Structures and Dissociation Channels of Protonated Mixed Clusters

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Structures and Dissociation Channels of Protonated Mixed Clusters around a Small Magic Number: Infrared Spectroscopy of ((CH3)3N)n− H+−H2O (n = 1−3) Ryunosuke Shishido,† Jer-Lai Kuo,‡ and Asuka Fujii*,† †

Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, 10617, Taiwan

‡

S Supporting Information *

ABSTRACT: The magic number behavior of ((CH3)3N)n− H+−H2O clusters at n = 3 is investigated by applying infrared spectroscopy to the clusters of n = 1−3. Structures of these clusters are determined in conjunction with density functional theory calculations. Dissociation channels upon infrared excitation are also measured, and their correlation with the cluster structures is examined. It is demonstrated that the magic number cluster has a closed-shell structure, in which the water moiety is surrounded by three (CH3)3N molecules. The ion core (protonated site) of the clusters is found to be (CH3)3NH+ for n = 1−3, but coexistence of an isomer of the H3O+ ion core cannot be ruled out for n = 3. Large rearrangement of the cluster structures of n = 2 and 3 before dissociation, which has been suggested in the mass spectrometric studies, is confirmed on the basis of the structure determination by infrared spectroscopy.

I. INTRODUCTION Structures of protonated clusters in the gas phase have been extensively studied to understand microscopic solvation structures of the proton and their dynamical behavior such as proton transfer and rearrangement of solvent molecules.1,2 When a specific size of clusters has unusually high stability, this size is called the “magic number”. Magic number clusters have been found in mass spectrometric studies including thermodynamical measurements, and closed solvation shell models have been often proposed to explain their stabilities.1,2 Since the pioneering study by Lee and co-workers,3 infrared (IR) spectroscopy has been applied to cluster ions, and experimental spectroscopy combined with quantum chemical calculations gives us detailed and direct information on cluster structures, which is complementary to mass spectrometry.4−12 For example, the famous magic number cluster of protonated water, H+(H2O)21, was first reported in the mass spectrometric studies, and closed cage structures were suggested.13−16 Afterward, such a cage structure was proved by IR spectroscopy of the dangling OH stretch region.17−19 Magic number behavior of molecular clusters can be seen even in very small sizes. It has been reported by many groups that Xn−H+−H2O shows clear magic number behavior at n = 3 when X is a one-coordinated (single acceptor) species in a hydrogen-bond network, such as trimethylamine (TMA) and acetone.20−25 A one-coordinated species usually locates at terminal sites in a hydrogen-bond network. Therefore, a closedshell structure, in which the water molecule is located at the center with the excess proton, can be formed at n = 3, as © 2012 American Chemical Society

schematically shown in Figure 1a. While many mass spectrometric studies have been accumulated so far, no

Figure 1. Schematic structures of the X3−H+−H2O magic number clusters suggested by mass spectrometric studies (refs 12−20). (a) Closed-shell-type structure. (b) Charge (induced) dipole-type structure. Note that two locations are possible for the excess proton in the closed-shell-type structure (see the text).

spectroscopic evidence for this very simple closed-shell structure has been reported.20−25 Moreover, though this closed-shell structure suggests that the ion core (protonated site) is H3O+ at a glance,22−25 we should note that even if the excess proton moves to the X (TMA or acetone) moiety, the Received: March 19, 2012 Revised: May 6, 2012 Published: May 25, 2012 6740

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processes are discussed on the basis of correlations between cluster structures and dissociation channels.

