Theoretical Study on the Excess Electron Binding Mechanism in the

Aug 3, 2010 - ... hydrogen bonding interactions in the [CH 2 XNO 2 ·H 2 O] clusters (X = H, F, Cl, Br, I). Zoi Salta , Demetrios K. Papayannis , Agni...
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J. Phys. Chem. A 2010, 114, 8939–8947

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Theoretical Study on the Excess Electron Binding Mechanism in the [CH3NO2 · (H2O)n](n ) 1-6) Anion Clusters Haruki Motegi and Toshiyuki Takayanagi* Department of Chemistry, Saitama UniVersity, 255 Shimo-Okubo, Sakura-ku, Saitama City, Saitama 338-8570, Japan

Takao Tsuneda AdVanced Science Institute, RIKEN, Wako, Saitama 351-0198, Japan, and CREST, JST (Japan Science and Technology Agency), Saitama 332-0012, Japan

Kiyoshi Yagi Yamanashi UniVersity, Fuel Cell Nanomaterials Center, Yamanashi UniVersity, 6-43 Miyamae-cho, Kofu 300-0021, Japan

Ryuzo Nakanishi and Takashi Nagata Department of Basic Science, Graduate School of Arts and Sciences, The UniVersity of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan ReceiVed: May 5, 2010; ReVised Manuscript ReceiVed: July 20, 2010

The excess electron binding mechanism of the anionic nitromethane-water clusters was theoretically investigated using the potential energy surfaces calculated by high-level electronic structure theories. The mechanism was first studied for the dipole-bound and valence-bound anionic states of the CH3NO2- monomer with the ab initio multireference configuration interaction method to reveal the electron transformation process between these anionic states in detail. As a result, it was found that both the NO2 tilting angle and NO distances play an essential role in this electron transformation. Following this result, various water solvation structures of the valence-bound CH3NO2- anion were optimized with up to six water solvents using the second-order Møller-Plesset (MP2) method. The calculated results predicted that the vertical detachment energy of the valence-bound CH3NO2- anion increases gradually with the hydration number, as is consistent with recent experimental observations. We also investigated metastable complexes composed of CH3NO2 and (H2O)6- by using the MP2 and long-range corrected density functional theory calculations. Two types of dipole-bound forms were obtained for the [CH3NO2 · (H2O)6] anion complex. In one form the excess electron is internally suspended between the two moieties while in the other form two dipolar moieties are cooperatively arranged to reinforce the electron-dipole interaction. 1. Introduction Nitromethane (CH3NO2) is a prototypical example of a polar molecule that can form two different types of molecular anion, i.e., the dipole-bound and conventional valence anions.1,2 In the dipole-bound anion, the excess electron is trapped by the longrange electrostatic potential of the neutral molecule and, as a result, resides in a diffuse orbital spreading out of the molecular frame. Then, it is generally known that the molecular geometry of the dipole-bound anion is almost the same as that of the neutral molecule.3 On the other hand, in the valence anion, the excess electron is accommodated in a molecular orbital localized mainly on the nitro group. The electron accommodation in the valence orbital induces a significant change in the molecular geometry: the CN bond lies in the ONO plane in the neutral molecule whereas the bond is inclined with respect to the plane in the anion.3-7 Recently, Weber et al. have directly observed * Author to whom correspondence should be addressed. Fax: +81-48858-3700. Electronic mail: [email protected].

the dipole-bound state of CH3NO2- by using low-energy photoelectron imaging spectroscopy and also determined the adiabatic electron affinity of CH3NO2 (172 ( 6 meV).8 Of particular interest is the coupling between these two forms of the CH3NO2- anion in that the dipole-bound state acts as a doorway into the valence state.9-11 A Rydberg electron transfer (RET) experiment has demonstrated that the dipole-bound state is initially prepared as a precursor in the electron attachment process of CH3NO2 to form its valence anion.1,2 This indicates unique electronic properties of nitromethane: namely, the LUMO of CH3NO2 is located at relatively low energy so that the valence anion electronic state can readily interact with the dipole-bound anion state. It has also been revealed that the coupling between the dipole-bound and valence electronic states is susceptible to solvation environments. In the RET experiment the formation rate of the valence anion was found to increase for a nitromethane solvated by few argon atoms.2 The enhanced formation rate due to Ar solvation indicates a rather important role of solvents to mediate electron attachment to polar

10.1021/jp1041124  2010 American Chemical Society Published on Web 08/03/2010

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molecules, especially when a certain degree of coupling exists between the dipole-bound and valence electronic states of the anions. On a related issue, it has been also shown that solvation can transform molecular anions from dipole-bound to valence forms in a variety of gas-phase systems including biologically relevant molecules. For example, Hendricks et al. have revealed by using negative ion photoelectron spectroscopy that solvated uracil anions undergo a gradual transformation from a dipole-bound to covalent form as the single atom solvating the anions is changed from Ar, Kr, to Xe.12 They have further observed a complete transformation into the valence anion when a water molecule is attached to the uracil anion. This result indicates that the most stable anion state of the uracil monomer is of a dipole-bound type, while the valence form is the global minimum structure on the potential energy surface for the hydrated anion (uracil-H2O)-.12 Stabilization of an otherwise unstable valence anion due to hydration is commonly observed for several organic molecules such as pyridine, acetaldehyde, and acetone;13,14 the minimum number of water molecules needed for the stabilization is in the range 3-4, depending on the species. Very recently, it was found by two of the present authors that a dipole-bound type of [CH3NO2 · (H2O)6] anion complex was formed in the reactions of hydrated electron cluster composed of six water molecules, (H2O)6-, with nitromethane via “Ar-mediated” association:15

