Vibrational Spectra and Fragmentation Pathways of Size-Selected, D2

Jul 23, 2013 - Sterling Chemistry Laboratory, Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520,. United Sta...
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Vibrational Spectra and Fragmentation Pathways of Size-Selected, D2‑Tagged Ammonium/Methylammonium Bisulfate Clusters Christopher J. Johnson 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: Particles consisting of ammonia and sulfuric acid are widely regarded as seeds for atmospheric aerosol nucleation, and incorporation of alkylamines has been suggested to substantially accelerate their growth. Despite significant efforts, little direct experimental evidence exists for the structures and chemical processes underlying multicomponent particle nucleation. Here we are concerned with the positively charged clusters of ammonia and sulfuric acid with compositions H+(NH3)m(H2SO4)n (2 ≤ m ≤ 5, 1 ≤ n ≤ 4), for which equilibrium geometry structures have been reported in recent computational searches. The computed harmonic vibrational spectra of such minimum energy structures can be directly compared with the experimental spectra of each cluster composition isolated in the laboratory using cryogenic ion chemistry methods. We present one-photon (i.e., linear) infrared action spectra of the isolated gas phase ions cryogenically cooled to 10 K, allowing us to resolve the characteristic vibrational signatures of these clusters. Because the available calculated spectra for different structural candidates have been obtained using different levels of theory, we reoptimized the previously reported structures with several common electronic structure methods and find excellent agreement can be achieved for the (m = 3, n = 2) cluster using CAM-B3LYP with only minor structural differences from the previously identified geometries. At the larger sizes, the experimental spectra strongly resemble that observed for 180 nm ammonium bisulfate particles. The characteristic ammonium- and bisulfate-localized bands are clearly evident at all sizes studied, indicating that the cluster structures are indeed ionic in nature. With the likely (3,2) structure in hand, we then explore the spectral and structural changes caused when methylamine is substituted for ammonia. This process is found to occur with minimal perturbation of the unsubstituted cluster. The thermal decomposition pathways were also evaluated using multiplephoton induced dissociation and are, in all cases, dominated (>100:1) by evaporation of a neutral ammonia molecule rather than methylamine. Spectra obtained for the product cluster ions resulting from this evaporation are consistent with the formation of a single hydrogen bond between two neighboring bisulfate ions, partially regenerating a sulfuric acid molecule. These results provide critical experimental benchmarks for ongoing theoretical efforts to understand the early stages of aerosol growth.

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sulfate and ammonium bisulfate).7,9 Models of growth based on these simple constituents often underpredict nucleation rates, however, and other classes of molecules,17,18 particularly amines,19−23 have been invoked to account for this discrepancy. Because the concentration of ammonia in the atmosphere is typically a hundred fold greater than that of amines, it has been further proposed that amine enrichment of ammonium sulfate particles occurs through exchange of amines with ammonia in the early stages of growth.24 In this paper, we present vibrational spectra and photofragmentation pathways of cold, mass selected ammonium and methylammonium bisulfate cluster ions, providing an exper-

INTRODUCTION New particle formation and growth is a topic of intense focus in atmospheric science because of its impact on climate and human health.1−4 Although models5−8 for atmospheric nucleation and measurements9−14 of the compositions of newly formed particles have yielded considerable insight into their formation mechanism, there are major gaps regarding the elementary processes controlling the first stages of nucleation.14 Standard field measurements, for example, are only sensitive to particles above a few nanometers in diameter,10,12,15 and therefore have not elucidated the structures and compositions of the much smaller clusters that initiate large particle growth. The dimensions of the seed particles fall into the size regime that can be best accessed in the laboratory using a bottom-up approach.3,16 These molecular clusters occur as both ions and neutrals,13 and typically contain sulfuric acid and ammonia,4 which in turn are thought to aggregate as salts (e.g., ammonium © 2013 American Chemical Society

Special Issue: Terry A. Miller Festschrift Received: April 29, 2013 Revised: July 22, 2013 Published: July 23, 2013 13265

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based methods.38 Although recent thermal decomposition25 and surface-induced dissociation measurements39 have provided more insight into the energetics of the decomposition pathways of ammonium bisulfate particles, the specific mechanism by which ammonia/amine exchange occurs remains elusive, particularly at the sub-10 nm level. The present study expands recent work in which infrared multiple-photon dissociation (IRMPD) has been used to obtain vibrational spectra of atmospherically relevant clusters of hydrated bisulfate and nitrate anions as well as hydrated magnesium nitrate cations.40−42 Because of its inherent nonlinearity, however, the IRMPD approach does not yield spectra that can be directly compared with those computed in the harmonic approximation. We overcome this limitation by employing a linear action approach that yields spectra directly comparable to ab initio predictions at the harmonic level. In this so-called cryogenic ion vibrational predissociation (CIVP) spectroscopic method,43 weakly bound adducts (typically rare gas atoms, H2 or D2) of cold, mass-selected ions are photodissociated with a single IR photon throughout the fingerprint region. The CIVP approach has recently been applied in the context of atmospheric chemistry to elucidate the water network rearrangements necessary to form HONO in hydrated NO+ clusters.44 Here we apply it to determine the structures of ammonium bisulfate and ammonium/methylammonium bisulfate particles ≲1 nm in diameter, which have been the subject of the recent computational and mass spectrometric studies discussed above.24−26,33−35,39 The specific target ions in this study [H+(NH3)m(H2SO4)n (2 ≤ m ≤ 5, 1 ≤ n ≤ 4)] and [H+(CH3NH2)m(NH3)(3−m)(H2SO4)2 (0 ≤ m ≤ 3)] were chosen because of their putative role in the formation of amine-containing particles in the early stage of their nucleation and the existing body of work characterizing their reactions.

