Disentangling the Contribution of Multiple Isomers ... - ACS Publications

Letter pubs.acs.org/JPCL

Disentangling the Contribution of Multiple Isomers to the Infrared Spectrum of the Protonated Water Heptamer Nadja Heine,†,⊥ Matias R. Fagiani,†,§ and Knut R. Asmis*,§ †

Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany Wilhelm-Ostwald-Institut für Physikalische und Theoretische Chemie, Universität Leipzig, Linnéstrasse 2, D-04103 Leipzig, Germany

§

S Supporting Information *

ABSTRACT: We use infrared/infrared double-resonance population labeling (IR2MS2) spectroscopy in the spectral region of the free and hydrogen-bonded OH stretching fundamentals (2880−3850 cm−1) to identify the number and to isolate the vibrational signatures of individual isomers contributing to the gas-phase IR spectra of the cryogenically cooled protonated water clusters H+(H2O)n·H2/D2 with n = 7−10. For n = 7, four isomers are identified and assigned. Surprisingly, the IR2MS2 spectra of the protonated water octa-, nona-, and decamer show no evidence for multiple isomers. The present spectra support the prediction that the quasi-2D to 3D structural transition occurs in between n = 8 and 9 in the cold cluster regime. However, the same models have difficulty explaining the remarkable size dependence of the isomer population reported here.

P

evidence for a transition from quasi-2D net-like to cage-like 3D structures was reported at around n = 11.8−10 High-level computational studies predict that already the protonated water nonamer should adopt a 3D structure at 0 K.21 However, open network structures are entropically favored at higher internal temperatures, and the contribution of multiple isomers to the IRPD spectra of protonated water clusters in this intermediate size range as well as the dependence of the relative isomer abundances on the internal temperature is actively discussed.8,10,21−27 The present study is therefore aimed at (i) experimentally identifying the number and nature of isomers in this intermediate cluster size range (n = 7−10) contributing to the corresponding vibrational spectrum and (ii) pinpointing the transition from quasi-2D to 3D structures. We do this by way of IR/IR double resonance population labeling (IR2MS2) spectroscopy28 of cryogenically cooled, H2-tagged protonated water clusters. IR2MS2 is a powerful variation of IR/UV ion dip spectroscopy,29−32 with the advantage that no UV chromophore is required. An overview of the (single-color) IRPD spectra of H+(H2O)n·H2/D2 with n = 6−10 from 3000 to 3850 cm−1 is shown in Figure 1. The sharp features above 3615 cm−1 are assigned to fundamental transitions involving free OH stretching vibrations, while the bands at lower energies are attributed to modes associated with hydrogen-bonded O−H oscillators.7 The bands are labeled according to their assignment to vibrational modes, either of the ionic core (H5O2+) or

rotonated water clusters are abundantly present in the earth’s atmosphere, where they impact new particle formation and may alter the radiative balance.1−3 They also serve as model systems for studying proton hydration, in general, and solvent-mediated charge migration along hydrogen-bond (HB) networks, in particular.4,5 The structures of isolated protonated water clusters, H+(H2O)n, have been extensively studied by means of infrared photodissociation (IRPD) spectroscopy.6−15 The use of messenger tagging allows probing of internally colder clusters and vibrations directly associated with the excess charge.11−17 Using molecular hydrogen (H2), in contrast to Ar, as a messenger is less perturbing and yields isomer distributions more similar to the bare, untagged cluster cations.13 These studies have shown convincingly that the vibrational spectra of small protonated water clusters up to n = 5 can be readily assigned to a single isomer containing either an equally shared proton as part of a H5O2+ core (Zundel-type isomer) or a hydronium ion core (Eigen-type isomer), although this finding has been challenged by recent computational studies.18,19 The protonated water hexamer, on the other hand, coexists in two isomeric forms, a Zundel-type and an Eigen-type isomer,7,13 and their IRPD spectra have recently been measured isomer-specifically over nearly the complete IR range.15,20 Adding additional water molecules intuitively should increase the number of energetically low-lying isomers. Indeed, evidence for the contribution of three isomers, both Zundel- and Eigen-type, to the IRPD spectrum for n = 7 has been found. Interestingly, only a single isomer with a Zundel core is observed for n = 8.7,14 For larger clusters, the complexity of the IRPD spectra prohibited an unambiguous assignment to one or more specific isomers, but © 2015 American Chemical Society

