Vibrational Optical Activity of (3S,6S)-3,6-Dimethyl-1,4-dioxane-2,5-dione

Abstract

Vibrational circular dichroism (VCD), absorption, Raman, and Raman optical activity (ROA) spectra for the title compound, a cyclic dimer, were measured in non-aqueous solution. The vibrational normal modes are assigned based on the result from ab initio force field calculation. Harmonic frequencies and atomic polar tensors for simulation of IR absorption were calculated both on the (Hartree−Fock SCF) HF/6-31G** level and using density functional theory (DFT) methods with the Becke3/LYP hybrid functional. Magnetic transition dipole derivatives were calculated on the HF/6-31G level, and the ROA polarizability tensors were calculated on the HF/4-31G level. Excellent agreement between the DFT calculated and experimental frequencies was obtained without a need for scaling. Furthermore, using the DFT force field, the correct VCD sign and intensity patterns were reproduced as compared to the experimental mid-IR spectra. Reasonable near-IR VCD and mid-IR ROA sign patterns for the intense peaks were also calculated. The excellent agreement for the mid-IR VCD results shows that medium-sized, biologically relevant molecules can have their spectra simulated using quantum mechanical techniques to a high level, certainly one suitable for conformational analyses by direct comparison of theory to experimental results. Comparison of DFT and HF level calculations suggests that the improvement found using DFT methods is primarily due to the force field and not to the intensity parameters. DFT atomic polar tensors were systematically weaker than the HF generated ones. Weak coupling between the subunits of this dimer implies dominance by local interactions which suggests that useful extension of these calculational techniques to larger oligomers might be accomplished by transfer of parameters.