Photoelectron Imaging Spectra of O2âÂ·VOC and O4âÂ·VOC Complexes

Article pubs.acs.org/JPCA

Photoelectron Imaging Spectra of O2−·VOC and O4−·VOC Complexes Kellyn M. Patros,† Jennifer E. Mann,‡ and Caroline Chick Jarrold*,† †

Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States Physical Electronics, 18725 Lake Drive East, Chanhassen, Minnesota 55317, United States

‡

S Supporting Information *

ABSTRACT: The anion photoelectron imaging spectra of O2−· VOC and O4−·VOC (VOC = hexane, isoprene, benzene, and benzene-d6) complexes measured using 3.49 eV photon energy, along with the results of ab initio and density functional theory results are reported and analyzed. Photodetachment of these anionic complexes accesses neutrals that model collision complexes, offering a probe of the effects of symmetry-breaking collision events on the electronic structure of normally transparent neutral molecules. The energies of O2−·VOC spectral features compared to the bare O2− indicate that photodetachment of the anion accesses a modestly repulsive region of the O2−VOC potential energy surface, with subtle VOC dependence on the relative energies of the O2 (X 3Σg−)·VOC ground state and O2 (a 1Δg)·VOC excited state. In contrast, a significantly higher intensity of the transition to the O2 (a 1Δg)·VOC excited state relative to the O2 (X 3Σg−)·VOC ground state is observed for VOC = benzene, with a less pronounced effect observed for VOC = isoprene. Similar spectral effects are observed in the O4−· benzene and O4−·isoprene PE spectra. Several explanations are considered, with involvement of a temporary anion state emerging as the most plausible.

I. INTRODUCTION The importance of molecular oxygen in atmospheric chemistry and photophysics cannot be overstated.1 Atmospheric O2 generally is not implicated in initiating volatile organic compound (VOC) oxidation reactions but participates in VOC oxidation alongside radical species such as OH. As a homonuclear diatomic molecule, O2 is not a greenhouse gas, but collision-induced absorption by O2 is nontrivial2 because of its abundance in the atmosphere.3−13 Collision-induced absorption cross sections of O2 with a variety of gases have been shown to be 106 to 107 greater than isolated O2, and absorption transitions include forbidden electronic transitions to the low-lying singlet states. In addition, the abundance and therefore contribution of bound molecular complexes such as O2·H2O in the atmosphere continues to be an area of active research.14 One bound complex that is a well-established atmospheric absorber is the O2·O2 dimol,15 observed in low-temperature expansions by Ewing over 40 years ago,16,17 with atmospheric profile measurements of forbidden electronic transitions reported in the last two decades.18 The complex electronic structure of this dimol is notorious, and has been the topic of numerous computational19−22 and experimental23−26 studies (a sampling of which is included in the references), including anion dissociative detachment studies27−29 that take advantage of the high abundance of O4− in anion sources.30 In addition, O2·X van der Waals complexes have exhibited ultraviolet photosensitivity of the complexes, with an enhanced a 1Δg formation31,32 and O2 photodissociation33−37 observed in © XXXX American Chemical Society

several studies, with X identities including Ar, H2O, and a range of volatile organic complexes such as isoprene and cyclohexane. Though equilibrium intermolecular distances in van der Waals complexes are longer than the nonequilibrium intermolecular distances transiently formed in collision complexes, the common theme of spin-forbidden O2 a 1Δg formation in both O 2·X UV studies and direct near-IR collision-induced absorption underscore the unique electronic properties of O2. Here, we report new photoelectron imaging (PEI) spectra of anionic precursors to the O2·hexane, O2·isoprene, O2·benzene, and O2·benzene-d6 collision complexes, which we will refer to generally as O2·VOC. Benzene and isoprene were chosen because of their relevance in atmospheric chemistry,38−41 whereas hexane was selected as a nonfunctionalized, saturated hydrocarbon with polarizability similar to those of isoprene and benzene for comparison. As shown qualitatively in Figure 1, the O2−·VOC anionic precursors are more strongly bound than the neutral van der Waals complexes, which typically have O2− VOC binding energies, D0, ca. 0.05 eV (D298 ∼ 0.02 eV).42 Because interactions present in the anion result in a shorter equilibrium intermolecular distance, photodetachment of the anion will prepare the neutral on a repulsive part of the potential, mimicking the distortion associated with neutral O2− VOC collisions. We note here that the kinetic energy distributions of neutral fragments formed by energy of the O2 Received: July 15, 2016 Revised: September 5, 2016

A

DOI: 10.1021/acs.jpca.6b07107 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

lene47 and a number of larger oxygen clusters.48 In this work, we detail the characterization of several oxygen encounter compounds through Franck−Condon simulations, photoelectron angular distributions, and theoretical calculations in comparison to the experimental PE spectra.

