Spectroscopy and picosecond dynamics of aqueous NO₂.

Spectroscopy and picosecond dynamics of aqueous NO2 Ane Riis Gadegaard, Jan Thøgersen, Svend Knak Jensen, Jakob Brun Nielsen, Naresh K. Jena, Michael Odelius, Frank Jensen, and Søren Rud Keiding Citation: The Journal of Chemical Physics 141, 064310 (2014); doi: 10.1063/1.4892342 View online: http://dx.doi.org/10.1063/1.4892342 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/141/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Reaction dynamics of O(1D) + HCOOD/DCOOH investigated with time-resolved Fourier-transform infrared emission spectroscopy J. Chem. Phys. 141, 154313 (2014); 10.1063/1.4897418 Monitoring equilibrium reaction dynamics of a nearly barrierless molecular rotor using ultrafast vibrational echoes J. Chem. Phys. 141, 134313 (2014); 10.1063/1.4896536 Dynamics of the reactions of O(1D) with CD3OH and CH3OD studied with time-resolved Fourier-transform IR spectroscopy J. Chem. Phys. 137, 164307 (2012); 10.1063/1.4759619 Infrared overtone spectroscopy and unimolecular decay dynamics of peroxynitrous acid J. Chem. Phys. 122, 094320 (2005); 10.1063/1.1854094 Characterization of dynamical product-state distributions by spectral extended cross-correlation: Vibrational dynamics in the photofragmentation of NH 2 D and ND 2 H J. Chem. Phys. 112, 3181 (2000); 10.1063/1.480902

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THE JOURNAL OF CHEMICAL PHYSICS 141, 064310 (2014)

Spectroscopy and picosecond dynamics of aqueous NO2 Ane Riis Gadegaard,1 Jan Thøgersen,1 Svend Knak Jensen,1 Jakob Brun Nielsen,1 Naresh K. Jena,2 Michael Odelius,2 Frank Jensen,1 and Søren Rud Keiding1,a) 1

Department of Chemistry, Aarhus University, Langelandsgade 140, DK 8000 Aarhus C, Denmark Department of Physics, Albanova University Center, Stockholm University, S-106 91 Stockholm, Sweden 2

(Received 22 April 2014; accepted 23 July 2014; published online 13 August 2014) We investigate the formation of aqueous nitrogen dioxide, NO2 formed through femtosecond photolysis of nitrate, NO− 3 (aq) and nitromethane CH3 NO2 (aq). Common to the experiments is the observation of a strong induced absorption at 1610 ± 10 cm−1 , assigned to the asymmetric stretch vibration in the ground state of NO2 . This assignment is substantiated through isotope experiments substituting 14 N by 15 N, experiments at different pH values, and by theoretical calculations and simulations of NO2 –D2 O clusters. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4892342] INTRODUCTION

In this work, we identify the unique spectral signatures of nitrogen dioxide in liquid water and study the dynamics of its hydration. The chemistry of nitrogen oxides is at the heart of many key processes in industry and in the environment. Furthermore, nitrogen oxides serve as illustrative examples of the unique properties of aqueous chemistry. In spite of the omnipresence of nitrogen dioxide in both atmospheric and aqueous chemistry, it has remained surprisingly elusive when it comes to direct observations in the aqueous phase. Perhaps the historically most important reaction involving aqueous NO2 is the Ostwald process that played a key role in the foundation of the modern chemical industries at the beginning of the 20th century.1 In this process hot ammonia gas is oxidized over a platinum catalyst to generate NO(g) which is then further oxidized to NO2 (g) and transferred to the liquid state through a water based absorber stage whereby aqueous NO2 is obtained: Pt

O2

H2 O

NH3 + O2 −−−−→ NO(g) −−−−→ NO2 (g) −−−−→ NO2 (aq). (1) Once NO2 is absorbed in water it rapidly dimerizes and hydrolysis ultimately leads to the formation of the desired nitric acid, HNO3 and nitrous acid, HNO2 . In spite of this prominent place in the history of chemistry, the detailed molecular mechanism behind the overall reaction described above still remains more or less unresolved even after numerous experimental and theoretical investigations.2–4 A key element in the mechanism is the dimerization of NO2 to form N2 O4 .4–6 This is the case not only in the aqueous phase but also in the gas phase and intermediate heterogeneous processes where the oxidation of NO2 takes place on water droplets or ice. These heterogeneous processes are responsible for the noxious effects of NO2 in the atmosphere enabling, for example, the formation of hydroxyl radicals and nitric acid.7, 8 Considerable efforts have been invested

a) [email protected]

0021-9606/2014/141(6)/064310/11/$30.00

in the experimental and theoretical characterization of these heterogeneous processes taking place in water, on water surfaces, and on aerosols.7, 9–13 As the first step to study NO2 one must, consequently, immobilize the nitrogen dioxide molecules in order to prevent the dimerization reactions. This was previously done using matrix isolation techniques, where NO2 was trapped in a noble gas ice to enable spectroscopic investigations.14–16 Matrix isolation experiments have thus been crucial in elucidating the primary photochemical reactions taking place in the atmosphere and have also provided crucial spectroscopic knowledge of NO2 in the condensed phases. Nitrogen dioxide was observed in cold matrices by Crawford et al.14, 15 and the asymmetric stretch vibration of NO2 is observed as a strong absorption between 1610 and 1624 cm−1 depending on the exact chemical composition of the matrix. The corresponding vibrational transition in the gas phase is located at 1618 cm−1 , indicating a relatively weak perturbation by the matrix host.17 Recently, Finlayson-Pitts et al. also reported infrared spectroscopic measurements of NO2 hydrolysis in D2 O and assigned a band at 1600 cm−1 to a HNO3 –NO2 complex.6 A somewhat different approach to immobilization is through the use of time resolved spectroscopic techniques.18 Once formed in a photochemical process aqueous NO2 molecules will diffuse freely until they encounter, for example, another NO2 molecule. The average time spent between such bimolecular encounters depends on concentrations and diffusion coefficients. Assuming NO2 concentration below 1 mM, typical for photochemical experiments, the average time between two encounters between NO2 molecules is well into the nanosecond range. Consequently, if a spectroscopic investigation is carried out within the first 100 ps of the photochemical generation process, NO2 is effectively immobilized, thus excluding the possibility of bimolecular reactions. Recently Adamczyk et al.19 used a similar approach to study the dynamical properties of the aqueous bicarbonate system and the formation of carbonic acid. In the present work, we will use transient absorption spectroscopy to study the properties of NO2 on the picosecond timescale, where bimolecular reactions can safely be excluded.

