Article
pubs.acs.org/accounts
Role of Double Hydrogen Atom Transfer Reactions in Atmospheric
Chemistry
Manoj Kumar,† Amitabha Sinha,*,‡ and Joseph S. Francisco*,†
†
Department of Chemistry, University of NebraskaLincoln, Lincoln, Nebraska 68588, United States
Department of Chemistry and Biochemistry, University of CaliforniaSan Diego, La Jolla, California 92093-0314, United States
‡
CONSPECTUS: Hydrogen atom transfer (HAT) reactions are ubiquitous and
play a crucial role in chemistries occurring in the atmosphere, biology, and
industry. In the atmosphere, the most common and traditional HAT reaction is
that associated with the OH radical abstracting a hydrogen atom from the
plethora of organic molecules in the troposphere via R−H + OH → R + H2O.
This reaction motif involves a single hydrogen transfer. More recently, in the
literature, there is an emerging framework for a new class of HAT reactions that
involves double hydrogen transfers. These reactions are broadly classified into
four categories: (i) addition, (ii) elimination, (iii) substitution, and (iv)
rearrangement. Hydration and dehydration are classic examples of addition and
elimination reactions, respectively whereas tautomerization or isomerization
belongs to a class of rearrangement reactions. Atmospheric acids and water
typically mediate these reactions.
Organic and inorganic acids are present in appreciable levels in the atmosphere
and are capable of facilitating two-point hydrogen bonding interactions with oxygenates possessing an hydroxyl and/or carbonyltype functionality. As a result, acids influence the reactivity of oxygenates and, thus, the energetics and kinetics of their HATbased chemistries. The steric and electronic effects of acids play an important role in determining the efficacy of acid catalysis.
Acids that reduce the steric strain of 1:1 substrate···acid complex are generally better catalysts. Among a family of monocarboxylic
acids, the electronic effects become important; barrier to the catalyzed reaction correlates strongly with the pKa of the acid.
Under acid catalysis, the hydration of carbonyl compounds leads to the barrierless formation of diols, which can serve as seed
particles for atmospheric aerosol growth. The hydration of sulfur trioxide, which is the principle mechanism for atmospheric
sulfuric acid formation, also becomes barrierless under acid catalysis. Rate calculations suggest that such acid catalysis play a key
role in the formation of sulfuric acid in the Earth’s stratosphere, Venusian atmosphere, and on heterogeneous surfaces.
Over the past few years, theoretical calculations have shown that these acid-mediated double hydrogen atom transfers are
important in the chemistry of Earth’s atmosphere as well as that of other planets. This Account reviews and puts into perspective
some of these atmospheric HAT reactions and their environmental significance.
1. INTRODUCTION
Hydrogen bonds are a common feature in many areas of
chemistry.1−4 These bonds are much weaker than a typical
covalent bond but are of sufficient strength to influence the
structural and dynamical properties of many molecular systems.
Hydrogen bonds can be of either the intermolecular or
intramolecular variety. In this Account, the focus is on exploring
the role of intermolecular hydrogen bonds in affecting
atmospheric chemistry. An important role played by such
hydrogen bonds is in the formation of small atmospheric
clusters, which can grow to form a critical nucleus and
eventually an aerosol particle, that impacts climate.5 The
abundance, volatility, and reactivity of atmospheric species are
important factors in determining their potential for contributing to the nucleation process. Because of its low vapor pressure
and propensity to form strong hydrogen bonds, a common
nucleating species is sulfuric acid. However, depending on the
environmental conditions, other nucleating agents are also
possible. For example, in the troposphere, volatile organic
© 2016 American Chemical Society
compounds emitted from anthropogenic and biogenic sources
undergo reactions with OH, NO3 and O3, which result in the
formation of oxidized products. The structural features in these
oxidized organics impart to the molecule their characteristic
chemical and physical properties. In particular, molecules with
polar functional groups (e.g., >CO and −OH) possess the
ability to participate in hydrogen bonding making these
oxidized organics also important contributors to the nucleation
process. Apart from affecting molecular aggregation, intermolecular hydrogen bonding can also impact the chemistry of
molecules within the clusters by lowering reaction barriers and
opening up new reactive pathways. The later topic is the main
focus of the current Account.
