Capturing transient structures in the elimination reaction of haloalkane in solution by transient X-ray diffraction

Article abstract of DOI:10.1021/ja710267u

This Communication reports simultaneous tracking of structural and kinetic information for the photoinduced elimination reaction of 1,2-diiodotetrafluoroethane in solution by transient X-ray diffraction. The transient structure of ·CF₂CF₂I is determined to be a classical mixture whereas ·CH₂CH₂I is bridged. Compared with the gas phase reaction, the secondary dissociation of ·CF₂CF₂I into C₂F₄ and I is slowed down by a factor of 6 in solution. Transient X-ray diffraction offers a complementary method for capturing transient structures in solution which might be invisible or optically silent in time-resolved optical spectroscopy. Copyright

Full text of DOI:10.1021/ja710267u

Published on Web 04/15/2008
Capturing Transient Structures in the Elimination Reaction of
Haloalkane in Solution by Transient X-ray Diffraction
†
‡
†
§
§
Jae Hyuk Lee, Tae Kyu Kim, Joonghan Kim, Qingyu Kong, Marco Cammarata,
§
§
,†
Maciej Lorenc, Michael Wulff, and Hyotcherl Ihee*
Center for Time-ResolVed Diffraction, Department of Chemistry, KAIST, Daejeon 305-701,
Korea, Department of Chemistry and Chemistry Institute for Functional Materials, Pusan
National UniVersity, Busan 609-735, Korea, and European Synchrotron Radiation Facility
(ESRF), BP220, Grenoble Cedex 38043, France
Received November 12, 2007; E-mail: hyotcherl.ihee@kaist.ac.kr
Here we report the simultaneous tracking of structural and kinetic
information for the photoinduced elimination reaction of 1,2-
diiodotetrafluoroethane (C2F4I2) in methanol solution using transient
1
X-ray diffraction (TXD). The iodine elimination reaction of C2F4I2
is known to produce a short-lived ( ·CF2CF2I) intermediate, which
2
can further dissociate into C2F4 and I. Whether the intermediate
is a bridged radical or an open structure, that is, the classical mixture
of anti and gauche conformers, is a critical issue for stereochemical
3
control in organic synthesis. The structure of a short-lived
·
CF2CF2I in the gas phase was determined to be classical form by
ultrafast electron diffraction, which is a powerful tool in investigat-
ing molecular structures of short-lived intermediates in the gas
4
phase. However, whether classical structures are also energetically
favored in solution remained to be revealed. In addition, the solvent
may greatly influence the reaction kinetics such as the time constant
5
for secondary C-I bond dissociation. These fundamental questions
have remained unanswered. For example, transient absorption
Figure 1. Time-resolved X-ray diffraction data at selected time-delays for
C F I in methanol. (a) Difference-diffraction intensities, q∆S(q), excited
minus nonexcited (black). Theoretical fits from global-fitting analysis are
also shown (red). (b) Difference radial intensities, r∆R(r) which are sine-
Fourier transforms of q∆S(q).
2b
spectroscopy in solution phase could not elucidate the structure
of the intermediate(s) and measure the time constant for the
secondary dissociation. With its proven ability to study spatiotem-
2
4 2
1a,b
poral kinetics in solution,
TXD was employed to unravel the
Figure 1 shows difference diffraction intensities q∆S(q,t), (q )
π/λ sin(θ) where λ is the X-ray wavelength, 2θ is the scattering
