Size-dependent proton localization in hydrated uracil clusters: A joint experimental and theoretical study

Area of Science:

• Physical Chemistry
• Chemical Physics
• Molecular Dynamics

Background:

• Protonated uracil clusters are important in understanding molecular interactions and reaction mechanisms.
• Hydration significantly influences the structure and reactivity of molecular ions.

Purpose of the Study:

• To investigate the collision-induced dissociation of hydrated protonated uracil clusters (H2O)nUH+.
• To determine how cluster size and structure affect fragmentation pathways and products.

Main Methods:

• Mass-selected cluster beams were subjected to collision-induced dissociation with water molecules and rare gases.
• Absolute fragmentation cross sections and branching ratios were measured.
• Density-functional based tight-binding and MP2 calculations were used to determine low-energy isomer structures.

Main Results:

• A transition in evaporation products was observed: only water evaporated from small clusters (n=1-4), while uracil also evaporated from larger clusters.
• Structural analysis revealed the proton is localized on uracil or strongly bound water in small clusters, but distant from uracil in larger clusters.
• Fragmentation channels correlate with low-energy isomer structures, suggesting impulsive dissociation dynamics.

Conclusions:

• The fragmentation behavior of hydrated protonated uracil clusters is strongly dependent on cluster size and the resulting proton localization.
• The observed transition in fragmentation products is explained by the structural differences between small and large clusters.
• Dissociation dynamics appear to be largely impulsive, with cluster geometry influencing fragmentation pathways.