II. METHODS II.A. Experimental Section. The anion photoelectron imaging apparatus has been described elsewhere.49 In brief, there are three main regions responsible for ion production, anion detection/mass selection, and electron detection by laser interaction. O2 (60 psig) seeded with room temperature VOC (hexane, isoprene, benzene, and benzene-d6) is supersonically expanded into the vacuum chamber using a pulsed (30 Hz) solenoid-type molecular beam valve (General Valve Series 9, 0.5 mm orifice). Ions are generated upon passage through an electrical discharge containing two needles (23G Precision Glide) similar to that of Duncan.50 The needles are separated by 1 mm and are approximately 2 mm from the orifice. Following production, anions are skimmed (2.5 mm diameter), accelerated to 1 keV, and then rereferenced to ground potential in a high-voltage switch before entering a 97 cm time-of-flight Bakker-type mass spectrometer.51,52 The molecular beam is focused and guided using deflectors and an Einzel lens prior to passage through a 3 mm mass-defining slit located 13 cm upstream of the interaction region. At the end of the flight path, the ions collide with a 25 mm dual microchannel plate. A mass resolution of 550 m/Δm was obtained in the mass region relevant for the current studies. Before colliding with the detector, anions of interest are selectively photodetached in the interaction region with the second and third harmonic output of a Nd:YAG laser (Continuum Surelite, 30 Hz). Photoelectrons are extracted using a velocity map imaging lens system similar to that of Eppink and Parker.53 The image on a 70 mm dual microchannel plate-phosphor screen detector is recorded using a CCD camera.54,55 Three dimensional PE velocity distributions are obtained using an inverse Abel transformation in BASEX56 and then converted to electron kinetic energy (e−KE). Photoelectron spectra are plotted as a function of electron binding energy, e−BE = hν − e−KE, which is independent of photon energy. Calibrations based on the well-known PE spectrum of O2− were performed for each complex. The pBASEX code57 was used to produce images that tend to have fewer artifacts on the polarization axis line. II.B. Computational Details. Despite the challenges associated with calculating the energies of a system strongly affected by electron correlation, the approach demonstrated by Jalbout and Adamowicz58 for O2−·benzene and the associated ground neutral state was implemented to determine which O2−· VOC and O2·VOC structures were favored, along with their relative energies. Multiple initial structures for each anion and neutral complex were explored using the GAUSSIAN09 program suite for electronic structure calculations.59 Geometries were optimized both using the second-order Møller− Plesset perturbation theory (MP2) level of theory with the ccpVDZ basis set followed by MP2/aug-cc-pVTZ single point calculations.60 In addition, density functional theory calculations using the B3LYP density functional with the aug-ccpVTZ basis set were also completed for comparison with previous calculations on the O2−·benzene (anion only) reported by Johnson and co-workers.61 To more accurately describe the extended electron density of the anions in both

Figure 1. Potential energy curves along the O2− (2Πu)−VOC and O2 (X 3Σg−, a 1Δg, and b 1Σg+)−VOC dissociation coordinate. The shaded area illustrates the Franck−Condon overlap between the initial anion state and a repulsive portion of the neutral ground state.

and VOC fragments presumably formed from O2−·VOC photodetachment is not measured in this experiment, and the Leonard-Jones potentials shown in Figure 1 are based on one set of calculated intermolecular distances (computational results are presented below), which vary significantly with method. With the potentials shown, the repulsion on the neutral surface would correspond to a hyperthermal collision, but they should be taken as qualitative rather than quantitative. The excess charge in the anion is anticipated to remain localized on the O2− portion of the O2−·VOC complex, because the VOCs chosen for this study are closed shell molecules with negative electron affinities (EAs). Therefore, as with the photoelectron (PE) spectrum of isolated O2−,43 the PE spectra of the complexes should exhibit transitions to the O2 (X 3Σg−)· VOC (S0), O2 (a 1Δg)·VOC (S0), and O2 (b 1Σg+)·VOC (S0), all of which are one-electron-allowed photodetachment transitions. Bearing in mind that spin-forbidden transitions are lit up in O2−X collisions, the overarching goal of this study is to determine whether the different collision partners affect the relative energies of the ground X 3Σg− state and low-lying singlet states of O2. The results show that the relative energies and, more strikingly, intensities of transitions to the O2 (X 3 − Σg )·VOC (S0) and O2 (a 1Δg)·VOC (S0) states do vary with collision partner. In addition, the PEI spectra of the O4−·VOC analogs are also presented and analyzed. Though less relevant to atmospheric physical chemistry, these complexes exhibit interesting variations in the different photophysical processes associated with O4− photodetachment (O4− direct detachment versus photodissociation followed by O2− autodetachment and/or photodetachment). Combined with the results of the O2−·VOC studies, benzene emerges as a unique collision partner. Anion PE and PEI spectroscopies have been applied to a number of On−·X complexes such as O2−·H2O,44,45 O2−·1,4difluorobenzene, O2−·p-xylene,46 O2−·benzene, O2−·naphthaB