An important mechanism by which hydrogen-bonding lowers
the reaction barrier is by forming cyclic molecular complexes
that involve lesser ring strain and facilitate intermolecular
Received: January 21, 2016
Published: April 13, 2016
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hydrogen atom transfers (HAT). The HAT reactions are quite
common and play a prominent role in a variety of
environments including biology, industry, combustion and
atmospheric chemistry.6 A central feature of atmospheric HAT
reactions is the presence of a two-point hydrogen bond, which
facilitates the transfer of a hydrogen atom from one location to
another within the complex, thus resulting in the formation of
new products. Acids are present in appreciable amounts
(∼parts per billion) in the atmosphere7 and can readily form
hydrogen bonds, which makes them potent HAT catalysts.The
main sources of these acids are anthropogenic and biogenic
emissions, hydrocarbon gas-phase oxidations, and aqueousphase oxidation of carbonyl compounds. The low molecular
weight carboxylic acids are abundantly available in tropospheric
aqueous and gaseous phases and in aerosol particles in various
environments.7 This Account puts into perspective several
examples of atmospherically relevant acid- and water-catalyzed
HAT reactions involving neutral and radical molecular
complexes, and highlights their impact on a variety of
atmospheric processes ranging from the formation of sulfuric
acid, organic diols relevant for aerosol growth, and the
isomerization and decomposition of free radicals.
Scheme 1. Prereaction Complexes Involved in the Waterand Acid-Assisted Reaction of Acetaldehyde with the
Hydroxyl Radical (Upper Panel), and Isomerization of
Methoxy Radical (Lower Panel)
2. FEATURES OF THE DOUBLE HYDROGEN BONDING
INTERACTION
Acids that promote HAT reactions share a common motif; they
form two-point hydrogen bonds with substrate in the activated
complex. These hydrogen-bonding interactions generally occur
between carbonyl (CO) and hydroxyl (OH)-like functionalities and result in pseudo cyclic structures. Note that these
cyclic structures contain both covalent and noncovalent bonds
and are different from cycloalkanes that are only covalently
bonded. The stability of these cyclic structures depends upon
the steric flexibility of the noncovalent ring formed by the twopoint hydrogen bonds. For catalysts, in which the hydrogen
exchange occurs through a single heteroatom, the smaller sized
(up to seven-membered) cyclic structures are formed, which
are less stable due to steric congestion (Scheme 1). Water
catalysis involves this type of mechanism.8−15On the other
hand; there are catalysts, which promote the exchange reaction
by employing unique functionalities in both HAT components
of the exchange reaction. Under such catalysis, the larger sized
(up to 10-membered) and hence, more stabilized cyclic
intermediates are formed (Scheme 1), that accounts toward
their enormous catalytic efficiency. Monocarboxylic-acid-based
catalysis16−28 or inorganic-acid-based catalysis21,26,29−31 belongs
to this category.
Most catalysts possess the H−O donor motif and the OY
(YC, N, S, P) acceptor motif,16−31 except for water, which
has a different acceptor motif (O−H).8−15 Depending upon the
nature of the reaction and catalyst, the most common donor−
acceptor motifs, which are operative in the hydrogen exchange
reactions, can be classified into four groups (Scheme 2). For
example, C−O(H)···H−O, O−H···OY and O−H···O−H are
the donor−acceptor motifs for the decomposition reactions,15,21,24−26 whereas the hydration reactions are only
mediated by the O−H···OY and O−H···O−H motifs. The
C−H···OY, C−H···O−H, and CO···H−O are the donor−
acceptor motifs for the tautomerization of oxygenates.16,24,25
The tautomerizations operative in biological media have the AH···OY, A−H···O−H, and CN···H−O donor−acceptor
motifs, respectively.28 The strength of hydrogen bonding in
these donor−acceptor motifs depends upon the bond polarity
Scheme 2. General Donor−Acceptor Motifs Involved in
Water- and Acid-Assisted Chemical Reactions
and the bond distance. For example, the SO acceptor motif
forms better hydrogen bonds than CO or NO because the
SO bond is relatively longer than the CO or NO
bond.21,26,29−31 On the other hand, the O−H donor motif
forms more stable hydrogen bonds than the S−H motif because
of its greater polarity.26
To gain molecular level insight into the double hydrogen
atom transfer reactions, the nature of interactions in the
methanediol···formic acid21 complex within the framework of
natural bond orbital (NBO)32 procedure is analyzed. The
interactions that most contribute to the stabilization of
hydrogen-bonded complex generally involve interactions
between the lone pair orbitals of donor oxygen and the σ
antibonding orbital of acceptor O−H bond (n → σ*OH). The
overlap between the oxygen lone pair and the σ*OH orbital is
examined by constructing the pre normalized NBOs. Figure 1a
shows the n → σ*OH interaction for the hydrogen bond where
a diol acts as the Lewis acid (acceptor) and HCOOH acts as a
Lewis base (donor). In the figure, the π-type lone pair of
oxygen (nπ) in HCOOH is interacting with the σ*OH orbital of
the diol monomer. The nπ → σ*OH interaction contributes 5.8
kcal/mol toward the total second order stabilization energy.
