Electron Affinities and Electronic Structures of o-, m-, and p

Jul 29, 2010 - Department of Physics, Washington State University, 2710 University Drive, Richland, ... University of Science and Technology of China...
0 downloads 0 Views 3MB Size
Download PDF
J. Phys. Chem. A 2010, 114, 9083–9089

9083

Electron Affinities and Electronic Structures of o-, m-, and p-Hydroxyphenoxyl Radicals: A Combined Low-Temperature Photoelectron Spectroscopic and Ab Initio Calculation Study Xue-Bin Wang,*,† Qiang Fu,‡ and Jinlong Yang*,‡ Department of Physics, Washington State UniVersity, 2710 UniVersity DriVe, Richland, Washington 99354, Chemical and Materials Sciences DiVision, Pacific Northwest National Laboratory, P.O. Box 999, MS K8-88, Richland, Washington 99352, and Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei, Anhui 230026, China ReceiVed: April 26, 2010; ReVised Manuscript ReceiVed: June 14, 2010

Hydroxyl substituted phenoxides, o-, m-, p-HO(C6H4)O-, and the corresponding neutral radicals are important species; in particular, the p-isomer pair, i.e., p-HO(C6H4)O- and p-HO(C6H4)O•, is directly involved in the proton-coupled electron transfer in biological photosynthetic centers. Here we report the first spectroscopic study of these species in the gas phase by means of low-temperature photoelectron spectroscopy (PES) and ab initio calculations. Vibrationally resolved PES spectra were obtained at 70 K and at several photon energies for each anion, directly yielding electron affinity (EA) and electronic structure information for the corresponding hydroxyphenoxyl radical. The EAs are found to vary with OH positions, from 1.990 ( 0.010 (p) to 2.315 ( 0.010 (o) and 2.330 ( 0.010 (m) eV. Theoretical calculations were carried out to identify the optimized molecular structures for both anions and neutral radicals. The electron binding energies and excited state energies were also calculated to compare with experimental data. Excellent agreement is found between calculations and experiments. Molecular orbital analyses indicate a strong OH antibonding interaction with the phenoxide moiety for the o- as well as the p-isomer, whereas such an interaction is largely missing for the m-anion. The variance of EAs among three isomers is interpreted primarily due to the interplay between two competing factors: the OH antibonding interaction and the H-bonding stabilization (existed only in the o-anion). 1. Introduction -

Hydroxyl substituted phenoxides, o-, m-, p-HO(C6H4)O , and the corresponding neutral radicals are important species; in particular, the p-isomer also known as reduced semiquinone is an intermediate species involved in the proton-coupled electron transfer for quinone reduction in biological photosynthetic centers1-8 and energy conversion.9-11 They are also important derivatives of the phenoxide anion/phenoxyl radical which play important roles in a wide range of combustion, atmospheric chemistry, and biological processes.12-14 The electron affinity (EA) and electronic structure of quinone15-24 and phenoxyl25-30 have been subjects of extensive experimental and theoretical studies. The EA of p-benzoquinone (p-•O(C6H4)O•) has been reported multiple times using various gas phase techniques15-19 and ranges from 1.37 to 1.99 eV,31 with the most recent and accurate value of 1.860 ( 0.005 eV.15,32 The EA of phenoxyl (C6H5O•) has been measured to be 2.253 ( 0.006 eV,27 and its electronic structure and vibrational frequencies have been investigated as well.25-27 In contrast, our knowledge about hydroxyphenoxyl is very limited. Only the EAs of HO(C6H4)O• have been estimated with relatively large uncertainties (>0.1 eV) via gas-phase proton or electron transfer bracketing techniques (indirect method)31,33 which nevertheless show significant differences in a comparison to the EAs of pbenzoquinone and phenoxyl and their dependence on different OH positions. With the importance of hydroxyl substituted * Corresponding authors. E-mail: [email protected] (X.-B.W.); [email protected] (J. Y.). † Washington State University and Pacific Northwest National Laboratory. ‡ University of Science and Technology of China.