same frame of the hydrogen-bond network can be held. Therefore, the precise form of the ion core cannot be distinguished by the magic number behavior. (Hereafter, irrespective to the proton location, all structures in which the central water is surrounded by three X molecules are categorized into “closed-shell structures”, though structures with the XH+ ion core are not closed-shell structures in the strict sense). Because the gas-phase proton affinity of water (167 kcal/mol) is smaller than that of TMA (225 kcal/mol) and acetone (197 kcal/mol),26 the proton location in these simple magic number clusters is an interesting problem to understand effects of solvation structures on a preferential site in protonation. Structures and ion cores in Xn−H+−H2O clusters have been studied by measurements of their metastable and collisioninduced dissociation (CID) channels.22−25 Wei et al. have found that in (TMA)n−H+−H2O and (acetone)n−H+−H2O, the metastable dissociation channel is the H2O loss in n ≤ 3, while it switches to the TMA or acetone loss in n ≥ 4 (both the channels are open only in (TMA)3−H+−H2O).23,24 For (acetone)n−H+−H2O, they have explained the H2O loss in n = 1 and 2 by the acetone ion core, and the water molecule in n = 2 has been considered to be loosely bound to the protonated acetone dimer core by the charge (induced) dipole interaction. The switch of the dissociation channel in n ≥ 4 well-supports closed-shell structures, since acetone or TMA should form the weakly bound second solvation shell once the first shell is closed at n = 3. The H2O loss in n = 3 is rather surprising and it seems to conflict with closed-shell structures, because the water moiety locates at the center of the hydrogen-bond network. Wei et al. have also measured the CID channel of (acetone)3− H+−H2O and have found a rise of the acetone loss channel with increase of the collision gas pressure.23 Therefore, they have attributed the acetone loss at high pressure to direct bond cleavage at closed-shell structures. For the metastable decay leading to the H2O loss, they have proposed large rearrangement from a closed-shell structure (Figure 1a) to a charge− dipole structure (see Figure 1b) prior to the dissociation. For protonated (TMA)n−H+−NH3 and (pyridine)n−H+−NH3, Wei et al. have obtained a similar conclusion.27 Very recently, Chiang et al. have studied metastable and collision-induced dissociation channels of a similar system, (acetone)n−H+− CH3OH, and their results also support the scenario by Wei et al.28 Chiang et al. have performed density functional theory (DFT) calculations, and they have shown that the loss of the central methanol moiety in the hydrogen-bond network is an energetically possible and favored process in the dissociation. Though the authors of the mass spectrometric studies have proposed a systematic scenario of the first solvation shell formation in X3−H+−H2O, spectroscopic studies of such clusters would be able to give us more detailed and firm information on the cluster structure and the preferential protonated location. Then, in this study, we apply size-selective IR predissociation spectroscopy to (TMA)n−H+−H2O (n = 1− 3). For comparison, IR spectra of H+(TMA)n (n = 2, 3) are also measured. We observe IR spectra in the OH and CH stretching vibrational region. Dissociation channels upon IR vibrational excitation are also measured. Possible cluster structures are examined by DFT calculations with different exchange and correlation functionals. Cluster structures and preferential proton locations are determined by comparisons between observed and simulated spectra. Vibrational predissociation

II. EXPERIMENTAL METHODS IR spectra of (TMA)n−H+−H2O (n = 1−3) and H+(TMA)n (n = 2, 3) were recorded by IR predissociation spectroscopy using a mass spectrometer which is equipped with linearly aligned tandem quadrupole mass filters connected by an octopole ion guide. Details of the experimental apparatus have been described elsewhere29 and only brief description is given here. Protonated mixed clusters were produced by pulsed discharge of the TMA/H2O mixed vapor seeded in the Ar buffer gas (total pressure of 5 atm). The gaseous mixture was expanded from a pulsed supersonic jet valve into a channel nozzle that equips a pin electrode in the channel. Pulsed voltage of −400 V relative to the channel was applied to the electrode. The pulse width of the voltage was set to be 40 μs. The application of the high voltage pulse was synchronized with the pulsed valve operation. Ionization by the discharge and successive proton transfer generated protonated species. Protonated mixed clusters were produced and cooled by the jet expansion through the channel nozzle. Intensities of unprotonated ionic species were almost negligible under the present source condition. Clusters of interest were selected by the first quadrupole mass filter and they were introduced into the octopole ion guide. The mass resolution of the first mass filter was set to be high enough (Δm/z < 1) to exclude the contamination of other cluster species. Within the octopole ion guide, the size-selected clusters were irradiated by the counterpropagating IR laser light and were transferred into the second quadrupole mass filter, which was tuned to pass only fragment ions of certain mass. Thus, an IR spectrum of the size-selected cluster was recorded by monitoring the fragment ion intensity while the IR laser frequency was scanned. The coherent IR light was generated by an IR optical parametric oscillator (Laser Vision) pumped by the fundamental output of a YAG laser (Continuum Powerlite 8000). Because of the limitation of the output power in the low-frequency region, reliable measurements were restricted above 2500 cm−1 in this experiment. All the observed spectra were normalized with the IR power30 and were calibrated to the vacuum wavenumber by simultaneous observations of atmospheric water absorption lines. Mass spectra of fragments produced by the IR vibrational excitation were also measured by scanning the second mass spectrometer. We eliminated the contribution of the metastable decay of hot clusters without the IR excitation by subtraction between the signals with and without the IR excitation. Stable cluster structures and their vibrational spectra were calculated by the Gaussian 09 program suite.31 The ωB97X-D, M06-2X, and B3LYP functionals with the 6-311+G(2d,p) basis set were used to optimize cluster structures.32−34 In the energy evaluation, the role of the dispersion interaction would be important in the present system because of the large molecular volume of TMA. Results of the dispersion-corrected functionals, ωB97X-D and M06-2X, are mainly referred to in the discussion on the energetics. Observed vibrational spectra are also compared with simulations at the ωB97X-D/6-311+G(2d,p) level. Simulations by M06-2X and B3LYP are shown in the Supporting Information. We confirmed that no significant difference is seen among the spectral simulations by these three functionals. The harmonic approximation was employed to calculate vibrational frequencies, and the scaling factor of 0.9394 was applied to the calculated frequencies at the ωB97X6741

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IR spectra, we calculated several low-energy optimized structures of each cluster and simulated their IR spectra. The optimized cluster structures and the comparison between the observed and simulated spectra are compiled in Figures 3−5, 7, and 8. The relative energies of the optimized structures are summarized in Table 1.

