9-Methyltriptycene

At ambient temperatures, the stochastic motions of methyl groups are generally fast on the NMR time scale. The only exceptions repotted so far involve the methyl group in 9-methyltriptycene derivatives (see Fig. 13) in which the torsional energy barriers concerned can be very high they can exceed 40 k.l/mol. In the latter compounds, the stochastic dynamics of the methyl rotor can be frozen out on the time-scale of NMR experiments in liquids. The liquid-phase NMR investigations on a number of 9-methyltriptycene derivatives, performed in the period up to the early 1990s, were reviewed by Oki. The experimental spectra of the methyl protons were interpreted in terms of the AB theory there were no mentions to the possible inadequacies of the AB model in the description of the observed line shapes. 

Fig. 13. The structure of 9-methyltriptycene derivatives with high torsional barriers for the methyl group re-orientation. The symbol X denotes halogen atoms, Y stands for either a halogen or hydrogen substituent, and Z denotes either H or CH3.

Fig. 15. The standard experimental spectra of l,2,3,4-tetrachloro-9-methyltriptycene 10, for the methyl proton B, superposed with the corresponding best-fit theoretical AB (top) and DQR (bottom) spectra. The impurity signal at 2.24 ppm was included in the fits, with assumed Lorentzian lineshape. (Reproduced with permission from Ref. 67. Copyright by the American Institute of Physics 2003).

The methyl group may experience hindered rotation with substantial barriers in congested environments. Thus 9-methyltriptycenes are not particularly crowded in the staggered ground state, but experience severe steric interactions in the eclipsed transition state for rotation. If one or two rings are labelled then the rotational process may be observed by H nmr spectroscopy. The methyl protons display A2B or ABC patterns when the... 

Recently, some of the methyltriptycene derivatives were re-investigated with use of the methodology of Ref. 51 (Section 4.2) to line shape analysis. In these liquid-phase studies, instead of the solid echo spectra, the Carr-PurceU (CP) echo spectra were... 

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