10 research outputs found
Influence of Intramolecular Hydrogen Bonding on OH-Stretching Overtone Intensities and Band Positions in Peroxyacetic Acid
Vapor phase absorption spectra and integrated band intensities of the OH stretching fundamental as well as first and second overtones (2Ī½<sub>OH</sub> and 3Ī½<sub>OH</sub>) in peroxyacetic acid (PAA) have been measured using a combination of FT-IR and photoacoustic spectroscopy. In addition, ab initio calculations have been carried out to examine the low energy stable conformers of the molecule. Spectral assignment of the primary features appearing in the region of the 2Ī½<sub>OH</sub> and 3Ī½<sub>OH</sub> overtone bands are made with the aid of isotopic substitution and anharmonic vibrational frequency calculations carried out at the MP2/aug-cc-pVDZ level. Apart from features associated with the zeroth-order OH stretch, the overtone spectra are dominated by features assigned to combination bands composed of the respective OH stretching overtone and vibrations involving the collective motion of several atoms in the molecule resulting from excitation of the internal hydrogen bonding coordinate. Integrated absorption cross section measurements reveal that internal hydrogen bonding, the strength of which is estimated to be ā¼20 kJ/mol in PAA, does not result in a enhanced oscillator strength for the OH stretching fundamental of the molecule, as is often expected for hydrogen bonded systems, but does cause a precipitous drop in the oscillator strength of its 2Ī½<sub>OH</sub> and 3Ī½<sub>OH</sub> overtone bands, reducing them, respectively, by a factor of 165 and 7020 relative to the OH stretching fundamental
Gas Phase Hydrolysis of Formaldehyde To Form Methanediol: Impact of Formic Acid Catalysis
We find that formic acid (FA) is
very effective at facilitating
diol formation through its ability to reduce the barrier for the formaldehyde
(HCHO) hydrolysis reaction. The rate limiting step in the mechanism
involves the isomerization of a prereactive collision complex formed
through either the HCHOĀ·Ā·Ā·H<sub>2</sub>O + FA and/or
HCHO + FAĀ·Ā·Ā·H<sub>2</sub>O pathways. The present study
finds that the effective barrier height, defined as the difference
between the zero-point vibrational energy (ZPE) corrected energy of
the transition state (TS) and the HCHOĀ·Ā·Ā·H<sub>2</sub>O + FA and HCHO + FAĀ·Ā·Ā·H<sub>2</sub>O starting reagents,
are respectively only ā¼1 and ā¼4 kcal/mol. These barriers
are substantially lower than the ā¼17 kcal/mol barrier associated
with the corresponding step in the hydrolysis of HCHO catalyzed by
a single water molecule (HCHO + H<sub>2</sub>O + H<sub>2</sub>O).
The significantly lower barrier heights for the formic acid catalyzed
pathway reveal a new important role that organic acids play in the
gas phase hydrolysis of atmospheric carbonyl compounds
UV Photochemistry of Peroxyformic Acid (HC(O)OOH): An Experimental and Computational Study Investigating 355 nm Photolysis
The photochemistry of peroxyformic
acid (PFA), a molecule of atmospheric
interest exhibiting internal hydrogen bonding, is examined by exciting
the molecule at 355 nm and detecting the nascent OH fragments using
laser-induced fluorescence. The OH radicals are found to be formed
in their ground electronic state with the vast majority of available
energy appearing in fragment translation. The OH fragments are vibrationally
cold (vā³ = 0) with only modest rotational excitation. The average
rotational energy is determined to be 0.35 kcal/mol. Further, the
degree of OH rotational excitation from PFA is found to be significantly
less than that arising from the dissociation of H<sub>2</sub>O<sub>2</sub> as well as other hydroperoxides over the same wavelength.