D/6-311+G(2d,p) level. This scaling factor was determined to reproduce the free OH stretch frequency.35 The limit of the harmonic approximation will be discussed later. Stick spectra from DFT calculations were convoluted with a Lorenzian function of 10 cm−1 full width at half-maximum to be compared with the observed spectra. Optimized structures were visualized by the MOLEKEL program.36

III. RESULTS AND DISCUSSION 1. IR Spectra and Structures of (TMA)n−H+−H2O. Figure 2b−e shows observed IR spectra of (TMA)n−H+−H2O

Figure 3. Comparison among (a) the observed spectrum of H+(TMA)2 and (b−d) its simulated spectra based on the optimized stable structures. All the calculations were performed at ωB97X-D/6311+G(2d,p). The calculated harmonic frequencies were scaled by the factor of 0.9394.

The observed IR spectrum of H+(TMA)2 (Figures 2a and 3a) shows a somewhat broadened and asymmetric band at 3000 cm−1, which is attributed to CH stretching vibrations. It should be noted that no band that can be assigned to the NH stretching vibration (the excess proton motion) is found in the

Figure 2. Infrared spectra of (a) H+(TMA)2, (b) (TMA)1−H+−H2O, (c) (TMA) 2 −H + −H 2 O, (d, e) (TMA) 3 −H + −H 2 O, and (f) H+(TMA)3. The TMA loss channel was monitored in spectra a, e, and f. The H2O loss channel was monitored in spectra b−d.

(n = 1−3). The spectra of n = 1 and 2 clusters were measured by monitoring the H2O loss channel. In n = 3 clusters, as discussed later, both the H2O and TMA loss channels were found, and the spectrum was measured by monitoring each channel (parts d and e of Figure 2, respectively). For comparison, observed IR spectra of H+(TMA)n (n = 2 and 3) are also shown in parts a and f of Figure 2, respectively. Both of them were observed by monitoring the TMA loss channel. To determine the cluster structures on the basis of the observed

Figure 4. Comparison between (a) the observed spectrum of (TMA)1−H+−H2O and (b) its simulated spectrum based on the optimized stable structure. All the calculations were performed at ωB97X-D/6-311+G(2d,p). The calculated harmonic frequencies were scaled by the factor of 0.9394. 6742

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Figure 5. Comparison among (a) the observed spectrum of (TMA)2−H+−H2O and (b−e) its simulated spectra based on the optimized stable structures. All the calculations were performed at ωB97X-D/6-311+G(2d,p). The calculated harmonic frequencies were scaled by the factor of 0.9394.

Then, the OH stretching vibrations in the water moiety are similar to those of a free water molecule, and the symmetric and antisymmetric stretch bands are calculated at 3637 and 3723 cm−1, respectively, as shown in Figure 4b. These band frequencies well agree with those in the observed spectrum. The antisymmetric stretch band in the observed spectrum seems to be broadened in comparison with the symmetric stretch band. Similar broadening (or splitting) of the antisymmetric stretch band in a one-coordinated (single proton acceptor) water (or ammonia) has been reported, and it has been attributed to internal rotation and coupling with torsional motion.37−40 The intense band at 3100 cm−1 in the observed spectrum is qualitatively reproduced in the simulated spectrum. This band is due to the hydrogen-bonded NH stretch. The NH stretch band at 3100 cm−1 is clear evidence for the hydrogenbonded structure of the cluster. The NH frequency is remarkably high-frequency shifted in comparison with that in H+(TMA)2 and it clearly reflects the smaller proton affinity of water than that of TMA. The N−H bond length in 1I (104 pm) is calculated to be much shorter than that in D1 (110 pm). This is consistent with the large difference of the NH frequency. Another intense band at 2820 cm−1 in the observed spectrum is missing in the simulation. A plausible assignment of this band is a symmetric CH stretch band, which is generally

