Infrared Predissociation Vibrational Spectroscopy ... - ACS Publications

Jan 9, 2015 - ... University of Illinois at Urbana Champaign, Urbana, Illinois 61801, ... Institut Universitaire de France IUF, 103 Blvd St Michel, 75...
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Infrared Predissociation Vibrational Spectroscopy of Li+(H2O)3−4Ar0,1 Reanalyzed Using Density Functional Theory Molecular Dynamics V. Brites,† J. M. Lisy,*,‡ and M.-P. Gaigeot*,†,§ †

Laboratoire Analyse et Modélisation pour la Biologie et l’Environnement, LAMBE UMR 8587 CNRS, Université d’Evry Val d’Essonne, 91025 Evry, France ‡ Department of Chemistry, University of Illinois at UrbanaChampaign, Urbana, Illinois 61801, United States § Institut Universitaire de France IUF, 103 Blvd St Michel, 75005 Paris, France S Supporting Information *

ABSTRACT: The experimental IR-PD (infrared predissociation) spectra of Li+(H2O)3−4Ar and Li+(H2O)3−4 clusters, monitoring two different loss channels and thus different temperatures, have been reanalyzed using DFT-MD (density functional theory based molecular dynamics) simulations for finite temperature and anharmonic theoretical spectroscopy. The use of DFT-MD to calculate IR-PD spectra at low and elevated temperatures was found remarkably accurate and useful in precise structural characterization. The dynamical spectra have in particular provided the opportunity to estimate the clusters temperatures in the IR-PD experiments. The temperatures for Li+(H2O)3−4Ar are estimated at 50−60 K whereas Li+(H2O)3 and Li+(H2O)4 have been estimated at around 500−600 and 400 K, respectively.

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INTRODUCTION Vibrational spectroscopy is one of the most important methods for structural characterization in the gas phase. For systems with low number densities such as clusters or cluster ions, monitoring changes in populations, as in action spectroscopy, is an extremely sensitive and useful approach. IR-PD (infrared predissociation) and IR-MPD (infrared multiphoton dissociation) spectroscopies are being broadly applied in various domains of chemistry, physics, and biology.1−6 A variety of techniques have found applications from cold neutral molecular assemblies6−10 to charged systems at finite temperatures.11−17 Structural assignments rely strongly on both experimental spectra and theoretical calculations to provide a clear and definitive picture on the structures of the molecular assemblies. We have shown in previous studies that a precise, detailed, and unambigous characterization of structures can be achieved when IR-MPD or IR-PD experiments are combined with theoretical finite temperature anharmonic spectra derived from DFT-based molecular dynamics simulations (DFT-MD). See for instance our results on flexible peptides,18,19 anharmonic peptides,9,20 and highly anharmonic ionic clusters.21,22 The two immediate advantages arising from DFT-MD for theoretical spectroscopy are the direct inclusion of temperature and anharmonic effects, without applying any scaling factors or adhoc corrections that are typically used in standard harmonic vibrational frequency calculations.23 Temperature is of particular relevance for IR-PD experiments in the present paper. Depending on the presence (or absence) of argon atoms tagged to the ionic cluster and the loss channel © XXXX American Chemical Society

monitored, we have shown that one can probe ionic clusters with temperatures in the range of 50−150 K (argon loss channel or argon + water loss channel) to 300−500 K (water loss channel).15,24 At the lower temperature range (or equivalently at lower internal energy) of the ionic clusters, there can be insufficient kinetic energy to overcome conformational barriers that separate higher energy conformers on the ground state electronic potential energy surface from the global minimum energy conformer, thus kinetically trapping these high-energy conformers.25−28 Higher temperatures are generally sufficient to overcome these barriers so that the global minimum energy conformer can be formed. Therefore, not only is temperature of prime importance for the formation of the lowest or higher energy conformers in IR-PD experiments, but also it is relevant to the observed spectral features. A theoretical spectroscopic method that directly includes temperature into the calculation is therefore necessary to quantify temperature effects and related anharmonicities on the final spectroscopic features. This is where the synergy between IRPD experimental spectroscopy and dynamical theoretical spectroscopy is revealed and becomes essential in the field of gas-phase vibrational cluster spectroscopy. Taking advantage of finite temperature DFT-MD, we thus revisit the interpretation of IR-PD spectroscopy of Li+(H2O)3−4 Special Issue: Markku Räsänen Festschrift Received: August 28, 2014 Revised: January 9, 2015

