Infrared and Electronic Spectroscopy of Benzene−Ammonia Cluster

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Infrared and Electronic Spectroscopy of Benzene-Ammonia Cluster Radical Cations [C6H6(NH3)1,2]+: Observation of Isolated and Microsolvated σ-Complexes† Kenta Mizuse, Hayato Hasegawa, Naohiko Mikami, and Asuka Fujii* Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Sendai 980-8578, Japan ReceiVed: February 1, 2010; ReVised Manuscript ReceiVed: March 18, 2010

We report infrared (IR) and electronic spectra of benzene-ammonia cluster radical cations [C6H6(NH3)n]+ (n ) 1 and 2) in the gas phase to explore cluster structures and chemical reactivity of the simplest aromatic radical cation with base (nucleophile) molecules. The electronic spectra in the visible region indicate that these cluster cations no longer have the benzene cation chromophore as a result of an intracluster reaction. Analyses of the IR spectra, on the basis quantum chemical calculations and the vibration-internal rotation analysis, reveal that both [C6H6(NH3)1,2]+ form σ-complex structures, in which the ammonia moiety is covalently bonded to the benzene moiety due to the intracluster nucleophilic addition. For [C6H6(NH3)2]+, it is also shown that the second ammonia molecule solvates the σ-complex core via a N-H · · · N hydrogen bond. Such σ-complex structures are generally supposed to be a key intermediate of aromatic substitution reactions. The observed mass spectra and energetics calculations, however, show that [C6H6(NH3)n]+ systems are inert for aromatic substitutions. The present experimental observations indicate the inherent stability of these σ-complex structures, even though they do not show the aromatic substitution reactivity. 1. Introduction Interactions between electron-deficient aromatics and Lewis base (nucleophile) molecules have been studied for more than 100 years.1-3 Such interactions play key roles in many chemical and biochemical reactions, in which oxidized/cationized aromatic rings (including damaged DNA bases) react with water, alcohol, and other reactants.1-11 For molecular-level understanding of the simplest aromatic cation-base (nucleophile) interactions, structures of [C6H6(L)n]+ (L ) H2O, CH3OH, CH3COOH, NH3) cluster radical cations in the gas phase have been studied experimentally and theoretically.12-24 Figure 1 shows the identified structures of these cluster cations. Infrared (IR) and electronic spectroscopy in addition to direct ab initio molecular dynamics calculations have shown that [C6H6(H2O)1]+ forms the C-H · · · O hydrogenbonded structure (Figure 1a).12-14,16-19,21 IR and electronic spectroscopy also have revealed that [C6H6(CH3OH)1]+ and [C6H6(CH3COOH)1]+ form the “on-ring” type structures, in which CH3OH or CH3COOH lies on the benzene ring (Figure 1b and c).17,21-23 These studies indicate that dominant intermolecular interactions strongly depend on the type of nucleophile molecules. For [C6H6(NH3)1]+, Tachikawa has predicted another type of structure by direct ab initio molecular dynamics calculation for the ionization process of the corresponding neutral cluster.15 His calculation has shown that the cationic cluster forms the σ-complex (cyclohexadienyl radical type) structure, in which the ammonia moiety is covalently bonded to the benzene moiety (Figure 1d). This σ-complex formation is a result of the intracluster nucleophilic addition following the ionization of the benzene moiety. In our previous letter, we have reported IR and electronic spectra of [C6H6(NH3)1]+, and we have experimentally demonstrated its σ-complex structure.24 †

Part of the “Klaus Mu¨ller-Dethlefs Festschrift”. * To whom correspondence should be addressed. E-mail: asukafujii@ mail.tains.tohoku.ac.jp.

Such σ-complex structures are generally presumed to be an intermediate or transition state in nucleophilic aromatic substitution reactions (Scheme 1).3-6,25-27 The nucleophilic aromatic substitution reaction is a major reaction of electron-deficient aromatics. Therefore, to understand chemical reactivities and reaction mechanisms of electron-deficient aromatics, it is obviously important to elucidate structures and inherent stability of σ-complexes in the isolated condition.

Figure 1. Structures of various benzene-nucleophile cluster cations, (a) [C6H6(H2O)]+, (b) [C6H6(CH3OH)]+, (c) [C6H6(CH3COOH)]+, and (d) [C6H6(NH3)]+. Green, white, red, and blue balls represent C, H, O, and N atoms, respectively.

SCHEME 1: General Mechanism of Nucleophilic Aromatic Substitution Reactionsa

a X ) leaving group; Y ) electron-withdrawing group; Nu ) nucleophile. Charge symbols and unpaired electrons are not shown here (see also ref 27).

