LETTER pubs.acs.org/JPCL
Revisiting Liþ(H2O)34Ar1 Clusters: Evidence of High-Energy Conformers from Infrared Spectra Oscar Rodriguez, Jr. and James M. Lisy* Department of Chemistry, University of Illinois at UrbanaChampaign, 600 South Mathews Avenue, Urbana, Illinois 61801, United States
bS Supporting Information ABSTRACT: New infrared predissociation (IRPD) spectra of Liþ(H2O)34Ar1 clusters with previously unknown hydrogen-bonding motifs are reported. For Liþ(H2O)4Ar1, high-energy conformers were detected by monitoring the [Ar þ H2O] fragmentation channel. Hydrogen-bonded OH stretching features were observed between 3200 and 3450 cm1, while a band at 3678 cm1 was assigned to a three-coordinated, or acceptoracceptordonor (AAD), H2O free OH stretching mode. Furthermore, spectroscopic evidence of a high-energy conformer of Liþ(H2O)3Ar1 containing a hydrogen-bonded OH stretching mode was also observed by monitoring the [Ar] fragmentation channel. MP2/aug-cc-pVDZ level calculations were used to identify possible conformers and determine relative energies and simulated vibrational spectra for Liþ(H2O)34Ar1 cluster ions. SECTION: Dynamics, Clusters, Excited States
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hen the internal energy in argon-bearing cluster ions (hereafter referred to as argon-tagged) is adequately low, there can be insufficient kinetic energy to overcome energetic barriers to rearrange into a lower or global-minimum-energy conformer. As a result, the cluster ion beam can contain multiple conformers, including kinetically trapped, high-energy clusters in addition to minimum-energy clusters. Previous studies of cluster ion conformers have been performed using a two-laser IRIR double resonance technique.1,2 A recent study on tagged Hþ(H2O)6 cluster ions has revealed that the relative populations of the Zundel and Eigen forms are affected by the type of species used for tagging.3 We have discovered that by monitoring different photofragmentation channels, selective IRPD spectra of kinetically trapped, high-energy conformers can be obtained using a single laser.46 It was proposed that the amount of energy stored in high-energy conformers can be released upon infrared excitation, resulting in unique fragmentation pathways.46 While these high-energy conformers may not be the dominant species in the cluster ion beam, their infrared spectra can reveal unique hydrogen-bonding motifs. The results can be used to gain a more complete description of ion solvation, the interplay between competing noncovalent interactions, and cluster photodissociation dynamics. In a previous IRPD study, our group reported spectroscopic features from a high-energy conformer of Liþ(H2O)4Ar17 which were only observed using the argon tagging method and monitoring the loss of argon. With our recent experience obtaining different IRPD spectra by monitoring additional fragmentation channels, we chose to re-examine some of the simplest hydrated ion species, Liþ(H2O)34Ar1, to r 2011 American Chemical Society
see if other high-energy conformers could be observed in IRPD experiments. Spectra of Liþ(H2O)4Ar1 obtained by monitoring the [Ar þ H2O] and [Ar] fragmentation channels are reported in Figure 1 (spectra will be referred to as [Ar þ H2O] and [Ar] herein). The differences between the two IRPD spectra are striking. In the region corresponding to hydrogen-bonded OH stretches (below 3600 cm1), the spectra are completely different. This suggests that the conformers responsible for these features exhibit different H2O 3 3 3 H2O interactions. In addition, the [Ar þ H2O] spectrum contains a band at 3678 cm1 that is absent in the [Ar] spectrum and is in a region associated with a free OH stretch of a three-coordinated, double-proton acceptor and single-proton donor8 (AAD), H2O molecule. This is intriguing given the presence of only four H2O molecules in these clusters. Typically, the three-coordinated H2O free OH stretching feature is observed in spectra of large bulk-like water clusters,912 surfaces or interfaces,13 and ice,14,15 but has been observed before in IRPD spectra of Hþ(H2O)7,16 NH4þ(H2O)6,8 and CH3NH3þ(H2O)417 clusters. The OH vibrational bands of the [Ar] spectrum have been previously reported and assigned7 (although we rescanned the spectrum below 3450 cm1 to ensure that there were no additional vibrational features). The [Ar] spectrum is shown in Figure 2 with simulated spectra for two lowest-energy conformers, 4a and 4b, calculated at the MP2/aug-cc-pVDZ level. Received: April 19, 2011 Accepted: May 25, 2011 Published: May 25, 2011 1444
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Figure 1. IRPD spectra of Li (H2O)4Ar1 obtained by monitoring the argon loss channel [Ar] and argon and water loss channel [Ar þ H2O]. A (red) dashed vertical line at 3678 cm1 is included to emphasize the three-coordinated water free OH stretching feature in the [Ar þ H2O] spectrum, which is absent in the [Ar] spectrum.