CH3NO2 + (H2O)6-Arm f [CH3NO2 · (H2O)6]- + mAr (1) The electronic properties of the product [CH3NO2 · (H2O)6]were probed by negative ion photoelectron spectroscopy. The photoelectron spectrum displayed two band components with the vertical detachment energy (VDE) being 3.54 and 0.65 eV, respectively, indicating coexistence of two types of anionic forms bearing different electron binding abilities. The 3.54 eV band was assigned to the photodetachment from a hydrated valence-bound CH3NO2- anion, where the excess electron is accommodated in the π* valence orbital. On the basis of several suggestive evidence, the 0.65 eV band was assigned to an anion complex represented as (H2O)6{e-}CH3NO2, where the excess electron was suspended between two dipole moments of (H2O)6 and CH3NO2. The anion complex is named “dual dipole-bound” since the diffuse electron bound by the dipolar field of (H2O)6 traps another polar molecule via the electron-dipole interactions. It was also found that the photodestruction of the anion complex near the electron detachment threshold occurred with a competition between electron detachment and fragmentation; the latter process was accompanied distinctively by a loss of three water molecules15 as [CH3NO2 · (H2O)6]- + hν f CH3NO2 · (H2O)3 + 3H2O

(2) This finding indicates a possibility that the [CH3NO2 · (H2O)6] anion complex serves as a precursor to the hydrated valence anion of CH3NO2-. In the present study, motivated by the experimental findings mentioned above,15 we performed extensive electronic structure calculations for the [CH3NO2 · (H2O)n]- (n ) 1-6) anions. Although several electronic structure calculations have already been performed for the CH3NO2- monomer

anion,16-20 those previous studies mainly focused on the transformation between the dipole-bound and valence bound states of CH3NO2- and on the dissociative electron attachment processes of CH3NO2. To the best of our knowledge, there have a pair of reports dealing with the optimized geometry of the hydrated valence anion, CH3NO2- · H2O, where the water molecule is bound along the symmetry axis of the NO2 group through a double ionic hydrogen-bonding (DIHB) configuration.17,18 We discuss here the excess-electron binding mechanism at play in the [CH3NO2 · (H2O)n]- anions in terms of the local minimum structures and their energetics calculated in the present study. 2. Computational Details Previous electronic structure studies for molecular anions as well as cluster anions showed that both higher-order electron correlation effects and flexible basis sets including diffuse basis functions are required to obtain accurate VDE values and energetics of anions.21 This is particularly true in the dipolebound anion case since both long-range dispersion interaction and exchange repulsion play an essential role in excess electron binding mechanisms. After extensive tests, we have determined to employ Dunning’s aug-cc-pVDZ basis set22 with additional diffuse functions for both oxygen and hydrogen atoms having exponents of 0.00987 for O(s), 0.00857 for O(p) and 0.00372 for H(s), which were obtained by reducing the exponents of the “aug-” basis by a factor of 1/8. This basis set is exactly the same as that used in the study on water cluster anions by Yagi et al.23 and is denoted as aug-cc-pVDZ+diff hereafter. It should be emphasized that Yagi et al. have already shown that this basis set can describe the diffuse dipole-bound anion states for various sized water clusters at a reasonably accurate level. We have also checked the effect of diffuse basis functions for carbon and nitrogen atoms on calculated VDE values by using the same scheme as H and O atoms. As a result, we found that such diffuse functions on C and N do not affect final results. Thus, we added diffuse basis functions only on H and O atoms throughout this study. The equilibrium geometries of various isomers were mostly computed at the MP2/aug-cc-pVDZ+diff level of theory. Singlepoint coupled-cluster singles, doubles, and perturbative triples (CCSD(T)) calculations with the same basis set were performed to obtain accurate VDE values for optimized anion clusters. In the case of the [CH3NO2 · (H2O)6] anion complexes, we have also employed the long-range correction (LC) scheme24 with Becke 1988 exchange25 + one-parameter progressive correlation26 functional (LC-BOP) in the Kohn-Sham calculations. Yagi et al.23 have shown that LC-BOP gives reliable VDE values for water anion clusters by systematically comparing with the CCSD(T) results. MP2, CCSD(T), and LC-BOP calculations have been carried out using a locally modified version of Gaussian 03 programs,27 in which the LC scheme was implemented. To make clear the transformation between the dipole-bound and valence-bound anion states of the nitromethane monomer, we have carried out multireference configuration interaction (MRCI) calculations with the aug-cc-pVDZ+diff basis set. The reference wave function was determined by the completeactive space self-consistent field (CASSCF) level of theory, where seven electrons were distributed among five active orbitals. These calculations were performed using MOLPRO program.28

Binding Mechanism in the [CH3NO2 · (H2O)n]- Anions

Figure 1. Molecular structures of dipole-bound and valence-bound states of nitromethane optimized by MP2/aug-cc-pVDZ+diff. Singly occupied molecular orbitals (SOMO) for both the anionic states are also shown with isosurface values being 0.0055 au (dipole-bound) and 0.0292 au (valence-bound), respectively. Thus, excess electron density values correspond to 50% and 90%, respectively.