Received: April 28, 2015 Accepted: June 2, 2015 Published: June 2, 2015 2298

DOI: 10.1021/acs.jpclett.5b00879 J. Phys. Chem. Lett. 2015, 6, 2298−2304

Letter

The Journal of Physical Chemistry Letters

of H2O molecules, in which case they are classified as a HB donor (D) and/or acceptor (A). The free O−H stretching region provides a clear diagnostic of structures with dangling waters and rings.8 For n = 6, this region is dominated by the symmetric (S) and antisymmetric (AS) free O−H stretches of singly accepting H2O molecules (A), so-called dangling waters. Bands associated with A-H2O persist up to n = 9 and are not observed in the spectrum of n = 10. The less intense band at 3714 cm−1 in the n = 6 spectrum originates from a single acceptor−single donor (AD) H2O and is observed throughout the spectra up to n = 10. Starting with n = 7, a new feature evolves at 3676 cm−1, attributed to a triply coordinated AADH2O. This band becomes the most intense feature in the free O−H stretching region for n = 10 and has been shown to be characteristic for ring formation.7 The absence of dangling waters for n = 10 can only be rationalized by the presence of a cage-like 3D structure.8,10,21,23 On the other hand, the presence of a dangling water at n = 9 does not exclude an assignment to a 3D structure, and indeed, such a cage-like structure is likely also for n = 9, as we will discuss in more detail later.21 The observation of cage-like 3D structures for n = 10 is in agreement with previously measured Ar-tagged spectra.11 It also confirms the prevalence of such structures at n = 10 in the cold cluster regime, independent of the way that the clusters are prepared, either by fast adiabatic cooling in a supersonic expansion described in ref 11 or by slow thermalization in a buffer-gas-filled ion trap described here. For bare protonated water clusters,8−10 cage-like 3D structures are observed only at larger cluster sizes (n > 10), which has been

Figure 1. IRPD spectra of H+(H2O)6−9·H2 and H+(H2O)10·D2 in the OH stretching region (3000−3850 cm−1). Assignments are based on previous studies.7,10,21,23 Free O−H oscillators are denoted with an apostrophe. A = HB acceptor, D = HB donor, S = symmetric, AS = antisymmetric.

Figure 2. Comparison of the IRPD spectrum (a) to IR2MS2 spectra (b−e) of H+(H2O)7·H2 from 2880 to 3850 cm−1. The IR2MS2 spectra are obtained by probing at (b, red) 3542, (c, orange) 3676, (d, blue) 3351, and (e, green) 3492 cm−1, indicated by vertical colored arrows. Data points are shown as dots, and a weighted three-point-running average (solid line) is added to guide the eye. Simulated vibrational spectra of four low-energy isomers of H+(H2O)7 are also shown (f−i). The intensities of the free OH stretches have been scaled (×8) for better comparability. See Table 1 for peak assignments. 2299


Letter


Table 1. Assignment and Position (in cm−1) of the Features Observed in the IRPD and the Four IR2MS2 Spectra of H+(H2O)7· H2 Shown in Figure 3 IR2MS2 band a0 a1 a2 a3 a4 a5 a6 a7 a8 a9 a10 a11 a12 a13 a14

