declare no competing interests.Data and materials availability:The
atomic structure coordinates have been deposited in the RCSB
Protein Data Bank (PDB) under the accession numbers 6XR8 and
6XRA, and the electron microscopy maps have been deposited in the
Electron Microscopy Data Bank (EMDB) under the accession numbers
EMD-22292 and EMD-22293. All materials generated during the
current study are available from the corresponding author under a
materials transfer agreement with Boston Children’s Hospital.
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SUPPLEMENTARY MATERIALS
science.sciencemag.org/content/369/6511/1586/suppl/DC1
Materials and MethodsFigs. S1 to S12
Table S1
References ( 49 – 58 )
MDAR Reproducibility Checklist20 June 2020; accepted 14 July 2020
Published online 21 July 2020
10.1126/science.abd4251CHEMICAL PHYSICS
Rotational resonances in the H 2 CO roaming reaction
are revealed by detailed correlations
Mitchell S. Quinn^1 , Klaas Nauta^1 , Meredith J. T. Jordan^2 , Joel M. Bowman^3 ,
Paul L. Houston^4 , Scott H. Kable^1 †
Since its discovery 16 years ago, roaming has become a ubiquitous mechanism in molecular photochemistry.
Its general features are now understood, but little detail is known about how the potential energy
surface (PES) determines reaction outcomes. We performed detailed experiments on formaldehyde
(H 2 CO) photodissociation and determined fully correlated quantum state distributions of the molecular
hydrogen and carbon monoxide products. These experiments reveal previously undetected bimodal
carbon monoxide rotational distributions. Insights from classical trajectory calculations demonstrate
that these features arise from resonances as the PES directs the reaction into cis and trans O–C–H···H
critical geometries, which produce rebound and stripping mechanisms, respectively. These subtle and
pervasive effects demonstrate additional complexity in this prototypical roaming reaction, which we
expect to be general. They also provide detailed benchmarks for predictive theories of roaming.
R
oaming is now an established term in
reaction dynamics ( 1 – 4 ) and is accepted
as a pervasive feature of chemical reac-
tions ( 5 – 17 ). Notably, roaming reactions
bypass any conventional tight–saddle
point transition state (TS). In formaldehyde,
H 2 CO, roaming occurs in the van der Waals
region of the H + HCO radical product channel
when there is insufficient energy in the rel-
ative translational degree of freedom for the
radicals to dissociate ( 5 ). Instead, the H atom
roams about the HCO fragment and abstracts
the second H to form H 2 + CO molecular pro-
ducts. This leads to large amplitude in the
C–H–H angle coordinate,qCHH,describingthe
roaming motion. Because abstraction occurs
at large H–H separation, roaming also results
in high vibrational excitation of the H 2 accep-
tor molecular product. These features are char-
acteristic of all roaming reactions. Roaming
necessarily involves large-amplitude angular
motion in a van der Waals region of the con-
figuration space of the system, and abstraction
at large separation leads to highly vibration-
ally excited acceptor products.
Our chemical and intuitive understand-
ingofroaminghasalwaysbeenguidedby
high-quality theory. Computational support
of roaming has generally been provided by
quasiclassical trajectory (QCT) calculations
on high-level potential energy surfaces (PESs)
( 5 , 7 ). However, strictly theoretical analyses are
challenging because of the large-amplitude
motion, which necessitates sampling large
volumes of configuration space and because
of the fact that many roaming pathways are
in the same region of configuration space as
conical intersections ( 3 , 13 , 18 , 19 ). Quantum
calculations of roaming in a three-atom reac-
tion ( 20 , 21 ) have provided both support and
insight about the large-amplitude nature of
roaming. There is also pronounced current in-
terest in using statistical mechanics approaches
to roaming dynamics through analysis of phase
space—rather than configuration space—which
is embedded in QCTs ( 19 , 22 – 24 ).
Seminal experiments in 1993 on the uni-
molecular dissociation of H 2 CO showed un-
expected bimodality of the CO rotational
distribution with a puzzling peak at low quanta
of CO angular momentum,j(CO) ( 25 ). Bimodal-
ity in chemical dynamics is often an indicator
of competing chemical or dynamical pathways.
Although the more pronounced high-j(CO) sig-
nal was well understood in 1993 as charac-
teristic of the conventional tight TS, it was not
until 2004 that the low-j(CO) feature was shownto be a signature of roaming ( 5 ). The 2004 exper-
iments were critical in showing that this low-j
(CO) peak was correlated with highly vibra-
tionally excited H 2. These results were confirmed
in dynamics calculations, which, by examination
of the trajectories, were able to identify the roam-
ing pathway. Similar observations of bimodal
distributions of final quantum states in the
products of a reaction are now often considered
to be diagnostic of possible roaming. Although
unexpected products are another signature of
possible roaming dynamics ( 11 ), bimodal distri-
butions are centrally important as they indicate
the presence of multiple reaction pathways.
This paper presents additional details of
the multimodality of roaming signatures and
shows that the previously reported low-j(CO)
rotational distribution is itself bimodal. Obser-
vation of the bimodality requires the mea-
surement of fully correlated, individual H 2 (v,j)
states with CO(v,j) states. Although aspects of
the correlation between CO and H 2 product
quantum states were revealed even in the first
roaming paper ( 5 ), these observations were
derived from interrogation of the CO product,
which allowed the H 2 states correlated with
individual CO states to be inferred. The corre-
lations between individual H 2 quantum states
and the full set of CO states have not been
previously revealed. The results in this paper
show that the CO rotational state distribution
from roaming is itself bimodal. Moreover, this
bimodality is present for all CO vibrational
states, for every H 2 rotation-vibration state, and
for all H 2 CO initial states. It is a pervasive fea-
ture of roaming in H 2 CO. Our observation of
multimodality implies previously unidentified,
and as yet unexplained, dynamics in the roam-
ing reaction.Experimental results
The experiments used in this study have large-
ly been described previously ( 16 ), except that
the H 2 photofragment was detected instead
of CO. Briefly, H 2 CO in a molecular beam of
He was excited by a laser to the 2^143 vibra-
tional level of the first excited singlet (S 1 ) state.
See supplementary materials and fig. S1 for
relevant details of H 2 CO spectroscopy. The
nascent, recoiling H 2 photofragments were
state-selectively ionized using (2 + 1) resonance
enhanced multiphoton ionization (REMPI)
via theE;F^1 Sþgstate. An example REMPI1592 25 SEPTEMBER 2020•VOL 369 ISSUE 6511 sciencemag.org SCIENCE
(^1) School of Chemistry, University of New South Wales,
Kensington, NSW, 2052, Australia.^2 School of Chemistry,
University of Sydney, Sydney, NSW, 2006, Australia.
(^3) Department of Chemistry, Emory University, Atlanta, GA,
USA.^4 Department of Chemistry and Biochemistry, Cornell
University, Ithaca, NY, USA.
*These authors contributed equally to this work.
†Corresponding author. Email: [email protected]
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