Absorption spectra and aqueous photochemistry of -hydroxyalkyl nitrates of atmospheric interest

Molecular Physics, 2015Vol. 113, Nos. 15–16, 2179–2190, Absorption spectra and aqueous photochemistry of β-hydroxyalkyl nitrates of atmospheric
Dian E. RomonoskLucas Q. NguyDorit Tran B. NguyScott A. EDavid B.C. Mar Christopher D. VanderwaR. Benny Gand Sergey A. Nizkorodov aDepartment of Chemistry, University of California, Irvine, CA, USA; bFritz Haber Center for Molecular Dynamics, The Hebrew University, Jerusalem, Israel (Received 25 December 2014; accepted 5 February 2015) Molar absorption coefficients were measured for select alkyl nitrates and β-hydroxyalkyl nitrates in methanol. The presenceof the β-hydroxyl group has a relatively minor effect on the absorption spectrum in the vicinity of the weak n π∗ transition,which is responsible for photolysis of organic nitrates in the atmosphere. For both alkyl nitrates and β-hydroxyalkyl nitrates,there is an enhancement in the absorption coefficients in solution compared to the gas-phase values. The effect of theβ-hydroxyl group on the spectra was modelled with molecular dynamics simulations using an OM2/GUGA-CI Hamiltonianfor ethyl nitrate and β-hydroxyethyl nitrate. The simulation provided a qualitatively correct shape of the low energy tail of theabsorption spectrum, which is important for atmospheric photochemistry. The role of direct aqueous photolysis in removalof β-hydroxyalkyl nitrates in cloud and fog water was modelled using a relative rate approach, and shown to be insignificantrelative to gas-phase photochemical processes and aqueous OH oxidation under typical atmospheric conditions.
Keywords: molar absorption coefficients; molecular dynamics; aqueous photolysis; cloud processing of aerosols;
Esters of nitric acid, better known as organic nitrates to R − CH (OH) − CH (OO·) − R + NO atmospheric chemists, represent an important group of at- → R − CH (OH) − CH (ONO2) − R mospheric organic compounds Oxidation of saturatedhydrocarbons in air by OH in the presence of NOx (NO + R − CH (OH) − CH (OO·) − R + NO NO2) is a common pathway to unsubstituted alkyl nitrates: → R − CH (OH) − CH (O·) − R + NO2 (4b) Reactions (3) and (4) are especially important in the oxida- 2) → ROO · +H2O tion of biogenically emitted isoprene, monoterpenes, and ROO · +NO → (ROONO)∗ → RONO other unsaturated volatile organic compounds in air masses affected by urban emissions. The resulting β-hydroxyalkyl ROO · +NO → (ROONO)∗ → RO · +NO nitrates have been observed in significant concentrations in both urban and remote environments in a number of fieldstudies Reactions of nitrate radicals with alkenes alsoserve as an important source of nitrates substituted The yield of reaction (2a) relative to that of reactions by a hydroxyl, hydroperoxyl (–OOH), or carbonyl group (2a + 2b) increases with the size of the alkyl group, R, Downloaded by [The UC Irvine Libraries], [Sergey A. Nizkorodov] at 09:43 11 September 2015 and approaches ∼30% for larger peroxy radicals, ROOࢫ β-position, for example, through the following sequence of reactions: Under the same conditions, the oxidation of unsatu-rated hydrocarbons commonly produces nitrates with a hy-droxyl (–OH) group in the β position relative to the nitroxy R − CH = CH − R + NO3 (+O2) → R − CH (OO·) − CH (ONO2) − R (5) R − CH (OO·) − CH (ONO → R − CH (OH) − CH (OO·) − R (3) → R − CH (OOH) − CH (ONO2) − R ∗ Corresponding author. Email:  2015 Taylor & Francis D.E. Romonosky et al.
R − CH (OO·) − CH (ONO2) − R + RO2 with additional functional groups. Indeed, there have been → R − CH (OH) − CH (ONO a number of observations of β-hydroxyalkyl nitrates in particle-phase products of oxidation of isoprene R − CH (OO·) − CH (ONO alpha-pinene and other terpenes.
