The dependence of the 13C and the 1H nuclear magnetic shielding on bond extension in methane

The mean bond displacements 〈Δr〉 in the methane isotopic homologous series 13/12CX4−nYn (X, Y=H, D, T) at 300 K, and the temperature dependence of 〈ΔrCH〉 in 13CH4 from 250 to 350 K were calculated. With the assumption that the linear terms are sufficient to account for the isotope shifts, we determine from the 2/1H-induced 13C isotope shift an empirical value of(∂σC/∂ΔrCH)e=−35±3 ppm/Å. This predicts a temperature dependence in the 13C resonance in CH4 gas in the zero-pressure limit of 2.7×10−2 ppm over 100 °, which explains why it could not be observed. We observed the 13/12C-induced 1H isotope shift in CH4, −0.0024 ppm. With the same mean bond displacements, this isotope shift gives an estimate of (∂σHi /∂ΔrCHi )e=−38±3 ppm/Å. From the reported 2/1H-induced two-bond 1H isotope shift in CH4, −0.016 ppm, using the mean bond displacements and the derivative obtained from the 13/12C-induced one-bond isotope shift, we get an estimate of (∂σHi/ ∂ΔrCHj) e=−1.3±0.2 ppm/Å.