# Sunlight-Initiated Photochemistry: Excited Vibrational States of Atmospheric Chromophores

Authors: Veronica Vaida; Karl J. Feierabend; Nabilah Rontu; Kaito Takahashi
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

Atmospheric chemical reactions are often initiated by ultraviolet (UV) solar radiation since absorption in that wavelength range coincides to typical chemical bond energies. In this review, we present an alternative process by which chemical reactions occur with the excitation of vibrational levels in the ground electronic state by red solar photons. We focus on the O–H vibrational manifold which can be an atmospheric chromophore for driving vibrationally mediated overtone-induced chemical reactions. Experimental and theoretical O–H intensities of several carboxylic acids, alcohols, and peroxides are presented. The importance of combination bands in spectra at chemically relevant energies is examined in the context of atmospheric photochemistry. Candidate systems for overtone-initiated chemistry are provided, and their lowest energy barrier for reaction and the minimum quanta of O–H stretch required for reaction are calculated. We conclude with a discussion of the major pathways available for overtone-induced reactions in the atmosphere.


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## Body

## 1. Introduction

The earth’s temperature, climate, and chemistry are determined by the balance of incoming
shortwave solar radiation and outgoing longwave infrared (IR). The emission of
the sun is modeled as a blackbody at 5800 K, with a maximum photon output in the
visible red [1]. Absorption by atmospheric
components, notably ozone, limits
the ultraviolet radiation reaching the earth’s surface to wavelengths greater than 300 nm. In
this review, we consider alternative photophysical and
photochemical processes driven by solar photons in the visible red and near IR [2].Traditionally, processes considered in atmospheric photochemistry are those requiring
excitation of electronic states by ultraviolet (UV) light as these high
energies are of the necessary energy to dissociate covalent bonds [3]. Reactive radicals including
hydroxyl radical, OH, the main oxidant of organic and inorganic compounds in
the earth’s atmosphere can be generated in this way [4, 5]. Chemistry in the
stratosphere and troposphere is, therefore, driven by direct
photochemical reactions or by photolytically generated OH originating from
photolysis of O3 to give O(D1) which reacts with H2O
and CH4 to give OH.In this review, we discuss a sunlight-initiated
process which occurs by excitation with red solar photons of vibrational levels
of the ground electronic state of atmospheric chromophores [2, 6]. It has recently been shown
that these ground electronic state photoreactions can be important in the
atmosphere when excited electronic
state reactions are precluded by the lack of appropriate electronic states
accessible in the solar spectrum [7–16]. Atmospheric conditions,
where UV photons are suppressed as is the case at high zenith angle which occurs at dusk, dawn, or the edge of a
polar vortex, are also auspicious for such ground state photochemistry [17–32]. Atmospheric chemists describe the rate of formation of photoproducts from
atmospheric processing as a first-order rate constant J (J-values), where J is
given by a convolution of the photoabsorption cross-section, the wavenumber
dependent photochemical quantum yield, and the photon flux of solar radiation [3]. In the present review, the experimental and
theoretical methods used to obtain accurate integrated O–H cross-sections for
vibrational transitions and the atmospheric impact of ground electronic state
chemistry will be discussed.This overtone-induced chemistry is important in
that it contributes to atmospheric aerosol processing. In general, hydrophilic
acids and alcohols promote aerosol nucleation and growth. The overtone-induced chemistry
leads to the production of hydrophobic byproducts such as CO2,
aldehydes, and ketones, which can evaporate, and therefore process these
aerosols. The resulting changes in aerosol size, composition, and surface
structure are paramount to understanding radiative transfer and consequently
climate.Our discussion focuses on sunlight-initiated
chemistry of significance to secondary organic aerosol (SOA) chemistry and
consequently to chemistry relevant to the earth’s climate. We propose
photochemistry of excited vibrational levels of the ground electronic state as
potential contributions to explain the gap between modeled and measured organic
concentrations [33–35]. Atmospheric aerosols influence global
radiative forcing by back scattering and/or absorption of solar radiation and
by enhancement of cloud albedo due to an increased number of cloud droplets [36].  Current aerosol models underestimate SOA mass
by up to two orders of magnitude [37–39]. Atmospheric oxidation processes are largely driven by oxidants such as
OH and O3. However, direct photolysis processes are also important
degradation pathways of potential SOA precursors and contributors. Photolysis
processes of a variety of highly oxidized organics might represent an
unrecognized chemical pathway in the multiphase (gas/particle/droplet) system
that might help to explain the large gap between measured and modeled organic
concentrations.