There is also a second interaction involving the σ-type lone pair
(nσ) of HCOOH and the σ*OH orbital of diol (nσ → σ*OH).
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Figure 1. Prenormalized NBOs for (a) nπ → σ*OH interaction in the first hydrogen bond, (b) nσ → σ*OH interaction in the first hydrogen bond, and
(c) nσ→ σ*OH interaction in the second hydrogen bond of the methanediol···HCOOH complex. The key NBO charges are given in panel (d).
hydration of SO3 and compared it with the uncatalyzed and
H2O-catalyzed reactions.17 They found that the unimolecular
isomerization of the SO3···H2O···HCOOH complex, whether it
is formed from SO3 + H2O···HCOOH or SO3···H2O +
HCOOH, is barrierless for producing the H2SO4···HCOOH
complex. The calculated barrier for the HCOOH-catalyzed
reaction is only 0.1 kcal/mol, which is 6.5 kcal/mol lower than
that for the H2O-catalyzed one. This makes acid catalysis an
efficient mechanism for the gas-phase hydration of SO3, which
may potentially be a major contributor to the atmospheric
formation of H2SO4. Long et al.18 also studied this reaction and
confirmed that the HCOOH-catalyzed reaction was barrierless.
For the HCOOH-catalyzed reaction, they calculated a rate
constant of 2.1 × 10−10 cm3 molecule−1 s−1 at a typical
atmospheric temperature of 260 K, which is 105 times greater
than that for the H2O-catalyzed reaction.
Once formed, H2SO4 itself can also catalyze the SO3
hydration. H2SO4 possesses SO and S−O−H functionalities,
which allows it to catalytically influence the SO3 hydration. A
recent study30 examined the autocatalytic mechanism of H2SO4
formation. The calculated rate constants for the H2SO4catalyzed reaction are 2 orders of magnitude larger than that
for the H2O-catalyzed one. The autocatalytic reaction
mechanism is expected to be particularly important in the
stratosphere, where the concentration of (H2O)2 drops to a
level similar to that of the H2SO4···H2O complex. This
autocatalytic reaction could also be significant in Venus’
atmosphere, where the estimated concentration of H2SO4···
H2O complexes (8.3 × 1010 molecules·cm−3) is significantly
greater than that of (H2O)2 (1.5 × 107 molecules·cm−3).30
ii. Hydration of Aldehydes to Form Diols, Triols, and
Tetrols. Organic compounds containing carbonyl moieties can
undergo hydration resulting in the formation of more polar OH
functionalities that can facilitate aerosol growth. The role of
organic acids in catalyzing the hydration of several aldehydes
including formaldehyde (HCHO), 19,21,31 acetaldehyde
(CH3CHO),20 ketene,22 and glyoxal ((HCO)2)23 to form
diols, triols, and tetrols has been analyzed. Here, the results for
the hydration of HCHO, CH3CHO, and (HCO)2 are
presented.
