(a) The 1 H NMR spectrum of [18]annulene shows two signals, at = 9.28 (12 H)...

(a) The ¹H NMR spectrum of [18]annulene shows two signals, at δ = 9.28 (12 H) and -2.99 (6 H) ppm. The negative chemical shift value refers to a resonance upfield (to the right) of (CH₃)₄Si. Explain this spectrum. (Consult Figure 15-9.) (b) The unusual molecule 1,6-methano[10]annulene (shown in the margin) exhibits two sets of signals in the ¹H NMR spectrum at δ = 7.10 (8 H) and -0.50 (2 H) ppm. Is this result a sign of aromatic character?

Figure 15-9

Transcribed Image Text:

H H 1,6-Methano[10]annulene