On the Electronic Structures and Electron Affinities of the m

ARTICLE pubs.acs.org/JPCA

On the Electronic Structures and Electron Affinities of the m-Benzoquinone (BQ) Diradical and the o-, p-BQ Molecules: A Synergetic Photoelectron Spectroscopic and Theoretical Study Qiang Fu,† Jinlong Yang,*,† and Xue-Bin Wang*,‡ ‡

Chemical & Materials Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, MS K8-88, Richland, Washington 99352, United States and Department of Physics, Washington State University, 2710 University Drive, Richland, Washington 99354, United States † Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China

bS Supporting Information ABSTRACT: Electron affinity (EA) is an important molecular property relevant to the electronic structure, chemical reactivity, and stability of a molecule. A detailed understanding of the electronic structures and EAs of benzoquinone (BQ) molecules can help rationalize their critical roles in a wide range of applications, from biological photosynthesis to energy conversion processes. In this Article, we report a systematic spectroscopic probe on the electronic structures and EAs of all three isomers—o-, m-, and p-BQ—employing photodetachment photoelectron spectroscopy (PES) and ab initio electronic structure calculations. The PES spectra of the three BQb
radical anions were taken at several photon energies under low-temperature conditions. Similar spectral patterns were observed for both o- and p-BQb
, each revealing a broad ground-state feature and a large band gap followed by well-resolved excited states peaks. The EAs of o- and p-BQ were determined to be 1.90 and 1.85 eV with singlet
triplet band gaps of 1.68 and 2.32 eV, respectively. In contrast, the spectrum of m-BQb
is distinctly different from its two congeners with no clear band gap and a much higher EA (2.89 eV). Accompanied theoretical study confirms the experimental EAs and band gaps. The calculations further unravel a triplet ground state for m-BQ in contrast to the singlet ground states for both o- and p-BQ. The diradical nature of m-BQ, which is consistent with its non-Kekule structure, is primarily responsible for the observed high EA and helps explain its nonexistence in bulk materials.

’ INTRODUCTION Benzoquinones (BQs) and their radical anions (BQsb
) are molecules of considerable biological importance due to their presence in all organisms as proton-coupled electron transfer agents in respiration and photosynthesis.1,2 Inspired by their biologic functions, BQ has been used as an electron acceptor in a synthetic triad, which includes a photochemically active porphyrin as well as a carotenoid polyene electron donor, to study photon-driven charge separation and conversion of light energy to proton potential in an artificial photosynthesis reaction center.3,4 In all of these processes, the electron affinities (EAs) and electronic structures of the BQ molecules and the BQb
anions play critical roles in determining the charge transfer path, the energy conversion efficiency, and the stability of the whole molecular assembly. Of the three BQ molecules, para-BQ (p-BQ) has been extensively studied to determine its EA and probe its electronic structure, employing both electronic structure calculations5
9 and experimental interrogation10 via a variety of spectroscopic techniques such as photodetachment,11 absorption,12 excitation emission and fluorescence,13 electron scattering,14 and photoionization.15 To date, r 2011 American Chemical Society

no spectroscopic studies have been reported for the ortho- (o-) and meta-BQ (m-BQ) molecules. Therefore, a systematic comparison study of the three BQ molecules is desirable and becomes more interesting considering the fact that although o- and p-BQ are normal organic molecules with standard Kekule structures as indicated in their crystal structures,16 m-BQ is an avatar of nonKekule species (Scheme 1). In contrast to the closed-shell, singlet ground states for both o- and p-BQ, a previous ab initio calculation predicted a triplet ground state for m-BQ (a diradical).17 In addition, there is an approximate 5.1 D dipole moment in o-BQ, 18 while there is no permanent dipole moment in the symmetric p-BQ species. These differences are expected to lead to a distinct, isomer-specific EA and electronic structure for each BQ molecule. Indeed, a recent experimental study based on a gas-phase proton or electron transfer bracketing technique reported an EA of 2.82 ( 0.10 eV for m-BQ,19 Received: December 19, 2010 Revised: March 17, 2011 Published: March 30, 2011 3201

