Thermodynamics of Water Dimer Dissociation in the Primary

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Thermodynamics of Water Dimer Dissociation in the Primary Hydration Shell of the Iodide Ion with TemperatureDependent Vibrational Predissociation Spectroscopy Conrad Tristan Wolke, Fabian S Menges, Niklas Tötsch, Olga Gorlova, Joseph A. Fournier, Gary H. Weddle, Mark A. Johnson, Nadja Heine, Tim K Esser, Harald Knorke, Knut R. Asmis, Anne B. McCoy, Francesco Paesani, Rita Prosmiti, and Daniel J Arismendi-Arrieta J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp510250n • Publication Date (Web): 03 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

Thermodynamics of Water Dimer Dissociation in the Primary Hydration Shell of the Iodide Ion with Temperature-Dependent Vibrational Predissociation Spectroscopy

Conrad T. Wolkea, Fabian S. Mengesa, Niklas Tötschb, Olga Gorlovaa, Joseph A. Fourniera, Gary H. Weddlec and Mark A. Johnsona,* a

Sterling Chemistry Laboratory, Yale University, New Haven, CT 06525, USA

b

Graduate School of Chemistry and Biochemistry, Ruhr-Universität Bochum, Universitätsstraße 150, D-44801 Bochum, Germany

c

Department of Chemistry, Fairfield University, Fairfield, CT 06824, USA

Nadja Heined, Tim K. Esserd,e, Harald Knorked,e and Knut R. Asmise,* d

Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany e

Wilhelm-Ostwald-Institut für Physikalische und Theoretische Chemie, Universität Leipzig, Linnéstraße 2, D-04103 Leipzig, Germany Anne B. McCoy* The Department of Chemistry and Biochemistry, The Ohio State University, Columbus, OH 43210, USA Daniel J. Arismendi-Arrietaf,g, Rita Prosmiti,f and Francesco Paesanig,* f

Instituto de Física Fundamental (IFF-CSIC), CSIC, Serrano 123, 28006 Madrid, Spain

g

Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093

*M. A. Johnson. Tel: +1 203 432 5226, Email: [email protected] A.B. McCoy. Tel: +1 614 292 9694, Email: [email protected] K. R. Asmis. Tel: +49 (0) 34197 36421, Email: [email protected] F. Paesani. Tel: +1 858 822 3383, Email: [email protected] 1 ACS Paragon Plus Environment


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Abstract: The strong temperature dependence of the I⁻·(H2O)2 vibrational predissociation spectrum is traced to the intracluster dissociation of the ion-bound water dimer into independent water monomers that remain tethered to the ion. The thermodynamics of this process are determined using van’t Hoff analysis of key features that quantify the relative populations of H-bonded and independent water molecules. The dissociation enthalpy of the isolated water dimer is thus observed to be reduced by roughly a factor of three upon attachment to the ion. The cause of this reduction is explored with electronic structure calculations of the potential energy profile for dissociation of the dimer, which suggest that both reduction of the intrinsic binding energy and vibrational zero-point effects act to weaken the intermolecular interaction between the water molecules in the first hydration shell. Additional insights are obtained by analyzing how classical trajectories of the I⁻·(H2O)2 system sample the extended potential energy surface with increasing temperature. Keywords: CIVP Spectroscopy, Infrared, Halide Solvation, Temperature, Van’t Hoff Analysis, Phase Transition