phenoxyl species, it is desirable to obtain more accurate, isomerspecific EAs, and to investigate their electronic structures, which for the p- isomer is directly relevant to the proton-coupled electron transfer reaction in photosynthetic reaction centers. Gas phase negative ion photoelectron spectroscopy (PES) has been demonstrated to be a powerful experimental technique to directly obtain EAs and electronic structures of neutral species (see ref 32 for a comprehensive review, also refs 15, 27, 34). With the coupling of an electrospray ionization source (ESI), many anions that exist in solutions are now easily transported into vacuum and have been investigated by PES.35-37 The new development of cold ion trap techniques38 makes vibrational cooling of molecular species possible,39-41 and has been shown to significantly sharpen the threshold region of PES spectra, often leading to vibrationally resolved features otherwise unachievable at room temperature.42-44 Thus, the EA of the neutral can be accurately determined from the 0-0 transition. In this paper, we report the first PES study on o-, m-, p-HO(C6H4)O- employing the recently developed low-temperature ESI-PES technique.45 Vibrationally resolved spectra are obtained for all three isomers, yielding adiabatic electron detachment energies (ADEs) of the anions or the EAs of the corresponding neutral radicals to be 2.315 ( 0.010, 2.330 ( 0.010, and 1.990 ( 0.010 eV for o-, m-, and p-species, respectively. Electronic structures of the ground as well as the excited states of the neutral radicals are probed. Theoretical calculations were performed to obtain the optimized structures, to calculate ADEs and energies of excited states, and to provide rational explanation of the observed experimental ADE changes. Excellent agreement is found between experimental data and theoretical predictions. The observed EA differences among

10.1021/jp103752t  2010 American Chemical Society Published on Web 07/29/2010

9084

J. Phys. Chem. A, Vol. 114, No. 34, 2010

Wang et al.

three isomers are interpreted largely due to two competing factors, i.e., an antibonding interaction within HOMO orbitals and the H-bonding stabilization effect. 2. Experimental and Theoretical Details 2.1. Low-Temperature ESI-PES. The low-temperature ESIPES apparatus has been described in several recent studies.44,45 A key feature of the new ESI-PES apparatus is a temperaturecontrolled ion trap that is used for ion accumulation and cooling. The anions of interest, o-, m-, p-HO(C6H4)O-, were produced via electrospray of 0.1 mM solutions of catechol, resorcinol, and hydroquinone in a mixture of methanol/water solvent (3/1 volume ratio) under slightly basic conditions (by titrating a small amount of NaOH aqueous solution). Anions produced were guided by two RF-only quadruples, directed by a 90° ion bender to the temperature-controlled ion trap, where they were accumulated and cooled via collisions with a background gas of ∼0.1 mTorr 20% H2 balanced in helium. Ions were trapped and cooled at 70 K trap temperature for a 20-80 ms period before being pulsed out into the extraction zone of a time-of-flight mass spectrometer at a repetition rate of 10 Hz. During the PES experiment, ions were mass-selected and decelerated before being intercepted by a probe laser beam in the photodetachment zone of a magnetic-bottle photoelectron analyzer. In the current experiment, several photon energies at 532 nm (2.331 eV), 355 nm (3.496 eV), and 266 nm (4.661 eV) from a Nd:YAG laser, and 193 nm (6.424 eV) from an ArF excimer 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 (∆E/E) was about 2%, i.e., ∼20 meV for 1 eV electrons. 2.2. Theoretical Methods. Density functional theory (DFT) calculations were employed to study the geometric and electronic structures of the o-, m-, p-HO(C6H4)O- anions as well as the corresponding neutral radicals. Geometry optimizations and single-point energy calculations were performed using the hybrid B3LYP exchange-correlation functional46 and the 6-311++G(d,p) basis set. Local minima were all verified through the vibrational frequency analysis. The ADE value of the anion was computed as the energy difference between the anion and the corresponding neutral radical at their respective optimized structures, while the vertical detachment energy (VDE) was calculated as the energy difference between the anion and the neutral at the optimized structure of the anion. Zero-point vibrational energy (ZPE) correction was included for the values of calculated ADEs and relative energies. The time-dependent DFT method47,48 was used to calculate the excitation energies of the neutral from its ground state. The electron binding energies for higher states were obtained by adding the vertical excitation energies of the neutral at the anion’s optimized structure to the experimental ADE values. All calculations were carried out with the Gaussian 03 program package.49 3. Experimental Results Strong mass peaks corresponding to o-, m-, and p-HO(C6H4)O- were observed when electrospraying the cor-

Figure 1. Photoelectron spectra of o-HO(C6H4)O- at (a) 355, (b) 266, and (c) 193 nm at 70 K. The circle in part c indicates the position of theoretical adiabatic detachment energy (ADE), and short bars represent the positions of calculated excited detachment transitions relative to the experimental ADE. Two vibrational modes are identified as shown in part a. The intensity anomalies observed in band X at 355 and 266 nm are likely due to the vibronic coupling effect (see text for details).

