12The reaction of Cl atoms, in the presence of Cl 2 and O 2 , with sub-micron squalane particles is 13 used as a model system to explore how surface hydrogen abstraction reactions initiate chain 14 reactions that rapidly transform the chemical composition of an organic particle.

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revealing changes in the appearance energy due to the amount of internal energy in the ion pairs.The aerosol source has a shift to higher threshold energy (~0.3 eV), attributed to reduced internal energy of the isolated ion pairs. The method of ionic liquid submicron aerosol particle vaporization, for reactive ionic liquids such as hypergolic species, is a convenient, thermally "cooler" source of isolated intact ion pairs in the gas phase compared to effusive sources.Keywords: ionic liquid, aerosol, ion pair, gas phase, photoionization efficiency, synchrotron Intact cations are observed whether the origin of the ion pair vapor is a bulk sample or a thin film heated for the vaporization. However, highly reactive ionic liquids show dissociative ionization as well as decomposition, and it becomes difficult to identify the intact cation signal and to distinguish thermal decomposition from fragmentation of ion pairs upon ionization. This makes it nearly impossible to study reaction mechanisms and kinetics of hypergolic ionic liquids because of the difficulty of detecting small changes in their complicated mass spectra.Typically ions with high internal energies fragment extensively producing a mass spectrum that contains a wide variety of abundant fragment ions. 35 The internal energy content of the molecular ion (M + ) is from two sources: the thermal energy from evaporation and the energy imparted by the ionization process. Molecular ions (M + ) of labile molecules can only be detected if the internal energy of the molecular ion is kept very low, by obtaining mass spectra with low photon energies and low temperatures. Aerosol particle generation [36][37][38][39][40][41] has previously been demonstrated as a new way to introduce fragile biomolecules into the gas phase with nearly fragmentation-free mass spectra by minimizing their internal energy imparted into the molecular ion in gas phase. We apply this new method to hypergolic IL studies, to produce isolated ion pairs in the gas phase from IL aerosol particles followed by thermal vaporization, and monitor the ion pair vapor by soft ionization using tunable vacuum ultraviolet (VUV) photoionization mass spectrometry. This report focuses on comparing the degree of fragmentation depending on the isolated ion pair vapor source of a hypergolic ionic liquid, 1-Butyl-3-Methyl-Imidazolium Experimental ApparatusIsolated ion pairs of ionic liquids are generated in the gas phase by thermal vaporization of IL aerosol particles and are monitored using soft ionization detection with tunable vacuum ultraviolet (VUV) photoionization mass spectrometry. Those ion pairs that are generated by aerosol particles are compared to those generated by a conventional effusive beam. 26,28,29 The aerosol experimental apparatus at the Chemical Dynamics Beamline 9.0.2.1 of the Advanced Light Source in Berkeley, California, previously described in detail, 35 includes a particle generation system, a particle size analyzer, and an aerosol mass spectrometer (AMS). In the other sets of experiments, i...

The controlled breaking of a specific chemical bond with photons in complex molecules remains a major challenge in chemistry. In principle, using the K-edge absorption of a particular atomic element, one might excite selectively a specific atomic entity in a molecule. We report here highly selective dissociation of the peptide bonds in N-methylformamide and N-methylacetamide on tuning the X-ray wavelength to the K-edge absorption of the atoms connected to (or near) the peptide bond. The high selectivity (56-71%) of this cleavage arises from the large energy shift of X-ray absorption, a large overlap of the 1s orbital and the valence π* orbital that is highly localized on a peptide bond with antibonding character, and the relatively low bond energy of the peptide bonds. These characteristics indicate that the high selectivity on bond dissociation following core excitation could be a general feature for molecules containing peptide bonds.

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Total-ion-yield near-edge X-ray absorption fine-structure (NEXAFS) spectra of three oligo-peptide model molecules were recorded. The NEXAFS spectra were also calculated with time-dependent density functional theory; the calculated spectra are consistent with the experimental measurements. The ionic products from core-excited molecules and their branching ratios as functions of excitation energy show branching ratios 42%-75% of dissociation of the peptide bond, especially at resonant excitations. The major destination orbitals assigned at these excitation energies show significant antibonding character along peptide bonds, which causes large probabilities of bond cleavage.

While examining the heterogeneous reaction of chlorine atoms with alkenes, in the presence of Cl2, we have observed an unexpectedly large enhancement of reactivity and the predominance of chlorinated reaction products even under high O2 conditions, where Cl atom recycling is expected to be minimal. These observations cannot be explained by known free radical oxidation or cycling mechanisms, but rather we find evidence for the multiphase catalytic coupling of free radical oxidation with electrophilic Cl2 addition. The mechanism entails the production of oxygenated reaction intermediates, which act as gas–liquid phase-transfer catalysts (gl-PTCs) by promoting the accommodation of gas-phase Cl2 by the aerosol, thereby enhancing electrophilic addition. Although the majority of PTCs typically couple chemistry between two immiscible liquid phases (aqueous/organic), there are few examples of PTCs that couple gas–liquid reactions. This work shows how multiphase reaction schemes of aerosols can be reimagined for understanding catalytic reaction mechanisms.

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