CHANGES IN INTENSITY OF SOME INFRARED BANDS OF BENZENE IN SOLUTION

Author Institution: Naval Research LaboratoryFundamental vibrational frequencies which are infrared-inactive in the vapor state are commonly observed in the liquid state. This breakdown of the selection rules is generally attributed to the influence of intermolecular forces. In an attempt to utilize this phenomenon as a means of studying intermolecular forces, data have been obtained on the intensities of some of the inactive fundamentals of benzene in several solvents. The Raman-active fundamental at $1179 cm^{-1}$ is less intense in carbon tetrachloride, chloroform, cyclohexane, and N-Decane solutions than it is in pure benzene while in carbon disulfide solutions it is stronger. The magnitude of the change is considerable. In a 10% by volume solution of benzene in carbon tetrachloride the extinction coefficient is 0.6 of its value in pure benzene, while in a 10% solution of benzene in carbon disulfide, the extinction coefficient is twice that of pure benzene. A similar effect is observed for other inactive fundamentals corresponding to both planar and non-planar vibrations. The infrared-active fundamental at $1037 cm^{-1}$, on the other hand, shows no such marked intensity change in solution. The observed intensity changes appear to correlate with the dispersion forces between molecules. In the case of the carbon disulfide solutions, where the inactive fundamentals are enhanced, the London dispersion force (induced-dipole-induced-dipole interaction) between solute and solvent molecules is greater than the dispersion force between two benzene molecules. In the case of the carbon tetrachloride, chloroform, and cyclohexane solutions where the bands are weakened, the solute-solvent interactions are less than in pure benzene. Some data on the effect of temperature on these bands in pure benzene has also been obtained. In going from $10^{\circ} C$ to $70^{\circ} C$, a decrease in intensity of approximately 30% is observed for the inactive fundamentals at 850, 1179, and $1310 cm^{-1}$. However, the two active fundamentals at 1030 and $1485 cm^{-1}$ show the same change so that a relationship between the effects of temperature and dilution in various solvents is not apparent