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The authors are grateful for financial support provided by the
NIH’s National Institute of General Medical Sciences (GM105938).
S.K.M. is grateful for a Canada Graduate Scholarship from the
Natural Sciences and Engineering Research Council of Canada.
We acknowledge C. M. Rathbun for contributions during the
optimization stage of reaction development.
Materials and Methods
References ( 36–54)
15 September 2014; accepted 6 November 2014
Extremely short-lived reaction
resonances in Cl + HD (v = 1) → DCl + H
due to chemical bond softening
Tiangang Yang,1,2 Jun Chen,1 Long Huang,1 Tao Wang,1 Chunlei Xiao,1† Zhigang Sun,1, 3†
Dongxu Dai,1 Xueming Yang,1, 3† Dong H. Zhang1, 3†
The Cl + H2 reaction is an important benchmark system in the study of chemical reaction
dynamics that has always appeared to proceed via a direct abstraction mechanism, with no
clear signature of reaction resonances. Here we report a high-resolution crossed–molecular
beam study on the Cl + HD (v = 1, j = 0) → DCl + H reaction (where v is the vibrational
quantum number and j is the rotational quantum number). Very few forward scattered
products were observed. However, two distinctive peaks at collision energies of 2.4 and
4. 3 kilocalories per mole for the DCl (v′ = 1) product were detected in the backward
scattering direction. Detailed quantum dynamics calculations on a highly accurate potential
energy surface suggested that these features originate from two very short-lived dynamical
resonances trapped in the peculiar H-DCl (v′ = 2) vibrational adiabatic potential wells that
result from chemical bond softening. We anticipate that dynamical resonances trapped
in such wells exist in many reactions involving vibrationally excited molecules.
Reaction resonances are quasi-trapped quan- tum states in the transition state region that profoundly influence both the rate and product distribution of a chemical reaction (1– 3). Since the landmark theoretical pre-
dictions of reaction resonances in the H/F + H2
reaction in the early 1970s ( 4, 5), extensive studies
have been carried out to detect the resonances
experimentally and to elucidate them theoreti-
cally. However, direct observations have proven
to be extremely challenging. Through a series
of crossed–molecular beam experiments ( 6–9), a
physical picture of reaction resonances in F + H2
(HD) beyond chemical accuracy has been estab-
lished. In addition, threshold photodetachment
spectroscopy has been used to probe resonances
in the I + HI reaction ( 10). Recently, resonance
signatures have also been detected in polyatomic
reactions ( 11–14). Forward scattering of reaction
products in crossed-beam scattering experiments
can be caused by long-lived resonances. However,
the presence of forward scattering does not nec-
essarily imply that there are resonances in a chem-
ical reaction. An intriguing question then is if
and how we can probe reaction resonances in
systems that show no or little forward scattering
product, in which the reaction intermediate is
very short lived.
Here we report a combined high-resolution
crossed-beam and accurate quantum reaction
dynamics study on the Cl + HD (v = 1, j = 0) →
DCl + H reaction (v, vibrational quantum number; j, rotational quantum number). Our study
provides very strong evidence for the existence
of short-lived quantum dynamical resonances in
this reaction. The Cl + H2 system has served as
one of the most important benchmark systems
in the study of chemical reaction dynamics ( 15),
along with the H + H2 and F + H2 reactions. It
has also played a special role in development of
the transition state theory and in the verification of kinetic isotope effects ( 16–19). In contrast to the F + H2 reaction, the Cl + H2 (v = 0)
reaction was shown to be a direct abstraction
with a colinear later reaction barrier ( 20–25).
60 2 JANUARY 2015 • VOL 347 ISSUE 6217 sciencemag.org SCIENCE
1State Key Laboratory of Molecular Reaction Dynamics, Dalian
Institute of Chemical Physics, Chinese Academy of Sciences,
Dalian, Liaoning 116023, China. 2School of Physics and
Optoelectric Engineering, Dalian University of Technology,
Dalian, Liaoning 116023, China. 3Center for Advanced Chemical
Physics and 2011 Frontier Center for Quantum Science and
Technology, University of Science and Technology of China, 96
Jinzhai Road, Hefei 230026, China.
*These authors contributed equally to this work. †Corresponding
author. E-mail: firstname.lastname@example.org (C.X.); email@example.com
(Z.S.); firstname.lastname@example.org (X. Y.); email@example.com (D.H.Z.)
Fig. 1. Time-of-flight spectra of the H atom
product from the Cl + HD (v = 1, j = 0) → DCl
(v′) + H reaction at the collision energy of
4. 3 kcal/mol at different laboratory angles.