The Eyring-type analysis of temperature-dependent phenomenon often serves as
an effective tool for elucidating the dynamic aspects of the system and indeed
provides us with further insights into the dynamics of the propeller chirality of
HABs. In Fig. 7.12, the CD intensity and the excitation energy (taken from the
UV spectra) for the lowest-energy
1 L b band of H1 and H6 are plotted against the
reciprocal temperature (T
À1 ) to all afford a bent line or two kinds of correlations for
each HAB, suggesting a switching of the major contributor at the critical temperature
(that corresponds to the bending point observed): T c % À50
C. We explain these
observations by assuming the “whizzing toroid” conformation, in addition to the
C and CC propeller geometries (Fig. 7.13).
Fig. 7.12 Temperature dependence of the excitation energy (absorption maximum) of the
1
L b
transition (E max ) and the CD intensity at the extremum (Δε ext ) for H1 (left) and H6 (right) in
methylcyclohexane. UV: open circle and dashed line. CD: solid circle and line
aR
aS
Clockwise (C) Propeller
Counterclockwise (CC) Propeller
Whizzing Toroids
Fig. 7.13 Conformer equilibrium in HABs
162
T. Mori