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Vibrational Signatures of Solvent-Mediated Deformation of the Ternary Core Ion in Size-Selected [MgSO4Mg(H2O)n=4−11]2+ Clusters

Joseph W. DePalma, Patrick J. Kelleher, Christopher J. Johnson,† Joseph A. Fournier, and Mark A. Johnson*

Sterling Chemistry Laboratory, Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520, United States S Supporting Information *

ABSTRACT: Elucidation of the molecular-level mechanics underlying the dissolution of salts is one of the long-standing, fundamental problems in electrolyte chemistry. Here we follow the incremental structural changes that occur when water molecules are sequentially added to the ternary [MgSO4Mg]2+ ionic assembly using cryogenic vibrational predissociation spectroscopy of the cold, mass-selected [MgSO4Mg(H2O)n=4−11]2+ cluster ions. Although the bare [MgSO4Mg]2+ ion could not be prepared experimentally, its calculated minimum energy structure corresponds to a configuration where the two Mg2+ ions attach on opposite sides of the central SO42− ion in a bifurcated fashion to yield a D2d symmetry arrangement. Analysis of the observed spectral patterns indicate that water molecules preferentially attach to the flanking Mg2+ ions for the n ≤ 7 hydrates, which results in an incremental weakening of the interaction between the ions. Water molecules begin to interact with the sequestered SO42− anion promptly at n = 8, where changes in the band pattern clearly demonstrate that the intrinsic bifurcated binding motif among the ions evolves into quasilinear Mg2+−O−S arrangements as water molecules H-bond to the now free SO groups. Although condensed-phase MgSO4 occurs with a stable hexahydrate in which water molecules lie between the ion pairs, addition of a sixth water molecule to one of the Mg2+ ions in the n = 11 cluster occurs with the onset of the second hydration shell such that the cation remains coordinated to one of the SO42− oxygen atoms. frequency generation (SFG) measurements36−39 at the air− water interface, which indicate the concentration of Mg2+ is enhanced at the surface, while SO42− ions lie deeper in the bulk, thus raising the issue of how their spectral signatures depend on the local hydration structure surrounding the ions. Our primary objective is to determine how water molecules bind to a “salt bridge” [MgSO4Mg]2+ ternary ion core and to explore how hydration affects the binding between the ions.34 In particular, we follow the incremental structural changes that occur upon sequential addition of water molecules in the cold, size-selected cluster regime through analysis of the vibrational signatures displayed by both the embedded SO42− ion and the H2O solvent molecules in [MgSO4Mg(H2O)n=4−11]2+ clusters. It is of particular interest to investigate the competition between insertion of water molecules between the ions and addition to outer hydration shells around the ternary ion core.

1. INTRODUCTION Although the solvent-induced charge separation of the ionic constituents in inorganic salts has been at the foundation of electrolyte chemistry since the idea was introduced by Arrhenius over 130 years ago,1 the accurate molecular-level mechanics underlying this process have only recently been addressed as experimental and theoretical methods become equal to the task.2−7 On the experimental side, early efforts by Castleman and co-workers8−10 failed to identify solventinduced ion separation in a series of elegant cluster studies that monitored the dipole moments of HNO3−H2O−NH3,8 NH4HS−NH3,8 acetic acid,9 and methanol/ethanol10 clusters with a molecular beam electric deflection method. Recent experimental advances in cluster ion spectroscopy, however, now routinely allow such assemblies to be generated, cooled near their vibrational zero-point levels, and structurally characterized with vibrational spectroscopy, thus providing direct observables with which to benchmark calculated behavior.11−21 Here we are concerned with the fundamental interactions between the MgSO4 salt and water, for which many experimental and theoretical studies have been performed to understand the hydration structures, energetics, and dynamics that mediate solvent-induced ion separation.22−34 Magnesium sulfate is a highly soluble salt with a Ksp of 4.67 at 25 °C,35 which makes it a particularly attractive candidate for following the onset of dissolution in the microhydration regime. Our interest in this system is also motivated by vibrational sum © XXXX American Chemical Society

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS Cryogenic ion vibrational predissociation (CIVP) spectra of size-selected [MgSO4Mg(H2O)n]2+ (n = 4−11) clusters were obtained using Yale’s double-focusing tandem time-of-flight photofragmentation mass spectrometer, which has been described in detail elsewhere.40,41 Briefly, ions were produced Received: May 13, 2015 Revised: June 30, 2015

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DOI: 10.1021/acs.jpca.5b04612 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A via electrospray ionization (ESI) of ∼10 mM magnesium sulfate in water in a humidity-controlled purge chamber. Hydrated ions were transported through differential pumping stages by quadrupole and octopole ion guides into a cryogenic, three-dimensional quadrupole ion trap (R. M. Jordan) held at 20 K.41,42 He buffer gas with a trace amount of D2 was pulsed into the trap to slow and collisionally cool the ions to ∼20 K; this temperature was optimized for the condensation of D2 molecules onto the cluster ions. Cluster ions were accumulated in the ion trap for ∼100 ms, ‘tagged’ during the accumulation time, then were ejected into the first time-of-flight region of the tandem mass spectrometer. Only clusters in the size range of n = 4−11 could be prepared in sufficient abundance for spectroscopic characterization using this protocol. The smaller clusters displayed a very rapid falloff intensity below n = 4, with the n = 3 cluster detectable but weak as illustrated by the parent ion mass spectrum in Supporting Information, Figure S1. A particular m/z target ion was then selectively irradiated with infrared light produced from a 10 Hz pulsed OPO/OPA laser system (LaserVision). The resonant absorption of a single photon caused efficient evaporation of the weakly bound D2 “tag,” producing a fragment ion that was monitored after separation by a second stage of mass selection. This fragment signal was monitored as a function of continuously scanned photon energy to yield linear action spectra.43 The spectra reported here are averages of ∼10 individual scans and include corrections for fluctuations in laser pulse energy over the scan range. The effective resolution of the spectrometer is ∼4 cm−1 (FWHM) due to laser bandwidth and scan reproducibility. Calculation of cluster minimum geometries and harmonic frequencies was performed using the B3LYP/6-311++G(d,p) level of theory as implemented in Gaussian09.44 The resulting frequencies for each minimum structure were scaled by 0.983 in the fingerprint region and 0.96 in the OH region. These scale factors were determined by taking the ratio of the experimental band position to the harmonic prediction for the water bend and antisymmetric OH stretch. The use of multiple scaling factors for OH versus heavy particle fundamentals is a common practice.45,46 It is known, however, that the 0 K harmonic approximation may be inadequate when predicting the behavior of strongly hydrogen bonded systems. A recent example is the large discrepancy between observed and harmonic spectra in the hydrated dihydrogen phosphate clusters which was traced to large structural fluctuations at finite temperature.47 Because of this, the present analysis relies largely on the behavior of the SO stretching and intramolecular HOH bending vibrations, which are much less susceptible to such anharmonic effects. Figure S2 in the Supporting Information offers a comparison of the scaled versus unscaled harmonic predictions for the [MgSO4Mg(H2O)4]2+ cluster. To ensure that the calculated frequencies can be compared directly to the experimental linear action spectra, the predicted IR intensities are divided by their associated vibrational frequencies.48