2) − R + NO (+O2) Despite the fact that electronic excitations play a major R − C (O) − CH (ONO2) − R role in the initiation of atmospheric reactions theo-retical predictions of the accurate shapes of the absorp- Other pathways to β-substituted alkyl nitrates also exist. A tion spectra of atmospheric compounds remain a major comprehensive review of these mechanisms is beyond the challenge. Significant radiation is available in the lower scope of this paper.
atmosphere only for wavelengths longer than 290 nm (pho- For unsaturated nitrates, such as the ones derived from ton energies below 4.3 eV) because higher energy photons isoprene, reaction with OH serves as the most important are efficiently screened by stratospheric ozone. For many daytime sink, while reaction with NO3 dominates at night atmospheric molecules, the lowest electronic transition is For saturated nitrates, other sink mechanisms may be- centred deeper in the ultraviolet (UV) region, and the ab- come competitive. One of the known degradation pathways sorption takes place in the red tail of the spectrum, far for alkyl nitrates is gas-phase photolysis by means of the removed from the absorption centre. Even though the ab- weak n π∗ transition sorption coefficient in the red tail of the spectrum is small,it may dominate the atmospheric photochemistry because RONO2 + hv → RO · +NO2 only the near-UV photons can make it through the ozoneshield. The importance of weak red tails in absorption spec- The absorption cross sections and photolysis quan- tra of atmospheric molecules was stressed in a study of tum yields of gas-phase alkyl nitrates have been photochemistry of methyl hydroperoxide This situa- studied extensively, making it possible to reliably predict tion applies to alkyl nitrates because their n π∗ transition the rate of reaction (9) under all relevant atmospheric con- is centred at ∼260 nm and only the tail of this transition ditions. The electronic states and photodissociation overlaps with the tropospheric actinic wavelength region dynamics of simple alkyl nitrates have also been The calculation of the shape of the red tail of the investigated. Because of the low oscillator strength of the n spectrum requires considerable computational effort. The → π∗ transition, the photolysis is relatively slow with typi- Franck–Condon region is greatly extended at ambient tem- cal lifetimes of days. In contrast to the simple alkyl nitrates, peratures by the internal motion of the molecule making it photochemistry of β-substituted nitrates is less well un- necessary to calculate vertical electronic transition energies derstood. Investigation of the neighbouring group effects and oscillator strengths at various molecular geometries. In on photochemistry of atmospheric organic compounds is condensed phases, the tail absorption may be affected by the important; for example, the synergetic interaction between shift of the electronic states due to the presence of solvent the carbonyl and nitroxy groups on the neighbouring car- molecules. References provide illustrative exam- bon atoms has been shown to lead to an efficient photolysis ples of accounting for these effects in predictions of absorp- of β-carbonyl nitrates that occurs at faster rates than reac- tion spectra of atmospheric compounds. For a recent review tion with OH In the case of β-hydroxyalkyl nitrates, on the applications of molecular dynamics (MD) meth- absorption cross sections have been measured only for a ods to photochemical problems, the reader is referred to few compounds, such as β-hydroxyethyl nitrate and trans-2-hydroxycyclopentyl-1-nitrate and no photoly- The main question addressed in this paper is whether sis studies have been done.
direct photolysis of β-hydroxyalkyl nitrates in the aqueous Depending on their solubility and volatility, organic phase or in the organic particle phase is atmospherically nitrates can remain in the gas-phase, partition into cloud relevant. With several notable exceptions, such as measure- and fog droplets, or partition into aerosol particles. The ments of molar absorption coefficients of simple alkyl ni- Downloaded by [The UC Irvine Libraries], [Sergey A. Nizkorodov] at 09:43 11 September 2015 presence of a hydroxyl group decreases the vapour pres- trates in hexane and photolysis of alkyl nitrates on ice sure and increases the solubility of small β-hydroxyalkyl surfaces condensed-phase photochemistry of alkyl ni- nitrates enough to make their wet and dry deposition a trates has not been studied enough to predict whether it can significant sink. The magnitudes of the measured Henry's compete with gas-phase photochemistry or heterogeneous solubility constants suggest that β-hydroxyalkyl nitrates oxidation. In this study, we begin to address the follow- partition into the aqueous phase to a significant extent ing important questions by examining the molar absorption whenever cloud and fog droplets are present Larger coefficients of atmospherically relevant β-hydroxyalkyl ni- β-hydroxyalkyl nitrates, such as ones derived from the ox- trates dissolved in methanol. Are solvatochromic effects idation of monoterpenes, may have sufficiently low vapour significant for these types of molecules? Does the presence pressures to efficiently partition into aerosols and onto of the solvent affect the photolysis quantum yields? Do environmental surfaces, especially if they are decorated additional photolysis channels open up in the condensed Molecular Physics phase? Does the β-hydroxyl group play a special role in the that was compound L that was studied in multiple solvents).
