Infrared Group Frequency Assignments Aided by Solvent Studies

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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.