degenerate f 2 SO4 antisymmetric stretching mode. This suggests the observed SO4 stretching pattern should be an excellent reporter for subtle asymmetries in the local environment of the anion when it is sequestered within a microhydrated ionic cluster, and indeed this aspect has been exploited in previous cluster studies.12,13,17−19,49,53 A calculated spectrum for a locally stable, minimum-energy Td SO42− structure is presented in Figure 1A, indicating the expected location of the antisymmetric f 2 SO stretching fundamentals and the OSO intramolecular bend.

Figure 1. Comparison of the symmetry of (a) the bare sulfate tetrahedron and (b) the anhydrous [MgSO4 Mg]2+complexes. Structures and spectra were calculated with B3LYP/6-311++G(d,p). Yellow are sulfur atoms, red are oxygen atoms, and pink are Mg2+ ions.

The [MgSO4Mg]2+ ion central to this study has not been isolated as a bare ion, perhaps because the formation mechanism here involves evaporation of water from the larger [MgSO4Mg(H2O)n]2+ ions in the ESI ion source.54 One likely possibility is that the very small clusters undergo fragmentation into H3O+(H2O)m and (MgOH)+(H2O)n−m−2 singly charged ions, which is known to be the fragmentation mechanism of the related Mg2+(H2O)n clusters starting at n = 3.55,56 A minimumenergy structure of the ternary ion can be identified with electronic structure calculations at the B3LYP/6-311++G(d,p) level, however, and it is predicted to adopt the configuration depicted in Figure 1B. This structure features the two Mg2+ ions flanking the central SO42− ion and attached to it in a bifurcated fashion, that is, equidistant from two of the oxygen atoms. As a point of reference, the computed full natural bond orbital (NBO) population analysis for this species places +1.83e on each Mg2+, +2.43e on the S atom, and −1.022e on each of the four O atoms, which yield a net +2 charge of the bare cluster. We note this composition has been suggested as the “triple ion” in conductance studies on bulk MgSO 4 solutions.32,57 The coordination of the Mg2+ ions to the SO42− ions in the calculated bifurcated bonding motif would therefore act to lower the symmetry of the SO42− ion from Td into D2d. The retention of D2d symmetry is evidenced by the calculated

3. RESULTS AND DISCUSSION 3.1. Theoretical Expectations for the Structure and Spectra of the [MgSO4Mg]2+ Ion Core. Although the isolated SO42− ion is unstable with respect to electron ejection,49,50 in sufficiently symmetrical anhydrous MgSO4 crystals,51 the sulfate ion adopts tetrahedral (Td) symmetry such that the SO stretching manifold is composed of a and f species.52 Consequently, the IR spectrum associated with this motion should consist of a single peak arising from the triply B


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The experimental [MgSO4Mg(H2O)4]2+ spectrum is in remarkable agreement with the harmonic prediction for the symmetrical structure shown in Figure 2. Specifically, the highenergy OH stretches are sharp, with the symmetric (νs) and antisymmetric (νas) fundamentals appearing as single peaks at 3604 and 3665 cm−1, respectively, as expected for equivalent, largely noninteracting water molecules. Similarly, the HOH bend appears as a single peak at 1650 cm−1. Note that in this high-symmetry structure, the water-based features are composed of essentially degenerate, collective motions involving both in-phase and out-of-phase displacements on nearly equivalent water molecules. In the lower-energy SO stretching region, two main bands are observed with a separation (∼77 cm −1 ) close to the harmonic prediction for e and b 2 fundamentals (115 cm −1 ), consistent with a local D 2d environment for SO42−. The lower energy e feature (at 1111 cm−1) is slightly split (5 cm−1) to form a close doublet below the b2 fundamental at 1188 cm−1. The presence of this small splitting both reinforces its assignment as the doubly degenerate e fundamental and also indicates the system is not rigorously D2d. Calculations indicate this residual symmetry breaking is due to the perturbation induced by the D2 tag, as it is predicted to attach close to one of the Mg2+ ions as well as one of the SO2 groups, as displayed in Supporting Information, Figure S4. 3.3. Survey of the Vibrational Spectra for [MgSO4Mg(H2O)n]2+, (n = 4−11). Figure 3 presents the evolution of the predissociation spectra with increasing hydration for the

decrease of the O−S−O bond angles from 109.5° in the tetrahedral structure to 101° upon Mg2+ attachment to two proximal oxygen atoms on opposite sides of the anion. This reduction in symmetry acts to split the triply degenerate f 2 SO4 antisymmetric stretch into a doubly degenerate e SO 2 antisymmetric stretch (nearly isoenergetic to the f 2) and an isolated b2 SO4 antisymmetric stretch fundamental higher in energy. The B3LYP calculated harmonic spectrum for the bare [MgSO4Mg]2+ cluster, shown in Figure 1B, predicts a 115 cm−1 splitting between the e and b2 fundamentals.