observed region. The hydrogen-bonded structure (isomer DI in Figure 3b), in which the excess proton is shared by two TMA molecules, is the minimum energy structure in all the calculation levels. The simulated spectrum of isomer DI reproduces the observed spectrum. The observed band at ∼3000 cm−1 includes a contribution from multiple CH stretches of the methyl moieties in TMA. The NH stretch band is calculated to be 1984 cm−1 with the harmonic approximation. The charge−dipole structures (isomers DII and DIII) have much higher energies at all the calculation levels, as shown in Table 1. For these structures, the free NH stretch band is expected at ∼3290 cm−1, and the missing of such a band in the observed spectrum clearly excludes the contribution of the charge−dipole structures. The unique optimized structure (1I) of (TMA)1−H+−H2O is shown in Figure 4b. In this structure, the excess proton is located on the TMA moiety, and the resulting NH group is hydrogen-bonded to the oxygen atom of the water moiety. When the optimization is started with the excess proton on the water moiety, the proton is transferred to the TMA moiety during the optimization, and no stable structure is found with the H3O+ ion core. It can be simply interpreted in terms of the larger proton affinity of TMA than water.26 The two OH groups in the water moiety are free from hydrogen bonds. 6743

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Table 1. Relative Energies of Isomers of (TMA)n−H+−H2O (n = 1−3) and H+(TMA)n (n = 2 and 3)a species TMA−H+− H2O (TMA)2− H+−H2O

(TMA)3− H+−H2O

H+(TMA)2

H+(TMA)3

B3LYP/6311+G(2d,p)

M06-2X/6311+G(2d,p)

ωB97X-D/6311+G(2d,p)

1I

0.0

0.0

0.0

2I

0.0

0.0

0.0

2II 2III 2IV 3I

25.6 18.9 39.7 −

5.6 7.1 31.2 0.0

0.3 1.0 24.4 0.0

3II 3III DI DII DIII TI TII TIII

0.0 40.4 0.0 50.5 73.7 0.0 6.8 57.9

5.6 26.1 0.0 51.0 88.5 0.0 0.3 −

2.0 21.0 0.0 52.8 89.2 0.0 0.3 70.9

isomers

seen in this frequency region of compounds including a methyl group.41 An alternative assignment is an overtone band of the NH bending vibration. In both the cases, this band would borrow the intensity by the anharmonic coupling with the intense NH stretch band, and this would be the reason for its missing in the harmonic simulation. In addition, flat absorption is actually seen in between the intense two bands at 2820 and 3100 cm−1, and it would be also attributed to CH stretches. The vibration of a shared proton can be strongly anharmonic42 and multidimensional calculations are requested to fully analyze observed spectra.43 Multidimensional anharmonic calculations of the present system are in progress, but further analyses of the spectrum is beyond the focus of the present paper. For (TMA)2−H+−H2O, two types of stable structures were found in our DFT calculations. The most stable one is a hydrogen-bonded chain-type structure (isomer 2I shown in Figure 5b), in which the water molecule locates in between two TMA molecules. The excess proton is localized on the TMA moiety (the H+−N bond length is 106 pm while the H+−O distance is 160 pm). No structure with the H3O+ ion core was found to be stable at any of the present computational levels. Three isomers of charge−dipole type, in which the hydrogenbonded TMA−H+−TMA or TMA−H+−H2O moiety is bound to the third molecule by the charge−dipole interaction, are found. Two of them (2II and 2III) have the TMA−H+−TMA

a

All units are kJ/mol. In each species, the energy of the minimum energy isomer is set to zero. The zero point energy correction is included for all the levels of theory. A dash indictates that the value was not found.

Figure 6. Comparison between the spectral simulations of (TMA)2−H+−H2O based on (b, d, f) harmonic approximation and (c, e, g) onedimensional potential scan. (b, c) ωB97X-D, (d, e) M06-2X, and (f, g) B3LYP functionals are employed for the calculations, respectively. The 6311+G(2d, p) basis set was used in all the calculations. Only the OH and NH stretch bands are calculated and the CH stretch bands are neglected in the simulations. Spectrum a is the observed spectrum of (TMA)2−H+−H2O. 6744

