title: n-sigma 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.
authors.affiliation: Gifu Pharmaceutical University, Mitahora-higashi, Gifu 502-8585, Japan. uno@gifu-pu.ac.jp
authors.forename: B
authors.initials: B
authors.lastname: Uno
authors.forename: N
authors.initials: N
authors.lastname: Okumura
authors.forename: M
authors.initials: M
authors.lastname: Goto
authors.forename: K
authors.initials: K
authors.lastname: Kano
mesh_terms: D000838:Anions / Q000737:chemistry; D016227:Benzoquinones / Q000737:chemistry* / Q000378:metabolism; D004563:Electrochemistry; D004583:Electrons; D006860:Hydrogen Bonding; D000432:Methanol / Q000737:chemistry* / Q000378:metabolism; D010084:Oxidation-Reduction; D011506:Proteins / Q000737:chemistry / Q000378:metabolism; D013053:Spectrophotometry; D013696:Temperature; D013816:Thermodynamics
chemical_list: D000838:Anions; D016227:Benzoquinones; D011506:Proteins; D000432:Methanol