October 27, 2009
Pattern recognition is an integral part of the process of structure elucidation. The quicker the elucidator can pick up on the pattern, the faster the elucidation can be accomplished and the less time wasted in elucidating the unknown.
Recap of the problem: The ESI+ MS shows a single [M+H]+ at m/z 102 allowing a maximum carbon count of 8. The 13C NMR shows there to be 12 carbons. How can the data from the MS and NMR present such different results for the same unknown?
The 13C NMR below shows a unique pattern that is not obvious at first. Each 13C signal seems to be paired to a nearby signal of similar peak intensity. The pattern is also noticeable in the 13C DEPT-135 NMR spectrum below.
This pairing pattern indicates that the unknown is a mixture at approximately a 1:1 ratio (note: the option of rotamers is also possible but is not being considered for this example). Therefore, instead of 12 carbons for a single unknown compound as perceived earlier, there are 6 carbons per unknown. This interpretation falls in line with what is being suggested by the MS data.
The last piece of the puzzle is to determine why the MS data shows only one molecular ion peak?