Figure 2. Vibrational predissociation spectra (upper) and computed harmonic spectra (lower) for the [MgSO4Mg(H2O)4]2+ cluster. Predicted transitions and experimental positions are labeled. Structure calculated with B3LYP/6-311++G(d,p) with the color scheme identical to that in Figure 1 with the addition of white for hydrogen atoms.

3.2. Observation of Symmetrical Hydration in [MgSO4Mg(H2O)4]2+. The top trace in Figure 2 presents the experimental predissociation spectrum for the D2-tagged [MgSO4Mg(H2O)4]2+ cluster. Figure 2 also displays the calculated (B3LYP/6-311++G(d,p)) minimum-energy structure and associated harmonic spectrum (inverted bottom trace). This structure retains the bifurcated binding of Mg2+ to the sulfate ion calculated for the bare species and features water molecule attachment only to the Mg2+ ions and in a symmetrical arrangement (i.e., with two on each Mg2+). We note that this (2:2) arrangement lies much lower (8.33 and 39.6 kcal/mol) in energy than antisymmetric hydration configurations (3:1 and 4:0, respectively), which are also local minima on the potential energy surface. The relative energies and structures are included in Supporting Information, Figure S3. It is likely this difference in energy leads to the selective preparation of the symmetrical system under the ambient evaporation conditions at play in the ion source as discussed above.

Figure 3. Experimental vibrational predissociation spectra of the [MgSO4Mg(H2O)n]2+ clusters for n = 4−11. The spectrum for n = 11 was obtained via IRMPD due to degrading ion signal. νDD is tentatively assigned to a double H-bond donor water molecule in the second hydration shell around one of the Mg2+ ions. C


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The Journal of Physical Chemistry A Table 1. Observed Vibrational Transitions (4 cm−1 FWHM) and Computed Harmonic Predictions for the [MgSO4Mg(H2O)n=4‑7]2+ Clusters SO2 antisym

SO4 antisym

clustera

observed (cm−1)

harmonic (cm−1)

observed (cm−1)

harmonic (cm−1)

W4 W5 W6 W7 W8 W9 W10 W11

1112 1097; 1135 1119 1100; 1142 1123 1123 1121 1120

1033 1013; 1066 1046 1017; 1058 1050 1040 1039 1035

1188 1188 1188 1186 1152 1158 1159 1158

1120 1122 1103 1102 1078 1081 1082 1091

HOH bend observed (cm−1) 1651 1638 1635 1631 1624; 1621; 1621; 1619;

1646 1644 1656 1652

OH sym

harmonic (cm−1)

observed (cm−1)

harmonic (cm−1)

1643 1629 1659 1658 1615; 1641 1611, 1647 1612; 1655 1605; 1660

3605 3613 3625 3625 3623 3611; 3633 3632 3629

3622 3639 3586 3588 3644 3645; 3660 3659 3662

OH antisym observed (cm−1) 3665 3675; 3700 3700 3692; 3702; 3704; 3714

3698

3712 3714 3718

harmonic (cm−1) 3700 3724; 3675 3679 3725; 3728; 3750; 3750

3695

3738 3744 3729

a The computed values are from scaled (0.983 for the fingerprint region and 0.96 for the OH region) B3LYP/6-311++G(d,p) calculations. Wn notation refers to the number of water molecule in the [MgSO4Mg(H2O)n]2+ clusters.

Figure 4. Calculated (B3LYP/6-311++G(d,p)) structures of the [MgSO4Mg(H2O)n=4−7]2+ clusters. O−S−O bond angles and Mg2+−S distances labeled in figure and summarized in the inset table. Table gives angles and distances for the left and right structural elements. See Figure 2 caption for color scheme.

[MgSO4Mg(H2O)n]2+, (n = 4−11) clusters. We note the spectrum for the n = 11 cluster is an IRMPD spectrum obtained by monitoring the loss of a single water molecule to generate action spectra and not through predissociation of a D2 tag due to degrading signal intensity. In general, bands present in the n = 4 cluster persist with size-dependent multiplet structure, while a new, broad feature emerges in the larger hydrates (n ≥ 8) around 3000 cm−1, signaling the onset of hydrogen bonding interactions that we describe in detail below. In the n = 11 cluster, a distinct sharp peak (νDD at 3450 cm−1) appears in the OH stretching region, while the remainder of the spectrum remains largely the same as those displayed by the n = 8−10 clusters. There is a clear break in the behaviors of both the SO stretching and HOH intramolecular bending regions in going n = 7 to 8, where the water HOH intramolecular bend splits into a doublet that persists at larger sizes with a concurrent change in the multiplet pattern in the SO stretching manifold. We first address the early hydration regime (n = 4− 7) in which the water-based features are relatively stable but the SO bands undergo an interesting alternation in pattern for the

even and odd hydrates. The band locations for all of these species are collected in Table 1. 3.3.1. Symmetry Breaking in the SO42− Ion by Asymmetric Hydration of Mg2+ Ions with Retention of the Bifurcated Metal Ion Attachment Motif in the n = 4−7 Clusters. While the higher energy, nondegenerate SO4 antisymmetric stretch occurs at roughly the same intensity and location throughout this size range (∼1188 cm−1), the lower-energy feature alternates between a single band for n = 4 and 6 and an open doublet that brackets this band for n = 5 and 7. (We noted earlier that the small splitting of the e band in the n = 4 spectrum can be traced to tag attachment close to the anion). The origin of this behavior is clear from the calculated minimum-energy structures and harmonic predictions, presented in Figures 4 and 5, respectively. Consistent with the persistence of the nondegenerate b2 band, these structures retain the bifurcated Mg2+ attachment motif, and the water molecules are all confined to the metal ions with minimal asymmetry in their distribution among the two ions (i.e., no D


