H-NMR spectroscopy of [18]annulene
The aromaticity of [18]-annulene1 was confirmed by room temperature 1H NMR (60 MHz) in 1964.2 [&tau; values have been converted to &delta; values; &delta; = 10 - &tau;] When the spectrum was recorded at room temperature the 12 deshielded, exterior hydrogens appeared at &delta; 8.9 while the 6-interior hydrogens were highly shielded (&delta; -1.8). Evidence for flexiblity in the ring at room temperature was confirmed by observing a further downfield shift of the 12 hydrogens (&delta; 9.28) and an upfield shift of the 6 hydrogen signal (&delta; -2.99) when the spectrum was recorded at -60oC. Recording the spectrum at 110oC caused the two signals to coalesce to a singlet peak at &delta; 5.45. This value is close to the weighted average of the resolved chemical shifts. Ostensibly, the coalesence is due to the interchange of external and internal hydrogens. When the sample is cooled, the resolved spectra are restored.

1) Sondheimer, F., Wolovsky, R., Amiel, Y. J. Am. Chem. Soc., 1962, 84, 274.
2) Gaoni, Y., Melera, A., Sondheimer, F., Wolovsky, R. Proc. Chem. Soc., 1964, 397.