## 2. Vibrationally Excited States

The vibrations of polyatomic molecules are
often represented as normal modes [40]. In the normal mode picture, the potential
energy surface is expanded to the second-order term at the minimum, and the
mass-weighted Cartesian force constant matrix is diagonalized to give the normal
modes. These normal modes are sets of concerted, collective, and harmonic
motions of all the atoms in the molecule with a fixed center of mass.
Vibrations are uncoupled, and each type of vibration possesses its own distinct
harmonic potential energy well. Spectra in the fundamental region have been assigned
successfully using this normal mode picture assuming a linear dipole moment surface
in these coordinates [40]. However, this double harmonic approximation
(harmonic potential energy surface and linear dipole moment surface) becomes
increasingly invalid for treatments of transitions to higher vibrational states
[41–43]. In real molecules, the mechanical and
electrical anharmonicities (the anharmonic terms of the potential energy
surface and the nonlinear terms of the dipole moment surface, resp.) relax the Δv=±1 vibrational selection rule and allow for
the observation of transitions involving multiple quanta in one vibrational
mode (i.e., overtone bands) or multiple quanta in two or more differing vibrational
modes (i.e., combination bands). However, in most cases, these anharmonicities
are relatively small, and, in general, overtone and combination bands are weak
and not easily observed.In the energy regime of about 30 to 60 kcal/mol, observations have shown that
the X–H (X=O, C, N) stretching overtones
dominate the ground electronic state vibrational spectra. These X–H stretching
vibrations have been studied and assigned successfully using the local mode
model [42–53]. Unlike in the normal mode view, local
oscillators are considered to be weakly coupled and each X–H oscillator is
treated independently in the local mode description. For example, the dominant
absorption in the overtone spectra of the water molecule is better described
with a linear combination of two independently oscillating O–H bonds (in the
local mode picture) instead of considering a high excitation in either
symmetric or asymmetric O–H stretching modes (in the normal mode picture). Albeit
weak, these X–H stretching overtones allow for a method of depositing energy into
a molecule by the absorption of sunlight, and thus may lead to reaction [54–60]. As mentioned above, sunlight has maximum photon
intensity in the visible region, where these overtone transitions are observed.
Therefore, in order to assess the importance of overtone-initiated chemistry,
it is important to understand the absolute absorption cross-section of these
X–H stretching overtones.In the mid 1980s, Crim et al. reported overtone-initiated
unimolecular decomposition for the hydrogen peroxide (HOOH) molecule, where the
O–H stretching excitation ofΔvOH=5 (+ torsion mode excitation) and 6 resulted
in a bond cleavage of the O–O bond (De∼50 kcal/mol) [61–63]. Their wavelength dependent product state
distribution showed initial state dependence. Slight differences in the product
distribution of OH were seen when the combination band of the O–H stretch
overtone and the torsion mode was excited compared to the excitation via pure O–H
stretch overtone. Following their work, overtone-induced photodissociation
reactions have been investigated in several molecules with the O–O–H bond, such
as peroxynitric acid (O2NOOH) [28, 32], peroxynitrous acid (ONOOH) [20, 64–66], methyl hydroperoxide (CH3OOH) [27, 67], as well as hydrogen trioxy radical (OOOH) [68, 69]. Furthermore, systems with weak bonds adjacent
to the O–H bond, such as N–O (De∼55 kcal/mol) in nitric acid (O2NOH)
[70, 71] and O–Cl (De∼43 kcal) in ClOH [72–75], have also been investigated as possible
candidates to dissociate following O–H stretching overtone excitation.The O–H bond with its strong polarity usually
results in the strongest absorption within the X–H stretching spectra (per X–H
bond) for a given quanta of excitation; thus, it is a good absorbing
chromophore for the deposition of photon energy into the molecule. For example,
the ratio between the average intensities for the O–H bond versus the C–H bond
is about 4:1 forΔv= 3, 4.5:1 for Δv= 4, and 6:1 for Δv= 5 [23, 76, 77]. Therefore, polyatomic molecules with O–H
bonds are candidates for absorbing solar red photons and reacting in the
atmosphere [2, 7, 9, 14, 15, 17, 22, 23, 26, 65, 78–81]. Since greater effort is needed to obtain an
absolute integrated cross-section in comparison to the peak position, only a
few papers report both [82–84]. The weak absorption intensities of these relevant
overtones also make it more difficult to obtain absolute values both
experimentally and theoretically. In the following work, we compile the
available experimental and theoretical integrated cross-sections of the O–H
stretching mode for several polyatomic molecules. A comparison on the molecular
dependence as well as an investigation of the accuracy of the calculated
values, which are usually obtained using the local mode model, will be
presented. In addition, abiding with the possibility that excitation through
combination bands may play an important role in the unimolecular reaction dynamics,
combination bands in nitric acid are examined as an example. Furthermore, calculated
reaction barriers of possible candidates for overtone-initiated reaction will
be presented. The possibility of overtone-induced concerted reactions, where
the excited O–H bond is strongly coupled to the reaction coordinate, will be
discussed [7, 16, 18, 85]. These reactions will be contrasted with
previous experimental results of overtone-induced dissociation reaction, where
the energy in the excited O–H bond is transferred to rupture a weak adjacent
bond [2, 17, 23, 24, 26, 30, 57, 64, 65, 70, 71, 78, 86, 87]. Finally, we will conclude with the discussion of the photochemical rate (J-values)
and important points to note for discussing these overtone-initiated reactions
in the atmosphere.