10.1021/jp1009466  2010 American Chemical Society Published on Web 03/31/2010

Infrared and Electronic Spectroscopy of [C6H6(NH3)1,2]+ Since the first report by Brutschy and co-workers,28 nucleophilic aromatic substitution has been observed in ionization processes of gas-phase clusters consisting of halobenzene and nucleophiles, such as C6H5F/CH3OH and C6H5Cl/NH3 systems.28-48 In these reactions, ionization forces the aromatics to be highly electron-deficient, and it ensures high reaction efficiencies (note that these reactions occur in the radical cationic state, and the charge sign is different from that in textbook cases; see ref 27). On the other hand, the nonhalogenated [C6H6(NH3)1]+ system shows no substitution reactivity because of its poor leaving group (X ) H).24 Instead, the production yield of the σ-complex is relatively high because the leaving step of Scheme 1 is inhibited. The stability of the σ-complex in this system is advantageous to probe its detailed properties and further microsolvated structures. Besides [C6H6(NH3)1]+, isolated σ-complex structures have been observed in various protonated aromatics,49-57 anionic complexes,58,59 and radical cationic complexes.48 To explore the stability of σ-complexes in a microsolvated environment, a study of clusters involving two or more ligands is requested. However, no microsolvated σ-complex structures have been identified, except for solvations by an inert ligand (Ar and N2).24,49,52,54,56,57 The stability of [C6H6(NH3)1]+ may enable us to explore microsolvation of the σ-complex with nucleophiles. In this article, following our previous letter, we report IR and electronic spectra of [C6H6(NH3)1,2]+. Although the spectra of [C6H6(NH3)1]+ have been reported in the previous letter,24 detailed discussion for the chemical reactivity and the reaction path has not been given. Recently, Chiang et al. reported the fragment mass spectra of [C6H6(NH3)n]+, and they analyzed the mass spectra on the basis of the σ-complex formation.60 However, structural information by mass spectrometry is still limited, and microsolvated structures of the σ-complex have not been experimentally confirmed yet. In this paper, in addition to the comprehensive analysis of the optical spectra of [C6H6(NH3)1]+, the structures of [C6H6(NH3)2]+ are studied in detail on the basis of IR and electronic spectroscopy. We provide the first spectroscopic evidence of the σ-complex microsolvated by nucleophiles. These results demonstrate the inherent stability of the σ-complex with or without a solvent molecule. 2. Methodology 2.1. Experimantal Method. IR and electronic spectra of size-selected [C6H6(NH3)n]+ were measured as photodissociation spectra by using a tandem quadrupole mass filter type spectrometer and coherent light sources, both of which have been described elsewhere.16,19,48 Cluster cations were produced by a supersonic jet expansion of photoionized bare benzene cations with ammonia. A gaseous mixture of benzene/ammonia/neon (the typical ratio was 1/1/ 30, and the total pressure was 0.3 MPa) was expanded into a vacuum chamber through a pulsed valve (General Valve Series 9). In the collisional region of the jet expansion, bare benzene was resonantly photoionized via the S1 61 level, and the benzene cation picked up ammonia molecules to form clusters during the jet-cooling. Cluster ions of a specific size were selected at the first quadrupole mass filter. The mass resolution was set to be high (∆m/z < ∼1) enough to completely eliminate other cluster ions. The mass-selected clusters were then introduced into an octopole ion guide, which was placed between the two mass filters. A counterpropagating light pulse irradiated the cluster cations in the ion guide. When the photon energy was resonant with a vibrational or electronic transition of the size-selected cluster

J. Phys. Chem. A, Vol. 114, No. 42, 2010 11061 cation, vibrational or electronic predissociation following the transition resulted in fragmentation of the cluster cation. The second quadrupole mass filter was tuned to pass only the fragment cluster cations. Thus, an action spectrum of the sizeselected cluster cation was recorded by scanning the light wavelength while monitoring the intensity of the ion current due to the fragment cluster cations. The single ammonia-loss channel was monitored for the IR spectroscopy, and the bare benzene cation fragment channel was detected for the electronic spectroscopy. The intensities of all of the spectra were normalized by the power of the light. We used two types of the coherent light source for IR spectroscopy; one was diffrence frequency generation with a LiNbO3 crystal between the fundamental output of a Nd:YAG laser (Spectra-Physics GCR-230) and that of a dye laser (Lambda Physik Scanmate with LDS759 and LDS821 dyes) to cover the region of 2800-3450 cm-1. The other was a KTP/KTA-based optical parametric oscillator/ amplifier (OPO/OPA) system (Laser Vision) pumped by a Nd: YAG laser (Continuum Powerlite 8000) to cover the region of 2400-3650 cm-1. The visible light for electronic spectroscopy was the output of an OPO (GWU Vis-IR2-170) pumped by the third harmonic of a Nd:YAG laser (Continuum Surelite III). 2.2. Theoretical Method. For detailed analyses of the observed spectra and the understanding of cluster structures, energetics, and reactivities, we carried out quantum chemical calculations. Geometry optimizations for both equilibrium points and transition states were carried out by density functional theory (DFT) calculations with the B3LYP functional. We used the 6-31+G(d) basis set for the initial explorations and the larger 6-311+G(2df,2pd) basis set for the final optimization. Optimized geometries at this level were used for further calculations. Harmonic frequencies and IR intensities were calculated at the same (B3LYP/6-311+G(2df,2pd)) level, and calculated frequencies were scaled by the single scaling factor of 0.953. For the transition states, it was confirmed that one imaginary frequency mode exists along the reaction coordinate. Electronic transitions were estimated by the time-dependent density functional theory calculations (TD-B3LYP/6-311+G(2df,2pd)). In the previous studies, the TD-B3LYP calculations with the suitable basis set successfully reproduced the observed electronic spectra of the [C6H6(L)n]+ type cluster cations.22,24 Relative energies were evaluated by the spin-restricted open-shell second-order Møller-Plesset perturbation theory (ROMP2/6-311+G(2df,2pd)) single-point energy calculations with the zero-point energy (ZPE) corrections at the DFT calculations, in which ZPEs were also scaled by the factor of 0.953. The spin-unrestricted MP2 (UMP2) calculations for aromatic radical cations were often affected by the spin contamination. We found that UMP2 calculations of σ-complex [C6H6(NH3)1]+ results in 〈S2〉 ≈ 1.10, while 〈S2〉 ≈ 0.78 at the unrestricted B3LYP. Therefore, we employed the ROMP2 energies at the B3LYP-optimized geometries. All of the calculations in this study were performed using the Gaussian 03 program package.61 The optimized cluster structures were visualized with the MOLEKEL program.62 3. Results and Discussion 3.1. Mass Spectra. Figure 2a shows the mass spectrum of the cluster ions produced by the present photoionization pickup type cluster ion source. This spectrum is similar to that of the electron ionization cluster source reported by Chiang et al.60 Figure 2b shows the expanded mass spectra in the region near the [C6H6(NH3)n]+ species. The peaks at ∆m/z ) 0 correspond to [C6H6(NH3)n]+, while ∆m/z ) -1 and -2 correspond to loss

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Figure 2. (a) Mass spectrum of the ions produced by the photoionization pickup ion source. (b) Expanded spectra in the region near the [C6H6(NH3)n]+ peaks.