responding solutions via deprotonation processes. For hydroquinone, appreciable portion solutes (about 1/3) were found to lose two protons to afford generation of the p-benzoquinone anion, i.e., p-O(C6H4)O-. Photoelectron spectra of o-, m-, and p-HO(C6H4)O- were recorded at 70 K and at 355, 266, and 193 nm, whereas an additional 532 nm spectrum was taken for the p-anion due to its low electron binding energy. The resulting spectra are displayed in Figures 1-3, respectively. The 70 K temperature was chosen because this temperature was low enough to afford elimination of hot bands in molecular species44,45 without a potential problem: the buffer-gas inlet tube could be blocked by a very small amount of residue air which sometimes occurred at temperatures below 70 K. 3.1. o-HO(C6H4)O-. The 193-nm spectrum (Figure 1c) shows two strong broad bands, labeled as X and A centered at electron binding energies of ∼2.5 and ∼4.1 eV, respectively, with a clear band gap between them. Some fine structures are partially resolved for the A band. A third weak feature, B, is revealed at the high binding energy of 5.9 eV. At 266 nm (Figure 1b), the X band is partially resolved, and a sharp peak at 3.92 eV is dominant in the A feature region. The X feature is much better resolved at 355 nm (Figure 1a), displaying a complicated set of fine structures. It is interesting to note that there are some intensity anomalies in the structure of band X, where the intensity of the second group of peaks is higher at 266 nm than at 355 nm. This effect is not characteristic of vibrational features and suggests that the second set of peaks is either a low-lying neutral electronic state or is affected by vibronic coupling. Because the first excited electronic state of the neutral is calculated to be ∼1.6 eV above the ground state (vide infra), the fine structures of band X are assigned due to vibrational progressions, and the uncharacteristic intensity change at different photon energies is likely due to the effect of vibronic coupling.

Electron Affinities and Electronic Structures

J. Phys. Chem. A, Vol. 114, No. 34, 2010 9085

Figure 2. Photoelectron spectra of m-HO(C6H4)O- at (a) 355, (b) 266, and (c) 193 nm at 70 K. The circle in part c indicates the position of theoretical adiabatic detachment energy (ADE), and short bars represent the positions of calculated excited detachment transitions relative to the experimental ADE. Only calculated data from the global minimum (m-1) are shown. One vibrational mode is resolved in part a.

Two vibrational sequences are assigned with frequencies 605 ( 40 and 1575 ( 40 cm-1 as shown in Figure 1a. Because of the complex nature of the X band associated with many modes and vibronic couplings, no Franck-Condon simulation is attempted. But the sharp rising edge of the X band suggests that the first vibrationally resolved peak at 355 nm represents the 0-0 transition. Thus, the ADE of o-HO(C6H4)O- or the EA of o-HO(C6H4)O• is measured to be 2.315 ( 0.010 eV (Table 1), in comparison to 2.41 ( 0.13 eV, estimated from a previous ion-molecule reaction study.33 The binding energies of all observed features are listed in Table 2. 3.2. m-HO(C6H4)O-. PES spectra of m-HO(C6H4)O- were recorded at 70 K and at 355, 266, and 193 nm as shown in Figure 2a-c, respectively. Four features labeled X and A-C in order of increasing binding energy are observed at 193 nm with their binding energies centered at ∼2.4, 3.5, 4.0, and 5.5 eV, respectively. The A and C features are weak compared to the intense X and dominant B features. At 266 nm, the B feature becomes very weak presumably due to the reduced detachment cross-section at 266 nm, and the X feature becomes dominant. Some fine structures are discernible for X and A. At 355 nm, only X and the threshold of the A band are accessible. The X feature is much better resolved showing a single vibrational progression with a frequency of 525 ( 20 cm-1. From the 0-0 transition in band X, the EA of the m-HO(C6H4)O• radical is determined to be 2.330 ( 0.010 eV (vs 2.42 ( 0.13 eV previously reported33). The threshold of A is also well-defined to be 3.335 eV (Table 2). 3.3. p-HO(C6H4)O-. The electron binding energy of p-HO(C6H4)O- is observed to be significantly smaller than its congeners (o- and m-). In addition to being studied at 355, 266, and 193 nm, the p-anion was studied at 532 nm as well to unravel fine structures in the threshold region. The 193-nm spectrum (Figure 3d) shows two intense bands X and B spanned over 1.9-2.6 and 4.2-5.0 eV ranges, one relatively weak feature A from 3.0 to 3.7 eV, and continuous signals beyond

Figure 3. Photoelectron spectra of p-HO(C6H4)O- at (a) 532, (b) 355, (c) 266, and (d) 193 nm at 70 K. The circle in part d indicates the position of theoretical adiabatic detachment energy (ADE), and short bars represent the positions of calculated excited detachment transitions relative to the experimental ADE. Two vibrational modes are resolved in part a. The extra peak (X*) in part b arises due to resonant autodetachment (see text).

