Primary photochemistry of peroxynitrite in aqueous solution

Chemical Physics Letters 641 (2015) 187–192

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Primary photochemistry of peroxynitrite in aqueous solution Jan Thøgersen a , Reinhard Kissner b , Thomas Nauser b , Willem H. Koppenol b , Bo Richter a , Frank Jensen a , Søren Rud Keiding a , Svend J. Knak Jensen a,∗ a b

Department of Chemistry, Langelandsgade 140, Aarhus University, Aarhus C DK-8000, Denmark Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zurich, Zürich CH-8093, Switzerland

a r t i c l e

i n f o

Article history: Received 4 August 2015 In final form 23 October 2015 Available online 3 November 2015

a b s t r a c t The photolysis of aqueous peroxynitrite, ONOO− (aq), by 266 nm UV radiation is explored by femtosecond UV–UV and UV–IR transient absorption spectroscopies. The experiments show that 90 ± 10% of the photolyzed ONOO− anions remain dissociated after 400 ps. We analyze the photolysis in terms of five potential reaction channels, using steady-state absorption spectra together with electron structure calculations on potential photoproducts in clusters of water molecules. In addition, calculations of all excited electronic states of ONOO− (g) below the excitation energy help to explore the reaction channels. The dominant reaction channel is h + ONOO− → O2 •− + NO• . © 2015 Elsevier B.V. All rights reserved.

1. Introduction Nitrogen monoxide is an intercellular messenger that controls blood flow, thrombosis, and neural activity, yet nitric oxide is also cytotoxic and is often associated with nonspecific host defenses against tumor cells and intercellular pathogens. However, the pioneering research of Beckman and Koppenol [1,2] indicates that the cytotoxicity thought to be caused by nitrogen monoxide is more likely to result from peroxynitrite, ONOO− , produced by the diffusion-limited reaction of NO• with O2 •− [3]. ONOO− is a powerful toxic oxidant and has been associated with oxidative stress. Also, in combination with H+ and CO2 , it hydroxylates, nitrates, and oxidizes biomolecules. These reactions are currently perceived as fundamental to disease and aging [4]. Obviously, the rate with which NO• reacts with O2 •− to form ONOO− is a key to the toxicity of ONOO− . Former studies with nanosecond time resolution have revealed that UV photolysis of ONOO− leads to the formation of NO• and O2 •− and the subsequent back-reaction has been used for determining the rate of ONOO− formation [3]. However, nanosecond spectroscopy may not necessarily provide the true picture of the primary photochemistry as the reactive photoproducts may undergo unimolecular or bimolecular reactions in the picosecond range [5]. Also, cage effects between photo fragments and solvent may lead to fast geminate recombination and isomerization, which will complicate the analysis [6]. In an effort to enhance the current understanding of the chemistry of

∗ Corresponding author. Tel.: +45 87155926; fax: +45 8619 6199. E-mail address: [email protected] (S.J.K. Jensen). http://dx.doi.org/10.1016/j.cplett.2015.10.056 0009-2614/© 2015 Elsevier B.V. All rights reserved.

peroxynitrite, we have utilized femtosecond ultraviolet and infrared transient absorption spectroscopy to address the primary photochemistry of peroxynitrite in aqueous solution after absorption of 266 nm radiation. The main objectives have been to determine the time-dependent yield of the primary photodissociation process and to establish the primary photo-products. We have explored the following potential reaction channels: h + ONOO− → NO− 3 −

h + ONOO →

•−

h + ONOO− → O −

•−

(2) •

+NO2

h + ONOO → O2 +NO −

(1)

O+NO− 2

−

h + ONOO → O2 +NO

•

(3) (4) (5)

2. Experimental and computational details 2.1. Experimental details The experimental results were obtained with UV pump–UV probe and UV pump–IR probe transient absorption spectroscopy in two separate experiments. During both experiments, a beam of 100 fs, 800 nm laser pulses from a 1 kHz amplified Ti: sapphire laser system with a pulse energy of 1 mJ was split into two equally powerful beams. One of the 800 nm beams was frequency tripled in two consecutive ␤-BaBO3 (BBO), crystals to generate the 266 nm pump pulses used for the photoexcitation. The beam of pump pulses was modulated at 0.5 kHz by a mechanical chopper synchronized to the 1 kHz pulse repetition rate, and sent through a scanning delay line

