The intrinsic intensity of the 0=0 infra-red band has been measured for various halogenated acetic acid derivatives dissolved in carbon tetrachloride. For reference purposes, formic and acetic acids were studied. D-Oarboxydichloroacetic acid was also measured. For measurement of the intensity, WILSON-WELLS extrapolation was used and a correction of the "wings" was made by RAMSAY'S method. Even in dilute solutions of carboxylic acids the 0=0 band is double. The more intense maximum in our solutions corresponds to the dimer and lies at lower frequency than that corresponding to the monomer. It was not possible to eliminate the monomer although in some cases its absorption was in the range of only 1 per cent of the dimer.
Since the effect of hydrogen bonding on the intensity of the carbonyl band in this case is expected to be small, the total intensity was measured for both bands. On the other hand, the overlap of the monomer band was negligible at the concentrations used and it was possible to measure the area of the dimer band without, any considerable error. The monomer and dimer concentrations required for calculating the intensities were determined, approximately, on the basis of the optical densities at the maximal absorptions. With these data, the maximal molecular extinction coefficients and the intensities of the dimer bands were calculated.
The intensity values were related to the chemical reactivity by using the Taft polar parameters.
THE band associated with the stretching vibration of the C=O bond whose frequency is located between 1580 and 1900 em"! appears isolated in the spectrum, and shows a high intensity. It has been studied in various compounds, such as amides, steroids, acetophenones, cyc1anones, esters, aliphatic and aromatic aldehydes, etc. [l]. There seems to be no previous data on the intensity of the C=O band in the series of halogenated derivatives of acetic acid. The intensity of the carbonyl band for acetic acid, which we also measured has already been determined by WENOGRAD and SPURR [2].
Infra-red intensities have also been related to Hammett's equation log Xi -log X o = pa where X o is the ionization constant of the non-substituted benzoic acid, Xi that of the substituted substance, p a constant depending on the reaction type, and
(1 a constant depending on the nature of the substituent [3]. However, it has been t This paper has been supported, in part, by the U.