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Photodissociation of diiodomethane and recombination into iso-diiodomethane in acetonitrile solution studied with ab initio molecular dynamics simulations
Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Chemistry, Department of Physical Chemistry.
Manuscript (Other academic)
URN: urn:nbn:se:uu:diva-90880OAI: oai:DiVA.org:uu-90880DiVA: diva2:163387
Available from: 2003-09-24 Created: 2003-09-24 Last updated: 2010-01-13Bibliographically approved
In thesis
1. Ultrafast Photo-induced Reaction Dynamics of Small Molecules
Open this publication in new window or tab >>Ultrafast Photo-induced Reaction Dynamics of Small Molecules
2003 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The main focus of this thesis is the investigation of the dissociation dynamics of aryl halides using femtosecond pump-probe spectroscopy. In the monohalogenated aryl halides, iodo-, bromo- and chlorobenzene, the rate of dissociation following excitation at 266 nm in the gas phase increased with increasing mass of the halogen atom. This process was assigned to predissociation of the initially excited singlet (π, π*) state via a repulsive triplet (n, σ*) state due to spin-orbit interaction. In addition to the predissociative mechanism, a direct dissociation channel was observed in iodobenzene. The rate of the predissociation in bromobenzene was found to be faster in the condensed phase than in the gas phase, which can be explained by solvent-induced symmetry perturbations. Ab initio calculations of the potential energy surfaces of the ground state and several low lying excited states in bromobenzene have been performed in order to verify the suggested mechanism. Substituting one of the hydrogen atoms in bromobenzene affected the predissociation rate significantly. In o-, m- and p-dibromobenzene the predissociation rate increased with decreasing distance between the bromine atoms in accordance with an increased spin-orbit interaction introduced by the bromine substituent. The fastest predissociation rate was observed in 1,3,5-tribromobenzene. With chlorine and fluorine substitution, inductive and conjugative effects were found to be of importance. In the o- and m-isomers of the dihalogenated aryl halides, an additional faster dissociation channel was observed. Guided by ab initio calculations of the potential energy surfaces in the dibromobenzene isomers, we ascribed the fast dissociation pathway to predissociation of an initially excited triplet state. Upon methyl group substitution in bromobenzene, the decreased lifetime of the initially excited state was attributed to an incresaed density of coupled states.

Another system which has been studied in the condensed phase is diiodomethane. Using Car-Parrinello molecular dynamics simulations we observed a prompt dissociation and subsequent recombination to the isomer, iso-diiodomethane, in acetonitrile solution.

Vibrational wavepacket dynamics in the C (1Σ+) state of NaK were studied using a direct ionization probing scheme. A simple analytical expression for the pump-probe signal was developed in order to see what factors that govern direct ionization of the vibrational wavepacket. Our experimental data was consistent with a photoionization transition dipole moment that varies with internuclear distance.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2003. 52 p.
Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1104-232X ; 891
Physical chemistry, pump-probe spectroscopy, reaction dynamics, photodissociation, aryl halides, spin-orbit coupling, ab initio calculations, molecular dynamics, vibrational wavepackets, Fysikalisk kemi
National Category
Physical Chemistry
urn:nbn:se:uu:diva-3597 (URN)91-554-5750-9 (ISBN)
Public defence
2003-10-17, B7:113a, BMC, Uppsala, 10:15
Available from: 2003-09-24 Created: 2003-09-24Bibliographically approved

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