7
70 Hyeongtaek Lim et al.
contribution of tunneling to ESDPT decreases to reduce the
KIE of ESDPT; the rate of proton tunneling can be veiled if
configurational optimization determines the overall rate.
Anomalous behaviors in the KIE of the solvent-assisted
ESDPT of 7AI in water or alcohols have been explained by
the two-step model, which consists of solvent reorganization
and intrinsic proton transfer (16,32). Thus, we suggest that the
overall ESDPT of Nt can be divided into two orthogonal steps:
pretunneling motion to make an optimized precursor config-
uration and intrinsic proton transfer via tunneling. In one limit,
the precursor-configurational optimization, which is indepen-
dent of isotope effects, can be the rate-determining step, so that
KIE becomes unity. In the opposite limit, the precursor-
configurational optimization is rapid relative to the overall
ESDPT of Nt and the intrinsic proton transfer dominates the
overall process. Thus, we suggest that the precursor-configu-
rational optimization of Nt* assisted by lattice vibrations, as
discussed with Fig. 6, determines the ESDPT rate of Nt
completely at low temperatures. As temperature increases, the
configurational reorganization of Nt* assisted by librational
motions becomes active to increase the role of the intrinsic
proton transfer such as tunneling in the rate-determining step
of the overall ESDPT. Thus, KIE increases exponentially with
temperature increase, although it is not large yet in the organic
glass of n-dodecane. KIE was reported to be as large as 7.4 in
n-heptane at room temperature (34). This suggests that the
intrinsic proton transfer via tunneling dominates the rate-
determining step at high temperatures. The important role of
tunneling in the ESDPT of 7AI dimers has been discussed
widely in previous reports (9–15,18,24).
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CONCLUSION
1
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´
The reaction mechanisms of the ESDPT of 7AI dimers in a
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ground-state conformations of the dimers at the moment of
excitation. Whereas planar normal dimers (Np) undergo
ESDPT within our temporal resolution (<10 ps) to form
tautomeric dimers (T*), twisted normal dimers (Nt) go
through ESDPT on the time scale of 250 ps at 8 K. The
precursor-configurational optimization of Nt*, which is
prerequisite to facile intrinsic proton transfer via tunneling,
determines the overall proton-transfer rate to reduce the KIE
as small as unity, and it is assisted by lattice vibrations below
doubly-H-bonded base-pair, and corresponding proton-transfer-
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1
50 K or by librational motions above 150 K. At cryogenic
temperatures where solvent motions are almost frozen, nuclear
motions such as heavy-atom rearrangement, essential prior to
intrinsic proton transfer, are highly retarded. Consequently,
whereas tunneling contributes mainly to the rate of ESDPT at
room temperature, configurational optimization does at cryo-
genic temperatures.
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femtosecond time scale: Proton transfer in model base pairs.
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20. Sakota, K. and H. Sekiya (2005) Excited-state double-proton
transfer in the 7-azaindole dimer in the gas phase. 1. Evidence of
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asymmetric isotopomers. J. Phys. Chem. A 109, 2718–2721.
Acknowledgements—This work was financially supported by research
grants through the National Research Foundation of Korea funded by
the Ministry of Education, Science and Technology (2010-0015806 and
2
1. Sakota, K. and H. Sekiya (2005) Excited-state double-proton
transfer in the 7-azaindole dimer in the gas phase. 2. Cooperative
nature of double-proton transfer revealed by H ⁄ D kinetic isotopic
effects. J. Phys. Chem. A 109, 2722–2727.
2011-0001216). H.L. is also thankful to the BK21 scholarship.
2
2. Sakota, K., C. Okabe, N. Nishi and H. Sekiya (2005) Excited-state
double-proton transfer in the 7-azaindole dimer in the gas phase.
3. Reaction mechanism studied by picosecond time-resolved RE-
MPI spectroscopy. J. Phys. Chem. A 109, 5245–5247.
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