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readily understood in the context of the predicted attachment sites as presented in Supporting Information, Figures S4−S6. A comparison of the D2 and N2 tagged predissociation spectra are included in Supporting Information, Figure S6, demonstrating that the D2 tag is even less perturbing than is calculated, while the N2 shifts are in line with harmonic predictions. As noted above, the splitting in the e species SO2 antisymmetric stretching band is traced to asymmetric D2 docking near the anion core. At larger sizes, the tag location is calculated to migrate onto the one of the OH groups of a water molecule bound to Mg2+. This leads to the observed doubling of the symmetric and antisymmetric OH stretches, as is typical for cation hydration.58−61 3.3.2. Water-Mediated Emergence of the Single-Contact Mg2+ Attachment Motif in the n = 8−10 Clusters. We now address the larger hydration regime (n = 8−10), where significant changes occur across the entire spectrum. For example, upon the addition of the eighth H2O molecule, the SO stretching bands change to a more closely spaced (∼30 cm−1) doublet that persists throughout this size range, and the upper member is red-shifted by ∼30 cm−1 compared to the analogous band in the n = 4−7 clusters. This major shift in the SO stretching region signals a significant change in the local environment around the core SO42− ion at n = 8 and is common to the larger hydrates. Concurrently, the HOH bend splits into two bands (labeled A and B in Figures 3 and 6) at n = 8 with a spacing of ∼25 cm−1, which also persists in the Figure 5. Comparison of the predissociation spectra and harmonic (B3LYP/6-311++G(d,p)) predictions for the [MgSO 4 Mg(H2O)n=4−7]2+ clusters.

more than one H2O difference between the two Mg2+ ions, which is of course unavoidable for odd hydrates). The key structural features underlying the spectral behavior of the core ion are the response of the Mg2+−S distance and the O−S−O bond angle with increasing hydration on the Mg2+ ions, which are included in the inset in Figure 4. In essence, increasing the number of water molecules attached to the metal ion acts to reduce the interaction strength between the metal and the sulfate ion. This, in turn, results in the lengthening of the distance between the Mg2+ ions and the S atom of the sulfate ion core and a concomitant increase of the O−S−O bond angle. When there are different numbers of water molecules on each Mg2+, this causes a symmetry breaking due to the different angles of the two SO2 domains bound to the cations. This uneven coordination on the Mg2+ ions in the oddnumbered hydrates has the effect of further reducing the symmetry of the core sulfate ion from D2d to C2v where there are three IR active species, thus accounting for the triplet structure of the n = 5 and 7 spectra. When symmetry is restored at n = 4 and 6, the D2d doublet is recovered. The harmonic spectra capture these effects very accurately, as evidenced by the comparison in Figure 5. This type of asymmetry-induced splitting of the sulfate Td triply degenerate SO4 antisymmetric stretch has been observed previously in the hydrated sulfate clusters,19,49,53 and related symmetry breaking in degenerate bands structures due to asymmetric solvation effects have been reported in hydration of the bisulfate ion as well as Mg(NO3)(H2O)n+ clusters.12,13,17,21 Although they do not affect the main conclusions about the structures operative in this size regime, we note there are significant tag effects in this size range (n = 4−7) that are

Figure 6. Calculated (B3LYP/6-311++G(d,p)) minimum-energy structures of larger [MgSO4Mg(H2O)n=8−10]2+ clusters. “A” water molecules are free, and “B” water molecules form a hydrogen bond with the sulfate anion core. Mg2+−S bond distances are included in the table (left, right). Green dotted lines indicate hydrogen bonds. E


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Mg2+ binding arrangement persists for the n = 9 and 10 clusters. Note that with the additional water molecules, one (n = 9) and then both (n = 10) Mg2+ ions reside in hexacoordinated environments, which is the preferred morphology for ligands around Mg2+.62,63 From the structural standpoint, the Mg2+−S distance continues to increase with hydration in this sequential fashion, as quantified by the values in the table inset in Figure 6. 3.4. Onset of the Second Hydration Shell: Comparison with the Solvent Separated Ion Pairs in the Bulk Hexahydrate Crystal. At n = 10, both Mg2+ ions reside in hexacoordinated sites with five of the oxygen ligands coming from water and one from sulfate. As such, addition of the 11th water molecule raises the issue of whether a water molecule can displace the SO group to form the hexahydrate, which is tethered to the anion through water H-bonds and thus corresponds to a solvent-separated ion pair motif. This arrangement is actually the stable form of the mineral hexahydrite, the hexahydrate MgSO4 crystal, as established by X-ray crystallography.64 If a water molecule cannot displace SO in the hexacoordinate coordination shell, it would appear that it could either begin a second solvation shell around Mg2+ or attach directly to an oxygen atom of sulfate. We were able to identify minimum-energy structures corresponding to the solvent-separated ion pair (SSIP), with a representative isomer displayed in Figure 8b, but these were calculated to occur at

spectra of the larger clusters. Finally, the n = 8 spectrum also displays the onset of very broad absorptions (spanning several hundred wavenmbers) around ∼3000 cm−1, consistent with the onset of strong ionic hydrogen bonding. The splitting in the H2O bending feature indicates the presence of two different classes of H2O molecules beginning at the n = 8 cluster, which in turn suggests the additional H2O molecule(s) are causing the ion contact motif to change when there are four or more on each Mg2+ ion. This empirical conclusion is well-supported by the calculated minimum-energy structures and harmonic predictions shown in Figures 6 and 7.

Figure 7. Predissociation (upper) and computed (lower) spectra for the larger [MgSO4Mg(H2O)n=8−10]2+ clusters. Spectra calculated with B3LYP/6-311++G(d,p).

