of the shielding with respect to internal coordinates. At the first-order level it is found for the carbon-13 shielding that σ r , the derivative with respect to the stretching of a single bond, is − 71.0 (±0.6) ppm Å −1; for the proton shielding σ r , the derivative with respect to the length of the bond involving the proton, is − 23.5 (±1.5) ppm Å −1 whilst σ s , the derivative with respect to the length of one of the other bonds is − 2.1(±0.5) ppm Å −1. The derivatives are used to predict the values of the nuclear motion corrections for 13CH 4 and to predict isotope shifts for 13CH 2D 2 which was not studied experimentally in this work.