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I. Introduction Over the past decade, considerable progress has been made in the determination of the structural motifs at play in the formation of the first hydration shell around small anions.1-13 In general, halide hydration occurs with one hydrogen atom pointing toward the ion in a “single ionic hydrogen bond” or SIHB donor configuration, leaving the second OH bond to form hydrogen bonding networks with the other water molecules, typically in an asymmetrical fashion. The calculated minimum energy structure of the iodide dihydrate is displayed in Fig. 1, and it has been evident since the earliest spectroscopic investigations of this system that cooling is essential to recover the pattern of fundamentals associated with this cyclic structure, thus ushering in a wave of renewed activity using rare gas tagging methods to reveal the spectral signatures of the global minima.4 The first such measurements on this system were reported over 15 years ago and established that, when cooled to near its ground state with Ar tagging, the zeropoint energy in the vibration that breaks the hydrogen bond between the water molecules is wellbelow the energy of the shelf region of the potential (region (II) in Fig. 1a). In this geometry, the experimental spectrum is welldescribed by the harmonic spectrum shown in Fig. 1b. As such, the band assignments of the four OH stretch fundamentals are labeled according to the calculated displacement vectors in Fig. 1b: the central two bands (IHB2D and IH) are traced to the symmetric and antisymmetric stretches of the double Hbond donor water molecule (DD) while the outer two bands arise from the acceptordonor (AD) molecule as indicated. The predissociation spectrum of the Ar-tagged complex reported earlier is in line with this harmonic expectation. Interestingly, patterns such as this were not even qualitatively present in the analogous predissociation spectra of the bare, warm hydrated halide clusters,14 indicating that the bands do not simply broaden upon internal excitation about Figure 1. (a) Relaxed PES (MP2/aug-cc-pVDZ) along the O-O distance, capturing both, the intramolecular a particular band origin, as is common in water bond cleavage ((I) to (II)) and the cluster polyatomic molecule vibrational dissociation upon loss of a neutral water molecule (III). 15 spectroscopy. Instead, the bare cluster The PES displays a broad and shallow shelf (II) on spectra displayed completely different which each water molecule can move as independent distributions of oscillator strengths. This monomers orbiting the iodide center, upon breaking the 14 inter-water hydrogen bond between the two water situation was rationalized as being due to monomers. The harmonic frequencies of the minimum break-up of the water dimer in the first energy structure using MP2/aug-cc-pVTZ and hydration shell in the warm cluster ensembles MP2/aug-cc-pVTZ-pp33 for iodide (scaled by 0.951) is such that the spectra evolve from that of an presented in (b) and displacement vectors for the four intact H-bonded dimer to two independently dominant transitions are shown in the insets. The transition most sensitive to the H-bond between the orbiting water molecules about the ion. two water molecules is highlighted in blue. Unfortunately, because it was not possible in 3 ACS Paragon Plus Environment


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that era to establish the temperature of the ions, such hypotheses could not be challenged or verified experimentally. A one dimensional cut through the potential energy surface (PES) along the oxygen – oxygen distance underlying such a scenario is presented in Fig. 1a, which features a shallow minimum associated with the cyclic, ground state structure that evolves into a relatively flat region lying far below the energy required to remove one of the water molecules. This is consistent with the observation that the dissociation energy of the bare water dimer (3.16 kcal/mol or 1105 cm-1)16 is much smaller than that required to dissociate the iodide monohydrate (3460 ± 50 cm-1).17-18 Over a decade ago, Xantheas and co-workers explored the spectroscopic implications of cluster temperature in a theoretical study of the Cl⁻·(H2O)2 vibrational spectrum,19-20 which introduces many aspects relevant to the iodide system of primary interest here. His analysis of the Cl⁻·(H2O)2 cluster predicted that the key band associated with the intermolecular linkage of the water molecules (analogous I⁻·(H2O)2 band denoted IH (blue) in Fig. 1b) would be lost in the temperature range 30 – 50 K. This suggests that the water dimer indeed becomes surprisingly fragile upon attachment to an anion, in part accounting for the fact that it was much more difficult to experimentally obtain well-resolved spectra of the hydrated ions as compared to the situation in neutral hydrates also prepared with supersonic jet cooling techniques.21-23 Certainly the increased binding energy of water molecules to the ion also places increased demands on the cooling capacity of the ion source, so both effects are in play. ; here we use recent advances in cryogenic ion processing24 to disentangle the relative contributions of cooling and the reduction in intrinsic strength of the inter water H-bond when the dimer attaches to an iodide ion. In particular, we report the evolution of the vibrational spectra of size-selected I⁻·(H2O)2 clusters over the temperature range 10 – 300 K. One of the important aspects of this study is that we have carried out completely complementary measurements of the temperature-dependent I⁻·(H2O)2 spectra in two laboratories, the Asmis Lab at the FHI in Berlin and the Johnson Lab at Yale. We chose this system because the cold clusters yield relatively well defined features associated with the formation of an inter-water hydrogen bond, and the binding energy of water molecules to the large iodide ion is sufficiently small (D0 [I⁻·(H2O)] = 3460 ± 50 cm-1)18 that most of the key bands can be observed in a linear action regime without a tag.17 We then carry out a van’t Hoff analysis of the temperature-dependent signatures of the water dimer and independent monomer moieties to quantify the enthalpy and entropy associated with dimer dissociation on the iodide “surface”. A more quantitative understanding of how the potential surface is sampled with increasing temperature is explored with classical trajectory simulations of the dynamics of the I⁻·(H2O)2 at temperatures ranging from 10 K to 300 K, where the shape of the extended surface is obtained using an ab Initio-based and polarizable model for the iodide-water interactions, and the ab Initio MB-pol many-body potential for the water-water interactions.25-27 II. Experimental Methods Both the FHI and Yale spectrometers record vibrational predissociation spectra of sizeselected I⁻·(H2O)2 clusters that are prepared in temperature controlled radio frequency ion traps. The experimental protocols and the trap configurations are quite different in the two instruments, however, with the schematic layouts of each displayed in Fig. 2. The Yale trap (Fig. 2b) is a standard RM Jordan three dimensional Paul trap, while the FHI instrument utilizes a custom built 4 ACS Paragon Plus Environment

