De-Zhan Chen

Theoretical studies of conjugate and substituent effects on the intramolecular proton transfer: an HF/CIS study

Abstract To find the effect of conjugate and substituents on the intramolecular proton transfer, fourteen systems are studied using HF and CIS methods. The results indicate that the intramolecular proton transfer barrier decreases firstly, and changes a little for larger conjugate systems when the combined phenyl cycles are added from three to five. Hereby, we conjecture that the larger conjugate systems (more than three combined phenyl cycles) have little effect on the proton transfer barrier. Theoretical studies on substituents effect show that electronic donor groups enhance the H-bond and electronic acceptor groups reduce the H-bond. The position of substituents is another factor in the proton transfer reaction. The effect of all the substituents on the proton transfer barrier is small, so the character of mother molecule is crucial to the proton transfer reaction. Moreover, the linear analysis results show that the charges on the acceptor oxygen and the differences between the labile hydrogen and the acceptor oxygen have good linear relationship with the proton transfer barrier.

Theoretical study on the trihydroxy-anthraquinone tautomerism in the ground and excited states

Abstract Proton tautomerism of 1,5,8-trihydroxy-3-methoxyl-6-methyl-9,10-anthraquinone was studied using HF and configuration interaction all single-excitations methods with 6-31g(d, p) basis set for the ground and singlet excited states. The calculations indicate that the compound exhibits three strong intramolecular hydrogen bonds (IHB), and shows similar characters in their proton transfer processes considering the geometries and Mulliken charge population. It exhibits normal intramolecular proton transfer in the region III where only one pair of adjacent hydroxyl group and carbonyl group exists in a peri region. However, intramolecular proton transfer (IPT) is not favored in view of the energy trend along IPT coordinate in the region I and II, where two hydrogens of hydroxyl groups bond with a common oxygen of carbonyl group and exists two IHBs in the peri region. This energy trend is verified by the potential energy surface. Hereby, it can be conjectured that IPT is absence in the similar structure such as hypericins peri region and exists in hypomycin B from a theoretical point of view. Calculation results on the photophysical process show that the isomerization process is competitive with the intersystem crossing process, which facilitates the increase of triplet state quantum efficiency and photosensitive activity.