Remote Control of Hydrogen Bond Strengths in Donor-bridge-Acceptor Systems: The Effects of Torsion and Metalation
Date Issued
2005
Date
2005
Author(s)
Huang, Yi-Ying
DOI
zh-TW
Abstract
Remote control of hydrogen bond strengths can be modulated in a three-component system consisting of a hydrogen bonding site (pyrrole), a conjugated bridge and a reaction
center (imine). Protonation of the reaction center can trigger intramolecular charge transfer thus altering the binding ability of the remote hydrogen-bonding site. The signaltransduction from the reaction center to the hydrogen-bonding site can be easily observed.
In the first part of the study, we investigated the effect of non-planarity in the π-conjugated system has on the binding strength in the three-component system. Computational study showed that the torsion angle between the binding site and the bridging unit could greatly
influence the remote signal communication in our three-component system. Dihedral angle drive calculations on the hydrogen-bonding site were performed to show how the
binding energies behaved as the torsion angle is varied. The results showed decay in the binding strength and transduction sensitivity as the torsion angle is increased. Small decrease in the binding energies is observed when the torsion angle is within 45°, so that
intramolecular charge transfer is not seriously affected by small distortion in the torsion angle. When we compare the CH=CH bridge and N=N bridge, the N=N bridge, which
displays more efficient charge-transfer ability than the CH=CH bridge in planar configuration, is less affected by the variation of the torsion angle, as long as the torsion
angle is within 30°. In general, the decrease of binding energy in the α-systems is slightly more than the β-systems as the three-component systems become nonplanar. In the second part of the study, we observed the extent of charge transfer that was induced by metalation
and compared it with protonation-induced charge transfer. We studied four three-component systems: system A (pure CH=CH-CH=CH bridge), system B (with two substituted CN groups in the CH=CH-CH=CH bridge), system C (with two substituted CN groups in the reaction center), and system D (with four substituted CN groups in the
reaction center). The results showed that the abilities of charge transfer and sensitivity for metalation with M2+ ions are similar to protonation in all four systems. However, the abilities of charge transfer and sensitivity for metalation with M+ ions are very weak and
are similar to the neutral state. The binding energies of different metals with the same oxidation state are similar (Ru2+ > Zn2+ = Mg2+ ; Li+ > Na+). The strengths of binding
energies ordered from high to low observed in the four systems are system B > system D > system C > system A and sensitivities ordered from high to low in four systems are system D > system C > system B > system A. When we observed the bridge length effect on the four systems, the charge-transfer ability of system B is the least affected as the length of the bridge increase. Our studies have provided great insights in the understanding of
remote hydrogen bond communication in our three-component systems and they could provide guidelines for the synthetic design of these materials.
Subjects
氫鍵
分子內電荷轉移
hydrogen bond
intramolecular charge transfer
Type
thesis
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