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configuration consisting of 24 rings separated by insulating sapphire spacers (Fig. 2c). The latter configuration was designed to maintain the favorable aspect of linear n-pole traps, where the buffer gas collisions are not strongly driven by the RF voltage and thus more readily equilibrate trapped ions close to the ambient temperature established by the second stage of the cryostat. In the Yale instrument, ions were stored initially in a room temperature octopole trap before injection into the cryogenic trap, where a short (1 ms) pulse of helium buffer gas is introduced into the trap just before the ions are injected. By subsequent storage for about 90 ms, the ions are cooled during the evacuation of the buffer gas so that they are extracted at low pressure in order to minimize reheating by collisions during extraction. In the ring electrode trap (RET) setup, ions undergo a first step of mass selection in a quadrupole mass filter, before being injected into the RET, filled with cold buffer gas held at a constant pressure. The ions are held in the trap for up to 100 ms and the 12 adjustable ring DC voltages are adjusted to independently optimize the trapping and extraction conditions before injecting the clusters into the photofragmentation mass spectrometer. Both instruments carry out the size-selective photodissociation using time-of-flight methods and the (LaserVision) OPO/OPA to generate tunable radiation over the range 600 – 4200 cm-1. III. Results and Discussion IIIA. Temperature dependence of the I⁻·(H2O)2 predissociation spectra from 10 to 250 K: Spectral signature of intracluster water dimer dissociation Figure 3 presents the predissociation spectra obtained from the two temperaturecontrolled ion traps described in the experimental section. The top gray trace corresponds to the H2 tagged spectrum, and it appears quite similar to the Ar predissociation spectrum (top black) reported earlier.14 The lower traces present the predissociation spectra of the bare I⁻·(H2O)2 ion over the range 10 - 250 K, and are clearly strongly dependent on temperature. Note that this dependence is not a situation where bands broaden and red-shift as vibrational hot bands arise from population of excited states in the 3N6 harmonic modes; rather, the coarse behavior is better described as a loss in the pattern of relatively sharp features present in the cold spectra with a concomitant rise of a broad feature centered at 3450 cm-1 starting at around 100 K.

Figure 2. Schematics of both experimental setups deployed to verify the conformity of the temperature in the micro solvated system. The Asmis setup (d) uses a QMS to preselect the ion of interest before injection into the ring electrode trap (RET) (c), while the Johnson apparatus (a) accumulates the ions in a linear octopole prior to ion storage in a 3D Paul trap (b). Both RF ion confinements are mounted to a temperature-controlled He cryostat and the temperature is measured by a 4 point diode attached to the trap. Ions are collisional cooled and thermalized by precooled helium buffer gas injected into the trap, either by a constant flow (FHI) or pulsed prior to ion injection (Yale).