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Figure 1. Optimized structures of Li+(H2O)3 (top) and Li+(H2O)4 (bottom) and relative electronic energies (kJ/mol). See text for nomenclature “X +Y”.

clusters.15,24,28 These ionic clusters are of particular interest for probing intermolecular forces, especially hydrogen bonds that can be enhanced by the presence of the cation. Moreover, the presence of a higher energy conformer in the IR-PD spectra of Li+(H2O)3Ar and Li+(H2O)4Ar, recorded through the Ar loss channel, was clearly identified.28 Recently, we have applied DFT-MD simulations to directly model the formation processes of these ionic clusters.29 The formation of highenergy conformers was confirmed and discussed with respect to temperature. Parts a−j of Figure 1 present the lowest energy conformers of Li+(H2O)3−4 optimized at the DFT/BLYP level (optimization with the mixed plane-waves and Gaussian basis set representation of CP2K, see the Methods section). Structures are labeled X+Y, with X representing the coordination of Li+ with respect to the oxygen atoms, and Y the number of hydrogen bonds between water molecules. Note that all electronic levels of representation tested (BLYP, BLYP-D2, MP2, CCSD(T)) provide the same potential energy surface to within 4.0 kJ/mol (see the Supporting Information, Tables S1− S2, relative electronic energies and Gibbs free energies at 100 K). Note also that the values presented in Figure 1 and Tables S1−S2 (Supporting Information) for the MP2 references compare to the ones from ref 24 for Li+(H2O)3,4 isomers within ∼5 kJ/mol. This arises from the use of different basis sets, taking into account zero point energies (ref 24) and different temperatures in Gibbs energies, between the two works. The BLYP representation slightly underestimates the relative energetics between conformers 4 + 0 and 3 + 2 of Li+(H2O)4 when compared to the reference MP2 and CCSD(T) values. Briefly, we have demonstrated in our previous investigation29 that in the range ∼60−100 K the lowest energy isomer 3 + 0 of Li+(H2O)3 is systematically formed through collision and evaporative cooling, and a higher energy conformer 3 + 2 is predominantly formed for Li+(H2O)4 together with a smaller population of the lowest energy isomer 4 + 0. These conclusions were obtained by 2 ps time scale DFT-MD direct dynamics of the collision between the cation and the water clusters (where an extensive set of different collision impact parameters and collisions angles with respect to the plane formed by the oxygens of the water clusters has been investigated), including evaporative cooling of the clusters as they form, combined with RRKM isomerization processes over

the microsecond time scale. The formation of the clusters by collision has been shown to be nonstatistical, thus requiring direct dynamics within the first picoseconds of the collisional processes. The temperature, collision impact parameter, and collision angle with respect to the plane of oxygens in the preformed cyclic water species influence the dynamical formation pathway and the resulting conformation distribution. When the lowest energy isomers are not formed by a direct process, a cascade process involving isomerization from high to lower energy conformers takes place, where temperature plays a crucial role in the potential trapping of higher energy conformers. The lower the temperature the higher the probability of trapping high-energy conformers: at low internal energy, clusters do not have enough energy for overcoming the barriers to isomerization and cannot proceed further along the path to lower energy configurations. In contrast, we have shown that the lower energy conformers of Li+(H2O)3,4 are systematically formed for cluster temperatures above 100 K, corresponding to the results observed for nonargonated Li+(H2O)3,4 clusters in the experiments. With these results from collisions in mind, we investigate here the spectroscopic features of these ionic clusters using the DFT-MD dynamical anharmonic approach. Temperature is key to dynamics and by its adjustment to get the best match between the experimental IR-PD signatures and the theoretical dynamical ones, we are able to provide an estimate of temperature of the Li+(H2O)3,4 clusters formed and probed in the IR-PD experiment. This investigation goes beyond those previously published on these systems,15,24,28 as we go beyond harmonic spectral calculations and directly include temperature effects in the spectroscopic analysis. Temperature effects on DFT-MD dynamical vibrational spectra have been shown in a few publications, see for instance refs 30−34.

