Rotation-internal rotation spectrum of p-fluorotoluene

Abstract

The rotational spectrum of p-fluorotoluene in the vibrational ground state is measured over the frequency range 7–63 GHz. For the lowest torsional state ( m = 0), 228 transitions up to J = 30 are assigned and fitted to a rigid rotor Hamiltonian to obtain accurate values of the rotational and five quartic centrifugal distortion constants. For the internal rotation states m = 1, 2, and 4, altogether 199 transitions are assigned up to J = 15. They are fitted to the rotation-internal rotation Hamiltonian, H = AP a 2 + BP b 2 + CP c 2 + Fp 2 − 2A′P ap + ( V 6 2 )·(1 + cos 6α) , together with quartic centrifugal distortion constants of the frame and the top. The least-squares fit of all the m ≠ 0 transitions yields A′ and four additional centrifugal distortion constants associated with the top motion. The value of A′ is lower than that of A, indicating the effect of vibrational perturbations. However, a separate least-squares fit of the transitions belonging to different values of m leads to a better standard deviation of frequency and the constants are determined with improved accuracies. Low- J R-branch transitions for | m| = 3, | k| = 1 are measured and used to obtain the barrier potential V 6 = 13.638(18) cal/mole. The energy eigenvalues and eigen-functions of m ≠ 0 states are calculated and the information is used for labeling the states. The effects of possible accidental degeneracies are discussed.