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The fact that the spectra exhibit marked changes continuously over such a large range in temperature provides a stringent test for the equilibration of the ion ensembles to the set temperatures of the traps. The spectra from the two instruments are overlaid in Fig. 3, with that obtained with the Yale Paul trap displayed in black and the FHI RET data presented in gray. Close inspection reveals that all features are very faithfully reproduced over the entire spectral range at all temperatures. As such, we conclude that temperature is indeed reliably established in both instruments, an observation that, to our knowledge, is the first of its type and thus represents a significant advance in the control and characterization of gas phase ions. Cursory inspection of the spectral evolution with increasing temperature reveals the qualitative origin of the strong thermochromic response. As anticipated by Xantheas and coworkers over a decade ago,19 the IH band (blue dashed line) is lost with increasing temperature, along with the other sharp features associated with the cyclic dimer ground state structure. These give way to a broad band centered around 3450 cm-1 (red dashed line), which appears very close to the location of the ion-bound OH stretch in the I⁻·(H2O) monomer.13, 28-31 Note that we are certain that the spectrum arises from the I⁻·(H2O)2 cluster, as this species is sizeselected prior to laser excitation, and the photodissociation signal is recorded above the minor background from metastable decay in the drift region of the TOF mass spectrometers. This spectral evolution is thus easily rationalized in a scenario where the two water molecules, initially docked together in the global minimum (0K) configuration, effectively dissociate while remaining attached to the iodide ion. This is clearly energetically possible since the shelf region of the potential that corresponds to two non-interacting water molecules complexed to the iodide anion is at Figure 3. Temperature-dependent vibrational an energy that is well-below the I⁻·(H2O) predissociation study of the I⁻·(H2O)2 anion at the dissociation threshold.

FHI (gray) and Yale (black). The top two traces compare a recent H2 tagged spectrum with the triple Ar tagged results reported earlier.14 The untagged traces present spectra, for which the ions are thermalized to temperatures between 10 and 250 K. Spectra of the same temperature are essentially the same, independent of the experimental apparatus used, an observation strongly supporting the idea of the cluster temperature being a well-defined quantity. The † features are assigned to combination band activity as described in supplementary Fig. S1.

To better quantify how the I⁻·(H2O)2 system behaves with increasing internal energy, we calculated classical trajectories for the system on an accurate Born-Oppenheimer potential energy surface, the details of which are provided in the supporting materials. Since we are primarily interested in the spectral manifestation of the dynamics, we investigated 6

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the time evolution of the cluster structure by calculating the local anharmonic frequencies of the four OH oscillators along the classical trajectories as a function of temperature. Specifically, for each molecular configuration visited during the classical dynamics, the instantaneous local frequencies were obtained independently for the four OH oscillators by carrying out a scan along each OH stretching displacement, while keeping the positions of all other atoms fixed. The corresponding onedimensional Schrödinger equation was then solved numerically for the first two eigenstates. Figure 4 presents the local frequencies of the four OH oscillators along representative trajectories corresponding to average temperatures of 10 K, 100 K and 300 K. In this analysis, the color scheme defines the arrangement of the four hydrogen atoms Figure 4. Time dependence of the local at the beginning of the trajectory (t = 0 ps) as frequencies of the four OH stretches calculated indicated in the inset structure at the right of the along classical MD trajectories at 10 K (top panel), 100 K (middle panel), and 300 K (bottom panel). lower panel. By following the time evolution of the The color scheme defines the position of the four I⁻·(H2O)2 hydrogen-bond topology along the hydrogen atoms at t = 0 ps according to the dynamical trajectories, we reveal how the positions structural schematic shown in the inset. of the four hydrogen atoms evolve as a function of temperature. At 10 K, the frequencies of the four oscillators remain constant throughout the simulation, indicating that the cluster is locked near its equilibrium position. At 100 K, one finds a correlated exchange of the two oscillators with low and with high frequencies. This corresponds to exchange between the water molecule that is identified as the hydrogen bond donor and the hydrogen bond acceptor in the water dimer. At the highest temperature shown here, the exchanges become more pronounced and it becomes difficult to differentiate the various bonding geometries, consistent with the breaking the water-water hydrogen bond in the complex. The spectral response is thus consistent with the breakup of the cyclic dimer structure with increasing energy, in turn raising the possibility to quantify the thermodynamics associated with this structural change by following the evolution of bands traced to each structural class. IIIB. Comparison of predissociation spectra of bare and tagged I⁻·(H2O)2 For the intensity of infrared features detected by action spectroscopy (predissociation) to be directly proportional to the population of the corresponding cluster geometry, it is critical that excitation over the entire spectral region yields fragmentation with essentially unity quantum yield. The wide popularity of the messenger method is, indeed, due to the fact that this condition is achieved quantitatively over the entire range of OH stretching fundamentals found in ionic hydrates. The situation here is more complex, however, in that weakly bound messengers such as H2 and Ar (Do = 500 cm-1)32-33 are readily evaporated at temperatures far below that where the I⁻·(H2O)2 spectra begin to show dramatic changes. We have therefore chosen to study a dihydrate of the largest halide (iodide) as it has the smallest binding energy and thus allows access to most OH bands upon excitation of the bare I⁻·(H2O)2 cluster.