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We have previously reported the photolysis of NO− 3 (aq) with femtosecond pulses at λ = 200 nm.18, 20, 21 The dominant yield in the photolysis of nitrate is isomerization to peroxynitrite ONOO− (∼48%) and internal conversion to vibrationally excited nitrate (∼44%). For the remaining ∼8% we recently suggested a third channel leading to the formation of NO2 +O− :20 − NO− 3 (aq) + ¯ω → NO2 (aq) + O (aq)

8%.

(2)

A similar reaction channel was previously also suggested by Goldstein and Rabani.22 We also proposed, that the strong infrared transition observed at ν˜ = 1610 cm−1 could be assigned to the fundamental asymmetric stretching mode of the aqueous NO2 molecule. In the present work we substantiate these claims and furthermore report the spectroscopic and dynamical characteristic of aqueous NO2 . We observe NO2 both from the photolysis of NO− 3 (aq) and CH3 NO2 (aq) using 200 nm radiation. In both cases the observation of NO2 in the vibronic ground state was delayed by 31 ps. When changing the solvent from water to acetonitrile and the excitation wavelength from 200 nm to 266 nm in the photolysis of CH3 NO2 , we were able to observe NO2 directly in the vibronic ground state without any delay. We attribute this to the formation of NO2 in an excited electronic state following photolysis at 200 nm. In addition, we have performed theoretical investigations of the excited electronic states of NO− 3 (aq) using high level ab initio methods. This enables a direct correlation between the initial excitation processes in NO− 3 (aq) and the population of the different exit channels described above. Finally, a Car-Parrinello molecular dynamics simulation was performed to investigate the hydration and spectroscopy of the aqueous NO2 observed in this work. The clear identification of NO2 in aqueous phase opens a range of new experimental possibilities for studying the fundamental chemical properties of this interesting and relevant molecule. EXPERIMENTAL AND THEORETICAL METHODS

The NO2 molecules are produced through photolysis of the parent molecules and detected using a femtosecond transient infrared absorption spectrometer, described in detail in Ref. 23. Briefly, the laser pulses are produced by a CPATi:Sapphire laser system operating at λ = 800 nm. The λ = 200 nm and 266 nm excitation pulses are generated by a combination of frequency doubling and sum-frequencymixing of the 800 nm laser pulses in β-barium borate (BBO) crystals. Typical pulse energies range from 2 to 10 μJ per pulse. The probe pulses are generated by difference frequency mixing the signal and idler pulses from an optical parametric amplifier (TOPAS), pumped by the residual 800 nm beam from the titanium-sapphire laser system. Using an AgSe mixing crystal the probe pulses cover the range from 2 to 11 μm with typical pulse energies below 75 nJ per pulse. The probe beam is divided into a signal and a reference beam. The signal beam is probing the sample inside the volume excited by the pump pulse, whereas the reference beam passes through the sample above the volume excited by the pump. The signal and reference beams are subsequently sent through a grating spectrometer and measured by a 2 × 32 channels cooled dual

array HgCdTe detector. With the present grating, the energy resolution is approximately 10 cm−1 , corresponding to 1 pixel on the HgCdTe array. The time resolution of the setup is estimated from the cross correlation between the IR probe and the UV pump and is between 200 fs and 325 fs depending on the exact wavelength of the pump and probe pulses. Nitrate solutions were based on 0.1M KNO3 in D2 O. The pH of the sample was adjusted using DNO3 . A standard pH-meter was used to measure the pH and no distinction between pD and pH was considered. An 80 mM solution of CH3 NO2 in D2 O was used corresponding to OD 1 at 200 nm. D2 O was used as the solvent in order to avoid the absorbance caused by the H2 O fundamental bending mode around 1640 cm−1 . Note that the hydrogen atoms on the methyl group in CH3 NO2 do not exchange deuterium atoms under our experimental conditions. Finally, to investigate the effects of pumping at 266 nm, a series of measurements were performed on CH3 NO2 dissolved in deuterated acetonitrile, CD3 CN. All the experiments in water were conducted in a thin liquid film obtained by carefully flowing the solution between two parallel, 50 μm thick titanium wires separated by 4 mm.24 A flow chamber using CaF2 windows was used for the acetonitrile measurements with sample thickness of 1 mm. The concentration of CH3 NO2 in d3-acetonitrile was raised due to the low extinction coefficient at 266 nm. In all experiments, sample degradation and isotopic dilution due to atmospheric exchange were monitored regularly using an ATR-FTIR spectrometer and the samples were replaced frequently. To facilitate the experimental identification of the various reactants and products, we performed electron structure calculations on the different molecules in micro-solvated environments composed of 11 water molecules (D2 O). The geometry of the micro-solvated system was optimized and the harmonic vibration frequencies were calculated for a range of solvent configurations. The calculated vibrational frequencies were multiplied with a scaling factor of 0.9664 before comparison with the experimental transition frequencies.25, 26 The chosen level of theory is the B3LYP implementation of the density functional theory with the basis set 6-31G(d). Gassian09 was used for the calculations.27 The excited electronic states in NO− 3 were calculated with the Dalton program package using a (24e,16o)CASSCF-NEVPT2 wave function28, 29 (denoted CAS-PT2 in the following) and the aug-cc-pVTZ basis set.30 A equilibrium continuum model for the solvation process was used to estimate the solvation energies for the excited states.25 Finally, the hydration of NO2 was investigated using CarParrinello molecular dynamics (CPMD) simulations in the CPMD program.31, 32 CPMD simulations were performed using KNO3 in 55 deuterated water molecules in a cubic box with 12.02 Å side length corresponding to a density of 1.15 g/ml and a concentration of 0.95 M. The BLYP functional with van der Waals corrections,33–35 a plane wave kinetic energy cut-off of 70 eV, and an NVT ensemble at 300 K was used. From an equilibrated simulation of aqueous KNO3 (80 ps), O− was manually removed to generate the starting configuration for K+ /NO2 and subsequently a 40 ps simulation was carried out for the later system for analysis of the hydration structure. After 10 ps of equilibration, the NO2