IRPD 3815 3800 3739 3716 3701 3676 3651 3619 3573 3544 3532 3495 3352 3341 3310 3185 3185 ∼3070 2957

7E4R

3737 3716 3703 3676 3651

7Z 3815 3800 3739 3714 3701 3651 3619

7E5R

7EC

3802 3735 3717 3702 3679 3651

3799 3737 3713 3703 3676 3651

3573 3542

3545 3532 3486 3352

3500 3352

3493 3354

previous exp. 3791a 3742a, 3717a, 3710a 3679a, 3652a,

3741b 3716b 3679b 3654b

3581a 3555a, 3555b 3544a 3502a 3360a, 3351/3570b

3339 3310/3325b

3312 3268 3171 3171 3073

3264 3189

3185

∼2960

3198a, 3194b 3198a, 3194b

assignmentc combination band combination band AAS′-H2O 2° AD′-H2O 1° AD′-H2O AAD′-H2O AS′-H2O AAS′-H2O AD-H2O 1° ADAS-H2O 1° ADS-H2O 2° ADS-H2O 1° AD-H2O 1° ADAS-H2O, AAD-H2O AAD-H2O 1° ADS-H2O, AAD-H2O 1° AD-H2O H5O2+ H5O2+ H3O+

Bare H+(H2O)7 data from ref 7. bNe predissociation data from ref 14. cA = HB acceptor, D = HB donor, ′ = free OH stretch, S = symmetric, AS = antisymmetric, 1° = first hydration shell, 2° = second hydration shell. a

is 7Z, a five-membered ring structure with a Zundel-type core. Three competing structural motifs with Eigen-type cores are found within 6 kJ/mol of 7Z containing either a fourmembered ring (7E4R), a five-membered ring (7E5R), or noncyclic chains (7EC). Note, the exact energetic ordering of the Eigen-type isomers depends on the model used.25,33 While each of these isomers yields a characteristic harmonic IR spectrum in the O−H stretching region (see Figure 2), more than one nearly isoenergetic conformer is found for each of these structural isomers, whose IR spectra are so similar that they cannot be discerned (see Figure S1, Supporting Information). We find evidence for all four structural motifs, that is, 7Z, 7E4R, 7EC, and 7E5R, in the IR2MS2 spectra. We first consider trace (b) in Figure 2, measured at a probe energy corresponding to band a7 (3542 cm−1), attributed to the global minimum structure 7Z. The observed bands are characteristic for 7Z and correspond to the symmetric and antisymmetric combination of the hydrogen-bonded O−H stretches of the two AD water molecules (a7), as well as the four terminal O−H stretches of the H5O2+ core, which contribute to the two broad features a13 and a14. This isomer also shows two weaker, isolated absorptions at 3619 (a5) and 3815 cm−1 (a0), which are assigned to the symmetric free O−H stretch of the AA water molecule, only present in 7Z,7,14 and a combination band (a0), respectively. Note, this spectrum shows no bands at wavenumbers corresponding to bands a3 and a8 in trace (a) and significantly less signal around a9, indicating that additional isomers must contribute to the IRPD spectrum in trace (a). The IR2MS2 spectrum probed at 3676 cm−1 (a3), characteristic for a triply coordinated AAD water molecule, is shown in trace (c) and is satisfactorily reproduced by the simulated IR spectrum shown in trace (g), corresponding to an Eigen-type isomer containing a four-membered ring (7E4R). The experimental spectrum reveals a relative enhancement of the

attributed to the higher internal energy of the clusters in those particular experiments, resulting in an enhanced population of the entropically favored quasi-2D structures.21 In the region of the hydrogen-bonded OH stretches between 3300 and 3700 cm−1, the complexity of the IR spectra increases with n. For n = 7, the observed features are due to the symmetric and antisymmetric stretches of AD-H2O’s in the first and second solvation shell.11 The broad absorption in between 3100 and 3300 cm−1 for n = 6−8 can be attributed to the four terminal hydrogen-bonded O−H stretches of a Zundel core.7,11 The sharper feature at around 3200 cm−1 for n = 9 and 10, which have Eigen cores, corresponds to a double acceptor− single donor (AAD) water,11,21,23 and for n = 10, the doublet at around 3600 cm−1 suggests the presence of an ADD water.23 H+(H2O)7. In order to isolate and identify the contributing isomers to the IRPD spectra, we measured IR2MS2 spectra of H+(H2O)n·H2/D2 with n = 7−10 in the OH stretching region (2880−3850 cm−1). Figure 2 compares the single-color H2 predissociation spectrum (a) to two-color IR2MS2 spectra (b− e) of H+(H2O)7·H2. Observed IRPD bands are labeled from a0 to a14 (see Table 1 for assignments). The IR2MS2 spectra were measured at probe energies indicated by the vertical arrows, corresponding to bands a7 (b), a3 (c), a9 (d), and a8 (e). These measurements yield four characteristically different IR2MS2 spectra, identifying the presence of (at least) four H+(H2O)7 isomers. The experimental band positions and their assignments are compared to values from previous experimental work in Table 1.7,14,15 In order to aid in the assignment of the IR2MS2 spectra, we performed electronic structure calculations of the known minimum-energy structures.25 A complete list of all calculated isomers including their structures, relative energies, and simulated absorption spectra are given in Table S2 and Figure S1 (Supporting Information). Four low-energy binding motifs (see Figure 3) are found. The global MP2/TZVPP ground state 2300


