Photochemical Sciences Ph.D. Dissertations

A Computational Study of Diiodomethane Photoisomerization

Date of Award

2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Alexander Tarnovsky (Advisor)

Second Advisor

Massimo Olivucci (Committee Co-Chair)

Third Advisor

Robert McKay (Committee Member)

Fourth Advisor

John Cable (Committee Member)

Abstract

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.

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