3. Results and Discussion 3.1. Transformation between the Dipole-Bound and Valence-Bound Anion States of the Nitromethane Monomer. Before we present computational results for the [CH3NO2 · (H2O)n]- anion clusters, it may be quite important to discuss the electron transformation mechanism between the dipolebound and valence-bound anion states of the nitromethane monomer. Figure 1 shows the geometries for both the dipolebound and valence-bound CH3NO2- anion states optimized at the MP2/aug-cc-pVDZ+diff level of theory along with singly occupied molecular orbitals (SOMOs). In the dipole-bound anion, the excess electron is loosely bound outside the CH3 group while the excess electron is trapped in the compact π* orbital in the valence-bound nitromethane anion. Previous theoretical studies have already shown that the equilibrium geometry of the valence-bound CH3NO2- anion state is different from that of the neutral CH3NO2 as well as of dipole-bound anion states. More specifically, in the dipole-bound CH3NO2anion, the CN bond lies in the ONO plane, while the nitro group is strongly tilted away from the CN bond in the valence-bound anion geometry. The tilting angle θtilt, which is defined as an angle between the ONO plane and the CN bond, was optimized to be 36.0° for the valence-bound anion. Another important difference between the dipole-bound and valence-bound anionic geometries is the NO bond length, RNO. The two NO distances in the CH3NO2- anion are stretched by ∼0.07 Å with respect to the equilibrium distances for the dipole-bound anion. This indicates that these internal coordinates, the tilting angle θtilt and the bond length RNO, play an essential role in the transformation mechanism between the dipole-bound and valencebound anion states of nitromethane. In the bottom trace of Figure 2, the 2D contour plot of the ground-state potential energy surface of CH3NO2- is displayed as a function of the NO bond length RNO and the tilting angle θtilt calculated at the MRCI/aug-cc-pVDZ+diff level of theory. We can see three local minima on the calculated potential energy surface. The local minimum at θtile ∼ 0° corresponds to the dipole-bound anion minimum while the two minima at RNO ∼ 1.3 Å correspond to the valence-bound anion structures. The result of the present MRCI calculation implies that the dipole-

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Figure 2. 3D perspective and contour plot of the ground-state and first excited-state potential energy surfaces of the nitromethane anion as a function of the NO distance and the tiling angle θtilt, obtained by MRCI/aug-cc-pVDZ+diff. The contour increment is 50 meV and zero energy is defined as the valence-bound anion minimum.

bound CH3NO2- anion can easily convert into the valence-bound anion structure because of the low barrier separating these minima and thus the lifetime of the dipole-bound CH3NO2anion is relatively short. In addition, it should be emphasized that the small change in the NO bond length obviously contributes to this electronic transformation. This may be the reason why the dipole-bound state was not clearly identified in the 1D potential energy curve calculated as a function of the tilting angle θtilt as revealed in the paper of Gutsev and Bartlett.3 Figure 2 also displays the 3D perspective plot of the potential energy surfaces of both the ground and first excited states of CH3NO2- obtained at the MRCI level. From this plot, it is found that the three minima on the ground-state potential energy surface are adiabatically constructed from the lowest two electronic states corresponding to the dipole-bound and valencebound anion states. We can clearly see that the transition states between the dipole-bound and valence-bound minima are formed through the avoided crossing between these two electronic states. Sommerfeld10 has previously pointed out that the energy difference between the ground and first excited states at the avoided crossing point reflects the coupling strength between these two electronic states. He has obtained the corresponding energy difference to be 30 meV at the equation-of-motion coupled-cluster level of theory and concluded that the nitromethane anion is in the weak coupling regime. This means that transformation dynamics between the dipole-bound and valence-bound anion states should be described on the basis of diabatic picture. The present MRCI calculation gives the corresponding energy splitting around the crossing point (at RNO ) 1.28 Å and θtilt ) 13°) to be 74 meV, which is somewhat larger than the result of Sommerfeld.10 Although we have obtained reasonable potential energy surface features as shown in Figure 2, it is worth pointing out that even MRCI calculation is still not accurate enough for discussing the adiabatic electron affinity of the neutral nitromethane monomer. Figure 2 clearly shows that the valencebound CH3NO2- anion state is slightly more stable than the neutral CH3NO2 monomer (notice that the energy levels of the