5.0 eV (C). The B band is well resolved. At 266 nm, band X becomes dominant and partially resolved, while A and B have similar intensity, most likely due to the smaller detachment cross-section for B at 266 nm. The threshold of B is well-defined as 4.34 eV with 1210 ( 40 cm-1 vibrational spacing (Table 2). Surprisingly, at 355 nm, while the A feature is better resolved with 485 ( 40 cm-1 spacing and 3.155 eV threshold binding energy (Table 2), the X band becomes more complicated (cf. that at 266 and 193 nm) with an extra peak of ∼2.6 eV (X*) and a few sharp peaks or steps discernible along the rising edge of the X band (Figure 3b). This extra peak (X*) is most likely due to an autodetaching process following the anion being resonantly excited at 355 nm. The fine structure discernible on the rising edge of band X is well resolved at 532 nm (Figure 3a) dominant with two vibrational modes with spacing of 425 ( 20 and 1735 ( 40 cm-1. The ADE of p-HO(C6H4)O- or the EA of p-HO(C6H4)O•, determined from the first resolved peak, is 1.990 ( 0.010 eV, in good accord with a previously estimated EA of 1.97 ( 0.14 eV.33 4. Theoretical Results Electronic structure calculations were performed to optimize the anions’ structures, to sort out possible isomers, and to compute the ADEs and VDEs to compare with the experimental spectra. The highest occupied molecular orbital (HOMO) of each anion was analyzed to rationalize the observed ADE and visualize the distribution of the extra electron within each

9086

J. Phys. Chem. A, Vol. 114, No. 34, 2010

Wang et al.

TABLE 1: Experimental Adiabatic Detachment Energies (ADEs), Calculated ADEs, and Vertical Detachment Energies (VDEs), (in eV) for Different Isomers and Their Relative Energies (in kcal/mol) for o-, m-, and p-HO(C6H4)O-a compd

ADE (expt)b

ADE (calcd)

VDE (calcd)

isomerd

relative energye

o-

2.315(10) [2.41(13)]c

m-

2.330 (10) [2.42 (13)]c

p-

1.990 (10) [1.97 (14)]c

2.263 2.057 2.308 2.323 1.936

2.378 2.112 2.376 2.368 2.069

o-1 o-2 m-1 m-2 p-

0 13.3 0 0.855 0

a

All values from this work unless otherwise noted. The numbers in parentheses represent experimental uncertainties in the last digits. Obtained from the 0-0 transitions. The ADE also represents the EA of the corresponding neutral radicals. c This value is from ref 33a. d Optimized structures for each isomers are shown in Figure 4. e At B3LYP/6-311++ G(d,p) level of theory with zero-point energy corrections. b

TABLE 2: Observed and Calculated Electron Binding Energies (EBEs) for o-, m-, and p-HO(C6H4)O-, Observed Vibrational Frequencies, and Final State Assignments EBE (eV) feature experiment o-

m-

p-

X A

2.315 (10) 3.920 (10)

B X A B C X X* A B C

∼5.9 2.330 (10) 3.335 (10) 3.99 (1) ∼5.5 1.990 (10) 2.6 3.155 (10) 4.34 (1) >5.0

a

calculationb

state 2

vib freq (cm-1)

2.315 (2.263) 3.919 3.963 5.866 2.330 (2.308, 2.323) 3.336 [3.306]c 3.930 [3.912]c 5.577 [5.628]c 1.990 (1.936)

A′′ A′′ 2 A′ 2 A′ 2 A′′ 2 A′′ 2 A′ 2 A′ 2 A

605 (40), 1575 (40)

3.122 4.423 5.293 6.064

2

485 (40) 1210(40)

2

A A 2 A 2 A 2

525 (20)

425 (20), 1735 (40)

a Obtained from the first vibrationally resolved peak available for each feature. Each number in parentheses represents the experimental uncertainty in the last digits. b Obtained using the time-dependent DFT method by adding the vertical excitation energies in the neutral at the optimized geometry of the anion to the respective experimental EBEs of the first X peaks. The calculated EBEs of X features are included in parentheses. c The numbers in brackets are calculated EBEs from isomer m-2, which is likely present in the experiments.

