AB-INITIO CALCULATION OF VIBRATIONAL ABSORPTION AND CIRCULAR-DICHROISM SPECTRA - 6,8-DIOXABICYCLO[3.2.1]OCTANE

Predictions of the unpolarized vibrational absorption and vibrational circular dichroism (VCD) spectra of the chiral molecule 6,8-dioxabicyc lo[3.2.1]octane (1) are reported. Harmonic force fields and atomic pol ar tensors are obtained using the density functional theory (DFT), MP2 and SCF methodologies, and the 3-21G and 6-31G basis sets. Three fun ctionals, LSDA, BLYP, and B3LYP, are used in DFT calculations. Atomic axial tensors are obtained using the Distribution Origin gauge; distri buted atomic axial tensors are calculated using gauge-invariant atomic orbitals (GIAOs) at the SCF level of approximation. The quality of th e predicted spectra is highly dependent on the methodology and the bas is set is employed. Spectra calculated using 6-31G MP2 and DFT/B3LYP force fields are very similar and in excellent agreement with experime ntal spectra. 6-31G SCF, DFT/LSDA and DFT/BLYP calculations are in si gnificantly worse agreement with experiment, as are 3-21G MP2 and DFT/ B3LYP calculations. When both accuracy and computational effort are co nsidered, at this time, predictions of vibrational absorption and VCD spectra of molecules comparable to 1 in size are optimally performed u sing 6-31G or equivalent basis sets and (i) harmonic force fields and atomic polar tensors calculated via DFT and a hybrid density function al; (ii) Distributed Origin gauge atomic axial tensors; and (iii) SCF GIAO-based distributed atomic axial tensors.