A dynamic nuclear magnetic resonance spectroscopic study of conformational properties of 1,3‐dioxepane and 4,4,7,7‐tetramethyl‐1,3‐dioxepane

The 251 MHz 1 H and the natural abundance 63.1 MHz 13 C NMR spectra of 1,3‐dioxepane (1) and 4,4,7,7‐tetramethyl‐1,3‐dioxepane (2) have been investigated over the temperature range of 5 to −180 °C. While the spectra of 1 show no dynamic NMR effect, compound 2 exists in solution as a 1:1 mixture of a symmetrical ( C 2 ) twist‐chair and its mirror image conformation. The free energy barrier for the conformational racemization of 2 is 43 kJ mol −1 (10.3 kcal mol −1 ). Interconversion paths between various conformations of 2 are discussed. Compound 1 is suggested to have a symmetrical ( C 2 ) twist‐chair conformation which is rapidly pseudorotating via a chair conformation to achieve a time averaged symmetry of C 2v , even at −180 °C.