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solution was sampled for 30 ps. Each step in the simulation was 0.1 fs. For the K+ /NO2 we have used the local spin density approximation (LSD) as implemented in the CPMD program. To further strengthen the vibrational analysis, we have sampled 360 snapshots for NO− 3 and 80 for NO2 , involving /NO with surrounding water molecules solvation of the NO− 2 3 employing a distance criterion of 5 Å around N atom, resulting in clusters with 14–21 water (D2 O) molecules. The extracted clusters are subjected to partial geometry optimization in Gaussian09 program27 at the level of BLYP/aug-cc-pVDZ. Only NO− 3 or NO2 are allowed to optimize and the coordinates of the water molecules are kept frozen. Subsequently, a vibrational analysis has been carried out to get the IR fre−1 quencies of either NO− 3 or NO2 . An ad hoc shift of 60 cm − has been added to both the calculated IR spectra (NO3 /NO2 ), which is based on alignment of the nitrate asymmetric stretching to experiment as in our previous report.36

estimate the temporal dynamics of the water background as it gradually cools by thermal diffusion.37 The transient absorption from 1475 cm−1 to 1580 cm−1 reflects the isomerization of ONOO− from the trans to the more stable cis isomer.20 The signal assigned to asymmetric stretch of NO2 is observed around 1610 cm−1 . The signal rises gradually from the background and attains its maximum absorption after ∼100 ps. This is also shown in the normalized kinetic trace of the total signal at 1610 cm−1 depicted in Fig. 1(b). The blue solid line shown in Fig. 1(b) represents a model including both a rise time for the NO2 signal and a water background signal composed of a fast and slow decay time. From the transient absorption above 1650 cm−1 we obtain a fast and slow decay time of 5 ± 1 ps and ∼1 ns for the water background. The rise time of the NO2 signal was determined to be τ = 31 ± 5 ps. Isotope substitution

In order to confirm the assignment of the 1610 cm−1 absorption to NO2 , we have repeated the photolysis experiment using isotopically substituted K15 NO3 . The transient signal of the 15 N-sample, obtained under conditions similar to those described above, is shown in Figs. 2(a) and 2(b). Isotopic substitution causes the NO2 signal to shift from 1610 cm−1 to 1576 cm−1 . The normalized kinetic trace of the 15 NO2 signal at 1576 cm−1 is shown in Fig. 2(b), along with the normalized kinetic trace of 14 NO2 at 1610 cm−1 for comparison. The similarity of the two kinetic traces illustrates that within experimental uncertainty, the rise times of 14 NO2 and 15 NO2 are identical following 200 nm excitation of their parent molecules. We have summarized the line position for the 14 N- and 15 N-samples in Table I. Here we also give the observed line positions of the parent NO− 3 molecules obtained using a Fourier transform infrared spectrometer. The experimental line positions are compared both to the calculated

RESULTS NO− 3 transient absorption

Fig. 1(a) shows the induced absorption of 0.1M solution of KNO3 in D2 O for a few representative time delays following the 200 nm pump pulse at t = 0. The transient absorption caused by the ONOO− and NO2 photoproducts stands out clearly and a transient signal caused by heating of the water solvent is present throughout the entire wavelength range probed. The transient signal from water gives rise to a negative (bleaching) contribution to the transient absorption. The bleaching is caused by the increase in temperature caused by the 200 nm photolysis pulse. The shape of the background signal, shown in Fig. 1(a), is caused by the general weakening of the hydrogen (deuterium) bond network with increasing temperature and the spectral response of the weak combination band in D2 O.20 Above 1650 cm−1 the transient signal is predominantly from water and this region can be used to 0.5

0

ΔA [Norm.]

-

Solvent Heat

-

ΔA [mOD]

NO 2

trans-ONOO

0

cis-ONOO

0.25

160ps 80ps 50ps 30ps 20ps 10ps

-0.25

NO2 Kinetic -0.5

Background fit constituent NO2 fit constituent

-1

1400 1450 1500 1550 1600 1650 1700 1750

Wavenumbers [cm-1]

Total fit 0

50 100

200

Δt [ps]

300

400

FIG. 1. (a) Transient absorption of 14 NO− 3 in D2 O following excitation at 200 nm at selected pump/probe delay times. Absorption bands pertaining to transand cis-ONOO− , NO2 and the water background are marked. The dotted line is the approximate shape of the background signal obtained from previous work.20 (b) Kinetic trace (stars) of the transient absorption at 1610 cm−1 , assigned to ground state NO2 absorption. The signal is normalized to −1. The blue line is a fitted model containing a rise time of 31 ± 5 ps for the NO2 component and a double exponential background contribution with time constants 5 ps and 1 ns.