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neutral CH3NO2 minimum and the dipole-bound anion minimum are very similar and the energy difference should be only ∼10 meV). The adiabatic electron affinity of CH3NO2 was calculated to be 0.031 eV at the MRCI level. It should be mentioned that this small electron affinity value is in serious disagreement with previous experimental results. Very recently, Weber and his coworkers8 obtained a new value of the adiabatic electron affinity of nitromethane of 0.172 ( 0.006 eV using a low-energy velocity map photoelectron imaging technique. This value is somewhat smaller than the older experimental value of 0.26 ( 0.08 eV, which was determined from the laser photodetachment photoelectron spectroscopy,1 although this old value contains somewhat larger uncertainty. On the other hand, from the theoretical side, Bull et al. have obtained a reliable computational valule of 0.176 eV for the adiabatic electron affinity at the MCQDPT2 level of theory with an aug-cc-pVTZ basis set.20 Thus, our study and previous theoretical studies suggest that the higher-order electron correlation effect is very important for obtaining an accurate adiabatic electron affinity value even for the simple CH3NO2- monomer anion case. Nevertheless, we emphasize that the present MRCI calculations are very useful for understanding the transformation mechanism between the dipole-bound and valence-bound anion states of CH3NO2-. 3.2. Geometries and VDE Values of the Valence-Bound CH3NO2- Anion Solvated by Water Molecules. On the basis of the above results of the monomer, we discuss the geometries and VDE values of the valence-bound CH3NO2- · (H2O)n (n ) 1-6) hydrated anion clusters. Recently, a photoelectron spectroscopic study15 showed that the CH3NO2- · (H2O)n (n ) 1-6) clusters give a bell-shaped band structure in all cases. This study also indicated that the VDE peak value increases with the increase in cluster size. We have carried out geometry optimization for the CH3NO2- · (H2O)n (n ) 1-6) clusters at the MP2/aug-cc-pVDZ+diff level of theory. The relative energies and VDE values for all the optimized structures are summarized in Table 1 but some energetically low-lying structures are displayed in Figure 3. Since the excess electron occupies the π* orbital of the NO2 group in the valence-bound nitromethane anion (see Figure 1), the excess negative charge is mainly localized on the O atoms. Thus, it is expected that the O atoms strongly accept hydrogen atoms of water through hydrogen-bond. In the case of n ) 1, the most stable structure (1A) has a double ionic H-bonding (DIHB) form, in which two hydrogen atoms in water are bound to the O atoms in the valence-bound nitromethane anion. However, in this case, the O · · · H-Ow hydrogen-bond structure is significantly distorted from the linear configuration, where Ow is the oxygen atom in water. The second lowest structure (1B) of the CH3NO2- · H2O cluster has one linear O · · · H-Ow hydrogen bond, but with a much shorter distance of ∼1.6 Å. Notice that the energy difference between these two structures is found to be relatively small. This suggests that these structures have similar hydrogen-bond energies. It is also interesting to note that these two clusters give close VDE values (1.34 eV vs 1.29 eV). In the case of the small CH3NO2- · (H2O)n (n < 4) anion clusters, all water molecules directly attach to the NO2 group through a hydrogen bond since the O · · · H-Ow hydrogen bond is stronger than the water-water hydrogen bond. As the cluster size increases (n g 4), it is seen that water-water hydrogen bonds are gradually formed. The relative energies of optimized structures are thus mainly determined by the hydrogen-bond network structures of water in these large clusters.

Motegi et al. TABLE 1: Relative Energies (Erel in meV) and VDE (in eV) of the Hydrated Valence-Bound Anions CH3NO2- · (H2O)n (n ) 1-6) MP2//MP2 structure

Erel

a

n)0 n)1 n)2

n)3

n)4

n)5

n)6

VDE

CCSD(T)//MP2 Erela

0.55

VDE

exp VDEb

0.87

0.94

1A 1B

0.0 74.6

1.34 1.29

0.0 91.6

1.65 1.58

1.57

2A 2B 2C 2D 2E 2F 2G 2H

0.0 7.9 22.3 27.2 31.2 113.5 141.3 141.8

1.85 1.90 1.93 1.99 1.87 1.78 1.73 1.72

0.0 9.1 26.0 40.4 68.1 122.1 153.2 153.3

2.16 2.20 2.22 2.27 2.15 2.09 2.03 2.03

2.18

3A 3B 3C 3D 3E 3F 3G

0.0 19.6 24.2 149.7 152.4 219.7 294.9

2.33 2.32 2.39 2.46 2.13 2.19 2.10

0.0 26.8 24.3 166.5 172.0 244.0 319.1

2.62 2.60 2.68 2.74 2.40 2.46 2.38

2.62

4A 4B 4C 4D 4E 4F

0.0 31.7 39.6 102.1 103.8 163.0

2.72 2.68 2.74 2.87 2.87 2.56

2.97

5A 5B 5C 5D 5E 5F 5G

0.0 60.8 82.8 106.7 197.3 206.2 212.4

3.00 3.12 2.89 3.14 3.18 3.07 2.81

3.24

6A 6B 6C

0.0 98.9 196.7

3.15 3.08 2.98

3.54

a

Relative to the most stable structure among anion structures with a given hydration number. b Experimental VDE values taken from ref 15.

