Microwave study of internal rotation in para-tolualdehyde: Local versus global symmetry effects at the methyl-rotor site

The rotational spectrum of para-tolualdehyde (CH3-C6H4-CHO) has been measured using three different microwave spectrometers, with the goal of quantifying the influence of the aldehyde group at the top of the benzene ring on the internal rotation barrier seen by the methyl group at the bottom of the ring. This barrier consists of a six-fold component, which results from the local C2vsymmetry of the benzene ring seen by the methyl top (as in toluene), and an additional three-fold component, which results from information on the non-C2vsymmetry at the aldehyde site at the top of the ring being transmitted to the methyl-group site at the bottom of the ring. The nearly-free internal rotation of the methyl group splits each of the rotational transitions into two components, one of A and one of E symmetry. Assignment and fit of 786 A-state and E-state transitions to an internal rotation Hamiltonian containing barrier terms of three-fold (V3= 28.111(1) cm−1) and six-fold (V6= −4.768(7) cm−1) symmetry with respect to the internal rotation angle, as well as the three rotational constants and a number of higher-order torsion-rotation interaction terms, resulted in residuals equal to experimental measurement uncertainty. Isotopic data from all eight mono-substituted13C species and the one18O species were obtained in natural abundance and used to determine an rssubstitution structure. Various chemical and physical implications of this structure and the two barrier parameters are discussed.