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cis-O15NOO15

ΔA [mOD]

0.25

NO

0


-0.25

-0.5 1400

1450

1500

0

2

ΔA (Norm.)

trans-O15NOO-

15

NO Kinetic

14

NO Kinetic

2 2

-1 1550

Wavenumbers [cm-1]

1600

0

Exponential Fit 50 100

200

Δt [ps]

300

400

FIG. 2. (a) Transient absorption of 15 NO− 3 in D2 O following excitation at 200 nm at selected pump/probe delay times. Absorption bands pertaining to transand cis-O15 NOO− and 15 NO2 are marked. (b) Normalized kinetic trace of 15 NO2 (blue circles) and 14 NO2 (red circles). Within experimental uncertainty, the kinetics of the two absorptions is identical. The green line is a sum of exponentials fit to the 15 NO2 kinetic trace, using parameters identical to those used in the 14 NO fit mentioned earlier. 2

values for molecules embedded in a small cluster of 11 D2 O molecules as described above and the IR-spectra sampled from the CPMD simulation. Table I also shows the calculated vibrational frequencies for the ν˜ 3 vibration of the parent NO− 3 molecule and the peroxynitrite isomers. The ν˜ 3 vibration is split into a doublet band38 at 1352 cm−1 and 1398 cm−1 due to a solvent induced lowering of the D3h symmetry of NO− 3. pH measurements

The slow formation of NO2 from photolysis of NO− 3 could either occur directly from the excited states of NO− 3 or occur as a result of the subsequent dissociation of the ONOO− primary photoproduct. The lifetime of ONOO− is known to depend on pH.18 The pKa of peroxynitrous acid (ONOOH) is 6.8 and under neutral or basic conditions the ONOO− molecules is stable for seconds or longer. However, under acidic conditions, ONOO− is rapidly protonated to ONOOH. At pH 1 the lifetime of ONOO− is below 100 ps. Hence, if NO2 is formed as a secondary reaction product from ONOO− we expect that the dynamics should mirror that of ONOO− . At low pH the concentration of ONOO− decreases rapidly and we would expect that the dynamics of the 1610 cm−1 signal, assigned to NO2 , should mimic this. In Fig. 3, we show the time evolution of the transient absorption signal at 1610 cm−1 obtained at pD ≈ 7 and pD ≈ 1. In addition, we show the

transient absorption from ONOO− at 1575 cm−1 obtained at similar pD. Such a large change in the pD value leads to a significant change in the vibrational properties of the solvent.39 In order to directly compare the kinetics obtained at different pD values, we have subtracted the solvent background from the pD = 1 kinetic trace shown in Fig. 3. We observe, in agreement with previous work,18 that ONOO− disappears at a rate corresponding to the bimolecular diffusional encounters with protons (deuterons). However, the signal at 1610 cm−1 assigned to NO2 is independent of the pD thus indicating that the formation of NO2 is independent of the presence or absence of ONOO− . Similar experiments, at pD ≈ 14 are hampered by the strong UV absorption of OD− . However, the data still confirm the hypothesis that the formations of ONOO− and NO2 are independent processes. Power/concentration dependencies

To further investigate the origin of the slowly rising signal at 1610 cm−1 assigned to NO2 we have measured the rise

TABLE I. The vibrational transition wavenumbers for nitrate, peroxynitrite, and nitrogen dioxide for fully isotope substituted compounds of 14- and 15nitrogen. The theoretical values were calculated for clusters containing 11 D2 O molecules. The vibrational frequencies sampled from the CPMD simulations (see text for computational details) are denoted 14 N-CPMD. Experiment (cm−1 )

NO− 3 NO2 ONOO− (cis) ONOO− (trans)

Calculation (cm−1 )

14 N

15 N

14 N

15 N

14 N-CPMD

1398 1352 1610 1576 1519

1372 1324 1576 1550 1492

26 28 37 26 27

1413 1403 1667 1651 1539

1381 1371 1630 1621 1511

32 32 37 30 29

1408 1322 1622 ... ...

FIG. 3. Kinetic traces pertaining to NO2 at pD ≈ 1 (red stars) and pD ≈ 7 (green stars), as well as cis-ONOO− at pD ≈ 1 (blue circles) and pD ≈ 7 (orange circles). The large solvent (water) background signal for pD ≈ 1 was obtained from spectral regions where only water contribute to the transient absorption. To facilitate comparison with the pD ≈ 7 data the large background at pD ≈ 1 is subtracted in the data shown. The ONOO− molecules disappears at a rate corresponding to the bimolecular diffusional encounters with protons/deuterons,18 whereas the NO2 absorption appears unaffected, indicating that the slow rise of the NO2 signal is uncorrelated with the presence of ONOO− .


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time of the signal as function of both excitation power and the concentration of NO− 3 . If the slow rise time is associated with the diffusional encounters between reactants, the rise time of the signal will depend on the excitation power as well as the concentration of NO− 3 . The pulse energy of the 200 nm pump pulse was varied from 1.2 to 7.4 μJ. For the range of pulse energies investigated, the rise time of the signal remained constant, 31 ps. Likewise, we measured the dynamics of the 1610 cm−1 signal at a range of NO− 3 concentrations from 0.1 to 1M. The rise time of the signal was independent of the concentration of NO− 3 , again indicating that a non-diffusional process is responsible for the formation of NO2 .

Nitromethane

To further substantiate the assignment of the peak at 1610 cm−1 to NO2 we have investigated the photolysis of aqueous nitromethane, CH3 NO2 . From previous experimental and theoretical investigations of the photochemistry of CH3 NO2 in both gas and liquid phase42–46 it is known that the dominant primary photochemical reaction is CH3 NO2 + ¯ω → CH3 + NO2

190 ≤ λ ≤ 270 nm. (3)

In this paper we focus on using CH3 NO2 as a photochemical source of NO2 , while a more detailed discussion of the photochemistry of CH3 NO2 will be given elsewhere. A pump wavelength of 200 nm drives a strong π →π ∗ transition and excites the 2 A state46 from where the molecule dissociates into the methyl radical, CH3 , and NO2 (2 B1 ). We again observe the signal at 1610 cm−1 assigned to NO2 thus confirming the above reaction scheme as the dominant primary photochemical reaction in CH3 NO2 . The relevant section of the transient spectrum is shown in Fig. 4(a), where the transient absorbance at 1610 cm−1 again is seen to grow on a picosecond timescale. In Fig. 4(b) we show the rise time of the NO2 signal observed from CH3 NO2 . A sum of exponentials fit to the NO2 trace is also shown in the figure together with the NO2 signal observed from NO− 3 . The parameters in the fit are identical to those obtained in Fig. 1 suggesting that the dynamics of NO2 formation is independent of the parent molecules. Experimental observation of fluorescence47 from NO2 as well as theoretical calculations46 confirm that NO2 formed photochemically from CH3 NO2 is formed in the excited 2 B1 state. The 31 ps rise time of the ground state signal from aqueous NO2 is thus indicative of the lifetime of the 2 B1 state. However, changing the pump wavelength from 200 nm to 266 nm changes the yield of NO2 . At 266 nm previous experimental and theoretical results suggest that the energy needed to