## 3. O–H Stretching Spectra and the Local  Mode Model

Due to the light
mass of the hydrogen atom, X–H stretching vibrations have considerably higher
frequencies than other vibrational modes. Also, the large difference in the
mass between the X (O atom in the present
case) atom and H atom allows for this vibrational mode to have relatively small
coupling with other modes. In the late
sixties, Siebrand and Williams [45], Swofford et al. [88], Henry [42], Burberry et al. [89], and Mortensen et al. [90] observed the following trends
in the X–H stretching spectra.First,
the fundamental and overtone transition energies of the X–H stretching spectra
were shown to fit to a two-parameter equation:(1)ΔEv0=v(A−Bv), where ΔEv0 is the transition energy between the ground
and vth excited state. This equation,
sometimes regarded as Birge-Sponer equation [91], is the solution of a
one-dimensional Morse oscillator [92] for diatomic molecules, where
the harmonic term is(2)ωe=A+B and the anharmonic term is
expressed as(3)χeωe=B. Second, Burberry et al. [89] observed the Δv=3−6 aromatic C–H stretching spectra of benzene, toluene,
xylene, and trimethyl benzene and reported that while the peak positions did
not vary, the absorption intensity showed systematic decrease in relation to
the number of aromatic C–H bonds.These
observations lead to the phenomenological “local mode model”, where the
vibrational spectra are understood in terms of localized excitation in the X–H
bond. For the O–H bond, both experimental and theoretical calculations have
shown that the peak positions can be given by a harmonic frequency of(4)ωe≅3700−3900cm−1 and an anharmonic frequency of(5)χeωe≅70−90cm−1. Therefore, to exceed the reaction
barriers of 30–60 kcal/mol discussed in the present review, overtone excitations of ΔvOH= 3–6,
which reside in the visible to near IR-region, need to be considered. Although we will not tabulate the actual
values here, recent high-level calculations give peak positions with errors of
less than few percent [51, 77, 93].