Briefly, the most intense band in the spectrum, at 3533 cm1, corresponds to bent hydrogen-bonded stretching modes associated with the two OH groups involved as proton-donors (see structure 4b in Figure 2). Bands observed at 3625 and 3655 cm1 are assigned to the symmetric OH stretches for second- and first-shell water molecules, respectively, that are not involved as proton donors. The remaining bands are due to the free OH groups (of the first-shell hydrogen-bonded waters) and asymmetric OH stretches of the non-proton-donating waters, which overlap and are centered at 3726 cm1. Conformer 4b, although 9 kJ/mol higher in energy than the global-minimum-energy conformer, 4a, appears to be the dominant species in the cluster ion beam based on the relative peak intensities in the IRPD spectrum. The simulated spectra of conformers 4a and 4b in Figure 2 do not contain any hydrogen-bonding features below 3450 cm1 and therefore cannot explain the OH stretching features observed in the [Ar þ H2O] spectrum (see Figure 1). In order to assign the observed experimental features in the [Ar þ H2O] spectrum, it was necessary to extend our search for stable conformers that contain other hydrogen-bonding motifs. Calculations at the MP2/aug-cc-pVDZ level revealed five additional stable conformers, 4c4g, of Liþ(H2O)4Ar1, as shown in Figure 3. Conformers 4c4g all exhibit H2O 3 3 3 H2O hydrogen bonding and are ∼42 53 kJ/mol higher in energy than the globalminimum-energy conformer, 4a (see Figure 2). These structures all share a common first-shell coordination about the lithium ion
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Figure 2. IRPD spectrum of Liþ(H2O)4Ar1 monitoring the [Ar] fragmentation channel (black). The simulated spectra (red) and structures 4a and 4b are calculated at the MP2/aug-cc-pVDZ level of theory. The relative energies are corrected for zero-point energy.
of two water molecules and the argon atom. The corresponding calculated spectra (harmonic frequencies) for conformers 4c4g are shown in Figure 4. The calculated spectra of conformers 4d and 4f each contain features between 3560 and 3630 cm1, and conformer 4g contains a hydrogen bond feature near 3054 cm1, features that are notably absent from the [Ar þ H2O] experimental spectrum. Therefore, we do not consider these conformers to be present in the cluster ion beam. The calculated spectra of the two remaining conformers, 4c and 4e, contains features that are present in the [Ar þ H2O] spectrum. While conformer 4e is the only one containing a feature correlating to a signature three-coordinated H2O free OH stretching mode (labeled with an asterisk in Figure 4), it is possible that 4c also contributes the spectrum. The number of features between 3200 and 3450 cm1 suggests that the water bend overtone bands, calculated to occur in the 32003400 cm1 region, are contributing by borrowing intensity from the hydrogen-bonded OH stretches. The threecoordinated free OH stretch and the number of hydrogenbonded OH bands require the presence of the 4e conformer, which is the most likely source of all of the spectral features. Conformer 4c could also be present, but there is no definitive or obligatory feature in the spectrum that requires it. A simple analysis of ligand binding energies of the minimumenergy conformer and relative energies of the high-energy conformers explains why spectral features from the [Ar] spectrum are absent in the [Ar þ H2O] spectrum and vice versa. First, at the MP2/aug-cc-pVDZ level, the argon binding energy in conformer 4a (see Figure 2) is only ∼5 kJ/mol, and the binding 1445
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Figure 3. Five stable conformers (4c4g) containing hydrogen bonds for Liþ(H2O)4Ar1 calculated at the MP2/aug-cc-pVDZ level. Relative energies (compared to the global-minimum-energy structure 4a from Figure 2) are corrected for zero-point energy. Liþ is shown in green and argon in light blue.