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isomers 2II and 2III would be also ∼2000 cm−1 or lower, since the anharmonicty of this proton vibration would be as large as that in isomer 2I. It is, therefore, most probable that the observed rise of the absorption at 2800 cm−1 is attributed to the hydrogen-bonded OH stretch of isomer 2I, though the 1-D calculations still overestimate the frequency. The highfrequency tail of the broadened NH stretch in 2I, 2II, and 2III may partially contribute to the rise, but this contribution is expected to be minor. This is supported by the complete missing of the NH stretch band in the observed spectrum of H+(TMA)2, of which the calculated harmonic NH frequency is close to those of 2I, 2II, and 2III. As seen in Figure 5b, isomer 2I shows only a single free OH stretch band, which corresponds to the 3680 cm−1 band in the observed spectrum. The observed spectrum, however, shows two free OH bands, and it means that another isomer should contribute to the observed spectrum. The other free OH band at 3640 cm−1 in the observed spectrum indicates the contribution of other isomer(s). This band is attributed to the symmetric stretch of isomer(s) 2II and/or 2III. This coexistence of the charge−dipole-type isomer(s) is consistent with their small energy difference from isomer 2I, as seen in Table 1. The frequency of the antisymmetric band in isomer 2II (2III) is close to that of the free OH band of isomer 2I, and they would overlap with each other in the observed spectrum. The 3640 cm−1 band comes only from 2II (2III). Therefore, by using the calculated intensity ratio between the symmetric and antisymmetric stretch bands in the charge−dipole isomers (we actually used the averaged value between those of 2II and 2III) and the observed intensity ratio, we estimated the relative contribution ratio between 2I and 2II (2III) to the 3680 cm−1 band intensity. This ratio was divided by the ratio between the calculated OH stretch band intensities of 2I and 2II (2III), and we finally evaluated the relative population of 2II (and/or 2III) to 2I to be ∼0.8. This evaluation should be qualitative because of the uncertainty of the observed signal intensities and the errors in the simulations. It is, however, shown that both of the hydrogen-bonded chain-type and charge−dipole-type isomers of (TMA)2−H+−H2O coexist in the same order of magnitude under the present experimental condition. Stable optimized structures of (TMA)3−H+−H2O are shown in Figure 7. The most stable isomer, isomer 3I, has a closedshell structure. The water moiety is protonated to form the H3O+ ion core (the H+−O bond length is 107 pm as an average of the three bonds, while the H+−N distance is 149 pm on average) and is located in the center of the hydrogen-bond network. All the molecules are bound by the hydrogen bonds. This structure is obtained with the ωB97X-D and M06-2X functionals, but is not available with the B3LYP functional. Another closed-shell structure, isomer 3II, in which the excess proton localizes on the TMA moiety (the H+−N bond length is 108 pm and the H+−O distance is 150 pm), is found for all the calculation levels. The energy of 3II is a little higher than 3I at ωB97X-D, but the energy difference is much larger at M06-2X. Isomer 3II is the most stable structure at B3LYP because of the missing 3I. A charge−dipole-type isomer, 3III, is also a stable structure with all the functionals. This isomer has the TMA− H+−TMA moiety and two other molecules (TMA and water) are bound to this moiety mainly by the charge−dipole interaction. The highest frequency OH stretch band of isomer 3I is predicted at 2140 cm−1 and the bands in the 2800 −3000 cm−1 regions are CH stretches (Figure 7c). No strong mode coupling exists between the CH and OH stretches. On the

moiety, and one isomer has the TMA−H+−H2O moiety (2IV). We should note that in this type of isomer, the water molecule is free from hydrogen bonds. In the previous work on (acetone)2−H+−H2O and other similar systems, Wei et al. have estimated that the charge-induced dipole-type structure is more stable than the hydrogen-bonded chain-type structure on the basis of the preferential fragmentation of the water moiety.23,27 In the present calculations, we evaluated the relative energies of the isomers by DFT calculations. As seen in Table 1, hydrogen-bonded-type isomer 2I is most stable. In the charge−dipole types, the dispersion interaction is also important, since two bulky TMA molecules are in close contact with each other. B3LYP calculations do not include the dispersion,44 and the relative binding energies in isomers 2II− 2IV should be largely underestimated by this functional. The ωB97X-D and M06-2X functionals are dispersion-corrected,32,33 and their results are similar to each other. With these functionals, the relative energies of isomers 2II−2IV are evaluated to be much closer to that of isomer 2I. Isomer 2II and 2III are slightly higher in energy only by 0.3 and 1.0 kJ/ mol, respectively, than isomer 2I at the ωB97X-D/6-311+G(2d,p) level of theory. The relative energy of isomer 2IV is much higher. The observed IR spectrum of (TMA)2−H+−H2O is shown in Figures 2c and 5a. Two bands at 3640 and 3680 cm−1 are assigned to free OH stretching modes. A CH stretch band appears at 3000 cm−1. This band is similar to that in the spectrum of H+(TMA)2, but it is very different from those in this region of (TMA)1−H+−H2O, as seen in Figure 2. This means that the strong coupling among the NH stretch and other modes in (TMA)1−H+−H2O is lifted in (TMA)2−H+− H2O. Isomer 2IV, which has the TMA−H+−H2O moiety, shows the NH stretch band at 3074 cm−1, and this band is quite similar to that observed in (TMA)1−H+−H2O. Such a band is missing in the observed spectrum of (TMA)2−H+−H2O, and contribution of isomer 2IV is clearly ruled out. Below 2800 cm−1, a rise of strong absorption is seen in the observed spectrum. This is attributed to a hydrogen-bonded XH stretch band. Around 2800 cm−1, isomer 2I, the hydrogen-bonded chain-type isomer, has two hydrogen-bonded XH stretches. One is the excess proton vibration (NH stretch) at 2376 cm−1 and the other is the OH stretch in the water moiety at 2694 cm−1, as shown in Figure 5b. The NH stretch frequency in isomer 2I is significantly low-frequency shifted in comparison with that of (TMA)1−H+−H2O (3100 cm−1) because the OH···N hydrogen bond cooperatively enhances the strength of the NH···O hydrogen bond, and it removes the strong coupling between the NH and CH stretches in (TMA)1−H+−H2O. On the other hand, the N−H stretch frequency in the charge− dipole isomers, 2II and 2III, is calculated at 2228 and 2150 cm−1, respectively. All these frequencies are obtained under the harmonic approximation, and their actual frequencies should be lower because of the anharmonicity. To evaluate the effect of the anharmonicity, we performed effective one-dimensional (1D) calculations to correct the peak positions and intensities of the NH and OH stretches in isomer 2I.45 The results are summarized in Figure 6. We performed the calculations with the three functionals, and all the functionals show the same trend. The NH stretch band is largely low-frequency shifted with the 1-D scan and is expected to appear at ∼2000 cm−1. The 1-D scan of the hydrogen-bonded OH results in the quite similar frequencies to the scaled harmonic frequencies (∼2600 cm−1). These results suggest the actual NH stretch frequency of 6745