Letter


Figure 3. Minimum-energy structures of the protonated water clusters H+(H2O)n with n = 7−10 discussed in the text. For n = 7, the water molecules are classified according to their function as a HB acceptor (A) and/or donor (D). The structures for n = 9 and 10 correspond to 54A (9E) and HW10A (10E) from ref 23.

more isomers for n = 8 and larger water clusters. However, the IRPD studies of Jiang et al.7 and Mizuse et al.14 already suggested that a single isomer, a Zundel-type one (8Z in Figure 3) derived from the 7Z structure with an additional water binding to the H2O (AA) site, can account satisfactorily for the observed experimental peaks in the OH stretching region. Our IR2MS2 spectra of n = 8 confirm that a single isomer contributes to the IRPD spectrum. The left column in Figure 4 shows four IR2MS2 spectra, measured at probe energies of 3713 (e), 3679 (d), 3489 (c) and 3204 cm−1 (b), respectively. All isomer-selective spectra (traces b−e) are identical within the experimental uncertainty. Even trace (d), probed at 3679 cm−1, an energy where solely the global minimum isomer is predicted to absorb (see Figure S2, Supporting Information), does not exhibit any change in intensity or band shape, supporting the previous assignment to this 2D isomer.7,14,22 The IR2MS2 spectra of the protonated water nonamer and decamer are also shown in Figure 4. The individual IR2MS2 traces, probed at 3546, 3199, 3355, and 3449 cm−1 for n = 9 and 3362, 3160, and 2741 cm−1 for n = 10, show no significant differences from the corresponding single-color IRPD spectrum, shown in the top row, suggesting that, similar to n = 8, predominantly a single independently absorbing isomeric species is present for n = 9 and 10, respectively. An unambiguous assignment of the n = 9 and 10 spectra to a particular isomer is difficult, also due to the limitations of the harmonic approximation in the available cluster models. On the basis of the highest-level calculations currently available, the most likely structures that can explain our experimental IRPD spectra for n = 9 and 10 are the cage-like 3D structures shown in Figure 3.21,23 Structure 9E represents the lowest-energy isomer containing a dangling water molecule (54A in ref 21). Isomers without dangling water molecules are predicted to be lowest in energy at 0 K, but those containing dangling waters are entropically favored. Structure 10E in Figure 3 corresponds to the currently accepted global minimum-energy structure for n = 10.23 A more reliable estimate of isomer populations and prediction of IR spectra for hydrogen-bonded clusters requires considering anharmonic effects as well as entropy-driven hydrogen-bond network fluctuations.34,35 Such calculations go beyond the scope of the present study but will hopefully be stimulated by the present results.