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The Journal of Physical Chemistry A Scheme 1. Structures of o-, m-, and p-Benzoquinone (BQ) Neutral Molecules

significantly higher than that of o-BQ (1.90 ( 0.13 eV)19 and p-BQ (1.860 ( 0.005 eV).11,20 Anion photoelectron spectroscopy (PES) is a powerful experimental technique that can be used to directly obtain EA values and probe both ground and excited states of neutral species.21 It has proven particularly useful for studying diradical species, which are generally short-lived and highly reactive.22 Photodetaching the BQb
radical anions (all doublet) results in either closed-shell singlet or triplet states for the neutral molecules. Such distinct transitions are expected to be directly reflected in the PES spectra. In this paper, we report a systematic spectroscopic investigation on the electronic structure of m-BQ along with its o- and p- congeners, employing a recently developed lowtemperature PES coupled with an electrospray ionization source (ESI),23 and ab initio electronic structure calculations. Their EA values and different electronic structures were directly obtained and probed. It is evident from the spectra that the EA of m-BQ is much higher than those of o- and p-BQ. The unraveled electronic structure of m-BQ in the ground state is a triplet, which differs distinctly from the singlet states of o- and p-BQ. Theoretical analyses reveal that the non-Kekule nature of m-BQ is responsible for its observed unique properties.

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photoelectron analyzer. In the current experiment, two photon energies at 355 nm (3.496 eV) and 266 nm (4.661 eV) from a Nd:YAG laser were used. The laser was operated at a 20 Hz repetition rate with the ion beam off at alternating laser shots for shot-by-shot background subtraction. Photoelectrons were collected at nearly 100% efficiency by the magnetic bottle and analyzed in a 5.2 m long electron flight tube. Time-of-flight photoelectron spectra were collected and converted to kinetic energy spectra calibrated by the known spectra of I
and ClO2
. The electron binding energy spectra were obtained by subtracting the kinetic energy spectra from the detachment photon energies used. The energy resolution (ΔE/E) was about 2%; i.e., ∼20 meV for 1 eV electrons with strong ions afforded full deceleration. It deteriorated to ∼3% for weak ions. Electronic Structure Calculations. Theoretical calculations were employed to study the geometries and electronic structures of the o-, m-, and p-BQb
anions as well as the corresponding neutral molecules. Geometrical optimizations were performed with density functional theory (DFT) calculations using the hybrid B3LYP exchange-correlation functional25 and the 6-311þþ G(d,p) basis set.26 The accurate energy values were obtained through single-point energy calculations at the coupled-cluster with single and double and perturbative triple (CCSD(T)) excitations level27 with a larger basis set of aug-cc-pVDZ.28 The adiabatic detachment energy (ADE) value of the anion is calculated as the energy difference between the anion and corresponding neutral radical at their respective optimized structures. The calculated band gap corresponding to the one observed in the PES spectrum (vide infra) is regarded as the energy difference between the singlet and the triplet of the neutral molecule at their respective optimized structures. The electronic structure analyses, zero-point vibrational energy corrections (ZPE), and natural bond orbital (NBO) analyses29 all were performed with the B3LYP hybrid functional25 and 6-311þþG(d,p) basis set.26 All calculations were conducted with the Gaussian 03 package.30

’ APPROACH Low-Temperature PES Spectra. The experiments were conducted using a low-temperature ESI
PES apparatus recently developed at Pacific Northwest National Laboratory (PNNL). The details of this apparatus have been described elsewhere.23 The desired o-, m-, and p-BQb
radical anions were produced (albeit in small amounts, particularly for m-BQb
) via electrospraying 0.1 mM alcoholic aqueous solutions of their corresponding quinols, i.e., catechol, resorcinol, and hydroquinone, under slightly basic conditions (by titrating a small amount of NaOH aqueous solution). The most intense mass signals generated were due to hydrobenzoquinone anions (o-, m-, and p-HBQ
) with one more atomic mass unit (amu).24 Anions produced were guided by two radio frequency only quadrupoles, directed by a 90° ion bender to a temperature-controlled ion trap, where they were accumulated and cooled via collisions with a ∼0.1 mtorr buffer gas consisting of 20% H2 balanced in helium. Ions were trapped and cooled at the 70 K trap temperature for a period of 20
80 ms before being pulsed out into the extraction zone of a time-of-flight mass spectrometer at a repetition rate of 10 Hz. During each PES experiment, isomer-specific BQb
radical anions were carefully mass-selected to make sure that no HBQ
contamination existed in the selected ion cloud. The selected anions were decelerated before being intercepted by a probe laser beam in the photodetachment zone of a magnetic-bottle