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features overlap with this broadened band. Since similar features are seen in H+(TMA)2 and H+(TMA)3, as shown in Figure 2, the sharp features are attributed to CH stretch vibrations. On the other hand, the broadened feature is assigned to hydrogen-bonded OH stretches in the water moiety. The simulated spectrum of isomer 3II is consistent with the observed features, as demonstrated in Figure 7d, while the simulation of 3I cannot reproduce the broadened feature since only the CH stretch bands are predicted in this region. In addition, it should be noted that the center frequency of the observed OH stretch band, 2900 cm−1, is higher than that of H+(H2O)4 (2700 cm−1),46 in which the H3O+ ion core is solvated by three water molecules. If the H3O+ ion core is actually formed as in isomer 3I, its hydrogen-bonded OH frequencies should be lower than that in H+(H2O)4 because the proton affinity of TMA is much larger than water. This qualitative discussion is consistent with the simulations of 3I and 3II. Therefore, the observed hydrogen-bonded OH band demonstrates the contribution of isomer 3II, which has the protonated TMA ion core. Though the observed spectra cannot prove the presence of the minimum energy isomer 3I of the H3O+ ion core, the coexistence of the isomers is considered to be probable based on the theoretical calculations. An IR spectrum of H+(TMA)3 was also observed for comparison, as shown in Figures 2f and 8a. Rich features are seen in the CH stretch region, and they are qualitatively

Figure 7. Comparison among (a, b) observed spectra of (TMA)3− H+−H2O and (c−e) its simulated spectra based on the optimized stable structures. Spectrum a was obtained by monitoring the H2O loss channel, while spectrum b was obtained by monitoring the TMA loss channel. Simulated spectrum (c−e) were calculated at ωB97X-D/6311+G(2d,p). The calculated harmonic frequencies were scaled by the factor of 0.9394.

other hand, the OH stretch bands of 3II are predicted in the 2800−3000 cm−1 region. Two OH stretch modes are strongly coupled with CH stretch modes and this coupling results in three strong bands and many weak bands, as shown in Figure 7d. The three strong bands include more components of the OH stretches, while the remaining weak bands are due to CH stretches, of which coupling with the OH stretches is much less. No free OH group exists in these two isomers, and missing of bands in the free OH stretch region is a spectral signature of the closed-shell structures. The NH stretch in 3II is predicted at 2303 cm−1. On the other hand, two free OH stretch bands in 3III are predicted at 3636 and 3724 cm−1. In the observed spectra of (TMA)3−H+−H2O, no band is seen in the free OH (and NH) stretch region. This means that all the OH (and NH) groups are hydrogen-bonded at this size. This is clear evidence of the closed-shell isomers 3I and/or 3II, which has been suggested by the magic number behavior in mass spectrometry.20−28 The coexistence of isomer 3III is clearly excluded. The broadened absorption centered at 2900 cm−1 appears in the observed spectra, and some relatively sharp

Figure 8. Comparison among (a) observed spectrum of H+(TMA)3 and (b−d) its simulated spectra based on the optimized stable structures. All the calculations were performed at ωB97X-D/6311+G(2d,p). The calculated harmonic frequencies were scaled by the factor of 0.9394. 6746

