Electronic Circular Dichroism of the Chiral Rigid Tricyclic Dilactam with Nonplanar Tertiary Amide Groups
Abstract
Electronic circular dichroism (ECD) of the spirocyclic dilactam 5,8-diazatricyclo[6,3,0,0(1,5)]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 pi-pi* transition region below 190 nm.
The interpretation using TDDFT calculations confirmed that effects of amide nonplanarity follow our previous observations on monolactams as far as amide n-pi* transitions are concerned. ECD band in the n-pi* transition region of the nonplanar diamide exhibits an identical bathochromic shift and its sign remains tied to the sense of nonplanar deformation in the same way.
As far as n-pi* 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 pi-pi* transitions (found between similar to 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 pi-pi* transitions is only minor and concentrates into the shorter wavelength lobe of the pi-pi* 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-sigma*, pi-sigma* or Rydberg type transitions) is only minor and is concentrated below 180 nm.