Femtosecond Elimination Reaction Dynmamics

Full text of DOI:10.1021/ja9710013

5978
J. Am. Chem. Soc. 1997, 119, 5978-5979
Femtosecond Elimination Reaction Dynamics
D. Zhong, S. Ahmad, and A. H. Zewail*
Arthur Amos Noyes Laboratory of Chemical Physics
California Institute of Technology
Pasadena, California 91125
ReceiVed March 31, 1997
In this paper, we report our femtosecond (fs) clocking of an
elimination reaction with multi-bond breakage:
X
F
F
F
elimination
(–XX)
F
C
C
C
C
F
F
X
F
F
ethanes
ethylenes
The elimination involves the (R, â) two C-X bonds (X ) I)
and leads to the transformation of ethanes to ethylenes, a general
class of halogen-atom elimination (and addition) in many
organic reactions.1 By measurements of (1) the fs time evolution
of products, (2) the product-velocity distributions, and (3) the
recoil anisotropy (at 277 nm), the reaction dynamics and
mechanism are microscopically elucidated: the two-center
elimination is a two-step, nonconcerted process with the reaction
†
involving the intermediate C2F4I . The time scales for the two
bond fissions are two orders of magnitude different: the primary
bond breakage only takes about 200 fs, but the secondary
elimination needs 25 ps. The former is due to the repulsive
force in the C-I bond caused by the promotion of the HOMO
n electron to the LUMO σ* orbital, while for the latter
elimination is directly governed by the energy redistribution and
†
Figure 1. TOF mass spectrum (top) from expansions of C
2
F
4
I
2
vapor
the rate of bond breakage of C2F4I .
+
at -35 °C and 700 Torr of He (monomer condition). The I KETOF
distributions are also shown for two typical delay times of 1 and 600
ps. The ø is the angle of the linear femtosecond laser polarization
relative to the TOF axis (z). The difference of each two KETOF
distributions (solid line) represents the contribution of the secondary
bond cleavage.
The reaction was studied in a supersonic molecular beam
2
integrated with the femtosecond apparatus. The velocity
distributions of iodine products were obtained using the ap-
proach of kinetic-energy resolved, time-of-flight (KETOF) mass
spectrometry with resonance-enhanced multiphoton ionization
(
centered at 304 nm, for both I and I* with about equal
efficiency) detection. The recoil anisotropy was determined by
using polarized femtosecond pulses, as detailed elsewhere.
dissociation times.4 This large translational energy release in
2
such short time excludes the possibility of breaking two C-I
A typical TOF mass spectrum is shown in Figure 1. With
bonds concertedly. The excitation energy is 103 kcal/mol, and
the femtosecond resolution, we clearly see the parent mass peak.
5
if two bonds (∆H ≈ 59 kcal/mol for the I channel) are involved,
+
The I KETOF distributions for the parallel (ø ) 0°) and magic
no such translational energy can be produced. This conclusion
is entirely consistent with the biexponential transient behavior
in Figure 2; the first “rise” (∼200 fs) is due to the primary
elimination. (For methyl iodide, this process actually takes 150
angle (ø ) 54.7°) excitation are also displayed for 1 ps vs 600
ps reaction times. For the magic angle KETOF shape at 1 ps,
the velocity distribution is essentially flat, as predicted by theory
for a direct femtosecond dissociation.2 Indeed, this distribution
reflects the primary C-I bond breakage. At longer times,
however, the distribution displays a buildup of intensity around
Vz ≈ 0, shown at 600 ps, reflecting the process of the secondary
C-I elimination. A similar conclusion is reached for the ø )
4
fs. ) From measurements of the polarization dependence of the
+
I
KETOF distribution (parallel vs magic angle), we also
deduced the anisotropy parameter: 2.0 e â e 1.9. The value
is close to an ideal parallel transition (â ) 2.0) (i.e., the transition
moment is parallel to the C-I bond direction) and again is
consistent with the observed prompt elimination process caused
by the pure repulsive force.
0° results. To obtain the complete temporal behavior, we gated
the slow velocity portion of the distribution (-250 e Vz e 250
m/s) at the magic angle while varying the time delay. The
biexponential rise of the femtosecond transient (Figure 2) gives
the two distinct reaction times for the two steps of C-I
elimination: τ1 ≈ 200 fs and τ2 ) 25 ps, consistent with the
behavior of the KETOF distributions.
