I have done the FT-IR characterization, but the wavenumber I get from this is not included in the IR table data. How do I know if the compound is not present in the literature?
Have you considered peak shifting? Peak shifts are generally related to structural changes, but there are other causes as well that can explain this behavioral response. Also, it would be useful to provide any data on the sample that you investigated. What is the physical state of the sample? Is it a mixture? How was the analysis carried out? Etc.
So, if I understood correctly, you have a powder sample of known composition, and one of the main absorption bands is shifted to lower wavenumbers compared to the literature. What band? Could this peak be affected by the particle size of your sample?