photochemistry, e.g., by hydrogen bonding to the nitroxy To improve baseline stability, the spectra were baseline- group? In addition to the experimental measurements, we corrected by setting the average measured absorbance in the explore the effect of the β-hydroxyl group on the shape of 500–700 nm range to zero. We acknowledge that it would the nπ∗ band in ethyl nitrate and β-hydroxyethyl nitrate be preferable to investigate the absorption spectra in water.
using on-the-fly molecular dynamics.
However, we elected to use methanol as the solvent becauseof the limited water solubility of the nitrates examined inthis work. Methanol is a reasonably polar solvent, and weexpect that its effects on the absorption spectra should be comparable to that of water.
The β-hydroxyalkyl nitrates labelled A-I in weresynthesised by nucleophilic epoxide ring opening withbismuth (III) nitrate The procedures included addition of the nucleophile at The structures were built in Avogadro a program room temperature under inert atmosphere to a solution of that includes a minimisation procedure with MMFF94 the selected, reagent-grade epoxide and acetonitrile. The force field and a conformer search option. The iden- reaction was quenched with deionised water and the result- tified conformers were further optimised at Møller- ing β-hydroxyalkyl nitrates were collected by extraction Plesset second order perturbation (MP2) theory level us- with ethyl acetate. All nitrates were purified using liquid ing the resolution of identity approximation The chromatography with a solvent system comprised of ethyl correlation-consistent polarized valence double-zeta (cc- acetate and hexanes. Solvents were removed using a rotary pVDZ) basis set was employed Vertical excitation evaporator. Proton nuclear magnetic resonance (NMR) was energies were computed with the with second-order ap- employed to verify the structure and purity of the result- proximate coupled-cluster (CC2) method at the ing product. The compound obtained in the highest yield MP2 optimised structures. For some conformers, the ver- and purity was 2-hydroxycyclohexyl nitrate (A), which was tical excitation energies were also computed with the in crystalline form. The rest of the compounds were ob- orthogonalisation-corrected orthogonalization method 2 tained as viscous liquids, and judging by their yellowish (OM2) Hamiltonian and the multireference configura- colour, may have contained impurities (estimated to be un- tion interaction procedure using the graphical unitary group der 5% based on NMR spectra) from the synthesis. Be- approach (GUGA-CI) using the modified neglect of di- cause of the possible presence of impurities we elected not atomic overlap (MNDO) program In GUGA-CI cal- to report molar absorption coefficients above 330 nm in culations, three reference configurations were used (closed this paper. Each sample was tested for the presence of ni- shell, single, and double highest occupied molecular orbital trate and other functional groups using a Mattson GL-5030 (HOMO) to lowest occupied molecular orbital (LUMO) ex- Fourier-transform infrared spectroscopy (FTIR) spectrome- citations) and the active space was chosen to include the ter. A sample FTIR spectrum of 4-hydroxytetrahydrofuran- highest five occupied orbitals and the lowest five unoc- 3-yl nitrate (F) is shown in Figure S1. The spectra were cupied orbitals with 10 electrons in 10 orbitals; in other consistent with spectra of alkyl nitrates reported by Bruns words, a complete active space of (10, 10) was employed.
et al. specifically the bands attributable to nitrates The absorption spectrum was obtained by running molecu- were observed at 1630, 1280, and 860 cm−1 for all com- lar dynamics with the OM2 Hamiltonian using a time step pounds. For compound F, the OH stretching band associated of 0.1 fs at 300 K for 10 ps. From each trajectory, 10,000 with the hydroxyl group was also present (Figure S1). Ni- structures were extracted (one structure every 1 fs of the trates were stored in a refrigerator at 5 ◦C. However, some simulation), and their vertical excitation energies and os- of the compounds, e.g., 3-hydroxy-3-methylbutan-2-yl ni- cillator strengths were calculated with the OM2/GUGA-CI Downloaded by [The UC Irvine Libraries], [Sergey A. Nizkorodov] at 09:43 11 September 2015 trate (B), were unstable under standard storage conditions.
