Clustering of Uracil Molecules on Ice Nanoparticles

We generate a molecular beam of ice nanoparticles (H₂O)_<i>N</i>, <i>N̅</i> ≈ 130–220, which picks up several individual gas phase uracil (U) or 5-bromouracil (BrU) molecules. The mass spectra of the doped nanoparticles prove that the uracil and bromouracil molecules coagulate to clusters on the ice nanoparticles. Calculations of U and BrU monomers and dimers on the ice nanoparticles provide theoretical support for the cluster formation. The (U)_<i>m</i>H⁺ and (BrU)_<i>m</i>H⁺ intensity dependencies on <i>m</i> extracted from the mass spectra suggest a smaller tendency of BrU to coagulate compared to U, which is substantiated by a lower mobility of bromouracil on the ice surface. The hydrated U_<i>m</i>·(H₂O)_<i>n</i>H⁺ series are also reported and discussed. On the basis of comparison with the previous experiments, we suggest that the observed propensity for aggregation on ice nanoparticles is a more general trend for biomolecules forming strong hydrogen bonds. This, together with their mobility, leads to their coagulation on ice nanoparticles which is an important aspect for astrochemistry

Biomolecule Analogues 2‑Hydroxypyridine and 2‑Pyridone Base Pairing on Ice Nanoparticles

Ice nanoparticles (H₂O)_<i>N</i>, <i>N</i> ≈ 450 generated in a molecular beam experiment pick up individual gas phase molecules of 2-hydroxypyridine and 2-pyridone (HP) evaporated in a pickup cell at temperatures between 298 and 343 K. The mass spectra of the doped nanoparticles show evidence for generation of clusters of adsorbed molecules (HP)_<i>n</i> up to <i>n</i> = 8. The clusters are ionized either by 70 eV electrons or by two photons at 315 nm (3.94 eV). The two ionization methods yield different spectra, and their comparison provides an insight into the neutral cluster composition, ionization and intracluster ion–molecule reactions, and cluster fragmentation. Quite a few molecules were reported <i>not to coagulate</i> on ice nanoparticles previously. The (HP)_<i>n</i> cluster generation on ice nanoparticles represents the first evidence for coagulating of molecules and cluster formation on free ice nanoparticles. For comparison, we investigate the coagulation of HP molecules picked up on large clusters Ar_<i>N</i>, <i>N</i> ≈ 205, and also (HP)_<i>n</i> clusters generated in supersonic expansions with Ar buffer gas. This comparison points to a propensity for the (HP)₂ dimer generation on ice nanoparticles. This shows the feasibility of base pairing for model of biological molecules on free ice nanoparticles. This result is important for hypotheses of the biomolecule synthesis on ice grains in the space. We support our findings by theoretical calculations that show, among others, the HP dimer structures on water clusters

Reactivity of Hydrated Electron in Finite Size System: Sodium Pickup on Mixed N₂O–Water Nanoparticles

We investigate the reactivity of hydrated electron generated by alkali metal deposition on small water particles with nitrous oxide dopant by means of mass spectrometry and ab initio molecular dynamics simulations. The mixed nitrous oxide/water clusters were generated in a molecular beam and doped with Na atoms in a pickup experiment, and investigated by mass spectrometry using two different ionization schemes: an electron ionization (EI), and UV photoionization after the Na doping (NaPI). The NaPI is a soft-ionization nondestructive method, especially for water clusters provided that a hydrated electron <i>e</i>_s^– is formed in the cluster. The missing signal for the doped clusters indicates that the hydrated electron is not present in the N₂O containing clusters. The simulations reveal that the hydrated electron is formed, but it immediately reacts with N₂O, forming first N₂O^– radical anion, later O^–, and finally an OH^• and OH^– pair

Photochemistry of Nitrophenol Molecules and Clusters: Intra- vs Intermolecular Hydrogen Bond Dynamics