imental foundation for studies of the more complex processes that are ultimately linked to aerosol formation starting from these fundamental species. The need for experimental benchmarks to guide the modeling efforts is underscored by the fact that the structures of bare ammonium sulfate and ammonium bisulfate clusters are not yet established, and at least three sets of recent calculations on small ammonium bisulfate cations arrive at different equilibrium structures for the H+(NH3)3(H2SO4)2 cluster, denoted A, B, and C in Figure 1.25−27 Although all three are based on electrostatic assembly of

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Figure 1. Survey of the published structures of the H+(NH3)3(H2SO4)2 cluster. Structures A, B, and C are the geometries reported in refs 25, 26, and 27, respectively. Note that a major difference among them is that all S−O groups in A are engaged as Hbond acceptors while one and two S−O groups (denoted *) are nonbonded in C and B, respectively.

EXPERIMENTAL SECTION Ions are generated by electrospray ionization of solutions of ∼20 mM ammonium sulfate (Baker >99%) and 20−40 mM methylamine (Aldrich 40 wt % in H2O) in pure water, and transported into a high vacuum chamber through a series of ion guides to a cryogenically cooled ion trap held at ∼10 K containing a helium buffer gas with 10% D2.45,46 After the ions have cooled and adsorbed D2 tags, they are injected into a double-focusing tandem time-of-flight photofragmentation mass spectrometer. The mass of interest is intercepted by a pulsed tunable infrared laser source at the first (transient) focus. When this light is resonant with a vibrational transition, absorption of a photon heats the cluster, resulting in evaporation of the tag molecule. The fragment ion is then separated in a second stage of mass selection using a reflectron, and its intensity is monitored as a function of laser wavelength, yielding an infrared action spectrum. It is possible to also condense trace N2 onto the clusters at 10 K, which in this application can lead to accidental mass degeneracies. Though undesirable, this N2 isobar does not contribute to or in any way complicate the spectra of the D2-adducts central to this study as the different fragments from the two are readily separated in the reflectron photofragment mass analyzer. Pulsed infrared light is supplied by a Nd:YAG-pumped KTP OPO/KTA OPA system (LaserVision) generating >10 mJ of IR at 3800 cm−1 and >1 mJ at 2500 cm−1. This range is extended by difference-frequency generation of the OPA signal and idler beams in AgGaSe2, producing 0.2 mJ to 1.5 mJ in the

intact ammonium and bisulfate ions, they differ according to the local H-bonding arrangement by which the NH groups are attached to the O atoms. The small cluster spectra and photochemical behavior reported here complement the growing body of work on the characterization of larger ammonium sulfate and ammonium bisulfate particles through spectroscopic and chemical experiments.28−32 Laboratory experiments on nanometer-scale particles have thus far been primarily carried out mass spectrometrically.24,33−35 Ammonium nitrate and ammonium bisulfate particles were found to rapidly exchange with amine vapor, while dimethylammonium bisulfate particles do not exchange with ammonia.34 Ammonia molecules on the exterior of the clusters were found to exchange with amines on time scales shorter than that required for particle growth under atmospheric conditions, confirming the amine exchange process.34 Computational studies have explored the structures and pathways by which this could occur.20,26,27,36,37 Matrix isolation infrared spectroscopy experiments have been performed by codepositing ammonia and sulfuric acid, identifying some characteristic vibrational bands in the mixed complexes, though the stoichiometry of these species is not as well-characterized as those studied using mass spectrometry13266

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range 600−2500 cm−1. The typical bandwidth of this system is ∼5 cm−1, and the pulse duration is ∼5 ns.