We have found that the magnitude of the calculated VDE value does not largely correlate with the relative energies of the optimized cluster structures. Instead, we found a general trend that the VDE value qualitatively depends on water binding structures. More specifically, the calculated VDE value is generally large in the case that water molecules are bound to the O atoms in nitromethane with a perpendicular configuration with respect to the NO2 plane. This behavior may be reasonable since such a binding configuration makes the π* orbital of nitromethane more compact. In Figure 4a, the VDE values obtained from the present electronic structure calculations are compared with the experimental VDE values in terms of the number of hydrating water molecules. This figure also shows the relative energy of the CH3NO2- · (H2O)n cluster in terms of the cluster size. Similar to the relative energy, the theoretical VDE values were calculated at the most stable structure result for a given cluster size (nA, see Table 1). As shown in the figure, the VDE values of MP2 are slightly smaller than the experimental VDE ones. On the other hand, the VDE values of CCSD(T) are in excellent

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Figure 4. (a) Comparison of the VDE values calculated for the most stable valence-bound CH3NO2- · (H2O)n (n ) 1-6) cluster structures to the experimental result. (b) Hydration energy for the CH3NO2- · (H2O)n (n ) 1-6) cluster as a function of the cluster size.

Figure 3. Molecular structures of the lowest valence-bound CH3NO2- · (H2O)n (n ) 1-6) clusters optimized by MP2/aug-cc-pVDZ+diff. Some hydrogen-bond distances are shown in the unit of Å. VDE values (in eV) obtained at the same level are also shown.

agreement with the experimental ones, although the CCSD(T) results are restricted to small clusters due to the limitation of our computer resources. This indicates that higher-order electron correlation effects should be included for obtaining accurate VDE values, as has already been pointed out in previous studies.21 3.3. Structures and Energetic of [CH3NO2 · (H2O)6] Anion Complexes. As described above, all the VDE values calculated for the possible isomeric forms of the hydrated valence anion of CH3NO2 with six water ligands, CH3NO2- · (H2O)6, lie in the range >3 eV (Table 1). On the other hand, VDE was determined experimentally to be 0.65 eV for the weakly electron-binding [CH3NO2 · (H2O)6]- species prepared in the CH3NO2 + (H2O)6-Arm reaction.15 This indicates that the CH3NO2- · (H2O)6 anions are not responsible for the observed VDE of 0.65 eV, and more importantly that the dipole-bound nature of (H2O)6remains intact in the weakly electron-binding species. By considering the above inference, we assumed two types of initial structures in geometry optimization for the weakly electron-binding [CH3NO2 · (H2O)6]- species; one type of structure where CH3NO2 interacts with the diffuse electron of (H2O)6- via the electron-dipole interaction, and the other where CH3NO2 interacts with the hydrogen-bonding framework of (H2O)6- via the dipole-dipole interaction. The former structure can be described in symbolic form as (H2O)6{e-}CH3NO2 while the latter as {e-}(H2O)6 · CH3NO2, where {e-}, (H2O)6 and CH3NO2 are written in series implying that each component interacts with next to each other. The notation {e-} was first

used by Tsurusawa and Iwata,29,30 who argued that only two water molecules can solvate an excess electron internally on the basis of the electronic structure calculations. They called the theoretically obtained electron-binding motif “OH{e-}HO bonding” as the excess electron bound two OH bonds through the attractive interaction between e- and positively charged H atoms of H2O. We started the geometry optimization with the (H2O)6{e-}CH3NO2 form. The initial geometries were constructed from (H2O)6- and CH3NO2. The structure of the (H2O)6- moiety was taken from the calculations made by Herbert and HeadGordon,31 where seven isomeric forms (denoted as isomer A-G, see ref 31) were obtained at the B3LYP/6-31(1+,3+)G* level. The CH3NO2 molecule was located so that its dipole moment was directed toward the excess electron lobe of (H2O)6- with the distance between the two moieties being ∼10 Å. This optimization strategy was employed since the electron binding energy of the (H2O)6 cluster is much larger than that of the CH3NO2 monomer. The geometry optimization was performed at two theoretical levels of MP2/aug-cc-pVDZ+diff and LCBOP/aug-cc-pVDZ+diff. We have eventually succeeded in optimizing two stable structures out of seven different initial geometries of (H2O)6- isomers. The optimized geometries are presented in Figure 5. They are denoted here as DDB-1 and DDB-2 after ref 15; DDB is an abbreviated form of “dual dipolebound”.15 In both geometries, the excess electron is suspended obviously between two polar moieties, as expected. It should be noted that the local minimum structures were obtained only for the initial geometries containing isomers E and F (see ref 31) having AA electron binding motif, where two OH bonds of the AA-water (accepting two hydrogen bonds) strongly capture the excess electron on the surface of (H2O)6. The calculated stabilization energies and VDEs for DDB-1 and DDB-2 are listed in Table 2. The MP2 and LC-BOP calculations provide comparable VDE values for these two structures, while the LC-BOP method gives slightly larger VDE values. The calculated VDEs are significantly larger than those of the

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Figure 5. Two optimized structures (DDB-1 and DDB-2) for the (H2O)6{e-}CH3NO2 dual dipole-bound anion cluster obtained by MP2/aug-ccpVDZ+diff and LC-BOP/aug-cc-pVDZ+diff. SOMOs are also shown, whose excess electron density values are 50%. Some hydrogen-bond distances are shown in units of Å. Numbers in parentheses are results obtained by LC-BOP.