## 4. Intensities of the O–h Stretching Mode

First, we present a brief summary of the experimental and theoretical methods used to obtain
absolute integrated cross-sections before discussing the results that have been collected.A combination of methods is usually needed to
obtain experimental intensities for allΔvOH= 1–5 transitions. Use of Fourier
transform infrared spectroscopy (FTS)
easily provides data for the fundamental and near-IR regions including ΔvOH= 1, 2, and 3, while methods such as
cavity ring-down (CRD) [94–98] or photoacoustic (PA) [99–105] spectroscopy grants access to
higher overtones into the visible region of the spectrum such as ΔvOH= 4, 5. Each method has its advantages and
disadvantages. Fourier transform
spectroscopy, for example, has high wavenumber resolution as well as the
Jacquinot advantage, enabling higher throughput of radiation [106]. Unlike conventional or laser
spectrometry where one must scan the full
wavenumber range, FTS information
on all frequencies simultaneously impinge upon the detector and the
entire spectrum of the species is recorded at once. In contrast to
laser spectroscopy such as CRD and PA,
FTS may be ∼106 less
sensitive due to its low-photon flux and the difficulty of measuring a small
difference between the light intensity with and without an absorber present. Increased
sensitivity, necessary for the detection of higher overtones due to their
diminishing intensity, is the main advantage of CRD and PA spectroscopy [24, 94, 95, 97, 98, 101–105, 107].
The use of highly reflective mirrors employed in the CRD setup in our lab, for
example [8, 10, 18, 108], increases the pathlength of
the experiment to orders of kilometers, thereby increasing the sensitivity by
at least a factor of 14000 in comparison to FTS experiments. However, CRD spectroscopy is limited in wavenumber range by the laser dye and by the mirrors that can be used. PA spectroscopy provides comparable
sensitivity to CRD spectroscopy and is also limited in range by the laser dye.
It provides the advantage of using small sample volume due to its ability to
detect at parts per trillion levels.A challenge often
encountered in determining experimental cross-sections, regardless of
spectroscopic method (FTS, CRD, or PA), is obtaining an accurate number
density. This value is usually obtained
using the available vapor pressure data and the ideal gas law. The accuracy of
vapor pressure measurements is low for compounds which readily form hydrogen-bonded
clusters or hydrates. However, vapor pressures inside the sample chamber
can readily fluctuate such that equilibrium
in the chamber may be achieved on
a different timescale by which point some sample can potentially be lost to the
atmosphere, and temperature effects may
cause the sample to degrade by thermal reaction. Often times, only
relative intensities are reported in the literature due to difficulties met in
determining an absolute cross-section.For the
theoretical calculation of the peak positions and integrated cross-sections,
one must first obtain the potential energy surface (PES) and the dipole moment
surface (DMS) from quantum chemistry ab
initio calculations. With
this PES, the vibrational eigenvalues and eigenfunctions are calculated by
solving the vibrational Schrödinger equation. Local and normal mode pictures result in different coordinate systems,
thereby giving different equation forms for the vibrational Schrödinger equation. The peak
position or transition energy is obtained from the difference between the respective initial and final state
energies. The integrated cross-section is obtained by [109](6)Scalc=8π33hc|〈Ψf|μ→|Ψi〉|2ν˜=4.16×10−19|〈Ψf|μ→|Ψi〉|2ν˜(cm molecule−1), where ν˜ is the transition energy in cm−1,
and |〈Ψf|μ→|Ψi〉|2 is the square of the absolute magnitude of the
transition moment in debye squared. The transition moment is obtained by the
integration of the aforementioned DMS
using the initial and final state eigenfunctions. It should be noted that there are numerous quantum chemistry methods that
can be used to calculate the PES and DMS ranging from the relatively “cheap” wave
function method of Hartree Fock [110, 111] and density functional theory methods [112], such as B3LYP [113, 114], to the “expensive” wave function methods,
such as QCISD(T) [115–117], CCSD(T) [116, 118–120], and MRSDCI [121]. In using these methods to obtain the PES/DMS
needed to calculate highly excited vibrational states, one needs to calculate
geometries that are displaced from the equilibrium geometry. At these geometries,
such as elongated X–H bond lengths, multireference methods are necessary to
accurately describe the electronic state, and instability may occur with the
use of single reference methods.Many different methods
have been used to solve the vibrational Schrödinger
equation, and one of the usual methods is the variational method where one diagonalizes the Hamiltonian matrix using a given basis set [122–124]. Quack [51] used harmonic oscillator
basis sets to obtain the eigenfunctions for the CH stretching overtone of CX3H.
Methods such as vibrational self-consistent
field [125] and their higher level expansions VCI [126] and VCCSD [127] have also been used for the calculation of OH
stretching overtone transitions of nitric acid [128]. Furthermore, several methods solve the time-dependent Schrödinger equation and obtain the
spectra from the Fourier transform of the autocorrelation function of the
wavepacket [129]. For the calculation of X–H stretching modes,
Henry et al. developed the harmonic coupled anharmonic oscillator model, where
the PES of the X–H stretching
motion is approximated as a Morse potential
and Taylor’s expansion fitting of the ab
initio dipole moments are used to
obtain the DMS as a function of bond
coordinates [90, 130].
Initially, experimental peak positions
were used to obtain the needed parameters for the Morse oscillators, but in
recent calculations these parameters are obtained from the fitting of ab initio energies. Recently, some of us used the local mode model and
calculated the intensities of the O–H stretching spectra for several simple
acids and alcohols using the grid variational method with the finite difference
approximation [131]. In this calculation, the PES and DMS were
obtained from the highly accurate numerical interpolation of the energy and
dipole moment calculated by B3LYP/6-311++G(3df,3pd)[113, 114, 132–136].A summary of the
O–H stretching intensities for