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COMPUTATIONAL METHODS Harmonic vibrational spectra for previously reported structures are recomputed at the same geometry, level of theory, and basis set as the original works25−27 to obtain infrared intensities to facilitate comparison with experimental spectra. These structures are then reoptimized in a variety of theoretical methods and basis sets to search for the best quantitative agreement. We find density functional theory calculations using the long-range corrected hybrid functional CAM-B3LYP (with a relatively large aug-cc-pVTZ basis set in Gaussian 09)47 to yield the closest match with the experimental spectrum of H+(NH3)3(H2SO4)2 (vide infra, without the use of empirical scaling factors for frequencies). We therefore report optimized structures and spectra for several systems at that level. Binding energies of ammonia and methylamine were calculated including corrections for basis-set superposition error (BSSE) between the daughter ion and neutral fragment.48 Three dimensional rotatable structures of the CAM-B3LYP calculated cluster geometries in this work are available in the Supporting Information.

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RESULTS AND DISCUSSION Spectral Signatures of the Ionic Components in H+(NH3)3(H2SO4)2. We begin the discussion with the CIVP spectrum of the H+(NH3)3(H2SO4)2 ion (denoted (3,2)), as this system presents a particularly clear example wherein many aspects of the structure can be empirically deduced by consideration of the band pattern in the context of the spectral signatures of the embedded molecular and ionic components. The upper trace in Figure 2 presents the experimental CIVP spectrum, which consists of a relatively simple pattern of sharp peaks, several of which can be assigned by inspection. The single free OH stretch is clearly evident at the high energy end of the spectrum at 3616 cm−1, as are the NH stretching bands at 3340 and 3410 cm−1, often observed for the symmetric and asymmetric stretches of an −NH2 group.49 The bands below 1350 cm−1 are typical for bisulfate ions embedded in H-bonded networks,41 but the exact assignments are not obvious based on comparison with other systems. We therefore turned to the reported harmonic structures for the three candidate structures reported previously.25−27 A comparison of these with the experimental spectrum is presented in Supporting Information, Figure S1, and the best qualitative agreement is found for structure A, for which we have repeated the calculation at the same (HF) level to generate the spectrum (scaled by the recommended value of 0.89 in ref 25) with intensities shown in Figure 2b. It is premature to conclude that this is the only structure whose calculated spectrum matches experiment, however, because different levels of theory were used in each case (HF for ref 25, B3LYP for 27 and PW91 for 26). To access the differences in the harmonic predictions for the various structures at the same level of theory, we carried out calculations for each structure with all three levels, with results compiled in Supporting Information, Figure S2. Structures A and B are essentially stable at all levels, while C optimizes to a connectivity similar to A at HF. Furthermore, optimization of the A structure at all three levels of theory yields the lowest energy isomer with only minor deformations of the high

Figure 2. Vibrational predissociation spectrum of D2-tagged H+(NH3)3(H2SO4)2 along with a comparison to the calculated spectrum of structure A.25 Harmonic frequencies are recalculated to obtain intensities for comparison with the experimental spectrum using the same method as that employed in ref 25. Bisulfate-localized modes are colored in red, and ammonium-localized modes are blue. Signature vibrations are highlighted with arrows.

symmetry arrangement of the starting geometry. The harmonic patterns are qualitatively similar for all three levels of theory as shown explicitly in Supporting Information, Figure S2, with the main differences occurring in the degree of splittings predicted for the bisulfate bands near 1200 cm−1 and the location of the single feature near 1100 cm−1. On this basis, we conclude that the structural motif derived from optimization of A is the dominant species prepared in the experiment. We then carried out a series of additional calculations to determine if any of the commonly used theoretical methods were capable of achieving a more quantitative agreement with the observed pattern. This series is displayed in Supporting Information, Figure S3, and the result obtained using CAMB3LYP is displayed in Figure 3a. The reoptimized structure (denoted A′) retains all essential binding features of the A starting structure with very small changes in structural parameters. With the exception of the broad structure near 2800 cm−1, the unscaled harmonic spectrum is in remarkably good agreement with the observed spectrum. In particular, it traces the closely spaced doublets near 1250 cm−1 to the −SO3 asymmetric stretches and allows assignment of the sharp band at 1114 cm−1 to the SOH bend fundamental. As these are unscaled harmonic spectra, the NH4+ vibrations near 1450 cm−1 are predicted to be higher in energy than the experimental bands, typical of anharmonic bands. However, the relative intensities and splittings of these bands are also well-recovered by the calculations. It is important to stress that accurate calculation of the vibrational spectrum could only be achieved with a large (aug13267