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To clarify the overall trends in the (experimental) temperature-dependent CIVP spectra, we merge the data into the two dimensional contour plot, displayed in Fig. 5, where each spectrum is normalized to its individual maximum peak. This presentation illustrates how bands expected for the cyclic structure are suppressed in the region highlighted by the red dotted line (low temperature and low photon energy), but regain intensity such that the more red shifted bands are incrementally accessed with increasing temperature. Two aspects of the fragmentation photophysics are important in assessing the differences between the tagged and bare ion spectra: The first issue concerns the role of the temperature-dependent internal energy in the complex, Eint(T), prior to photoexcitation to yield a distribution of energies E = Eint(T) + hν. The second issue involves the internal energy dependence of the unimolecular dissociation rate constant, k (E - Do), as this determines the energy range over which the action spectra lag behind the absorption spectra due to slow dissociation on the experimental timescale. The details of this analysis are included in the supporting information, where we estimate the unimolecular dissociation rate with a simple RRKM model, and address how vibrational hot bands affect the relative band intensities with increasing temperature. The main conclusion from this exercise is that, consistent with the experimental observation, all dissociations are prompt. In that case, the suppression of the low energy (IHB1D and IHB2D ) bands can be understood if excitation of the OH stretching fundamentals fall below the dissociation threshold, such that only the fraction of clusters with sufficient internal energy (Eint(T) ≥ Do - hν) contribute to the IHBM features. Here Eint(T) is accommodated by thermal population of low frequency modes, ωi, with quanta vi, which at high resolution would be resolved into a series of closely spaced OH v=0 to 1 transitions with ∆vi=0. These transitions would be displaced from the vibrational band origin according to the difference in frequencies between the OH(v=0) and (v=1) levels due to anharmonic coupling. Because this coupling is small, these sequence bands fall quite close to the vibrational origin, and at low resolution, the IHBD features will appear at their expected locations, but with lower intensities that reflect the fact that only the sequence bands associated with members of the ensemble with critical internal energies required for each hνIHB actually contribute to this intensity. A harmonic analysis of the Boltzmann weighted vibrationally excited states indicates that the IHB2D band of the cyclic form can be efficiently accessed with soft mode excitation available at an ensemble temperature of about 75 K, which is substantially below the value (≈ 125 K) where the strong nearby band associated with breakup of the dimer becomes dominant. An explicit analysis of the thermally excited vibrational populations is presented in Fig. S2 of the Supporting Information. The situation at play in the weak low energy bands in the

Figure 5. Contour plot of the CIVP spectra normalized to the individual maximum for each trap temperatures ranging from 10 – 300 K. The suppression of the low frequency (IHB1D and IHB2D ) bands at low temperature (dotted line) is traced to the minimum internal energy needed for IR excitation to reach the dissociation threshold. These features regain their intensities at around 75 K. Most importantly, the feature at 3620 cm-1, associated with the intermolecular water hydrogen bond (IH), disappears at around 100 K while a new feature arising from independent water molecules (IHBM at 3450 cm-1) dominates the spectrum at high temperature.