various alcohols, carboxylic acids, and peroxides is presented in Figures1 and
2 which illustrate the trend in O–H stretching integrated cross-sections as a
function of upper quantum number [8, 18, 20, 24, 28, 67, 76, 78, 82–84, 102, 137, 138, 139, 140]. Typically, intensities of the O–H stretches
are comparable for most organic compounds. Figure 1 summarizes the experimental
results from a variety of sources, [8, 18, 24, 78, 83, 102, 137, 141]
including work from our group, while
Figure 2 compiles theoretically derived integrated cross-sections [77, 131]. It should be noted that these figures list molecules in which both
experimental and theoretical values are available. Furthermore, as mentioned
above, many different quantum chemistry methods are available for the
calculation of the PES and DMS, and the same applies for solving the
vibrational problem. In order to keep the comparison consistent in terms of
calculation method, the results given in Figure 2 are given by the grid
variational method using the PES and DMS calculated by the B3LYP/6-311++G(3df,3pd)
method. In addition, due to the limits in the experimental methodology
mentioned above, for most molecules we were unable to collect literature values
for all ΔvOH= 1–5 transitions.Figure 1
Compilation of experimental integrated cross-sections of
O–H stretching overtones.Figure 2
Compilation of theoretical O–H stretching integrated
cross-sections.Both Figures1 and
2 illustrate that the O–H stretch preserves
its intensity even though it decreases with each overtone. The
“order-of-magnitude” rule [61], which states that the
intensities of O–H stretching vibrational transitions successively decrease in
magnitude by approximately a factor of 10, is illustrated in both figures by a linear decrease in log scale. This
trend has also been observed in C–H and N–H stretching transitions. In general,
the “order-of-magnitude” rule works best for higher energy overtones, and one sees great variance when comparing the
fundamental and first overtone intensities. For example, Figure 1
reveals that the drop in intensity for sulfuric
acid from the ΔvOH= 1 to the ΔvOH= 2 transition is 28 [137], while that for propanol is 4.7 [82]. Previously, we have shown that for propionic and pentafluoropropionic acid, this factor drops by
about 14 [9, 79] and 18 for longer chained perfluorocarboxylic acids [15].As shown in Figure1, intensities for the ΔvOH= 1 vary over an order of magnitude, from 1.54×10−18 cm molec−1 for tert-butanol [82] to 1.49×10−17 cm molec−1 (per O–H stretch) for sulfuric acid [137]. Upon observation of several simple acids and alcohols, Lange et al. [82] have discussed the
correlation of the fundamental O–H stretching intensities with the electron withdrawing
ability of the substituent bound to the O–H group. Theoretical work on the same set of molecules also showed the same trend
that the stronger the electron withdrawing ability of the substituent, the
greater the fundamental intensity [77]. Lange et al. [82]
also found that molecules with
more electron withdrawing substituents have a larger decrease in successive O–H
overtone intensities relative to the fundamental O–H stretch. As can be seen from Figures 1 and 2, overtone
intensities show much smaller variance between molecules than fundamental
intensities, and this trend seen by Lange et al. [82] is probably the result of variance in the
fundamental intensity. For the C–H stretching vibration, Burberry et al. [89] have mentioned the “universal intensity concept”
[142], in which the ΔvCH= 3, 4, 5, and 6 transition absorption intensities
per C–H oscillator become nearly equal regardless of type of molecule. A similar
trend is seen for the O–H bond, where overtone intensity loses its molecular
dependence, but it should be noted that in peroxide molecules (ROO–H), the
theoretical results show stronger overtone intensity compared to the acids and
alcohols (R–OH) [77]. Experimental results do not show clear trends
as discussed and more study is needed to obtain a full understanding of the
factors that determine overtone intensities.As mentioned above, the intensity trend seen in
the fundamental region is different from that seen in the overtones region. As
seen in Figures1 and 2, the O–H cross-section of sulfuric acid has the largest intensity at ΔvOH= 1 compared with the compounds
investigated here, but decreases with increasing vibrational quanta more
rapidly until possessing the smallest value at ΔvOH= 5. We
believe this behavior is a result of the onset of dehydration reaction and Section 6 addresses this in detail.Since theoretical values are used when experimental
limitations (low vapor pressure, no laser in the absorption range, and so on)
make it impossible to obtain the integrated cross-sections experimentally, it
is important to gauge the accuracy of these calculations. Methods such as mean
absolute percentage error and logarithmic deviation have been used to evaluate the accuracy of the
calculated versus the experimental values since they span many orders of
magnitude [122]. In the present paper, we compare the
accuracy by using the mean absolute percentage error:(7)MAPE=1Ndata∑i=1Ndata|Aicalc−AiexpAiexp|. Using all the available
experimental values shown in Figure 1, we obtain an error of 45%, signifying that the calculated results are within a factor of 2 of the
experimental values. It should be noted that if we only use the
fundamental intensities, the error is a much smaller value of 15%, signifying that the experimental values are
reproduced with high accuracy. The use of a more sophisticated and time-consuming
quantum chemistry methods to obtain the PES and DMF, as well as a more detailed
treatment in the vibrational calculation
(such as consideration of coupling to other vibrational modes) will
probably lead to a slightly better agreement with experiment, but it is
expected that at the present moment the error limit for the calculated
integrated cross-sections for overtone
transitions is about 30%. In
systems where the relative intensities of the overtones with respect to the
fundamental transition may be obtained experimentally, a hybrid method of using the theoretical fundamental
intensity and the experimental relative intensities may be used to obtain an
absolute cross-section for the overtones. As mentioned above for the
fundamental transition, the theoretical values can reproduce experimental
values with high accuracy.


