Knowledge Bank

By use of a general formula for Circular Dichroism (CD) spectra in terms of cross-correlation functions of electric and magnetic dipole $moments(1)$, we have calculated the semiclassical vibrational CD spectra associated to the two coupled CH-stretching modes of a chiral $HC\ast CH$ molecular fragment with a chiral carbon $C\ast$. Modes with well defined symmetric character (either normal or local and transition) show a CD spectrum which is composed by a dominant monosignated line in the fundamental and successive overtones: the sign of this feature depends on the absolute configuration of the fragment and not on the sign of the interaction force constant of the CH-stretchings. Modes with strong antisymmetric character show a single line at each overtone alternating sign. Normal modes presenting mixed symmetric/antisymmetric character, show bisignated couples in the fundamental and multiplets in the overtone region with alternating-sign components. The relevance of these calculations for the interpretation of the few existing near infrared CD spectra of chiral cyclic molecules will be pointed out, and the question will be addressed about the possible perturbation from other degrees of freedom.