At n = 8, the bifurcated bonding motif that drove the symmetry of the sulfate ion core in the smaller cluster hydrates has been replaced by a “quasilinear” bonding motif between the sulfate and Mg2+ ions. In this arrangement, the cations bind preferentially to a single SO group, which leaves two of the SO4 oxygen atoms free to interact with the water molecules. This scenario creates two classes of water molecules, where the A molecules in Figure 6 are tethered to the Mg2+ ions as in the smaller clusters, while the B water molecules bridge the Mg2+ and SO groups with a single dangling OH group. At n = 8, four of the water molecules are H-bonded to the sulfate ion, and the driving force for this radical morphology change appears to be driven by the competition between the Mg2+ ions binding to sulfate oxygen atoms or the formation H-bonds between the sulfate oxygen atoms and nearby water molecules. The computed structures indicate that it requires four of the latter (B-type) molecules to offset the intrinsic preference for the bifurcated binding motif. Once the four-fold hydration motif is established at n = 8, the next water molecules appear to add in A-type sites, thus accounting for the increasing intensity of the corresponding peak in the bending doublet from n = 8−10. The persistence of the telltale HOH bend and SO antisymmetric stretch doublets indicate that the single contact

Figure 8. IRMPD spectrum of [MgSO4Mg(H2O)11]2+ (upper) and harmonic structures and spectra for (b) the solvent-separated ion pair and (c) second solvation shell binding arrangements. The water molecules that donate a hydrogen bond to the second solvation shell give rise to the νDD (DD for double donor) bands through the inphase (νDD‑IP) and out of phase (νDD‑OOP) motion of the donor OH groups. Spectra calculated with B3LYP/6-311++G(d,p).

much higher (>3.12 kcal/mol) energy relative to the global minimum shown in Figure 8c, which results from formation of the second hydration shell around the cation. Searching for a structure with a water molecule on the sulfate ion core resulted in the location of higher-energy local minimum structures relative to our identified global minimum structure for the formation of the second hydration shell. The calculated spectra for the SSIP and second hydration shell isomers are displayed in Figure 8b and c, respectively, and the telltale feature that should readily distinguish which is F


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present are the strong feature(s) in the OH stretching region (νDD) arising from the OH groups donating H-bonds to the second shell water molecule. As noted in Section 3.3, the n = 11 spectrum was obtained by IRMPD of the untagged cluster due to degrading signal intensity. The experimental spectrum is displayed in Figure 8a, which exhibits many similar features to those from the n = 8−10 clusters that feature the single contact binding motif between the ions. The most striking feature in the spectrum is the appearance of a new sharp peak in the OH stretching region at 3540 cm−1, lower in energy than the vibrations observed thus far for the free OH groups. While there are no peaks expected in this region for the SSIP (Figure 8b), there is a doublet in the calculated spectrum for the second solvation shell structure. This doublet corresponds to the inphase and out-of-phase OH stretches, which are labeled as νDD‑IP and νDD‑OOP (DD for double donor), respectively, connecting the second shell water molecule to the cluster. These stretches arise from the two water molecules H-bonding to the 11th water molecule, with the lower-energy band (3600 cm−1) corresponding to the out-of-phase OH stretches and the higher-energy band (3646 cm−1) to the in-phase OH stretches. The weaker νDD‑OOP band is not observed in the experimental spectrum for n = 11. We note that this is an IRMPD spectrum, and as such the spectrum may be complicated by the transparency effect if oscillator strength at the one-photon level is suppressed after absorption of the first photon.12,13,65 The water binding energies on the bare Mg2+(H2O)n=3=10 clusters have been reported to fall rapidly after completion of the first hydration shell at n = 6, converging on a value around 4000 cm−1 by n = 10, which is approaching the photon energy of the free OH stretching bands in question.56,66−70 Although not definitive because of these experimental limitations, the data are most consistent with the 11th water molecule starting the onset of the second hydration shell around one of the Mg2+ ions, which is consistent with the expectation that this form corresponds to the global minimum.

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*E-mail: [email protected]. Present Address †

Department of Chemistry, Stony Brook University, Stony Brook, NY 11794. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge C. M. Leavitt for his contributions in the early phases of this work. J.W.D. and M.A.J. gratefully acknowledges financial support from the National Science Foundation through the Centers of Chemical Innovation Program under Grant No. CHE1305427. C.J.J. gratefully acknowledge support from the National Science Foundation American Competitiveness in Chemistry Fellowship (Grant No. CHE-1137404). Construction of the apparatus used to collect data was funded in part by the Air Force Office of Scientific Research (AFOSR) chemical dynamics program through Grant No. FA9550-13-1-0007. Calculations in this work were supported by the facilities and staff of the Yale University Faculty of Arts and Sciences High Performance Computing Center.

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REFERENCES

(1) Arrhenius, S. A. Ueber die Dissociation der im Wasser Gelösten Stoffe. J. Phys. Theor. Appl. 1887, 1, 631. (2) Jungwirth, P.; Tobias, D. J. Ions at the Air/Water Interface. J. Phys. Chem. B 2002, 106, 6361−6373. (3) Jungwirth, P.; Tobias, D. J. Molecular Structure of Salt Solutions: A New View of the Interface with Implications for Heterogeneous Atmospheric Chemistry. J. Phys. Chem. B 2001, 105, 10468−10472. (4) Tobias, D. J.; Jungwirth, P.; Parrinello, M. Surface Solvation of Halogen Anions in Water Clusters: An Ab Initio Molecular Dynamics Study of the Cl−(H2O)6 Complex. J. Chem. Phys. 2001, 114, 7036− 7044. (5) Callahan, K. M.; Casillas-Ituarte, N. N.; Roeselova, M.; Allen, H. C.; Tobias, D. J. Solvation of Magnesium Dication: Molecular Dynamics Simulation and Vibrational Spectroscopic Study of Magnesium Chloride in Aqueous Solutions. J. Phys. Chem. A 2010, 114, 5141−5148. (6) Marx, D.; Chandra, A.; Tuckerman, M. E. Aqueous Basic Solutions: Hydroxide Solvation, Structural Diffusion, and Comparison to the Hydrated Proton. Chem. Rev. 2010, 110, 2174−2216. (7) Tuckerman, M. E.; Marx, D.; Parrinello, M. The Nature and Transport Mechanism of Hydrated Hydroxide Ions in Aqueous Solution. Nature 2002, 417, 926−929. (8) Kay, B. D.; Hofmann-Sievert, R.; Castleman, A. W. A MolecularBeam Electric Deflection Study of Clusters: Acid and Salt Molecules in Microscopic Aqueous and Ammonia Clusters. Chem. Phys. 1986, 102, 407−415. (9) Sievert, R.; Cadez, I.; Van Doren, J.; Castleman, A. W. Mass Spectral and Electric Deflection Study of Acetic Acid Clusters. J. Phys. Chem. 1984, 88, 4502−4505. (10) Kay, B. D.; Castleman, A. W. Molecular-Beam Electric Deflection Study of the Hydrogen-Bonded Clusters (H2O)N, (CH3OH)N, and (C2H5OH)N. J. Phys. Chem. 1985, 89, 4867−4868. (11) Johnson, C. J.; Johnson, M. A. Vibrational Spectra and Fragmentation Pathways of Size-Selected, D2-Tagged Ammonium/ Methylammonium Bisulfate Clusters. J. Phys. Chem. A 2013, 117, 13265−13274.