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assembly motif has been regarded as the dominant arrangement, the possibility has also been raised that some neutral components could remain intact at this stoichiometry.25,51 The H-bonding arrangement in A features three ammonium ions that form a bridge between the two flanking bisulfate ions. This linkage is formed with a double-donor H-bonding motif in which NH2 bridges oxygen atoms on the two opposing anions. The two −SO3 groups are staggered in this arrangement such that the overall symmetry is slightly skewed from C3, largely because of the orientation of the bent OH groups at the exterior (best viewed using the rotatable structures in the Supporting Information). The assignment of the observed bands to vibrations of the embedded ions is indicated by the labels and color scheme in Figures 2 and 3, in which those due to bisulfate and ammonium are colored red and blue, respectively. The simple band structure is a consequence of the fact that this cluster is formed in a single conformation with relatively high symmetry, which in turn gives rise to quasi-degenerate transitions for collective motions involving in-phase/out-of-phase vibrations on nearly equivalent ions. Thus, the high-energy OH stretching vibrations near 3700 cm−1 appear in Figure 2 as a single, near resolutionlimited feature (denoted Free OH) at 3616 cm−1, and the two SOH moieties yield single features for the SOH bends and S− OH stretches at 1113 and 863 cm−1, respectively in Figures 2 and 3. The SO-based stretches of the two H-bonded SO3 groups on the flanking bisulfate ions also manifest as a single, narrow feature at 1065 cm−1 for the symmetric stretching bands shown most clearly in the expanded view in Figure 3. This indicates that both bisulfate ions not only occur in similar local environments, but also reside in sites where the three oxygen atoms in the SO3 group are in nearly equivalent H-bonding interactions. In fact, the only evidence that the two bisulfates are not actually in C3 symmetry sites lies in observation that the four members of the asymmetric stretching manifold (i.e., two molecules, each with two nearly degenerate −SO3 asymmetric stretching fundamentals) are split into a staggered quartet structure centered at 1235 cm−1 and labeled SO3 asym in the expanded view provided in Figure 3b. This quartet pattern is

Figure 3. Comparison of the fingerprint region of the H+(NH3)3(H2SO4)2 spectrum to the predicted harmonic spectrum (unscaled) from reoptimization of structure A at the CAM-B3LYP/ aug-cc-pVTZ level of theory. The structure, denoted A′, resulting from this procedure is shown at the top, and is very similar to A from Figure 1.

cc-pVTZ) basis set, likely because of the pervasive effect of basis set superposition error as pointed out by Kurten et al.50 This aspect of the study thus provides detailed and specific experimental targets for the critical evaluation of theoretical approaches designed to predict behaviors in much more complex systems derived from these constituents. Structure A (and A′) is best identified as a primitive salt with composition (NH4+)3(HSO4−)2. In essence, two of the basic neutral ammonia molecules abstract protons from acidic H2SO4, while the excess proton is accommodated on the third ammonia molecule. We note that, although this salt

Figure 4. Evolution of the infrared spectrum of the (3,2) cluster upon substitution of methylamine for ammonia, where the labels (k,m,n) denotes H+(MeNH2)k(NH3)m(H2SO4)n. Modes associated with bisulfate, ammonium and methylammonium are colored red, blue, and teal, respectively. 13268

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Figure 5. Size evolution of the spectra for (2,1), (3,2), (4,3), and (5,4) clusters, with (m,n) denoting H+(NH3)m(H2SO4)n, compared to FTIR spectra of ∼180 nm dry ammonium bisulfate particles, adapted with permission from ref 28, copyright 2000 American Chemical Society. Bisulfate bands are colored red, and ammonium bands are blue. Assignments of bands are given at the top of the figure. The appearance of the single NH stretch of triply hydrogen-bonded ammonium is denoted by *. The telltale sharp NH2 wag in the (4,3) spectrum is highlighted by a dagger † symbol.

cluster remains intact, with the two bisulfate molecules still tethered by the three −NH2 bridging groups. Consistent with the preservation of the overall morphology established in (3,2) upon incorporation of methylamine, the main differences down the series occur in the bands arising from the cations, namely, the NH bending features near 1500 cm−1, the nonbonded NH stretches around 3400 cm−1, and the broad H-bonded NH stretches near 2800 cm−1. Because the NH2 group on each cation is retained in the H-bonded bridge, the equal intensity doublet arising from the free NH2 stretches (NH2 sym and NH2 asym) near 3400 cm−1 in (0,3,2) gradually gives way to a single free NH stretch peak at 3352 cm−1 in the (3,0,2) spectrum. This occurs as methyl groups simply replace one of the free NHs on the three cations. This trend is also evident in the 1350−1700 cm−1 range of the spectrum, where the NH2 bending features (NH4+ bends, colored blue near 1400 cm−1) smoothly give way to a blue-shifted feature traced mostly to the single NH bending fundamental in the embedded CH3NH3+ ions around 1550 cm−1 (turquoise). Finally, the centroid of the broad H-bonded NH stretch envelope around 2900 cm−1 blue-shifts with increasing alkylation, which is readily understood as a consequence of the higher basicity of methylamine compared to ammonia [pKa(NH4+) = 9.25, pKa(CH3NH3+) = 10.66].55 This increase in pKa acts to reduce the shared-proton character of these bands as the bridging protons are pulled closer to the N atom.52 At the same time, the broad envelope breaks up into rather well-defined flanking peaks at ∼2800 cm−1 and ∼3100 cm−1, which lie in the range where transitions have been reported in gas phase, protonated methyl amine hydrate clusters.56 Size Evolution of the Ammonium Bisulfate Clusters: (NH4+)m(HSO4−)n. Using the characteristic spectral markers established in the previous section, we extended the study to reveal some general features of larger ammonium bisulfate clusters with stoichiometries (NH4+)m(HSO4−)n, where m = n + 1. The spectra for the n = 1−4 clusters are shown in Figure 5, along with a comparison to the FTIR spectra of significantly larger (mean diameter ∼180 nm) dry ammonium bisulfate particles adapted from ref 28. Interestingly, the rather diffuse

