n−σ Charge-Transfer Interaction and Molecular and Electronic Structural Properties in the Hydrogen-Bonding Systems Consisting of p-Quinone Dianions and Methyl Alcohol

Abstract

Molecular and electronic structural properties of the hydrogen-bonded complexes of p-quinone dianions (PQ(2)(-)) were investigated by electrochemistry and spectroelectrochemistry of PQ in MeCN combined with ab initio MO calculations. Hydrogen bonding between PQ(2)(-) and MeOH was measured as the continuous positive shift of the apparent second half-wave reduction potentials with increasing concentrations of MeOH. Detailed analyses of the behavior reveal that PQ(2)(-) forms the 1:2 hydrogen-bonded complexes at low concentrations of MeOH and the 1:4 complexes at high concentrations, yielding the formation constants. Temperature dependence of the formation constants allows us to yield the formation energy as 76.6 and 118.9 kJ mol(-)(1) for the 1:2 and 1:4 complex formation of the 1,4-benzoquinone dianion (BQ(2)(-)) with MeOH, respectively. These results show that the pi-dianions involving the quinone carbonyl groups exhibit very strong hydrogen-accepting ability. The longest wavelength band of the spectra of BQ(2)(-) and the chloranil dianion (CL(2)(-)) is assigned to the (1)B(3u) <-- (1)A(g) band mainly contributed from an intramolecular charge-transfer (CT) configuration. Hydrogen bonding allows the band of BQ(2)(-) and CL(2)(-) to be blue-shifted, depending on the strength of the hydrogen bonds. CNDO/S-CI calculations reveal that the blue shift is ascribed to stabilization of the ground state by the hydrogen bonding involving strong n-sigma-type CT interaction. The HF/6-31G(d) calculation results show that the structure of PQ(2)(-) is characterized by a lengthening of the C=O bonds and a benzenoid ring. The geometrical properties of the hydrogen-bonded complexes of PQ(2)(-) are a slight lengthening of the C=O bonds and a short distance of the hydrogen bonds. It is demonstrated that this situation is due to the strong n-sigma CT interaction in the hydrogen bonds. The results suggest that the differing functions and properties of biological quinones are conferred by the n-sigma CT interaction through hydrogen bonding of the dianions with their protein environment.

Mechanistic Study on the Electrochemical Reduction of 9,10-Anthraquinone in the Presence of Hydrogen-bond and Proton Donating Additives

“…[1][2][3][4][5] Hydrogen-bonding and proton transfer are of key importance for controlling the reduction potential and reaction path. [5][6][7][8][9][10][11][12][13][14][15] In well-buffered aqueous media, quinone-hydroquinone couples provide familiar, reversible two-electron redox systems in which half-wave reduction potentials vary with the pH in a straightforward Nernstian manner. 16 On the other hand, in dry, neutral aprotic media, quinones typically show two cathodic waves corresponding to reversible formation of the radical anion and the dianion.…”

“…It is well documented that thermodynamic stabilization of these ions by hydrogen donors significantly affects the electrochemical behavior of quinones. [6][7][8][9][10][11][12][13][14][15] Clearly different types of behavior are observed for the electrochemistry of quinones in the presence of various kinds of additives. Electrochemical and spectroelectrochemical investigations on redox systems composed of the quinone-additive pair in aprotic media, involving electron transfer coupled with hydrogenbonding and proton transfer, give much information concerning the effect of the molecular structure and environment on these basic processes.…”

“…Electrochemical and spectroelectrochemical investigations on redox systems composed of the quinone-additive pair in aprotic media, involving electron transfer coupled with hydrogenbonding and proton transfer, give much information concerning the effect of the molecular structure and environment on these basic processes. [6][7][8][9][10][11][12][13][14][15]17 Much attention has been paid to the characteristics of the redox behavior of quinones by hydrogen-bonding with weakly interacting proton donors, such as CH3OH and C2H5OH, [6][7][8][9][10][11][12][13][14][15] which is implicated in controlling both intra-and intermolecular structures in biological systems and biological function as an active site of quinoenzymes. [3][4][5] Such studies demonstrate that the first and second reduction waves of quinones move towards less-negative potential values as the concentration of the proton donor is increased, with no loss of reversibility.…”

“…We first considered the models used to describe this type of interaction involving the hydrogen-bonding of unreduced and reduced quinones with CH3OH, and evaluated the hydrogen-bond formation constants using electrochemical techniques. 6,7 It is concluded that the characteristic voltammograms are associated with strong hydrogen-bonded complexes of the electrogenerated dianion with CH3OH involving the n-σ type charge transfer interaction. 7 Similar models were used to get a deeper insight into the characterization of the electron-transfer coupled with hydrogen-bond formation in quinone redox systems.…”

“…6,7 It is concluded that the characteristic voltammograms are associated with strong hydrogen-bonded complexes of the electrogenerated dianion with CH3OH involving the n-σ type charge transfer interaction. 7 Similar models were used to get a deeper insight into the characterization of the electron-transfer coupled with hydrogen-bond formation in quinone redox systems. [10][11][12][13][14][15] It is well-recognized that the presence of a strongly interacting donor, such as benzoic acid (BA), is characterized by disappearance of the second wave and the appearance of new waves prior to the first wave, together with a new anodic wave.…”