Electronic
Circular Dichroism of the Chiral Rigid
Tricyclic Dilactam with Nonplanar Tertiary Amide Groups
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Abstract
Electronic circular dichroism (ECD)
of the spirocyclic dilactam
5,8-diazatricyclo[6,3,0,0<sup>1,5</sup>]undecane-4,9-dione has been
measured in the extended wavelength range (170–260 nm) utilizing
far-UV CD instrumentation including synchrotron radiation light source.
The data of this model of two nonplanar tertiary amide groups interacting
within the rigid chiral environment provided new information particularly
about the shorter wavelength π–π* transition region
below 190 nm. The interpretation using TDDFT calculations confirmed
that effects of amide nonplanarity follow our previous observations
on <i>mono</i>lactams as far as amide n−π*
transitions are concerned. ECD band in the n−π* transition
region of the nonplanar <i>di</i>amide exhibits an identical
bathochromic shift and its sign remains tied to the sense of nonplanar
deformation in the same way. As far as n−π* transitions
are concerned amide nonplanarity acts as a local phenomenon independently
reflecting sum properties of single amide groups. On the other hand,
CD bands associated with π–π* transitions (found
between ∼170 to 210 nm) form an exciton-like couplet with the
sign pattern determined by mutual orientation of the associated electric
transition moments. This sign pattern follows predictions pertaining
to a coupled oscillator. The influence of amide nonplanarity on π–π*
transitions is only minor and concentrates into the shorter wavelength
lobe of the π–π* couplet. The detailed analysis
of experimental ECD with the aid of TDDFT calculations shows that
there is only little interaction between effects of inherent chirality
caused by nonplanarity of amide groups and amide–amide coupling.
Consequently these two effects can be studied nearly independently
using ECD. In addition, the calculations indicate that participation
of other type of transitions (n−σ*, π–σ*
or Rydberg type transitions) is only minor and is concentrated below
180 nm