Excited-state hydrogen relay along a blended-alcohol chain as a model system of a proton wire: deuterium effect on the reaction dynamics

The excited-state deuteron transfer (ESDT) of deuterated 7-hydroxyquinoline (7DQ) along a heterogeneous hydrogen (H)-bonded chain composed of two deuterated alcohol (ROD) molecules having different acidities, as a model system of a proton wire consisting of diverse amino acids, has been investigated. To understand dynamic differences between deuteron transfer and proton transfer, solvent-inventory experiments have been performed with variation of the combination as well as the composition of alcohols in a H-bonded mixed-alcohol chain. Deuteron transfer from the adjacent ROD molecule to the basic imino group of 7DQ via tunneling, which is the rate-determining step, initiates ESDT, and subsequent barrierless deuteron relay from the acidic enolic group of 7DQ to the alkoxide moiety along the H-bonded chain completes ESDT. Whereas the acceleration of the reaction has been observed in excited-state proton transfer because of the accumulated proton-donating abilities of two alcohol molecules in a H-bonded chain by a push-ahead effect, such acceleration is not observed in ESDT. Because the energy barrier of deuteron relay is much higher than that of proton relay due to the low zero-point energy of 7DQ·(ROD)(2) and a deuteron is twice as heavy as a proton, it is hard for a deuteron to pass through the barrier via tunneling. Moreover, both the H-bonding ability and the acidity of ROD molecules are so weak that their deuteron-donating abilities cannot be accumulated at the rate-determining step of ESDT. Consequently, the rate constant of ESDT is determined mostly by the acidity of the ROD molecule H-bonded directly to the imino group of 7DQ.