probed band a3 together with a characteristic doublet at 3352 (a9) and 3312 cm−1 (a11) and significant absorption in the 3380−3575 cm−1 range. These absorptions correspond to the hydrogen-bonded OH stretches of the AAD and the three nonequivalent AD water molecules of 7E4R. The third IR2MS2 spectrum, trace (d) in Figure 2, was probed at 3352 cm−1 (a9) and reveals a third distinct IR signature. It is characterized by a simpler absorption spectrum with a single dominant absorption (a9) in the hydrogen-bonded O−H stretching region. Comparison to the simulated IR spectra reveals that this IR spectrum is best assigned to the structural motif 7EC, which contains three equivalent AD water molecules (C3 symmetry), and hence, a single IR-active absorption band is predicted in the relevant hydrogen-bonded O−H stretching region of trace (h). Note, the previously two identified isomers, 7Z and 7E4R, also absorb significantly at this probe energy, and therefore, their contributions are also picked up in trace (d). For example, 7Z is likely responsible for bands a7 and a13 and 7E4R for band a3 as 7EC contains no AAD water molecule. The fourth IR2MS2 spectrum, shown in trace (e), was measured at 3492 cm−1 (a8) and identifies the presence of a fourth isomer, an Eigen isomer containing a five-membered ring. Even though the signal/noise ratio of this spectrum is less favorable compared to that of the previous three spectra, it is sufficient to identify another characteristic absorption pattern consisting of at least two features at 3493 (a8) and 3339 cm−1 (a10), which are not observed with similar relative intensities in any of the other IR2MS2 spectra. Our calculations yield two types of low-energy isomers within 1.9 kJ/mol of each other containing five-membered rings but differing in the arrangement of the exocyclic water molecules (see Table S2, Supporting Information) that can account for these two absorptions (see Figure S1, Supporting Information). However, only the spectrum of the energetically slightly higher lying one, labeled 7E5R in Figure 3 and shown in trace (i) of Figure 2, accounts for band a3 (AAD water molecule) as well as for the absorption below 3300 cm−1, favoring an assignment to this particular structure. H+(H2O)8−10. After observing contributions from two isomers in the IRPD spectra of the protonated water hexamer15 and from at least four isomers for the protonated water heptamer, one would intuitively expect the presence of even 2301


Letter


Figure 4. H2 predissociation spectra (upper trace) and IR2MS2 spectra of H+(H2O)8·H2 (left), H+(H2O)9·H2 (center), and H+(H2O)10·D2 (right) from 2780 to 3860 cm−1. Probe energies are indicated by vertical arrows (see text for energies). Data points are shown as dots, and a weighted threepoint-running average (solid black line) is added to guide the eye.

ion packet is irradiated with a first IR laser pulse (scan pulse). All ions are accelerated into the 180° reflectron stage, separate out in time and space according to their mass/charge ratio, and are refocused at the original interaction zone. A second laser pulse, tuned to the probe wavelength, is applied at the fly-by time of the (undissociated) parent ions. All ions in between the acceleration plates are reaccelerated by a second high-voltage pulse into the linear TOF region, and a TOF mass spectrum is measured for each laser shot. IR2MS2 spectra are recorded by averaging over 50−100 TOF mass spectra per wavelength and scanning the wavelength of the first laser pulse.15,20 The photodissociation cross section σIRPD is determined as described previously.20

Summarizing, the present study resolves two important issues regarding the structure of protonated water clusters in the intermediate size range and at low internal energies. First, the protonated water heptamer exhibits the most pronounced structural variability of the clusters in this size range. The spectral signatures of four characteristically different structural motifs contributing to the IRPD spectrum are isolated and assigned. Moreover, the presence of an Eigen-type isomer containing a five-membered ring is identified for the first time. Second, we show that predominantly a single isomeric species contributes to the low-temperature IRPD spectra of the protonated water clusters with n = 8−10. This, together with the highest-level calculations up-to-date, suggests that the transition between the net-like 2D structures and the closed cube-like 3D structures already occurs in between n = 8 and 9 when sufficiently cold protonated water clusters are probed.

■

■

ASSOCIATED CONTENT

S Supporting Information *

EXPERIMENTAL METHODS IRPD experiments are carried out using a previously described ion trap tandem mass spectrometer,36,37 enhanced with a custom-built reflectron time-of-flight (TOF) mass spectrometer15 and using two widely tunable nanosecond OPO/OPA IR lasers (LASERVISION) operated at 10 Hz.38 Protonated water clusters are produced by electrospray of a 10 mM solution of HNO3 in a 1:4 water/acetonitrile mixture. Ions are collimated in a He-filled radio frequency (RF) ion guide, mass-selected in a quadrupole mass filter, and focused into a RF ring-electrode ion-trap. To allow for continuous ion loading, ion thermalization, and ion-messenger-complex formation, the trap is continuously filled with a buffer gas (H2 or D2) at 15 K.39 In a 10 Hz cycle, ions are extracted and focused into the center of the extraction region of a TOF mass spectrometer. Here, the

