Ultraviolet and Infrared Absorption Spectra of Substituted Acetophenones and Benzoic Acids

Influences of substituents on the ultraviolet and infrared absorptions of acetophenone and benzoic acid are investigated from theoretical and experimental points of view. In the present paper, all theoretical considerations are made by the molecular energy-level diagram method developed by the authors. Ultraviolet absorption spectra of some substituted acetophenones are measured in n-heptane, alcohol, water, and sulfuric acid. It is found that most molecules studied here have absorption bands corresponding to the n→π* transition of the carbonyl group, A1g→B2u and A1g→B1u transition of the benzene ring, and intramolecular charge-transfer absorption. Especially concerning last absorption, a somewhat quantitative theoretical consideration is presented and satisfactory agreement is obtained between the theoretical and observed excitation energies. Infrared absorption spectra of carbonyl groups in substituted acetophenones are measured in carbon tetrachloride. The observed carbonyl frequencies are compared with the bond orders of this bond calculated on the basis of the above method. Good correlation is also found between these two quantities. A similar consideration is also made with substituted benzoic acids.