High resolution measurements of 14N, D quadrupole coupling in CH3CN and CD3CN

A molecular beam maser spectrometer was used to measure hyperfine structure on the $\text{J=1→ 0}$ rotational transitions in CH₃CN and CD₃CN. Measured nitrogen quadrupole coupling strengths are 1${\mathrm{eq}}_{\mathrm{aa}}\mathrm{Q(}\mathrm{N}\text{)=−4224.4 ± 1.5\hspace{0.17em}}\mathrm{kHz}$ for CH₃CN and ${\mathrm{eq}}_{\mathrm{aa}}\mathrm{Q(}\mathrm{N}\text{)=−4229.3 ± 1.5\hspace{0.17em}}\mathrm{kHz}$ for CD₃CN. The deuterium quadrupole coupling strength along the C–D bond direction is ${\mathrm{eq}}_{\mathrm{zz}}\mathrm{Q(}\mathrm{D}\text{)=167.5 ± 4.0\hspace{0.17em}}\mathrm{kHz}$ Nitrogen quadrupole coupling is discussed in relation to the electronic structure. Spin‐rotation interaction strengths are reported. A convenient method for calculating matrix elements for hyperfine interactions in an arbitrary coupling scheme is presented.