Computational methods. Comparison between experimental and calculated frequencies (Table S1). Minimum-energy structures and absolute and relative energies (Tables S2 and S3). Comparison between the experimental IRPD spectrum and harmonic IR spectra for H+(H2O)7 (Figure S1) and H+(H2O)8 (Figure S2). Cartesian coordinates (Table S4). Calculated frequencies and intensities (Table S5). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b00879.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 2302


Letter

The Journal of Physical Chemistry Letters Present Address

Knorke, H.; Asmis, K. R. Site-Specific Vibrational Spectral Signatures of Water Molecules in the Magic H3O+(H2O)20 and Cs+(H2O)20 Clusters. Proc. Natl. Acad. U.S.A. 2014, 111, 18132−18137. (18) Kulig, W.; Agmon, N. Both Zundel and Eigen Isomers Contribute to the IR Spectrum of the Gas-Phase H9O4+ Cluster. J. Phys. Chem. B 2014, 118, 278−286. (19) Kulig, W.; Agmon, N. Deciphering the Infrared Spectrum of the Protonated Water Pentamer and the Hybrid Eigen−Zundel Cation. Phys. Chem. Chem. Phys. 2014, 16, 4933−4941. (20) Heine, N.; Asmis, K. R. Cryogenic Ion Trap Vibrational Spectroscopy of Hydrogen-Bonded Clusters Relevant to Atmospheric Chemistry. Int. Rev. Phys. Chem. 2015, 34, 1−34. (21) Karthikeyan, S.; Kim, K. S. Structure, Stability, Thermodynamic Properties and IR Spectra of the Protonated Water Cluster H+(H2O)9. Mol. Phys. 2009, 107, 1169−1176. (22) Karthikeyan, S.; Park, M.; Shin, I.; Kim, K. S. Structure, Stability, Thermodynamic Properties, and Infrared Spectra of the Protonated Water Octamer H+(H2O)8. J. Phys. Chem. A 2008, 112, 10120−10124. (23) Karthikeyan, S.; Kim, K. S. Structure, Stability, Thermodynamic Properties, and IR Spectra of the Protonated Water Decamer H+(H2O)10. J. Phys. Chem. A 2009, 113, 9237−9242. (24) Luo, Y.; Maeda, S.; Ohno, K. Quantum Chemistry Study of H+(H2O)8: A Global Search for Its Isomers by the Scaled Hypersphere Search Method, and Its Thermal Behavior. J. Phys. Chem. A 2007, 111, 10732−10737. (25) Luo, Y.; Maeda, S.; Ohno, K. Automated Exploration of Stable Isomers of H+(H2O)n (n=5−7) via Ab Initio Calculations: An Application of the Anharmonic Downward Distortion Following Algorithm. J. Comput. Chem. 2009, 30, 952−961. (26) Christie, R. A.; Jordan, K. D. Finite Temperature Behavior of H+(H2O)6 and H+(H2O)8. J. Phys. Chem. B 2002, 106, 8376−8381. (27) Nguyen, Q. C.; Ong, Y. S.; Kuo, J. L. A Hierarchical Approach to Study the Thermal Behavior of Protonated Water Clusters H+(H2O)n. J. Chem. Theory Comput. 