Figure 3. Electronic spectra of [C6H6(NH3)n]+ (n ) 0-2) in the visible region. Spectrum (a) has been measured as an argon-mediated dissociation spectrum.19

of one hydrogen atom and one hydrogen molecule from the [C6H6(NH3)n]+ species, respectively. It is clearly seen that elimination of atomic or molecular hydrogen is negligible in this system. This makes a sharp contrast with the ionized halobenzene base clusters, in which HX or X (X ) F, Cl, Br, I) loss is observed as main mass peaks.28-48 The mass spectra suggest that the second step of the aromatic substitution reaction, namely, loss of the leaving group, is inhibited in the present system. The mass spectra, however, provide no information on the structures of [C6H6(NH3)n]+. 3.2. Overview of Electronic and Infrared Spectra. Figure 3 shows electronic spectra of [C6H6(NH3)n]+ (n ) 0-2) in the visible region. The electronic spectrum of the benzene cation (n ) 0) (Figure 3a), measured as an argon-mediated dissociation spectrum,19 is shown for comparison. Electronic spectra of [C6H6(NH3)1,2]+ show very different features from that of the benzene cation. The absorption of the benzene cation at around 24000 cm-1 is relatively strong in intensity and is assigned to the π r π (C r X) transition.19,63-65 On the other hand, both [C6H6(NH3)1,2]+ have a relatively weak transition at around 20000 cm-1, and the strong absorption is missing over 23000 cm-1. This feature is also different from those of [C6H6(H2O)1-3]+, [C6H6(CH3OH)1]+, and [C6H6(CH3COOH)1]+.19,22,23 These clusters show a relatively strong absorption as the benzene cation, and they have the benzene cation core weakly perturbed by the microsolvation. The electronic spectra of [C6H6(NH3)1,2]+ indicate that π-orbitals of the aromatic ring are significantly perturbed in these systems, and the ion core is no longer regarded as a benzen cation. Furthermore, the spectral similarity between n ) 1 and 2 suggests that the clusters might have the same ion core. Figure 4 shows IR spectra of [C6H6(NH3)1,2]+ in the X-H (X ) C, H) stretch region. In both spectra, two relatively sharp

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Figure 4. Infrared spectra of [C6H6(NH3)n]+ (n ) 1 and 2) in the X-H (X ) C, N) stretch region.

bands are seen in the range of 3200-3400 cm-1 (at 3243 and 3328 cm-1 for [C6H6(NH3)1]+ and 3278 and 3334 cm-1 for [C6H6(NH3)2]+), and they are assigned to the free N-H stretch vibrations. There is also relatively weak and broad absorption in the higher-frequency region at around 3400 cm-1, and this absorption is more remarkable in [C6H6(NH3)2]+. The broad and strong band below 3100 cm-1 in the [C6H6(NH3)2]+ spectrum is assigned to the hydrogen-bonded N-H stretch. In the following sections, we will discuss cluster structures by analyzing these spectra with a help of quantum chemical calculations. 3.3. [C6H6(NH3)1]+. Both the IR and electronic spectra of [C6H6(NH3)1]+ are essentially the same ones as we have reported in the previous letter24 except for the extension of the IR frequency region. Because the basic interpretations of the spectrum have been described previously, here, we briefly analyze the spectra. Instead, the energetics of the cluster cation is newly discussed in detail. In the IR spectrum, two bands appear at 3243 and 3328 cm-1 and are assigned to free N-H stretches. Aromatic C-H stretch bands are generally expected to appear in the 3000-3100 cm-1 region;66 however, no strong absorption is observed in this region. In the case of [C6H6(H2O)1]+, the C-H stretch bands have comparable intensity to the O-H stretching because of the C-H · · · O type hydrogen bond formation.13,14 The absence of strong C-H stretch bands means that there are no C-H · · · N type hydrogen bonds in [C6H6(NH3)1]+. To analyze the spectra and cluster structures, we carried out DFT calculations at the B3LYP/6-311+G(2df,2pd) level. Although only two isomers have been considered in the previous studies of [C6H6(NH3)1]+,15,24,60 we found 11 types of stable isomers in this study. Figure 5 shows structures and relative energies of these isomers. Figure 6 compares experimental spectra and simulated spectra for four selected isomers, of which structures are expected to lie in the reaction path of the aromatic substitution (Scheme 1). Cluster structures and simulated spectra of all of the isomers are shown in the Supporting Information. In Figures 5 and 6, “Intermediate” has the σ-complex structure having the C-N covalent bond. “Entrance”, which is essentially the same structure as those observed in [C6H6(CH3OH)]+ and [C6H6(CH3COOH)]+,17,22,23 corresponds to a local minimum in the entrance channel of the substitution reaction because the C-N distance in this structure is much longer (∼2.3 Å) than the typical covalent bond distance (∼1.5 Å). In Entrance, the charge-dipole interaction and the interaction between the nonbonding orbital of the ammonia moiety and the π-orbitals of the benzene moiety play key roles. “Exit-1” and “Exit-2” are the minima in the reaction exit channel, in which the reaction products (C6H5NH3+ or C6H5NH2+) are formed in the cluster. Although Exit-1 is energetically most stable among all of the isomers, mass spectra in Figure 2 show that both Exit-1 and Exit-2 are not plausible under the present experimental condition.

Infrared and Electronic Spectroscopy of [C6H6(NH3)1,2]+

Figure 5. Calculated 11 stable structures of [C6H6(NH3)1]+ and their relative energies. Geometry optimizations were carried out at the B3LYP/6-311+G(2df,2pd) level. Relative energies at the ROMP2/6311+G(2df,2pd) level are given in kJ/mol with the zero-point energy corrections at the B3LYP level.

In Figure 6, the calculated spectra of the “Intermediate” (σ-complex) structure are in excellent agreement with the observed spectra. This agreement is evidence that the σ-complex structure is formed in the cluster. Then, the observed bands in the spectra are assigned on the basis of the calculations of “Intermediate”. The 3243 and 3328 cm-1 bands in the IR spectrum are assigned to the symmetric and asymmetric stretches of the ammonia moiety, respectively. The broad band at around 20000 cm-1 in the electronic spectrum is attributed to the transition mainly from the π-orbital of the benzene ring to the nonbonding orbital of the C-N covalent bond. Molecular (Kohn-Sham) orbitals of the “Intermediate” structure are shown in the Supporting Information. In the IR spectrum, the broad and relatively weak band is seen at around 3430 cm-1. No corresponding band is predicted in the calculation based on the harmonic approximation. This band is assigned to a combination of the N-H stretch mode and an intermolecular mode such as those found in IR spectra of various ionic clusters reported so far.16,67 It might be also possible to attribute this band to minor contribution of other isomers such as Entrance and Exit-1 structures. The electronic spectrum, however, shows no apparent signature for the Entrance isomer. It should be also noted that clear signature of the energetically most stable Exit-2 structure is not observed in the IR spectrum, consistent with the missing of the reaction products in the mass spectra. A possible explanation for the absence of Exit-1 and Exit-2 is that the observed spectra were measured by monitoring the C6H6+ fragment intensity. However, the IR spectrum of the Ar-tagged ion, which was measured as the action spectrum of the Ar loss, was reported in the previous letter, and it clearly shows the same spectral feature as bare [C6H6(NH3)1]+.24 Then, we conclude that neither the Exit-1 nor Exit-2 structures are formed in the present condition. By the IR and electronic spectra, the σ-complex structure is identified in [C6H6(NH3)1]+. On the other hand, no substitution products are observed in the mass spectra of this system. In contrast, several cationic halobenzene-ammonia systems have been reported to produce substitution reaction products.28-48 To evaluate the intrinsic reactivity of the [C6H6(NH3)1]+ σ-complex without solvation, we calculated energetics of the nucleophilic substitution reaction (Scheme 1) in this system. Figure 7 shows the calculated energy diagram at the ROMP2/6-311+G(2df,2pd)//