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photon energy. Raw images for these spectra, along with raw images, reconstructed images, and PE spectra measured using 2.33 eV photon energy are included in the Supporting Information. No additional vibrational structure was resolved in the PE spectra measured with lower photon energy, so they will not be discussed further. The PE spectrum of O2− is wellknown43 and is included for direct comparison with the complex spectra measured on a single instrument. Band X in the spectrum is the X 3Σg− + e− ← X 2Πg transition, and bands a and b are the a 1Δg + e− ← X 2Πg and b 1Σg+ + e− ← X 2Πg transitions, respectively. As expected, the spectra of all of the O2−·VOC complexes have the same qualitative profile as the O2− PE spectrum, with distinct bands X and a (band b lying within the tail of band a, as it does in the PE spectrum of O2−). But the O2 stretch progressions are no longer resolved, and both bands X and a are shifted to higher e−BE values. Both of these features are readily rationalized by the potential energy curves shown in Figure 1. The O2− anion is stabilized by interactions with the polarizable neutral partner significantly more than interactions on the neutral surfaces; thus O2−·VOC has a higher binding energy than bare O2−. Anion detachment prepares neutrals in a broad range of O2-local vibrationally and electronically excited states, all of which are above the O2−VOC dissociation energy, so the resulting PE spectrum is broadened due to short neutral lifetimes. As pointed out by Bowen and co-workers,47 it is difficult to identify the origin of broad, unresolved electronic transitions, because Franck−Condon overlap at the origin may be negligible. To systematically describe the relative changes in electron binding energies and band intensities in the spectra, analyses of the O2−·VOC PE spectra were carried out using spectral simulations. Simple Franck−Condon simulations of all three electronic bands in the O2− spectrum were generated and are shown in the upper panel of Figure 3a using wellestablished spectroscopic parameters.43 The lower panel of Figure 3a shows the sum of the three simulations (solid black trace) superimposed on the experimental spectrum (dotted trace). The bands were then modified by adding an unresolved low-frequency mode to affect the blurring of vibrational structure. Without adjusting any other spectroscopic parameters, the relative energies and intensities of the three bands were then varied to match the profiles of the O2−·VOC spectra. Simulations of the O2−·VOC spectra are shown in Figure 3b−d. The top panel of each frame shows the three electronic states separately, prior to shifting the origin of band X to match the overall profile, and the lower panel shows the sum of simulations after shifting the bands. Table 1 summarizes the origins of all three bands, the relative energies, and the scaling factors used to match the band intensities relative to the bare O2− spectral simulation. All three VOC partners result in a comparable shift in the origin of band X, with isoprene having the largest effect, 0.51 eV, and hexane having the smallest, 0.40 eV. The polarizabilities of all three partners are comparable (in Å3: hexane, 11.9; isoprene, 9.99; benzene, 10.33)66 so the O2−· VOC electron binding energies should be comparable. The contribution from simulated band b in all three O2−·VOC simulations is small, and it appears that most of the PE signal is due to bands X and a. In the case of O2−·isoprene, the intensity of band b is larger than in the other two complex spectra, to match the relatively higher electron intensity at higher e−BE values.

types of calculations, diffuse functions were added to all atomic centers (s and p functions) using an exponent ratio of 0.3 to maintain even-tempered basis set behavior.62 Frequency calculations were performed to verify that global minima were found. The energies reported below include zero-point corrections. We note here that basis set superposition error (BSSE) corrections in comparable systems63,64,65 are on the order of several millielectronvolts for neutrals. In our own calculations on O2−·isoprene, along with the triplet and singlet neutrals, BSSE correction energies ranged from 0.02 eV for the triplet neutral to 0.05 eV for the anion and singlet neutral. Because we are interested in the relative energies of O2−·X and O2 (X 3Σg− and a1Δg)·VOC measured with fairly low resolution (vide inf ra), we do not include BSSE corrections.

