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A. R. HIGHT WALKER AND JON T. HOUGEN, Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-8441; JENS-UWE GRABOW, Universität Hannover Institut für Physikalische Chemie und Elektrochemie, Callinstr 3-3a, D-30167 Hannover, Germany.
The rotational spectrum of paratolualdehyde (CH3-C6H4-CHO) has been observed using a pulsed-molecular-beam Fourier-transform microwave spectrometer. The nearly free internal rotation of the methyl group splits each rotational transition into two components, A and E, separated by up to 2.5 GHz. The > 75 A-state a and b-type transitions fit an asymmetric-rotor Hamiltonian to better than 1 kHz to give A= 5128.6429(4) MHz B=985.22385(3) MHz C=827.28476(3). The inertial defect of -0.61 uÅ2 is significantly smaller than the expected rigid-molecule value of -3.4 uÅ2 and is consistent with nearly free internal rotation of the methyl top. The E state lines are strongly perturbed from a rigid-rotor pattern due to the low barrier to internal rotation of the methyl top. The potential barrier inhibiting the free rotation is approximated by V(
) = (1/2)V3(1-cos3
) + (1/2)V6(1-cos6
), where
is the internal rotation angle. For toluene (CH3-C6H5) the V3 term is zero by symmetry, while the V6 term is approximately 5 cm-1. In paratolualdehyde, the presence of the aldehyde group leads to a nonzero V3 term due to a combination of long-range forces and the effect of the aldehyde substituent on the electronic structure of the benzene. Preliminary combined fits of the A and E state lines suggest that the V3 and V6 terms are of comparable magnitude. Searches are underway to observe the |m|>1 internal rotor states to aid in the determination of the internal-rotation potential. Also, A-state transitions from the eight different 13C isotopomers have been assigned to provide additional structural information.