Self-Induced Recognition of Enantiomers (SIRE) in NMR Spectroscopy

In the general practice it is widely assumed that because of the nonchiral nature of the NMR experiment, the enantiomers of a nonracemic mixture are only distinguishable by NMR in the presence of some chiral auxiliary agent. In reality, enantiomers may be naturally distinguishable by NMR without using such additives due to the formation of weak solute-solute associates—a mechanism that we call self-induced recognition of enantiomers (SIRE). Although this mechanism has been known for a long time and was actually utilized by a few groups, it is generally ignored on the basis of the instilled belief that the effect is extremely rare and small. This chapter discusses a case study of structure determination where the Mental Traps due to the strongly suggestive chemical context of the problem as well as to our initial dismissal of SIRE, could have led to a plausible-looking solution that in fact turned out to be erroneous. The correct solution involved SIRE, indicating that this effect may be more frequent and larger than generally realized if one is willing to “kindle” it by searching for the proper experimental conditions. The mechanism of SIRE is also explained.