III. RESULTS AND ANALYSIS III.A. PEIs and PE Spectra. III.A.1. O2−·VOC. Reconstructed PEIs of O2− and O2−·VOC (VOC = hexane, isoprene, benzene, and benzene-d6) are shown along with the resulting PE spectra in Figure 2. The spectra were all measured using 3.49 eV

Figure 2. Reconstructed PEIs and PE spectra of (a) O2− and (b)−(e) O2−·VOC obtained using 3.49 eV photon energy. The O2− spectrum is superimposed on the complex spectra to qualitatively illustrate the shift in binding energy. The origins of transitions to the X 3Σg−, a 1Δg, and b 1Σg+ states of O2 are indicated in the PE spectrum of O2−. Because of spectral broadening and threshold effects, contributions from the O2 (b 1Σg+)·VOC are not readily discerned in (b)−(e). C


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Figure 3. Spectral simulation of (a) O2− showing contributions from transitions to the X 3Σg− (blue), a 1Δg (red), and b 1Σg+ (green) states of O2, along with the sum (solid black trace) superimposed on the experimental spectrum (dotted black trace). Details on the simulations of the (b) O2−· hexane, (c) O2−·isoprene, and (d) O2−·benzene spectra are included in the text. The legends in (b)−(d) indicate by how much the intensities of bands a (red traces) and b (green traces) were scaled relative to the O2− simulated spectrum, holding the intensity of band X (blue trace) constant.

Variations in the origins of band a relative to band X in O2−· VOC transitions are small but nontrivial. The X−a energy difference [T0 for the O2 (a 1Δg)·VOC state] for the O2−· isoprene simulation is 0.07 eV larger than that of O2−·hexane, for example, with the O2−·benzene X−a interval being intermediate, and the same as the O2− X−a interval. The experimental vertical detachment energy (VDE) values for bands X and a in all the spectra, also summarized in Table 1, reflect this trend. In addition to transition energies, the PEIs show the photoelectron angular distributions (PADs) for the various transitions in the spectra. For a randomly oriented species, the differential cross section is given by67

The most striking detail that emerges from the spectral simulations is the variation in the relative intensity of band a to band X in the PE spectrum of O2−·benzene. Compared to band a in the O2− simulation, band a in the O2−·benzene spectrum is 1.6 times as intense, if the intensity of band X is held constant. In addition, the relative energies of band a in both the O2−· isoprene and O2−·benzene could be established firmly due to the partially resolved O2 stretch progression observed on the rising edge of band a in both spectra. The implication of this observation is that the O2 (a 1Δg)·VOC neutral is longer lived than the O2 (X 3Σg−)·VOC for VOC = isoprene, benzene. Although the electrons associated with the transition to the neutral O2 (a 1Δg)·VOC state are in a higher resolution region of the spectrum than those associated with the transition to the O2 (X 3Σg−)·VOC state, we note that no vibrational features were observed in the PE spectra measured using 2.33 eV photon energy (Figure S2), which would place the transitions to the O2 (X 3Σg−)·VOC state at slightly lower e−KE, and therefore higher resolution.

σ ⎡ ⎛3 1 ⎞⎤ ∂σ = total ⎢1 + β(E)⎜ cos2 θ − ⎟⎥ ⎝2 4π ⎣ 2 ⎠⎦ ∂Ω

(1)

where σtotal is the total photodetachment cross section and β(E) is an energy-dependent asymmetry parameter ranging from −1 for perpendicular transitions to 2 for parallel transitions. As D


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Table 1. Summary of Origins Used in the Simulation of the Three Electronic Transitions Contributing to the O2−·VOC PE Spectraa Simulated T0 (eV)

O−·VOC

T0 2

O−·VOC

− T0 2

(eV)

Intensity relative

Experimental

to O2−

VDE (eV)

β

Exp −

−

VDEO2 ·VOC − VDEO2 (eV)

O2− X a b O2−·hexane X a b O2−·isoprene X a b O2−·benzene X a b

−0.8 −0.6

0.448 1.425 2.075

2.40

0.85 1.80 2.48

0.40 0.37 0.40

1 0.93 0.25

1.48 2.23

−0.55 −0.3

0.66 0.45

0.96 1.98 2.59

0.51 0.56 0.51

1 1 0.5

1.50 2.51

−0.5 −0.25

0.68 0.73

0.92 1.90 2.55

0.48 0.48 0.48

1 1.6 0.25

1.44 2.38

−0.6 −0.2

0.62 0.60

Band origins for the O2− simulations were set to values reported in ref 43. Also included are the ratios of the intensity of the three bands in the simulated O2−·VOC spectra to the intensity of the same bands in the bare O2− spectral simulations.

a

Figure 4. Photoelectron angular distributions (PADs) of the vertical portions of bands X (solid line) and a (dotted line) in the (a) O2− and (b)−(d) O2−·VOC spectra.