’ EXPERIMENTAL RESULTS Electrospraying a slightly basic hydroquinone solution readily generated the p-BQb
anion with about one-third of the intensity compared to p-HBQ
. A relatively weak o-BQb
signal (∼1/6 in abundance, cf. o-HBQ
) was produced in a similar way via spraying its corresponding quinol solution. The spraying voltage and basicity of the resorcinol solution needed to be optimized to produce a trace amount of the m-BQb
anion with its abundance less than one-tenth relative to that of m-HBQ
. This degree of difficulty in generating the gaseous m-, o-, and pBQb
anions is in agreement with the trend of their corresponding enthalpies of formation reported in ref 19. Photoelectron spectra of o-, m-, and p-BQb
were recorded at 70 K (this temperature was chosen for convenience and was sufficiently low to afford elimination of hot bands for the molecules studied here23,24) and at 266 and 355 nm. Comparing the PES spectra between m-BQ and o- and p-BQ uncovered significantly different electronic structure information. Figure 1 shows the PES spectra of o-, m-, and p-BQb
at 266 (red) and 355 nm (blue). A broad band centered at ∼2.1 eV (X) is observed for p-BQb
at 266 nm followed by a clear band gap and two, well-resolved sets of sharp peaks above 4.0 eV (A and B, Figure 1c). Each set of the fine structures resolved in both A and B is equally spaced and assigned due to the associated vibrational progression in the transition from the ground state of the anion 3202

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Table 1. Experimental Electron Affinities (EAs), Calculated EAs, and Comparison with the Literature-Reported Experimental Values for o-, m-, and p-BQ Neutral Molecules (unit in eV)a compound

EA (expt.)b

EA (calc.)c

EA (G3)d

(expt. in literature)

o-BQ

1.90 ( 0.01

1.84

1.96

1.90 ( 0.13;e 1.62f

m-BQ

2.89 ( 0.01

2.75

2.88

2.82 ( 0.10e

p-BQ

1.85 ( 0.01

1.76

1.90

1.860 ( 0.005g

a

All values are from this work unless otherwise noted. b Obtained from the threshold of each spectra (see text for details). The EA also represents the adiabatic detachment energy (ADE) of the corresponding radical anions. c Energy values are obtained from single-point calculations at the CCSD(T)/aug-cc-pVDZ level, while the optimized structures are obtained based on B3LYP/6-311þþG(d,p) calculations. Zero-point energy corrections are added according to the unscaled vibrational frequencies from B3LYP/6-311þþG(d,p) calculations. d The G3 calculated EA values from ref 19. e EA from ref 19. f EA from ref 18. g EA from ref 11.

than o- and p-BQ and consistent with the recently reported value, 2.82 ( 0.10 eV.19

Figure 1. Photoelectron spectra of (a) o-, (b) m-, and (c) p-BQb
radical anions at 70 K at 266 nm (4.661 eV, red) and 355 nm (3.496 eV, blue). The vibrational progressions and band gaps are indicated.