structure of the intermediates and their kinetics. Since the transient
4
structure of ·CH2CH2I in solution was shown by TXD to be
1a
angle, and t the time-delay) and the corresponding radial difference
curves, r∆R(r,t), where r is the interatomic distance, which is the
sine-Fourier transform of q∆S(q,t) at selected time-delays. It is
intuitively helpful to examine the Fourier transform whose peaks
are a measure of the radial-density-change as seen from the center
bridged, a comparative study of ·CF2CF2I in solution can directly
show the effect of fluorine substitution on the molecular structure
3d
of a radical. However, in solution, the solute is surrounded by
solvent molecules and the diffraction signal is more complicated
since the signals from the solvent cage and the thermally excited
1c
1a,b
of an average excited atom. The broad negative peak at ∼3.0 Å
corresponds to a superposition of depletions of C-I, C · · ·I and
F· · ·I interatomic distances and the negative peak at ∼5.0 Å to
the initial I · · ·I interatomic distance in the parent molecule. The
positive peak around ∼4.0 Å is assigned to a reorganization of the
solvent around the truncated solute (new cage). However, structure
determination in solution requires quantitative analysis of three
signals that are mutually constrained by energy conservation in
the X-ray illuminated volume. The three components are (i) the
solute-only term, (ii) the solute-solvent cross term, and (iii) the
solvent-only term from the hydrodynamics; changes in the solvent
are mathematically correlated by energy and mass conservation.
The first term is due to the change in the internal solute structure
which can be described by Debye scattering, that is, the scattering
solvent has to be considered, making the data analysis nontrivial.
In addition, this molecular system offers a challenge for quantitative
analysis because C2F4I2 exists as a mixture of anti and gauche
conformers.
Time-resolved X-ray diffraction data were collected on
1
a,b
beamline ID09B at the ESRF using a pump-probe scheme.
Picosecond laser pulses (2 ps, 267 nm, 50 µJ/pulse) were used
to initiate iodine elimination in C2F4I2, and delayed X-ray pulses
8
(
5 × 10 photons per pulse, 100 ps (fwhm), 3% bandwidth
around the wavelength of 0.68 Å, repeated at 986.3 Hz) from
the synchrotron were used to interrogate the evolving structures
in the sample. The liquid sample was injected into the laser/
X-ray beam using a recirculating open jet with 60 mM of C2F4I2
dissolved in methanol. Diffraction data were collected for 10
time-delays from -100 ps up to 1 µs, and each delay was
interleaved by a measurement at -3 ns, which served as a
reference for the unperturbed sample.
4a,b
from isolated molecules as in the gas phase. The second term
can be simulated by molecular dynamics simulations by considering
the interactions between solute-solvent atomic pairs. The last term
is due to the temperature and pressure change in the solvent which
can be obtained in a separate experiment where the pure solvent
†
KAIST.
Pusan National University.
European Synchrotron Radiation Facility.
6
‡
is vibrationally excited by near-infrared light. The details of the
§
1a,b
global-fitting procedure can be found in our previous publications.
5
834 9 J. AM. CHEM. SOC. 2008, 130, 5834–5835
10.1021/ja710267u CCC: $40.75  2008 American Chemical Society