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and a /2 wave plate before it was focused 5 mm behind the sample by a concave mirror. The other part of the 800 nm beam was used for making the probe pulses as follows: In the UV pump–UV probe experiment, the 800 nm beam pumped an optical parametric amplifier (OPA). The beam of tunable UV probe pulses was generated by frequency doubling and sumfrequency mixing the output of the OPA in two consecutive BBO crystals. The probe beam was divided into a signal and a reference beam. The signal beam was focused on the sample by a concave mirror, thus probing the sample inside the area defined by the pump beam, whereas the reference beam passed through the sample next to the pump beam. Signal- and reference-pulses were subsequently detected by matched photodiodes and phase-sensitive amplifiers that were referenced to the 0.5 kHz of the mechanical chopper. In the UV pump-IR probe experiments, the beam of IR probe pulses was generated by difference-frequency mixing the signal and idler pulses from the OPA. As in the UV pump–UV probe setup, the probe beam was divided into a signal- and a reference-beam. The signal beam was focused on the sample by a concave mirror, thus probing the sample inside the volume excited by the pump pulse, whereas the reference beam passed through the sample next to the pump beam. The signal- and reference-beams were subsequently sent through a grating spectrometer and measured by a 2 × 32 channels cooled dual-array HgCdTe detector referenced to the 0.5 kHz of the mechanical chopper. All measurements were taken with the pump beam polarization at the magical angle (54.7◦ ) relative to that of the probe beam. The samples consisted of a constantly flowing film of aqueous tetramethylammoniumperoxynitrite, N(CH3 )4 ONOO, solution suspended between two parallel, 50 ␮m thick titanium wires separated by 4 mm. The sample was kept at 5 ◦ C and replaced frequently to reduce the buildup of permanent photoproducts. The equilibrated IR spectra are obtained with a Thermo-Fisher 380 ATR FTIR spectrometer.

calculated vibrational frequencies, which are also needed for free energy estimates. The calculations are performed on a small number of geometry optimized clusters consisting of a photoproduct and 11 water molecules using the DFT method B3LYP/6-31G(d), implemented in the Gaussian09 package [10]. The starting configurations for the geometry optimization of the clusters were designed to facilitate hydrogen bonding between the photoproduct and the solvent molecules and among the solvent molecules. The obtained harmonic frequencies were scaled by the empirical scaling factor 0.9614 relevant for B3LYP/6-31G (d) [11]. In most cases, three clusters were investigated for a given photoproduct and the thermal average of the relevant frequency was obtained. This technique has proved useful in other situations [12–14]. The intensities of the vibrational transitions used for estimating the quantum yields of reactions (1)–(5) are obtained from geometry optimizations using a DFT method, wB97xd, which incorporates dispersion forces along with a basis set with diffuse functions, augcc-pvdz. This combination is expected to give more accurate results than the B3LYP/6-31G(d) method [15]. In order to probe for bound excited states of ONOO− , the potential energy surface of all excited states of ONOO− in the gas phase with energies less than the applied excitation energy of 4.5 eV were explored within the coupled cluster framework at the CCSD and CC3 levels, as described in [16]. The calculations were done with the software package Dalton2013 [17] using the aug-cc-pvtz basis set. Both singlet and triplet states were considered. The justification for performing calculations on a single ONOO− ion, rather than on a more realistic and demanding aqueous cluster rests on the result presented in the next section that the quantum yield is close to one for this system, which indicates that cage back reactions are unimportant.

