Vibrational Spectra and Fragmentation Pathways of Size-Selected, D₂‑Tagged Ammonium/Methylammonium Bisulfate Clusters

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)_<i>m</i>(H₂SO₄)_<i>n</i> (2 ≤ <i>m</i> ≤ 5, 1 ≤ <i>n</i> ≤ 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 (<i>m</i> = 3, <i>n</i> = 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 multiple-photon 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

Integration of Cryogenic Ion Vibrational Predissociation Spectroscopy with a Mass Spectrometric Interface to an Electrochemical Cell

Cryogenic ion vibrational predissociation (CIVP) spectroscopy is used to structurally characterize electrochemically (EC)-generated oxidation products of the benchmark compound reserpine. Ionic products were isolated using EC-electrospray ionization (ESI) coupled to a 25 K ion trap prior to injection into a double-focusing, tandem time-of-flight photofragmentation mass spectrometer. Vibrational predissociation spectroscopy was carried out by photoevaporation of weakly bound N₂ adducts over the range 800–3800 cm^–1 in a linear (i.e., single photon) action regime, thus enabling direct comparison of the experimental vibrational pattern with harmonic calculations. The locations of the NH and OH stretching fundamentals are most consistent with formation of 9-hydroxyreserpine, which is a different isomer than considered previously. This approach thus provides a powerful structural dimension for the analysis of electrochemical processes detected with the sensitivity of mass spectrometry

Anharmonic Densities of States for Vibrationally Excited I^–(H₂O), (H₂O)₂, and I^–(H₂O)₂

Monte Carlo sampling calculations were performed to determine the anharmonic sum of states, <i>N</i>_anh(<i>E</i>), for I^–(H₂O), (H₂O)₂, and I^–(H₂O)₂ versus internal energy up to their dissociation energies. The anharmonic density of states, ρ_anh(<i>E</i>), is found from the energy derivative of <i>N</i>_anh(<i>E</i>). Analytic potential energy functions are used for the calculations, consisting of TIP4P for H₂O···H₂O interactions and an accurate two-body potential for the I^–···H₂O fit to quantum chemical calculations. The extensive Monte Carlo samplings are computationally demanding, and the use of computationally efficient potentials was essential for the calculations. Particular emphasis is directed toward I^–(H₂O)₂, and distributions of its structures versus internal energy are consistent with experimental studies of the temperature-dependent vibrational spectra. At their dissociation thresholds, the anharmonic to harmonic density of states ratio, ρ_anh(<i>E</i>)/ρ_h(<i>E</i>), is ∼2, ∼ 3, and ∼260 for I^–(H₂O), (H₂O)₂, and I^–(H₂O)₂, respectively. The large ratio for I^–(H₂O)₂ results from the I^–(H₂O)₂ → I^–(H₂O) + H₂O dissociation energy being more than 2 times larger than the (H₂O)₂ → 2H₂O dissociation energy, giving rise to highly mobile H₂O molecules near the I^–(H₂O)₂ dissociation threshold. This work illustrates the importance of treating anharmonicity correctly in unimolecular rate constant calculations

Persistence of Dual Free Internal Rotation in NH₄⁺(H₂O)·He_{<i>n</i>=0–3} Ion–Molecule Complexes: Expanding the Case for Quantum Delocalization in He Tagging

To explore the extent of the molecular cation perturbation induced by complexation with He atoms required for the application of cryogenic ion vibrational predissociation (CIVP) spectroscopy, we compare the spectra of a bare NH₄⁺(H₂O) ion (obtained using infrared multiple photon dissociation (IRMPD)) with the one-photon CIVP spectra of the NH₄⁺(H₂O)·He_1–3 clusters. Not only are the vibrational band origins minimally perturbed, but the rotational fine structures on the NH and OH asymmetric stretching vibrations, which arise from the free internal rotation of the −OH₂ and −NH₃ groups, also remain intact in the adducts. To establish the location and the quantum mechanical delocalization of the He atoms, we carried out diffusion Monte Carlo (DMC) calculations of the vibrational zero point wave function, which indicate that the barriers between the three equivalent minima for the He attachment are so small that the He atom wave function is delocalized over the entire −NH₃ rotor, effectively restoring <i>C</i>₃ symmetry for the embedded −NH₃ group

Vibrational Signatures of Solvent-Mediated Deformation of the Ternary Core Ion in Size-Selected [MgSO₄Mg(H₂O)_{<i>n</i>=4–11}]²⁺ Clusters

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)_{<i>n</i>=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 SO₄^2– ion in a bifurcated fashion to yield a <i>D</i>_2<i>d</i> symmetry arrangement. Analysis of the observed spectral patterns indicate that water molecules preferentially attach to the flanking Mg²⁺ ions for the <i>n</i> ≤ 7 hydrates, which results in an incremental weakening of the interaction between the ions. Water molecules begin to interact with the sequestered SO₄^2– anion promptly at <i>n</i> = 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 <i>n</i> = 11 cluster occurs with the onset of the second hydration shell such that the cation remains coordinated to one of the SO₄^2– oxygen atoms

Coordination-Dependent Spectroscopic Signatures of Divalent Metal Ion Binding to Carboxylate Head Groups: H₂- and He-Tagged Vibrational Spectra of M²⁺·RCO₂¯ (M = Mg and Ca, R = −CD₃, −CD₂CD₃) Complexes