Temperature dependence

Finally, we investigated the dynamics of the signal formation by changing the temperature of the sample. If a vibrational relaxation process or a solvent relaxation process in liquid water is involved in the formation of NO2 , we expect that changing the temperature will affect the rise time of the 1610 cm−1 signal. The measurements were conducted at a series of temperatures in the interval 278–313 K. The temperatures were limited by the stability range of the liquid jet. In spite of the limited range of temperatures, one would still expect to see significant changes in processes depending on vibrational relaxation or thermal relaxation processes. In water, for example, the rotational diffusion time or Debye time constant, τ D , changes by more than a factor of 2 in this interval40 and the reorientation time of a hydrated solute in water will also change significantly in this temperature range.41 However, we find that the 31 ps rise time of the 1610 cm−1 signal is independent of temperature. The only noticeable difference is a slight broadening of the transition at 1610 cm−1 at higher temperatures. -1

0

-1.5 NO2

ΔA (Norm.)

ΔA [mOD]

-2

-2.5

-3


-3.5

-4

1590

1600

1610

1620

1630

Wavenumbers [cm-1]

1640

NO2 from 14NO3NO2 from nitromethane

-1 0

Exponential Fit 50

100

200

Δt [ps]

300

400

FIG. 4. (a) Transient absorption of nitromethane in D2 O following excitation at 200 nm at selected pump/probe delay times. The absorption band pertaining to NO2 is marked. The negative transient absorption is again caused by thermal effects in the water solvent. The solvent signals consistently decay to lower values with time, whereas the small NO2 signal increase with time relative to the background indicated by arrows in the figure. (b) Normalized kinetic trace of NO2 formed from photolysis of nitromethane (blue circles) and nitrate (red dots). Within experimental uncertainty, the kinetics of the two absorptions are identical. The green line is an exponential fit using parameters equal to those used to fit the model to the 14 NO2 data described earlier.


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7

1.5ps 1.7ps 2.1ps 3.3ps 8ps 15.6ps

NO2

6

ΔA [mOD]

5 4 3 2 1 0 1500

1550

1600

1650

1700

1750

1800

Wavenumbers [cm-1]

FIG. 5. Transient absorption of nitromethane in d3-acetonitrile following excitation at 266 nm at selected pump/probe delay times. The NO2 absorption appears abruptly upon excitation, and disappears completely within 10 ps.

populate the 2 B1 state in the NO2 fragment is not available instead leading to the formation of NO2 in the ground state. The extinction coefficient of CH3 NO2 at 266 nm is low and a sample cell with 1 mm path length is used. This precludes the use of water as solvent and instead we use d3-acetonitrile which is transparent in the infrared region of interest. The transient absorption of CH3 NO2 following the 266 nm pump pulse is shown in Fig. 5. The transient absorption is dominated by a very large induced absorption pertaining to the CaF2 window materials. The spectral shape of this contribution is unstructured and decays to a constant level in ∼ 15 ps. Relative to the background, we observe a strong induced absorption at 1600 cm−1 corresponding to the asymmetric stretch vibration of NO2 in acetonitrile. We also observe weak bleaching signals at 1550 cm−1 , 1650 cm−1 , and 1700 cm−1 . The bleaching signal at 1550 cm−1 is also observed in CH3 NO2 in d3-acetonitrile and it decays with same time constant as the induced absorption at 1600 cm−1 and we thus tentatively assign this signal to bleaching of the CH3 NO2 ground state. In contrast to the NO2 signals observed following photolysis at 200 nm, the signal now appears immediately after the 266 nm pump pulse. Subsequently, the NO2 signal decays while the bleaching signal at 1550 cm−1 recovers on the same time scale. This observation suggests that diffusive geminate recombination of CH3 +NO2 is the origin of the spectral dynamics shown in Fig. 5. Again the induced absorption due to NO2 appears much stronger than the corresponding bleach in CH3 NO2 . In summary, we observe the photochemical generation of NO2 molecules from both NO− 3 and CH3 NO2 . When pumping at 200 nm the transient absorption signals from NO2 appear to rise slowly with a time constant of 31 ps. We attribute this to the formation and subsequent decay of NO2 in the excited 2 B1 state. However, when pumping at 266 nm, the energy to reach the 2 B1 state is no longer available and NO2 is formed in the ground state and observable immediately after the pump pulse. In CH3 NO2 we observe geminate recombination following the 266 nm pump pulse, whereas we do not observe recombination following pumping at 200 nm. DISCUSSION

The preceding sections focused on the experimental identification of the elusive aqueous NO2 molecule. Due to its

efficient dimerization to dinitrogen tetraoxide (N2 O4 ) and the subsequent disproportionation to HNO3 + HNO2 , aqueous NO2 has, to the best of our knowledge, remained undetected in aqueous environments until now. The strong transition at 1610 cm−1 assigned to the asymmetric stretch vibration of NO2 provides a useful marker band for future studies of the kinetics involving NO2 in aqueous solutions. We begin by considering the nature of the solvated/hydrated NO2 molecules. The line width, the line strength, and the small shift in the line position compared to the gas phase value indicate that NO2 is only very weakly solvated. To investigate this further, we have performed CPMD simulations of NO2 in water (H2 O). We start out with an equilibrated solution of KNO3 dissolved in 55 H2 O molecules at room temperature and standard densities. We then manually remove O− from the simulation and let it equilibrate for 10 ps to establish equilibrium hydration structures for both K+ and NO2 . Subsequently, the NO2 solution is simulated for 30 ps and the relevant radial distribution functions (RDF) pertaining to the NO2 –H2 O structures are sampled. In Fig. 6 we show the RDF for both NO− 3 and NO2 . The NO− 3 RDF from the CPMD simulation shows a distinct hydration shell with short (i.e., strong) hydrogen bonds between the oxygens on NO− 3 and the hydrogens on H2 O (rO–H = 2Å) also seen in the RDFs between nitrogen and hydrogen. This creates a structurally well-defined hydration shell around the NO− 3 ion. We can compare this structure with − the structure of the small water-NO− 3 clusters (NO3 -(D2 O)11 ) used to estimate the vibrational frequencies of the hydrated molecules calculated using density functional theory. Here we obtain hydrogen bond lengths of rO–H = 1.88 Å and rN–H = 2.3 Å for oxygen and nitrogen on NO− 3 accepting hydrogen bonds from water. These values are in excellent agreement with the RDF’s obtained from the CPMD simulation. Similarly, the oxygen-oxygen and nitrogen-oxygen bond lengths (rO–O = 2.82 Å, rN–O = 3.23 Å) between water and nitrate are in agreement with the size and geometry of the NO− 3 -water hydration shell. However, this structure changes completely when NO2 is considered by removing the O− ion from the