We investigate both experimentally and theoretically the structure and photodynamics of nitrophenol molecules and clusters, addressing the question how the molecular photodynamics can be controlled by specific inter- and intramolecular interactions. Using quantum chemical calculations, we demonstrate the structural and energetic differences between clusters of 2-nitrophenol and 4-nitrophenol, using phenol as a reference system. The calculated structures are supported by mass spectrometry. The mass spectra of 2-nitrophenol clusters provide an evidence for a stacked structure compared to a strong O–H···O hydrogen bonding for 4-nitrophenol aggregates. We further investigate the photodynamics of nitrophenol molecules and clusters by means of velocity map imaging of the H-fragment generated upon 243 nm photodissociation. The experiments are complemented by <i>ab initio</i> calculations which demonstrate distinct photophysics of phenol, 2-nitrophenol, 4-nitrophenol. The measured H-fragment kinetic energy distributions (KEDs) from 2-nitrophenol molecules are compared to the KEDs from phenol. The comparison points to the intramolecular O–H···O hydrogen bond in 2‑nitrophenol, stimulating fast internal conversion into the ground electronic state. This reaction channel is marked by exclusive appearance of slow statistical hydrogen fragments in 2-nitrophenol, which contrasts with fast hydrogen atoms observed for phenol. The photodissociation of 2-nitrophenol clusters yields a fraction of H-fragments with higher kinetic energies than the isolated molecules. These fragments originate from the caging effect in the clusters leading to multiphoton dissociation of molecules excited by the previous photons. We also propose a new <i>ab initio</i> based value for the O–H bond dissociation enthalpy in 2-nitrophenol (4.25 eV), which is in excellent agreement with the maximum measured H-fragment kinetic energy

Lack of Aggregation of Molecules on Ice Nanoparticles

Multiple molecules adsorbed on the surface of nanosized ice particles can either remain isolated or form aggregates, depending on their mobility. Such (non)­aggregation may subsequently drive the outcome of chemical reactions that play an important role in atmospheric chemistry or astrochemistry. We present a molecular beam experiment in which the controlled number of guest molecules is deposited on the water and argon nanoparticles in a pickup chamber and their aggregation is studied mass spectrometrically. The studied molecules (HCl, CH₃Cl, CH₃CH₂CH₂Cl, C₆H₅Cl, CH₄, and C₆H₆) form large aggregates on argon nanoparticles. On the other hand, no aggregation is observed on ice nanoparticles. Molecular simulations confirm the experimental results; they reveal a high degree of aggregation on the argon nanoparticles and show that the molecules remain mostly isolated on the water ice surface. This finding will influence the efficiency of ice grain-mediated synthesis (e.g., in outer space) and is also important for the cluster science community because it shows some limitations of pickup experiments on water clusters

Nucleation of Mixed Nitric Acid–Water Ice Nanoparticles in Molecular Beams that Starts with a HNO₃ Molecule

Mixed (HNO₃)_<i>m</i>(H₂O)_<i>n</i> clusters generated in supersonic expansion of nitric acid vapor are investigated in two different experiments, (1) time-of-flight mass spectrometry after electron ionization and (2) Na doping and photoionization. This combination of complementary methods reveals that only clusters containing at least one acid molecule are generated, that is, the acid molecule serves as the nucleation center in the expansion. The experiments also suggest that at least four water molecules are needed for HNO₃ acidic dissociation. The clusters are undoubtedly generated, as proved by electron ionization; however, they are not detected by the Na doping due to a fast charge-transfer reaction between the Na atom and HNO₃. This points to limitations of the Na doping recently advocated as a general method for atmospheric aerosol detection. On the other hand, the combination of the two methods introduces a tool for detecting molecules with sizable electron affinity in clusters

Clustering and Photochemistry of Freon CF₂Cl₂ on Argon and Ice Nanoparticles

The photochemistry of CF₂Cl₂ molecules deposited on argon and ice nanoparticles was investigated. The clusters were characterized via electron ionization mass spectrometry, and the photochemistry was revealed by the Cl fragment velocity map imaging after the CF₂Cl₂ photodissociation at 193 nm. The complex molecular beam experiment was complemented by ab initio calculations. The (CF₂Cl₂)_<i>n</i> clusters were generated in a coexpansion with Ar buffer gas. The photodissociation of molecules in the (CF₂Cl₂)_<i>n</i> clusters yields predominantly Cl fragments with zero kinetic energy: caging. The CF₂Cl₂ molecules deposited on large argon clusters in a pickup experiment are highly mobile and coagulate to form the (CF₂Cl₂)_<i>n</i> clusters on Ar_<i>N</i>. The photodissociation of the CF₂Cl₂ molecules and clusters on Ar_<i>N</i> leads to the caging of the Cl fragment. On the other hand, the CF₂Cl₂ molecules adsorbed on the (H₂O)_<i>N</i> ice nanoparticles do not form clusters, and no Cl fragments are observed from their photodissociation. Since the CF₂Cl₂ molecule was clearly adsorbed on (H₂O)_<i>N</i>, the missing Cl signal is interpreted in terms of surface orientation, possibly via the so-called halogen bond and/or embedding of the CF₂Cl₂ molecule on the disordered surface of the ice nanoparticles