3. Results and analysis 2.2. Preparation of the aqueous solution 3.1. Absorption spectra of the potential species The synthesis of tetramethylammoniumperoxynitrite is described elsewhere [7–9]. Solutions of N(CH3 )4 ONOO in NaOH/D(aq) (pH > 13, c = 0.1 M) were prepared under strictly inert conditions. N(CH3 )4 ONOO (stored in N2 (l)) was transferred to a darkened, Ar-filled glovebox (O2 /H2 O < 0.1 ppm) in which it was immediately weighed with a plastic spatula and weighing boats, and transferred to a dark plastic tube. The sample tube was removed from glove box and NaOH/D(aq) was added immediately with inert syringe techniques; a balloon with Ar was attached through the cap of the sample plastic tube, and the NaOH/D solution was added directly by syringe. The tube was sealed and the contents subjected to measurements as a stable yellow solution. The NaOH/D solutions were prepared from triple destilled H2 O or D2 O and NaOH/D. All handling of N(CH3 )4 ONOO ex-solution was carried out avoiding light, CO2 , moisture, air, ambient heat, and metallic parts (e.g., metal spatulas). Solutions were prepared with concentrations between 40 and 70 mM and the pH of the solution was larger than 13. In some cases, the solvent was chosen as D2 O in order to be able to observe photoproducts that absorb in the region 1550–1750 cm−1 , where the bending vibration of H2 O(l)) leads to an obscuring, broad absorption band. The low concentration of peroxynitrite assures that bimolecular reactions among photoproducts from different parent species can be safely ignored. 2.3. Computational details In order to assist the assignment of the absorption transients and estimate the quantum yields of reactions (1)–(5), we

As transient absorption spectroscopy identifies the primary photoproducts of aqueous peroxynitrite by their absorption spectra, we first present the equilibrated IR and UV spectra of the species involved in reactions (1)–(5). Ideally, we would determine, which of the reactions (1)–(5) are important by analyzing our spectroscopy data using literature data for the potential photoproducts in aqueous solution. However, such information is not available in all cases, and we therefore complement the missing information with electronic structure calculations. Fig. 1a shows the IR spectrum of tetramethylammoniumperoxynitrite dissolved in D2 O. The N O stretch vibration of ONOO− (aq) is clearly observed at 1574 cm−1 , while the distinct peaks of the N(CH3 )4 + cation are observed at 1488 and 1421 cm−1 . The three lines are superimposed on a background from the D2 O combination band. In addition to the absorption bands of the peroxynitrite solution, Table 1 lists the dominant IR absorption frequencies of likely photoproducts along with calculated intensities. The characteristic double peaked absorption band associated with the asymmetric stretch of NO3 − (aq) has two strong maxima at 1340 and 1390 cm−1 . The absorption band associated with the asymmetric stretch of the aqueous nitrite anion, NO2 − (aq), peaks at 1240 cm−1 , while the NO2 • (aq) radical has a strong absorption band at 1610 cm−1 associated with its asymmetric stretch transition [18]. The nitrogen monoxide radical, NO• (H2 O), has a weak absorption band at 1912 cm−1 and the oxidonitrate anion, NO− (H2 O) absorbs at 1424 cm−1 . Fig. 1b displays the UV spectra for most of the species in (1)–(5). The absorption of NO3 − (aq) shows a maximum at 200 nm,

J. Thøgersen et al. / Chemical Physics Letters 641 (2015) 187–192

a

189

1.0 +

A (arb. units)

0.8

N(CH3)4

-

ONOO

0.6 0.4 0.2

D2O

0.0

10000 8000

ε (l/mol cm)

1600 1800 -1 Frequency (cm )

-

NO3

ε (l/mol cm)

b

1400

6000 4000

-

NO2

1600 O2 1400 1200 1000 800 600 400 200 O 0 200 250

-

-

300

350

NO2 400

450

-

O

250

-

NO

ONOO NO

-

0 200

ONOO

Wavelength (nm)

O2

2000

2000

300 350 400 Wavelength (nm)

NO2

450

500

Fig. 1. a: IR absorption spectrum of a 70 mM solution of tetramethylammonium peroxynitrite in D2 O, b: The UV–VIS absorption spectrum of the peroxynitrite anion [24] and the potential photoproducts reproduced from [19, 25–29]. Photoproducts from N(CH3 )4 + are not included, as the cation does not absorb radiation at 266 nm.

