Doctor of Philosophy
Dr. Katharine Moore Tibbetts
Dr. Ka Un Lao
A radical cation is a molecule that has one unpaired electron that holds a positive charge. The unpaired electron within a radical cation causes the molecule to be reactive. The high reactivity of these species allows for radical cations to be commonly studied experimentally using mass spectrometry and other multi-mass imaging techniques. However, these methods often cannot resolve the reaction mechanisms for these fast reactions. Specifically, radical cation rearrangement mechanisms are particularly unresolved within experiments. For this reason, radical cation rearrangements are computationally investigated to explain complex reaction pathways for processes to understand reactions leading to the initiation of detonation in organic explosives and DNA radiation damage.
Rearrangement and fragmentation of nitroaromatic energetic molecules have been subject to extensive theoretical investigation to gain insight into the fragmentation pathways of the military explosive 2,4,6-trinitrotoluene (TNT). However, the decomposition mechanism of TNT is quite expensive to model outright. Therefore, smaller, more simplified models of TNT such as nitrotoluenes are studied. In the o-nitrotoluene cation, detonation is simulated with the direct cleavage of the C-NO2 bond. Interestingly, dissociation also occurs as the methyl group can undergo cleavage of the C-H bond creating a hydrogen transfer (HT) from the methyl to the nitro group. When investigated with DFT and pump-probe measurements with mass spectrometric detection, the H atom attack formed the aci-nitro tautomer as soon as 20−60 fs after ionization. Once formed, the aci-nitro tautomer spontaneously loses -OH to form C7H6NO+ through proposed singlet and triplet pathways. Subsequent fragmentation pathways leading to the formation of dissociation products C7H6NO, C7H7, and C6H6N were also calculated.
As compared to o-nitrotoluene, m- and p-nitrotoluene dissociate similarly as they undergo a nitro-nitrite rearrangement to create nitrite groups on the ring, rather than nitro groups. This nitro-nitrite rearrangement (NNR) was studied in nitromethane, the smallest organic-nitro compound. Using pump-probe femtosecond laser photoionization mass spectroscopy, coupled-cluster theory, and ab initio molecular dynamics, the nitromethane cation (NM+) fragments into CH3+, NO2+, and NO+. From theoretical analysis, NO2+ and CH3+ were formed through direct cleavage of the C-N bond, whereas NO+ forms spontaneously upon NNR of the NM+ cation. Direct ionization into the electronically excited D1 or D2 states by the pump pulse provides sufficient excess energy to initiate the NNR pathway. With excess energy stored in the NNR transition state, molecular dynamics simulations indicated that NNR requires 660 ± 230 fs and was typically followed by rapid NO+ loss 100-200 fs later. Experimentally, the fragmentation pathways to NO+ and CH3+ were in competition, with an associated decay timescale of ~ 480 ± 209 which is similar to the computed NNR timescale of ~ 435 ± 209 fs with added excess energy. This result suggests that CH3+ is formed by further excitation of the NM+ initially ionized into the D1 or D2 states before it undergoes NNR. Finally, the dissociation to NO+ from NM+ was assigned to a D0 → D2 transition.
As with TNT, the DNA backbone is quite expensive to computationally investigate outright. Therefore, smaller organic phosphates and phosphonates, such as DMMP, DIMP, DEMP, and TMP, were computationally studied to understand radiation-induced damage along the DNA backbone. Radiation damage occurs when one-electron oxidation of the phosphate forms sugar radicals and induces lesions, thus contributing to single and double-strand breaks along the backbone. From preliminary computational investigations, coupled with experimental data using pump-probe techniques, there are different fragmentation pathways between DMMP, DIMP, and DEMP. Experimentally, hydrogen atom shifts were observed for all atoms. This mechanism could be the mechanism for which experimentally observed sugar radicals are formed.
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