Rotational Diffusion Tensor of Liquid Methylene Chloride from Its Infrared Spectrum

Abstract

The rotational motion of the three inertial axes of liquid methylene chloride (CH₂Cl₂, point group $C_{\text{2υ}})$ has been studied by computing the autocorrelation functions through the Fourier inversion of the observed band contours. Three fundamentals [symmetry species $a_1\text{(283}{\mathrm{cm}}^{\text{−1}}{\mathrm{),\; b}}_1\text{(895}{\mathrm{cm}}^{\text{−1}})$ , and $b_2\text{(1265.5}{\mathrm{cm}}^{\text{−1}}\mathrm{)]}$ , as well as three summation bands [symmetry species $b_1\text{(2308}{\mathrm{cm}}^{\text{−1}}\text{, 2414}{\mathrm{cm}}^{\text{−1}})$ and $b_2\text{(2688}{\mathrm{cm}}^{\text{−1}}\mathrm{)]}$ , one overtone [symmetry species $a_1\text{(2525}{\mathrm{cm}}^{\text{−1}}\mathrm{)]}$ , and one difference band [symmetry species $b_2\text{(452}{\mathrm{cm}}^{\text{−1}})$ ], 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.