to each respective electronic excited state of the neutral. From their first resolved peaks, the electron binding energies (EBEs) of the excited states corresponding to A and B features are determined to be 4.17 ( 0.01 and 4.34 ( 0.01 eV with vibrational frequencies of 480 and 420 cm
1, respectively. At 355 nm, only the X band is accessible and better resolved, exhibiting a few sharp peaks along the rising edge. Thus, the ADE of p-BQb
or the EA of p-BQ is determined from the first resolved peak in X to be 1.85 ( 0.01 eV (Table 1), which is consistent with the recently reported value of 1.86 eV.11 The band gap, defined as the difference between the EBE of the A peak and the ADE, is 2.32 eV (Figure 1c and Table 2). The 266 nm PES spectrum of o-BQb
is similar to that of p-BQb
with a broad and weak feature (X) spanning from 1.9 to 3.5 eV, a sharp and intense peak (A) at 3.58 eV, and a band (B) centered at ∼3.8 eV (Figure 1a, Table 2). The X feature was slightly better resolved at 355 nm (blue curve in Figure 1a), displaying discernible structures. The ADE of o-BQb
is measured from the rising edge of the 355 nm spectrum to be 1.90 ( 0.01 eV (Table 1), and the X
A energy gap is 1.68 eV. The EA derived from the current work is 0.28 eV larger than an earlier reported value (1.62 eV), 18 but it is in an excellent agreement with the recent measurement of 1.90 ( 0.13 eV.19 A drastically different spectrum is obtained for m-BQb
at 266 nm (Figure 1b). In contrast to the rather weak and broad X features in both o- and p- cases, the first two sharp peaks at 2.89 (X) and 3.06 eV (A) are intense and well-resolved. The two peaks are followed by a continuous broad band, roughly grouped into two features, B (3.2
3.6 eV) and C (3.7
4.5 eV). No band gap is evident in the spectrum. The EA of m-BQ measured from the first peak X is 2.89 ( 0.01 eV, which is significantly higher

’ THEORETICAL RESULTS AND DISCUSSIONS The anions and neutral species were first optimized at the DFT B3LYP/6-311þþG(d,p) level of theory. Then, singlepoint CCSD(T) calculations with the basis set of aug-cc-pVDZ were performed to compute the ADEs (or EAs) and singlet
triplet band gaps for direct comparison with the experimental data. In addition, NBO analyses were performed to unravel the physical insight responsible for the observed isomer-specific electronic structure among the three isomers. Electron Affinities of Neutral BQ Molecules. First, we conducted computational studies to investigate the EA values using several different theoretical methods. The results are listed in Table 1 and Table S1 (Supporting Information). In these calculations, DFT methods with the B3LYP functional25 initially were employed. Although the result of m-BQ agrees well with the experimental data (2.849 versus 2.89 eV, Table S1), the EA values of o- and p-BQ are overestimated by ∼0.3 eV in the calculations. This result could not be improved even when a larger basis set (aug-cc-pVTZ31) was used (Table S1). Therefore, the influence of the basis set can be eliminated, and the inherent insufficiency of the B3LYP method is assumed to be the main reason. In fact, the difficulty of DFT calculations in the description of p-BQ has been encountered before. Boesch et al. (1996) carried out DFT calculations to study the EA value of p-BQ.6 Although the value of 1.85 eV was obtained using the B3LYP/ 6-311G(3d,p) method, it was later found that the exception of error cancellation was responsible for the agreement.6 After the basis set was increased to 6-311þþG(d,p), the EA value was changed to 2.17 eV,6 which compared very well with our calculated number, 2.22 eV (Table S1). The small difference comes from the zero-point vibration energy correction added in our calculations. Cramer et al.8 (1999) calculated the EA value of the 2,3-didehydro-1,4-benzoquinone molecule, a derivative of p-BQ, and determined that the calculated result, 1.95 eV at the CCSD(T) level, agreed quite well with the available experimental results. Inspired by the Cramer’s results, the CCSD(T) calculations were performed to calculate the EA values of the three BQ molecules. Much improvement has been achieved, especially for the o- and p- isomers, which the B3LYP method has described 3203

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Table 2. Observed Electron Binding Energies (EBEs in eV) for the o-, m-, and p-BQb
Anions, Band Gaps (in eV), Vibrational Frequencies (cm
1), and Final State Assignments for the Corresponding Neutral BQ Moleculesa EBE b

feature o-BQb


m-BQb


p-BQb


state

band gap

expt.

calc.

d

X

1

1.90(1)c

1.84

A

3

3.58(1)

3.57

B X

1

B2 3 B2

3.8(1) 2.89(1)c

2.75

A

1

3.06

2.94

Bg

1

3.2
3.6

3.3

C

1

3.7
4.5

3.9h

X

1

1.85(1)c

1.76

A

3

3

4.17(1)

4.34

B

1

1

4.34(1)

A1 B1

B2 A A1 Ag B1g/ Au B1g/ Au

e

expt.