= 0 becoming more predominant at lower e−KE resulting in β(E) approaching zero]. Figure 4 shows plots of peak intensity versus angle relative to laser polarization for the vertical portions of bands X and a for O 2− and O2−·VOC. Plots showing slices through the reconstructed images at 10° increments are included in the

shown recently by Mabbs and co-workers,67 β(E) for the O2− photodetachment transitions in the range of e−KE values sampled in this study range from −0.7 to −1 [the molecular orbital associated with the detachment transition has λ = ±1, resulting in interference between S = 0, 2 photoelectrons, with S E


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The Journal of Physical Chemistry A Supporting Information. All plots show the same general trend giving β(E) < 0, as determined using β (E ) =

I0 − I90 1 I + I90 2 0

(2)

The PADs of the O2−·VOC spectra are less pronounced than the bare O2− spectrum; their asymmetry parameters are closer to zero, which may in part be due to lower electron kinetic energies. However, band a in the PE spectrum of O2− appears at lower e−KE (higher e−BE) values than band X in the complex spectra and has a more negative asymmetry parameter than both O2−·hexane and O2−·isoprene. Band a in both the spectra of O2−·isoprene (Figure 4c) and O2− benzene (Figure 4d) have the most near-isotropic PADs with β(E) = −0.25 and −0.2, respectively. Asymmetry parameters determined for the most vertical portion of bands X and a are summarized in Table 1. III.A.2. O4−·VOC. The PEIs and PE spectra of O4− and O4−· (VOC) complexes obtained using 3.49 eV photon energy are shown in Figure 5. Again, spectra obtained using lower photon energy did not show any improved vibrational structure; reconstructed images and resulting PE spectra obtained using 2.33 eV photon energy are included in the Supporting Information. The O4− spectrum46 (along with more informative photoelectron-photofragment coincidence measurements)27−29 has been published previously, and it is included here for a direct comparison with the complex spectra obtained on the same apparatus. Neutral O4 is very weakly bound compared to the O4− anion, in which the additional charge is delocalized symmetrically between the two O2 molecules.68−70 The most intense feature in the O4− PE spectrum (Figure 5a), labeled X, is due to direct detachment of O4− to produce the dissociating dimol; it features a progression of shoulders that are spaced by the O2 neutral vibrational frequency. The position of band a relative to band X in the PE spectrum of isolated O2− is indicated on the spectrum to illustrate that an analogous band in the O4− PE spectrum is not pronounced. O4− also undergoes photodissociation to O2− + O2. At low e−BE values, direct detachment signal (“DD”) from O2− photofragments is observed, and at the highest e−BE values, autodetachment signal (“AD”) from O2− photofragments in vibrationally excited levels that lie above the detachment continuum (υ ≥ 4) is observed. The PEI and PE spectrum of O4−·hexane (Figure 5b) are very similar in profile to those of O4−, with band X shifted to higher e−BE by 0.32. The O4−·isoprene spectrum (Figure 5c) exhibits subtle differences: Band X is 0.42 eV higher in energy than band X in the O4− spectrum, and the full-width at halfmaximum is 0.15 eV broader. The intensity of the continuum signal at higher e−BE values is also greater in the O4−·isoprene spectrum. O2− vibrational autodetachment features in the O4−, O4−· hexane, and O4−·isoprene spectra have comparable intensities; the O2− direct detachment signal is slightly more pronounced in the O4−·isoprene spectrum. Because the profiles of band X in the O4−, O4−·hexane, and O4−·isoprene spectra are similar in profile, we conclude the O4− portion remains intact in the complex, an indication that [O2−O2]− is more strongly bound than O2−−VOC. VDEs and asymmetry parameters are summarized in Table 2. The O4−·benzene (Figure 5d) and O4− benzene-d6 (Figure 5e) PEIs and PE spectra are different: The primary signal appears broadened and plateau-like. The higher e−BE side of

Figure 5. Reconstructed PEIs and PE spectra of (a) O4− and (b)−(e) O4−·VOC obtained using 3.49 eV photon energy. The bare of O4− spectrum is superimposed on the complex spectra to qualitatively illustrate the shift in binding energy of the main O4− detachment feature (X). Photodissociation resulting in O2− formation is evident in all spectra: Signals from autodetachment (“AD”) of O2−(v ≥ 4) photofragments at the high-e−BE (low-e−KE) end of the spectrum and from direct detachment (“DD”) of the O2− photofragments in the lowe−BE region indicate that bare O2− is produced, independent of VOC identity.

the plateau of signal, however, is distinct from the lower e−BE side in terms of PAD. The portion of the spectrum that aligns with band X in the O4−·hexane an O4−·isoprene spectra have comparable PADs, with β ∼ −0.8, whereas β ∼ −0.1 for the higher e−BE portion. This large change in anisotropy parameter across this feature suggests two overlapping electronic transitions are contributing to what appears to be one broad band. Slices through the reconstructed images taken at different angles relative to laser polarization for O4− and O4−·VOC, included in the Supporting Information, show that the VDE for the high-e−BE feature in the O4−·benzene spectrum, labeled a, F