-

NO3 NO2 O-Hw

g(r)

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N-Hw

N-Ow

1

2

3

4

5

6

r [Ang]

FIG. 6. CPMD simulation of NO3 − and NO2 in H2 O. The dashed curves depict equilibrium distances between the N and O atoms of the nitrate ion and the atoms of its nearest water molecules, Hw and Ow. The solid curves depict the situation following the removal of O− from the nitrate ion, and then letting the resulting NO2 molecule and the surrounding water equilibrate for 30 ps. It follows that the average distance between solute and solvent increases during the reaction.


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CPMD simulation. First of all, the volume of the hydration shell increases by nearly a factor of 4 and all the RDF’s attain their maxima between 3.5 Å and 4 Å. In addition, no particular difference is seen between the different RDF’s indicating an unstructured hydration shell without any dominating interactions. Consequently, unlike the case with the NO− 3 ion, water molecules close to NO2 do not have any definite solvation structure. Due to the weak interactions with NO2 the solvent molecules are instead favoring water-water hydrogen bonding. This is in qualitative agreement with the force field calculations of the NO2 adsorption by water molecules by Murdachaew et al.48 They also estimated the binding energy of a H2 O–NO2 molecule to be ∼9.6 kJ/mole, i.e., only a few times the thermal energy kT. When considering (NO2 –(D2 O)11 ) clusters the potential minima structures all show NO2 residing on the surface of the clusters, indicative of a hydrophobic interaction with the water molecules. This indicates that the observed narrow line width of the NO2 asymmetric stretch vibration at 1610 cm−1 and its weak solvatochromatic shift is a direct consequence of the weak interactions in the hydration of NO2 . In addition, the observed asymmetric stretch transition appears very strong compared to the corresponding transitions observed in NO− 3 , CH3 NO2 , and ONOO− . Part of this is caused by the narrower line width. The weak hydration of NO2 gives rise to less broadening of the vibrational transition. Compared to NO− 3 the line width is ∼3 times narrower and thus, with comparable oscillator strength, three times stronger in the spectrum. The broadening of the transition with increasing temperature is also indicative of a solvent-solute interaction of hydrophobic character. Having established the structure and spectroscopic characteristics of the aqueous NO2 molecules, we turn to photolytic production of NO2 and, in particular, the observation that NO2 is solely formed in an excited electronic state with a lifetime of 31 ps, and independent of the parent molecule. Before we discuss the results of the theoretical investigation of the excited electronic states in NO− 3 we address the photolysis of CH3 NO2 : The photochemistry of gas phase CH3 NO2 has been studied extensively.47, 49–52 The very rapid dissipation of vibrational energy in the aqueous phase makes the comparison between gas phase and aqueous phase yields problematic.53 However, gas phase results can still provide excellent guidelines for identification of the available dissociation channels for the primary photochemistry in aqueous phase. In spite of different views of the state specific photochemistry of gas phase CH3 NO2 , there is consensus that the primary photochemical processes below 220 nm leads to breaking of the C–N bond and formation of CH3 +NO2 . Excitation leads to the population of the S3 state and through a conical intersection the S2 state is populated. This leads to the formation of the methyl radical in the ground state and NO2 in one of the low lying electronic states (12 B2 , 12 A2 , or 12 B1 ). In particular, the results from Rodriguez et al.50 and Arenas et al.46, 49 indicate that none of the accessible conical intersections at high excitation energies connects the photo excited state with electronic states correlating with CH3 +NO2 in their respective ground states. Furthermore, their work suggests that the 12 B1 state is the most likely exit channel for NO2 as a result of the S2 /S3 conical intersection. Thus the gas phase

J. Chem. Phys. 141, 064310 (2014)

result strongly indicates that when pumping above 5 eV the photolysis will yield NO2 in an electronically excited state. This is in excellent agreement with our observation of the CH3 NO2 (aq) photolysis at 200 nm where a delayed formation of ground state NO2 is observed and attributed to the decay of an excited electronic state populated in the photolysis process. At 266 nm we observe the prompt formation of ground state NO2 indicating that the 4.66 eV pump energy available is short of the energy needed to populate the 12 B1 state and, furthermore, it is too low to access the S2 /S3 intersection at ∼5 eV. This, in combination with lower lying conical intersections connecting to the electronic ground state, could be responsible for the prompt observation of ground state NO2 following photolysis at 266 nm. As seen in Fig. 5 the signal rapidly decays again and we attribute this to a rapid geminate cage back reaction53–55 among the ground state CH3 and NO2 photoproducts. The time evolution of the recombination can be fitted to a standard expression for geminate recombination involving an error function dependence on time.53 The rapid recombination observed indicates that the two photo fragments hardly separate and recombine after a very short diffusion time. Recombination is not observed when pumping at 200 nm and we attribute this to the larger excess kinetic energy available compared to pumping 266 nm. We speculate that this gives a larger initial separation between the fragments and subsequently reduces the chance for a diffusional encounter leading to recombination. Furthermore, the recombination of CH3 and NO2 in the excited 12 B1 state does not lead to the formation of ground state CH3 NO2 . The combination of a larger initial separation and the 31 ps lifetime of the 12 B1 state thus effectively preclude recombination to the ground state of CH3 NO2 . Having established good agreement between the observations reported in this work and the previous experimental and theoretical work on CH3 NO2 we will return to a discussion of the observations following photolysis of NO− 3 with 200 nm pulses. Based on a theoretical investigation of the excited states in NO− 3 this corroborates our interpretation of the experimental observations pertaining to NO2 and the structures of the excited states also sheds light on the internal conversion and formation of ONOO− from the photolysis. To simplify the analysis we have divided the calculation of excited states in NO− 3 into four segments: calculation of the excitation process, solvation of the excited states, geometry relaxation of the excited and solvated states, and finally calculation of the asymptotic exit channels pertaining to the rupture of one of the N–O bonds in NO− 3 . This division also reflects the different time scales involved in the processes, as the excitation step is faster than the solvent reorganization. The solvent relaxation is, in a simple picture, composed of a contribution from electronic polarizability and orientational (Debye) reorganization. For water the electronic polarization dominates the solvation effect. After excitation and solvent relaxation using the fixed NO− 3 ground state geometry, we alto relax geometrically. This is, naturally low the excited NO− 3 a gross simplification, but nevertheless it provides a useful insight into the dynamics of the internal conversion of the excited states and the transition states leading to the formation of both ONOO− and NO2 . Finally, we consider the total