In the primary C-I breakage, we observe that the I* and I
branching is 0.7 and 0.3, respectively. This branching is similar
3,4,6
to that observed in CH3I.
If all available energy is converted
to translational motion, the expected speeds are indicated by
the arrows in Figure 2, and the observed distributions do indeed
reach these maximum values (1100 m/s for I* and 1460 m/s
for I). The average translational energy release, 〈ET〉/Eavl, for
+
2
From the magic angle I KETOF distribution, we obtained
the laboratory recoil speed distribution of I atoms at 1 ps reaction
time (Figure 2). Two distributions peaked at 850 and 1200 m/s
were obtained for the primary elimination which correspond to
the formation of the product in both spin-orbit states, I* and I,
(
3) See: Riley, S. J.; Wilson, K. R. Faraday Discuss. Chem. Soc. 1972,
3, 132.
4) Zhong, D.; Cheng, P. Y.; Zewail, A. H. J. Chem. Phys. 1996, 105,
5
(
3
a characteristic of alkyl halide bond breakage with femtosecond
7864.
(
5) Nathanson, G. M.; Minton, T. K.; Shane, S. F.; Lee, Y. T. J. Chem.
(
1) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in Organic
Phys. 1989, 90, 6157.
Chemistry, 3rd ed.; Harper & Row: New York, 1987.
(6) For example, see: Godwin, F. G.; Paterson, C.; Gorry, P. A. Mol.
Phys. 1987, 61, 827. Hess, W. P.; Kohler, S. J.; Haugen, H. K.; Leone, S.
R. J. Chem. Phys. 1986, 84, 2143. Sparks, R. K.; Shobatake, K.; Carlson,
L. R.; Lee, Y. T. J. Chem. Phys. 1981, 75, 3838.
(
2) Zewail, A. H. Femtochemistry: Ultrafast Dynamics of the Chemical
Bond, Vol. I and II; World Scientific: Singapore, 1994. Cheng, P. Y.; Zhong,
D.; Zewail, A. H. J. Chem. Phys. 1996, 105, 6216.
S0002-7863(97)01001-9 CCC: $14.00 © 1997 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 119, No. 25, 1997 5979
between 2 and 8 ps, shorter than the observed 25 ps reaction
time, the secondary elimination produces an isotropic, small
speed distribution of I atoms. The addition of this small velocity
to the large velocity (see below) of the center-of-mass velocity
of the C2F4I radical does not destroy the initial (t ) 0) alignment
(
i.e., the anisotropy of the secondary process “remembers” the
primary elimination anisotropy and is determined mainly by the
larger center-of-mass velocity of C2F4I). Thus, even though
the measured anisotropy is very high, it does not reflect the
long-lived state of the intermediate and would have erroneously
predicted a short-lived intermediate.
The internal energy distribution of the C2F4I intermediate can
be obtained from conservation of energy and is shown in the
insert of Figure 2. This distribution is broad with two peaks at
1
2 and 17 kcal/mol for the primary I* and I channels.
Comparing the I atom yield (Figure 1, ø ) 54.7°) from the
secondary elimination with the total primary iodine product,
only ∼30% of the intermediate dissociates into C2F4 and I. From
P(E) vs E in the insert, we therefore deduce an activation energy
9
of ∼15 kcal/mol; the area under the distribution function of E
greater than 15 kcal/mol is about 30%. This indicates that a
small portion of C2F4I dissociates when is created from the I*
channel, while a large fraction decomposes if the I channel is
involved; the secondary elimination produces ground state I
8
atoms because of the energetics. The weakness of the second
C-I bond is due to a concerted path that breaks the C-I bond
10
while forming a new CdC π bond, in contrast with the primary
elimination which involves C-I cleavage. Preliminary RRKM
calculations of the radical dissociation rate give the picosecond
time scale. However, it is not clear that the secondary elimi-
nation is akin to complete thermal behavior, as discussed below.
Considering the decomposition threshold of ∼15 kcal/mol
(see the insert in Figure 2), the dissociating C2F4I radicals
resulting from the primary I* and I channels have two peak
speeds at 450 and 670 m/s (arrows in Figure 2, bottom),
respectively. The recoil speed distribution of the secondary I
atoms shows two peaks at ∼450 and ∼800 m/s. These peak
values of the radical and the secondary I atom for the I* channel
are the same. This equality supports the previous description
of the secondary elimination process. However, the values for
the I channel differ by 130 m/s (800 vs 670 m/s). In addition
to the expected ∼670 m/s peak distribution for the secondary I
atoms, it appears that a higher speed component (∼800 m/s) is
part of the distribution which reflects a faster dissociation of
the intermediate with higher internal energies, possibly in a
nonthermal distribution.