1.68 (X
A)

calc.d

vib. freq.

1.73

1.9 (X
B) 0.17 (X
A)

0.19 0.55

2.32 (X
A) [2.28, 2.32]f 2.49 (X
B) [2.48, 2.49]f

2.58

480(40) 420(40)

a All values are from this work unless otherwise noted. Numbers in parentheses represent the experimental uncertainties in the last digits. b Final state assignments are based on ref 5 for p-BQ. c Also adiabatic detachment energy (ADE) or electron affinity (EA) for the o-, m-, and p-BQ neutrals. d Energy values are obtained from single-point calculations at the CCSD(T)/aug-cc-pVDZ level, while the optimized structures are obtained based on B3LYP/ 6-311þþG(d,p) calculations. Calculated band gap is regarded as energy difference between the singlet and triplet at their respective optimized geometries. Zero-point energy corrections are added according to the unscaled vibrational frequencies from B3LYP/6-311þþG(d,p) calculations. e Defined as difference between the EBE (A or B) and ADE (also see Figure 1). f Band gaps obtained from ref 5. g The C2v group point symmetry does not hold in this band. h By adding the experimental EA value and the most reliable calculated term values from ref 17.

Figure 2. Optimized structures of the o-, m-, and p-BQb
radical anions (top) and the corresponding neutral molecules in their ground states (bottom) at the B3LYP//6-311þþG(d,p) level of theory (bond lengths in Å are indicated). The dashed lines represent mirror symmetries.

poorly. The calculated CCSD(T) EA values agree quite well (within 0.1 eV) with the experimental data (Tables 1 and S1). It is worth noting that Fattahi et al. (2005) also calculated the EA values of the three BQ isomers with G3 theory, and excellent agreement has been achieved between the calculations and experiments.19 Ground States of the BQ Molecules: Singlet of o- and p-BQ  Strucversus the m-BQ Diradical Triplet with Non-Kekule ture. The ground state of o- and p-BQ is found to be singlet, while m-BQ is a triplet (3B2). Their optimized structures are shown in Figure 2 with the bond lengths indicated (see Figure S1 for the related bond angles). Our results are consistent with previous works.6,17 Through the NBO analysis, the bond order in o-, m-, and p-BQ were obtained and are shown in Figure S2. In the

case of o- and p-BQ, all electrons are paired, and the chemical bonding between C and O atoms are all double-bonded with a short CdO bond length (1.21 and 1.22 Å for o- and p-, respectively). The total bond order for both the o- and p- neutrals is 16. The NBO analysis here further supports that both o- and p-BQ have classical Kekule structure with the single bond and double bond appearing in an alternating way (Scheme 1) consistent with their bulk crystal structures.16 However, in the case of m-BQ, the situation changes in that each oxygen atom has one unpaired electron, the bond order between O and C is 1.5 instead of 2 (Figure S2), and the C
O bond length increases to 1.25 Å. Thus, the total bond order of m-BQ is only 15. The NBO analysis clearly shows that m-BQ has a non-Kekule structure.32,33 The two unpaired electrons occupy so-called nondisjoint nonbonding 3204