TABLE 2: MP2 and LC-BOP Energies of the CH3NO2- · (H2O)6 Anion Complexesa Erelb

Est MP2

LC-BOP

MP2

µ0c

VDE

LC-BOP

MP2

LC-BOP

MP2

LC-BOP

0.41 0.42

0.51 0.49

10.0 9.5

10.4 9.8

-

(H2O)6 type E type F (H2O)6{e-}CH3NO2 DDB-1 DDB-2 {e-}(H2O)6 · CH3NO2 ADB-1 ADB-2

-244d -251e

-217d -217e

201 252

164 215

0.75 0.78

0.82 0.82

6.7 6.5

7.0 6.6

-445d -464e

-382d -395e

0 39

0 37

0.59 0.64

0.69 0.72

14.4 13.7

14.9 14.4

a Est and Erel are given in units of meV, VDE in units of eV, and µ0 in units of D. b The relative energy, Erel, is calculated with the reference to the most stable ADB anion (ADB-1). Note that (H2O)6- (type F) is located 58 meV above (H2O)6- (type E) at the MP2 level (50 meV at the LC-BOP level). c The total dipole moment of neutral species at the optimized anion geometries. d The stabilization enegy, Est, is defined as ∆H with reference to the (H2O)6- (type E) + CH3NO2 dissociation limit. e The stabilization enegy, Est, is defined as ∆H with reference to the (H2O)6- (type F) + CH3NO2 dissociation limit.

(H2O)6- anions. This comes from the fact that the attractive charge-dipole interaction between {e-} and CH3NO2 stabilizes the (H2O)6{e-}CH3NO2 anion complex whereas the interaction between the (H2O)6 and CH3NO2 moieties tends to be repulsive in the upper neutral state due possibly to the antiparallel orientation of the dipole moments at the anion geometry (see Figure 7). As for the {e-}(H2O)6 · CH3NO2 form, the initial geometries were prepared in such a way that CH3NO2 and (H2O)6 were

aligned so as to maximize the magnitude of the total dipole moment. The geometry optimization provided two local minimum structures for {e-}(H2O)6 · CH3NO2, where the excess electron is trapped by a large dipole moment generated cooperatively by the CH3NO2 and (H2O)6 constituents (Figure 6). We call here these types of [CH3NO2 · (H2O)6] anion complexes “assisted dipole-bound (ADB)” anions. As listed in Table 2, the MP2 stabilization energies of the ADB anions are calculated to be larger by 200-250 meV than those of the DDB

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Figure 6. Optimized structures (ADB-1 and ADB-2) for the assisted dipole-bound anion clusters, {e-}(H2O)6 · CH3NO2, obtained by MP2/augcc-pVDZ+diff and LC-BOP/aug-cc-pVDZ+diff. SOMOs are also shown, whose excess electron density values are 50%. Some hydrogen-bond distances are shown in units of Å. Numbers in parentheses are results obtained by LC-BOP.

anions. On the other hand, the calculated VDE values are slightly smaller for the ADB anions. This is attributable mainly to the stabilization of the upper neutral state due to the dipole-dipole interaction exerted attractively between CH3NO2 and (H2O)6 (see Figure 7). The VDE values calculated in the present study can be compared directly with the experimentally determined value of 0.65 eV.15 The calculated VDEs for the ADB anions are apparently in reasonable agreement with the experimental data (Table 2). The ADB isomers are also thermodynamically favored because of their larger stabilization energies. However, by considering the accuracy of the calculations together with the experimental finding mentioned below, we prudently withhold a decisive conclusion about which of the DDB and ADB anions are formed in the CH3NO2 + (H2O)6-Arm reaction and were detected as the spectral carrier in the previous photoelectron measurement.15 In a preliminary infrared predissociation spectroscopic study of the weakly electron-binding [CH3NO2 · (H2O)6] anion complex, it has been revealed that vibrational excitation of the NO2 moiety of the complex results exclusively in the formation of CH3NO2- · (H2O)3.32 The (H2O)6{e-}CH3NO2 configuration has a structural advantage in

