The low internal rotation barriers of halogenated toluenes: Rotational spectrum of 2,4-difluorotoluene

Abstract

The rotational spectrum of 2,4-difluorotoluene in the region 5–25 GHz has been studied by pulsed supersonic jet using Fourier transform microwave spectroscopy. The tunneling splitting due to the methyl internal rotation in the ground torsional state could be unambiguously identified and the threefold (V3) potential barrier hindering the internal rotation of the methyl top was determined as 2.80144 (82) kJ/mol. The ground-state rotational parameters for the parent and seven 13C isotopic species in natural abundance were determined with high accuracy, including all quartic centrifugal distortion constants. The electric dipole moment μ = 1.805(42) D was obtained from Stark effect measurements. The molecular structure was derived using the substitution (rs) method. Supporting ab initio (MP2) calculations provided comparative values for the potential barrier and molecular parameters.