Figure 4. Optimized structures, relative energies (kcal/mol), calculated adiabatic detachment energies (ADE), and vertical detachment energies (VDE) (in eV) at the B3LYP/6-311++G(d,p) level of theory with ZPE corrections for the o-, m- and p-HO(C6H4)O- anions.

molecular frame. All anions are closed-shell, and their respective neutrals are radicals with one unpaired electron (doublet). 4.1. Optimized Structures and Calculated ADEs. Figure 4 displays the optimized structures of the low-lying isomers along with their relative energies, the calculated ADEs, and the first VDEs for each anionic species. For o-HO(C6H4)O-, the global minimum (o-1) is a planar structure with the hydrogen

atom from the hydroxyl group pointing to the neighboring oxygen, forming O-H · · · O H-bonding. The next minimum (o2), obtained via rotating the OH group 180° along the C-O axis and breaking the H-bond, is 13.3 kcal/mol higher in energy, and not expected to be present under experimental conditions. The calculated ADE of 2.263 eV and VDE of 2.378 eV from o-1 agree excellently (within 0.05 eV) in comparison with the experimental data (Table 1 and Figure 1). On the other hand, the calculated ADE from o-2 is 2.057 eV, i.e., ∼0.3 eV smaller compared to the experimental data, suggesting that indeed this minimum is not present in the ion beam. Two close-lying minima are found for m-HO(C6H4)O-, one with OsH and CdO in cis position (m-1), and the other in trans (m-2). Both are planar structures with OH lying within the benzene plane. At the B3LYP//6-311++G(d,p) level, m-1 is 0.855 kcal/mol more stable than m-2. The calculated ADE and VDE from m-1 are nearly identical to the respective ones from m-2 (2.308, 2.376 eV for m-1 vs 2.323, 2.368 eV for m-2). The calculated ADEs from both minima are also in an excellent agreement with the experimental data (2.330 ( 0.010 eV). The near degeneracy of m-1 and m-2 and their ADEs is expected since the m-OH has little interaction with CdO. Both isomers may exist in the experiments. Only one minimum is located for p-HO(C6H4)O-, which is not a planar structure as contrasted with the o- and m-isomers, with the H atom from OH group tilted 28.5° relative to the phenoxide plane (see the side view of the optimized structure in the Supporting Information). The calculated ADE of 1.936 eV again agrees excellently with the measured one (1.990 eV). Furthermore, the calculations successfully reproduced the experimental ADE trend among three anions, i.e., ADE (p-) < ADE (o-) e ADE (m-), and the existence of the ∼0.3 eV ADE difference between the p-isomer and its two congeners. 4.2. Electronic Structures and Simulated Spectra. The electron binding energies of higher electronic state transitions for each neutral species were obtained using the time-dependent DFT method by adding the vertical excitation energies in the neutral at the optimized geometry of the anion to the respective experimental ADE. The calculated binding energies for each species are listed in Table 2, and compared with the corresponding 193-nm spectra as solid vertical bars in Figures 1-3. Excellent agreement is achieved, and the calculations account for all observed features. 5. Discussion The three hydroxyphenoxyl radicals (o-, m-, and p-HO(C6H4)O•) studied here have close connections with two relatively well characterized molecules, i.e., phenoxyl (C6H5O•) and p-benzoquinone (p-O(C6H4)O) in many aspects (compositions, structures, properties, and applications). The EA of the p-isomer (1.99 eV) is 0.13 eV higher than that of p-benzoquinone (1.86 eV),15 but 0.26 eV lower than that of phenoxyl

Electron Affinities and Electronic Structures

J. Phys. Chem. A, Vol. 114, No. 34, 2010 9087

Figure 5. Top (top) and side (bottom) views of the highest occupied molecular orbitals (HOMOs) for the o-1 (left), m-1 (middle, only m-1 is shown here, since that of m-2 looks very similar), and p- (right) HO(C6H4)O- anions with calculated MO energies at the B3LYP/6-311++G(d,p) level.

