Molecular Structure from Molecular Vibrations with Density Functional Theory

Density functional theory (DFT) can calculate Raman spectra of large molecules, although the accuracy for different spectral bands is not well established. Here we show that the DFT-calculated Raman spectra of several anthraquinones, flavones, and steroids can be highly accurate for low-frequency fingerprint vibrations of the entire molecule. By fitting linear combinations of calculated spectra to experimental measurements, distributions of conformational structures and hydrogen bond networks can be determined. Results on extremophile biosignature molecules parietin and quercetin establish the accuracy of DFT-calculated spectra for different conformers. Further mesaurements on cholesterol in phospholipid membranes shows that the resulting cholesterol conformer distributions and hydrogen bond effects are consistent with biophysical models of cholesterol's role in membranes. Experiments and DFT calculations on polarization- and orientation-dependent Raman spectra from organic crystals will also be presented to further test the accuracy of the calculated Raman tensors.