| Popis: |
This work gives the detailed description of the dynamics and mechanism of the previouslyunsuspected photochemical reaction path of diiodomethane (CH2I2), a paradigmatic haloalkane,which is direct intramolecular isomerization upon the excitation of this molecule to the lowestsinglet S1 state. The previous liquid-phase ultrafast spectroscopy experiments on the UVphotochemistry 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 ona 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 ultrafastsub-100 fs timescale observed suggests two main considerations:• The sub-100 fs photoisomerization in polyhalomethanes is direct, i.e. proceeds via theintramolecular reaction mechanism proceeding without any intermediates (such as a radicalpair) 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 aforementionedisomerization process is shorter than the 100-200 fs timescale for a single collisionalencounter between solvent and solute molecules.iiiFemtosecond transient absorption spectroscopy is a very valuable tool in studying thephotochemical reactivity on short timescales. The measured ultrafast time-resolved spectra arecomplicated by relaxation processes in far from equilibrium solutes, such as intramolecular energyredistribution and flow, and can be understood in detail with the help from state-of-the-artquantum-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 themodern quantum-chemical computations. |