HCHO is the simplest aldehyde and the most abundant
carbonyl compound in our atmosphere.36 The reaction
between HCHO and H 2 O produces methanediol
(CH2(OH)2). This reaction is not only important in the fields
of chemistry and biochemistry, but is also atmospherically
relevant because CH2(OH)2 is the smallest diol and diols have
been implicated in aerosol growth.19,23 Williams et al.37
reported the first theoretical study on the uncatalyzed and
H2O-catalyzed hydration reaction in the gas and aqueous
phases. The calculated barrier for the uncatalyzed reaction in
the gas-phase was 42.2 kcal/mol relative to separated reactants,
which was reduced to only 0.8 kcal/mol in the presence of an
additional H2O molecule. The inclusion of entropy effects
This interaction, which is shown in the Figure1b, contributes
3.9 kcal/mol toward the second order stabilization energy.
Figure 1c shows the n → σ*OH interaction for the second
hydrogen bond in the methanediol···formic acid complex, in
which the diol acts as a Lewis base and HCOOH acts as a
Lewis acid. In this case, the nσ lone pair of oxygen in diol
interacts with the σ*OH of the HCOOH monomer. The nσ →
σ*OH interaction contributes 23.0 kcal/mol toward the overall
second order stabilization energy of the complex. The NBO
charge on O6 is 0.13 unit more negative than on O10.
Moreover, H12 is bonded to relatively less negative O11. Both
these factors account toward the larger second order energy for
the interaction shown in Figure 1c.
3. ATMOSPHERIC CHEMICAL PROCESSES UTILIZING
DOUBLE HYDROGEN ATOM TRANSFER
Many atmospheric reactions involving hydrogen atom transfer
can be broadly classified on the basis of structural changes
occurring in the reactant molecule. This classification does not
require knowledge of reaction path or mechanism. The three
main reaction classes discussed below are addition, elimination/
decomposition, and rearrangement/isomerization.
a. Addition Reactions
Water vapor is the third most abundant molecule in the Earth’s
atmosphere, and hence, many addition reactions involve the
incorporation of a water molecule. These hydration reactions
can lead to the formation of important atmospheric molecules
that can facilitate nucleation and aerosol growth.
i. Hydration of SO3 and Sulfuric Acid Formation.
Sulfuric acid (H2SO4) is a key contributor to acid rain33 and is
known to impact atmospheric nucleation processes.34 In the
atmosphere, H2SO4 is mainly formed by the hydration of sulfur
trioxide (SO3).35 Both the uncatalyzed and the catalyzed
hydrations have been extensively examined computationally.9−12,17,18,30 Morokuma and Muguruma theoretically studied
the effect of water catalysis on the reaction and showed that a
second water molecule significantly modifies the potential
energy surface for the SO3 hydration.9 With two water
molecules, the rate-limiting step for H2SO4 formation involves
the unimolecular isomerization of the prereactive SO3···H2O···
H2O complex to form H2SO4···H2O. Various calculations have
estimated the barrier for this step to be in the range between
∼6.6−13.0 kcal/mol.10−12 The presence of additional water
molecules has been shown to further reduce the hydration
barrier, with four or more water molecules effectively making
the reaction barrierless.12
Since the basic mechanism of water catalysis involves
shuttling of a hydrogen atom between H2O and SO3, carboxylic
acids, which contain carbonyl and hydroxyl functionalities and
are present at the parts per billion level in the troposphere,7
may also catalyze the SO3 hydration. Hazra and Sinha recently
examined the formic acid(HCOOH)-catalyzed gas-phase
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carbonyl compound, HCHO, under H2O catalysis. The barrier
for the H2O-catalyzed NH3 reaction is 8.5 kcal/mol, whereas
the analogous CH3NH2 addition has only a 0.1 kcal/mol
barrier. With (CH3)2NH, the transition state is 5.4 kcal/mol
submerged below reactants. Thus, by tuning the R group on the
amine nitrogen, the barrier can be varied over a range of ca.14
kcal/mol. The H2O-catalyzed (CH3)2NH addition to HCHO
has a rate constant of 1 × 10−6 s−1 under tropospheric
temperatures (200−300 K), which is only 1/60th of the
bimolecular reaction rate for the OH + (CH3)2NH reaction (6
× 10−5 s−1). This makes the H2O-catalyzed reaction an
important loss process for the atmospheric (CH3)2NH at night
when OH levels are quite low.
significantly reduced the effect of H2O catalysis; the barrier for
the H2O-catalyzed reaction was only reduced by 27 kcal/mol as
compared to the uncatalyzed reaction (ΔG⧧ = 53.4 kcal/mol).
