Interpretation of Infrared Spectra: A Practical and Systematic Approach

Abstract The vibrational spectrum of a molecule is a unique physical property and is characteristic of the molecule. As such, the infrared spectrum can be used as a fingerprint for identification by the comparison of the spectrum from an “unknown” with previously recorded reference spectra. This is the basis of computer‐based spectral searching. In the absence of a suitable reference database, it is possible to perform a basic interpretation of the spectrum from first principles, leading to characterization, and possibly even identification of an unknown sample. This first‐principles approach is based on the fact that structural features of the molecule, whether they are the backbone of the molecule, or the functional groups attached to the molecule, produce characteristic and reproducible absorption features in the spectrum. This information can indicate whether there is backbone to the structure and, if so, whether the backbone consists of linear or branched chains. Next, it is possible to determine if there is unsaturation and/or aromatic rings in the structure. Finally, it is possible to deduce whether specific functional groups are present. If detected, one is also able to determine local orientation of the group and its local environment and/or location in the structure. The origins of the sample, its history, and the manner in which the sample is handled all have an impact on the final result. Basic rules of interpretation exist and, if followed, a simple, first‐pass interpretation leading to material characterization is possible. This article addresses these issues in a simple, logical fashion. Practical examples are included to help guide the reader through the basic concepts of infrared spectral interpretation. Finally, armed with the knowledge obtained from this first‐pass interpretation, further refinement using modern mathematical approaches is described.