Hamiltonian. For each excitation energy, the vertical tran- Therefore, we performed all the measurements shortly after sitions were convoluted with a Lorentzian line shape with a the preparation. Nitrates labelled J, K, and L in were width of 0.001 eV, and all of the resulting Lorentzians were purchased and used without further purification.
added to yield the excitation spectrum. The width was arbi- The ultraviolet-visible (UV-vis) absorption spectra were trarily chosen to get a continuous spectrum; the qualitative taken by a Shimazdu UV-2450 spectrometer with an accu- shape of the spectrum did not depend on the exact value racy of ±0.003 absorbance units in the base-10 absorbance of the width. Similar OM2/multi-reference configuration range of 0–1. Each sample was scanned in the 200–700 interaction (OM2/MRCI) approach were recently used for nm wavelength range with a rate of 210 nm/min. Each ex- calculation of the absorption spectrum of methyl hydroper- perimental run involved taking spectra for several volume oxide in frozen water clusters and for simulations of dilutions of the nitrate in methanol (the only exception to dynamics of atmospheric photochemical reactions D.E. Romonosky et al.
Summary of synthesised (A–I) and purchased (J–L) organic nitrates studied in this work. The first column contains letter abbreviations by which different nitrates are referred to in other tables and figures.
Faint yellow liquid, crystallises Acquired colour during storage 2-Hydroxyhexyl nitrate Faint yellow liquid Viscous yellow liquid, crystallises Was not able to purify Viscous yellowish liquid Viscous yellowish liquid Viscous yellowish liquid Clear, colourless ‘gel'; was not ableto purify 2-Ethylhexyl nitrate Downloaded by [The UC Irvine Libraries], [Sergey A. Nizkorodov] at 09:43 11 September 2015 Isopropyl nitrate Isosorbide mononitrate

Molecular Physics Representative UV-vis spectra of 2-hydroxycyclohexyl nitrate (compound A) at different solution concentrations. The in-set shows an example of calculating the molar absorption coeffi-cient from Beer's law at 325 nm; such calculations have been doneat every wavelength for each nitrate investigated in this work.
Results and discussion
shows an example of determination of mo-lar absorption coefficients (the terminology followsthe recommendations described in Ref. for 2- Panel (a): wavelength dependent molar absorption co- hydroxycyclohexyl nitrate (A).
efficients (molar absorptivity) for β-hydroxyalkyl nitrates A, B, Absorption spectra were recorded at multiple dilution C, D (in various shades of red with markers) and alkyl nitrates J, K(in shades of blue without markers) measured in methanol. Gas- levels in methanol to verify linearity over the experimental phase data (in black and green) for isopropyl nitrate (K) ethyl range of concentrations. For each wavelength, the molar nitrate (EN) hydroxyethyl nitrate (HEN) and trans-2- absorption coefficient was determined by a linear fit of the hydroxycyclopentyl-1-nitrate (HCPN) are provided for com- base-10 absorbance vs. molar concentration, as shown in the parison (only a limited number of points were reported for HCPN).
inset in The absorbance increases sharply towards Panel (b) contains the measured molar absorption coefficients forcompound L measured in various solvents.
the UV range. To avoid deviations from Beer's law, onlypoints with the absorbance values below ∼1 were includedin the fit. reports the molar absorption coefficientsfor the examined nitrates between 270 and 330 nm.
Molar absorption coefficients (in L mol−1 cm−1) for the investigated nitrates (the labels are defined in No data for compounds E and I are included because we could not purify them. The solvent is methanol, except for compound L, which wasadditionally investigated in water (w), acetonitrile (acn), octanol (oct), and tetrahydrofuran (thf). Each molar absorption coefficient isobtained from a fit of the available absorbance vs. concentration data as shown in Downloaded by [The UC Irvine Libraries], [Sergey A. Nizkorodov] at 09:43 11 September 2015

D.E. Romonosky et al.
We do not report values outside this range because the for other compounds because the absorption coefficients of measured absorbance values were too small for a reliable fit alkyl nitrates tend to increase with the size of the substituent above 330 nm, and there were too few measurement points chain and have an unknown dependence on the with acceptably low absorbance below 270 nm. solvent. However, based on the solution-phase compares the molar absorption coefficient for synthesised absorption coefficients in methanol appear to be larger on β-hydroxyalkyl nitrates A, B, C, and D and commercially average than the gas-phase values. According to the ex- obtained compounds J and K without the β-hydroxyl group.
isting gas-phase measurements for β-hydroxyethyl nitrate The values are comparable in the vicinity of 300 nm but di- and ethyl nitrate, the β-hydroxyl group could be expected verge at 330 nm, where the absorption coefficients become to have a suppressing effect on the absorption coefficients.
small and difficult to measure reliably.