This led to the conclusion that the catalytic effect of two or
more H2O molecules would be nominal.
Hazra et al. recently examined the HCHO hydration in the
absence and presence of H2O and HCOOH.19 The uncatalyzed
and H2O-catalyzed reactions have electronic barriers of 39.2
and 23.1 kcal/mol, respectively at the MP2/6-311++G(3df,3pd) level of theory. The calculated barrier for the
HCOOH-catalyzed reaction is dramatically lowered to 9.8 kcal/
mol. Moreover, the transition state is only 1.0−1.5 kcal/mol
above the energy of HCHO···H2O + HCOOH reactants,
implying that the acid-catalyzed hydration of HCHO could lead
to the facile formation of diol under gas-phase conditions. This
new mechanism may also be relevant in cold water-rich
interfaces such as ice and atmospheric aerosols where the
probability of HCHO···H2O complex formation and its
subsequent interaction with HCOOH is highly likely.
Rypkema et al. also examined the gas-phase hydration of
CH3CHO with and without catalysts.20 CH3CHO is an
important contributor to the budget of ozone, HOx,38 and
peroxyacetyl nitrate,39 and its photochemical tautomerization to
vinyl alcohol can serve as a source of organic acids in
troposphere.40The barrier for the uncatalyzed CH3CHO
hydration is 36.9 kcal/mol. This is substantially reduced in
the presence of a catalyst. The H2O-catalyzed reaction has a
barrier of 15.5 kcal/mol, whereas the HCOOH-catalyzed one
occurs barrierlessly. Additional calculations with other carboxylic acids indicate that the barrier to the catalyzed hydration
correlate strongly with the pKa of the acid, providing useful
insight into the predictive capacity of the effectiveness of acid
catalysts. A qualitative kinetic analysis indicates a lifetime of
years for the CH3CHO···H2O complex against the HCOOHcatalyzed reaction, which is suggestive of the fact that these
reactions are unlikely to have any atmospheric impact in the
free gas-phase. These reactions may, however, be important on
surfaces and in the bulk-phase droplets, where the stability of
pre- and postreactive complexes would be enhanced by
additional stabilization through hydrogen bonding from the
solvent cage.
Finally, (HCO)2 is the simplest α-dicarbonyl and is produced
in the atmosphere through the oxidation of biogenic and
anthropogenic oxygen species.41 The two carbonyl groups of
(HCO)2 can be hydrated in a stepwise manner leading to the
formation of (HCO)2-diol and(HCO)2-tetrol, which may play a
role in the formation and growth of secondary organic aerosols.
The calculations suggest that the catalytic effect of a HCOOH
molecule on the diol formation is enormous: the barriers for
the two possible reaction channels, (HCO)2···H2O + HCOOH
and (HCO)2 + H2O···HCOOH, are only 0.5 and 1.5 kcal/mol,
respectively.23 This implies that HCOOH catalysis could be a
viable mechanism for the (HCO)2 hydration in H2O-restricted
environments.
iii. Addition of Amines to Carbonyls and Carbinolamine Formation. Apart from water, amines can also
participate in addition reactions. The water catalyzed reaction
between an amine (R1NH2) and a carbonyl compound
(R2CHO), leading to carbinolamine R1NHC(R2)(OH) formation, is speculated to be important for certain aerosol
growth.42−45A recent theoretical study14 calculated and
compared the energetics of the reaction between the simplest
amines, NH3, CH3NH2, and (CH3)2NH, and the simplest
b. Radical Isomerizations
Methoxy radical (CH3O) reactions play an important role not
only in atmospheric and combustion chemistry, but also in lowtemperature matrix reactions in radiation chemistry.46 In the
atmosphere, CH3O is mainly produced by the oxidation of
hydrocarbons.47 Because of its gas-phase profile, the isomerization of CH3O into CH2OH has been extensively studied
using high-level ab initio theoretical calculations.31,46,48 Buszek
et al. examined this isomerization in the catalytic presence of
H2O, HCOOH, and H2SO4.29 Though all catalysts lowered the
isomerization barrier, HCOOH and H2SO4 produced better
catalysis than H2O because of their ability to form sterically
more favorable transitions states. The calculated rate constants
for HCOOH and H2SO4-catalyzed reactions are 10 and 12
orders of magnitude larger than the H2O-catalyzed one,
implying that acid catalysis provides an efficient mechanism
for making the radical isomerization energetically accessible
under atmospheric conditions.