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(TMA)2−H+−H2O have only the H2O loss channel, three dissociation channels (H2O, TMA, and TMA−H2O loss channels) were observed in (TMA)3−H+−H2O. (Both the detection of the H2O loss and TMA loss channels yields essentially the same IR spectra, as seen in parts d and e of Figure 2, respectively). These open dissociation channels agree with the previous mass spectrometric studies on the metastable decay by Wei et al.23,24 In the previous mass spectrometric studies, since the H2O loss occurs exclusively in the metastable decay of the similar system, (acetone)2−H+−H2O, the charge-induced dipole structures have been assumed to be more stable than the hydrogen-bonded chain structure.23−25,27 In the present IR study of (TMA)2−H+−H2O, it was shown that both the structural isomers coexist. In addition, the DFT calculations showed that the hydrogen-bonded structure is the minimum energy isomer. The IR photon being resonant with the CH stretch (2980 cm−1) or OH stretch band (3680 cm−1) should excite both the isomers. Though the hydrogen-bonded chain isomer (isomer 2I), in which the water moiety is located at the center of the chain, is excited, only the H2O loss occurs and no TMA loss is detected. This means large rearrangement of the cluster structure does occur before the dissociation. In (TMA)3−H+−H2O, only the closed-shell isomer(s) (isomer 3II and possibly 3I) exists, and existence of the charge−dipole type (isomer 3III) is clearly ruled out, as demonstrated by IR spectroscopy. In this closed-shell cluster(s), the loss of the central water competes with that of the terminal TMA loss. This is firm evidence for the large rearrangement in the dissociation process, which has been suggested by the previous mass spectrometric studies.23−25,27 No time-dependent information is available in this study, and intermediate processes in the dissociation are unknown at present. The previous mass spectrometric studies have suggested a conversion from a closed-shell structure to a charge−dipole structure prior to the dissociation.23,27 Chiang et al. have calculated dissociation energies in a similar system, (acetone)2−H+−CH3OH, which also has a hydrogen-bonded closed-shell structure and the central methanol evaporates in the metastable decay, and they have shown that the fragmentation of the methanol is the energetically favored dissociation path.28 We also calculated the dissociation energies of (TMA)2−H+−H2O (isomer 2I and 2II) and (TMA)3−H+− H2O (isomer 3I and 3II) at the ωB97X-D/6-311+G(2d,p) and M06-2X/6-311+G(2d,p) levels. Both the DFT methods give quite similar trends and the results are summarized in Table 2. Both in 2I and 2II, the dissociation energy of the H2O loss channel is the minimum, and the TMA loss channel requires at least two times larger energy. In the case of 3I and 3II, though the H2O loss is the minimum energy dissociation channel, the dissociation energy of the TMA loss is much closer. These dissociation energies are consistent with the observed fragmentation channels; that is, the H2O loss is the unique channel in n = 2 but the TMA loss becomes competitive in n = 3. These dissociation energy evaluations demonstrate that only the low dissociation energy channels are actually open. This suggests that the barrier in the rearrangement of the cluster structure in the dissociation is lower than the dissociation energy. This is consistent with the energy evaluation of the dissociation path by Chiang et al. for (acetone)2−H+− CH3OH.28

reproduced by the simulation based on the charge-dipole structures shown in Figure 8. Isomers TI and TII have the hydrogen-bonded TMA−H+−TMA moiety, and the third TMA molecule is bound mainly by the charge−dipole interaction. The NH stretch band is predicted to be 2106 and 2250 cm−1, respectively. Isomer TIII has no hydrogen bonds, and all the molecules are bound by charge−dipole and dispersion. The NH stretch band is calculated at 3209 cm−1. The relatively strong band at 2688 cm−1 is due to the CH stretch, which faces the NH group. Though the CH stretch bands in the observed spectrum are quite similar to those in (TMA)3−H+−H2O, no broad component is seen in this region. This supports that the broad absorption seen in (TMA)3−H+−H2O is attributed to the hydrogen-bonded OH stretches (of isomer 3II). It is hard to distinguish isomers TI and TII by the spectra in the observed region, and their energies are evaluated to be very close. Therefore, both the isomers are plausible. A small hump appears at around 3220 cm−1 in the observed spectrum. Its band position well agrees with the NH stretch in isomer TIII, and the small population of isomer TIII cannot be excluded. But the relative energy of isomer TIII is quite high, and the assignment of the small hump is tentative. 2. Dissociation Channels Following Vibrational Excitation. Figure 9 shows mass spectra of dissociation fragments from (TMA)n−H+−H2O following IR vibrational excitation of the OH or CH stretch band. While (TMA)1−H+−H2O and

Figure 9. Fragment distributions upon the infrared vibrational excitation. The parent ion is (a, b) (TMA)1−H+−H2O, (c, d) (TMA)2−H+−H2O, (e) (TMA)3−H+−H2O, and (f) H+(TMA)3. The abscissa plots the loss of mass, and the origin corresponds to the mass of the parent ion. The broken lines are eye guides. 6747