However, our measurements indicate that the β-hydroxyl Ideally, the measured molar absorption coefficients in group has a relatively minor effect on the absorption spec- solution should be compared to their corresponding gas- trum in solution. The authors of Ref. noted the difficul- phase values. However, gas-phase absorption cross sections ties of measuring absorption cross sections for β-hydroxy for any of the synthesised compounds listed in nitrates arising from their low vapour pressure. Indeed, the are not available. We are aware of only two gas-phase ab- examination of suggests that the existing absorp- sorption cross section measurements for β-hydroxyalkyl tion cross sections for β-hydroxy nitrates may be underes- nitrates, specifically for β-hydroxyethyl nitrate and timated; for example, the low value of the reported 275 nm for trans-2-hydroxycyclopentyl-1-nitrate The data absorption cross section for trans-2-hydroxycyclopentyl-1- for both of these β-hydroxyalkyl nitrates are included in nitrate seems to fall out of the general trend. Therefore, for comparison. In addition, we include the additional measurements of gas-phase absorption cross sec- recommended data for ethyl nitrate and for isopropyl tions for β-hydroxy nitrates are desirable.
nitrate (K). In all cases, we converted the base e gas-phase To further investigate the solution effects, we exam- absorption cross sections (σ , in cm2 molec−1) to base-10 ined absorption spectra of isosorbide mononitrate (L) in molar absorption coefficients (ε, in L mol−1 cm−1), various solvents. This compound has two ether groups inβ positions, which should have similar electron withdraw-ing effects on the photochemistry of the nitroxy group as ε(λ) = σ (λ) × the β-hydroxyl group. shows that the absorp-tion spectrum of L does not strongly depend on the type where NA is Avogadro's number.
of the solvent across the range of solvent polarities (water, The direct comparison can only be done for isopropyl methanol, acetonitrile, octanol, and tetrahydrofuran). The nitrate (K), for which the gas-phase molar absorption coef- absorption coefficient appears to be systematically smaller ficients are smaller than the ones in methanol by a factor of in water compared to the less polar solvents in this group ∼1.6 The comparison is less straightforward Optimised geometries, relative energies, and dipole moments of ethyl nitrate conformers as calculated with MP2/cc-pVDZ.
N-O-C-C dihedral angle Dipole moment (debye) Downloaded by [The UC Irvine Libraries], [Sergey A. Nizkorodov] at 09:43 11 September 2015

Molecular Physics The lowest electronic excited states of conformer 1 of ethyl nitrate. All parameters are calculated at CC2 level, but the OM2 energies are also provided for comparison. A similar table for conformer 3 is given in the supporting information section (Table S1).
Transitions involved HOMO-2 → LUMO 64% HOMO-3 → LUMO 31% HOMO-3 → LUMO 62% HOMO-2 → LUMO 23%HOMO-6 → LUMO 11% HOMO-1 → LUMO 46% HOMO-5 → LUMO 26%HOMO → LUMO 16% HOMO → LUMO 48% HOMO-1 → LUMO 12%HOMO-2 → LUMO + 5 11% HOMO-2 → LUMO + 5 22% HOMO → LUMO 16%HOMO-3 → LUMO + 5 11%HOMO-2 → LUMO + 1 10% suggesting a reduction in the excited state dipole moment(confirmed by calculations, see below). However, on thewhole, the solvent effect on the absorption spectrum ap-pears to be minimal.
Computed structures and absorption spectra
The geometries, dihedral angles, relative ground-state en-ergies calculated at the MP2/cc-pVDZ level, and dipolemoments of the lowest energy conformers of ethyl nitrateare summarised in The –ONO2 group of all the three conformers is pla- nar; the primary difference between them lies in the N-O-C-C dihedral angle. The conformer 2 (gauche + ) andconformer 3 (gauche−) are stereoisomers, therefore theirenergies are the same. The conformer 1 is a global minimumat this level of theory, but its energy is within a fraction ofa few meV from that of conformers 2 and 3. The verticalexcitation energies for conformer 1 calculated at the cou-pled cluster CC2 level are provided in and thosefor conformer 3 are given in the supporting informationTable S1.