(2.253 eV),27 whereas EAs of the o- and m-isomers are slightly higher than that of phenoxyl by 0.062 and 0.077 eV, respectively. Clearly, hydroxyl substitution at the p-position (relative to C-O in phenoxyl) has altered remarkably the resulting molecular properties. On the contrary, the same OH substitutions at the o- and m-positions only slightly change their EAs from that of phenoxyl. It is also surprising that electron binding energies of o- and m-HO(C6H4)O- are roughly the same, considering the existence of one H-bonding interaction in the o-anion that should significantly increase the electron binding energy in the former. Hence, other factors are playing roles in determining their electron binding energies too. 5.1. Underlying Reasons for the Variance of ADEs. The ADE is defined as the energy difference between the anion and the neutral in their respective optimal geometries. In Koopman’s approximation (single particle picture), it corresponds to the transition energy detaching one electron from HOMO to the vacuum level. Therefore, the HOMO property plays an overwhelming role to determine the ADE. The respective HOMOs for the o-, m- and p-anions at their optimal geometries are shown in Figure 5, revealing that they are all extended delocalized MOs with the π character and with a nodal plane on each molecular frame. The three HOMOs on the phenoxide moiety are very similar to each other with each having two perpendicular nodal planes. For both o- and p-HO(C6H4)O-, the pz orbital of the oxygen atom from the hydroxyl group (perpendicular to the molecular plane) interacts with the phenoxide moiety as an antibonding composition, causing a rise of orbital energy of the HOMO, whereas there is essentially no such contribution for m-HOMO. From the above analysis, it is expected qualitatively that the ADE of m-HO(C6H4)O- should be similar to that of (C6H5)O-, whereas the ADE of p-HO(C6H4)O- should be smaller in an appreciable amount than that of phenoxide because of the increase of HOMO orbital energy. This expectation is borne out from the experimental data. The observation that ADE of o-HO(C6H4)O- is ∼0.3 eV higher than that of the p-anion is due to forming O · · · H-O H-bonding in o-1 (H-bond stabilization effect). In fact, without H-bonding, the ADE calculated from the o-2 structure is 2.057 eV, only 0.07 eV higher than that of p-HO(C6H4)O- (1.99 eV).

5.2. Vibrational Modes of Neutral Radicals and Resonant Anionic States in the p-Isomer. The vibrational structures observed in the PES spectra reflect the corresponding vibrational modes being excited upon electron detachment, which usually involve the modes with appreciable bond-length or bond-angle changes. According to the selection rule, only those with total symmetry are allowed. Two vibrational modes have been identified in the ground state of the o- and p-neutrals with 605/ 1575 cm-1 and 425/1735 cm-1 frequencies, respectively, whereas only one vibrational mode is excited in the m-isomer (525 cm-1) (Table 2). Similar vibrational frequencies of 513/ 1476 cm-1 were observed in the PES spectrum of phenoxide with assignments to the C-C-C bending (the central C connected to O) and C-O stretching modes,27 as well as in the low-temperature IR matrix measurement with the same assignments (520/1481 cm-1).26 We carried out theoretical calculations to obtain vibrational frequencies and modes for the neutral radicals. As shown in the Supporting Information, for both o- and p-neutrals, we found two groups of vibrational modes involving the CsCsC bending and CdO stretching with the calculated frequencies close to the observed ones, whereas only the bending mode was identified for the m-radical with the corresponding CdO stretching mode not existing (see the Supporting Information for a complete list of calculated frequencies and selected vibrational modes of the neutral molecules). While the exact reason of the missing CdO stretching mode for the m-neutral is not clear at this point, it correlates well with the fact that only one vibrational mode at 525 cm-1 is observed in the m-isomer spectrum. The different vibrational patterns observed among the three isomers also accord well with the calculated anion-to-neutral geometric changes. While the CsO bond-length change is isomer-specific with the changes for o and p (moderate) and for m (the smallest) (i.e., -0.035, -0.017, and -0.022 Å for o-, m-, and p-, respectively), sizable CsCsC bond-angle increases upon detaching extra electrons are found for all three species, which is nicely in line with the observation that the bending mode is excited in all spectra and the stretching mode is exhibited only for the o- and p-spectra. Therefore, we assign