2009, 5, 2629−2639. (28) Elliott, B. M.; Relph, R. A.; Roscioli, J. R.; Bopp, J. C.; Gardenier, G. H.; Guasco, T. L.; Johnson, M. A. Isolating the Spectra of Cluster Ion Isomers using Ar-“Tag”-Mediated IR−IR Double Resonance within the Vibrational Manifolds: Application to NO2−· H2O. J. Chem. Phys. 2008, 129, 094303. (29) Page, R. H.; Shen, Y. R.; Lee, Y. T. Local Modes of Benzene and Benzene Dimer, Studied by Infrared−Ultraviolet Double-Resonance in a Supersonic Beam. J. Chem. Phys. 1988, 88, 4621−4636. (30) Riehn, C.; Lahmann, C.; Wassermann, B.; Brutschy, B. IR Depletion Spectroscopy  A Method for Characterizing a Microsolvation Environment. Chem. Phys. Lett. 1992, 197, 443−450. (31) Tanabe, S.; Ebata, T.; Fujii, M.; Mikami, N. OH Stretching Vibrations of Phenol−(H2O)N (N=1−3) Complexes Observed by IR−UV Double-Resonance Spectroscopy. Chem. Phys. Lett. 1993, 215, 347−352. (32) Pribble, R. N.; Zwier, T. S. Size-Specific Infrared-Spectra of Benzene-(H2O)N Clusters (N=1−7)  Evidence for Noncyclic (H2O)N Structures. Science 1994, 265, 75−79. (33) Shin, I.; Park, M.; Min, S. K.; Lee, E. C.; Suh, S. B.; Kim, K. S. Structure and Spectral Features of H+(H2O)7: Eigen versus Zundel Forms. J. Chem. Phys. 2006, 125, 234305. (34) Jiang, L.; Sun, S.-T.; Heine, N.; Liu, J.-W.; Yacovitch, T. I.; Wende, T.; Liu, Z.-F.; Neumark, D. M.; Asmis, K. R. Large Amplitude Motion in Cold Monohydrated Dihydrogen Phosphate Anion H2PO4−(H2O): Infrared Photodissociation Spectroscopy Combined with Ab Initio Molecular Dynamics Simulations. Phys. Chem. Chem. Phys. 2014, 16, 1314−1318. (35) Sun, S.-T.; Jiang, L.; Liu, J. W.; Heine, N.; Yacovitch, T. I.; Wende, T.; Asmis, K. R.; Neumark, D. M.; Liu, Z.-F., Microhydrated Dihydrogen Phosphate Clusters Probed by Gas Phase Vibrational Spectroscopy and First Principles Calculations Phys. Chem. Chem. Phys. accepted. (36) Goebbert, D. J.; Meijer, G.; Asmis, K. R. 10K Ring Electrode Trap−Tandem Mass Spectrometer for Infrared Spectroscopy of Mass Selected Ions. AIP Conf. Proc. 2009, 1104, 22−29.

