Spectroscopic signatures of proton transfer dynamics in the water dimer cation Page: 2 of 32
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sponsible for the double-helix structure of the DNA and for many unique properties of the
most important solvent, water. Hydrogen bonds can facilitate hydrogen or proton transfer
between molecules (or within the same molecule), a process commonly occurring in biochem-
istry, enzyme catalysis, and materials science. Proton transfer is often coupled to electronic
dynamics, and can be induced by electronic excitation or ionization. For example, ioniza-
tion of some hydrogen-bonded nucleobase pairs leads to barrierless proton transfer, which
is believed to play a role in the hole migration through the DNA[6-14]. Reduced barriers
along proton transfer coordinate have also been reported in electronically excited nucleobase
Ionization of liquid water, which has been investigated extensively since the 1960s, also
involves proton transfer, however, the entire process is still not clearly understood. Ex-
perimentally, it is observed that ionizing radiation strips an electron from water, resulting
in a thermalized electron, a hydroxyl radical (OH), and a hydronium ion (H30+). It is
believed that this results from the formation of a nascent water cation (H2O+), which sub-
sequently reacts by transferring a proton along one of its hydrogen bonds to a neighboring
water monomer. The water dimer cation is a minimal model system (readily amendable to
experimental and theoretical studies) that exhibits similar photoinduced dynamics. In the
neutral state, the water dimer exists in the classic donor-acceptor configuration with one of
the water monomers oriented so that it points a hydrogen at the second water. Ionization
of the dimer induces a proton transfer reaction. Representing the vertically ionized water
dimer as [(H20)2]+ and the proton-transferred product as a H3O+ . OH pair, the reaction
can be written as:
(H2D)2 + hi -> [(H20)2]+ + e - (H30)+ ... OH - (H30)+ + OH.
The H3O+ . OH complex is a bound complex, and may or may not dissociate depending
on the energy of the impinging radiation. In bulk water, the proton transfer is believed
to be complete within 100 femtoseconds and has been followed by transient electronic
spectroscopy, although it has been difficult to establish definitive evidence for this trans-
As a prototypical H-bonded system, the water dimer is of great fundamental interest [18,
19]. While the dynamics and vibrational spectroscopy (including hydrogen tunneling) of the
neutral water dimer has been extensively studied [20-22], previous dimer cation work has
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Kamarchik, Eugene; Kostko, Oleg; Bowman, Joel M.; Ahmed, Musahid & Krylov, Anna I. Spectroscopic signatures of proton transfer dynamics in the water dimer cation, article, December 21, 2009; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc1014509/m1/2/: accessed January 19, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.