The HOCO radical is another important species in
combustion and atmospheric environments. It is formed as
an intermediate in the oxidation of CO through its reaction
with OH. The existence of HOCO radicals was first
experimentally verified through its detection in low-temperature matrices.49 Theoretical calculations and spectroscopic
measurements confirmed that the HOCO radical exists in two
stable conformers: trans and cis. High-level ab initio calculations
showed that the potential well for HOCO is 30 kcal/mol
submerged below the separated HO + CO reactants.50,51 The
estimated barrier for the trans-HOCO → cis-HOCO
interconversion is ∼7−10 kcal/mol depending upon the
theoretical method used.51 Among the two conformers, transHOCO is predicted to be more stable. A recent study27
examined the effect of acids on the relative stability of the
HOCO conformers as well as on the barrier for their
interconversion in the gas-phase. These calculations found
that though cis-HOCO forms better complex with acids than
trans-HOCO, it would be readily interconverted into the more
stable trans-HOCO in acid-rich environments. This may
explain why the experimental characterization of cis-HOCO
in acidic conditions has been a challenge.52
c. Alkyl Peroxy Radical Decomposition
Alkyl peroxy radicals (RO2) are key intermediates in the
hydrocarbon oxidation processes.53A detailed knowledge of
peroxy radical chemistry is crucial for understanding the
oxidative capacity of atmosphere.54 The decomposition of RO2
radical, which yields the HO2 radical as one of the products,
could be an important degradation pathway in troposphere
because of its role in O3 cycle and hydrogen peroxide
formation. The hydroxymethylperoxy radical, HOCH2O2 is
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5. CONCLUSION AND OUTLOOK
The studies surveyed in this Account show that acid-catalyzed
double hydrogen transfer reactions occur across the major
classes of atmospheric reactions. There is an underlying theme
that runs through these reactions involving the commonality of
the potential energy surfaces for the double hydrogen transfer.
Traditionally, a chemical reaction is viewed as the passage of
reactants from a minimum energy state through a transition
state to the minimum energy state of products. The effect of
water or acids in the HAT reactions is found to be catalyzing.
Changes in the potential energy surface topography under
catalysis illustrate how lowering the barrier heights can have a
major influence on the reaction rates. There is another unique
feature of the acid-catalyzed double hydrogen transfer
reactions; they are mediated by deep potential wells. The
prereaction and postreaction complexes involved in these
systems are significantly more stable than the separated
reactants and products due to hydrogen bonding between the
oxygenates and acids. Since these entrance/exit complexes are
the acid-stabilized forms of the reactants and products, they
could be used to trap reactive atmospheric species, which may
otherwise be difficult to detect. For example, isolated HO3 is
unstable, though when complexed with water it is readily
detectable.61,62 In another example, Liu et al.24 have reported
the direct detection of stabilized vinyl hydroperoxide formed
via the deuterated carboxylic acid-catalyzed tautomerization of
alkyl-substituted Criegee intermediates. These acid-stabilized
vinyl hydroperoxides may aid in improving our understanding
of the mechanism for OH production from olefin ozonolysis in
the troposphere.
The deep potential energy wells associated with the acidcatalyzed reactions also have potential implications for aerosol
formation. Recent reports17,18,30 indicate that the H2SO4
dimers play an important role in atmospheric aerosol
formation. Moreover, the nucleation of H2SO4 is enhanced in
the presence of aromatic carboxylic acids.63 Recent theoretical
calculations17,18,30 show that the postreaction complexes
involved in the autocatalytic or acid-catalyzed hydrolysis of
SO3 are ∼20−29 kcal/mol more stable than the separated
reactants. This makes the subsequent reaction, which may
occur between H2SO4···H2SO4 or its hydrates or H2SO4···
HCOOH and SO3, favorable and hence a potential contributor
toward aerosol growth in the atmosphere.
an example of the RO2 radicals and its decomposition results in
the HO2 radical (eq 1).
