Chapter 7: Analysis of Chiral Systems by Theoretical Calculations

Circular dichroism (CD) spectra can often not be readily analysed in conceptual terms on the basis of the CD intensity mechanisms such as excition coupling and the analysis of the system; therefore, it is necessary to depend entirely on theoretical calculations. The problems faced in carrying out these calculations are described. In general, it is best to used coordinates derived from X-ray crystallography when suitable single crystals can be obtained. In the absence of suitable data in this regard, molecules with intrinsic chirality based on twisted π-systems are best suited to a calculation-based approach. Several examples based on the use of both semi-empirical and TD-DFT calculations are described in detail. Analyses of the experimental and calculated CD spectra of 1,8a-dihydro-3,8-dimethylazulene and halenaquinones provide examples of the use of the semi-empirical π-electron self consistent field-configuration interaction-dipole velocity (SCF-CI-DV) method. The CD spectroscopy of oxo(phthalocyaninato)vanadium(IV), doubly linked porphyrin dimers and three-layered [3,3]paracyclophane provide examples of the more recent use of TD-DFT calculations to analyse systems with inherent chirality.