Vibrational circular dichroism study of (-)-sparteine

Absorption and vibrational circular dichroism (VCD) spectra of (-)-sparteine were measured and interpreted on the basis of ab initio calculations. An excellent agreement of the theoretical and experimental frequencies and intensities was observed in the mid-IR region. Geometries of the lowest energy conformers were calculated and compared to known X-ray structures. For the simulation of VCD intensities an excitation scheme (EXC) based on the sum over states (SOS) formalism was used and compared with a calculation using the magnetic field perturbation (MFP) theory. A modified formulation, the EXC theory, was proposed and implemented, which avoids an explicit dependence of VCD intensities on the gradient of the electronic wave function. Thus the dependence of VCD intensities on the size of the basis set is reduced without an introduction of computationally expensive magnetic field-dependent atomic orbitals. The accuracy of the EXC method, however, is severely limited by the approximations used for the excited electronic states. Future applications of VCD for (-)-sparteine chemistry and conformational studies of large systems are discussed.