## VIBRATIONAL INTENSITIES IN $CH_{3-}$ AND $CD_{3-}$ CHLORIDE, BROMIDE, AND IODIDE

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Ohio State University##### Abstract:

Absolute intensities have been measured for all the fundamental vibrations of the six molecules $CH_{3}Cl, CD_{3}Cl, CH_{3}Br, CD_{3}BR, CH_{3}I$, and $CD_{3}I$. The intensities were measured by integrating the optical density over each band, high pressures of non-absorbing gas being used to broaden the fine-structure and so overcome the spectrometer slit-effect. Nitrogen, helium and argon were used at 1200 psi to pressure broaden the samples, and some special high-pressure absorption cells designed for this work will be described. The band areas $\Gamma_{i}$ were determined by integrating against the logarithm of the frequency, according to the relation \Gamma_{i}=\frac{1}{{n}l}\int_{band}\qquad l{n}({I}_{o}/{I})\cdot{d}l{n}\nu Here n is the concentration of sample gas and $l$ is the path length. $\Gamma_{i}$ is then related to ($\partial{p}/\partial{Q}_{i}$) for the $i^{th}$ fundamental vibration as follows: \Gamma_{i}=(\frac{{d}_{i}}{\omega_{i}}\cdot\frac{{N}\pi}{3{c}^{2}}\cdot\frac{\partial{p}}{\partial {Q}_{i}})^{2} where $d_{i}$ is the degeneracy and $\omega_{i}$ the harmonic vibration frequency. These relations have certain advantages over the definitions used previously by Wilson and $Wells.^{1}$ The individual areas of overlapping bands were determined by assuming symmetrical shapes for the perpendicular bands, and a complete error treatment was carried out in order to ascertain the effects of possible errors in the separations. Normal coordinates were calculated from the potential function recently derived by Chang2, which was adjusted to fit both the vibration frequencies, corrected for anharmonicity, and in certain cases the Coriolis $\zeta$ values. These normal coordinates were used to derive values of ($\partial{p}/\partial{S}_{i}$) where $S_{j}$ is a symmetry coordinate in the molecule; in the degenerate symmetry class, which contains a pair of infrared-active rotations, the known dipole moments of the molecules were used to correct the observed ($\partial{p}/\partial{S}_{j}$) values to a standard state in which there is no rotation of the carbon-halogen bond during the deformation. Ambiguities arising from the unknown relative signs of the ($\partial{p}/\partial{Q}_{i}$) were eliminated by comparing data on the isotopic species. Finally bond-effective moments were calculated for each symmetry coordinate.

##### Description:

1E. B. Wilson and A. J. Wells, J. Chem. Phys. 14, 57H (1946). 2Tsu-Shen Chang, Ph. D. Thesis, University of Michigan, Ann Arbor, Michigan; 1953.""

Author Institution: School of Chemistry, University of Minnesota

Author Institution: School of Chemistry, University of Minnesota

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