9088

J. Phys. Chem. A, Vol. 114, No. 34, 2010

the observed frequencies due to the CsCsC bending and CdO stretching modes. The observation of the extra peak X* at ∼2.6 eV in the 355nm spectrum for p-HO(C6H4)O- is interesting (Figure 3b). This is certainly due to an autodetachment process where the p-anion was first resonantly excited by absorbing one 355 nm photon to an excited anionic state, followed by autoionization, resulting in an extra peak otherwise not accessible under a direct detaching process. The existence of the excited states ∼3.5 eV above the ground state for the p-HO(C6H4)O- anion is confirmed from our calculations, which show a strong absorption at 3.56 eV for the p-anion (see the Supporting Information for calculated anion absorption). Similar resonance states for p-benzoquinone and phenoxide were observed and reported previously.15,27 The existence of rich resonant states in both p-HO(C6H4)O- and p-O(C6H4)O- anions is directly relevant to their roles as electron transfer mediators in a variety of biological and energy conversion processes. 6. Conclusions Low-temperature anionic photodetachment photoelectron spectroscopy combined with ab initio calculations has been employed to probe electronic structures of three hydroxyphenoxyl radicals in the gas phase. Their EAs (or ADEs of the corresponding anions) are determined directly from the resolved 0-0 transitions in the PES spectra owing to the cooling techniques recently developed for molecular anions.38-41 The ADE of the p-anion is ∼0.3 eV smaller than that of the m-species primarily due to the antibonding interactions of the OH group with the phenoxide moiety, whereas the formation of the O · · · H-O H-bond in the o-isomer stabilizes the extra electron, resulting in roughly the same ADEs for the o versus m species. Resonant excited states are observed for p-HO(C6H4)O-, which, together with previously reported excited states for p-O(C6H4)O-, may shed light on the electron transfer biological function for this pair of molecules. Acknowledgment. This work was supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences, and was performed at the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory, which is operated by Battelle for the DOE. Computational work was supported by the National Natural Science Foundation of China (Grants 50721091, 20803071, 20873129), the National Key Basic Research Program (2006CB922004), the USTC-HP HPC Project, SCCAS, and the Shanghai Supercomputer Center. Supporting Information Available: Structures and Cartesian coordinates of the optimized anions and neutrals, vibrational frequencies as well as vibrational modes of the neutral radicals, and calculated anion absorption spectra. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Graige, M. S.; Paddock, M. L.; Bruce, J. M.; Feher, G.; Okamura, M. Y. J. Am. Chem. Soc. 1996, 118, 9005–9016. (2) Steinberg-Yfrach, G.; Liddell, P. A.; Hung, S.-C.; Moore, A. L.; Gust, D.; Moore, T. A. Nature 1997, 385, 239–241. (3) Himo, F.; Babcock, G. T.; Eriksson, L. A. J. Phys. Chem. A 1999, 103, 3745–3749. (4) Mallardi, A.; Giustini, M.; Palazzo, G. J. Phys. Chem. B 1998, 102, 9168–9173.

