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Surface-induced dissociation (SID) is a mass spectrometry technique that has been employed to fragment and structurally characterize ions via collisions with surfaces. In particular, the SID technique has been experimentally applied to examine the dissociation of protonated peptides via collisions with a fluorinated alkylthiol self-assembled monolayer (F-SAM) on a Au(111) surface. These studies found that the efficiency of kinetic to internal energy transfer to the ion during the collision with the surface depends on the properties of both the ion and the surface. An atomic level understanding of the dynamics of the collisional energy transfer that occurs upon SID could be obtained from classical trajectory simulations. In the current study, an F-SAM surface is built from a sum of simple model interactions. The MP2/6-311++G(2df,2pd) level of theory was used to calculate intermolecular potential curves between CF 4 , as a model for the C and F atoms of a fluorinated alkylthiol surface, while CH 4 , NH 3 , NH 4 + , H 2 CO, and H 2 O were employed as models for the different various functional groups comprising protonated peptide ions. This level of theory was tested by comparisons with MP2/aug-cc-pVTZ and CCSD(T)/aug-cc-pVTZ theories. Explicit-atom (EA) analytical potential energy functions were then derived by fitting these potential energy curves with two-body potentials between the atoms of the two interacting molecules. An intermolecular potential for the interaction of a protonated peptide ion with a fluorinated alkylthiol surface may be constructed from these two-body potentials. Intermolecular potentials, for which CF 4 is treated as a united-atom (UA), were developed by isotropically averaging the CF 4 orientation for each of the EA potential energy curves. The intermolecular potential energy curves calculated for CF 4 are compared with curves calculated previously for CH 4 interacting with the same molecules, to consider the relative efficiency of energy transfer for protonated peptide ion collisions with hydrogenated and fluorinated alkylthiol surfaces. Classical trajectory simulations are performed to study energy transfer in collisions of protonated peptide ions with the F-SAM surface over a broad range of collision energies and various incident angles. The distribution of energy transfer to vibrational/rotational degrees of freedom, to the surface, and of energy remaining in peptide ion translation is compared for the hydrogenated and fluorinated alkylthiol surfaces |