J. Phys. Chem. A, Vol. 114, No. 42, 2010 11063 B3LYP/6-311+G(2df,2pd) level. TS1-3 are the maxima in the potential energy curve along the reaction path. In Figure 7, TS2 and TS3 lie above the “initial” state (isolated benzene cation and ammonia). The cluster starting from the initial state meets the high barriers at TS2 and TS3. Furthermore, if the system goes over the TS2, another dissociation channel [C6H5NH3+ + H] opens. The direct one-step reaction path from Intermediate to Exit-2 was not found. Figure 7 shows that aromatic substitution in the [C6H6(NH3)1]+ system is strongly forbidden in energetics. These high barriers also explain the absence of Exit-1 and Exit-2. On the other hand, ionized benzene reacts with an ammonia molecule via the nearly barrierless path to form the σ-complex structure. As for this barrier, it has been proposed that the potential curve crossing between the ground state (C6H6+/NH3) and the charge-transferred excited state (C6H6/ NH3+) lowers the barrier height.39,68,69 Here, we discuss the origin of the structural difference among various solvated benzene cation clusters, [C6H6(L)]+. Figure 1 collects the various cluster structures experimentally identified so far.13,14,16-24 We can attribute these structural variations to the difference of ionization energies (IE). The IE of H2O (12.62 eV) is much higher than that of C6H6 (9.24 eV).70 Such a large difference of IE makes the excess charge almost localized at the hole of the π-orbital of the benzene moiety in [C6H6(H2O)]+. This charge localization forces to the cluster structure to take the C-H · · · O type, in which the electrostatic interactions are dominant and only negligible charge-transfer (orbital) interaction exists. In contrast, the IEs of CH3OH (10.84 eV) and CH3COOH (10.65 eV) are closer to that of C6H6,70 and the excess charge is expected to be partially delocalized from the benzene moiety to the CH3OH/CH3COOH moiety. This charge transfer is carried by the interaction between the nonbonding orbital of the oxygen atom and the π-orbitals of benzene (n-π interaction) (Figure 1b and c).23 In the case of [C6H6(NH3)]+, the IE of NH3 (10.07 eV)70 is much closer to the IE of C6H6, and the stronger n-π interaction is expected. Then, the ammonia moiety and the benzene ring get closer, resulting in the covalent bond formation (Figure 1d). Such a picture is confirmed by the population analysis. Table 1 shows natural charge in the C6H6 moiety calculated at the B3LYP/6-311+G(2df,2pd) level. IE values are also collected. In [C6H6(NH3)]+, 60% of the total charge is transferred to the ammonia moiety because of the covalent interaction. On the other hand, in the other clusters, transferred charge is much smaller (0-23%). This analysis indicates the cluster structure, that is, the type of intermolecular interactions, is governed by the magnitude of charge transfer, which increases with decreasing IE of nucleophile molecules. Recently, El-Shall et al. suggested similar C-N covalent bond formation in the (benzene-pyridine)+ system on the basis of the mass spectrometric technique and DFT calculations.71 They also reported that the C-N bond dissociation energy in the (benzene-pyridine)+ system (∼130 kJ/mol) is larger than that in the present (benzene-ammonia)+ system (78.7 kJ/mol).71 Because the IE of pyridine (9.26 eV) is much closer to that of benzene, the result of El-Shall et al. is consistent with our finding described above. 3.4. [C6H6(NH3)2]+. The σ-complex structure is identified in [C6H6(NH3)1]+; however, it is not obvious that larger clusters also have the σ-complex moiety. In the case of larger-sized clusters with H2O and CH3OH, intracluster proton transfer and charge transfer have been suggested to occur,16,18,19,22,72-75 and such intracluster reactions potentially compete with the σ-complex formation.

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Figure 6. Comparison between observed and simulated spectra of [C6H6(NH3)1]+. Left column: (a) Observed IR spectrum. (b-f) Simulated IR spectra obtained with B3LYP/6-311+G(2df,2pd) and the scaling factor of 0.953. Right column: (a′) Observed electronic spectrum. (b′-f′) Simulated electronic spectra obtained with TD-B3LYP/6-311+G(2df,2pd). The stick spectra in the simulations were transformed into the continuous spectra by using the Lorentzian functions with full-width at half-maximum values of 10 cm-1 for the IR spectra and 2000 cm-1 for the electronic spectra.

Figure 7. Calculated energy diagram of the nucleophilic substitution reaction in the [C6H6(NH3)1]+ system. Calculations were carried out at the ROMP2/6-311+G(2df,2pd)//B3LYP/6-311+G(2df,2pd) level. Zero-point corrections are included in the presented energy values.

To interpret the experimental spectra of [C6H6(NH3)2]+, we explored its isomeric structures by using DFT calculations. Initial structures were manually constructed by adding one more ammonia molecule to the isomers of [C6H6(NH3)1]+. We tested

more than 70 initial structures and found 23 stable isomers. The structures of all of the stable isomers are presented in the Supporting Information with their relative energies. From the experimental results, some isomers should be excluded.

Infrared and Electronic Spectroscopy of [C6H6(NH3)1,2]+ TABLE 1: Natural Charge in the C6H6 Moiety Calculated for Various [C6H6(L)]+a cluster [C6H6(H2O)]+ [C6H6(CH3OH)]+ [C6H6(CH3COOH)]+ [C6H6(NH3)]+

natural charge nucleophile ionization in the C6H6 moiety molecule energy (eV) ∼1.00 0.77 0.77 0.40

H 2O CH3OH CH3COOH NH3 C 6 H6

12.62 10.84 10.65 10.07 9.24

a Ionization energies of ligand molecules taken from ref 70 are also shown.

