On the conformation of 6-ketononanolide and 1,4-cyclohexanedione in solution by dipole moment studies

The dipole moments of 6-ketononanolide (1) and its 2,4,7 and 8-Me derivatives (2–5) are found to be 1·8 – 1·9±0·2 D. Utilizing the known solid-state geometry of 1, a dipole moment of 1·6 D is calculated by the INDO quantum mechanical method. Thus the conformation of 1 in solution does not differ much from its solid-state conformation which is diamond-lattice derived with the CO groups 174° apart and in “Type III” positions. However, the IR spectrum of 1 in solution shows differences from that in the solid state (Nujol or KBr).The dipole moment of 1,4-cyclohexanedione (10) is calculated to be 1·2 D for the experimental 156° twist boat conformation (found in the solid-state). This is in excellent agreement with the experimental value found by several groups. 1,6-Cyclodecanedione (6), however, is found to have an experimental dipole moment of 0·7 D, as contrasted with a calculated value of 0 D for 180° opposed carbonyls.