Infrared Group Frequency Assignments Aided by Solvent Studies

Abstract

The antisymmetric NO₂ or SO₂ stretching modes or the out-of-phase N(C=O)₂ or O(C=O)₂ stretching mode for phthalic anhydride can be readily distinguished from their corresponding symmetric stretching mode or the in-phase stretching mode by recording infrared spectra of these compounds in CHCl₃ and CCl₄ solutions. In all cases the antisymmetric stretching mode or the out-of-phase stretching mode exhibits a significant decrease in frequency in CHCl₃ solution, compared with its frequency in CCl₄ solution, while the corresponding symmetric stretching mode or in-phase stretching mode either increases or decreases in frequency. When the symmetric or in-phase stretching frequency decreases, the decrease is significantly less than that exhibited by the antisymmetric stretching mode or the out-of-phase stretching mode in the same two solvents. The acceptor number (AN) values do not correlate with NO₂ stretching frequencies.