The rotational spectra, electric dipole moments and molecular structures of anisole and benzaldehyde

Abstract

The rotational spectra of anisole and of benzaldehyde were investigated in supersonic expansion at frequencies up to 41 GHz, and at room temperature in the millimetre-wave region, from 170 to 330 GHz. Accurate spectroscopic constants for the parent isotopomers in the ground vibrational state and for the first excited torsional state were determined for both molecules. The supersonic expansion spectrum allowed measurement, in natural abundance, of all singly substituted ¹³C isotopomers, as well as of the ¹⁸O isotopomer for both anisole and benzaldehyde. The rotational constants were used to determine the r_s and the r_m⁽¹⁾ gas-phase geometries, which are found to be consistent with prediction of bond length alternation in the phenyl ring induced by the asymmetric substituent. Stark measurements were made on the supersonic expansion spectrum resulting in electric dipole moment determination, |μ_a| = 2.9061(22) D, |μ_b| = 1.1883(10) D, μ_tot = 3.1397(24) D for benzaldehyde and |μ_a| = 0.6937(12) D, |μ_b| = 1.0547(8) D, μ_tot = 1.2623(14) D for anisole. During the investigation it was found that use of a carrier gas mixture consisting of 30% Ar in He carries significant advantages for studies of weak lines, and pertinent experimental details are reported.