Table 1 Vibration frequencies (cm−1 ) of ONOO− (D2 O) and potential photoproducs in the 266 nm induced photodissociation of ONOO− (aq). Species

Frequency/cm−1

Mode

Intensity/KM/Mole

ONOO− (D2 O) NO3 − (H2 O)

1574 1390 1340 1240 1610 1912 1424

NO stretch Asym. stretch

303 610 460 582 441 40 76

NO2 − (H2 O) NO2 • (D2 O) NO• (H2 O) NO− (H2 O)

Asym. stretch Asym. stretch Stretch Stretch

The frequencies in bold are scaled frequencies, calculated at the B3LYP/6-31G(d) level, otherwise experimental. The intensities are in all cases calculated at the wB97xd/aug-cc-pvdz level, in the harmonic approximation on clusters with 11 water molecules (light or heavy, as the case may be).

while the absorption associated with NO2 − (aq) peaks at 210 nm, and that of O2 − (aq) peaks around 245 nm. O− (aq) has a relatively modest absorption with a maximum at 210 nm, and the absorption of ONOO− (aq) is maximum at 308 nm. The absorption of NO− (aq) has only been determined in the range from 250–320 nm [19]. We show here that it increases with decreasing wavelength and attains a maximum value of about half of that of ONOO− (aq). NO2 • (aq) has a weak and very wide absorption band with a maximum around 400 nm. The UV spectrum of O2 (aq) is not included in Fig. 1b because the absorption is only appreciable below 200 nm. Likewise, the UV spectrum of NO• (aq) is not included, as no literature value could be found. However, NO• (g) absorbs in the VUV and CIS(d)/aug-cc-pvdz calculations on a cluster, NO,7H2 O,OH− , mimicking NO• in a strong basic solution, indicate that the UV absorption takes place below 200 nm.

Fig. 2. a: Comparison of the transient absorption after 2 ps (red line) and the (inverted) steady-state absorption spectrum of N(CH3 )4 ONOO from Fig. 1a (black line), b: Contour plot of the transient absorption pertaining to the NO stretch of the ONOO− (aq) ground state. The constant bleaching of the ground state absorption indicates that all of the photoexcited ONOO− anions remain dissociated after 100 ps.

3.2. UV pump–IR probe transient absorption dynamics We first assess the time-dependent change in the ground state ONOO− concentration induced by the 266 nm photolysis pulse. If one assumes that the extinction coefficient of the individual ground state ONOO− molecules is not perturbed by the excitation, then the change in the concentration of ground state ONOO− molecules is proportional to the induced absorption change, AONOO (t). Fig. 2 shows the induced absorption dynamics of the N O stretch of ONOO− when peroxynitrite is excited at 266 nm. The induced absorption pertaining to the peroxynitrite ions is superimposed on a background from heated water molecules situated in the vicinity of the photoexcitation site [20]. This solvent background has been removed in Fig. 2. Fig. 2a compares the transient absorption spectrum after 2 ps, to the steady-state absorption spectrum of ONOO− (aq) from Fig. 1. The transient absorption of the ONOO− band is in full agreement with the steady-state spectrum, which indicates that the induced absorption change observed after 2 ps is entirely due to the removal of ground state peroxynitrite ions by the excitation pulse. The lack of induced absorption at 1488 cm−1 further suggests that the N(CH3 )4 + counter ion is not excited by the pump pulse in agreement with the fact that N(CH3 )4 + does not absorb at 266 nm. Fig. 2b shows the contour plot of the transient absorption from 1500 to 1700 cm−1 during the first 400 ps after the excitation pulse. The data shows that the negative absorption change (blue and green colors) associated with the excitation of ONOO− attains its minimum value of A = −0.8 mOD after 3 ps. The magnitude as well as the spectral profile of the negative absorption signal remains virtually constant over the next 400 ps, which indicates that none of the excited ONOO− molecules return to

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a

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the contribution to the product yield from a potential NO3 − anion is below 15%.

0.001

ΔA (OD)

0.000 -0.001 -0.002 -0.003 -0.004 1250

3.4. The ONOO− → O + NO2 − channel

-

NO3 (H2O) 1.1 ps 10.4 ps 104 ps 405 ps

1300

1350

1400

1450

1500

-1

b

Frequency (cm ) 0.001

ΔA (OD)

0.000 -0.001

-

NO2 (H2O) 1.1 ps 10.4 ps 104 ps 405 ps

-0.002 -0.003 -0.004 1150

1200

1250

1300

1350

-1

Frequency (cm )

c

0.004

ΔA (OD)

