Calculated Absorption and Vibrational Circular Dichroism Spectra of Fundamental and Overtone Transitions for a Chiral HCCH Molecular Fragment in the Hypothesis of Coupled Dipoles

摘要

We establish the general behavior of absorption and vibrational circulardichroism spectra (VCD) of a chiral HCCH fragment lacking all symmetry elements; thestudy is limited to CH-stretching modes, and the Hamiltonian employed is written interms of normal-mode coordinates and momenta and approximates two different Morseoscillators interacting through a harmonic coupling term; rotational strengths areevaluated within a hypothesis of coupled electric dipoles. Van Vleck contact transformationswritten in terms of raising and lowering operators are used to calculatespectra up to the manifold Dv = 4. Three transformations are necessary to obtain fourthorderterms in the relevant operators, namely, the electric and magnetic dipolemoments. The dynamics of the system exhibits 1:1 resonance terms in addition to theDarling–Dennison coupling term. We discuss the importance of coupling between CHstretches with respect to differences in their local mechanical characteristics indetermining the aspect of the absorption and VCD fundamental and overtone spectra ofincreasing quantum number.