The molecular orbitals involved in these transitions are Downloaded by [The UC Irvine Libraries], [Sergey A. Nizkorodov] at 09:43 11 September 2015 shown in and Figure S2, respectively. The orbitalshave similar features but there are subtle differences as well.
For example, the first excited state for conformer 3 corre-sponds mainly to an excitation from HOMO-3 to LUMO,whereas for conformer 1, the main excitation involvesHOMO-2 and the LUMO orbital. The lowest electronic Molecular orbitals (obtained by MP2) involved in elec- exited state of conformer 3 is predicted around 5.07 eV at tronic transitions of conformer 1 of ethyl nitrate listed in the CC2 level. For comparison, the first excitation energy A similar figure for conformer 3 is provided in the supporting is 4.60 eV at the OM2 level. A comparison of the theoreti- information section (Figure S2).
cally predicted excitation spectrum to the experimental oneshows that the OM2/MRCI method reasonably describes

D.E. Romonosky et al.
Relative energies (as calculated with MP2) and relative Boltzmann populations of β-hydroxyethyl nitrate conformers at300 K.
Boltzmann distribution Comparison of theoretically predicted absorption cross sections of ethyl nitrate (red noisy trace) and β-hydroxyethyl ni- trate (blue noisy trace). The absorption cross sections for ethyl nitrate (red solid line) and hydroxyethyl nitrate (bluesolid line) are shown for comparison. The two theoretical resultshave been arbitrarily scaled by the same factor. Note that the ex-periments and simulations predict spectral shifts in the opposite The lowest electronic exited state is predicted at around direction for the two compounds.
5.09 eV at the CC2 level, which is very similar to the corre-sponding value for ethyl nitrate. The first excitation energy the actual excitation energies Normally, the ab at the OM2 level (4.59 eV) is also essentially identical to initio CC2 method is supposed to be more reliable than that of ethyl nitrate. However, there are differences between the OM2/MRCI method. However, in the present case, we the two systems at higher excitations energies; for example, suspect that the CC2 states are not covering the relevant or- transition 5 in is unique to β-hydroxyethyl nitrate, bitals of the different states involved. As a result, we believe and is not present in ethyl nitrate. The correspondence be- that the CC2 calculation is less accurate. An alternative ex- tween the molecular orbitals of ethyl and β-hydroxyethyl planation is that this is a case where the multireference nitrates is shown in Table S2.
description is essential; the semiempirical MRCI has the The absorption spectra of ethyl nitrate and β- advantage of more fully covering the active space.
hydroxyethyl nitrate from the MD simulations using the As many as 13 minima were found for β-hydroxyethyl OM2 semiempirical Hamiltonian are shown in nitrate. Their geometries are shown in Figure S3, and their The most important wavelength range for photochem- relative energies and the Boltzmann populations at 300 K istry in the lower atmosphere is the low energy tail of the are listed in The vertical excitation energies at the absorption spectrum. The oscillator strength for the lowest CC2 level for the lowest energy conformer are provided in n π∗ electronic transition responsible for this tail in ni- and Figure S4 shows the corresponding molecular trates is quite low, which is responsible for the increased noise in the predicted spectrum. The calculations predict The electronic excited states of the lowest energy conformer of β-hydroxyethyl nitrate as calculated by CC2. All parameters are calculated at CC2 level, but the OM2 energies are also provided for comparison.
Main transitions involved Downloaded by [The UC Irvine Libraries], [Sergey A. Nizkorodov] at 09:43 11 September 2015 HOMO-4 → LUMO 50% HOMO-3 → LUMO 23%HOMO-2 → LUMO 13% HOMO-4 → LUMO 37% HOMO-5 → LUMO 35%HOMO-2 → LUMO 19% HOMO-3 → LUMO 47% HOMO-1 → LUMO 11%HOMO-5 → LUMO 11% HOMO-1 → LUMO 30% HOMO → LUMO 74%

Molecular Physics but photolysis may become competitive for certain types ofsaturated nitrate compounds. To examine potential fates ofthe compounds listed in we performed a scalinganalysis to determine their most significant atmosphericsinks. This method was previously developed in Refs.