Ab initio calculation at the EOM-CCSD level is used to investigate
the first few electronic excited states of PFA. Differences in the
computed torsional potential between PFA and H<sub>2</sub>O<sub>2</sub> help rationalize the observed variation in their respective OH fragment
rotational excitation. The calculations also establish that the electronic
excited state of PFA accessed in the near UV is of <sup>1</sup>Aā³
symmetry and involves a Ļ*<sub>(OāO)</sub> ā n<sub>(O)</sub> excitation. Additionally, the UV absorption cross section
of PFA at 355 and 282 nm is estimated by comparing the yield of OH
from PFA at these wavelengths to that from hydrogen peroxide for which
the absorption cross sections is known
Carboxylic Acid Catalyzed Hydration of Acetaldehyde
Electronic
structure calculations of the pertinent stationary points on the potential
energy surface show that carboxylic acids can act effectively as catalysts
in the hydration of acetaldehyde. Barriers to this catalyzed process
correlate strongly with the p<i>K</i><sub>a</sub> of the
acid, providing the potential to provide the predictive capacity of
the effectiveness of carboxylic acid catalysts. Transition states
for the acid-catalyzed systems take the form of pseudo-six-membered
rings through the linear nature of their hydrogen bonds, which accounts
for their relative stability compared to the more strained direct
and water-catalyzed systems. When considered as a stepwise reaction
of a dimerization followed by reaction/complexation, it is likely
that collisional stabilization of the prereactive complex is more
likely than reaction in the free gas phase, although the catalyzed
hydration does retain the potential to proceed on water surfaces or
in droplets. Lastly, it is observed that postreactive diolāacid
complexes are significantly stable (ā¼12ā17 kcal/mol)
relative to isolated products, suggesting the possibility of long-lived
hygroscopic species that could act as a seed molecule for condensation
of secondary organic aerosols
Organic Acid Formation from the Atmospheric Oxidation of Gem Diols: Reaction Mechanism, Energetics, and Rates
Computational chemistry
is used to investigate the gas phase reaction
of several gem diols in the presence of OH radical and molecular oxygen
(<sup>3</sup>O<sub>2</sub>) as would occur in the Earthās troposphere.
Four gem diols, represented generically as RāHCĀ(OH)<sub>2</sub>, with R being either āH, āCH<sub>3</sub>, āHCĀ(O),
and āCH<sub>3</sub>CĀ(O) are investigated. We find that after
the abstraction of the hydrogen atom from the CāH moiety of
the diol by atmospheric OH, molecular oxygen quickly adds onto the
resulting radicals leading to the formation of a geminal diol peroxy
adduct (RāCĀ(OO)Ā(OH)<sub>2</sub>), which is the key intermediate
in the oxidation process. Unimolecular reaction of this RāCĀ(OO)Ā(OH)<sub>2</sub> radical adduct, occurs via a proton-coupled electron transfer
(PCET) mechanism and leads to the formation of an organic acid and
a HO<sub>2</sub> radical. Further, the barrier for the unimolecular
reaction step decreases along the R substitution series: āH,
āCH<sub>3</sub>, āHCĀ(O), āCH<sub>3</sub>CĀ(O);
this trend most likely arises from increased internal hydrogen bonding
along the series. The reaction where the R group is CH<sub>3</sub>CĀ(O), associated with methylglyoxal diol, has the lowest barrier
with its transition state being ā¼4.3 kcal/mol above the potential
energy well of the corresponding CH<sub>3</sub>CĀ(O)-CĀ(OO)Ā(OH)<sub>2</sub> peroxy adduct. The rate constants for the four diol oxidation
reactions were investigated using the MESMER master equation solver
kinetics code over the temperature range between 200 and 300 K. The
calculations suggest that once formed, gem diol radicals react rapidly
with O<sub>2</sub> in the atmosphere to produce organic acids and
HO<sub>2</sub> with an effective gas phase bimolecular rate constant
of ā¼1 Ć 10<sup>ā11</sup> cm<sup>3</sup>/molecule
s at 300 K
Role of Torsion-Vibration Coupling in the Overtone Spectrum and Vibrationally Mediated Photochemistry of CH<sub>3</sub>OOH and HOOH
The yield of vibrationally excited
OH fragments resulting from
the vibrationally mediated photodissociation of methyl hydroperoxide
(CH<sub>3</sub>OOH) excited in the vicinity of its 2Ī½<sub>OH</sub> and 3Ī½<sub>OH</sub> stretching overtones is compared with
that resulting from excitation of the molecule to states with three
quanta in the CH stretches and to the state with two quanta in the
OH stretch and one in the OOH bend (2Ī½<sub>OH</sub> + Ī½<sub>OOH</sub>). We find that the OH fragment vibrational state distribution
depends strongly on the vibrational state of CH<sub>3</sub>OOH prior
to photodissociation. Specifically, dissociation from the CH stretch
overtones and the stretch/bend combination band involving the OH stretch
and OOH bend produced significantly less vibrationally excited OH
fragments compared to that produced following excitation of CH<sub>3</sub>OOH to an overtone in the OH stretch. While the absence of
vibrationally excited OH photoproducts following excitation of the
CH overtone is not surprising, the lack of vibrationally excited OH
following excitation to the 2Ī½<sub>OH</sub>+Ī½<sub>OOH</sub> combination band is unexpected given that photodissociation following
excitation to the lower-energy 2Ī½<sub>OH</sub> state produces
OH products in <i>v</i> = 1 as well as in its ground state.