0.002 0.000 -0.002 -0.004 1800

1.1 ps 10.4 ps 104 ps 405 ps

1900

2000

2100

2200

-1

Frequency (cm ) Fig. 3. The transient absorption spectra measured at three spectral intervals for four representative delays after the UV excitation pulse. Fig. 3a compares the transient spectra from 1250 to 1500 cm−1 to the stationary absorption spectrum of ground state NO3 − (H2 O; pH > 13). Fig. 3b compares the transient spectra from 1150 to 1350 cm−1 to the stationary absorption spectrum of ground state NO2 − (H2 O; pH > 13) and Fig. 3c shows the transient spectra from 1800 to 2250 cm−1 , where NO(aq) is expected to absorb.

the ground state during this time. Based on arguments presented below, we assume that excitation at 266 nm lead to dissociation. Accordingly, we estimate that 90 ± 10% of the peroxynitrite ions excited by the 266 nm pump pulse remain dissociated for at least 400 ps. We now turn to the possible product channels. 3.3. The ONOO− → NO3 − channel We first look for the formation of NO3 − , which has a strong double-peaked absorption with maxima around 1340 and 1390 cm−1 due to its asymmetric stretch vibration. Fig. 3a presents the transient absorption spectra from 1250 to 1500 cm−1 at four representative delays after the UV excitation pulse. Apart from a slowly evolving change in the solvent background signal, no apparent change in the absorption spectrum is observed. Hence, there is no evidence that supports the formation of NO3 − by the photodissociation. Considering that absence of evidence is not necessarily evidence of absence, we entertain the possibility that a small amount of NO3 − could be hidden within the experimental detection limit. Using the calculated intensities of the NO stretch transition of ONOO− and the asymmetric stretch transition of NO3 − listed in Table 1 together with the measured spectral widths of the same transitions taken from Fig. 2 and ref [21], we estimate that

Next, we look for the formation of O + NO2 − . Since the O atom has no infrared absorption and its UV absorption lies beyond the spectral range of our experiment, this channel may only be identified by the IR and UV absorption bands of NO2 − . Fig. 3b compares the transient spectra from 1150 to 1350 cm−1 at four characteristic delays after the UV excitation pulse to the stationary absorption spectrum of ground state NO2 − (H2 O). The transient spectra show no absorption features that would indicate the formation of NO2 − during the photodissociation. Furthermore, transient UV spectroscopy showed no sign of NO2 − . Based on the experimental uncertainty, we estimate an upper value of 10% for the photodissociation of ONOO− into NO2 − + O, using the calculated intensities of the NO stretch transition of ONOO− and the asymmetric stretch transition of NO2 − listed in Table 1 together with their measured spectral widths taken from Fig. 2 and [20]. 3.5. The ONOO− → O•− + NO2 • channel The third possible channel is dissociation of peroxynitrite into O•− + NO2 • . As O•− (aq) has no infrared absorption and as its UV absorption is rather modest, the identification of this channel relies entirely on the observation of NO2 • by IR spectroscopy. The absorption band of NO2 • (aq) at 1610 cm−1 is positioned on the high-frequency flank of the O N stretch band of ONOO− at 1574 cm−1 , which makes it difficult to discern it from the absorption band of ONOO− in the ground state. Nevertheless, the excellent agreement between the transient data after 2 ps and the steadystate absorption of ONOO− (aq) shown in Fig. 2a leaves little room for an additional absorber at 1610 cm−1 , which suggests that NO2 • is not formed by photodissociation. Applying the same approach as for NO3 − and NO2 − , we estimate that less than 4% of the excited peroxynitrite anions dissociate into O•− + NO2 • within the first 2 ps. Previous photodissociation studies of nitrate and nitromethane have revealed that NO2 • may be formed in an excited state having a lifetime of tens of picoseconds [18]. It is therefore possible that ground state NO2 • first forms after some 50 ps. However, our transient absorption measurements are unable to positively identify NO2 • at any time after photodissociation within the first 400 ps. Accordingly, we estimate that the dissociation to O•− + NO2 • constitutes at most 4% of the excited peroxynitrite ions within the first 400 ps. 3.6. The ONOO− → O2 + NO− channel Having determined that reactions (1)–(3) are of minor importance, if any, we now turn to the potential formation of O2 + NO− . Molecular oxygen has no dipole moment and thus no IR absorption. Moreover, the UV absorption of O2 (aq) lies beyond the spectral range of our instrument. The identification of the ONOO− → O2 + NO− channel therefore depends on the observation of NO− . According to our calculations, NO− (aq) absorbs around 1424 cm−1 and has a rather weak infrared absorption due to its relatively small dipole moment (see Table 1). Not surprising, the infrared transient absorption measurements represented by the transient spectrum after 2 ps in Fig. 2a fail to identify NO− . However, the small intensity of the NO− IR band along with the lack of infrared identification leads to liberal limits on the yield. In fact, based on the infrared measurements alone, we estimate that all excited peroxynitrite anions could dissociate into O2 + NO− without being detected by our IR spectrometer.

