Ultrasonic Relaxation in Molecules Containing a Six-Membered Ring

Abstract

The ultrasonic absorption in a number of liquids whose molecular structure is a six-membered ring has been measured over the frequency range 0.6–45 MHz at temperatures in the range −100 to +150°C. In piperidine and piperazine derivatives, a large relaxation with a low relaxation frequency is found at the higher temperatures of measurement. The activation energy of the relaxation indicates a barrier to inversion of the chair form of the ring of ∼10 kcal/mole. In cyclohexene and cyclohexanone derivatives, a different relaxation is found at the lower temperatures with a high relaxation frequency. Its molecular mechanism is believed to be the interconversion of chair and boat configurations of the ring, and these measurements represent, apparently, the only direct physical measurements of the elusive and important boat form in relatively simple molecules.