Conformational analysis and TDDFT calculations of the chiroptical properties of tris [1,2-propanediolato(2-)-kappaO,kappaO'] selenium/tellurium and related compounds.

The tris(didentate) chelates [E(OCR1R2CR3R4O)3], with E = Se and Te, display both configurational (delta or lambda; R or S) and conformational (delta or lambda) chirality. In order to assess the contributions of these three chiral arrays to the Cotton effects of the chelates and to elucidate their stereochemistry in the gas phase and in solution, calculations of the UV and CD spectra (down to 180 nm), and also of the relative stability of the chelates, have been performed at the TDDFT/TZVP/B-P86 level. An extensive conformational analysis has supplied additional information on the relevant conformers in the conformational manifold. It was found that the dominant CD effect reflects mostly the delta/lambda twists of the three five-membered ligand rings, and less so the influence of the A/A core configuration, while the contributions of any R/S chiral carbon atoms of the ligand rings are negligible. The sign, the intensity, and the energy of this dominant CD band are found to depend on the stereochemistry of the chelates in a predictable way. Among the conformers, those with equatorially disposed methyl substituents are much preferred. These results make it possible to determine the absolute configuration (A/A) and conformation (delta/lambda) of the chelates from the CD data.