Infrared spectral evidence and DFT calculations of hydrogen-bonding and molecular structures of acetogenins

FTIR spectra have been measured for 31 different five component - simplex centroid design solvent mixture extracts of shaded and sun-exposed Annonaceous leaves harvested in all four seasons. The spectral frequencies are characteristic of anonnaceous acetogenins known to be a major component of these leaves. OH stretching spectral bands in the 3100-3600 cm−1 region provide evidence of notable intensity changes for the shaded and sun-exposed leaves. Chemometric principal component analysis involving 264 spectra show that shaded samples tend to have more intense OH stretching bands than those grown in the sun. B3LYP density functional calculations indicate significant OH stretching band changes in this region owing to hydrogen bond formation. Weak OH intensity enhancements, around 40 km mol−1, occur when an OH group forms a hydrogen bond with the oxygen atom of an adjacent tetrahydrofuran ring oxygen atom. Much more intense enhancements, 400-500 km mol−1, are predicted to occur for acetogenins with two tetrahydrofuran rings for which the OH group hydrogen bonds with its fartherest removed tetrahydrofuran ring oxygen. Whereas weak or moderate H-bond stretching intensities are obtained for acetogenins with slightly bent carbon chain structures the strongest hydrogen bond intensities are calculated for molecules with a 45° V-type backbone structure. These important structural modifications as well as significant changes in bond lengths and angles owing to hydrogen bonding are detailed.