Figure 2. The experimental femtosecond transient (top), together with
a biexponential theoretical fit (solid line). Three structures are shown
to indicate the reaction: prior to the femtosecond excitation (t
the primary bond breakage (t ), and when the final elimination (t ) is
-
), after
f
†
reached. The recoil speed distributions from the primary and secondary
C-I bond breakage, deduced (and smoothed) from the magic angle
data in Figure 1, are shown in the bottom half; the solid lines are
Gaussian fits of the observed distributions. The two arrows in the
secondary speed distribution indicate the peak speeds of the dissociating
C
2
F
4
I radicals produced from the I* and I channels (see the text). The
internal energy distributions of the C I radical after the primary
elimination are shown in the insert. The dashed line shows the deduced
decomposition threshold of C I into C and I.
2 4
F
2
F
4
2 4
F
each channel is found to be 59% for I* and 67% for I. Both
translational energy distributions are broad, and the full-width
at half-maximum is 8.5 kcal/mol for I* and 19.5 kcal/mol for
I. Theoretically, we predict that 86% of the available energy
should appear as translation for a rigid radical (no vibrational
excitation) following an impulsive bond breakage; for a “soft”
In conclusion, with femtosecond-resolved mass spectrometry,
we are able to study in real time the elementary dynamics of
two-center elimination processes and to resolve the state,
Velocity, and angular evolution of products. The reaction is
highly nonconcerted and involves an intermediate with a barrier
determined by the concerted breakage of a σ(C-I) bond and
the making of a π bond. The intermediate is formed in 200 fs
7
radical, we obtained 13%. Our experimental values are more
toward the rigid radical limit.
The secondary elimination dynamics have different reaction
times and velocity characteristic, but surprisingly similar recoil
anisotropy. After the femtosecond primary elimination, the
observed 25 ps rise of the I atom (Figure 2) represents the time
scale for the redistribution to deposit enough energy in the
4
at the initial structure after the detachment of one atom, and
dynamical entry of the transition state is critical to any
nonstatistical behavior. Molecular dynamics should be of
1
1
interest to elucidate reaction trajectories.
†
reaction coordinate of the intermediate [CF2I-CF2] to break
Acknowledgment. This work was supported by a grant from the
National Science Foundation.
the second C-I bond. This is consistent with the previous
8
picosecond studies and is further supported by the observation
JA9710013
of the broad speed distribution of I atoms from the secondary
elimination (Figure 2).
The velocity distribution for I atoms from the secondary bond
breakage (ø ) 0° of Figure 1) shows a very high angular
anisotropy, and the deduced anisotropy parameter (â) is ∼1.9,
nearly the same as that for the primary bond breakage. Since
the rotational time of the C2F4I intermediate is estimated to be
5
(
9) An activation energy of 7.1 ( 2.5 kcal/mol has been reported. The
difference may be due to the difficulty in separating the I* and I channels
at 308 nm excitation (very broad distributions).5 Note that stimulated
dissociation of the intermediate is, in our case, negligible as we do not
observe the 470 m/s peak at 1 ps.
(
10) The weak C-I bond energy in the C2F4I radical can be estimated
from D0(C-I) - D0(CdC) ) ∼8 kcal/mol. With this ∆H value and the
fact that I addition to CdC is endothermic, our value of the effective barrier
(
potential + centrifugal) is consistent with being larger than ∼8 kcal/mol.
(
7) Busch, G. E.; Wilson, K. R. J. Chem. Phys. 1972, 56, 3626, 3638.
8) Knee, J. L.; Khundkar, L. R.; Zewail, A. H. J. Chem. Phys. 1985,
For discussion of the CdC double bond energy, see: Carter, E. A.; Goddard,
W. A. J. Am. Chem. Soc. 1988, 110, 4077.
(
8
3, 1996. Khundkar, L. R.; Zewail, A. H. Ibid. 1990, 92, 231.
(11) Polanyi, J. C. Acc. Chem. Res. 1972, 5, 161.