Symmetrical molecule NMR challenge

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In the present challenge, symmetrical molecules is the topic. And please note that there is a prize to be won (a Springer book of your choice up to a value of €100,- given to one winner selected randomly). Please read on…

Meet the challenge

Structure elucidation of symmetrical molecules can often be accomplished using NMR with few such examples given in Fig. 1.

Fig. 1

Examples of symmetrical molecules whose structure can be elucidated using NMR

While symmetry dramatically simplifies the ¹H-NMR spectra of many molecules, it also introduces features that might not be trivial to inexperienced spectroscopists. As an example, ¹H-NMR spectra of both E and Z isomers of symmetrical alkenes (Fig. 1) display a single signal corresponding to both sp² protons; however, its coupling pattern depends only on the substituent group R, and not on the coupling between the sp² protons. In addition, one cannot employ NOESY experiments to distinguish between the E and Z isomers because both sp² protons are chemically equivalent. Hence, one must resort to other approaches to interpret the NMR data.

The challenge

Figure 2 shows a ¹H-NMR spectrum of a mixture of E and Z isomers of an aliphatic amine hydrochloride whose structures are also shown in Fig. 2. The NMR spectrum was obtained using a routine 600-MHz NMR in DMSO-d₆ solvent.

Fig. 2

¹H-NMR spectrum (600 MHz in DMSO-d₆) for a mixture of E and Z isomers of an aliphatic amine hydrochloride

Can you determine the approximate composition of this mixture (for example, E/Z = 1:1, 10:1)? Explain your reasoning.