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METHODS The experimental apparatus is described in detail elsewhere.15,24,28 Briefly, a gas mixture containing argon and a small amount of water was supersonically expanded through a conical nozzle into a source vacuum chamber, creating neutral clusters. The adiabatic expansion process typically generates neutral clusters with temperatures on the order of 10−20 K. Approximately 2 cm downstream from the nozzle after collisional cooling of the molecular beam has ceased, the neutral clusters were impacted with a lithium ion produced B

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clusters can be seen as quasi-rigid, without too much dynamical motion. IR intensities might be affected by the low statistics, although again as the clusters are quasi-rigid, increasing the statistics is not going to change the results presented. At higher temperatures, increasing the statistics might be necessary, especially for the more fluxional clusters as Li+(H2O)4 at 400 K for which an isomerization is observed along the 20 ps trajectory. In our previous investigation of the collisional processes leading to the formation of Li+(H2O)3−4 clusters by DFT-MD, combined with RRKM statistical calculations,29 we have estimated that the Li+(H2O)3−4Ar argonated species probed through the Ar loss channel in IR-PD have a temperature around 60 K. We have consequently chosen this temperature for the DFT-MD trajectories to get the theoretical spectroscopic features of the low-temperature clusters. For the theoretical spectroscopic features of non-argonated species Li+(H2O)3−4, corresponding to a water loss channel in the IRPD experiment, we have run different temperatures and searched for the best match to the experimental features, so that we could provide an estimate of the clusters temperature probed in IR-PD.

from a homemade ion gun, creating cluster ions. Excess energy in these nascent cluster ions (arising from the ion impact and solvation processes) was dissipated by either argon or water evaporation, on the basis of the initial composition of the neutral cluster, until the cluster ions reached a quasi-stable state (i.e., had a lifetime sufficient to traverse the experimental apparatus). Electrostatic lenses and an octupole ion guide were used to guide the ions into a detection chamber housing three quadrupoles. The first quadrupole mass filter was used to massselect a parent cluster ion of interest from among the stable ions produced in the source chamber. In the second quadrupole, the parent ions interacted with a single ∼10 ns pulse, from a gently focused tunable mid-IR laser (LaserVision OPO/A pumped by a 10 Hz Surelite II Nd:YAG laser). Resonant photon absorption provided sufficient energy for the parent cluster to dissociate, creating a fragment ion. The third quadrupole mass filter was tuned to select the fragment ion, resulting from the loss of argon, water or both. Laser fluencecorrected photodissociation cross sections are reported as a function of IR frequency. DFT-based Born−Oppenheimer molecular dynamics simulations (DFT-MD) have been performed with the CP2K package,35 where the nuclei are treated classically and the electrons quantum mechanically within the DFT (density functional theory) formalism. The BLYP functional36,37 combined with Goedecker−Teter−Hutter (GTH) pseudopotentials38,39 were used. A hybrid Gaussian and plane wave representation of the electronic wave function is employed with the aug-TZV2P Gaussian basis set (TZV2P for Li) and a planewave density cutoff of 450 Ry. A cubic cell with a side of 16 Å was chosen. This length has been obtained by checking the absolute convergence of the energy of conformer 3 + 2 of Li+(H2O)4 (somewhat the spatially larger cluster) with respect to the increase in the cubic box length, as well as the relative energies between the conformers of Li+(H2O)3 and Li+(H2O)4 clusters with respect to the box length (compared to the MP2 reference values presented in Tables S1 and S2 in the Supporting Information). Anharmonic spectra have been calculated for Li+(H2O)3−4 clusters using finite temperature DFT-MD simulations, to be compared to the experimental IRPD spectra, and to assign the experimental features in terms of structural arrangements of the ionic clusters. Dynamical anharmonic IR spectra are calculated through the Fourier transform of the dipole−dipole correlation function.23 No harmonic approximations are made, either on the potential energy surface or on the dipole moment calculation. No scaling of vibrational frequencies is made. Assignments of the active IR bands in terms of atomic displacements is achieved through VDOS (velocity density of states), calculated as the Fourier transform of velocity correlation functions. These are the standard procedures discussed in ref 23. We recall that the VDOS provides all vibrational bands but does not include the IR selection rules for the band intensities; thus they are used only for assigning internal motions to vibrational bands. As shown in Figures S1, S3, and S4 of the Supporting Information, some supplementary assignments have been directly achieved in terms of symmetric and antisymmetric motions of the O−H groups. See the legends for the underlying calculations. Trajectories are accumulated over 10−20 ps with a time-step of 4 fs, after an equilibration period of 4−5 ps used for thermalization of the clusters at a given temperature. Results presented are based on one trajectory per system. At the low temperature of ∼60 K, this can be considered enough as the