HOCH 2O2 → HO2 + HCHO
(1)
15
Recent electronic structure calculations investigated this
reaction in the absence and presence of water-, water dimer-,
the HO2 radical- and the water·HO2 radical complex. The
calculations revealed that the bimolecular decompositions
involve relatively larger barriers than the uncatalyzed one, and
the HO2 radical is generated in the complexed state that must
be released into the atmosphere. But, interestingly, the
prereaction and postreaction complexes that are more stable
than separated reactants and products respectively, mediate
these reactions. In these complexes, the peroxy radical and
catalyst are stabilized via H-bonds. The key H-bonds are O−H
H-bonds, which result in the binding energies of 7−18 kcal/
mol for these complexes. Vibrational overtone excitation of any
reagent’s X−H (X = O, N, S, and C) bond can give rise to an
increase in the rate coefficient of many orders of magnitude.55
The calculations revealed that the overtone excitation of vOH ≥
1 would be sufficient to make the HO2 radical- and the H2O·
HO2 radical complex-mediated decompositions feasible under
atmospheric conditions. Alternatively, the electronic excitation
of the organic peroxy species in these complexes occur in the
1044−1182 nm range that might also provide sufficient energy
for the decomposition. Results from this study suggested, for
the first time, an important chemical role of the H2O·HO2
radical complex that exist in significant abundance in the
troposphere.56 These results are expected to help resolve the
long-standing discrepancy between measurements and model
calculations on the photolytic source of the HOx radical at high
solar zenith angles.57
4. WATER-MEDIATED RADICAL-MOLECULE
REACTION
The reaction of hydrochloric acid (HCl) with the OH radical
reactivates chlorine radicals, which destroy stratospheric O3.
The experimental rate constant for the reaction fall in the range
6.8−8.5 × 10−13 cm3 molecule−1 s−1 at room temperature. The
value recommended by the NASA panel for Data Evaluation58
is 7.8 × 10−13 cm3 molecule−1 s−1 at 298 K with an uncertainty
factor of 1.1. Theoretical calculations59,60 indicate that the
reaction has a barrier of 2.4−2.6 kcal/mol and the rate
constants of 7.9 × 10−13 and 7.8 × 10−13 cm3 molecule−1 s−1,
calculated within conventional59 and variational60 transition
state theory frameworks, respectively.
A recent study examined the effect of H2O catalysis on the
chlorine reactivation.13 The calculations suggests that the
reaction barrier is reduced from 2.1 to 1.8 kcal/mol in the
presence of a H2O molecule, and two new reaction channels
open up at even lower energies: 4.2 and 4.4 kcal/mol below the
energy of the reactants. These new channels differ in the
orientation of the pendant hydrogen of H2O. The possibility of
two additional pathways arises from the fact that the three-body
reaction, HCl + OH + H2O occurs via three distinct pathways:
(i) OH···HCl + H2O → Cl + 2H2O, (ii) H2O···HCl + OH →
Cl + 2H2O, and (iii) HCl + H2O···OH → Cl + 2H2O. The
pathways (i) and (ii) correspond to the uncatalyzed and the
H2O-catalyzed reactions, whereas the pathway (iii) involves a
direct HAT between the OH, which is hydrogen-bonded with
H2O, and HCl, leading to the two lowest energy reaction
channels. Interestingly, H2O does not directly participate in the
pathway (iii), but stabilizes the radicals via hydrogen bonding.
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected].
*E-mail: [email protected].
Notes
The authors declare no competing financial interest.
Biographies
Manoj Kumar obtained his Ph.D. with Prof. P. M. Kozlowski at the
University of Louisville in 2012. He is currently working as a
postdoctoral researcher with Prof. Joseph S. Francisco.
Amitabha Sinha obtained his BS degree from Case Western Reserve
University and Ph.D. from MIT. After postdoctoral studies at
NOAABoulder and the University of WisconsinMadison, he
joined U.C.San Diego in 1992.
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Joseph S. Francisco is currently Elmer H. and Ruby M. Cordes Chair
in Chemistry and Dean of the College of Arts and Sciences at the
University of NebraskaLincoln.
■
ACKNOWLEDGMENTS
We thank Holland computing center of the University of
NebraskaLincoln for providing computing resources. A.S.
thanks the UCSD Academic Senate for support of his portion
of the work.
■
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