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10K spectra are less clear, as the band intensities can only be understood if the system has an internal energy quantum of around 10 cm-1, which is considerably below the harmonic values calculated for the minimum energy structure. In light of the photodissociation energetics discussed above, we conclude that the relative photodissociation yields for excitation of the 3620 cm-1 IH and 3450 cm-1 IHBM bands (Fig. 3) can be used in a van’t Hoff analysis of temperature-dependent open and cyclic populations without kinetic corrections above a temperature of approximately 75 K. To carry this out, we corrected the raw (laser fluence normalized) intensities of the two transitions for the calculated (harmonic) intensities of the fundamentals. This procedure clearly does not include the temperature dependence of these intrinsic features, and as such must be considered an approximation. However, given the large spectral changes in play and the fact that this analysis is the first (to our knowledge) of this type in size-selected clusters, we suggest that this rather crude analysis captures the underlying mechanics of the thermochromicity at play in this problem. IIIC. Van’t Hoff analysis of I⁻·(H2O)2 and comparison to neutral water dimer Figure 6 presents a van’t Hoff analysis of the two populations recovered using the approximation outlined above. Indeed, this analysis reveals an extended region of linearity in the logarithmic dependence of the equilibrium constant over a factor of two in ion temperature. The slope of this plot yields a ∆Ho value of 1.11 ± 0.09 kcal/mol and ∆So = 16.0 ± 0.7 cal/(K·mol). If we take ∆Ho to be associated with the binding energy of the water dimer in the vicinity of the iodide ion, we note that this value is about a factor of three smaller than that recently reported for the bare water dimer (Do = 3.16 ± 0.03 kcal/mol),16 in turn suggesting that the dimer binding energy is dramatically reduced in the presence of the iodide ion. This reduction is of interest in that it provides a compelling explanation for the fact that in the early days of sizeselected ion vibrational spectroscopy, it was much more difficult to obtain sharp spectra of the ion hydrates compared to the neutral water counterparts.21-23 The qualitative reason for this reduction likely reflects the distorted arrangement of the dimer when it is attached to the ion, which acts to weaken the inter-water H-bond. Calculations of the binding energy of the water dimer in the I⁻ complex place this value at 2.4 kcal/mol (MP2/aug-cc-pVTZ with aug-cc-pVTZpp34 basis set for I), or roughly 60% of the best calculations of the binding energy of bare water dimer.35 This value is more than twice the value of ∆Ho obtained from analysis of the temperature dependence of the spectrum, described above. Much of this difference can be accounted for by the relative zero-point energies of the two isomers. In fact, accounting for this zero-point energy at the harmonic level gives a value of Do = 1.3 kcal/mol, which is in good agreement with the measured value of ∆Ho. It is interesting to note that the leading contribution to the lowering of Do compared to De comes from the low-frequency intermolecular vibrations, and not the OH stretches. This reflects the highly constrained structure of the water dimer when it is complexed with I⁻, leading to an increase in the zero-point energy of the dimer compared to the two separate I⁻·(H2O)2 monomers. It is also of interest to note that the weakening of the inter-water H-bond is reflected in the vibrational frequency of the donor OH group on the DD water molecule, which evolves from 3597 cm-1 in the bare dimer36 to 3620 cm-1 when this dimer is attached to the iodide.



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The large positive ∆So value extracted from the van’t Hoff analysis is also qualitatively informative. It is instructive, for example, to consider the analogous situation involving the thermal dissociation of a dimer such as N2O4 → 2 NO2. In that case, the entropy change arises largely from the introduction of two translating triatomic molecules for each dissociated dimer. This occurs such that the separated monomers dominate the thermal ensemble at the characteristic temperature of T = ∆H/∆S ≈ 70 K, reminiscent of the rule that applies to a bulk first order phase transition. The key point here is that, because the open Figure 6. The intensity ratios of the bands associated configuration occupies a phase space volume with the inter-water hydrogen bond IH and the ionic much larger than that of the cyclic form, the hydrogen bond for the independent monomers IHBM, open form dominates the behavior of the weighted by their harmonic fundamental transition dipole moment, define the equilibrium constants keq. equilibrium ensemble. In this sense, the Shown is a van’t Hoff plot, in which the logarithm of microscopic dihydrate exhibits the observed eq is plotted against the inverse temperature of the ion spectroscopic behavior in which the spectra at ktrap. A linear fit from 100 - 200 K (blue datapoints) elevated temperatures are not simply yielded a value for ∆Ho =1.11 ± 0.09 kcal/mol and ∆So broadened about normal mode fundamentals = 16.0 ± 0.7 cal/(K·mol), which results in an internal by excitation of sequence bands involving soft modes that built upon the OH stretching band origins with ∆v = 0 selection rules. Rather, the entire spectrum evolves to reflect the average nature of the fluctuating assembly. IIID. Hot bands, isomers or a phase transition: A continuum of spectral response in microscopic clusters The thermodynamic perspective outlined above has important implications regarding the analysis of temperature-dependent spectra of fragile clusters, especially hydrated ions. For example, it is often the case that isomers are formed in ion sources, and it is often desirable to determine the relative energies of these species by monitoring how they change with source conditions (e.g., tag identity33, 37). The present demonstration of temperature control thus raises the important distinction between the populations of persistent isomers (i.e., with a large barrier between them) and the current situation regarding a dynamical evolution of the spectra. The open structure here is not an isomer per se, but an extension of the ground state to a configuration that dominates the ensemble as the temperature is raised. In the latter case, we do not expect a Boltzmann populated ensemble according the energy differences between isomers (which would saturate at a 50 % mixture at high T for similar overall structures), but rather the character of the ensemble will simply reflect the properties of the nuclear configurations that dominate the distribution, quite comparable to the observation of a roaming water molecule in the entropically favored transition region for the monohydrated dihydrogen phosphate anion.38 This also clarifies the issues of “transparency” in infrared multiple photon dissociation (IRMPD),37 where IR-active vibrations predominantly involving distortions of the hydrogen bound network are notably missing from the IRMPD spectra of these ions but are fully recovered 10 ACS Paragon Plus Environment