J. Thøgersen et al. / Chemical Physics Letters 641 (2015) 187–192

191

0.006 0.005 0.004

-

ONOO +O2

ΔA(OD)

0.003

-

0.002 0.001 0.000 -0.001 -0.002 -0.003 240

-

-

260

270

ONOO +NO

250

280

290

300

310

320

Wavelength (nm) Fig. 4. Comparison of the measured transient absorption spectrum after 1.1 ps (red dots) and the sum of the steady-state absorption spectrum of ONOO− and O2 •− (black line), and ONOO− and NO− (green line) adjusted for the best overall agreement with the transient spectrum after 1.1 ps. The measurements cover a spectral range from 245 to 320 nm in 16 individual probe wavelengths. The measurements are placed on a common scale by keeping the experimental parameters constant during all measurements and by measuring the individual wavelength in random order to reduce systematic effects. The error bars indicate the typical reproducibility of the individual measurements. The data have been corrected for minor contribution from hydrated electrons and hydroxyl radicals resulting from two-photon ionization of water.

In contrast to the weak IR absorption, NO− (aq) has a substantial UV absorption, potentially allowing its identification by UV spectroscopy (see Fig. 1b). The red dots in Fig. 4 indicate the transient ultraviolet absorption 1.1 ps after the photolysis of the peroxynitrite solution. The measured data points are compared to the combined change in the absorption of ONOO− (aq) and NO− (aq) assuming all excited ONOO− dissociate into O2 + NO− (green curve). Considering the uncertainties involved, the agreement between the measurements and the total steady-state spectra is satisfactory, and thus suggests that NO− could be the product of photolysis. 3.7. The ONOO− → O2 •− + NO• channel Like in the previous case, the infrared absorptions of O2 •− and NO• are rather modest. As demonstrated by the transient spectra presented in Fig. 3c, no absorption is observed around 1912 cm−1 , where NO• (aq) is expected to absorb, but the calculated intensity of the absorption is so low that even if this channel dominated the dissociation, we would still be unable to observe it with our setup. On the other hand, the known UV absorption of O2 •− (aq) is significant and stretches from about 220 to 300 nm. In Fig. 4, the combined steady-state absorption spectra of ONOO− (aq) and O2 •− (aq) reported in the literature is compared to the UV transient absorption after 400 ps under the assumption of a 100% yield of O2 •− (aq). The good agreement with the transient data suggests that O2 •− + NO• may indeed be formed by photodissociation of ONOO− . Thus, an interim conclusion of the analysis in the preceding sections is that photodissociation of peroxynitrite proceeds most probably via channel (4) and/or channel (5). 3.8. Dynamics of the photoproducts We now address the photodissociation dynamics derived from the UV–UV measurements in an attempt to differentiate between the possible reaction channels ONOO− → O2 •− + NO• (reaction 4) and ONOO− → O2 + NO− (reaction 5). Fig. 5 shows the

Fig. 5. Transient absorption dynamics at two wavelengths. The red curve is taken at 300 nm and is associated mainly with the loss of ONOO− due to photo discociation. The blue curve is taken at 260 nm and is related to the formation of O2 •− or NO− . a: Time span up to 400 ps and b: Time span up to 40 ps.