the CH3NO2- · (H2O)3 formation via the “dipole-bound to valence” transformation because the CH3NO2 moiety interacts directly with the diffuse excess electron. In the {e-}(H2O)6 · CH3NO2 configuration, a rapid IVR should be followed by a long-distance migration of the excess electron for the CH3NO2- · (H2O)3 formation. Whatever the case, further experimental and theoretical studies will be necessary to receive a clear answer to this problem. Finally, we discuss the stability of the [CH3NO2 · (H2O)6] anion complexes and their photodestruction processes. Here, isomers DDB-1 and ADB-1 are chosen for each representative of the two types of anion complexes, (H2O)6{e-}CH3NO2 and {e-}(H2O)6 · CH3NO2. Figure 7 shows the schematic potential energy curves obtained from the relative energies at the MP2 level of theory. The left half of the curves display the stability of the anion-complex states with reference to the (H2O)6- + CH3NO2 dissociation asymptote, and the right half the potential energy curves along the rearrangement reaction coordinate from the anion complexes to the valence-bound CH3NO2- · (H2O)6 anion. It is expected that there may be a substantial barrier separating the anion-complex states and the CH3NO2- · (H2O)6 state since they differ significantly in the electronic structures,

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Motegi et al. each other. To understand the overall process involving the hydration rearrangement and water evaporation more quantitatively, molecular dynamics simulations are certainly required. Such computational studies are currently in progress in our laboratory. 4. Summary

Figure 7. Schematic potential energy profiles of the [CH3NO2 · (H2O)6]- anion clusters (DDB-1 and ADB-1) obtained from the present electronic structure calculations. Relative energies and VDE values are taken from MP2/aug-cc-pVDZ+diff calculations.

similar to the CH3NO2- monomer anion case (see Figure 2). Also shown in Figure 7 are the energy levels of the fragment states of CH3NO2- · (H2O)m + (6 - m)H2O (m ) 2-5). As seen in Figure 7, the stabilization energy attributable to the excess electron is ≈240 meV in DDB-1 and ≈450 meV in ADB-1. These values are comparable to typical hydrogen-bonding energies between two water molecules. It is also seen that the anion-complex states are metastable and their relative energies lie 1600-1800 meV above the global minimum of the valencebound CH3NO2- · (H2O)6 anion. Experimentally, photoexcitation of the [CH3NO2 · (H2O)6] anion complex at 2.1 µm (581 meV) leads to the production of the CH3NO2- · (H2O)3 + 3H2O fragments.15 This suggests that the exothermicity of the dissociation process is consumed as the H2O evaporation energy, and that the energy level of the CH3NO2- · (H2O)3 + 3H2O dissociation limit should be close to that of the anion-complex state. The present computational result given in Figure 7 qualitatively supports this inference. It is also interesting to note that only the photodestruction channel observed is the CH3NO2- · (H2O)3 + 3H2O dissociation asymptote.15 This means that the evaporation of 3H2O molecules may occur before the available excess energy is fully dissipated into all the internal degrees of freedom of the most stable CH3NO2- · (H2O)6 species. Presumably, the H2O evaporation as well as the hydration structure rearrangement would occur during the isomerization from the photoexcited anion complex into the valence-bound CH3NO2- since the structures of the anion complex and CH3NO2- · (H2O)6 are quite different from

In this study, we have performed systematic electronic structure calculations for CH3NO2- and its hydrated clusters [CH3NO2 · (H2O)n]- (n ) 1-6) to make clear the excess electron binding mechanism of these anion clusters. First, we have calculated the 2D potential energy surfaces for the ground and first excited states of the CH3NO2- monomer anion as a function of the NO bond length and the titling angle of the NO2 group to make clear the transformation mechanism between the dipole-bound and valence-bound anionic states. The ground-state potential energy surface of the CH3NO2- anion is clearly formed by these two anionic states through electronically nonadiabatic couplings. The corresponding transformation barrier height was found to be small. However, since the nonadiabatic coupling strength was calculated to be about 70 meV around the avoided crossing region, the transformation dynamics may be in the range of nonadiabatic region as previously suggested by Sommerfeld.10 Second, hydration structures of the valence-bound CH3NO2anion were investigated up to six water solvents. It was found that three or four water molecules can directly attach to the NO2 group in the CH3NO2- anion through the strong hydrogen bonds and that water-water network structures can be seen in larger clusters for n > 3. The vertical detachment energy was predicted to increase gradually with the number of hydrating molecules. This result is consistent with the recent experimental result.15 Finally, we have found two types of metastable dipole-bound forms for the [CH3NO2 · (H2O)6] anion complex. In one form the excess electron is internally suspended between the two dipolar moieties while in the other form two dipolar moieties are cooperatively arranged to reinforce the excess electron-dipole interaction. It is expected that there exists a substantial barrier between the dipole-bound anion minimum and the valencebound anion minimum due to the significant difference in geometric structure. Photoexcitation processes of these dipolebound anions have been discussed on the basis of the obtained energetics. We presume that molecular dynamics studies should be definitely needed for the better understanding of the rearrangement process as well as dissociation processes for these metastable anions. This is an important issue in the future. Acknowledgment. This work was supported by the Grantin-Aid for Scientific Research of the Ministry of Education, Culture, Sports, Science, and Technology of Japan (Grant No. 20038011, 21550005). Supporting Information Available: Figures S1-S7 with optimized structures of the CH3NO2-(H2O)n (n ) 1-6) and (H2O)6- anion clusters. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Compton, R. N.; Carman, H. S.; Desfranc¸ois, C.; Abdoul-Carmine, H.; Schermann, J. P.; Hendricks, J. H.; Lyapustina, S. A.; Bowen, K. H. J. Chem. Phys. 1996, 105, 3472. (2) Lecomte, F.; Carles, S.; Desfranc¸ois, C.; Johnson, M. A. J. Chem. Phys. 2000, 113, 10973. (3) Gutsev, G. L.; Bartlett, R. J. J. Chem. Phys. 1996, 105, 8785. (4) Weber, J. M.; Robertson, W. H.; Johnson, M. A. J. Chem. Phys. 2001, 115, 10718.

