Ground-state proton-transfer dynamics governed by configurational optimization

The ground-state proton transfer (GSPT) of 7-hydroxyquinoline along a hydrogen-bonded alcohol chain has been investigated in n-alkanes using time-resolved transient-absorption spectroscopy with variation of alcohols, media, isotopes, and temperatures. As a 7-hydroxyquinoline molecule associates with two alcohol molecules via hydrogen bonding to form a cyclic complex in a nonpolar aprotic medium, the intrinsic GSPT dynamics of the cyclic complex in a n-alkane has been observed directly without being interfered with by solvent association to form the cyclic complex. GSPT occurs concertedly without accumulating any reaction intermediate and yet asymmetrically with a rate-determining tunneling process. Both the rate constant and the kinetic isotope effect of GSPT increase rapidly with the proton-donating ability of the alcohol but decrease greatly with the molecular size of the alcohol. The reorganization of the hydrogen-bond bridge to form an optimal precursor configuration for efficient proton tunneling takes place prior to intrinsic GSPT, and configurational optimization becomes more important as the molecular size of the alcohol increases. Consequently, the larger contribution of configurational optimization to GSPT leads to the weaker asymmetric character of GSPT.