recovered remarkably well in the calculated harmonic spectrum of structure A′ (Figure 3a), allowing us to assign the two highest energy peaks to the split (∼10 cm−1) in-phase and outof-phase SO stretches, while the two lower energy peaks are dominated by similarly split in-phase/out-of-phase SO 2 asymmetric stretches. Note that the SO and SO2 stretches are qualitative descriptions of the two nearly degenerate SO3 stretches in the isolated bisulfate ion. Turning to the bands associated with the embedded ammonium ions, the A′ structure involves an H-bonding configuration in which the two flanking bisulfate ions are tethered by three double-donor NH2 groups, each with a single H-bond to one of the oxygen atoms on the anion. This leaves a nonbonded NH2 group on each NH4+ ion, which is clearly evidenced in Figure 2 by the relatively sharp bands (denoted NH2 stretch) at 3340 and 3410 cm−1. These are traced to nearly degenerate triplets derived from the symmetric and asymmetric NH stretches on the three cations, respectively. Finally, the NH groups involved in H-bonding (labeled H-bond stretch in Figure 2) account for the very strong and relatively broad envelopes over the 2600−3100 cm−1 range that are typical for strongly hydrogen-bonded stretches.52 In fact, the location, breadth, and asymmetry of these bands appear remarkably similar to those displayed by the embedded hydronium ion in the H9O4+ “Eigen” cation.53,54 Evolution of the Spectra upon Sequential Substitution of NH3 by CH3NH2 in (NH4+)3(HSO4−)2. Having established the structural motif and spectral signatures of the ionic components in the (NH4+)3(HSO4−)2 system, we next follow how this relatively symmetrical structure evolves when the NH4+ ions are replaced with methylammonium molecules. In this discussion, the composition H+(MeNH2)k(NH3)m(H2SO4)n is given by the notation (k,m,n). The predissociation spectra for the (0,3,2)−(3,0,2) series are shown in Figure 4. It is immediately clear that the signature bands of the HSO4− groups highlighted in red (SO3 symmetric stretches, SOH bend, and OH stretch) are essentially identical throughout this series, strongly suggesting that overall structural motif established in the triammonium 13269

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features in the low energy region of the large particle spectrum (bottom trace of Figure 5) are already evident in the (5,4) spectrum. This observation suggests that the species produced in this experiment bear some useful resemblance to those studied using standard atmospheric analysis techniques, and consequently, the sharp bands displayed by the clusters reveal the distinct transitions that likely underlie the broader spectral features in the larger particles. As in the case of methylamine substitution (Figure 4) it is clear from the persistence of the telltale (red-colored) bands near 840, 1060, and 1110 cm−1 in Figure 5 that the anions in these clusters retain bisulfate geometries for all cluster sizes studied. The lower-energy member of the doublet around 3600 cm−1 found in the OH stretch region of the (2,1) spectrum was identified as due to the presence of a second isomer using IR-IR double-resonance.57 The distinct spectrum of each isomer was isolated, and the two spectra differ only in their OH stretching and broad hydrogen bonded NH stretching regions (see Supporting Information, Figures S4 and S5). These isomers likely result from the location of the D2 adduct, where attachment to a free OH group is known to induce a significant red shift.58 Thus, D2 attachment to the (2,1) cluster appears to occur at both the NH (as observed in protonated peptides, for example)46 and OH positions. The evolution of the nonbonded NH stretching bands near 3400 cm−1 is also revealing as the doublet due to the symmetric and asymmetric NH2 stretching modes at the smaller sizes gives way to a new peak (denoted by *) in the (4,3) and (5,4) spectra. This falls between the two NH2 peaks and is consistent with formation of a single free NH group as ammonium ions adopt a triple-donor H-bonding arrangement in the growing salt structure. The relative intensity of this free NH peak compared to the NH2 asymmetric stretching peak at lower energy suggests that the number of triple-donor ammonium ions is greater in (5,4) than (4,3). Unfortunately, the increasing contribution of the broad H-bonded NH stretching band to the baseline of these sharper free NH peaks makes more quantitative analysis difficult. The larger clusters do exhibit some broadening and fine structure splitting on the bisulfate bands below 1100 cm−1, potentially signaling the emergence of lower symmetry sites with increasing size. For example, new peaks appear slightly to the red of the S−OH stretch at 834 cm−1 in (4,3) and also below the SO3 symmetric stretch at 1035 cm−1 in (5,4). The most likely cause for these splittings is the formation of two hydrogen bonds to a single oxygen in one of the bisulfate anions, as calculations indicate that this oxygen double-acceptor motif should exhibit a red shift of 10−30 cm−1, consistent with those observed (23 and 27 cm−1, respectively). Significant changes are also apparent in the H-bonded NH stretching region, but these do not yield qualitative structural implications as does the stretching behavior of the free hydrogen atoms. It is interesting, nonetheless, that the (4,3) spectrum displays the largest red shift of the series with considerable substructure, thus presenting an attractive target for further spectroscopic analysis. While comparison of the experimental and computed infrared spectra provide a compelling assignment of the (3,2) structure and qualitative structural information about the larger clusters, identification of specific minimum energy structures becomes increasingly difficult as the clusters grow. We present in Figure 6 a selection of computed structures reoptimized from those previously reported25,26 for the (4,3) composition.

