The Raman Effect and the Carbon-Halogen Bond

Abstract

By far the most intense line in the Raman spectra of (organic) aliphatic bromides exhibits a Raman wave number ($\Delta \overline{\nu }$) of about 600 ${\mathrm{cm}}^{-1\mathrm{}}$ in the methyl derivative, of 564 ${\mathrm{cm}}^{-1\mathrm{}}$ if from two to five carbon atoms are present in the normal chain, and of about 539 ${\mathrm{cm}}^{-1\mathrm{}}$ if the chain is branched sufficiently close to the bromine atom. These frequencies are the most characteristic of the carbon-bromine bond. It is assumed that these Raman lines, and the corresponding most intense lines in the spectra of the chlorides and of the iodides, are produced by a decrease (or increase in anti-stokes lines) by unity in the vibrational quantum number. On the basis of this assumption the use in the calculations of this paper of only the first term in the brackets of the more general equation ${\epsilon }^v=hc\left[{\overline{w}}_e\right(v+\frac{1}{2})-x_2{(v+\frac{1}{2})}^2+\cdots ]$ may be considered to give only a small error, possibly of the order of one percent. Here $w$ is the wave number, but the $w$ used elsewhere in this paper gives the frequency, since for simplicity it includes the $\mathrm{hc}$ term. Although the fundamental frequency associated with the carbon-halogen bond in normal aliphatic compounds decreases by about 8 percent if the number of carbon atoms in the molecule is increased from one to two, a further increase in the length of the molecule up to 5 carbon atoms does not give any further decrease; that is, the frequency is independent of the length of the molecule. The fundamental frequencies which for the methyl halides are associated with this bond are 2.13×${10}^{13}$ for the chloride, 1.81×${10}^{13}$ for the bromide, and 1.60×${10}^{13}$ per second for the iodide. The value 1.81×${10}^{13}$ for the bromide with one carbon atom, is reduced to 1.69×${10}^{13}$ if three, four or five carbon atoms are present in the molecule. If it is assumed that the mechanical frequency ($w_0$) is, for a value of $\Delta v=\pm 1$, related approximately by the equation $\Delta \nu =w_0(v^{\prime }-v^{\prime \prime })=\frac{1}{2\pi }{\left(\frac{f}{\mu }\right)}^{\frac{1}{2}}(v^{\prime }-v^{\prime \prime })$ to the Raman frequency ($\Delta \epsilon$) which is associated with this bond, then the vibration of the bromine atom with respect to the adjacent part of the hydrocarbon chain, has a frequency which is independent of the length of the molecule, provided more than one carbon atom is present. The conclusion from these results is that the force constant which corresponds to the carbon-bromine bond, and therefore, presumably, the strength of the bond, either remains constant, or else increases only slightly as the length of the molecule increases. However, the force constant for methyl halides may be appreciably higher than those for the longer chain compounds. This indicates that the characteristic frequency of this bond corresponds to an inner vibration: that is, in general only a part of the rest of the organic molecule vibrates with respect to the halogen atom to give this frequency. The force constant characteristic of the carbon-halogen bond is found to have, for the methyl halides, values of about 3.0 for the chloride, 2.6 for the bromide, and 2.2×${10}^{-5\mathrm{}}$ dynes per cm for the iodide. Thus the values decrease in the same order as the corresponding heats of dissociation. The values are much less than that (5×${10}^{-5\mathrm{}}$ dynes per cm) previously found for single bonds (C:C, C·O, and C·N) in organic compounds. Another frequency, about 53 percent of that more distinctly characteristic, and with a value of about 300 ${\mathrm{cm}}^{-1\mathrm{}}$ for the wave number, seems to be also associated with carbon-bromine bond, possibly with some type of transverse vibration. A not too distant branching of the chain of the aliphatic hydrocarbon causes a decrease of about 5 percent in the characteristic frequency associated with the carbon-bromine bond, and probably indicates a decrease in the strength of the bond. The value 1638 ${\mathrm{cm}}^{-1\mathrm{}}$ is obtained for the wave number which is associated with the double bond of allyl bromide, while Dadieu and Kahlausch obtain 1639 ${\mathrm{cm}}^{-1\mathrm{}}$ for the chloride and 1645 ${\mathrm{cm}}^{-1\mathrm{}}$ for the alcohol. This illustrates the smallness of the effect of the change of mass of the substituted group upon the frequency at the double bond, which seems to indicate an inner and not an outer vibration. Experimental data are presented for the Raman spectra of nine (organic) aliphatic bromides. The spectra were taken by a large three prism Steinheil G. H. glass spectrograph, by the use of two constricted quartz mercury arcs. The ultraviolet light from these arcs was filtered out by special glass filters in order to prevent the appearance of a brown color in the liquid, and the resultant continuous fluorescence spectrum.