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the anion ground state is higher in energy than the neutral ground state, but single-point calculations with larger basis sets (only the single point calculations with the aug-cc-pVTZ basis set are included in Table 3; additional results obtained with other basis sets are included in the Supporting Information) using the MP2/cc-pVDZ optimized structures do give reasonable agreement with the observed ADE values. Both the MP2/aug-cc-pVTZ and B3LYP/aug-cc-pVTZ correctly predict the trends in the adiabatic electron affinities (ADE values) for the complexes, with O2·isoprene having the largest electron affinity, O2·hexane having the smallest. The B3LYP/ aug-cc-pVTZ calculations overestimate both the electron binding energy and the O2(a 1Δg)·VOC term energies and also give significantly longer O2 (X 3Σg−)−VOC equilibrium intermolecular distances than the MP2/cc-pVDZ optimized structures. Figure 6 shows the depictions of the O2-local πg orbitals in the lowest energy structures predicted for the O2−·VOC

Table 2. Summary of Origins Used in the Simulation of the Three Electronic Transitions Contributing to the O2−·VOC PE Spectra Experimental VDE (eV)

β

Exp VDE

−

O2 ·VOC

− VDE

−

O2

(eV)

O4− X O4−·hexane X O4−·isoprene X O4−·benzene X a

−0.65

1.57 1.89

0.32

−0.50

1.92

0.35

−0.45

1.97 2.49

0.40

−0.8 −0.1

a

Also included are the ratios of the intensity of the three bands in the simulations.

coincide energetically with the maximum of band a in the O2−· benzene spectrum. III.B. Computational Results. Calculations on molecular oxygen complexes are vexed by the challenges associated with electron correlation. However, calculations on O2−·benzene and the triplet ground state of O2·benzene reported previously by Jalbout and Adamowicz58 using ab initio methods were in fair agreement with the anion PE spectrum of O2−·benzene reported by Bowen and co-workers.47 Concomitantly, Johnson and co-workers reported results of DFT calculations61 that predicted less symmetric structures for the anion, which were validated by their infrared predissociation spectra of O2−· benzene. We have used both methods to get a qualitative picture of the structures of the anion and neutral complexes for all three VOCs, and including both the triplet and singlet neutral states. In terms of O2−·benzene, we have been able to reproduce the structures reported previously, and found that despite differences in structure, qualitatively, the O 2−(X2Πg)· benzene electronic structures predicted by the two methods are in agreement, in that the doubly occupied O2-local πg orbital points toward the benzene partner, and the singly occupied πg orbital is orthogonal to the O2−−benzene dissociation coordinate. The relative energies of the anion and neutral states calculated with the various methods are summarized in Table 3. As had been reported by Jalbout and Adamowicz,58 results of calculations at the MP2/cc-pVDZ level incorrectly predict that

Figure 6. Optimized structures of (a) O2−·hexane, (b) O2−·isoprene, and (c) O2−·benzene with depictions of two O2-local orbitals correlating to the O2 πg orbitals. In all three cases, the πg orbital pointing toward the VOC partner is doubly occupied.

complexes, based on MP2/cc-pVDZ calculations. As with O2−· benzene, the excess charge in O2−·hexane and O2−·isoprene is

Table 3. Relative Energies (eV), O2 Bond Lengths, and Nearest O−H Bond Distance Determined from O2−·VOC and O2·VOC Calculations

O2 (1Δg) O2 (3Σg−) O2− (2Πg) O2 (1Δg)·hexane O2 (3Σg−)·hexane O2− (2Πg)·hexane O2 (1Δg)·isoprene O2 (3Σg−)·isoprene O2− (2Πg)·isoprene O2 (1Δg)·benzene O2 (3Σg−)·benzene O2− (2Πg)·benzene

MP2/cc-pVDZ (opt)

r(O−O) (Å)

1.36 0 1.18 1.45 0 0.36 1.31 0 0.27 1.31 0 0.42

1.26 1.23 1.37 1.26 1.23 1.37 1.26 1.23 1.36 1.37 1.23 1.23

r(O−H) (Å)

MP2/aug-cc-pVTZ (SP)

B3LYP/aug-cc-pVTZ (opt)

r(O−O) (Å)

2.78 2.75 2.25 2.60 2.81 1.88 2.48 2.66 2.10

1.33 0 −0.29 1.26 0 −0.70 1.24 0 −0.82 1.25 0 −0.74

1.67 0 −0.56 1.67 0 −0.90 1.56 0 −1.06 1.66 0 −0.98

1.21 1.21 1.34 1.21 1.21 1.34 1.22 1.21 1.33 1.21 1.21 1.34

G

r(O−H) (Å)

exp T0 (eV)

