VIBRATIONAL OPTICAL-ACTIVITY OF (3S,6S)-3,6-DIMETHYL-1,4-DIOXANE-2,5-DIONE

Vibrational circular dichroism (VCD), absorption, Raman, and Raman opt ical activity (ROA) spectra for the title compound, a cyclic dimer, we re measured in non-aqueous solution. The vibrational normal modes are assigned based on the result from ab initio force field calculation. H armonic frequencies and atomic polar tensors for simulation of IR abso rption 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 calcul ated on the HF/6-31G level, and the ROA polarizability tensors were ca lculated on the HF/4-31G level. Excellent agreement between the DFT ca lculated 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 f or 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 t echniques to a high level, certainly one suitable for conformational a nalyses by direct comparison of theory to experimental results. Compar ison of DFT and HF level calculations suggests that the improvement fo und using DFT methods is primarily due to the force field and not to t he 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 t hat useful extension of these calculational techniques to larger oligo mers might be accomplished by transfer of parameters.