Figure 6. Overview of the structures discussed for the (4,3) cluster. Structure A′ is reoptimized from Isomer A proposed in ref 25, B′ is reoptimized from Isomer B from that same work. Structure C′ is reoptimized from the lowest energy structure proposed in ref 26.

Reference 25 reported two structures for the (4,3) cluster (reoptimized to obtain A′ and B′) within 1.8 kcal/mol while ref 26 suggested a third structure (reoptimized to obtain C′). Structure A′ for the (4,3) cluster, again displays approximate C3 symmetry, with one ammonium ion forming a hydrogen bond with an oxygen on each bisulfate, which themselves are connected by doubly hydrogen bonded ammonium ions in a ring-like fashion. Structure B′ differs only in the formation of an additional partial hydrogen bond across the ring between a free NH and a bisulfate oxygen on the opposite side of the ring that already has one hydrogen bond (more easily seen in the rotatable structures in the Supporting Information). Structure C′ is similar to B′ but features several free SO groups. When all structures are reoptimized using the same level of theory (noted in the Computational Methods section) and corrected for zero-point energy, we find that A′ is higher in energy than B′ by 1.3 kcal/mol, while C′ is still higher at 11.6 kcal/mol above B′. A comparison of the computed and experimental spectra is provided in Supporting Information, Figure S6. We find better overall agreement with structure A′, particularly in the recovery of the ammonium modes. The high symmetry of this structure is particularly manifested in a sharp NH2 rocking band at 1376 cm−1 (denoted by a dagger † symbol in Figure 5), which is split in the lower-symmetry structures. However, the aforementioned split in the S−OH stretching band suggests a double-acceptor oxygen as in structures B′ or C′, and the highenergy bisulfate peak near 1320 cm−1 is reminiscent of a free SO group, so the contribution of any of the three isomers to this spectrum cannot be ruled out. Future IR-IR double resonance studies, similar to those mentioned in the discussion of the (2,1) spectrum, should resolve this issue. Assignment of the (5,4) spectrum is even more difficult, and no structures are presented here because of the large number of potential configurations and the computationally intensive nature of the calculations required to properly reproduce the experimental spectra. Photochemical Decomposition: Selective Ejection of NH3 from the Mixed Ammonium/Methyl-Ammonium Clusters. The decomposition pathways of size-selected ammonium bisulfate particles in the small size range of interest here have been explored by thermal dissociation as well as surface-induced dissociation to elucidate their decomposition 13270

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mechanisms and energetics.25,39 Central to those studies is the assumption, based upon the principle of microscopic reversibility, that the energetics governing decomposition are identical to those governing growth. Fragmentation was observed to occur through loss of neutral ammonia and then sulfuric acid, with a possible concerted mechanism in play. The barriers for these processes were estimated to be on the order of 2 eV based on modeling of the unimolecular decay kinetics.39 In the present study, we explored multiphoton-induced decomposition of the (0,3,2) to (3,0,2) clusters through observation of neutral ammonia loss in addition to the loss of the D2 tag. For cold, untagged clusters (i.e., prepared under the same conditions as those that yielded the adducts), ammonia loss was found to be the dominant process for (3,2), (4,3), and (5,4) clusters, and the ammonia loss action spectra recorded for these clusters are included in the Supporting Information, Figure S7. The identity of the fragment was confirmed by substitution of 15N ammonia, leading to loss of 18 instead of 17. An example fragment mass spectrum for the (2,1,2) cluster is given in Figure 7, showing that for excitation of mixed

Table 1. Comparison of Computed Zero-Point (0 K) Binding Energies of Ammonia and Methylamine in the (0,3,2) through (3,0,2) Clusters, Corrected for Basis Set Superposition Errora binding energy (kcal/mol) cluster

NH3

CH3NH2

(0,3,2) (1,2,2) (2,1,2) (3,0,2)

25.8 25.4 24.2 N/A

N/A 32.5 31.4 30.7

a

The structures used for these calculations are based on structure A discussed above, reoptimizing the geometry upon each exchange of methylamine for ammonia.

