Structure and methyl torsion of halogenated toluenes: Rotational spectrum of 3,4-difluorotoluene

Abstract

The rotational spectrum of 3,4-difluorotoluene, final in the series of difluorotoluenes to be studied in the microwave region, has been investigated using Fourier transform microwave spectroscopy on a pulsed supersonic jet in the region 5–25 GHz. The tunneling splitting due to the methyl internal rotation in the ground torsional state could be unambiguously identified and the three-fold (V3) potential barrier hindering the internal rotation of the methyl top was determined as 406.280(37) J mol−1. The ground-state rotational constants for the parent and seven 13C isotopic species in natural abundance were determined with high accuracy. The electric dipole moment μ = 3.0396 (51) D was obtained from Stark effect measurements. The structure of the C-atom frame was derived using the substitution (rs) method. Supporting ab initio (B3LYP/MP2) calculations provided comparative values for the potential barrier and molecular parameters.