Acid-base cluster chemistry drives atmospheric new particle formation (NPF), but the details of the growth mechanisms are difficult to experimentally probe. Clusters of ammonia, alkylamines, and sulfuric acid, species fundamental to NPF, are probed by infrared spectroscopy. These spectra show that substitution of amines for ammonia, which is linked to accelerated growth, induces profound structural rearrangement in clusters with initial compositions (NH) (HSO) (1 ≤ n ≤ 3). This rearrangement is driven by the loss of N-H hydrogen bond donors, yielding direct bisulfate-bisulfate hydrogen bonds, and its onset with respect to cluster composition indicates that more substituted amines induce rearrangement at smaller sizes. A simple model counting hydrogen bond donors and acceptors explains these observations. The presence of direct hydrogen bonds between formal anions shows that hydrogen bonding can compete with Coulombic forces in determining cluster structure. These results suggest that NPF mechanisms may be highly dependent on amine identity.

The formation of ternary aqua complexes of metal-based diagnostics and therapeutics is closely correlated to their in vivo efficacy but approaches to quantify the presence of coordinated water ligands are limited. We introduce a general and high-throughput method for characterizing the hydration state of para- and diamagnetic coordination complexes in the gas phase based on variable-temperature ion trap tandem mass spectrometry. Ternary aqua complexes are directly observed in the mass spectrum and quantified as a function of ion trap temperature. We recover expected periodic trends for hydration across the lanthanides and distinguish complexes with several inner-sphere water ligands by inspection of temperature-dependent speciation curves. We derive gas-phase thermodynamic parameters for discernible inner- and second-sphere hydration events, and discuss their application to predict solution-phase behavior. The differences in temperature at which water binds in the inner and outer spheres arise primarily from entropic effects. The broad applicability of this method allows us to estimate the hydration states of Ga, Sc, and Zr complexes under active preclinical and clinical study with as-yet undetermined hydration number. Variable-temperature mass spectrometry emerges as a general tool to characterize and quantitate trends in inner-sphere hydration across the periodic table.

Organic acids play an important role in atmospheric chemistry, particularly in the formation of aerosol particles. Here we explore the reactivity of adipic acid, an analogue to the alpha-pinene oxidation product pinic acid, upon complexation with ammonium. Collision-induced dissociation mass spectra and mass-selective vibrational spectra show that relatively mild activation, consistent with breaking hydrogen bonds, yields (adipic acid)H+ and neutral ammonia. This is consistent with a higher proton affinity for adipic acid than ammonia that we trace to a specific structural motif in which both protonated carboxylic acid carbonyl groups combine to form a basic site that supports an additional bridging proton. Further mild collisional activation yields sequential loss of two water molecules, similar to the behavior of carboxylic acids in superacids, necessitating abstraction of at least one hydrogen from a CH group. Deuterium labeling experiments confirm that the second step indeed involves CH hydrogen atoms. Comparison of vibrational spectra and quantum chemical calculations allows us to assign structures for each step, identifying several ring structures but notably not forming the minimum energy structure upon the first loss of water. We propose a mechanism that explains this reactivity and discuss possible atmospheric implications of these observations.

Organic acids play an important role in atmospheric chemistry, particularly in the formation of aerosol particles. Here we explore the reactivity of adipic acid, an analogue to the alpha-pinene oxidation product pinic acid, upon complexation with ammonium. Collision-induced dissociation mass spectra and mass-selective vibrational spectra show that relatively mild activation, consistent with breaking hydrogen bonds, yields (adipic acid)H+ and neutral ammonia. This is consistent with a higher proton affinity for adipic acid than ammonia that we trace to a specific structural motif in which both protonated carboxylic acid carbonyl groups combine to form a basic site that supports an additional bridging proton. Further mild collisional activation yields sequential loss of two water molecules, similar to the behavior of carboxylic acids in superacids, necessitating abstraction of at least one hydrogen from a CH group. Deuterium labeling experiments confirm that the second step indeed involves CH hydrogen atoms. Comparison of vibrational spectra and quantum chemical calculations allows us to assign structures for each step, identifying several ring structures but notably not forming the minimum energy structure upon the first loss of water. We propose a mechanism that explains this reactivity and discuss possible atmospheric implications of these observations.

It is estimated that ˜50% of climatically relevant atmospheric aerosols arise from new particle formation (NPF), the process by which trace atmospheric gases such as sulfuric acid and ammonia cluster and grow. Amines are expected to enhance NPF, with greater enhancement from larger amines. Using cryogenic ion vibrational predissociation (CIVP) spectroscopy, we studied the structural evolution of clusters with up to 3 sulfuric acids. It is shown that substitution of amines for ammonia can induce structural rearrangement, which is driven by the ability of the alkylamines (MA, DMA, TMA) to form hydrogen bonds, and can lead to direct bisulfate-bisulfate hydrogen bonds. This direct interaction between formal anions indicates that hydrogen bonding can compete with Coulombic force in determining cluster structure. From these observations we have developed a model to predict when these arrangements may arise in ionic and neutral clusters with a variety of compositions. This structural motif is correlated with the fastest growing amines, and could play a role in the mechanism of NPF.