Binding Mechanism in the [CH3NO2 · (H2O)n]- Anions (5) Schneider, H.; Vogelhuber, K. M.; Schinle, F.; Stanton, J. F.; Weber, J. M. J. Phys. Chem. A 2008, 112, 7498. (6) Adams, C. L.; Schneider, H.; Weber, J. M. J. Phys. Chem. A 2010, 114, 4017. (7) Goebbert, D. J.; Pichugin, K.; Sanov, A. J. Chem. Phys. 2009, 131, 164308. (8) Adams, C. L.; Schneider, H.; Ervin, K. M.; Weber, J. M. J. Chem. Phys. 2009, 130, 74307. (9) Adamowicz, L. J. Chem. Phys. 1989, 91, 7787. (10) Sommerfeld, T. Phys. Chem. Chem. Phys. 2002, 4, 2511. (11) Sommerfeld, T. J. Phys. Conf. Ser. 2005, 4, 245. (12) Hendricks, J. H.; Lyapustina, S. A.; De Clercq, H. L.; Bowen, K. H. J. Chem. Phys. 1998, 108, 8. (13) Han, S. Y.; Kim, J. H.; Song, J. K.; Kim, S. K. J. Chem. Phys. 1998, 109, 9656. (14) Balaj, O. P.; Siu, C.-K.; Balteanu, I.; Beyer, M. K.; Bondybey, V. E. Int. J. Mass Spectrom. 2004, 238, 65. (15) Nakanishi, R.; Nagata, T. J. Chem. Phys. 2009, 130, 224309. (16) Gutsev, G. L.; Jena, P.; Bartlett, R. J. J. Chem. Phys. 1999, 110, 403. (17) Robertson, W. H.; Price, E. A.; Weber, J. M.; Shin, J.-W.; Weddle, G. H.; Johnson, M. A. J. Phys. Chem. A 2003, 107, 6527. (18) Myshakin, E. M.; Jordan, K. D.; Sibert, E. L., III; Johnson, M. A. J. Chem. Phys. 2003, 119, 10138. (19) Arenas, J. F.; Otero, J. C.; Pela´ez, D.; Soto, J.; Serrano-Andre´s, L. J. Chem. Phys. 2004, 121, 4127.

J. Phys. Chem. A, Vol. 114, No. 34, 2010 8947 (20) Bull, J. N.; Maclagan, R. G. A. R.; Harland, P. W. J. Phys. Chem. A 2010, 114, 3622. (21) Simons, J. J. Phys. Chem. A 2008, 112, 6401. (22) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007. (23) Yagi, K.; Okano, Y.; Sato, T.; Kawashima, Y.; Tsuneda, T.; Hirao, K. J. Phys. Chem. A 2008, 112, 9845. (24) Iikura, H.; Tsuneda, T.; Yanai, T.; Hirao, K. J. Chem. Phys. 2001, 115, 3540. (25) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (26) Tsuneda, T.; Suzumura, T.; Hirao, K. J. Chem. Phys. 1999, 110, 10664. (27) Frisch, M. J.; et al. Gaussian03, Revision D.02; Gaussian, Inc.: Pittsburgh, PA, 2004. (28) Werner, H.-J.; Knowles, P. J.; Lindh, R.; Manby, F. R.; Schu¨tz, M.; Celani, P.; Korona, T.; Rauhut, G.; Amos, R. D.; Bernhardsson, A.; Berning, A.; Cooper, D. L.; Deegan, M. J. O.; Dobbyn, A. J.; Eckert, F.; Hampel, C.; Hetzer, G.; Lloyd, A. W.; McNicholas, S. J.; Meyer, W.; Mura, M. E.; Nicklass, P.; Palmieri, R.; Pitzer, U.; Schumann, H.; Stoll, H.; Stone, A. J.; Tarroni, R.; Thorsteinsson, T. MOLPRO, version 2006.1, a package of ab initio programs. (29) Tsurusawa, T.; Iwata, S. Chem. Phys. Lett. 1998, 287, 553. (30) Tsurusawa, T.; Iwata, S. Chem. Phys. Lett. 1999, 315, 433. (31) Herbert, J. M.; Head-Gordon, M. Phys. Chem. Chem. Phys. 2006, 8, 68. (32) Johnson, M. A. Private communication.

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