This trend persists even when the electronic photodissociation wavelength
is changed from 532 to 355 nm and thus suggests that the observed
disparity arises from differences in the nature of the initially populated
vibrational states. This lack of vibrationally excited OH products
likely reflects the enhanced intramolecular vibrational energy redistribution
associated with the stretch/bend combination level compared to the
pure OH stretch overtone. Consistent with this hypothesis, photodissociation
from the stretch/bend combination level of the smaller HOOH molecule
produces more vibrationally excited OH fragments compared to that
resulting from the corresponding state of CH<sub>3</sub>OOH. These
results are investigated using second-order vibrational perturbation
theory based on an internal coordinate representation of the normal
modes. Consistent with the observations, the first-order correction
to the wave function shows stronger coupling of the 2Ī½<sub>OH</sub>+Ī½<sub>OOH</sub> state to states with torsion excitation compared
to the other bands considered in this study
Oxygenate-Induced Tuning of Aldehyde-Amine Reactivity and Its Atmospheric Implications
Atmospheric
aerosols often contain a significant fraction of carbonānitrogen
functionality, which makes gas-phase aldehyde-amine chemistries an
important source of nitrogen containing compounds in aerosols. Here
we use high-level ab initio calculations to examine the key determinants
of amine (ammonia, methylamine, and dimethylamine) addition onto three
different aldehydes (acetaldehyde, glycolaldehyde, and 2-hydroperoxy
acetaldehyde), with each reaction being catalyzed by a single water
molecule. The model aldehydes reflect different degrees of oxygenation
at a site adjacent to the carbonyl moiety, the Ī±-site, and represent
typical oxygenates that can arise from atmospheric oxidation especially
under conditions where the concentration of NO is low. Our results
show that the reaction barrier is influenced not only by the nature
of the amine but also by the nature of the aldehyde. We find that,
for a given amine, the reaction barrier decreases with increasing
oxygenation of the aldehyde. This observed trend in barrier height
can be explained through a distortion/interaction analysis, which
reveals a gradual increase in internal hydrogen bonding interactions
upon increased oxygenation, which, in turn, impacts the reaction barrier.
Further, the calculations reveal that the reactions of methylamine
and dimethylamine with the oxygenated aldehydes are barrierless under
catalysis by a single water molecule. As a result, we expect these
addition reactions to be energetically feasible under atmospheric
conditions. The present findings have important implications for atmospheric
chemistry as amine-aldehyde addition reactions can facilitate aerosol
growth by providing low-energy neutral pathways for the formation
of larger, less volatile compounds, from readily available smaller
components
A Computational Study Investigating the Energetics and Kinetics of the HNCO + (CH<sub>3</sub>)<sub>2</sub>NH Reaction Catalyzed by a Single Water Molecule
High-level
ab initio calculations are used to explore the energetics
and kinetics for the formation of 1,1-dimethyl urea via the reaction
of isocyanic acid (HNCO) with dimethyl amine (DMA) catalyzed by a
single water molecule. Compared to the uncatalyzed HNCO + DMA reaction,
the presence of a water molecule lowers the reaction barrier, defined
here as the energy difference between the separated HNCO + DMA + H<sub>2</sub>O reactants and the transition state (TS), by ā¼26 kcal/mol.
