In my organic lab we recently isolated and crystallized the natural product usnic acid from dried lichen. An IR spectrum of the crystals dissolved in mineral oil was obtained, but it came out looking a bit strange. The thing that's really confusing me is that there is no band that would be suggestive of a C=O bond, which one would expect to see granted the structure of the molecule (http://en.wikipedia.org/wiki/Usnic_acid) My question is: what are some of the reasons for an unusual/unexpected IR spectrum? I would assume that contamination would be one factor, but are there others? The actual spectrum can be found here by searching for usnic acid: http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi See my spectrum attached.