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The Journal of Physical Chemistry A molecular orbitals in m-BQ,34 in which Hund’s rule is applied. This results in their spin directions paralleling and interacting with each other through the exchange interaction; i.e., the spin coupling of the two unpaired electrons within the two oxygen atoms is ferromagnetic. Exchange interaction of two electrons with the same spin direction can stabilize the molecule and, as a result, renders m-BQ a triplet ground state. It is interesting to note that m-benzoquinodimethane, an isoelectronic species with m-BQ, also is a diradical with a triplet ground state as reported by Lineberger and co-workers via a detailed photoelectron spectroscopic analysis.35 Considering that the total bond order is 16 for o- and p-BQ but 15 for m-BQ, one can expect that m-BQ is less stable than both o- and p-BQ. This expectation is substantiated by our calculations, which show that m-BQ is 1.59 eV higher in energy than p-BQ. In view of the electrostatic repulsion between two oxygen atoms, o-BQ also is less stable than p-, but the energy difference is only 0.34 eV. The relative energies among the three BQ molecules are p-BQ (0.00) < o-BQ (0.34) , m-BQ (1.59) (eV, in the order of reducing stability). It should be noted that these calculated energy differences among the three neutral isomers agree quite well with the previous calculations.19 Conversely, attaching one extra electron smears out the difference between the non-Kekule m-BQ diradical and the classical Kekule o- and p-BQ molecules, resulting in all-doublet radical anions (BQb
). According to our calculations, the relative energy difference among the three radical anions is significantly reduced: p-BQb
(0.00) < o-BQb
(0.26) < m-BQb
(0.54) (eV). These results also agree quite adequately with the previous calculations as well as the experimental measurement.19 The relatively high energy of the m-BQb
radical anion reported here seems to related to its relatively high reactivity as being reported experimentally.36 Taking the EA of 1.85 eV for p-BQ, the above relative energy scale between the neutrals and anions leads to EA of 1.93 eV for o-BQ and 2.90 eV for m-BQ, which is in excellent agreement with the measured EA values (Table 1). Excited States and Band Gaps. The triplet states of both o- and p-BQ molecules are regarded as electronic excited states. Our calculated singlet
triplet band gaps are 1.73 and 2.58 eV for o- and p-BQ, respectively, and in good agreement with the observed values of 1.68 and 2.32 eV (Table 2). For m-BQ, the peak at 3.06 eV (A) is assigned to originate from the first excited state, a singlet 1B2 state, which is calculated to be 0.19 eV higher in energy than the ground triplet state at the CCSD(T)/aug-ccpVDZ level of theory. This result also explains that no clear band gap is observed in the spectrum of m-BQb
. The singlet
triplet gap of p-BQ, 2.32 eV, is observed appreciably larger than that of o-BQ (1.68 eV). We attribute this fact to the different strength of the exchange coupling between the two spin-parallel electrons. In the triplet states of the o- and p-BQ neutrals, the spin directions of the two single electrons within the two oxygen atoms are parallel to each other (Figure 3). Therefore, the two electrons can interact with each other through the exchange interaction, which reduces the energy and stabilizes the molecule. As the distance between the two oxygen atoms is smaller in o- than in p-, the coupling between the two electrons is stronger in the case of o-BQ, which makes the exchange interaction larger in o-BQ. As a result, the energy of the o-BQ triplet is expected to be lower than that of p-BQ. Our calculation confirmed this expectation, showing that the triplet o-BQ is 0.44 eV lower in energy than the triplet p-BQ. Considering that the singlet o-BQ is 0.34 eV higher in energy than that of p-BQ

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Figure 3. NBO orbitals of the two unpaired electrons within each two oxygen atoms for both o- and p-BQ neutral molecules in their triplet states.

