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

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, /mu a/ = 2.9061(22) D, /mu b/ = 1.1883(10) D, /mu tOt/ = 3.1397(24) D for benzaldehyde and /mu a/ = 0.6937(12) D, /mu b/ = 1.0547(8) D, mu 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.

The Gas-Phase Electric Dipole Moments of the Symmetric Top Tertiary Butyl Molecules (t)BuX, X=F, Cl, Br, I, CN, and NC.

The electric dipole moments of symmetric top tertiary butyl derivatives were determined from Stark effect measurements made with cavity Fourier transform microwave spectroscopy under conditions of supersonic expansion. The resulting values are 1.9562(15), 2.1817(16), 2.2574(17) and 2.2122(17) D for tertiary butyl fluoride, chloride, bromide, and iodide, and 4.0129(30), and 4.0640(31) for tertiary butyl cyanide and isocyanide, respectively. The experimental values are compared with ab initio calculations and with experimental and calculated dipole moments for the corresponding methyl derivatives. Copyright 2001 Academic Press.