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RESULTS AND DISCUSSION Figure 2 presents the IR-PD spectrum of Li+(H2O)4Ar recorded through the Ar loss channel (bottom of Figure 2)

Figure 2. IR-PD spectrum of Li+(H2O)4Ar recorded through the Ar loss channel (bottom; from ref 28) and dynamical IR spectra calculated at 50−60 K for the 4 + 0 and 3 + 2 conformers of Li+(H2O)4.

together with the dynamical IR spectra computed at 50−60 K for the two lowest energy conformers of Li+(H2O)4, namely 4 + 0 (middle of Figure 2) and 3 + 2 (top of Figure 2). Note that the intensity of the spectrum associated with the 4 + 0 conformer has been scaled for comparison with the 3 + 2 spectrum. These two conformers are separated by about ∼2−7 kJ/mol (electronic energies), depending on the level of theory employed (Table S2, Supporting Information), with the 4 + 0 C

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The Journal of Physical Chemistry A being the lowest energy conformer. The direct dynamics of the collisions for the formation of Li+(H2O)4 coupled with RRKM statistical calculations29 have shown, at low temperature (100 K), that the higher energy conformer 3 + 2 is predominantly formed together with a smaller population of the lowest energy isomer 4 + 0. Comparison between the experimental and calculated spectra clearly support the formation of the 3 + 2 conformer when the argonated cluster and the Ar loss channel in the IR-PD experiment are considered. For the four bands ranging from 3600 to 3800 cm−1, the difference between IR-PD and computations is 5 cm−1, i.e., comparable to the resolution of the experiment. The band located at 3533 cm−1 in IR-PD associated with the hydrogen bonds between water molecules is computed at 3506 cm−1, i.e., 27 cm−1 lower than the experiment. In considering the role of dispersion, we have also performed calculations that included Grimme’s D2 dispersion.40 This resulted in a larger red shift in frequency from experiment for that band (43 cm−1). One has to keep in mind that such empirical corrections are not adjusted on vibrational properties and therefore are not deemed to systematically improve the theoretical spectra with respect to experimental ones. To make a better one-to-one comparison to experiments, we thus decided not to include D2 corrections in our calculations. At any rate, electrostatic interactions are the leading terms for the structures and energetics of charged clusters of interest here and for the associated red-shifts of stretching modes of H-bonded O−H groups. The rather excellent agreement between experiment and the dynamical spectrum of the 3 + 2 conformer confirms the anharmonicities of the H-bonds and the couplings between these H-bonded modes. Considering the very low temperature, there are no conformational dynamics. Note also the two symmetric and antisymmetric stretching signatures of the free O−H of the four water molecules in conformer 4 + 0 are identically located to the ones of the O−H streches of the free water molecule in conformer 3 + 2. See Figure S1 in the Supporting Information for the decomposition of the IR peaks in terms of symmetric and antisymmetric motions of the water O−H bonds. This illustrates very well that the lowest energy conformer 4 + 0 of Li+ (H 2 O) 4 cannot be completely ruled out from the distribution of conformers formed in the experiment, as nicely shown by the collisions.29 Figure 3 now presents the IR-PD spectrum of Li+(H2O)3Ar recorded through the Ar loss channel (bottom) together with the dynamical IR spectra computed at 50 K for the three lower energy conformers of Li+(H2O)3, namely 3 + 0, 2 + 2, and 2 + 1, respectively, second, third, and fourth traces in Figure 3. The 2 + 2 and 2 + 1 conformers were found to be quasi isoenergetic (Table S1, Supporting Information), separated by a barrier of about 4.0 kJ/mol (free energy barrier from metadynamics simulations, see Figure S2 in the Supporting Information), and about 30 kJ/mol higher in energy than the 3 + 0 lowest energy conformer (Table S1, Supporting Information). The DFT-MD direct dynamics of the collisions combined with RRKM29 have shown that the lowest energy isomer 3 + 0 of Li+(H2O)3 is systematically formed at a temperature above 60 K, but that below that temperature the isomerization between 2 + 2 and 3 + 0 isomers is reduced and a small proportion of population of 2 + 2 isomer remains. And indeed our theoretical spectra calculated at a temperature of 50 K (Figure 3) support the fact that isomer 3 + 0 is dominant in the final ensemble of cluster ions but with the minor presence of the 2 + 2 isomer. This