We explore the intramolecular distortions present in divalent metal ion–carboxylate ion pairs using vibrational spectroscopy of the cryogenically cooled, mass-selected species isolated in the gas phase. The spectral signatures of the C–O stretching modes are identified using the perdeutero isotopologues of the acetate and propionate anions to avoid congestion arising from the CH₂ fundamentals. Both Ca²⁺ and Mg²⁺ are observed to bind in a symmetrical, so-called “bidentate” arrangement to the −CO₂¯ group. The very strong deformations of the head groups displayed by the binary complexes dramatically relax when either neutral water molecules or counterions are attached to the Mg²⁺RCO₂¯ cation. These results emphasize the critical role that local coordination plays when using the RCO₂¯ bands to deduce the metal ion complexation motif in condensed media

Unmasking Rare, Large-Amplitude Motions in D₂‑Tagged I^–·(H₂O)₂ Isotopomers with Two-Color, Infrared–Infrared Vibrational Predissociation Spectroscopy

We describe a two-color, isotopomer-selective infrared–infrared population-labeling method that can monitor very slow spectral diffusion of OH oscillators in H‑bonded networks and apply it to the I^–·(HDO)·(D₂O) and I^–·(H₂O)·(D₂O) systems, which are cryogenically cooled and D₂-tagged at an ion trap temperature of 15 K. These measurements reveal very large (>400 cm^–1), spontaneous spectral shifts despite the fact that the predissociation spectra in the OH stretching region of both isotopologues are sharp and readily assigned to four fundamentals of largely decoupled OH oscillators held in a cyclic H-bonded network. This spectral diffusion is not observed in the untagged isotopologues of the dihydrate clusters that are generated under the same source conditions but does become apparent at about 75 K. These results are discussed in the context of the large-amplitude “jump” mechanism for H-bond relaxation dynamics advanced by Laage and Hynes in an experimental scenario where rare events can be captured by following the migration of OH groups among the four available positions in the quasi-rigid equilibrium structure

Isolating the Spectral Signatures of Individual Sites in Water Networks Using Vibrational Double-Resonance Spectroscopy of Cluster Isotopomers

We report the spectral signatures of water molecules occupying individual sites in an extended H-bonding network using mass-selective, double-resonance vibrational spectroscopy of isotopomers. The scheme is demonstrated on the water heptamer anion, (H₂O)₇¯, where we first randomly incorporate a single, intact D₂O molecule to create an ensemble of isotopomers. The correlation between the two OD stretching frequencies and that of the intramolecular DOD bending transition is then revealed by photochemical modulation of the isotopomer population responsible for particular features in the vibrational spectrum. The observed patterns confirm the assignment of the dominant doublet, appearing most red-shifted from the free OD stretch, to a single water molecule attached to the network in a double H-bond acceptor (AA) arrangement. The data also reveal the unanticipated role of accidentally overlapping transitions, where the highest-energy OD stretch, for example, occurs with its companion OD stretch obscured by the much stronger AA feature

Diffuse Vibrational Signature of a Single Proton Embedded in the Oxalate Scaffold, HO₂CCO₂^–

To understand how the <i>D</i>_2<i>d</i> oxalate scaffold (C₂O₄)^2– distorts upon capture of a proton, we report the vibrational spectra of the cryogenically cooled HO₂CCO₂^– anion and its deuterated isotopologue DO₂CCO₂^–. The transitions associated with the skeletal vibrations and OH bending modes are sharp and are well described by inclusion of cubic terms in the normal mode expansion of the potential surface through an extended Fermi resonance analysis. The ground state structure features a five-membered ring with an asymmetric intramolecular proton bond. The spectral signatures of the hydrogen stretches, on the contrary, are surprisingly diffuse, and this behavior is not anticipated by the extended Fermi scheme. We trace the diffuse bands to very strong couplings between the high-frequency OH-stretch and the low-frequency COH bends as well as heavy particle skeletal deformations. A simple vibrationally adiabatic model recovers this breadth of oscillator strength as a 0 K analogue of the motional broadening commonly used to explain the diffuse spectra of H-bonded systems at elevated temperatures, but where these displacements arise from the configurations present at the vibrational zero-point level

Hiding in Plain Sight: Unmasking the Diffuse Spectral Signatures of the Protonated N‑Terminus in Isolated Dipeptides Cooled in a Cryogenic Ion Trap

Survey vibrational predissociation spectra of several representative protonated peptides and model compounds reveal very diffuse absorptions near 2500 cm^–1 that are traced to pentagonal cyclic ionic hydrogen bonds (C₅ interactions) involving the excess charge centers. This broadening occurs despite the fact that the ions are cooled close to their vibrational zero-point levels and their spectra are obtained by predissociation of weakly bound adducts (H₂, N₂, CO₂) prepared in a cryogenic ion trap. The C₅ band assignments are based on H/D isotopic substitution, chemical derivatization, solvation behavior, and calculated spectra. We evaluate the extent to which this broadening is caused by anharmonic coupling in the isolated molecules by including cubic coupling terms in the normal mode expansion of the potential energy surface. This analysis indicates that the harmonic H-bonded stretching vibration is mixed with dark background states over much of the energy range covered by the observed features. The difficulty with identifying these features in earlier studies of dipeptides is traced to both the breadth and the fact they are calculated to be intrinsically weaker than cases involving linear variations of the N···H⁺···O motif