therefore, we will only provide a summary of theassumptions used for this particular analysis. Oxidation byOH radicals is typically the most dominant chemical sinkfor most atmospheric organics in both the aqueous andgaseous phases suggesting that OH oxidation is areasonable benchmark to determine the significance of di-rect aqueous photolysis. We chose to compare the rates ofchemical reaction via gas-phase photolysis, aqueous-phasephotolysis, gas-phase oxidation by OH, and aqueous-phaseoxidation by OH. Note that we are not considering loss bydry deposition, which may be an important loss mechanismin the boundary layer. We are also ignoring hydrolysis ofhydroxyalkyl nitrate isomers with the nitrate group in thetertiary position, which has been shown to occur with atmo-spherically relevant rates in both aqueous solutions and water-containing aerosols Henry's Law constant is used to determine the equilib- rium partitioning between each phase in an air mass with aspecific liquid water content. The parameter ‘Z' is definedas the ratio between the gas-phase photolysis rate and theaqueous-phase photolysis rate: Z = dt = Jgas (R · T · LWC Decomposition of the predicted absorption spectra of v · KH )−1 ethyl nitrate (a) and β-hydroxyethyl nitrate (b) into contribution from the four lowest electronic states.
where n represents the moles of the compound of interest, trepresents time, J is the computed photolysis rate constant a small red shift of the spectrum upon addition of the β- using the measured absorption coefficients (see below), R hydroxyl group, whereas the available experiments suggest is the gas constant, T is the temperature, LWCv is the (di- that the spectrum of β-hydroxyethyl nitrate is slightly blue mensionless) liquid water content in volume of liquid water shifted from the spectrum of ethyl nitrate. However, there per volume of air, and KH is the Henry's Law constant. The is reasonable agreement in the overall shape of the spec- Henry's law constant for isopropyl nitrate was obtained tra across the entire range over which measurements are from the experimental measurements of Hauff et al. Henry's law constants of the remaining nitrates were un- We should note that the absorption spectra represent a available and thus were predicted from HENRYWIN convolution of different overlapping transitions. using bond contribution methods (Table S3). The experi- shows a decomposition of the predicted absorption spectra mentally measured Henry's constants for C2-C5 β- of ethyl nitrate and β-hydroxyethyl into contribution from hydroxyalkyl nitrates without other functional groups range Downloaded by [The UC Irvine Libraries], [Sergey A. Nizkorodov] at 09:43 11 September 2015 the four lowest electronic states.
from 6 × 103 to 4 × 104 M/atm, which compares reasonably The low energy tail of the spectrum is dominated by the well to the range of HENRYWIN predictions of 3 × 104 to lowest electronic state (see and Figure S5). These 8 × 104 M/atm for compounds A, B, C, D, E, and G which results demonstrate that the shape of the low energy tail in have no polar functional groups other than hydroxyl and the spectrum can be adequately predicted by using only one nitroxy. We should note that HENRYWIN Henry's constant state, which greatly decreases the computational expenses.
predictions of a series of β-hydroxyalkyl nitrates from Ref.
overestimated the measured values by factors rangingfrom 2.6 to 17, so the treatment presented here should be Photochemical fates of β-hydroxyalkyl nitrates
viewed as approximate at best. The method predicts higher As discussed in Section 1, the OH reaction is the most im- solubility for the compound H with an aromatic substituent, portant removal mechanism for unsaturated nitrates which may be an artefact of the bond contribution method.
D.E. Romonosky et al.