Wang et al. (5) Feher, G.; Allen, J. P.; Okamura, M. Y.; Rees, D. C. Nature 1989, 339, 111–116. (6) Ishikita, H.; Knapp, E.-W. J. Am. Chem. Soc. 2005, 127, 14714– 14720. (7) Palazzo, G.; Francia, F.; Mallardi, A.; Giustini, M.; Lopez, F.; Venturoli, G. J. Am. Chem. Soc. 2008, 130, 9353–9363. (8) Kurisu, G.; Zhang, H.; Smith, J. L.; Cramer, W. A. Science 2003, 302, 1009–1014. (9) 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. (10) Wu, H.; Zhang, D.; Su, L.; Ohkubo, K.; Zhang, C.; Yin, S.; Mao., L.; Shuai, Z.; Fukuzumi, S.; Zhu, D. J. Am. Chem. Soc. 2007, 129, 6839– 6846. (11) Osycczka, A.; Moser, C. C.; Daldal, F.; Dutton, P. L. Nature 2004, 427, 607–612. (12) Platz, J.; Nielsen, O. J.; Wallington, T. J.; Ball, J. C.; Hurley, M. D.; Straccia, A. M.; Schneider, W. F.; Sehested, J. J. Phys. Chem. A 1998, 102, 7964–7974. (13) Itoh, S.; Taki, M.; Fukuzumi, S. Coord. Chem. ReV. 2000, 198, 3–20. (14) Shimazaki, Y.; Huth, S.; Odani, A.; Yamauchi, O. Angew. Chem., Int. Ed. 2000, 39, 1666–1669. (15) Schiedt, J.; Weinkauf, R. J. Chem. Phys. 1999, 110, 304–314. (16) Heinis, T.; Chowdhury, S.; Scott, S. L.; Kebarle, P. J. Am. Chem. Soc. 1988, 110, 400–407. (17) Cooper, C. D.; Naff, W. T.; Compton, R. N. J. Chem. Phys. 1975, 63, 2752–2757. (18) Batley, M.; Lyons, L. E. Nature 1962, 196, 573–574. (19) Marks, J.; Comita, P. B.; Brauman, J. I. J. Am. Chem. Soc. 1985, 107, 3718–3719. (20) El Ghazaly, M. O. A.; Svendsen, A.; Bluhme, H.; Nielsen, S. B.; Andersen, L. H. Chem. Phys. Lett. 2005, 405, 278–281. (21) (a) Stanton, J. F.; Sattelmeyer, K. W.; Gauss, J.; Allan, M.; Skalicky, T.; Bally, T. J. Chem. Phys. 2001, 115, 1–4. (b) Suzuka, I.; Sanekata, M.; Ito, M.; Ohta, N. Laser Chem. 1994, 14, 143–154. (22) (a) Boesch, S. E.; Grafton, A. K.; Wheeler, R. A. J. Phys. Chem. 1996, 100, 10083–10087. (b) Nonella, M. J. Phys. Chem. B 1998, 102, 4217–4225. (c) Nonella, M.; Brandli, C. J. Phys. Chem. 1996, 100, 14549– 14559. (23) Cramer, C. J. J. Chem. Soc., Perkin Trans. 2. 1999, 2273–2283. (24) (a) Pou-Ame´rigo, R.; Mercha´n, M.; Ortı´, E. J. Chem. Phys. 1999, 110, 9536–9546. (b) Itoh, T. Chem. ReV. 1995, 95, 2351–2368. (25) Radziszewski, J. G.; Gil, M.; Gorski, A.; Spanget-Larsen, J.; Waluk, J.; Mroz, B. J. J. Chem. Phys. 2001, 115, 9733–9738. (26) Spanget-Larsen, J.; Gil, M.; Gorski, A.; Blake, D. M.; Waluk, J.; Radziszewski, J. G. J. Am. Chem. Soc. 2001, 123, 11253–11261. (27) Gunion, R. F.; Gilles, M. K.; Polak, M. L.; Lineberger, W. C. Int. J. Mass. Spectrom. Ion Processes 1992, 117, 601–620. (28) Moylan, C. R.; Dodd, J. A.; Han, C.-C.; Brauman, J. I. J. Chem. Phys. 1987, 86, 5350–5357. (29) Richardson, J. H.; Stephenson, L. M.; Brauman, J. I. J. Am. Chem. Soc. 1975, 97, 2967–2970. (30) Suter, H. U.; Nonella, M. J. Phys. Chem. A 1998, 102, 10128– 10133. (31) NIST Chemistry Webbook (http://webbook.nist.gov), accessed April 10, 2010. (32) Rienstra-Kiracofe, J. C.; Tschumper, G. S.; Schaefer, H. F., III; Nandi, S.; Ellison, G. B. Chem. ReV. 2002, 102, 231–282. (33) (a) Fattahi, A.; Kass, S. R.; Liebman, J. F.; Matos, M. A. R.; Miranda, M. S.; Morais, V. M. F. J. Am. Chem. Soc. 2005, 127, 6116– 6122. (b) McMahon, T. B.; Kebarle, P. J. Am. Chem. Soc. 1977, 99, 2222– 2230. (34) Davico, G. E.; Schwartz, R. L.; Ramond, T. M.; Lineberger, W. C. J. Am. Chem. Soc. 1999, 121, 6047–6054. (35) Wang, L. S.; Wang, X. B. J. Phys. Chem. A 2000, 104, 1978– 1990. (36) Wang, X. B.; Yang, X.; Wang, L. S. Int. ReV. Phys. Chem. 2002, 21, 473–498. (37) Wang, X. B.; Wang, L. S. Annu. ReV. Phys. Chem. 2009, 60, 105– 126. (38) Gerlich, D. AdV. Chem. Phys. 1992, 82, 1–176. (39) Wang, X. B.; Woo, H. K.; Wang, L. S. J. Chem. Phys. 2005, 123, 051106-1-4. (40) Boyarkin, O. V.; Mercier, S. R.; Kamariotis, A.; Rizzo, T. R. J. Am. Chem. Soc. 2006, 128, 2816–2817. (41) Zhou, J.; Santambrogio, G.; Brummer, M.; Moore, D. T.; Woste, L.; Meijer, G.; Neumark, D. M.; Asmis, K. R. J. Chem. Phys. 2006, 125, 111102-1–4. (42) Wang, X. B.; Woo, H. K.; Wang, L. S.; Minofar, B.; Jungwirth, P. J. Phys. Chem. A 2006, 110, 5047–5050. (43) Wang, X. B.; Woo, H. K.; Kiran, B.; Wang, L. S. Angew. Chem., Int. Ed. 2005, 44, 4968–4972.

Electron Affinities and Electronic Structures (44) Wang, X. B.; Matheis, K.; Ioffe, I. N.; Goryunkov, A. A.; Yang, J.; Kappes, M. M.; Wang, L. S. J. Chem. Phys. 2008, 128, 114307-1–6. (45) Wang, X. B.; Wang, L. S. ReV. Sci. Instrum. 2008, 79, 0731081-8. (46) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (47) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem. Phys. 1998, 109, 8218–8224. (48) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys. Lett. 1996, 256, 454– 464. (49) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;

J. Phys. Chem. A, Vol. 114, No. 34, 2010 9089 Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.; Gaussian 03, Revision B.04; Gaussian, Inc.: Pittsburgh, PA, 2003.

JP103752T