requires absorption of at least three photons at the OH stretching energy to reach the dissociation threshold, while methylamine loss requires four photons, likely accounting for the difference in efficiencies between ammonia and methylamine photodissociation. Note that these energies lie well below the barrier to ammonia dissociation reported in recent surface-induced dissociation measurements.39 Work is ongoing to experimentally characterize the energetics of this process. Characterization of the Ionic Photoproducts: Clusters with Bisulfate-Bisulfate H-Bonds. The observation of efficient neutral ammonia loss from the ammonium bisulfatebased salt clusters is interesting since the product cannot be based on the building blocks of ammonium and bisulfate ions. For this discussion we revert to the nomenclature H+(NH3)m(H2SO4)n to emphasize this fact. In an effort to elucidate the structures of these species, we prepared clusters with n = m (which we will denote as (2,2), (3,3), and (4,4)) directly in the electrospray source. This was accomplished by increasing the acceleration voltage of the nascent ions into the first ion guide, which is held at ∼100 mTorr. It is likely that this protocol yields sufficiently energetic collisions between ions and background gas to heat the ions above the ammonia dissociation threshold. The CIVP spectra of these ions are shown in Figure 8, and it is immediately apparent from the very diffuse band structures that the high symmetry of the (3,2) cluster is completely broken in the (2,2) fragment. In losing ammonia, an H-bond between bisulfates can form by accommodating the excess proton in a base-H+-base motif. This type of proton bound dimer of the conjugate bases is characterized by formation of a double-well potential for the shared proton with a low-to-nonexistent transition state.52 In this case, the parallel stretching band arising from vibration of the central proton is expected to appear at very low frequencies, and thus the broad feature in the (2,2) spectrum around 800 cm−1 is a good candidate for this motion. The OH stretching region of this spectrum shows three distinct transitions (red), though only two would be expected for the free bisulfate OH groups. This extra peak could signify the presence of multiple isomers, or strong tag effects as seen in the (2,1) spectrum. As the cluster size increases, the telltale bisulfate features (highlighted in red in Figure 8) reappear, indicating that the geometric distortions caused by the bisulfate-sufuric acid hydrogen bond are primarily localized to the two species involved in hydrogen bonding, leaving the remaining bisulfate(s) in essentially symmetric environments. Ongoing work to characterize these more complex spectra should yield insight into the nature and signature of the intracluster bisulfate-bisulfate hydrogen bond.

Figure 7. Example mass spectrum of methylamine-substituted ammonium bisulfate clusters. See Figure 4 caption for notation describing compositions. Inset is a fragment mass spectrum of the (2,1,2) cluster upon absorption of multiple photons at the OH stretching frequency (3616 cm−1). Analysis of the fragment mass spectrum shows >100:1 preference of ammonia (17) over methylamine (31) loss, despite the 1:2 stoichiometry of the cluster. The loss 28 channel corresponds to the accidentally mass-degenerate N2-tagged (0,3,2) clusters, and spectra collected monitoring this loss channel reproduce the D2-tagged (0,3,2) spectrum. A small peak at loss 45 indicates loss of N2 and ammonia.

ammonia/methylamine clusters at the OH stretching frequency (3625 cm−1), there is essentially no loss of methylamine (m/z = 31), with an upper limit of ∼0.5% relative to ammonia loss based on the signal-to-noise. This result is consistent with stronger binding of amines than ammonia, which has been studied extensively both experimentally and computationally.20,22,23,25,26,31−36 The (3,0,2) cluster, which has no ammonia to lose, displayed a small but observable signal due to methylamine loss, amounting to less than 5% of the ammonia loss yield for any other composition. That is, when ammonia is not available as a loss channel, it is possible to dissociate methylamine but with much lower efficiency. The calculated dissociation energies of ammonia and methylamine from the (0,3,2) to (3,0,2) clusters are summarized in Table 1. These values are consistent with previously reported calculations22,23,25,26,39 and imply that ammonia fragmentation 13271

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (203) 436-4930. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to acknowledge assistance in data collection from Christopher M. Leavitt and Joseph A. Fournier, aid with calculations by Wei Lin and Francesco Paesani, and helpful discussions with Leif D. Jacobson and Kimberly A. Prather. C.J.J. gratefully acknowledges support from the National Science Foundation American Competitiveness in Chemistry Fellowship (Grant CHE-1137404) in collaboration with the NSF Center for Aerosol Impacts on Climate and the Environment (Paesani, Prather). This work was supported in part by the facilities and staff of the Yale University Faculty of Arts and Sciences High Performance Computing Center, and by the National Science Foundation under Grant CNS 0821132 that partially funded acquisition of the facilities. M.A.J. acknowledges support from the United States Department of Energy under Grant DE-FG02-06ER15800.