Figure 3. IR-PD spectrum of Li+(H2O)3Ar recorded through the Ar loss channel (bottom; from ref 28) and dynamical IR spectra calculated at 50 K for the 3 + 0, 2 + 2, and 2 + 1 conformers.

strongly supports the fact that Li+(H2O)3Ar cluster ions are probed in IR-PD at a rather low temperature, estimated here to be 50−60 K. There are two prominent bands in the IR-PD spectrum of Li+(H2O)3Ar at 3648 and 3723 cm−1 that are accurately reproduced by the dynamical spectrum of isomer 3 + 0 within 7 and 9 cm−1, respectively. They are due to the symmetric and antisymmetric OH stretches arising from the free OH of the water molecules, as already assigned in the case of conformer 4 + 0 of Li+(H2O)4 (Figure 2, see also Figure S3 (Supporting Information) for the decomposition of modes). The third feature in the experiment, located at 3508 cm−1, can only reflect a conformer with hydrogen bonds between water molecules, as already observed in the case of conformer 3 + 2 of Li+(H2O)4 (Figure 2). The low intensity of this feature, compared to the intensity of the other two bands, suggests that hydrogenbonded conformers are minor contributors to the total cluster ion ensemble. The dynamical anharmonic spectrum of isomer 2 + 2 has a band at 3482 cm−1 that is compatible with the experimental feature. It is located 26 cm−1 lower than experiment, as already noticed for conformer 3 + 2 of Li+(H2O)4 in Figure 2: both the 2 + 2 conformer of Li+(H2O)3 and the 3 + 2 conformer of Li+(H2O)4 have similar bands because of the similar arrangement of the H-bonds between water molecules. See also Figure S4 (Supporting Information) for band assignments. This assigment is the most consistent with the experiment, because the band associated with the H-bond of isomer 2 + 1 is located below 3200 cm−1. The experimental spectrum was not recorded below 3400 cm−1, giving no information in the potential formation of this 2 + 1 conformer. However, previous experimental investigations of M+(H2O)3Ar, with M = Na, K, and Cs15,24 failed to reveal any evidence for the presence of a similar 2 + 1 conformer. Also, the IR spectrum of conformer 2 + 2+1 (Figure S5, Supporting Information) is not compatible with the IR-PD experiment, also ruling out this conformer. D

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provide as good an agreement for the water rotational bands, therefore bracketing the Li+(H2O)3 cluster temperature in the 500−600 K range. It is interesting to note that the temperature analysis done in our previous publication,24 using the RRKMEE model, of this cluster ion yields a value of 485 K, fairly close to the value from the dynamics. The analysis of the trajectory reveals that conformer 3 + 0 does not isomerize at this temperature (at least over the time-scale of the DFT-MD trajectory). The band assigned to conformer 2 + 2 at lower temperature (3533 cm−1) is not present anymore at higher temperature, therefore showing only the presence of the lowest energy conformer of Li+(H2O)3 cluster. Note that conformer 2 + 2 readily isomerizes into conformer 3 + 0 at this high temperature. Figure 5 presents the IR-PD spectrum of the Li+(H2O)4 cluster ion recorded through the water loss channel (bottom).