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TABLE II. NO− 3 excitation energies E in eV. Oscillator strengths, Osc, in atomic units. CCSD denotes results obtained with a HF reference wave function and electron correlation taken into account by the CCSD method, while CASSCF denotes results obtained with a full valence CASSCF reference wave function (24 electrons in 16 orbitals). CAS-PT2 denotes results obtained by optimizing the lowest energy wave function of each symmetry and including dynamical correlation by multi-reference second order perturbation theory. Gas phase CCSD

CASSCF

State

E

Osc

E

Osc

X1 A1 11 B 1 21 B 1 31 B 1 11 B 2 21 B 2 11 A2

6.16 6.06 6.67 7.59 5.53 6.16 4.05

0.094 0.000 0.000 0.064 0.000 0.094 0

6.13 6.33 8.01 8.86 6.13 7.42 4.62

0.196 0.000 0.000 0.065 0.196 0.000 0

TABLE III. CASSCF geometry optimization of NO− 3 within C2v symmetry. and 1 B1 states are minima within C2v . Na O is the unique NO distance while Nb O is the two symmetric NO distance (Å), ONO angles in degrees. CAS-PT2 relative energies in eV. The superscripts a/b on the oxygen atoms are depicted on the figure adjacent to the table. 1A 1

Geometry

Aqueous phase CAS-PT2 E 6.33 6.84

5.42 3.98

CASSCF E

Osc

5.88 6.54 9.38 10.96 5.09 7.40 3.59

0.213 0.000 0.000 0.001 0.010 0.007 0

a

CAS-PT2 E

E(CAS-PT2)

State

Na O

Nb O

a ONb O

b ONb O

Gas

Aqueous

1A 1 1B 1 1B 2 1A 2

1.263 1.332 1.169 1.420

1.263 1.225 1.280 1.130

120 116 127 113

120 128 107 133

0 6.70 5.41 4.01

0 7.61 6.37 4.08

O

N

7.60 7.76

b

6.96

point group. The geometries in the gas- and aqueous phases are identical within the number of digits reported in Table III. We have also performed analogous calculations of the 3 lowest triplet states, which all correlate with the 1 NO− 2 + O dissociation channel. As nitrite was never observed as a primary photoproduct in the photolysis of nitrate,21 we conclude, as expected, that intersystem crossing plays a negligible role in the photolysis of NO− 3 (aq), and triplet state results are not reported. The lowest excited 1 B1 state is a minimum within the C2v symmetric geometry, while both the 1 B2 and 1 A2 states are first order saddle points. The C2v optimized 1 B2 state corresponds to a strong shortening of the unique N–O bond, which puts the molecule into a region where several states of different symmetry are close in energy. The optimized geometry of the 1 B2 state has an imaginary frequency belonging to the b2 representation, corresponding to an in-plane distortion of the two symmetric N–O bond lengths. Lowering the symmetry to Cs along this normal mode correlates the excited state with a wave function of 1 A symmetry, and thus provides a path for dissipating the excitation energy into non-reactive vibrational 22, 36 We hypothesize that the primary excienergy of NO− 3. tation to the 1 B2 state leads to a dynamical situation where it becomes possible to couple to the lower energy 1 A2 state by shortening the unique NO bond length, or alternatively break the C2v symmetry leading to non-reactive internal energy transfer. The optimized geometry of the 1 A2 state has an imaginary frequency belonging to the b1 representation corresponding to an out-of-plane distortion. Lowering the symmetry to Cs along this normal mode correlates the excited state with the lowest state of 1 A symmetry which upon geometry relaxation leads to a stable pyramidal structure 3.59 eV above the NO− 3 ground state. The lowest vibrational mode of this structure has a calculated frequency of 175 cm−1 and corresponds to a symmetry breaking twisting motion. A scan along this normal mode leads to a transition structure 4.36 eV above the NO− 3 ground state energy surface (C1 symmetry). − This connects the NO− 3 and ONOO (trans) structures. The − ONOO structures in the trans configuration are calculated to be 2.54 eV higher in energy than NO− 3 . The calculations therefore suggest that the formation of the ONOO− occurs from the 1 A2 state that relaxes to a pyramidal structure and