The highest solubility (1.7 × 1010 M/atm) is predicted forcompound L, which has the largest O/C ratio. To inves-tigate the significance of aqueous photochemistry duringideal conditions, we used a cloud liquid water content of0.5 g m−3, typically the largest value experienced in thetroposphere The aqueous- and gas-phase photolysis rate constants are a function of the actinic flux, the absorption crosssection, σ , and the photolysis quantum yield, : Jgas = FA(λ) · gas(λ) · σgas(λ) · FA(λ) · aq(λ) · σaq(λ) · where λ represents wavelength. Due to the absence of gas-phase absorption cross sections for the investigated com-pounds, we initially assumed that the gas-phase absorptioncross sections were identical to the solution-phase values Likely photo-induced atmospheric sinks of studied reported in To test the bounds of this framework, we compounds at a solar zenith angle of 65◦. Q is defined as the ratioof the aqueous oxidation by OH rate and the aqueous photolysis repeated the calculations with a small bathochromic shift rate. Z is defined as the ratio of the gas photolysis rate and aqueous in the cross sections. In this variation, gas-phase absorption photolysis rate. The lack of points with Q < 1 and Z < 1 indicates cross sections were estimated by applying a 10 nm blue shift that liquid-phase photolysis of nitrate compounds considered in to the measured cross sections in methanol The this work is too slow relative other sink processes under typical application of this 10 nm solvent-induced shift did not sig- nificantly affect the conclusions of this analysis. The actinicflux was calculated with the Tropospheric Ultraviolet andVisible (TUV) radiation model at a 24-hr average solar ing experimental data from Ref. for two hydroxyl ni- zenith angle of 65o representative of Los Angeles, USA at trates: E-2-methyl-4-nitrooxybut-2-ene-1-ol and 3-methyl- the summer solstice using a similar procedure described 2-nitroxybut-3-ene-1-ol. While SARs did a reasonable job in Ref. Both aqueous- and gas-phase quantum yields predicting the rate constants for the aqueous reaction of are unknown, but after making the simplification that they OH with simpler alkyl nitrates, SARs overpredicted the are independent of photon energy over the relevant wave- rate constants for these two compounds by a factor of 23 lengths (segment of wavelengths where the actinic flux and and 57, respectively. Therefore, this SAR is satisfactory for the absorption cross section are non-zero), we can treat the the purposes of examining the relative influence of aqueous quantum yields as a ratio. If gas- and aqueous-phase photol- photolysis but may be an order of magnitude off with respect ysis occur with the same chemical mechanism, we expect to the absolute rate of reactions with OH. Aqueous OH con- that the aqueous-phase quantum yields should be less than centrations were set to 10−13 M, the daytime cloud-water or equal to the gas-phase values In certain cases, value estimated in Ref. To understand the maximum gas- and aqueous-phase photolysis mechanisms may dif- contribution of aqueous photolysis, the unknown aqueous fer significantly, leading to a breakdown in this assumption photolysis quantum yields were set to unity. Comparison If the gas-phase and aqueous-phase quantum yields of Z and Q on the same axis can illustrate the potential are of the same magnitude, the simplified factor Z reveals significance of aqueous photolysis. reveals that the significance of aqueous-phase photolysis relative to gas- for the nine compounds with measured absorption coeffi- phase photolysis.
cients, aqueous oxidation by OH is significantly faster than Downloaded by [The UC Irvine Libraries], [Sergey A. Nizkorodov] at 09:43 11 September 2015 We can also compare the rate of aqueous photolysis aqueous photolysis even under conditions that will lead with the rate of aqueous oxidation by OH by defining a to an enhancement in aqueous photolysis rates (wet clouds, strong actinic radiation, and large aqueous photolysis quan-tum yields).
Depending on the compound and the ratio in quantum Q = dt = kOH[OH] yields, aqueous photolysis may be faster than gaseous pho- tolysis, but in all cases, oxidation by OH appears to be the dominant photo-induced sink. Comparison of predicted where kOH is the rate constant for aqueous oxidation by and measured Henry's law constants and kOH values of a OH. The values of kOH are not available experimentally few similar compounds indicates that there is the poten- in the literature, so we used group contribution structure– tial that each of these values may be overestimated by the activity relationships (SAR) that were developed to predict predictive methods we employed. A potential overestima- values for alkyl nitrates We tested these SARs us- tion in the Henry's law constant does not affect the overall Molecular Physics conclusions, as the studied compounds could be less soluble port via the graduate fellowship program. Research at the Hebrew than predicted. However, an overestimation in k University was supported by the Israel Science Foundation [grant would suppress the significance of aqueous photolysis and number 172/12].
could potentially modify the conclusion that aqueous pho-tolysis is not a significant sink.
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Source: http://aerosol.chem.uci.edu/publications/Irvine/2015_Romonosky_MolPhys_RONO2.pdf

Absorption spectra and aqueous photochemistry of -hydroxyalkyl nitrates of atmospheric interest

Molecular Physics, 2015Vol. 113, Nos. 15–16, 2179–2190, Absorption spectra and aqueous photochemistry of β-hydroxyalkyl nitrates of atmospheric Dian E. RomonoskLucas Q. NguyDorit Tran B. NguyScott A. EDavid B.C. Mar Christopher D. VanderwaR. Benny Gand Sergey A. Nizkorodov aDepartment of Chemistry, University of California, Irvine, CA, USA; bFritz Haber Center for Molecular Dynamics, The Hebrew


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