energy of a water molecule from the subsequent Liþ(H2O)4 fragment cluster ion is ∼67 kJ/mol; thus, ∼72 kJ/mol of energy is required to induce [Ar þ H2O] fragmentation. Absorption of an IR photon in the OH stretching region imparts ∼38 45 kJ/mol of energy into the cluster ion. Clearly, from the binding energies, the absorbed photon energy of conformer 4a is only sufficient to cause argon fragmentation. Conformer 4b (see Figure 2) is ∼9 kJ/mol higher in energy than 4a. Thus, the total energy required to access the [Ar þ H2O] fragmentation channel for 4b is reduced from ∼72 to ∼63 kJ/mol, still well above the energy provided by the IR photon (∼38 45 kJ/mol). Therefore, spectral features of conformer 4b are only observed in the [Ar] spectrum. However, conformers 4c and 4e (Figure 3) reside ∼42 44 kJ/mol higher in energy above the global-minimum-energy conformer, reducing the energy required to access the [Ar þ H2O] fragmentation channel from ∼72 to ∼2830 kJ/mol. Because the energy (∼3845 kJ/mol) from the absorbed photon in the OH stretching region exceeds this threshold, spectral features from conformers 4c and 4e are observed in spectra obtained by monitoring the [Ar þ H2O] fragmentation channel. As the [Ar] fragmentation channel requires only 5 kJ/mol, there is substantial energy remaining for further fragmentation. The available energy of 7584 kJ/mol exceeds the water binding energy of ∼67 kJ/mol; therefore, water loss will also occur for these conformers. Thus, the [Ar þ H2O] channel is unique to these high (þ4244 kJ/mol) conformers. As the conformers, 4c4g, all have the argon directly bound to the lithium ion, we were interested in seeing if the position of the argon was responsible for the stability of these calculated structures. We performed additional calculations without argon and identified stable structures similar to those shown in Figure 3 (see Supporting Information for calculated structures for nonargonated Liþ(H2O)4 clusters). These stable argonated and nonargonated structures have not been previously reported in the literature, to the best of our knowledge, and represent new minima on the potential energy surface of Liþ(H2O)4Ar01 at the MP2/aug-ccpVDZ level of theory. The results from Liþ(H2O)4Ar1 experiments encouraged us to search for high-energy conformers and unique fragmentation
Figure 4. IRPD spectrum for Liþ(H2O)4Ar1 monitoring the [Ar þ H2O] fragmentation channel along with the calculated spectra (MP2/ aug-cc-pVDZ level) for conformers 4c4g from Figure 2. The threecoordinated water free OH stretching feature in the calculated spectra for 4e is labeled with an asterisk. Spectra are scaled so that the most intense peak in each spectrum has the same maximum intensity.