Because no substitution products are observed in the mass spectra (see Figure 1), isomers of the substitution product types are ruled out. The electronic spectrum shows that the ion core of the cluster no longer has the benzene cation moiety. In the IR spectrum, the broad and strong absorption is observed below 2800 cm-1. Because C-H stretch bands are generally sharp in IR spectra,16 this significantly broadened band is assigned to a hydrogen-bonded N-H stretch. Then, the cluster should have a hydrogen-bonded N-H bond. These requests are satisfied only in three types of the isomers. Figure 8 shows plausible structures of the cluster, their relative energies, and simulated spectra. “Intermediate-1” (b) and “Intermediate-2” (c) have the σ-com-

J. Phys. Chem. A, Vol. 114, No. 42, 2010 11065 plex ion core with an N-H · · · N hydrogen bond. In “Charge transferred” (d), positive charge is almost completely transferred to the ammonia moiety, resulting in the structure of [(neutral benzene)-(ammonia dimer cation)]. Here, the ammonia dimer cation moiety prefers to form the proton-transferred structure.76,77 On the other hand, in “Proton transferred” (e), the proton is attracted from the benzene cation to the ammonia moiety, resulting in [(phenyl radical)-(protonated ammonia dimer)]. Figure 8 also shows two important isomers, (f) and (g). Chiang et al. have carried out DFT calculations for [C6H6(NH3)2]+ at the B3LYP/6-311+G(d,p) level and have reported the structures of “Intermediate-3” (f), in addition to Intermediate-1.60 Intermediate-3 also has the σ-complex structure with a weaker C-H · · · N hydrogen bond. “Exit” (g) is the global minimum in the system. Although these two isomers are finally ruled out on the basis of the spectroscopic results, we pick them up for comparison. For the electronic spectra, calculations for all of the isomers except Exit show weak absorption at around 20000 cm-1, and they agree with the experimental spectrum. The simulated IR spectra, on the other hand, remarkably vary in the isomer structures. Then, the IR spectra would be more helpful to determine the structure of the observed cluster. Among all of the simulated spectra, Intermediate-1 and Intermediate-2 repro-

Figure 8. Observed and simulated spectra of [C6H6(NH3)2]+. Optimized cluster structures and relative energies at the ROMP2/6-311+G(2df,2pd) level of isomers are also shown. Left column: (a) Observed IR spectrum. (b-g) Simulated spectra obtained with B3LYP/6-311+G(2df,2pd) and the scaling factor of 0.953. Right column: (a′) Observed electronic spectrum. (b′-g′) Simulated electronic spectra obtaimed with TD-B3LYP/6311+G(2df,2pd). The stick spectra in the simulations were transformed into the continuous spectra by using the Lorentzian functions with the full-width at half-maximum values of 10 cm-1 for the IR spectra and 2000 cm-1 for the electronic spectra. Dashed lines are eye guides.

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duce the experimental spectrum well, especially in its band frequencies, whereas Intermediate-3 does not have a hydrogenbonded N-H band, and Charge transferred and Proton transferred have an extra band in the 2800-3200 cm-1 region, which is missing in the observed spectrum. The simulated IR spectra of Intermediate-1 and Intermediate2, however, do not reproduce the band widths and relative intensities of the lowest-frequency band (below 2600 cm-1) and the highest-frequency band (at around 3400 cm-1) well in comparison with those in the experimental spectrum. As for the lower-frequency band, this band is assigned to the hydrogenbonded N-H stretch. Broader bandwidth is ordinarily observed for the hydrogen-bonded X-H stretch band, and the broadening is attributed to enhanced anharmonic couplings.78 On the other hand, the band at around 3400 cm-1 is assigned to the free N-H stretch of the terminal (proton-acceptor) ammonia molecule, and the broadening of this band cannot be attributed to a hydrogen bond. All of the calculated IR spectra in Figure 8 are based only on the vibrational analyses, and the contribution of internal rotation of the terminal ammonia moiety is neglected. However, in the previous study for various hydrated and ammoniated clusters, such as NH4+(NH3)n and NH4+(H2O)n, IR spectra show that sub-band structures come from the internal rotation along a hydrogen bond.79-82 Because Intermediate-1 and Intermediate-2 have very similar spectral features (Figure 8), the discussion on the internal rotation is done only for Intermediate-1. Figure 9a shows a schematic picture of the internal rotation in Intermediate-1, and Figure 9b shows the calculated potential energy curve for the internal rotation at the ROMP2/6-311+G(2df,2pd) level (using the B3LYP-optimized structures). From this potential energy curve, the barrier height of the internal rotation is estimated to be ∼40 cm-1. Such a low barrier height means that the terminal ammonia molecule can be regarded as a free rotor along the hydrogen bond axis. For a one-dimensional free rotor, the following selection rules are held66

∆k ) k' - k'' ) 0 ∆k ) k' - k'' ) (1

for parallel bands

(1)

for perpendicular bands

(2) where k′ and k′′ are the quantum numbers of the internal rotation in the vibrational excited state and in the ground state, respectively. Figure 9c and d shows expansion of the observed spectrum and the simulation for Intermediate-1 without the contribution of the internal rotation. The assignments of the bands are also shown. The band at around 3400 cm-1 is the asymmetric stretch of the terminal (proton-acceptor) ammonia molecule, and the transition dipole is perpendicular to the molecular axis. Then, ∆k ) (1 sub-bands are expected. Note that the symmetric stretch is a parallel band, and no sub-band structures are expected because of the overlap of the rotational bands (∆k ) 0). Sub-bands in the perpendicular transition are denoted by P(k′′) and R(k′′) for ∆k ) -1 and ∆k ) +1 transitions, respectively. Though these notations are not generally adopted for internal rotation, we use them for convenience. Within the rigid rotor approximation, the energy level of the internal rotation is denoted as follows

Ek ) Ck2

(3)