In addition to the HNCO + DMA + H<sub>2</sub>O reaction, the energetics
of the analogous reactions involving, respectively, ammonia and methyl
amine were also investigated. Comparing the barriers for these three
amine addition reactions, which can be represented as HNCO + R-NH-Rā²
+ H<sub>2</sub>O with R and Rā² being either āCH<sub>3</sub> or āH, we find that the reaction barrier decreases
with the degree of methylation on the amine nitrogen atom. The effective
rate constants for the bimolecular reaction pathways HNCOĀ·Ā·H<sub>2</sub>O + DMA and HNCOĀ·Ā·DMA + H<sub>2</sub>O were calculated
using canonical variational TS theory coupled with both small curvature
and zero-curvature tunneling corrections over the 200ā300 K
temperature range. For comparison, we also calculated the rate constant
for the HNCO + OH reaction. Our results suggest that the HNCO + H<sub>2</sub>O + DMA reaction can make a non-negligible contribution to
the gas-phase removal of atmospheric HNCO under conditions where the
HNCO and water concentrations are high and the temperature is low
Hydrolysis of Ketene Catalyzed by Formic Acid: Modification of Reaction Mechanism, Energetics, and Kinetics with Organic Acid Catalysis
The hydrolysis of ketene (H<sub>2</sub>Cī»Cī»O) to
form acetic acid involving two water molecules and also separately
in the presence of one to two water molecules and formic acid (FA)
was investigated. Our results show that, while the currently accepted
indirect mechanism, involving addition of water across the carbonyl
Cī»O bond of ketene to form an eneādiol followed by tautomerization
of the eneādiol to form acetic acid, is the preferred pathway
when water alone is present, with formic acid as catalyst, addition
of water across the ketene Cī»C double bond to directly produce
acetic acid becomes the kinetically favored pathway for temperatures
below 400 K. We find not only that the overall barrier for ketene
hydrolysis involving one water molecule and formic acid (H<sub>2</sub>C<sub>2</sub>O + H<sub>2</sub>O + FA) is significantly lower than
that involving two water molecules (H<sub>2</sub>C<sub>2</sub>O +
2H<sub>2</sub>O) but also that FA is able to reduce the barrier height
for the direct path, involving addition of water across the Cī»C
double bond, so that it is essentially identical with (6.4 kcal/mol)
that for the indirect eneādiol formation path involving addition
of water across the Cī»O bond. For the case of ketene hydrolysis
involving two water molecules and formic acid (H<sub>2</sub>C<sub>2</sub>O + 2H<sub>2</sub>O + FA), the barrier for the direct addition
of water across the Cī»C double bond is reduced even further
and is 2.5 kcal/mol <i>lower</i> relative to the eneādiol
path involving addition of water across the Cī»O bond. In fact,
the hydrolysis barrier for the H<sub>2</sub>C<sub>2</sub>O + 2H<sub>2</sub>O + FA reaction through the direct path is sufficiently low
(2.5 kcal/mol) for it to be an energetically accessible pathway for
acetic acid formation under atmospheric conditions. Given the structural
similarity between acetic and formic acid, our results also have potential
implications for aqueous-phase chemistry. Thus, in an aqueous environment,
even in the absence of formic acid, though the initial mechanism for
ketene hydrolysis is expected to involve addition of water across
the carbonyl bond as is currently accepted, the production and accumulation
of acetic acid will likely alter the preferred pathway to one involving
addition of water across the ketene Cī»C double bond as the
reaction proceeds
Dimethylamine Addition to Formaldehyde Catalyzed by a Single Water Molecule: A Facile Route for Atmospheric Carbinolamine Formation and Potential Promoter of Aerosol Growth
We
use ab initio calculations to investigate the energetics and
kinetics associated with carbinolamine formation resulting from the
addition of dimethylamine to formaldehyde catalyzed by a <i>single</i> water molecule. Further, we compare the energetics for this reaction
with that for the analogous reactions involving methylamine and ammonia
separately. We find that the reaction barrier for the addition of
these nitrogen-containing molecules onto formaldehyde decreases along
the series ammonia, methylamine, and dimethylamine. Hence, starting
with ammonia, the reaction barrier can be ātunedā by
the substitution of an alkyl group in place of a hydrogen atom. The
reaction involving dimethylamine has the lowest barrier with the transition
state being 5.4 kcal/mol <i>below</i> the (CH<sub>3</sub>)<sub>2</sub>NH + H<sub>2</sub>CO + H<sub>2</sub>O separated reactants.
This activation energy is significantly lower than that for the bare
reaction occurring without water, H<sub>2</sub>CO + (CH<sub>3</sub>)<sub>2</sub>NH, which has a barrier of 20.1 kcal/mol. The negative
barrier associated with the single-water molecule catalyzed reaction
of dimethylamine with H<sub>2</sub>CO to form the carbinolamine (CH<sub>3</sub>)<sub>2</sub>NCH<sub>2</sub>OH suggests that this reaction
should be energetically feasible under atmospheric conditions. This
is confirmed by rate calculations which suggest that the reaction
will be facile even in the gas phase. As amines and oxidized organics
containing carbonyl functional groups are common components of secondary
organic aerosols, the present finding has important implications for
understanding how larger, less volatile organic compounds can be generated
in the atmosphere by combining readily available smaller components
as required for promoting aerosol growth