Simulation of the structures and calculation of IR Spectra of (22 s,23 s)-Homobrassinolide conformers

Frequencies and intensities of normal vibrations of (22 S,23 S)-homobrassinolide, a biologically active representative of steroidal phytohormones, were calculated within the framework of an original approach that combined a classical analysis of normal vibrations by a molecular mechanics method with a quantum-chemical estimation of absolute intensities. Two molecular structures with different side-chain conformations were considered. The molecular IR absorption bands in the range 1500–900 cm–1 were interpreted for the first time and the influence of the side-chain conformation on the IR spectrum was analyzed based on a comparison of the experimental and calculated spectra.