## 5. Significance of Combination Bands

As mentioned in the introduction, product
distributions of the photoinduced dissociation of HOOH showed initial state
dependence [61–63]. It is expected that near the dissociation
threshold, the dissociating product may show mode dependence on the band used
to excite it. Therefore, Homitsky et al. [102] performed detailed analysis on the O–H stretch
and torsional excitation on the overtone and combination bands of
methylhydroperoxide. The observation of combination bands suggests mode
coupling [144] and many combination bands
involving the O–H stretching overtones of HNO3 are shown as an
example in Figure 3. Feierabend et
al. [128, 143] showed that the O–H
stretching overtones are not the only important absorbers in the near-IR (NIR) and visible regions; the
appearance of combination bands involving the O–H stretch is also prevalent in the higher energy
regions. These have fairly large absorption intensities for combination bands,
although in general the local mode model often neglects mode mixing. The
following mode assignments are made as shown in Figure 3: 2v1= first O–H stretching
overtone, v2= NO2 asymmetric stretch, v3= NO2 symmetric stretch, v4= H–ON bend, and v9= O–H torsion. A hot
band of the O–H torsional mode in combination with the first O–H stretching
overtone was attributed to the band centered at approximately 6500 cm−1.
The work of Havey et al. [145] showed that molecules which
contain many low-frequency torsional modes may have significant hot band
contributions.These hot band contributions
can add to the total intensity of the  O–H stretching
overtones region. A
surprising result of the HNO3 work revealed that the intensities of
the combination bands do not drop off in the O–H stretching overtone region.Figure 3
Illustration of combination bands
observed for HNO3 in the NIR. Assignments taken from Feierabend et al. [128, 143].Figure 4
Calculated energy barrier for sulfuric acid
dehydration.Interestingly,
the absorption intensities of these types of combination bands scale roughly
the same as the intensities of the overtones. For example, the intensity of
overtones is about the same as that of combination bands;I[nvOH/(n+1)vOH]∼I[vx+vOH/vx+(n+1)vOH], where vx is an arbitrary mode with
fundamental absorption intensity. The fact that these combination modes are
observed with significant absorption intensities in the NIR suggests a few key
ideas. First, these combination modes should not be ruled out and regarded as
bright states for overtone-induced photodissociation, especially if the
vibrational mode involved is closely coupled to the reaction coordinate. Next,
more local mode simulations have begun
treating the coupling of the O–H stretch with other modes [43, 80, 138, 146, 147]. These types of treatments are
expected to be more accurate due to the
qualitative evidence for mode coupling/mixing in the observed spectra. Finally,
while the combination modes cannot provide any direct quantitative measurement
of mode coupling, their frequencies and intensities can serve as a test for theoretical calculations. This will allow for the assessment of the
accuracy in the theoretical methods, providing information on the method to be
used for calculating the spectra of molecules in which experimental
determination is not possible.