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Table 2. Dissociation Energies of Isomers of (TMA)2−H+−H2O and (TMA)3−H+−H2Oa dissociation energy species

isomers

(TMA)2−H −H2O

2I

(TMA)2−H+−H2O

2II

(TMA)3−H+−H2O

3I

+

H2O H2O H2O TMA H2O H2O H2O TMA H2O TMA

(TMA)3−H+−H2O

3II

dissociation channel

M06-2X/6-311+G(2d,p)

ωB97X-D/6-311+G(2d,p)

H (TMA)2 (DI) + H2O H+(TMA)2 (DII) + H2O H+(TMA)2 (DIII) + H2O TMA−H+−H2O (1I) + TMA H+(TMA)2 (DI) + H2O H+(TMA)2 (DII) + H2O H+(TMA)2 (DIII) + H2O TMA−H+−H2O (1I) + TMA H+(TMA)3 (TI) + H2O H+(TMA)3 (TII) + H2O (TMA)2−H+−H2O (2I) + TMA (TMA)2−H+−H2O (2II) + TMA (TMA)2−H+−H2O (2III) + TMA (TMA)2−H+−H2O (2IV) + TMA H+(TMA)3 (TI) + H2O H+(TMA)3 (TII) + H2O (TMA)2−H+−H2O (2I) + TMA (TMA)2−H+−H2O (2II) + TMA (TMA)2−H+−H2O (2III) + TMA (TMA)2−H+−H2O (2IV) + TMA

34.2 85.2 122.7 72.4 28.9 79.9 117.4 67.1 55.3 55.6 55.6 61.2 62.8 86.8 50.1 50.4 50.4 56.0 57.6 81.6

29.7 82.5 118.9 68.4 29.7 82.5 118.8 68.4 47.9 48.2 57.7 58.0 58.7 82.2 42.0 42.3 51.8 52.1 52.9 76.3

fragment

H2O TMA

+

a

All units are in kJ/mol. The zero point energy and basis set superposition error corrections are included. Isomer structures are displayed in Figures 3−5, 7, and 8.

IV. SUMMARY

to the evaporation of the central moiety in the hydrogen-bond network.

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IR spectra of size-selected (TMA)n−H −H2O (n = 1−3) were measured by IR dissociation spectroscopy. The cluster structures were determined by the observed IR spectra and DFT calculations. (TMA)1−H+−H2O has the hydrogenbonded structure with the TMA ion core (isomer 1I). On the other hand, the hydrogen-bonded isomer (2I) and the charge-dipole isomer(s) (2II and/or 2III) coexist in (TMA)2− H+−H2O. The closed-shell structure of the magic number cluster, (TMA)3−H+−H2O, was clearly proved by the IR spectra. The observed IR spectra of (TMA)3−H+−H2O showed that the isomer of the protonated TMA ion core (3II) exists while the coexistence of the isomer of the H3O+ ion core (3I) is also plausible. Though these clusters are rather simple systems and their structures have been inferred by the mass spectrometric data, IR spectra reported in this work provide the first firm evidence of the structures. Moreover, detailed and reliable information on the preference of the protonated site (ion core) was also obtained. In all the clusters, the H2O loss is the unique or dominant dissociation channel following the IR vibrational excitation. Even in the n = 2 and 3 clusters, in which the water moiety locates on the center of the hydrogen-bond network, the preferential H2O loss is still observed. These results demonstrate that large rearrangement of the cluster structure occurs in the vibrational predissociation of these clusters. This rearrangement has been suggested by the previous mass spectrometric studies, but the present measurements of the structure-specific clusters provide the additional and firm evidence. The (TMA)n−H+−H2O clusters are adequate systems to study the competition between the hydrogen bond and the charge−dipole interaction. Though the present study focuses on the static structures of the clusters, we hope the present results can stimulate elucidation of the role of the competition in the rearrangement of the clusters, leading +

ASSOCIATED CONTENT

S Supporting Information *

The spectral simulations of (TMA)n−H+−H2O (n = 1−3) and H+(TMA)n (n = 2 and 3) at the M06-2X/6-311+G(2d,p) and B3LYP/6-311+G(2d,p) levels. These materials are available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS A.F. acknowledges funding support by the Grant-in-Aid for Scientific Research (Project No. 19056001 from MEXT Japan, and No. 2235001 from JSPS). J.L.K. thanks National Science Council (NSC98-2113-M-001-029-MY3) of Taiwan and NanoScience Research Project of Academia Sinica for funding support. Computational resources are in part supported by National Center for High Performance Computing in Taiwan.

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REFERENCES

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