3.24 4.98 2.32 3.08 5.00 2.18 2.88 4.98 2.04

0.977 0 −0.448 0.95 0 −0.85 1.02 0 −0.96 0.98 0 −0.92


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The Journal of Physical Chemistry A localized in the O2 πg orbital oriented toward the VOC partner (left column, Figure 6). Detachment of the electron from the singly occupied orbital (Figure 6, right column) accesses the O2 (a 1Δg)·VOC neutral state. Table 3 includes the shortest O−H internuclear distances in the complexes. As expected, the O2− VOC intermolecular distance is predicted to be shorter for the anion. The observation that the increase in ADE is greatest for the O2−·isoprene complex can be rationalized by the lowest energy structure in which the O2− is nestled between the methyl group and a H−C bond from the terminal carbon. The calculations also suggest delocalization of electrons from O2− into the conjugated π-bonds of isoprene. In agreement with Jalbout and Adamowicz,58 results on the O2−·benzene complex suggest some delocalization in the σ-bonding skeleton of benzene. The anion and neutral O2−VOC potentials are very flat. The Supporting Information includes a large number of local minimum energy structures calculated for O2−·isoprene and both the triplet and singlet neutrals. In addition to the structure shown in Figure 6b, eight additional O2−·isoprene local minima were found within 0.2 eV of the lowest energy structure. On the triplet neutral surface, eight structures converged within a 0.05 eV energy window. The range of energies of local minimum structures found for the singlet state is slightly larger (ca. 0.15 eV). The minimum energy structure has the O2 parallel to the butadiene plane, with numerous additional structures lying in a narrower window 0.1−0.15 eV higher in energy. The spectra of both O2−·hexane and O2−·isoprene may have contributions from several close-lying structures on the anion surface, though the transition energies to the associated neutral states are predicted to be nearly identical for different initial structures. On the contrary, different initial structures of O2−·benzene converged to the coplanar structure shown in Figure 6c. Calculations on O4·VOC anion and neutral complexes require a much higher level of theory. With the methods described here, O4−·VOC complexes optimized to O2−·VOC· O2 structures.

lying triplet states, and temporary ion states of benzene and isoprene. First, the ionization energies of hexane, benzene and isoprene are 10.29, 9.244, and 8.86 eV, respectively.71 Given the close proximity of O2− and isoprene in the complex anion, ca. 2−3 Å, the O2−·(isoprene)+ charge transfer state, based on IEisoprene − EAO2 − 14.4 eV·Å/r(Å) would be 1.2−3.6 eV above the ground O2·isoprene neutral state, placing the O2−·(isoprene) → O2−· (isoprene)+ transition at a minimum e−BE value of 2.2 eV. However, because this region of the spectrum features the clearest evidence of O2−-local detachment, which is a partially resolved O2 a 1Δg neutral vibrational progression, we assert that although energetically feasible, a charge transfer state is probably not the source of broader and slight enhancement of band a in the O2−·isoprene spectrum. The charge transfer bands would appear at higher e−BE values in the O2−·hexane and O2−·benzene spectra because of their higher IEs and longer O2−−VOC intermolecular distances. Second, in terms of the neutral electronic structures of the collision complexes, both benzene and isoprene have low-lying triplet states that may result in mixing between the triplet and singlet states of O2. The ã 3B1u+ triplet state of benzene is 3.47 eV above the ground X̃ 1A1g− state,72 whereas the lowest lying triplet state of isoprene, based on the butadiene triplet state energy, would be approximately 2.5 eV.73 In benzene, the X̃ 1 A1g− → ã 3B1u+ transition is both spin- and dipole-forbidden. The presence of the X 2Πg O2− molecular anion breaks the D6h symmetry of the benzene molecular orbitals (the symmetry of the two orbitals involved is b1 in the C2v point group, so μz dipole-allowed), and the change of spin is possible through exchange. The 3.49 eV photon energy used in this study is in near-resonance with the X̃ 1A1g− → ã 3B1u+ transition. Therefore, one explanation for the enhancement and nearisotropic PAD of band a in the O2−·benzene spectrum is electronic autodetachment from the O2−(X 2Πg)·benzene (ã 3 B1u+) anion state to the neutral O2 (a 1Δg)·benzene (X̃ 1A1g) + e− (e−KE ≈ 1 eV) final state. However, the appearance of band a in the O4−·benzene spectrum at the same energy, in spite of additional energy being required to dissociate O4−, does not support this mechanism. Finally, Jordan and Burrow73 summarized numerous studies describing experiments exploring temporary anion states of polyatomic hydrocarbons: Temporary anion states of benzene and 1,3-butadiene were found at 1.1 and 0.6 eV, respectively. Therefore, electrons detached from the O2−·benzene or O4−· benzene complexes with kinetic energy of ca. 1.1 eV (e−BE ∼ 2.4 eV) might be affected by the benzene-local resonance, which would lead to a less pronounced PAD. This explanation is consistent with band a in both the O2−·benzene and O4−· benzene spectra being at the same position. In the case of O2−· isoprene, temporary anion state enhancement would be observed toward e−BE ∼ 2.9 eV, and indeed, the higher e−BE end of the O2−·isoprene PE spectrum has more intensity than the other O2−·VOC spectra (this feature is most evident in Figure 3). The signal at e−BE ∼ 0.6 eV in the O4−·isoprene spectrum is also twice as intense as the signal in the O4−·hexane spectrum. Although the hypothesis that a temporary anion state may affect the appearance of the anion PE spectra of these complexes is not supported unambiguously by our observations, it is both reasonable and not contradicted by any observations.

