Gas phase ion energetic studies via photoelectron imaging and energy resolved mass spectrometry

Abstract

Nitro containing compounds are well known energetic materials widely used in explosives. The shock sensitivity of nitro containing explosives has been linked to energetic properties such as bond dissociation energies, heats of formations, and molecular electronegativities. Measuring the energetic properties of nitro containing compounds can aide in the development and deployment of these materials, as well as influence practices in safe handling and use. The primary focus of this dissertation is investigating the energetic properties of nitro containing molecules using two experimental gas phase techniques. The first part of this dissertation describes experiments carried out on a homebuilt negative ion photoelectron imaging spectrometer. A detailed description of the construction of this instrument is described in this dissertation. Two studies included in this dissertation highlight the utility of photoelectron imaging. The first photoelectron imaging study was conducted on the CH- molecule. The results from this work allowed for the determination of fundamental properties such as the electron binding energy associated with two separate electronic transitions corresponding to electron detachment from the 1π and 3σ orbitals of CH-. In addition to this, analysis of the photoelectron angular distributions revealed evidence for a temporary excited state of CH- previously undetected. The second photoelectron imaging study focused on nitromethane anion and the dimer, trimer, and hydrated monomer cluster anions. In this work, vertical detachment energies, estimated electron affinities, and solvation energies for each cluster anion were identified from the photoelectron spectra. Theoretical calculations were used to predict cluster structures with calculated detachment energies in agreement with the experimental values. This work suggests the nitromethane clusters are formed by a single anion solvated by additional neutral molecules held together by O--C-H interactions. The later part of this dissertation describes collision induced dissociation and energy resolved mass spectrometry experiments using a commercial triple quadrupole mass spectrometer. Fragmentation pathways in copper nitrate cluster anions, nitrotoluene radical anions, and deprotonated nitrophenols anions as well as dissociation energies were determined from these studies. Dissociation energies were determined from threshold measurements where the apparent cross section of a fragment was measured as a function of the collision cell voltage. The values obtained from these works can be used in the negative ion thermochemical cycle to provide information on corresponding neutral species and are potential useful in the field of energetic materials.