Solution to the Ariadne's thread NMR challenge.

Abstract

The substance of the Ariadne’s thread NMR challenge [1] is linalool (Fig. 1). For the structural determination we should first consider the C NMR spectrum (Fig. 2 in Ref. [1]). Ten carbon signals were detected here. In addition, the DEPT_135 NMR spectrum reveals that two of them are quaternary carbon atoms and three are methylene (CH2) carbon atoms. The other carbon atoms are part of two CH groups and three CH3 groups. With this knowledge, 17 hydrogen atoms are expected for the structure, with an empirical formula C10H17. This is inconsistent with the 18 hydrogen atoms observed in the H NMR spectrum (Fig. 1 in Ref. [1]). Therefore, the hydrogen atom with the broad signal at 2.6 ppm must bond to a heteroatom and the empirical formula becomes C10H18X. Without the molecular mass given, the determination of this heteroatom is difficult. We can only use the chemical shifts of the carbon atoms. Fortunately an INADEQUATE spectrum was given in this challenge. Here, by means of the chemical shifts (horizontal auxiliary lines) and double quantum coherences (vertical auxiliary lines), the whole carbon skeleton of a compound may be easily traced out. However, beginning with the least shielded carbon signal (1) at 145 ppm, the following skeleton will be obtained (Fig. 2). Here, the carbon atoms are numbered consecutively in descending order of their C NMR chemical shifts. Not all connectivities were revealed as clearly as the connectivity of the carbon atoms at chemical-shift-position 5 to the carbon