4. CONCLUSIONS We have investigated the vibrational spectral spectra of the cold (20 K) [MgSO4Mg(H2O)n=4−11]2+ cluster ions to establish how water molecules attach to a ternary salt bridge with divalent ions. Analysis of the spectra indicates that the first four water molecules fill the first solvation shell around the two Mg2+ ions, which flank the core sulfate ion in a bifurcated binding geometry. When n is even, these water molecules are evenly distributed between the two cations, but the odd clusters break the inherent D2d geometry of the [MgSO4Mg]2+ core ion to C2v where one Mg2+ has one more water than the other. This effect is evidenced by a strong alternation in the multiplet structure of the SO4 stretching bands from n = 4 to 7. Upon the addition of an eighth water molecule, the bifurcated binding motif in the ion core gives way to a quasilinear arrangement with concomitant formation of a partial H-bond network. When an additional water molecule is added to the n = 10 cluster, which contains two fully coordinated Mg2+ ions, it begins the second solvation shell around the cation. The latter result is in contrast to preferred arrangement of the bulk hexahydrate crystal, which features an SSIP motif.

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The Journal of Physical Chemistry A (12) Yacovitch, T. I.; Heine, N.; Brieger, C.; Wende, T.; Hock, C.; Neumark, D. M.; Asmis, K. R. Vibrational Spectroscopy of Bisulfate/ Sulfuric Acid/Water Clusters: Structure, Stability, and Infrared Multiple-Photon Dissociation Intensities. J. Phys. Chem. A 2013, 117, 7081−7090. (13) Yacovitch, T. I.; Wende, T.; Jiang, L.; Heine, N.; Meijer, G.; Neumark, D. M.; Asmis, K. R. Infrared Spectroscopy of Hydrated Bisulfate Anion Clusters: HSO4−(H2O)1−16. J. Phys. Chem. Lett. 2011, 2, 2135−2140. (14) Rozenberg, M.; Loewenschuss, A.; Nielsen, C. J. Trimethylamine/Sulfuric Acid/Water Clusters: A Matrix Isolation Infrared Study. J. Phys. Chem. A 2014, 118, 1004−1011. (15) Rozenberg, M.; Loewenschuss, A.; Nielsen, C. J. Complexes of Molecular and Ionic Character in the Same Matrix Layer: Infrared Studies of the Sulfuric Acid/Ammonia System. J. Phys. Chem. A 2011, 115, 5759−5766. (16) Rozenberg, M.; Loewenschuss, A. Matrix Isolation Infrared Spectrum of the Sulfuric Acid-Monohydrate Complex: New Assignments and Resolution of the ″Missing H-Bonded υ(OH) Band″ Issue. J. Phys. Chem. A 2009, 113, 4963−4971. (17) Yacovitch, T. I.; Heine, N.; Brieger, C.; Wende, T.; Hock, C.; Neumark, D. M.; Asmis, K. R. Communication: Vibrational Spectroscopy of Atmospherically Relevant Acid Cluster Anions: Bisulfate Versus Nitrate Core Structures. J. Chem. Phys. 2012, 136, 136. (18) Asmis, K. R.; Neumark, D. M. Vibrational Spectroscopy of Microhydrated Conjugate Base Anions. Acc. Chem. Res. 2012, 45, 43− 52. (19) Miller, Y.; Chaban, G. M.; Zhou, J.; Asmis, K. R.; Neumark, D. M.; Gerber, R. B. Vibrational Spectroscopy of SO42‑·(H2O)N Clusters, N=1−5: Harmonic and Anharmonic Calculations and Experiment. J. Chem. Phys. 2007, 127, 094305/1−094305/11. (20) Heine, N.; Kratz, E. G.; Bergmann, R.; Schofield, D.; Asmis, K. R.; Jordan, K. D.; McCoy, A. B. Vibrational Spectroscopy of the Water-Nitrate Complex in the OH Stretching Region. J. Phys. Chem. A 2014, 118, 8188−8197. (21) Jiang, L.; Wende, T.; Bergmann, R.; Meijer, G.; Asmis, K. R. Gas-Phase Vibrational Spectroscopy of Microhydrated Magnesium Nitrate Ions [MgNO3(H2O)1−4]+. J. Am. Chem. Soc. 2010, 132, 7398− 7404. (22) Jahn, S.; Schmidt, C. Speciation in Aqueous MgSO4 Fluids at High Pressures and High Temperatures from Ab Initio Molecular Dynamics and Raman Spectroscopy. J. Phys. Chem. B 2010, 114, 15565−15572. (23) Yan, H.; Yuan, S. L.; Xu, G. Y.; Liu, C. B. Effect of Ca2+ and Mg2+ Ions on Surfactant Solutions Investigated by Molecular Dynamics Simulation. Langmuir 2010, 26, 10448−10459. (24) Matthews, R. P.; Naidoo, K. J. Experimentally Consistent Ion Association Predicted for Metal Solutions from Free Energy Simulations. J. Phys. Chem. B 2010, 114, 7286−7293. (25) Ghoufi, A.; Malfreyt, P. Calculations of the Potential of Mean Force from Molecular Dynamics Simulations Using Different Methodologies: An Application to the Determination of the Binding Thermodynamic Properties of an Ion Pair. Mol. Phys. 2006, 104, 3787−3799. (26) Rudolph, W. W.; Irmer, G.; Hefter, G. T. Raman Spectroscopic Investigation of Speciation in MgSO4(aq). Phys. Chem. Chem. Phys. 2003, 5, 5253−5261. (27) Pye, C. C.; Rudolph, W. W. An Ab Initio and Raman Investigation of Sulfate Ion Hydration. J. Phys. Chem. A 2001, 105, 905−912. (28) Pye, C. C.; Rudolph, W. W. An Ab Initio and Raman Investigation of Magnesium (II) Hydration. J. Phys. Chem. A 1998, 102, 9933−9943. (29) Tomsic, M.; Bester-Rogac, M.; Jamnik, A.; Neueder, R.; Barthel, J. Conductivity of Magnesium Sulfate in Water from 5 to 35 Degrees C and from Infinite Dilution to Saturation. J. Solution Chem. 2002, 31, 19−31.

