PROBING VIBRATIONAL WAVE PACKETS IN ORGANOPHOSHOROUS MOLECULES USING FEMTOSECOND TIME-RESOLVED MASS SPECTROMETRY (FTRMS)
Doctor of Philosophy
Dr. Katharine Moore Tibbetts
Dr. Scott Gronert
Dr. Puru Jena
Dr. Dusan Bratko
The study of organophosphorus and nitroaromatic compounds is essential due to their suitability as models for understanding the dynamics of radiation induce damage to DNA sugar-phosphate backbone, and the detection of chemical warfare agents as well as nitroaromatic explosives (such as TNT). The ultrafast dynamics of these polyatomic radicals were studied using femtosecond time-resolved mass spectrometry (FTRMS), a versatile method that is capable of monitoring the ultrafast dynamics of rapid dissociation in the gas phase after the removal of an electron. The method uses a technique called pump-probe, which makes use of two time-delayed laser pulses. In this work, the strong ionizing pump pulse and the weak non-ionizing probe pulse were in the near infrared and the visible region respectively. The pump pulse ionizes the molecule to produce a parent radical cation, which is vertically excited in the Frank Condon region to the ground electronic state. A wave packet is generated as a result of the coherent vibrational excitation that populates several excited vibrational states at the same time. A delayed probe pulse monitors the wave packet as it moves over the potential energy surface giving information on a particular molecule including the vibrational frequency.
Four organophosphorus compounds, dimethyl methylphosphonate (DMMP), diethyl methylphosphonate (DEMP), diisopropyl methylphosphonate (DIMP) and trimethyl phosphate (TMP) were studied. Upon ionization, each molecular radical cation exhibited unique oscillatory dynamics in its transient ion yield resulting from coherent vibrational excitation. The results for DMMP revealed a particularly well-resolved frequency at 732 ± 28 cm-1 with a weak feature at 610-650 cm-1 and an oscillation period of 45 fs. DIMP exhibited bimodal oscillation with period of ~55 fs and two frequency features at 554 ± 28 and 670-720 cm-1. A bimodal frequency oscillation at 770 and 880 cm−1 was observed in TMP with an oscillational period of ~ 40 fs. In contrast, the oscillations for DEMP were barely visible due to rapid decay. Based on our DFT calculations, the high- and low- frequency oscillations observed in DMMP and DIMP were assigned to coherent excitation of P-C and O-P-O stretching and bending modes respectively. The high and low frequencies in TMP were assigned to an asymmetric and symmetric P-O stretching modes respectively.
Four nitroaromatic derivatives of 2,4,6-trinitrotoluene (TNT), 4-Nitrotoluene (PNT), 3-Nitrotoluene (MNT), 2-Nitrotoluene (ONT) and nitrobenzene (NB) were studied. Oscillation frequencies of the transient ion yield were observed for these compounds upon ionization. A torsional motion of the NO2 group with respect to the phenyl and benzene ring (in the case of NB) was identified as the main excited vibrational mode in three compounds, PNT (85 cm-1), ONT (91 cm-1) and NB (80 cm-1), based on DFT calculations. The vibrational frequency in MNT was roughly two times faster than in the others and was assigned to a symmetric bending mode of the CH3 and NO2 with respect to the benzene ring. In ONT, the aci-tautomer exhibited a unique oscillation frequency at 114 cm-1, faster than the frequency of the parent ion. The aci-tautomer preserved the torsional coherence that was initially exhibited by the parent ion.
All the assignments of vibrational motion were supported by DFT calculations utilizing various functionals such as BPW91, B3LYP, CAM-B3LYP and the basis set Def2-tzvpp[(11s6p2d1f)/5s3p2d1f], 6-311+G* and 6-311G*.
Collectively, results obtained from this research may support several applications such as in national security. For example, procedures can be developed to specifically detect explosives, and enhanced techniques could detect and decontaminate nerve agents, which would help save lives and property. It could also help build photo-labile explosives, which could be initiated by a laser. In addition, quality of life could be enhanced via early intervention from radiation induced damage to DNA.
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