The data presented show that infrared group frequencies are only applicable for specific spectra structure identification when the infrared spectrum of the unknown material is recorded in the same manner as that used to record the infrared spectra used to build the IR group-frequency data base. IR spectra recorded of compounds in different phases such as the liquid, solution, and vapor phase are useful in helping one to determine whether a compound exists in more than one rotational molecular configuration such as rotation of a CHCl<sub>2</sub> group about a C-C=O single bond in compounds of form CHCl<sub>2</sub>-C(|O-R)=O.
Richard A. Nyquist, "Factors Affecting Infrared Group Frequencies: Carbonyl Stretching Absorption Bands," Appl. Spectrosc. 40, 336-339 (1986)