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by messenger-tagging the clusters, because the warm ensemble displays only weak absorption in the vicinity of the minimum energy fundamentals, similar to the I⁻·(H2O)2 system. IV. Conclusions The bond enthalpy for dissociation of the water dimer attached to the iodide ion is determined by following the temperature dependence of vibrational features arising from the intact dimer relative to those from independent water molecules bound to the ion. The effective bond enthalpy is about a factor of three lower than that reported for dissociation of the bare water dimer into free monomers.16 This reduction is traced to a three-body distortion of the intrinsic intermolecular (H2O-H2O) potential upon attachment to the ion as well as a large contribution from the vibrational zero-point energy when the free rotation of the water monomers is lost upon formation of the dimer. Acknowledgments MAJ thanks the Air Force Office of Scientific Research (AFOSR) under grant number FA9550-13-1-0007 for the establishment of a temperature controlled ion spectrometer at Yale. JAF thanks the U.S. Department of Defense for support through a National Defense Science and Engineering Graduate Fellowship. KRA and HK acknowledge the support from the German Research Foundation within the Collaborative Research Center 1109. ABM thanks the NSF for support under grant number CHE-1213347. MJ and FP thank the NSF for support under grant number CHE-1305427 (“CCI: Center for Aerosol Impacts on Climate and the Environment”), while FP thanks the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by the National Science Foundation grant number ACI-1053575 (allocation TGCHE110009). DJAA acknowledge the support by MICINN grants No. FIS2011-29596-C02-01 and to the FPI-MEC predoctoral fellowships No. BES-2012-054209 and EEBB-I-14-08152. Supporting Information Available

S1. Comparison of the calculated anharmonic (VPT2) vibrational spectrum of I⁻·(H2O)2 to those of the bare (10 K) and argon-tagged ions and assignments of extra bands in the experimental spectrum of the bare ion as described in table T1. XYZ data and harmonic frequencies obtained with the Gaussian09 software package39 for the three geometries along the slice through the PES, depicted in Fig. 1. S2. Description of the RRKM formalism and the programs used to explain the suppression of low energy bands in the spectrum of the bare complex along with examples of typical input and output data. Boltzmann weighted density of states population for the I⁻·(H2O)2 cluster as a function of temperature. S3. Simulation of the intensity loss observed for the low energy bands using the results from S2 for different dissociation threshold energies. S4. Description of the potential energy surface used in the classical MD simulations. This material is available free of charge via the Internet at http://pubs.acs.org. References 1. Weber, J. M.; Kelley, J. A.; Nielsen, S. B.; Ayotte, P.; Johnson, M. A., Isolating the Spectroscopic Signature of a Hydration Shell using Clusters: Superoxide Tetrahydrate. Science 2000, 287, 2461-2463. 11 ACS Paragon Plus Environment


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TOC Figure


Thermodynamics of Water Dimer Dissociation in the Primary

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