## 6. Candidates Systems of Overtone-Initiated Chemistry

Previous studies [7] have revealed that overtone-induced photolysis
of sulfuric acid is the source of SO2 in the stratosphere and
mesosphere [12, 13]. UV photolysis of sulfuric acid was ruled out [13, 137] because the electronic excitation lies well
above the energies available from the sun in the earth’s atmosphere. Sulfuric
acid only absorbs in the IR and the near-IR regions, where the O–H stretching
vibration plays a dominant role in its atmospheric chemistry. These studies on overtone-induced photolysis of
sulfuric acid have predicted that the photochemical reaction is a concerted
dehydration reaction, where the product is H2O and SO3 rather than a bond cleavage reaction where the product would be OH and SO3H.
This concerted reaction only requires ∼32–40 kcal/mol, so, consequently, excitation
of ΔvOH= 4 and higher overtones provides sufficient
energy to drive the reaction and generate SO3 and H2O. The strong S–O (about 80 to 90 kcal/mol) [148] bond requires significantly more energy for
direct bond cleavage that cannot be provided from overtone chemistry because of
the low cross-sections expected for the 9th overtone required to reach this
threshold. Figure 3 shows the calculated energy barrier, including the minimum
and the transition state structures for the dehydration reaction of sulfuric
acid H2SO4→SO3 + H2O. It should be
noted that the occurrences of reactions may cause the absorption spectrum to
broaden, thereby making it difficult to obtain an accurate cross-section due to
the low signal-to-noise.Similar to sulfuric acid, other atmospherically
relevant compounds, specifically oxidized organic species such as acids and
alcohols, are capable of undergoing concerted reaction by overtone pumping of
the O–H bond [2, 7, 14, 16, 18, 30, 108]. Table 1 shows several organic acids, their
lowest energy barrier for reaction, the reaction products, and the minimum
quanta of O–H stretch required for the chemistry to occur [7, 14, 16, 30, 57, 149–155]. The values for the energy barrier do not
include the zero point energy corrections, therefore they are possibly higher
than the actual reaction thresholds.Table 1
Organic acids and barriers for their chemistry.
CompoundChemical formulaLowest energy barrier for reaction (kcal/mol)Reaction productsOH quanta exceeding barrierReferenceSulfuric acidH2SO432–40SO3,
H2OvOH= 4[7, 149, 150]Nitric acidHNO347.7NO2,
OHvOH= 5[57]Pernitric acidHO2NO221.2HO2,
NO2vOH= 3[30]Formic acidHCOOH41.5(a), 63.0(b)CO,H2OvOH= 5,7[151](a), [152](b)Glycolic acidOHCH2COOH50.9H2CO,
CO, H2OvOH= 6[153]Lactic acidOHCHCH3COOH47.4HCOCH3,
CO, H2OvOH= 5[153]Malonic acidHOOCCH2COOH27.6H2CC(OH)2,
CO2vOH= 3[16]Acetic acidCH3COOH67.01(c), 71.5(d)CH4,
CO2vOH= 7,8[154](c),
[155](d)Trifluoroacetic acidCF3COOH50.3CF2CO2,
HFvOH= 6[14]In general, if there is a lack of competing processes
(such as collisional deactivation), the energy provided by overtone pumping is
sufficient to drive the reaction to completion [12, 156, 157]. For example, in nitric and pernitric acid,
theoretical calculations and experimental data have shown that visible
wavelength excitation of O–H overtone vibrations contain sufficient energy to
cleave the O–O and N–O bonds [26, 158]. Similarly, the work of Staikova et al. [16] showed that for malonic acid excitation of ΔvOH≥3 has the ability to decarboxylate the acid, although experimental validation
of the reaction is not yet available [159]. In the case where a water molecule is
available and forms a complex with the acid, it lowers the energy for reaction
by one quantum of an O–H stretch [7, 16, 160]. Briefly, the presence of the water molecule
forms an eight-membered hydrogen-bonded cyclic ring with the chromophore and
relaxes the strain on the transition state as compared to the pure acid which
forms a six-membered hydrogen-bonded cyclic ring on its own. In doing so, the transition state energy for
decarboxylation of malonic acid is lowered from 25.6 to 17.6 kcal/mol [16]. The energetic difference is equal to one quantum
of an O–H stretching vibrational overtone and in theory, excitation to ΔvOH=2 can promote the reaction when a water
molecule is present. Several other systems such as sulfuric acid,
fluoromethanol, and perfluoromethanol have shown similar decrease in barrier
heights with hydrate complex formation [160]. Abundance of water in the atmosphere point to
the high possibility of the existence of hydrated complexes indicating the likelihood
of water catalyzed overtone reaction being a factor in atmospheric chemistry. These
hydrated clusters may serve as a model to mimic the environment that these
acids are in/on the surface of aerosols. More detailed experimental investigations
of these overtone-initiated concerted reactions are needed to fully assess their importance in the
atmosphere.


