Rotational Diffusion Tensor of Liquid Methylene Chloride from Its Infrared Spectrum

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Abstract
The rotational motion of the three inertial axes of liquid methylene chloride (CH2Cl2, point group C2υ) has been studied by computing the autocorrelation functions through the Fourier inversion of the observed band contours. Three fundamentals [symmetry species a1 (283 cm−1), b1 (895 cm−1), and b2 (1265.5 cm−1)], as well as three summation bands [symmetry species b1 (2308 cm−1, 2414 cm−1) and b2 (2688 cm−1)], one overtone [symmetry species a1 (2525 cm−1)], and one difference band [symmetry species b2 (452 cm−1)], were investigated. The experimental results indicate that the liquid-phase molecules do not obey Debye-type diffusion but, on the contrary, are able to make orientational jumps of 27° – 38° around their inertial axes. The decay of the coherence of the rotational motion takes between 0.6 × 10−12 and 1.1 × 10−12 sec. Further-more, the anisotropy of the rotational motion in the liquid is observed to be about the same as that of freely rotating CH2Cl2 molecules. The results imply that rotational motion is not sensitive to molecular association, weak hydrogen-bond formation, and similar phenomena. A method of adjusting the integration limits of the Fourier transformation to the theoretical value of the second moment of the bands has been applied. Intermolecular torques were estimated.