Figure 8. Spectra of H+(NH3)m(H2SO4)n clusters with (m,n) = (2,2), (3,3), and (4,4), prepared by collision-induced dissociation of (3,2), (4,3), and (5,4) clusters, respectively. The (2,2) cluster spectrum bears little resemblance to any of the spectra presented in Figures 3 and 4, while bisulfate bands (red) are recovered in the (3,3) and (4,4) cluster spectra. The OH stretching bands of the (2,2) and (3,3) clusters consist of several peaks, potentially indicating the existence of multiple isomers for these compositions.

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(1) Kulmala, M. How Particles Nucleate and Grow. Science 2003, 302, 1000−1001. (2) Zhang, R. Y. Getting to the Critical Nucleus of Aerosol Formation. Science 2010, 328, 1366−1367. (3) Bzdek, B. R.; Johnston, M. V. New Particle Formation and Growth in the Troposphere. Anal. Chem. 2010, 82, 7871−7878. (4) Zhang, R. Y.; Khalizov, A.; Wang, L.; Hu, M.; Xu, W. Nucleation and Growth of Nanoparticles in the Atmosphere. Chem. Rev. 2012, 112, 1957−2011. (5) Doyle, G. J. Self-Nucleation in Sulfuric Acid-Water System. J. Chem. Phys. 1961, 35, 795−799. (6) Korhonen, P.; Kulmala, M.; Laaksonen, A.; Viisanen, Y.; McGraw, R.; Seinfeld, J. H. Ternary Nucleation of H2SO4, NH3, and H2O in the Atmosphere. J. Geophys. Res.: Atmos. 1999, 104, 26349− 26353. (7) Anttila, T.; Vehkamaki, H.; I, N.; Kulmala, M. Effect of Ammonium Bisulphate Formation on Atmospheric Water-Sulphuric Acid-Ammonia Nucleation. Boreal Environ. Res. 2005, 10, 511−523. (8) Kuang, C.; Riipinen, I.; Sihto, S. L.; Kulmala, M.; McCormick, A. V.; McMurry, P. H. An Improved Criterion for New Particle Formation in Diverse Atmospheric Environments. Atmos. Chem. Phys. 2010, 10, 8469−8480. (9) Kulmala, M.; Pirjola, U.; Makela, J. M. Stable Sulphate Clusters as a Source of New Atmospheric Particles. Nature 2000, 404, 66−69. (10) Kulmala, M.; Riipinen, I.; Sipila, M.; Manninen, H. E.; Petaja, T.; Junninen, H.; Dal Maso, M.; Mordas, G.; Mirme, A.; Vana, M.; et al. Toward Direct Measurement of Atmospheric Nucleation. Science 2007, 318, 89−92. (11) Sipila, M.; Berndt, T.; Petaja, T.; Brus, D.; Vanhanen, J.; Stratmann, F.; Patokoski, J.; Mauldin, R. L.; Hyvarinen, A. P.; Lihavainen, H.; et al. The Role of Sulfuric Acid in Atmospheric Nucleation. Science 2010, 327, 1243−1246. (12) Jiang, J. K.; Zhao, J.; Chen, M. D.; Eisele, F. L.; Scheckman, J.; Williams, B. J.; Kuang, C. A.; McMurry, P. H. First Measurements of Neutral Atmospheric Cluster and 1−2 nm Particle Number Size Distributions During Nucleation Events. Aerosol Sci. Technol. 2011, 45, iii−v. (13) Kirkby, J.; Curtius, J.; Almeida, J.; Dunne, E.; Duplissy, J.; Ehrhart, S.; Franchin, A.; Gagne, S.; Ickes, L.; Kurten, A.; et al. Role of Sulphuric Acid, Ammonia and Galactic Cosmic Rays in Atmospheric Aerosol Nucleation. Nature 2011, 476, 429−433.

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SUMMARY We have presented the first experimental characterization of the structures of small ammonium bisulfate clusters thought to be active in aerosol nucleation. This work provides highly resolved experimental benchmarks against which the accuracy of computational efforts may be compared, and lays the groundwork for further spectroscopic investigations of nucleating particles to determine the critical chemical interactions involved in the activation of a particle to take up water. We also demonstrate a multiphoton evaporation mechanism that leads to loss of neutral ammonia with high selectivity relative to methylamine loss. Further investigation of this photochemical process should yield complementary, and potentially much more accurate information on the decomposition energetics of the clusters since this can be carried out in a microcanonical regime.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

Comparison of calculated spectra at various levels of theory to the experimental spectrum for the (3,2) cluster. Isomerselective spectra for the (2,1) cluster. Comparison of the experimental spectrum of the (4,3) cluster to calculations for the three candidate structures. Infrared multiple photon dissociation spectra of the (3,2), (4,3), and (5,4) clusters. Calculated atomic coordinates and rotatable images of all clusters. Tabulated experimental and computed vibrational frequencies and thermochemistry. This material is available free of charge via the Internet at http://pubs.acs.org. 13272

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