LAUSR

Abstract The microwave spectrum of 2,5-difluorotoluene was investigated in a pulsed Molecular Beam Fourier Transform Microwave (MB-FTMW) spectrometer in the frequency region 4.5–24 GHz. Rotational lines show large internal rotation splittings indicating that the potential barrier hindering internal rotation of the methyl group is comparatively low. The torsional splittings could be unambiguously identified and the potential barrier hindering internal rotation of the methyl top was determined. Rotational constants for the parent species, together with quartic centrifugal distortion terms were determined for a total of eight isotopologues, comprising the parent and all monosubstituted 13C species. Supporting ab initio (MP2) and density functional (B3LYP) calculations are provided. The experimental potential barrier V3 = 2580(12) J/mol compares satisfactorily with the ab initio prediction of 2568 J mol−1.