Quantum Chemical Insights into the Dissociation of Phenol: Shedding Light on Impact Ionization Mass Spectrometry for Icy Moon Exploration

Ice grains emitted by the Saturnian moon Enceladus were sampled by Cassini’s Cosmic Dust Analyser (CDA) using impact ionization mass spectrometry. CDA revealed that Enceladus hosts a rich organic and inorganic chemical inventory in its subsurface ocean, hinting at its potential habitability. Analysis of fragmentation patterns with laser desorption experiments for the interpretation of CDA data has been essential; however, theoretical insights regarding both fragmentation and ionization processes are often missing. Here, we use density functional theory methods to investigate the energies for dissociation channels of phenol, a model aromatic compound for the features observed by CDA. The fragmentation channels are compared to experimental spectra obtained by using laser-induced liquid beam ion desorption (LILBID) mass spectrometry, an analogue for ice impact mass spectra. Our findings suggest that protonation is the dominant mechanism of ionization, that dissociation from the radical cation and neutral phenol molecule is limited, and that multiple isomers of the protonated molecule act as starting points for dissociation. The highest-intensity organic fragments observed in the LILBID spectrum─arising from the losses of CO, [M + H–CO]+, and water, [M + H–H2O]+─are found to be both thermodynamically and kinetically accessible. We examined water–molecule interactions during the initial production of the protonated molecule. The presence of water significantly influences the preferred site of protonation and causes variation in the relative energy ordering of the protomers. This work builds toward a computational model of ice grain impact ionization mass spectrometry, relevant for missions such as Europa Clipper and ESA’s L4 mission to Enceladus.