## 7. Conclusions

Solar radiation provides sufficient energy to drive chemical reactions in the atmosphere and
determines the earth’s climate and temperature. Typically, the UV region of the
solar spectrum (200–400 nm) is
considered for chemical reactions since electronic transitions occur in this
region and absorption in that wavelength range corresponds to normal chemical
bond energies [3]. At high solar zenith angles,
UV light is suppressed and low-energy visible radiation becomes important [2, 26]. Therefore, absorption of radiation in the
near-IR and visible regions, which are usually dominated by X–H
stretching vibrational overtones, play a significant role in determining
the atmospheric chemistry and fate of certain organic compounds [7, 14, 16, 161].Atmospheric
chemistry via O–H overtone pumping can be driven by three mechanisms. In the
first case, an O–H stretch (or any other strongly absorbing local mode
oscillator) is excited into a vibrational overtone followed by intramolecular
vibrational energy redistribution (IVR) which transfers sufficient energy into
a weaker bond in the molecule to cause dissociation [2, 22, 26, 30]. Even though the O–H stretch
itself is excited, it is ultimately a different weaker bond in the molecule
that is broken. This is the case for compounds such as nitric acid [57], pernitric acid [17, 32], and hydrogen peroxide [61–63] which all have relatively
weak bonds. A second scenario involves exciting the O–H overtone state which is strongly coupled to the reaction coordinate [2]. Strong coupling of the excited state with the reaction coordinate allows for
the concerted excitation of low-frequency modes (heavy atom motion)
causing the photoexcited O–H bond to break at energies
significantly lower than the bond dissociation energy at its equilibrium geometry. The mechanism is best described as a
concerted reaction. In sulfuric acid, for example, excitation well above vOH=6 induces the hydrogen of the O–H bond to migrate to a different location in
the molecule followed by dissociation [11]. This occurs on the timescale
of picoseconds, while the process of IVR requires a significantly longer time
on the order of nanoseconds [12]. Malonic acid is another
example in which case the reaction is also concerted [16]. Lastly, the third process is a mechanism entailing solar radiation
in sequential two-photon absorption [162]. The first photon excites a
vibrational overtone into an intermediate state, while the second photon
promotes the system to a repulsive dissociative first excited electronic state.In the atmosphere, the
excited species are under conditions, where deactivation can occur through
collisions with water and other “cold” molecules (N2, O2,
etc.). Therefore, the quantum yield will be defined by how fast the reaction
proceeds in comparison to the deactivation time defined by collisions to these “cold”
surrounding bath molecules [11, 12, 160, 163]. In discussing the time scales of overtone-induced
unimolecular reaction, a key aspect to consider is whether the reaction
proceeds directly or statistically. It would be expected that concerted
reactions which have the excited O–H bond strongly coupled to the reaction
coordinate will likely occur more directly than bond dissociation, although
this will be left for future studies. Detailed study of the reaction dynamics
following overtone excitation is needed in order to calculate the quantum yield
and accurate J-vales for overtone-induced reactions which appear to be possible
and probable in the atmosphere.



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*Source: 138091-2008-06-16.xml*