IV. DISCUSSION The overarching goal of this study was to determine how the low-lying electronic states of O2 would be affected by different VOC collision partners. Overall, the neutral O2·VOC states accessed by detachment of O2−·VOC appear qualitatively similar to bare O2 neutral states, with what can be characterized as a solvent shift in the electron binding energy. However, there are small differences in the relative energies and more notable differences in the intensities of the X 3Σg− ← X 2Πg and a 1Δg ← X 2Πg profiles of VOC = isoprene and benzene partners. Partially resolved vibrational structure observed in band a of the O2−·isoprene and O2−·benzene spectra suggest the O2 (a 1Δg)· VOC lifetime is longer than the ground state for these two species. In light of the computational results, the degeneracy of the O2-local πg orbitals is clearly broken. The doubly occupied πg orbital favors orientation along the O2−−VOC dissociation coordinate, and the calculations predict that the a 1Δg−X 3Σg− splitting is decreased slightly [i.e., the O2 (a 1Δg)·VOC complex is more strongly bound than O2 (X 3Σg−)·VOC, consistent with a longer lifetime]. This effect is observed in the positions of band a in the O2−·hexane and O2−·benzene spectra, but not in the O2−·isoprene spectrum. The calculations give no insight into the disparate relative intensities of transitions to the triplet and singlet states. We now consider other effects that may have an impact on the spectra, including charge transfer states, lowH



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V. CONCLUSIONS Anion PEI and PE spectra of O2−·VOC (VOC = hexane, isoprene, benzene, and benzene-d6) complexes along with ab initio and DFT calculations on the anions and lowest energy triplet and singlet states of the neutrals have been reported and analyzed. Qualitatively, the PE spectra of O2−·VOC are similar to the PE spectrum of O2−, with transitions to nominally the O2 (X 3Σg−)·VOC and O2 (a 1Δg)·VOC neutral states shifted to ca. 0.5 eV higher e−BE values, consistent with typical stabilization of the O2− anion in the O2−·VOC complex and modest repulsion energy on the neutral surface. In addition, loss of O2 stretch progressions are washed out by low-frequency mode activation and dissociation of the neutral. Detachment of the anion therefore prepares the neutral electronic states as though the O2 and VOC molecules are undergoing a collision. The PE spectra exhibit subtle VOC-dependent spectral shifts in the relative ground O2 (X 3Σg−)·VOC and excited O2 (a 1 Δg)·VOC energies. Computationally, the broken degeneracy of the O2-local πg orbital favors the component pointing toward the VOC, which is predicted to stabilize the O2 (a 1Δg)·VOC state relative to the O2 (X 3Σg−)·VOC state. This effect is borne out in the O2−·hexane spectrum, whereas the O2−·iosprene and O2−·benzene/benzene-d6 spectra exhibit different spectral anomalies associated with the O2 (a 1Δg)·VOC bands. Partially resolved O2 stretch progressions are observed in the O2 (a 1 Δg)·VOC bands of the O2−·iosprene and O2−·benzene spectra, suggesting that these states are longer lived. Additionally, the O2 (a 1Δg)·benzene band is significantly higher in intensity and the O2 (a 1Δg)·isoprene band is slightly more intense and broader than the O2 (a 1Δg)·hexane band. Both benzene and isoprene have electronic structures that feature low ionization energies, low-lying triplet states, and low-lying temporary anion states. The possible role of these three features in the enhancement of the O2 (a 1Δg)·isoprene and O2 (a 1Δg)· benzene bands is considered. Comparable features observed in O4−·isoprene and O4−·benzene spectra are more consistent with temporary anion states of isoprene and benzene contributing to this enhancement.

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ACKNOWLEDGMENTS The authors gratefully acknowledge generous support for this work from the National Science Foundation, Grant No. CHE1265991.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge at The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b07107. Raw photoelectron images obtained with both 2.33 and 3.49 eV photon energies of all species considered in this paper, reconstructed PEIs of O2−, O2−·VOC, O4−, and O4−·VOC along with PE spectra obtained using 2.33 eV photon energy, slices through the reconstructed images obtained using 3.49 eV taken at 10° increments, structures of O2−·isoprene, computational results for O2−·VOC and O2·VOC (PDF)

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Article

AUTHOR INFORMATION

Corresponding Author

*C. C. Jarrold. E-mail: [email protected]. Phone: (812) 856-1190. Fax: (812)855-8300. Notes

The authors declare no competing financial interest. I


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