OPTICAL ROTATORY DISPERSION STUDIES—131: DEMONSTRATION OF A CONFORMATIONAL ISOTOPE EFFECT IN DEUTERIUM SUBSTITUTED CYCLOPENTANONES††This contribution commemorates our collaborative research in the late 1950's with R. B. Woodward which led to the development of the octant rule (Ref. 7).

The synthesis and chiroptical properties of three chiral, deuterium-substituted cyclopentanones are reported which shed light on subtle conformational changes. Thus, the CD spectra of (3R)-3-deuteriocyclopentanone (1), (3S, 4S)-3,4-dideuteriocyclopentanone (2) and (3S, 4S)-2,2,3,4,5,5-hexadeuteriocyclopentanone (3) are found to undergo an increase (in the absolute sense) of the rotational strength on lowering the temperature. This change is associated with a shift in the conformational equilibrium towards the twist conformation with the deuterium in the quasi-axial position. Using literature estimates for the rotational strength of the involved twist conformation, enthalpy differences of approximately –1, –4 and − 6 cal/mol were calculated for the respective conformational equilibria of 1, 2 and 3. Empirical force field calculations were found to be in qualitative agreement with the experimental results when weaker non-bonded interactions for deuterium were used. The gas-phase spectra of 1 and 2 are reported which reveal a surprisingly well resolved vibrational fine structure. Two superimposed main progressions with a spacing of 1250 cm −1 extending up to the sixth overtone are identified. It is suggested that they represent progressions in the excited state carbonyl stretch mode built upon two pseudo origins.