Methyl tunnelling rotation in a series of ketones

✍ Scribed by K.J. Abed; S. Clough; A.J. Horsewill; M.A. Mohammed

• Publisher: Elsevier Science
• Year: 1988
• Tongue: English
• Weight: 354 KB
• Volume: 147
• Category: Article
• ISSN: 0009-2614
• DOI: 10.1016/0009-2614(88)80280-x

✦ Synopsis

The frequencies are reported of tunnelling rotation of the terminal methyl groups of n-alkyl chains in polycrystalline samples of ten members of the series n-alkane-2-one. The dependence of the frequency on chain length exhibits a sharp change. Alkyl chains longer than seven carbon atoms have methyl tunnel frequencies close to 300 kHz, as found earlier for long n-alkanes. Shorter chains have frequencies less than 160 kHz, except for ethyl, which has a frequency of 495 kHz. All of the ketone series give a single tunnel frequency per sample for the terminal methyl group of the alkyl chain, though up to four tunnel frequencies per sample have been observed for the much less hindered COCHX methyl group, reflecting distinguishable molecular sites in the unit cell. It is concluded that the molecular packing adapts to equalise the tunnel frequencies of strongly hindered groups and that the 300 kHz observed for longer chains may indicate alkane-like molecular packing.