C O M M U N I C A T I O N S
observed in the investigated time-regime. Then 20 ( 1.3% of
the transient classical C2F4I radical decays further into C2F4 +
I in picoseconds with the time constant of 153 ( 24 ps. These
values can be compared with 55 ( 5% and 26 ( 7 ps
4
a
determined from gas-phase ultrafast electron diffraction,
showing that the solvent reduce the rate and yield of secondary
dissociation significantly. These results might be assigned to
intermolecular energy transfer to the solvent which typically
5
occurs on a time scale of several tens of picoseconds.
Subsequently molecular iodine I2 is formed by recombination
of two I atoms which takes about 100 ns as shown in Figure
2
b. The time constant for the formation of I2 molecules is 8.8
10
-1 -1
(
(2.5) × 10
M
s , which is comparable to the value of
the nongeminated recombination of iodine atoms in CCl4 from
8
optical spectroscopy.
The time constant for the secondary dissociation obtained in
this study relies on a rather small number of measured time
delays. To estimate the associated error, we generated mock
diffraction data with various time constants (from 50 to 500 ps
in steps of 25 ps) for the secondary dissociation. The same
analysis applied to the real experimental data was used to fit
the time constant out of the mock data, and the fitted time
constants agree with the true time constants of the mock data
within ( 2 ps accuracy when no noise was added. If we add
noise comparable to experimental noise, then the time constants
from the global fit analysis deviate from the true values with a
standard deviation of 20 ps. We also note that the convolution
by the X-ray temporal pulse width of ∼100 ps does not affect
the time constant significantly in the investigated time range.
In summary, we determined accurate structural and kinetic
information in the halogen elimination reactions in the solution-
phase by using TXD. The transient structure of ·CF2CF2I in
the photoinduced elimination reaction is determined to be a
classical mixture whereas ·CH2CH2I is bridged. Compared with
the gas-phase reaction whose structural dynamics was revealed
by ultrafast gas electron diffraction, the secondary dissociation
of ·CF2CF2I into C2F4 and I is slowed down by a factor of 6 in
methanol solution.
Figure 2. (a) Identification of the transient structures of dissociated C2F4I2
in solution at 100 ps by transient X-ray diffraction (TXD). The solute-only
terms for the two candidate models (classical and bridged) are shown. The
2
ꢀ
value of the classical structure model is much smaller than that of the
bridged showing that the C2F4I radical has the classical structure. (b)
Reaction kinetics and population changes of the relevant chemical species
during the photoelimination reaction for C2F4I2 in methanol as a function
of time. The measured time delays are indicated with symbols. (c-f)
Schematic reaction mechanism determined by TXD. (c) C2F4I2 in methanol
before laser excitation. (d) Upon photo dissociation, the classical mixtures
of C2F4I and iodine atom are formed. (e) Some of the C2F4I intermediate
are further dissociated into the C2F4 + I species. (f) After 3 ns, molecular
iodine (I2) is generated by nongeminate recombination.
All putative structures (anti-, gauche-C2F4I2, anti-, gauche-, and
bridged-C2F4I, C2F4, and I2) for the solutes were provided by
7
density functional theory calculations.
According to the previous ultrafast electron diffraction studies
4a,b
in the gas phase,
the molecular structure of the C2F4I radical
was determined to be classical and not bridged. To unravel its
structure in solution, we performed the global-fitting analysis
using two candidate models for the transient structures: the anti-
and gauche-mixture (classical structure) and the bridged struc-
2
ture. The ꢀ value for the fit with the classical model was smaller
than that of the bridged structure for all the investigated time-
Acknowledgment. This work was supported by the Creative
Research Initiatives (Center for Time-Resolved Diffraction) of
MOST/KOSEF and the EU grant FLASH(FP6-503641).
2
2
delays. Moreover the ꢀ value (ꢀ ) 1.72) for the classical model
2
was almost a factor of 2 smaller than for the bridged (ꢀ )
3
.24) at 100 ps where C2F4I is the major transient chemical
species. In addition, when a mixture of the two models is
included in the fit, the fraction of bridged radical converges to
zero. These findings confirm the formation of the classical mixed
structure in the elimination reaction of C2F4I2 in solution. The
discrimination in the fits between the classical and bridged forms
is enhanced when the solute-only contribution is extracted from
the total difference diffraction intensity (Figure 2a). The negative
peak near 5 Å corresponding to the I · · ·I interatomic distance
in parent C2F4I2 is common in both models, but the broad
negative peak between 2.0 and 3.0 Å can only match the
classical model. The position and line shape of this peak are
indeed very sensitive to the position of the iodine atom relative
to the two carbon and four fluorine atoms.
Besides the structural identification of the transient structure,
the relative contributions of anti/gauche conformers in the C2F4I
radical can be obtained from global-fitting analysis. In the global
analysis of the TXD data, the fraction of each conformer in the
transient molecule was optimized as fitting parameters. The anti:
gauche conformer ratio of the C2F4I radical was determined to
be 86((4.2):14. Figure 2b shows the populations versus time
for all the species in the reaction as determined by global-fitting.
According to the fit results, the classical-C2F4I mixture and I
dominate at 100 ps as they are essentially created within a few
picoseconds. No three-body dissociation to C2F4 + 2I is
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JA710267U
J. AM. CHEM. SOC. 9 VOL. 130, NO. 18, 2008 5835