A density functional theory study on the hydrogen bonding interactions between luteolin and ethanol

Ethanol is one of the most commonly used solvents to extract flavonoids from propolis. Hydrogen bonding interactions play an important role in the properties of liquid system. The main objective of the work is to study the hydrogen bonding interactions between flavonoid and ethanol. Luteolin is a very common flavonoid that has been found in different geographical and botanical propolis. In this work, it was selected as the representative flavonoid to do detailed research. The study was performed from a theoretical perspective using density functional theory (DFT) method. After careful optimization, there exist nine optimized geometries for the luteolin - CH3CH2OH complex. The binding distance of X - H center dot center dot center dot O, and the bond length, vibrational frequency, and electron density changes of X - H all indicate the formation of the hydrogen bond in the optimized geometries. In the optimized geometries, it is found that: (1) except for the H2', H5', and H-6', CH3CH2OH has formed hydrogen bonds with all the hydrogen and oxygen atoms in luteolin. The hydrogen atoms in the hydroxyl groups of luteolin form the strongest hydrogen bonds with CH3CH2OH; (2) all of the hydrogen bonds are closed-shell interactions; (3) the strongest hydrogen bond is the O3' - H3'center dot center dot center dot O in structure A, while the weakest one is the C3 - H3(center dot center dot center dot)O in structure E; (4) the hydrogen bonds of O3' - H3'center dot center dot center dot O, O - H center dot center dot center dot O4, O - H center dot center dot center dot O3' and O - H center dot center dot center dot O7 are medium strength and covalent dominant in nature. While the other hydrogen bonds are weak strength and possess a dominant character of the electrostatic interactions in nature.