Figure 9. The vibration-internal rotation analysis for Intermediate-1 structures. (a) Structure of Intermediate-1 and atom labels used in (b). (b) Potential energy curve for the internal rotation of the proton-acceptor ammonia along the hydrogen bond. (c) Observed IR spectrum of [C6H6(NH3)2]+. (d) Calculated IR spectrum and band assignments based on pure vibrational analysis using the scaled harmonic frequencies. νsym(D) shows the symmetric N-H stretch of the proton-donor ammonia, and νasym(A) shows asymmetric stretch of the proton-acceptor ammonia. (e) Simulated IR spectrum including the vibration-internal rotation analysis. See text for details.

where k ()k′′, k′) ) 0, 1, 2, ... and C is the rotational constant for the internal rotation.66 Then, transition energies of P(k′′) and R(k′′) sub-bands, that is, ∆E[P(k′′)] and ∆E[R(k′′)], are given by

∆E[P(k'')] ) hν + C(k'' - 1)2 - Ck''2 ) hν - C - 2Ck''

(4)

∆E[Q(k'')] ) hν + C(k'' + 1)2 - Ck''2 ) hν + C + 2Ck''

(5)

where hν is the pure vibrational frequency of the perpendicular transition. C is assumed to be identical in both the vibrationally excited and ground states. As seen in eqs 4 and 5, sub-bands appear with the 2C spacing. For the transition intensities, Sheppard and Woodman have derived the dipole transition matrix elements associated with the perpendicular vibration of an internal rotor.83 By using the model system, in which a symmetric top rotates against an infinitely heavy framework, they showed that the relative intensities of sub-bands are independent of k′′ and are dependent only on the populations of the lower states. Then, we calculated the Boltzmann distribution of k′′ at 200 K, which is the typical temperature of ionic clusters formed in a supersonic jet.81 According to the k′′ distribution, the IR transition intensity obtained by DFT

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Figure 10. Calculated energy diagram of the nucleophilic substitution reaction in the [C6H6(NH3)2]+ system. Calculations were carried out at the ROMP2/6-311+G(2df,2pd)//B3LYP/6-311+G(2df,2pd) level. Zero-point energy corrections are included in the presented energy values.

calculation was divided into each sub-band. Statistical weight was taken into account, assuming that the terminal ammonia moiety has the C3 axis (the weight factors of the k′′ ) 0, 3, 6, 9, ... levels are twice as large as those of k′′ ) 1, 2, 4, 5, 7, ...).66 The hν used is the band center in Figure 9d, 3397.6 cm-1, which is the scaled harmonic frequency. C was taken from the rotational constant of the NH3 monomer around the C axis (6.3 cm-1).66 The above-mentioned simulation for Intermediate-1 results in the calculated spectra in Figure 9e. The red stick spectrum in Figure 9e shows the simulated sub-band structure. The continuous spectrum in Figure 9e agrees well with the observed spectrum. It should be noted that the same argument is applicable to Intermediate-2. These results indicate that the microsolvated σ-complex of Intermediate-1 and Intermediate-2 reproduce the observed spectra well, and these isomers are formed in [C6H6(NH3)2]+. The structure of the internal rotation results in the broad band feature in the observed spectrum of [C6H6(NH3)2]+, and no subband splitting was actually seen. This is in contrast to the previously reported spectrum of NH4+(NH3)n, where clear band splitting appears.79,80 This difference would be attributed to the coexistence of two isoenergetic isomers, Intermediate-1 and Intermediate-2. These two isomers have a little (∼5 cm-1) difference in the N-H stretch frequencies because of the different interaction strength between the N-H bond and the π-electrons. This difference of the frequency avoids the overlap between sub-bands of two isomers and makes it difficult to resolve them. Furthermore, degeneracy of two asymmetric stretches of the ammonia moiety would be lifted in [C6H6(NH3)2]+. This also enhances the complexity of the band. With the vibration-internal rotation analysis, it is shown that [C6H6(NH3)2]+ forms the σ-complex structures solvated by an

ammonia molecule via the N-H · · · N hydrogen bond. According to this conclusion, the observed electronic band at around 20000 cm-1 is assigned to the same transition as that in [C6H6(NH3)1]+. The related Kohn-Sham orbitals are shown in the Supporting Information. As in [C6H6(NH3)1]+, the stability and reactivity of [C6H6(NH3)2]+ are examined by the theoretical calculation. Figure 10 shows the potential energy diagram of the nucleophilic substitution reaction in the [C6H6(NH3)2]+ system calculated at the ROMP2/6-311+G(2df,2pd) level. In the previous mass spectrometric and theoretical study, Chiang et al. have concluded that the second ammonia molecule strengthens the C-N bond of the σ-complex.60 This conclusion is confirmed by the absence of the Entrance structure, which is a local minimum in [C6H6(NH3)1]+. This means further stabilization of the σ-complex by solvation. On the other hand, the barrier height of the elimination step is similar to that in [C6H6(NH3)1]+. This shows that the second ammonia molecule does not affect the reactivity of the σ-complex. Again, we should note that the low reactivity makes it easy to observe the σ-complex structures in the present system. 4. Conclusions Structures of the benzene-ammonia cluster cations [C6H6(NH3)1,2]+ were investigated by IR and electronic photodissociation spectroscopy. The electronic spectra show the signature of the intracluster reaction. Detailed analyses of the IR spectra aided by quantum chemical calculations show that both [C6H6(NH3)1,2]+ form the σ-complex structure. For [C6H6(NH3)2]+, it was also demonstrated that the second ammonia molecule solvates the σ-complex core via a N-H · · · N hydrogen bond. The stabilities of the observed σ-complex structure were estimated by mass spectrometry and quantum

