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NEARLY FREE INTERNAL ROTATION IN PARATOLUALDEHYDE

Please use this identifier to cite or link to this item: http://hdl.handle.net/1811/19268

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Title: NEARLY FREE INTERNAL ROTATION IN PARATOLUALDEHYDE
Creators: Walker, A. R. Hight; Hougen, Jon T.; Grabow, Jens-Uwe
Issue Date: 1999
Abstract: The rotational spectrum of paratolualdehyde $(CH_{3}-C_{6}H_{4}-CHO)$ 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 asymmetricrotor 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 $-0.61 u{\AA}^{2}$ is significantly smaller than the expected rigid-molecule value of $-3.4 u{\AA}^{2}$ 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 $V(\alpha) = (1/2)V_{3} (1 -cos3\alpha) + (1/2) V_{6}(1-cos6\alpha)$, where $\alpha$ is the internal rotation angle. For toluenc $(CH_{3}-C_{6}H_{5})$ the $V_{3}$ term is zero by symmetry, while the $V_{6}$ term is approximately $5 cm^{-1}$. In paratolualdchyde, the presence of the aldehyde group leads to a nonzero $V_{3}$ term due to a combination of long-range forces and the effect of the aldehyde substituent on the electronic structure of the benzenc. Preliminary combined fits of the A and E state lines suggest that the $V_{3}$ and $V_{6}$ 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 $^{13}C$ isotopomers have been assigned to provide additional structural information.
URI: http://hdl.handle.net/1811/19268
Other Identifiers: 1999-MH-04
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