EXPERIMENTAL AND THEORETICAL-STUDY OF THE CIRCULAR-DICHROISM SPECTRA OF OXA-[2.2]METACYCLOPHANE AND THIA-[2.2]METACYCLOPHANE

The excited electronic states of (-)-(M)-1-oxa[2.2]metacyclophane and (-)-(M)-1-thia[2.2]metacyclophane are discussed in the light of experi mental UV/VUV optical absorption and circular dichroism (CD) spectra a nd multireference configuration interaction (MRD-CI) calculations of e xcitation energies and rotatory strengths. As one-particle basis, semi -empirical AM1 and MNDOC wavefunctions expressed in terms of localized molecular orbitals (LMO) are used. Good agreement between experimenta l and calculated CD spectra is obtained by correlating only a small po rtion of the 80 valence electrons of the compounds. The dominant CD tr ansitions of the oxa[2.2] metacyclophane can be characterized as separ ate and coupled pipi transitions localized in the aromatic rings. The CD spectrum of the thia[2.2] metacyclophane is mainly described by va lence-type excitations involving the lone-pair orbital of the sulphur atom (3p) and/or anti-bonding sigma MOs of C-S bonds; localized pipi* transitions are of lower intensity.