(30) Rull, F.; Balarew, C.; Alvarez, J. L.; Sobron, F.; Rodriguez, A. Raman-Spectroscopic Study of Ion Association in Aqueous Magnesium-Sulfate Solutions. J. Raman Spectrosc. 1994, 25, 933−941. (31) Zhang, Y. H.; Chan, C. K. Study of Contact Ion Pairs of Supersaturated Magnesium Sulfate Solutions Using Raman Scattering of Levitated Single Droplets. J. Phys. Chem. A 2000, 104, 9191−9196. (32) Marcus, Y.; Hefter, G. Ion Pairing. Chem. Rev. 2006, 106, 4585− 4621. (33) Choi, M. Y.; Chan, C. K. Continuous Measurements of the Water Activities of Aqueous Droplets of Water-Soluble Organic Compounds. J. Phys. Chem. A 2002, 106, 4566−4572. (34) Zhang, X. C.; Zhang, Y. H.; Li, Q. S. Ab Initio Studies on the Chain of Contact Ion Pairs of Magnesium Sulfate in Supersaturated State of Aqueous Solution. J. Mol. Struct.: THEOCHEM 2002, 594, 19−30. (35) Haynes, W. M. CRC Handbook of Thermophysical and Thermochemical Data; CRC Press: Boca Raton, FL, 2013. (36) Jubb, A. M.; Hua, W.; Allen, H. C. Environmental Chemistry at Vapor/Water Interfaces: Insights from Vibrational Sum Frequency Generation Spectroscopy. Annu. Rev. Phys. Chem. 2012, 63, 107−130. (37) Jubb, A. M.; Hua, W.; Allen, H. C. Organization of Water and Atmospherically Relevant Ions and Solutes: Vibrational Sum Frequency Spectroscopy at the Vapor/Liquid and Liquid/Solid Interfaces. Acc. Chem. Res. 2012, 45, 110−119. (38) Verreault, D.; Hua, W.; Allen, H. C. From Conventional to Phase-Sensitive Vibrational Sum Frequency Generation Spectroscopy: Probing Water Organization at Aqueous Interfaces. J. Phys. Chem. Lett. 2012, 3, 3012−3028. (39) Jubb, A. M.; Allen, H. C. Bisulfate Dehydration at Air/Solution Interfaces Probed by Vibrational Sum Frequency Generation Spectroscopy. J. Phys. Chem. C 2012, 116, 13161−13168. (40) Kamrath, M. Z.; Relph, R. A.; Guasco, T. L.; Leavitt, C. M.; Johnson, M. A. Vibrational Predissociation Spectroscopy of the H2Tagged Mono- and Dicarboxylate Anions of Dodecanedioic Acid. Int. J. Mass Spectrom. 2011, 300, 91−98. (41) Wolk, A. B.; Leavitt, C. M.; Garand, E.; Johnson, M. A. Cryogenic Ion Chemistry and Spectroscopy. Acc. Chem. Res. 2014, 47, 202−210. (42) Wang, X. B.; Wang, L. S. Development of a Low-Temperature Photoelectron Spectroscopy Instrument Using an Electrospray Ion Source and a Cryogenically Controlled Ion Trap. Rev. Sci. Instrum. 2008, 79, 073108. (43) Johnson, M. A. In Encyclopedia of Mass Spectrometry; Armentrout, P. B., Ed.; Elsevier: San Diego, CA, 2003; Vol. 1. (44) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian 09, Revision D.01; Gaussian, Inc.: Wallingford, CT, 2009. (45) Laury, M. L.; Carlson, M. J.; Wilson, A. K. Vibrational Frequency Scale Factors for Density Functional Theory and the Polarization Consistent Basis Sets. J. Comput. Chem. 2012, 33, 2380− 2387. (46) Halls, M. D.; Velkovski, J.; Schlegel, H. B. Harmonic Frequency Scaling Factors for Hartree-Fock, S-VWN, B-LYP, B3-LYP, B3-PW91 and MP2 with the Sadlej pvtz Electric Property Basis Set. Theor. Chem. Acc. 2001, 105, 413−421. (47) Sun, S.-T.; Jiang, L.; Liu, J.-W.; Heine, N.; Yacovitch, T. I.; Wende, T.; Asmis, K. R.; Neumark, D. M.; Liu, Z.-F. Large Amplitude Motion in Cold Monohydrated Dihydrogen Phosphate Anions H2PO4-(H2O): Infrared Photodissociation Spectroscopy Combined with Ab Initio Molecular Dynamics Simulations. Phys. Chem. Chem. Phys. 2015, 16, 1314−1318. (48) DeBlase, A. F.; Bloom, S.; Lectka, T.; Jordan, K. D.; McCoy, A. B.; Johnson, M. A. Origin of the Diffuse Vibrational Signature of a Cyclic Intramolecular Proton Bond: Anharmonic Analysis of Protonated 1,8-Disubstituted Naphthalene Ion. J. Chem. Phys. 2013, 139, 024301. H


Article


(69) Rodriguez-Cruz, S. E.; Jockusch, R. A.; Williams, E. R. Hydration Energies of Divalent Metal Ions, Ca2+(H2O)N (N=5−7) and Ni2+(H2O)N (N=6−8), Obtained by Blackbody Infrared Radiative Dissociation. J. Am. Chem. Soc. 1998, 120, 5842−5843. (70) Bush, M. F.; O’Brien, J. T.; Prell, J. S.; Wu, C.-C.; Saykally, R. J.; Williams, E. R. Hydration of Alkaline Earth Metal Dications: Effects of Metal Ion Size Determined Using Infrared Action Spectroscopy. J. Am. Chem. Soc. 2009, 131, 13270−13277.

