The infrared spectrum of the C-type 819 cm-1 band of p-fluorotoluene is measured by Fourier transform infrared spectrometry with a resolution of 0.002 cm-1. For the methyl rotor m = 0 state, 786 vibration-rotation transitions are assigned up to J = 50 and Ka = 35 and fitted to a rigid rotor Hamiltonian to obtain the center frequency and the upper state rotational and quartic centrifugal distortion constants. The accurately known ground state constants obtained from a recent microwave study are used to derive the combination-difference frequencies. For the m = 1 and 2 states, the vibration-rotation transition frequencies are calculated by using the Hamiltonian, H = APa2 + BPb2 + CPc2 + Fm2 - 2A′Pap. The effect of the small barrier V6 = 4.77 cm-1 is found to be negligible in these cases. A plot of the frequency difference between the transitions PR-k(J) and RRk(J) against k in the symmetric top limit is used to obtain the change in A′ in the upper state compared to that of the ground state. With the help of the ground state value of A′ known from the microwave work the value of A′ in the upper state is obtained. This leads to assignment of a number of R- and P-branch transitions for m = 1 and 2 up to J = 14 and high k. A number of pairs of transitions of the type [PR-J(J, m = 2) and PR-J-1(J + 1, m = 1)] are identified as characteristic features of the m = 1 and 2 transitions. The effect of a symmetry-breaking three-fold potential term originating from asymmetric deformation of the ring around the symmetry axis, when the out-of-plane A″1 CH deformation mode is excited, is discussed. © 1990.