time-dependent absorptions at 260 and 300 nm, which essentially represent the concentrations of O2 •− /NO− and ONOO− , respectively. The excitation of ONOO− at 266 nm induces a drop in the absorption at 300 nm, as part of the ONOO− molecules are excited from the ground state. The negative absorption at 300 nm attains its minimum within 300 fs after the excitation pulse. Subsequently, about 15% of the absorption recovers after 400 ps. Under the assumption that the induced absorption at 300 nm is a direct measure of the ONOO− concentration, the UV absorption at 300 nm indicates that about 85% of the photoexcited ONOO− ions remain dissociated after 400 ps. This is in good agreement with the 90 ± 10% dissociation yield determined from the UV–IR measurements. The instrument-limited (300 fs) drop in the UV absorption pertaining to ONOO− stands in contrast to the initial 3 ps drop to the minimum IR absorption of ONOO− . We speculate that the slower drop in the IR absorption may be due to incomplete subtraction of the substantial solvent background at early delays, or may be caused by the absorption of short-lived transient species. The O2 •− /NO− absorption trace at 260 nm attains its maximum within 300 fs and then decreases by about 20% in 2.3 ps before rising to a second maximum in 20 ps (Fig. 5). Since O2 •− and NO− are the only ground state species with significant absorption at 260 nm (see Fig. 1), the initial absorption dynamics at 260 nm are likely due to short-lived ONOO− configurations on the path to dissociation or to the ground state. Having reached its second maximum, the 260 nm absorption drops by the same relative amount and on the same timescale as the absorption recovery that pertains to ONOO− . This suggests that the absorption dynamics at 260 and 300 nm are due to the recombination of O2 •− with NO• or, alternatively, to the recombination of NO− with O2 to form ground state ONOO− . Ideally, the UV–UV data could differentiate between the two, but by an unfortunate coincidence, the combined absorption of ONOO− together with NO− and the combined absorption of ONOO− together with O2 •− , are too close to discern within the experimental uncertainty, as seen in Fig. 4.

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4. Discussion Some assistance in differentiating between reaction 4 and 5 can be obtained from theoretical considerations. First, we have calculated the change in Gibbs energy for the reaction •

•

O2 + NO− → O2− +NO

(6)

in aqueous solution. The standard Gibbs energies of formation for O2 , NO− , O2 •− , and NO• in water are +16.4, +180.1, +33.8, and +102.0 kJ/mol [22,23], respectively, which leads to G◦ = −61 kJ/mol. This indicates that the ONOO− → O2 •− + NO• channel is the thermodynamically favored reaction channel. To further explore the excitation dynamics, we have calculated all the excited states (singlets and triplets) of ONOO− with energies below the excitation energy of 4.5 eV. Geometry optimizations of the excited states show that they all result in barrier-less dissociations to the O2 •− + NO• products. The theoretical gas phase considerations therefore support that the ONOO− → O2 •− + NO• channel is the dominant one. The present study thus shows that the photoproducts found in photolysis studies with nanosecond time resolution are indeed the primary products formed within the first picoseconds after the photolysis. The finding of a quantum yield close to one is relevant for assessing the assumptions made to determine the rate constant for the formation of ONOO− by measuring the reaction rate of NO• and O2 •− produced by photodissociating ONOO− [3]. The analysis of experimental data assumed a uniform distribution of NO• and O2 •− reactants, i.e., without local overrepresentation of NO• and O2 •− around the photodissociation sites. The virtual lack of geminate recombination in the present studies with femtosecond time resolution suggests that the NO• and O2 •− fragments indeed travel so far apart that reactions with products from other photolysis sites are just as likely as diffusive recombination of geminate pairs of products. Hence the assumed uniform distribution of the reactants is largely fulfilled. 5. Summary The primary photodissociation of peroxynitrite in aqueous solution has been investigated by femtosecond spectroscopy. Five reaction channels have been considered. IR analysis indicates that the dominant photoproducts are O2 + NO− or O2 •− + NO• rather than NO3 − , NO2 − , NO2 • , O•− , or O. The experimental data show that the products are formed within 1 ps picosecond, but one cannot

