Rotational spectroscopy of p-cyanotoluene: Sixfold internal rotation, hyperfine coupling and electric dipole moment

The rotational spectrum of p -cyanotoluene was measured in supersonic expansion conditions at 2–18 GHz, and at room temperature at 8–18 GHz, both with Fourier transform microwave techniques. The room temperature mm-wave spectrum at 98-135 GHz was also measured, allowing combined access to rotational transitions in internal rotation states | m | =0 to 3, with J =1–75 and K a =0–28. Over 2000 transitions were successfully fitted with a sixfold internal rotation program accounting for nuclear quadrupole hyperfine splitting, resulting in determination of sixfold internal rotation barrier V 6 =4.781694(6) cm −1 , which is slightly lower than 4.8378399(5) cm −1 in toluene. The resolved nuclear quadrupole splitting structure was fitted with χ a a =–4.2084(6) MHz, also slightly lower than –4.2374(4) MHz in benzonitrile. Stark measurements in supersonic expansion allowed the electric dipole moment of p -cyanotoluene to be determined at μ =5.0895(22) D, and the dipole moment of toluene was redetermined to be μ =0.37264(15) D. Comparisons with benzonitrile allow additivity of these values to be explored. Several tutorial examples based on experimentally observed hyperfine splitting patterns and Stark behaviour are also provided for correlation between the symmetric top and asymmetric top labeling in low-barrier internal rotation.