channels for Liþ(H2O)3Ar1 clusters. A spectrum of Liþ(H2O)3Ar1 has been previously reported7 in the frequency region of 36003850 cm1. We have obtained a new spectrum monitoring the [Ar] fragmentation channel and extended the frequency range to 32003850 cm1, as shown in Figure 5. The bands at 3648 and 3723 cm1 correspond to symmetric and asymmetric OH stretching modes and are indicative of water molecules where the OH groups are not involved in hydrogen bonds. However, by scanning to lower frequency, we observed an additional minor feature corresponding to a hydrogen-bonded OH stretching feature, at 3508 cm1 Two stable conformers were identified at the MP2/aug-ccpVDZ level. Their simulated spectra (see Figure 5) correspond to structures containing an argon atom. Calculations indicated that these structures did not require the presence of an argon atom and thus represent true minima on the Liþ(H2O)3 potential energy surface. The calculated bands, at 3634 and 3740 cm1, of the minimum-energy conformer, 3a, are assigned as H2O symmetric and asymmetric vibrational stretches, respectively. The high-energy conformer (þ28 kJ/mol), 3b, contains the characteristic bent hydrogen bond out-of-phase/in-phase modes near ∼3500 cm1, a symmetric stretch of the second-shell H2O at 3617 cm1, and free OH and H2O asymmetric stretches centered at 3723 cm1. We note that the feature at 3508 cm1 is analogous to the 3533 cm1 band in conformer 4b of Liþ(H2O)4Ar1. 1446
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for a three-coordinated H2O and hydrogen-bonded OH modes arising from previously unknown structural motifs in such a small cluster size. The features in the [Ar þ H2O] spectrum were noticeably absent from the [Ar] spectrum and vice versa. Using ligand binding energies and relative conformer energies, we have shown the energetic requirements behind the unique fragmentation channels for Liþ(H2O)4Ar1. For Liþ(H2O)3Ar1, a bent hydrogen-bonded OH stretching mode was observed, albeit weakly, for the first time. It was assigned to a conformer calculated to be þ28 kJ/mol higher in energy than the global-minimum-energy conformer and observed by monitoring the [Ar] fragmentation channel. No other photofragmentation channels were detected for Liþ(H2O)3Ar1. We have reported a simple way to obtain IRPD spectra of high-energy conformers utilizing unique fragmentation channels. The Liþ(H2O)34Ar1 clusters reported here are prime candidates for further theoretical exploration. Stable high-energy conformer analogues for nonargonated Liþ(H2O)3,4 were also found in the computational studies. These structures and energies will be useful in mapping out the potential energy surface and determining the barrier heights between different conformers in this deceptively complex cluster ion system. While this has been done previously in flexible neutral biomolecules,20 it is a new frontier for clusters and ion clusters. Figure 5. IRPD spectrum of Liþ(H2O)3Ar1 cluster ions monitoring the [Ar] fragmentation channel. MP2/aug-cc-pVDZ level structures and calculated spectra for two stable conformers are shown in 3a and 3b with their respective relative energies (corrected for zero-point energy).
The lower degree of hydration in Liþ(H2O)3Ar1 leads to a stronger electrostatic enhancement of the hydrogen bonds and thus a slightly lower value for the hydrogen-bonded OH stretching mode. The bent hydrogen-bonded OH stretching feature intensity is weak relative to the other bands in the experimental spectrum. Because the formation of OH 3 3 3 O hydrogen bonds is accompanied by a significant increase in infrared intensity,18,19 we conclude that conformer 3b is only a minor constituent in the cluster ion beam. IRPD for Liþ(H2O)3Ar1 was only detected by monitoring the argon loss channel. According to MP2/aug-cc-pVDZ level calculations, the binding energies of argon and water in the minimum-energy conformer are ∼7 and 88 kJ/mol, respectively (or 95 kJ/mol total). Thus, an absorbed photon in the OH stretching region of interest (35003800 cm1 for Liþ(H2O)3Ar1), providing ∼4245 kJ/mol of energy to the cluster ion, is only capable of inducing [Ar] fragmentation. For the highenergy conformer, 3b, that lies 28 kJ/mol above the globalminimum-energy conformer, we estimate that the minimum energy required to access the [Ar þ H2O] fragmentation channel is 67 (i.e., 95 minus 28) kJ/mol. The imparted energy from an absorbed IR photon (∼4245 kJ/mol) does not meet the energetic requirement to access the [Ar þ H2O] fragmentation channel; therefore, only the [Ar] fragmentation channel (