At higher temperature, i.e., when the formation of nonargonated species and the H2O loss channel in the IR-PD experiment are considered, the lowest energy conformer 3 + 0 of Li+(H2O)3 is formed, as shown by the combined DFT-MD direct dynamics of collisions and RRKM calculations.29 The IRPD spectrum of Li+(H2O)3 recorded through the H2O loss channel (Figure 4, bottom) still displays the symmetric OH

Figure 4. IR-PD spectrum of Li+(H2O)3 recorded through the H2O loss channel (bottom; from ref 24) and dynamical IR spectrum calculated at 650 K for the 3 + 0 conformer. See Figure S6 in the Supporting Information for more temperature comparisons.

stretch observed at lower temperature for isomer 3 + 0 (Figure 3, bottom), whereas the well-resolved antisymmetric band is now replaced by a rotational sub-band structure due to free rotation of the water molecules around their symmetry axes. The perpendicular vibrational transition moment gives rise to ΔK = ±1 free rotor transitions. See Figure S7 in the Supporting Information for a direct assignment of the bands to rotational motion and the broadening of the antisymmetric band. In this figure, one can indeed clearly see the 3610 cm−1 band solely arising from the OH symmetric stretches of the water molecules, whereas the 3660 and 3680 cm−1 bands arise from antisymmetric motions. The signatures of the rotational motions of each individual water molecule (see legend of Figure S7 (Supporting Information) for its definition) clearly overlap only with the antisymmetric motions, thus broadening the associated bands. Such rotational structure clearly results from a higher temperature of the ionic cluster formed in the experiment. The dynamical IR spectrum of conformer 3 + 0 has been calculated at several temperatures to find which provides the best match to the IR-PD experiment. See the Supporting Information (Figure S6) for all results. The spectra calculated in the range 500−650 K provide good to remarkable agreements with the experiment. The rotational bands in the 500 K theoretical spectrum appear somehow too intense compared to the symmetric band, but there is already an overall good agreement with the experiment. The spectrum calculated around 650 K and reported in Figure 4 (top) provides a remarkable agreement with the experiment, although the symmetric band is slightly too broad. Presumably an intermediate temperature between 500 and 650 K would provide the best agreement with experiment. Such agreement is readily available from dynamical spectra, including temperature effects in the motion of the molecules and thus directly taken into account into the vibrational signatures. Note (Figure S6, Supporting Information) that lower temperature spectra do not

Figure 5. IR-PD spectrum of Li+(H2O)4 recorded through the H2O loss channel (bottom; from ref 24) and dynamical IR spectrum calculated at 400 K (starting from the 4 + 0 conformer). See Figure S8 in the Supporting Information for more temperature comparisons and comments on 4 + 0/3 + 2 isomerization depending on temperature.

Two main differences are notable when this is compared with the IR-PD spectrum of Li+(H2O)3 in Figure 4 (bottom): (1) the band associated with the antisymmetric OH stretch is now clearly visible, suggesting that the free rotation of the water molecules is substantially reduced, possibly due to a lower temperature for Li+(H2O)4 compared to that for Li+(H2O)3; (2) there is a band in the 3350−3400 cm−1 range, clearly suggesting the presence of H-bonded conformer(s). This band is down-shifted from its counterpart in the spectrum of the argonated Li+(H2O)4Ar at lower temperature (3508 cm−1). As in the Li+(H2O)3 case, dynamical IR spectra have been computed at different temperatures, systematically starting from the 4 + 0 conformer as the initial structure of the dynamics. See Figure S8 in the Supporting Information. In Figure 5, the spectrum computed around 400 K (top) is compared to the spectrum from the IR-PD experiment. In the free OH region, the positions, shapes, and relative intensities of the computed bands remarkably match the IR-PD experiment, whereas a broad band is obtained around 3300 cm−1, very similar to the experimental feature though red-shifted from experiment. Below 400 K this H-bonded spectral feature is not present (Figure S8, Supporting Information). Interestingly, the temperature analysis using the RRKM-EE model24 of this cluster ion yields a value of 325 K, fairly close to the 400 K value from the dynamics. At 400 K, the analysis of the trajectory reveals an E

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Together with the present paper, they provide a definitive, comprehensive assessment of how the clusters form in our experimental setup, of which isomers can be formed on the basis of temperature, at what temperature higher energy conformers can be kinetically trapped, and of how the respective vibrational bands can be accurately predicted at the relevant temperatures.