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chemical calculations. Although σ-complex structures are generally supposed to be an intermediate of aromatic substitution, no substitution reactivity was found in [C6H6(NH3)1,2]+. The present results exhibit the inherent stability of the σ-complex structure even in the inactive system for aromatic substitution. Acknowledgment. The authors are grateful to Dr. T. Maeyama and Dr. Y. Matsuda for their helpful discussion. This study was supported by the Grant-in-Aid for Scientific Research (Project No. 19056001 on Priority Areas [477] “Molecular Science for Supra Functional Systems” from MEXT Japan and Nos. 20 · 5015, 19205001, and 20550005 from JSPS) and the Mitsubishi Foundation. K.M. is supported by JSPS Research Fellowships for Young Scientists. Most of the calculations were performed at the Research Center for Computational Science, Okazaki (Japan). Supporting Information Available: The calculated optical spectra of all of the stable [C6H6(NH3)1]+ isomers; the optimized structures of [C6H6(NH3)2]+ and their relative energies; the Kohn-Sham orbitals of the σ-complex structures. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Jackson, C. J.; Gazzolo, F. H. Am. Chem. J. 1900, 23, 376. (2) Meisenheimer, J. Justus Liebigs Ann. Chem. 1902, 323, 205. (3) Buncel, B.; Crampton, M. R.; Strauss, M. J.; Terrier, F. Electron Deficient Aromatic- And Hetroaromatic-Base Interactions; Elsevier: Amsterdam, The Netherlands, 1984. (4) Burnett, J. F.; Zahler, R. E. Chem. ReV. 1951, 49, 273. (5) Hepworth, J. D.; Waring, D. R.; Waring, M. J. Aromatic Chemistry; Royal Society of Chemistry: London, 2002. (6) Miller, J. Aromatic Nucleophilic Substitution, Elsevier: Amsterdam, The Netherlands, 1968. (7) Buck, P. Angew. Chem., Int. Ed. Engl. 1969, 8, 120. (8) Kasai, H.; Yamaizumi, Z.; Berger, M.; Cadet, J. J. Am. Chem. Soc. 1992, 114, 9692. (9) Barnett, R. N.; Bongiorno, A.; Cleveland, C. L.; Joy, A.; Landman, U.; Schuster, G. B. J. Am. Chem. Soc. 2006, 128, 10795. (10) Cadet, J.; Douki, T.; Ravanat, J.-L. Acc. Chem. Res. 2008, 41, 1075. (11) Sainsbury, M. Aromatic Chemistry; Oxford University Press: Oxford, U.K., 1991. (12) Tachikawa, H.; Igarashi, M. J. Phys. Chem. A 1998, 102, 8648. (13) Miyazaki, M.; Fujii, A.; Ebata, T.; Mikami, N. Chem. Phys. Lett. 2001, 349, 431. (14) Solca`, N.; Dopfer, O. Chem. Phys. Lett. 2001, 347, 59. (15) Tachikawa, H. Phys. Chem. Chem. Phys. 2002, 4, 6018. (16) Miyazaki, M.; Fujii, A.; Ebata, T.; Mikami, N. Phys. Chem. Chem. Phys. 2003, 5, 1137. (17) Solca`, N.; Dopfer, O. J. Phys. Chem. A 2003, 107, 4046. (18) Miyazaki, M.; Fujii, A.; Ebata, T.; Mikami, N. J. Phys. Chem. A 2004, 108, 10656. (19) Miyazaki, M.; Fujii, A.; Ebata, T.; Mikami, N. Chem. Phys. Lett. 2004, 399, 412. (20) Miyazaki, M.; Fujii, A.; Mikami, N. J. Phys. Chem. A 2004, 108, 8269. (21) Dopfer, O. Z. Phys. Chem. 2005, 219, 125. (22) Enomoto, S.; Miyazaki, M.; Fujii, A.; Mikami, N. J. Phys. Chem. A 2005, 109, 9471. (23) Kosugi, K.; Inokuchi, Y.; Nishi, N. J. Chem. Phys. 2001, 114, 4805. (24) Mizuse, K.; Fujii, A.; Mikami, N. J. Phys. Chem. A 2006, 110, 6387. (25) Wheland, G. W. J. Am. Chem. Soc. 1942, 64, 900. (26) Strauss, M. J. Chem. ReV. 1970, 70, 667. (27) In Scheme 1, charge signs and unpaired electrons are removed to keep the generality of reaction scheme. In this study, the aromatics is a radical cation and the nucleophile are neutral, and the resultant σ-complex is a radical cation. On the other hand, in general (textbook) nucleophilic aromatic substitutions, the aromatics are neutral, and the nucleophile is an anion, resulting in the negatively charged σ-complex (refs 1-6). (28) Dimopoulou-Rademann, U.; Rademann, K.; Bisling, P.; Brutschy, B.; Baumgartel, H. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 215.