4.03

energy of the different exit channels pertaining to the rupture of an N–O bond in order to identify the threshold for the different dissociation limits and, at least to a crude approximation, the correlation to the excited states in NO− 3. Using the D3h optimized ground state geometry of NO− 3 (rN−O = 1.263 Å) we calculated the lowest excited states of the four representations within the C2v subgroup to which the NO2 +O− asymptotic limit correlates. The B2 representation is chosen as being symmetric with respect to reflection in the molecular plane, while the B1 representation is antisymmetric. The excitation energies and associated oscillator strengths calculated with the aug-cc-pVTZ basis set are shown in Table II. Results at the coupled cluster single double (CCSD) and partly triple (CC3)56 levels with the smaller aug-cc-pVDZ basis set indicate that inclusion of correlation beyond CCSD lowers the excitation energies by ∼0.2 eV. The CCSD and complete active space self-consistent field (CASSCF) results disagree on which of the two lowest 1 B2 states has an oscillator strength significantly different from zero, while they both predict that the two lowest 1 B1 states have vanishing transition probabilities. Excitation to the 11 A2 state is symmetry forbidden. Including solvation effects by the continuum solvation model suggests that excitation is possible to both 1 B2 states, while the two lowest 1 B1 states are still dark. The reliability of a non-equilibrium solvent model (neglecting the orientational contribution to solvation) was tested using time dependent density functional theory (TDDFT).25 The energy differences relative to the equilibrium model was found to be smaller than ∼0.1 eV. Given that the photolysis is initiated with a 200 nm light pulse, equivalent to 6.2 eV, the results in Table II indicate that the primary excitation process populates the 1 B2 state, which is in line with earlier work.57 Excitation into the 1 A1 state is also a possibility, but that will most likely just lead to internal conversion to the ground 1 A1 state. Table III shows the result of geometry relaxation of the lowest singlet energy state of NO− 3 of each symmetry within the C2v

O

b

O


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via the lowest vibrational mode leads to a transition to the ground state electronic surface. Here NO− 3 has a geometry that can either lead to (i) peroxynitrite,20, 58 (ii) relax to ground 2 2 − state NO− 3 , or (iii) dissociate into NO2 + O . The partition21 ing between these channels is most likely determined by dynamical factors during the transition between the excited and ground state electronic surface, which our present calculations cannot quantify.

TABLE V. Dissociation channels.a Dissociation

Gas

Aqueous

Spin-state(s)

1 NO − + 3 O 2 2 NO + 2 O− 2 1 NO − + 1 O 2 3 NO − * (3 B ) + 3 O 2 1 1 NO − * (1 B ) + 3 O 2 1 2 NO * (2 B ) + 2 O− 2 1

3.80 4.33 6.14 6.07 6.93 6.07

3.63 3.70 5.97 5.93 6.81 5.30

Triplet Singlet, triplet Singlet Singlet, triplet Triplet Singlet, triplet

a

CAS-PT2 relative energies in eV.

GROUND AND EXCITED STATES OF NO2

Table IV shows the CASSCF optimized structures of the lowest state of the four representations within C2v symmetry of NO2 . A number of low lying doublet states in NO2 are found in good agreement with the well-known observations of visible absorption in NO2 . Note also that the effect of solvation is very modest in good agreement with the observation of very narrow vibrational transitions in NO2 and the modest solvent shifts between water, other solvents, and gas phase. The 2 A1 state is a minimum within C2v while the 2 B1 state is linear. 2 B2 and 2 A2 distort to Cs symmetry and correlate with the lowest state of 2 A and 2 A symmetry, respectively. The calculated asymmetric stretch frequency of the 2 A1 ground state is in good agreement with observation. The 2 B1 state was observed in early gas phase work by Huber and Douglas who reported a vibrational transition at 880 cm−1 which is outside the spectral coverage of our experimental setup. In general, the spectroscopic signatures of the excited states in gaseous NO2 are complicated to assign due to the complexity of the vibronic interactions among the low lying doublet states in NO2 .59, 60 The 2 B1 state is coupled to the ground state through the Renner-Teller effect, thus paving the way for 31 ps decay observed. It is important to note that the recombination of ground state NO2 with ground state O− yields ground state ni1 trate; NO2 (2 A1 ) + O− (2 P) → NO− 3 ( A1 ) whereas recombi2 nation of NO2 in the excited B1 state and ground state O− does not correlate to ground state nitrate. Consequently, in case of geminate recombination, or a cage back reaction, all ground state NO2 molecules will result in ground state NO− 3. Table V shows the CAS-PT2 energies of the various exit channels relative to the ground state NO− 3 energy as referred to in the discussions above.

SUMMARY

Our experimental results show the formation of aqueous NO2 formed through photolysis of either nitrate or nitromethane. Aqueous NO2 is characterized by a strong infrared transition at 1610 cm−1 corresponding to the asymmetric stretch vibration of ground state NO2 . The appearance of ground state NO2 is delayed by 31 ps when pumping at 200 nm, whereas the molecule appears promptly when pumped at 266 nm. We attribute this to the population and subsequent decay of the excited 2 B1 state in NO2 when pumping at 200 nm and assign the 31 ps decay to the lifetime of the excited 2 B1 state. When pumping at 266 nm, the energy available is insufficient to populate the excited 2 B1 state. The strong and narrow vibrational transition observed for NO2 , and the similarity between the gas phase and liquid phase transition frequency is caused by the solvation structure of aqueous NO2 . In marked contrast to the hydration structure of NO− 3 , the solvent shell surrounding ground state NO2 is much larger, unstructured, and only weakly interacting, indicating a very weak solvation of NO2 allowing it to attain a gas phase like structure in aqueous solution. Our experimental and theoretical results provide an explanation of the complex mechanism responsible for the primary photochemistry 1 of nitrate. The aqueous NO− 3 molecules are excited to the B2 ∗ (π π state character) at 200 nm. This state couples both to the ground state of NO− 3 giving rise to the observed rapid recombination/internal conversion (44%), and to the 1 A2 state from where it can either isomerize to ONOO− (48%) or it can dissociate into NO2 +O− fragments. If the NO2 +O− fragments are in their electronic ground states fast recombination 1 to the NO− 3 A1 ground state is possibly giving rise to vibrationally excited NO− 3 , i.e., contributing to the 44% yield. The remaining NO2 molecules, formed in the excited 2 B1 state,

TABLE IV. CASSCF geometry optimization of NO2 within C2v symmetry, 2 A1 and 2 B1 states are minima within C2v . R(N–O) is the bond length in NO2 in Å,

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