Microwave spectra of o-fluorotoluene and its 13C isotopic species:: Methyl internal rotation and molecular structure

The rotational spectra of o-fluorotoluene and its seven C-13 isotopic species were recorded in the frequency range from 4 to 20 GHz with employment of pulsed molecular beam Fourier-transform microwave (MB-FTMW) spectrometers. The analysis of the spectra in the two lowest states of methyl internal rotation (torsional ground state, A and E species) was based on a asymmetric frame-rigid symmetric top Hamiltonian with inclusion of centrifugal distortion terms, yielding structural rotational constants, as well as the threefold barrier V-3 to internal rotation and the angle not less than (a,i) between the principal moment of inertia a axis and the internal rotor axis i. The rotational constants of all eight isotopomeres were used to derive the seven C-13 r(s) coordinates of the molecule.