The Effect of the Isotopic Exchange of the Oxygen Atom on the Absorption Intensity of the Vibration Bands of Benzophenone

According to theory the sum of the integrated intensities of all absorption bands of a species, divided over the square of their respective frequencies should, under certain conditions, be independent of the isotopic constitution of the vibrating molecule. It was observed that of all the benzophenone bands in the 3500–370 cm⁻¹ region only the CO stretching frequency at 1667 cm⁻¹ and its CO bending vibration at 640 cm⁻¹, shifted significantly as a result of labelling with O¹⁸; hence only the absorption intensity of these seems to be affected by this exchange. The planimetrically integrated intensity of the 1637 cm⁻¹ CO¹⁸ stretching band, measured under a resolution of 1/10th of its half‐width, was found to be about 25% less than of the corresponding CO¹⁶ band. This decrease is not balanced by the tiny increase in the intensity of the 640/637 cm⁻¹ band and disagrees with the theoretically expected value.