(vide supra), the energy difference between the singlet and triplet for o-BQ should be 0.78 eV smaller compared to that of p-BQ, which is in qualitative agreement with the measured band gap difference of 0.64 eV (1.68 versus 2.32 eV) (Table 2). As shown in Figure 1 and Table 2, our measured X
A and X
B band gaps for p-BQ are in an excellent accord with the gas-phase emission spectral data and theoretical calculations.5 We can safely assign peak A (4.17 eV) as corresponding to the two nearly degenerate triplet states, 3B1 g and 3Au, and peak B (4.34 eV) to the two degenerate singlet states, 1B1g and 1Au. For m-BQ, the first excited state is assigned as the 1B2 state, which is 0.19 eV higher in energy and corresponds to the A peak. The second excited state is the 1A state (the C2v group symmetry does not hold in this state) residing 0.55 eV above the ground state (Table 2), which matches the B band in the spectrum. This assignment is supported by the qualitative agreement between the calculated term values of these two states and the A and B band positions (Table 2). The C band observed in the m-BQb
spectrum (3.7
4.5 eV) is likely due to the 1A1 state, which is predicted to be ∼3.9 eV by adding experimental EA and the most reliable calculated term value from ref 17 (Table 2). The excited states of o-BQ have not been assigned before. Here, we make a tentative assignment for the o-BQ molecule based on its closeness in spectral pattern compared to p-BQ. The A peak (3.58 eV) is attributed to the triplet 3B1 state, which is calculated at 3.57 eV, and the B peak (3.8 eV) is assigned to the singlet 1B2 state (Table 2).  Isomerization. PhotodetachSpectral Reflection on Kekule ing the o- and p-BQb
radical anions leads to perfectly localized Kekule neutral molecules. This Kekule isomerization for the ground state of both o- and p-BQ is clearly manifested in the spectra. Due to the expected large geometric changes from the radical anions to the localized Kekule neutrals (Figure 2), the first band (X) of each, which reflects the Franck
Condon factors between the anion and the neutral, is expected to be very broad. This expectation is validated in the spectra. Conversely, the triplet state for each o- and p-BQ retains a diradical nature, and their expected geometries are not so different from the anions. Indeed, sharp and well-resolved peaks (A) appear in each o- and p- spectra. However, the situation is reversed for m-BQ. Because of its triplet ground state, little geometric changes are expected upon detaching the extra electron from m-BQb
(Figure 2). 3205

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The Journal of Physical Chemistry A Therefore, the first PES feature (X) is rather sharp and wellresolved.

’ CONCLUSIONS In summary, this report documents the first spectroscopic investigation on the elusive m-BQ diradical in the gas phase. By comparing its anion PES spectrum with that of o- and p-BQb
, drastic differences in the electronic structure and EA have been observed. Theoretical analysis reveals that the non-Kekule structure characteristic of m-BQ produces its unique properties, such as a triplet ground state and a large EA value. As highly reactive intermediate species, diradicals often play critical roles in a variety of chemical reactions and processes, but, in general, they are short-lived and difficult to study. Again, this work demonstrates that valuable electronic structure information and EA of diradical species can be obtained via a synergic investigation combining photodetachment of radical anions and ab initio electronic structure calculations. ’ ASSOCIATED CONTENT

bS

Supporting Information. Materials regarding the calculated EA values at different levels of theory, Cartesian coordinates of optimized anions and neutrals, bond angles of optimized anions and neutrals, and bond order for the ground state of the neutrals. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (X.-B.W.); [email protected] (J.Y.).

’ ACKNOWLEDGMENT The experimental work was supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. A portion of the research was performed using EMSL, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle for the DOE under Contract No. DE-AC05-76RL01830. The computational work was supported by the National Natural Science Foundation of China (Grant Nos. 50721091, 20803071, and 20873129), the National Key Basic Research Program (2006CB922004), the USTC-HP HPC Project, SCCAS, and Shanghai Supercomputer Center. ’ REFERENCES (1) Graige, M. S.; Paddock, M. L.; Bruce, J. M.; Feher, G.; Okamura, M. Y. J. Am. Chem. Soc. 1996, 118, 9005–9016. (2) Kurisu, G; Zhang, H.; Smith, J. L.; Cramer, W. A. Science 2003, 302, 1009–1014. (3) Seta, P.; Bienvenue, E.; Moore, A. L.; Mathis, P.; Bensasson, R. V.; Liddell, P.; Pessiki, P. J.; Joy, A.; Moore, T. A.; Gust, D. Nature 1985, 316, 653–655. (4) Osycczka, A.; Moser, C. C.; Daldal, F.; Dutton, P. L. Nature 2004, 427, 607–612. (5) Pou-AmeRigo, R.; Merchan, M.; Ortí, E. J. Chem. Phys. 1999, 110, 9536–9546. (6) Boesch, S. E.; Grafton, A. K.; Wheeler, R. A. J. Phys. Chem. 1996, 100, 10083–10087.

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