Chiroptical studies. Part 98. The short-wavelength circular dichroism of ketones in 2,2,2-trifluoroethanol

Abstract

C.d. curves in the range 205–185 nm for a series of 67 chiral ketones, in 2,2,2-trifluoroethanol (TFE) as solvent, show marked differences from those in hexane. A red-shift of the c.d. maximum, averaging 7 nm, is normally accompanied by a significant change in Δε or in many cases by an inversion of its sign. The values of Δε in TFE are analysed in the usual empirical way to derive ‘group increments’(δΔε) which can be used to calculate Δε for trans-decalones and their extended polycyclic derivatives in TFE.

It is suggested that c.d. curves previously reported for the same ketones in hexane may result from superimposition of two absorption bands. One is presumed to derive from the n→σ*(or n→ 3s) transition, which would be blue-shifted in TFE to beyond the range of observation with the available instruments, exposing the red-shifted component which has not been assigned, but which appears to derive from a transition not previously recognised. An attempt is made to derive the separate c.d. characteristics of the two transitions by comparing the values of Δε in TFE with those in hexane.