A Computational Study of Diiodomethane Photoisomerization
Date of Award
Doctor of Philosophy (Ph.D.)
Alexander Tarnovsky (Advisor)
Massimo Olivucci (Committee Co-Chair)
Robert McKay (Committee Member)
John Cable (Committee Member)
This work gives the detailed description of the dynamics and mechanism of the previously unsuspected photochemical reaction path of diiodomethane (CH2I2), a paradigmatic haloalkane, which is direct intramolecular isomerization upon the excitation of this molecule to the lowest singlet S1 state. The previous liquid-phase ultrafast spectroscopy experiments on the UV photochemistry of di- and polyhalomethanes suggest that following excitation of these molecules, the carbon-halogen bond breaks, leading to formation of the initial radical pair. The radical pair, trapped by a solvent cage collapses into an isomer product species with halogen-halogen bond on a picoseconds timescale (1 ps = 10-12 s). Yet, the results recently obtained in our research group, clearly suggest that in addition to this conventional, in-cage isomerization process, there is another, unconventional isomerization mechanism, which occurs on a sub-100 fs timescale (1 fs = 10-15 s) and does not require the solvent environment around the excited CH2I2 solute. Indeed, the ultrafast sub-100 fs timescale observed suggests two main considerations:
- The sub-100 fs photoisomerization in polyhalomethanes is direct, i.e. proceeds via the intramolecular reaction mechanism proceeding without any intermediates (such as a radical pair) and, likely, is mediated by a crossing of excited and ground electronic states.
- The solvent cage may not be needed, because the timescale of the aforementioned isomerization process is shorter than the 100-200 fs timescale for a single collisional encounter between solvent and solute molecules.
Femtosecond transient absorption spectroscopy is a very valuable tool in studying the photochemical reactivity on short timescales. The measured ultrafast time-resolved spectra are complicated by relaxation processes in far from equilibrium solutes, such as intramolecular energy redistribution and flow, and can be understood in detail with the help from state-of-the-art quantum-chemical modeling. Thus, in order to gain the detailed interpretation of the observed (photo)chemical dynamics it is necessary to complement the femtosecond experiments with the modern quantum-chemical computations.
Borin, Veniamin Aleksandrovich, "A Computational Study of Diiodomethane Photoisomerization" (2016). Photochemical Sciences Ph.D. Dissertations. 89.