isomerization between 4 + 0 and 3 + 2 isomers over time (Figure S9, Supporting Information), showing that the system has enough internal energy to overcome the ∼20.0 kJ/mol of free energy barrier for the 4 + 0 → 3 + 2 isomerization. See Figure S2 (Supporting Information) for the free energy profile calculated by means of metadynamics simulations and description of the meta-variables used. The direct dynamics for the formation of Li + (H 2 O) 4 species and RRKM calculations29 show that isomer 4 + 0 is systematically generated at 300 K, and the dynamics at 400 K further shows that the cluster has enough energy to overcome the 4 + 0 → 3 + 2 isomerization barrier. Once this isomerization has occurred, the free energy barrier for the backward 3 + 2 → 4 + 0 isomerization is ∼10 kJ/mol, half the 4 + 0 → 3 + 2, easily leading to a conformational dynamics between these two conformers. The 20 ps DFT-MD performed here only shows one 4 + 0 → 3 + 2 isomerization event. Running supplementary/longer trajectories would provide a more definitive answer on the relative population of these two isomers probed in the experiment. Such conformational isomerization between these two structures is at the origin of the spectral features of Li+(H2O)4 species recorded in the IRPD experiment. Note also (Figures S10 and S11, Supporting Information) that at the temperature of 400 K, the spectrum of the 4 + 0 isomer alone (Figure S10 (Supporting Information), spectrum recorded over the first part of the dynamics where the 4 + 0 isomer only exists) already displays the water rotational substructures we discussed in Figure 4 in the case of Li+(H2O)3 species. For the 3 + 2 isomer, the presence of the hydrogen bonds restricts internal rotation of three of the four water molecules. As a result (as shown in Figure S11, Supporting Information), this gives rise to a free OH band near 3715 cm−1, which joins the more intense symmetric stretch at 3640 cm−1 as the dominant features in the non-hydrogen-bonded region of the spectrum.

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ASSOCIATED CONTENT

S Supporting Information *

Supporting Information provides tables of energy differences and figures showing the decomposition of IR peaks, free energy profiles, IR and IR-PD spectra, and the evolution with time of the coordination of Li+. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Authors

*J. M. Lisy. E-mail: [email protected]. *M.-P. Gaigeot. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the initial support of the National Science Foundation under Grant Nos. CHE-0748874 and CRIF 0541659. This work was performed using HPC resources from GENCI-France [IDRIS] (Grant 2012-2013 [072484]) and NICS/XSEDE under the grant TG-CHE120083. J.M.L. and M.P.G. acknowledge the United States National Science Foundation (CHE-1124821) and the French ANR SPIONCLUS, respectively, for research support through the International Collaboration in Chemistry Program (ANR-NSF). J.M.L. also acknowledges that this material is based on work supported while he served at the National Science Foundation.

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CONCLUSIONS The experimental IR-PD spectra of argonated Li+(H2O)3−4Ar and non-argonated Li+(H2O)3−4 clusters, monitoring two different loss channels and thus different temperatures, have been reanalyzed using DFT-MD to account for finite temperature effects and anharmonicity in simulated vibrational spectra. The use of DFT-MD to calculate IR-PD spectra at low and elevated temperatures once again demonstrates its power to provide accurate structural characterization. Agreement between the calculated and experimental spectra yields reliable vibrational assignments, without scaling factors or other ad hoc approximations. The dynamical spectra have given us the opportunity to estimate the cluster temperatures formed and probed in the IR-PD experiments. The temperature of Li+(H2O)3−4Ar cluster ions is estimated at 50−60 K whereas the Li+(H2O)3 and the Li+(H2O)4 species have been estimated at around 500−600 and 400 K, respectively. These values compare well to the cluster temperature analysis performed in ref 24 using the RRKM-EE model. This paper complements our previous investigation on combined DFT-MD and RRKM calculations for the direct dynamics of the formation of the clusters by collision between the water cluster and Li+, including the argon evaporative cooling as the collision proceeds, supplemented by RRKM population evolution over the microsecond time scale,29 in which we have demonstrated how higher energy conformers can be kinetically trapped as they form by evaporative cooling, when the temperature is low.

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DOI: 10.1021/jp508699m J. Phys. Chem. A XXXX, XXX, XXX−XXX

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DOI: 10.1021/jp508699m J. Phys. Chem. A XXXX, XXX, XXX−XXX