conclude which of the two dominant channels is the most important. Theoretical studies strongly favor the ONOO− → O2 •− + NO• channel and thus support the findings of earlier studies [3] with nanosecond time resolution. References [1] S. Beckman, T.W. Beckman, J. Chen, P.A. Marshall, B.A. Freeman, J. Proc. Natl. Acad. Sci. 87 (1990) 1620. [2] W.H. Koppenol, J.J. Moreno, W.A. Pryor, H. Ischiropoulos, J.S. Beckman, Chem. Res. Toxicol. 5 (1992) 834. [3] T. Nauser, W.H. Koppenol, J. Phys. Chem. A 106 (2002) 4084. [4] P. Pacher, J.S. Beckman, L. Liaudet, Physiol. Rev. 87 (2007) 315. [5] S.R. Keiding, D. Madsen, J. Larsen, S. Knak Jensen, J. Thøgersen, Chem. Phys. Lett. 390 (2004) 94. [6] J. Larsen, D. Madsen, J.A. Poulsen, T.D. Poulsen, S.R. Keiding, J. Thøgersen, J. Chem. Phys. 116 (2002) 7997. [7] D.S. Bohle, B. Hansert, S.C. Paulson, B.D. Smith, J. Am. Chem. Soc. 116 (1994) 7423. [8] D.S. Bohle, P.A.Q. Glassbrenner, B. Hansert, Methods Enzymol. 269 (1996) 302. [9] D.S. Bohle, E.S. Sagan, Inorg. Synth. 34 (2004) 36. [10] M.J. Frisch, et al., Gaussian 09, Revision A.02, Gaussian, Inc, Wallingford, CT, USA, 2009. [11] C.J. Cramers, Essentials of Computational Chemistry, 2nd edn., Chichester, Wiley, UK, 2004. [12] S. Knak Jensen, S.R. Keiding, J. Thøgersen, Phys. Chem. Chem. Phys. 12 (2010) 8926. [13] J.B. Nielsen, J. Thøgersen, S. Knak Jensen, S.B. Nielsen, S.R. Keiding, Phys. Chem. Chem. Phys. 13 (2011) 13821. [14] J.B. Nielsen, J. Thøgersen, S. Knak Jensen, S.R. Keiding, Chem. Phys. Lett. 567 (2013) 50. [15] F. Jensen, J. Chem. Phys. 117 (2002) 9234. [16] O. Christiansen, A. Halkier, H. Koch, P. Jørgensen, T. Helgaker, J. Chem. Phys. 108 (1998) 2801. [17] K. Aidas, et al., WIREs Comput. Mol. Sci. 4 (2014) 269. [18] A.R. Gadegaard, J. Thøgersen, S. Knak Jensen, J.B. Nielsen, N.K. Jena, M. Odelius, F. Jensen, S.R. Keiding, J. Chem. Phys. 141 (2014), 064310. [19] W.A. Seddon, J.W. Fletcher, F.C. Sopchyshyn, Can. J. Chem. 51 (1973) 1123. [20] C. Petersen, J. Thøgersen, S. Knak Jensen, S.R. Keiding, P. Sassi, Phys. Chem. Chem. Phys. 10 (2008) 990. [21] J. Thøgersen, J. Réhault, M. Odelius, T. Ogden, N.K. Rena, S.J. Knak Jensen, S.R. Keiding, J. Helbing, J. Phys. Chem. B 117 (2013) 3376. [22] V. Shafirovich, S.V. Lymar, Proc. Natl. Acad. Sci. USA 99 (11) (2002) 7340. [23] D.A. Armstrong, R.E. Huie, W.H. Koppenol, S.V. Lymar, G. Merényi, P. Neta, B. Ruscic, D.M. Stanbury, S. Steenken, P. Wardman, Pure Appl. Chem. (2015), http://dx.doi.org/10.1515/pac-2014-0502. [24] R. Kissner, T. Nausser, P. Bugnon, P.G. Lye, W.H. Koppenol, Chem. Res. Toxicol. 10 (1997) 1285. [25] J. Mack, J.R. Bolton, J. Photochem. Photobiol. A128 (1999) 1. [26] M. Fischer, P. Warneck, J. Phys. Chem. 100 (1996) 18749. [27] J. Rabani, Adv. Chem. Ser. 81 (1968) 131. [28] B.H.J. Bielski, J. Photochem. Photobiol. 28 (1978) 645. [29] M.V. Grätzel, A. Henglein, J. Lilie, G. Beck, Ber. Bunsen-Ges. Phys. Chem. 73 (1969) 646.