Mizuse et al. (29) Brutschy, B.; Janes, C.; Eggert, J. Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 74. (30) Maeyama, T.; Mikami, N. J. Am. Chem. Soc. 1988, 110, 7238. (31) Tho¨lmann, D.; Gru¨tzmacher, H. F. Chem. Phys. Lett. 1989, 163, 225. (32) Brutschy, B. J. Phys. Chem. 1990, 94, 8637. (33) Eggert, J.; Janes, C.; Wassermann, B.; Brutschy, B.; Baumgartel, H. Ber. Bunsen-Ges. Phys. Chem. 1990, 94, 1282. (34) Maeyama, T.; Mikami, N. J. Phys. Chem. 1990, 94, 6973. (35) Brutschy, B.; Eggert, J.; Janes, C.; Baumgaertel, H. J. Phys. Chem. 1991, 95, 5041. (36) Maeyama, T.; Mikami, N. J. Phys. Chem. 1991, 95, 7197. (37) Martrenchard, S.; Jouvet, C.; Lardeux-Dedonder, C.; Solgadi, D. J. Phys. Chem. 1991, 95, 9186. (38) Tho¨elmann, D.; Gru¨tzmacher, H. F. J. Am. Chem. Soc. 1991, 113, 3281. (39) Brutschy, B. Chem. ReV. 1992, 92, 1567. (40) Tho¨lmann, D.; Gru¨tzmacher, H.-F. Int. J. Mass Spectrom. Ion Processes 1992, 117, 415. (41) Dedonder-Lardeux, C.; Dimicoli, I.; Jouvet, C.; Martrenchard-Barra, S.; Richard-Viard, M.; Solgadi, D.; Vervloet, M. Chem. Phys. Lett. 1995, 240, 97. (42) Martrenchard-Barra, S.; Dedonder-Lardeux, C.; Jouvet, C.; Rockland, U.; Solgadi, D. J. Phys. Chem. 1995, 99, 13716. (43) Dedonder-Lardeux, C.; Jouvet, C.; Martrenchard-Barra, S.; Solgadi, D.; Talbot, F.; Vervloet, M.; Dimicoli, I.; Richard-Viard, M. Chem. Phys. 1996, 212, 371. (44) Fornarini, S. Mass Spectrom. ReV. 1996, 15, 365. (45) Djafari, S.; Barth, H.-D.; Buchhold, K.; Brutschy, B. J. Chem. Phys. 1997, 107, 10573. (46) Gunther, P. A.; Oppel, M.; Baumgartel, H. Z. Phys. Chem. 2000, 214, 1569. (47) Gru¨tzmacher, H. F. Curr. Org. Chem. 2003, 7, 1565. (48) Hasegawa, H.; Mizuse, K.; Hachiya, M.; Matsuda, Y.; Mikami, N.; Fujii, A. Angew. Chem., Int. Ed. 2008, 47, 6008. (49) Solca`, N.; Dopfer, O. Angew. Chem., Int. Ed. 2002, 41, 3628. (50) Jones, W.; Boissel, P.; Chiavarino, B.; Crestoni, M. E.; Fornarini, S.; Lemaire, J.; Maıˆtre, P. Angew. Chem., Int. Ed. 2003, 42, 2057. (51) Solca`, N.; Dopfer, O. J. Am. Chem. Soc. 2003, 125, 1421. (52) Solca`, N.; Dopfer, O. Angew. Chem., Int. Ed. 2003, 42, 1537. (53) Dopfer, O.; Solca`, N.; Lemaire, J.; Maıˆtre, P.; Crestoni, M.-E.; Fornarini, S. J. Phys. Chem. A 2005, 109, 7881. (54) Dopfer, O. J. Phys. Org. Chem. 2006, 19, 540. (55) Dopfer, O.; Lemaire, J.; Maıˆtre, P.; Chiavarino, B.; Crestoni, M. E.; Fornarini, S. Int. J. Mass Spectrom. 2006, 249-250, 149. (56) Lorenz, Ulrich, J.; Solca`, N.; Lemaire, J.; Maıˆtre, P.; Dopfer, O. Angew. Chem., Int. Ed. 2007, 46, 6714. (57) Douberly, G. E.; Ricks, A. M.; Schleyer, P. v. R.; Duncan, M. A. J. Phys. Chem. A 2008, 112, 4869. (58) Chiavarino, B.; Crestoni, Maria, E.; Fornarini, S.; Lanucara, F.; Lemaire, J.; Maıˆtre, P. Angew. Chem., Int. Ed. 2007, 46, 1995. (59) Chiavarino, B.; Crestoni, Maria, E.; Fornarini, S.; Lanucara, F.; Lemaire, J.; Maıˆtre, P.; Scuderi, D. Chem.sEur. J. 2009, 15, 8185. (60) Chiang, C.-T.; Freindorf, M.; Furlani, T.; DeLeon, R. L.; Richard, J. P.; Garvey, J. F. J. Phys. Chem. A 2007, 111, 6068. (61) Frisch, M. J., Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. J. A.; 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.; 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 E.01; Gaussian, Inc.: Wallingford, CT, 2004. (62) Flukiger, P.; Luthi, H. P.; Portmann, S.; Weber, J. MOLEKEL 4.3; Swiss Center for Scientific Computing: Manno, Switzerland, 2000-2002. (63) Åsbrink, L.; Lindholm, E.; Edqvist, O. Chem. Phys. Lett. 1970, 5, 609. (64) Karlsson, L.; Mattsson, L.; Jadrny, R.; Bergmark, T.; Siegbahn, K. Phys. Scr. 1976, 14, 230. (65) Walter, K.; Weinkauf, R.; Boesl, U.; Schlag, E. W. Chem. Phys. Lett. 1989, 155, 8. (66) Herzberg, G., Molecular Spectra and Molecular Structure II. Infrared and Raman Spectra of Polyatomic Molecules; Krieger: Florida, 1945.

Infrared and Electronic Spectroscopy of [C6H6(NH3)1,2]+ (67) Yeh, L. I.; Okumura, M.; Myers, J. D.; Price, J. M.; Lee, Y. T. J. Chem. Phys. 1989, 91, 7319. (68) Shaik, S. S.; Pross, A. J. Am. Chem. Soc. 1989, 111, 4306. (69) Shaik, S.; Shurki, A. Angew. Chem., Int. Ed. 1999, 38, 586. (70) NIST Chemistry WebBook; National Institute of Standards and Technology: Gaithersburg, MD, 2008. (71) El-Shall, M. S.; Ibrahim, Y. M.; Alsharaeh, E. H.; Meot-Ner, M.; Watson, S. P. J. Am. Chem. Soc. 2009, 131, 10066. (72) Courty, A.; Mons, M.; Le Calve, J.; Piuzzi, F.; Dimicoli, I. J. Phys. Chem. A 1997, 101, 1445. (73) Ibrahim, Y.; Alsharaeh, E.; Dias, K.; Meot-Ner, M.; El-Shall, M. S. J. Am. Chem. Soc. 2004, 126, 12766. (74) Ibrahim, Y. M.; Meot-Ner, M.; Alshraeh, E. H.; El-Shall, M. S.; Scheiner, S. J. Am. Chem. Soc. 2005, 127, 7053. (75) Shimizu, M.; Yamashita, E.; Mitani, M.; Yoshioka, Y. Chem. Phys. Lett. 2006, 432, 22. (76) Park, J. K. J. Phys. Chem. A 2000, 104, 5093.

J. Phys. Chem. A, Vol. 114, No. 42, 2010 11069 (77) Hachiya, M.; Matsuda, Y.; Suhara, K.-i.; Mikami, N.; Fujii, A. J. Chem. Phys. 2008, 129, 094306. (78) Mare´chal, Y. The Hydrogen Bond and the Water Molecule; Elsevier: Amsterdam, The Netherlands, 2007. (79) Price, J. M.; Crofton, M. W.; Lee, Y. T. J. Chem. Phys. 1989, 91, 2749. (80) Price, J. M.; Crofton, M. W.; Lee, Y. T. J. Phys. Chem. 1991, 95, 2182. (81) Wang, Y. S.; Chang, H. C.; Jiang, J. C.; Lin, S. H.; Lee, Y. T. J. Am. Chem. Soc. 1998, 120, 8777. (82) Pankewitz, T.; Lagutschenkov, A.; Niedner-Schatteburg, G.; Xantheas, S. S.; Lee, Y. T. J. Chem. Phys. 2007, 126, 074307. (83) Sheppard, N.; Woodman, C. M. Proc. R. Soc. London, Ser. A 1969, 313, 149.

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