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This material is based on work supported by the National Science
Foundation under grant no. AST-0908237, as well as observations
made with the NASA/European Space Agency HST associated
with Guest Observer program 10286 (Principal Investigator,
R. Fesen) and obtained from the data archive at the Space
Telescope Science Institute (STScI). STScI is operated by the
Association of Universities for Research in Astronomy under
NASA contract NAS 5-26555. Visual modeling of our observations
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We thank anonymous reviewers for providing suggestions that
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D. Patnaude for helpful discussions.
Materials and Methods
1 October 2014; accepted 23 December 2014
Vibrational relaxation and
microsolvation of DF after F-atom
reactions in polar solvents
G. T. Dunning,1 D. R. Glowacki,1,2,3,4 T. J. Preston,1 S. J. Greaves,5 G. M. Greetham,6
I. P. Clark,6 M. Towrie,6 J. N. Harvey,1 A. J. Orr-Ewing1*
Solvent-solute interactions influence the mechanisms of chemical reactions in solution,
but the response of the solvent is often slower than the reactive event. Here, we report
that exothermic reactions of fluorine (F) atoms in d3-acetonitrile and d2-dichloromethane
involve efficient energy flow to vibrational motion of the deuterium fluoride (DF) product
that competes with dissipation of the energy to the solvent bath, despite strong
solvent coupling. Transient infrared absorption spectroscopy and molecular dynamics
simulations show that after DF forms its first hydrogen bond on a subpicosecond time
scale, DF vibrational relaxation and further solvent restructuring occur over more than
10 picoseconds. Characteristic dynamics of gas-phase F-atom reactions with hydrogen-containing molecules persist in polar organic solvents, and the spectral evolution of the DF
products serves as a probe of solvent reorganization induced by a chemical reaction.
Elementary reactions of fluorine atoms are central to the development of our under- standing of rates, dynamics, and mecha- nisms of chemical reactions (1, 2). Evidence for rich and subtle dynamical behavior has
come from studying hydrogen atom abstractions
by F atoms from molecules such as H2, H2O, and
CH4 (or deuterium abstraction from their iso-
topologs) under isolated-collision conditions in
the gas phase. In partnership with quantum-
mechanical scattering calculations, sophisticated
experiments have probed the transition state
(TS) region directly (3, 4), identified nonclassical
processes that contribute to reaction (5–8), and
observed breakdown of the Born-Oppenheimer
approximation (9). Early TSs for these exother-
mic F-atom reactions favor highly vibrationally
excited HF or DF molecules (10, 11), consistent
with expectations from the Polanyi rules (12). For
reactions of F atoms with hydrocarbons, similar
dynamics persist at the gas-liquid interface (13).
Here, we extend mechanistic studies of F-atom
reactions to the bulk liquid phase and report
coupled DF-product and solvent dynamics on
the picosecond time scale.
Bimolecular chemical reactions in solution are
of considerable importance in both chemical
synthesis and the biochemistry of living orga-
nisms. Under thermal conditions, these reactions
typically occur on the ground-state potential en-
ergy surface (PES). The mechanisms of the re-
actions and the influence of the solvent can be
explored using time-resolved spectroscopy and
nonequilibrium molecular dynamics (MD) simu-
lations (14–18), but examples of ultrafast studies
of the dynamics of bimolecular reactions of ther-
malized reagents in liquids remain rare. The current
study shows that exothermic F-atom reactions
in CD3CN or CD2Cl2 solutions, to produce DF
and an organic radical [DrH0 ≈ −150 kJ mol−1 (11)],
exhibit comparably rich dynamics to their gas-
phase counterparts and examines the evolving
postreaction microsolvation environment. The pro-
pensity of nascent DF to hydrogen bond promotes
strong solute-solvent coupling, which might be
expected to quench the state-specific dynamics.
Nevertheless, we observe the formation of highly
vibrationally excited DF, with subsequent rapid
relaxation by energy transfer to the solvent bath.
Evidence for a solvent response to the chemical
reaction also emerges.
Figure 1 presents frames from an MD simulation that illustrate the early-time dynamics of
reaction in d3-acetonitrile. Despite reagents that
are thermalized, the reaction results in strikingly
nonthermal microscopic dynamics in the wake of
transition-state passage. At the instant of reactive
formation, DF molecules have multiple quanta of
vibrational excitation, and the surrounding solvent molecules are not oriented to solvate the DF
optimally. The solvent environment is thus intermediate between the noninteracting gas-phase
limit and the strongly interacting equilibrium
limit. Initial hydrogen-bond formation is the first
step toward equilibrium solvation of the nascent
DF and occurs within a few hundred femtoseconds.
Subsequent time-dependent shifts in the DF vibrational frequency over several picoseconds signify
both vibrational relaxation and restructuring of the
microsolvation environment to accommodate this
Figure 2 shows time-resolved infrared (TRIR)
absorption spectra of DF after F-atom reactions
in CD3CN and CD2Cl2. One-photon photolysis of
XeF2 with 50-fs laser pulses centered at 267 nm
promptly generated F atoms in solution (19), and
DF products were probed using 50-fs IR laser
pulses with > 500 cm−1 bandwidth (20). The
broad, unstructured IR bands are characteristic of DF solutes in organic solvents (21). However, the spectral features evolve in time, with a
component that moves from lower to higher
wave number within ~5 ps and a further shift of
~90 cm−1 from higher to lower wave number
over the first 30 ps.
1School of Chemistry, University of Bristol, Cantock’s Close,
Bristol BS8 1TS, UK. 2Department of Computer Science,
Merchant Venturers Building, Woodland Road, Bristol BS8
1UB, UK. 3Photon Ultrafast Laser Science and Engineering
(PULSE) Institute and Department of Chemistry, Stanford
University, Stanford, CA 94305, USA. 4SLAC National
Accelerator Laboratory, Menlo Park, CA 94025, USA. 5School
of Engineering and Physical Sciences, Heriot-Watt University,
Edinburgh EH14 4AS, UK. 6Central Laser Facility, Research
Complex at Harwell, Science and Technology Facilities Council,
Rutherford Appleton Laboratory, Harwell Oxford, Didcot,
Oxfordshire OX11 0QX, UK.
*Corresponding author. E-mail: firstname.lastname@example.org (A.J.O.-E.);