⊥

N.H.: Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, U.S.A. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS We gratefully acknowledge Harald Knorke (University of Leipzig) for structure analysis. REFERENCES

(1) Arijs, E.; Ingels, J.; Nevejans, D. Mass-Spectrometric Measurement of Positive-Ion Composition in Stratosphere. Nature 1978, 271, 642−644. (2) Goken, E. G.; Castleman, A. W., Jr. Reactions of Formic Acid with Protonated Water Clusters: Implications of Cluster Growth in the Atmosphere. J. Geophys. Res. D: Atmos. 2010, 115, D16203. (3) Aplin, K. L.; Lockwood, M. Cosmic Ray Modulation of Infra-Red Radiation in the Atmosphere. Environ. Res. Lett. 2013, 8, 015026. (4) Fujii, A.; Mizuse, K. Infrared Spectroscopic Studies on HydrogenBonded Water Networks in Gas Phase Clusters. Int. Rev. Phys. Chem. 2013, 32, 266−307. (5) Knight, C.; Voth, G. A. The Curious Case of the Hydrated Proton. Acc. Chem. Res. 2012, 45, 101−109. (6) Yeh, L. I.; Okumura, M.; Myers, J. D.; Price, J. M.; Lee, Y. T. Vibrational Spectroscopy of the Hydrated Hydronium Cluster Ions H3O+(H2O)N (N = 1, 2, 3). J. Chem. Phys. 1989, 91, 7319−7330. (7) Jiang, J. C.; Wang, Y. S.; Chang, H. C.; Lin, S. H.; Lee, Y. T.; Niedner-Schatteburg, G.; Chang, H. C. Infrared Spectra of H+(H2O)5−8 Clusters: Evidence for Symmetric Proton Hydration. J. Am. Chem. Soc. 2000, 122, 1398−1410. (8) Shin, J. W.; Hammer, N. I.; Diken, E. G.; Johnson, M. A.; Walters, R. S.; Jaeger, T. D.; Duncan, M. A.; Christie, R. A.; Jordan, K. D. Infrared Signature of Structures associated with the H+(H2O)n (n=6 to 27) Clusters. Science 2004, 304, 1137−1140. (9) Miyazaki, M.; Fujii, A.; Ebata, T.; Mikami, N. Infrared Spectroscopic Evidence for Protonated Water Clusters forming Nanoscale Cages. Science 2004, 304, 1134−1137. (10) Lin, C. K.; Wu, C. C.; Wang, Y. S.; Lee, Y. T.; Chang, H. C.; Kuo, J. L.; Klein, M. L. Vibrational Predissociation Spectra and Hydrogen-Bond Topologies of H+(H2O)9−11. Phys. Chem. Chem. Phys. 2005, 7, 938−944. (11) Headrick, J. M.; Diken, E. G.; Walters, R. S.; Hammer, N. I.; Christie, R. A.; Cui, J.; Myshakin, E. M.; Duncan, M. A.; Johnson, M. A.; Jordan, K. D. Spectral Signatures of Hydrated Proton Vibrations in Water Clusters. Science 2005, 308, 1765−1769. (12) Douberly, G. E.; Walters, R. S.; Cui, J.; Jordan, K. D.; Duncan, M. A. Infrared Spectroscopy of Small Protonated Water Clusters, H+(H2O)n (n = 2−5): Isomers, Argon Tagging, and Deuteration. J. Phys. Chem. A 2010, 114, 4570−4579. (13) Mizuse, K.; Fujii, A. Infrared Photodissociation Spectroscopy of H+(H2O)6·Mm (M = Ne, Ar, Kr, Xe, H2, N2, and CH4): MessengerDependent Balance between H3O+ and H5O2+ Core Isomers. Phys. Chem. Chem. Phys. 2011, 13, 7129−7135. (14) Mizuse, K.; Fujii, A. Tuning of the Internal Energy and Isomer Distribution in Small Protonated Water Clusters H+(H2O)4−8: An Application of the Inert Gas Messenger Technique. J. Phys. Chem. A 2012, 116, 4868−4877. (15) Heine, N.; Fagiani, M. R.; Rossi, M.; Wende, T.; Berden, G.; Blum, V.; Asmis, K. R. Isomer-Selective Detection of Hydrogen-Bond Vibrations in the Protonated Water Hexamer. J. Am. Chem. Soc. 2013, 135, 8266−8273. (16) Fournier, J. A.; Johnson, C. J.; Wolke, C. T.; Weddle, G. H.; Wolk, A. B.; Johnson, M. A. Vibrational Spectral Signature of the Proton Defect in the Three-Dimensional H+(H2O)21 Cluster. Science 2014, 344, 1009−1012. (17) Fournier, J. A.; Wolke, C. T.; Johnson, C. J.; Johnson, M. A.; Heine, N.; Gewinner, S.; Schoellkopf, W.; Esser, T. K.; Fagiani, M. R.; 2303


Letter

The Journal of Physical Chemistry Letters (37) Goebbert, D. J.; Wende, T.; Bergmann, R.; Meijer, G.; Asmis, K. R. Messenger-Tagging Electrosprayed Ions: Vibrational Spectroscopy of Suberate Dianions. J. Phys. Chem. A 2009, 113, 5874−5880. (38) Bosenberg, W. R.; Guyer, D. R. Broadly Tunable, SingleFrequency Optical Parametric Frequency Conversion System. J. Opt. Soc. Am. B 1993, 10, 1716−1722. (39) Brümmer, M.; Kaposta, C.; Santambrogio, G.; Asmis, K. R. Formation and Photodepletion of Cluster Ion-Messenger Atom Complexes in a Cold Ion Trap: Infrared Spectroscopy of VO+, VO2+, and VO3+. J. Chem. Phys. 2003, 119, 12700−12703.

2304


Disentangling the Contribution of Multiple Isomers ... - ACS Publications

Recommend Documents