(49) Zhou, J.; Santambrogio, G.; Brümmer, M.; Moore, D. T.; Wöste, L.; Meijer, G.; Neumark, D. M.; Asmis, K. R. Infrared Spectroscopy of Hydrated Sulfate Dianions. J. Chem. Phys. 2006, 125, 111102. (50) Whitehead, A.; Barrios, R.; Simons, J. Stabilization Calculation of the Energy and Lifetime of Metastable SO42‑. J. Chem. Phys. 2002, 116, 2848−2851. (51) Rentzeperis, P. J.; Soldatos, C. T. The Crystal Structure of the Anhydrous Magnesium Sulphate. Acta Crystallogr. 1958, 11, 686−688. (52) Herzberg, G. Molecular Spectra and Molecular Structure II. Infrared and Raman Spectra of Polyatomic Molecules; Van Nostrand Reinhold Company Inc.: New York, 1945; Vol. II. (53) Bush, M. F.; Saykally, R. J.; Williams, E. R. Evidence for Water Rings in the Hexahydrated Sulfate Dianion from IR Spectroscopy. J. Am. Chem. Soc. 2007, 129, 2220−2221. (54) Konermann, L.; Ahadi, E.; Rodriguez, A. D.; Vahidi, S. Unraveling the Mechanism of Electrospray Ionization. Anal. Chem. 2013, 85, 2−9. (55) Johnson, C. J.; Dzugan, L. C.; Wolk, A. B.; Leavitt, C. M.; Fournier, J. A.; McCoy, A. B.; Johnson, M. A. Microhydration of Contact Ion Pairs in M2+OH−(H2O)N=1−5 (M = Mg, Ca) Clusters: Spectral Manifestations of a Mobile Proton Defect in the First Hydration Shell. J. Phys. Chem. A 2014, 118, 7590−7597. (56) Carl, D. R.; Armentrout, P. B. Threshold Collision-Induced Dissociation of Hydrated Magnesium: Experimental and Theoretical Investigation of the Binding Energies for Mg2+(H2O)X Complexes (X=2−10). ChemPhysChem 2013, 14, 681−697. (57) Madekufamba, M.; Tremaine, P. R. Ion Association in Dilute Aqueous Magnesium Sulfate and Nickel Sulfate Solutions under Hydrothermal Conditions by Flow Conductivity Measurements. J. Chem. Eng. Data 2011, 56, 889−898. (58) Hammer, N. I.; Diken, E. G.; Roscioli, J. R.; Johnson, M. A.; Myshakin, E. M.; Jordan, K. D.; McCoy, A. B.; Huang, X.; Bowman, J. M.; Carter, S. The Vibrational Predissociation Spectra of the H5O2+· RGn (RG = Ar, Ne) Clusters: Correlation of the Solvent Perturbations in the Free OH and Shared Proton Transitions of the Zundel Ion. J. Chem. Phys. 2005, 122, 244301. (59) Headrick, J. M.; Bopp, J. C.; Johnson, M. A. Predissociation Spectroscopy of the Argon-Solvated H5O2+ ″Zundel″ Cation in the 1000 - 1900 cm−1 Region. J. Chem. Phys. 2004, 121, 11523−11526. (60) McCunn, L. R.; Roscioli, J. R.; Elliott, B. M.; Johnson, M. A.; McCoy, A. B. Why Does Argon Bind to Deuterium? Isotope Effects and Structures of Ar·H5O2+ Complexes. J. Phys. Chem. A 2008, 112, 6074−6078. (61) McCunn, L. R.; Roscioli, J. R.; Johnson, M. A.; McCoy, A. B. An H/D Isotopic Substitution Study of the H5O2+·Ar Vibrational Predissociation Spectra: Exploring the Putative Role of Fermi Resonances in the Bridging Proton Fundamentals. J. Phys. Chem. B 2008, 112, 321−327. (62) Neilson, G. W.; Enderby, J. E. The Coordination of Metal Aquaions. In Advances in Inorganic Chemistry; Academic Press: Waltham, MA, 1989; Vol. 34, pp 195−218. (63) Skipper, N. T.; Neilson, G. W. X-ray and Neutron-Diffraction Studies on Concentrated Aqueous-Solutions of Sodium-Nitrate and Silver-Nitrate. J. Phys.: Condens. Matter 1989, 1, 4141−4154. (64) Batsanov, A. S. Magnesium Sulfate Hexahydrate at 120 K. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 2000, 56, E230−E231. (65) Oomens, J.; Sartakov, B. G.; Meijer, G.; Helden, G. V. Gas Phase Infrared Mulitiple Photon Dissociation Spectroscopy of Mass Selected Molecular Ions. Int. J. Mass Spectrom. 2006, 254, 1−19. (66) Rodriguez-Cruz, S. E.; Jockusch, R. A.; Williams, E. R. Hydration Energies and Structures of Alkaline Earth Metal Ions, M2+(H2O)N, N=5−7, M = Mg, Ca, Sr, and Ba. J. Am. Chem. Soc. 1999, 121, 8898−8906. (67) Dalleska, N. F.; Tjelta, B. L.; Armentrout, P. B. Sequential BondEnergies of Water to Na+(3s0), Mg+(3s1), and Al+(3s2). J. Phys. Chem. 1994, 98, 4191−4195. (68) Wong, R. L.; Paech, K.; Williams, E. R. Blackbody Infrared Radiative Dissociation at Low Temperature: Hydration of X2+· (H2O)N, for X = Mg, Ca. Int. J. Mass Spectrom. 2004, 232, 59−66. I


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