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The mechanism for iron-catalyzed alkene isomerization in solution

Description: Here we report nano- through microsecond time-resolved IR experiments of iron-catalyzed alkene isomerization in room-temperature solution. We have monitored the photochemistry of a model system, Fe(CO){sub 4}({eta}{sup 2}-1-hexene), in neat 1-hexene solution. UV-photolysis of the starting material leads to the dissociation of a single CO to form Fe(CO){sub 3}({eta}{sup 2}-1-hexene), in a singlet spin state. This CO loss complex shows a dramatic selectivity to form an allyl hydride, HFe(CO){sub 3}({eta}{sup 3}-C{sub 6}H{sub 11}), via an internal C-H bond-cleavage reaction in 5-25 ns. We find no evidence for the coordination of an alkene molecule from the bath to the CO loss complex, but do observe coordination to the allyl hydride, indicating that it is the key intermediate in the isomerization mechanism. Coordination of the alkene ligand to the allyl hydride leads to the formation of the bis-alkene isomers, Fe(CO){sub 3}({eta}{sup 2}-1-hexene)({eta}{sup 2}-2-hexene) and Fe(CO){sub 3}({eta}{sup 2}-1-hexene){sub 2}. Because of the thermodynamic stability of Fe(CO){sub 3}({eta}{sup 2}-1-hexene)({eta}{sup 2}-2-hexene) over Fe(CO){sub 3}({eta}{sup 2}-1-hexene){sub 2} (ca. 12 kcal/mol), nearly 100% of the alkene population will be 2-alkene. The results presented herein provide the first direct evidence for this mechanism in solution and suggest modifications to the currently accepted mechanism.
Date: May 27, 2008
Creator: Sawyer, Karma R.; Glascoe, Elizabeth A.; Cahoon, James F.; Schlegel, Jacob P. & Harris, Charles B.
Partner: UNT Libraries Government Documents Department

Determining Transition State Geometries in Liquids Using 2D-IR

Description: Many properties of chemical reactions are determined by the transition state connecting reactant and product, yet it is difficult to directly obtain any information about these short-lived structures in liquids. We show that two-dimensional infrared (2D-IR) spectroscopy can provide direct information about transition states by tracking the transformation of vibrational modes as a molecule crossed a transition state. We successfully monitored a simple chemical reaction, the fluxional rearrangement of Fe(CO)5, in which the exchange of axial and equatorial CO ligands causes an exchange of vibrational energy between the normal modes of the molecule. This energy transfer provides direct evidence regarding the time scale, transition state, and mechanism of the reaction.
Date: December 11, 2007
Creator: Harris, Charles; Cahoon, James F.; Sawyer, Karma R.; Schlegel, Jacob P. & Harris, Charles B.
Partner: UNT Libraries Government Documents Department

DFT and time-resolved IR investigation of electron transfer between photogenerated 17- and 19-electron organometallic radicals

Description: The photochemical disproportionation mechanism of [CpW(CO){sub 3}]{sub 2} in the presence of Lewis bases PR{sub 3} was investigated on the nano- and microsecond time-scales with Step-Scan FTIR time-resolved infrared spectroscopy. 532 nm laser excitation was used to homolytically cleave the W-W bond, forming the 17-electron radicals CpW(CO){sub 3} and initiating the reaction. With the Lewis base PPh{sub 3}, disproportionation to form the ionic products CpW(CO){sub 3}PPh{sub 3}{sup +} and CpW(CO){sub 3}{sup -} was directly monitored on the microsecond time-scale. Detailed examination of the kinetics and concentration dependence of this reaction indicates that disproportionation proceeds by electron transfer from the 19-electron species CpW(CO){sub 3}PPh{sub 3} to the 17-electron species CpW(CO){sub 3}. This result is contrary to the currently accepted disproportionation mechanism which predicts electron transfer from the 19-electron species to the dimer [CpW(CO){sub 3}]{sub 2}. With the Lewis base P(OMe){sub 3} on the other hand, ligand substitution to form the product [CpW(CO){sub 2}P(OMe){sub 3}]{sub 2} is the primary reaction on the microsecond time-scale. Density Functional Theory (DFT) calculations support the experimental results and suggest that the differences in the reactivity between P(OMe){sub 3} and PPh{sub 3} are due to steric effects. The results indicate that radical-to-radical electron transfer is a previously unknown but important process for the formation of ionic products with the organometallic dimer [CpW(CO){sub 3}]{sub 2} and may also be applicable to the entire class of organometallic dimers containing a single metal-metal bond.
Date: April 30, 2008
Creator: Cahoon, James B.; Kling, Matthias F.; Sawyer, Karma R.; Andersen, Lars K. & Harris, Charles B.
Partner: UNT Libraries Government Documents Department

Direct observation of photoinduced bent nitrosyl excited-state complexes

Description: Ground state structures with side-on nitrosyl ({eta}{sup 2}-NO) and isonitrosyl (ON) ligands have been observed in a variety of transition-metal complexes. In contrast, excited state structures with bent-NO ligands have been proposed for years but never directly observed. Here we use picosecond time-resolved infrared spectroscopy and density functional theory (DFT) modeling to study the photochemistry of Co(CO){sub 3}(NO), a model transition-metal-NO compound. Surprisingly, we have observed no evidence for ON and {eta}{sup 2}-NO structural isomers, but have observed two bent-NO complexes. DFT modeling of the ground and excited state potentials indicates that the bent-NO complexes correspond to triplet excited states. Photolysis of Co(CO){sub 3}(NO) with a 400-nm pump pulse leads to population of a manifold of excited states which decay to form an excited state triplet bent-NO complex within 1 ps. This structure relaxes to the ground triplet state in ca. 350 ps to form a second bent-NO structure.
Date: June 28, 2008
Creator: Sawyer, Karma R.; Steele, Ryan P.; Glascoe, Elizabeth A.; Cahoon, James F.; Schlegel, Jacob P.; Head-Gordon, Martin et al.
Partner: UNT Libraries Government Documents Department

Two-photon Photo-emission of Ultrathin Film PTCDA Morphologies on Ag(111)

Description: Morphology- and layer-dependent electronic structure and dynamics at the PTCDA/Ag(111) interface have been studied with angle-resolved two-photon photoemission. In Stranski-Krastanov growth modes, the exposed wetting layer inhibited the evolution of the vacuum level and valence band to bulk values. For layer-by-layer growth, we observed the transition of electron structure from monolayer to bulk values within eight monolayers. Effective masses and lifetimes of the conduction band and the n=1 image potential state were measured to be larger for disordered layers. The effective mass was interpreted in the context of charge mobility measurements.
Date: November 29, 2007
Creator: Yang, Aram; Yang, Aram; Shipman, Steven T.; Garrett-Roe, Sean; Johns, James; Strader, Matt et al.
Partner: UNT Libraries Government Documents Department

Determination of Band Curvatures by Angle-resolved Two-photonPhotoemission in thin films of C(60) on Ag(111)

Description: The thickness-dependent interfacial band structure wasdetermined for thin films of Co(6) on Ag(111) by angle-resolvedtwo-photon photoemission spectroscopy.
Date: March 1, 2005
Creator: Shipman, Steven T.; Szymanski, Paul; Garrett-Roe, Sean; Yang,Aram; Strader, Matthew L. & Harris, Charles B.
Partner: UNT Libraries Government Documents Department

The nature and role of bridged carbonyl intermediates in theultrafast photo-induced rearrangement of Ru3(CO)12.

Description: The photochemistry of the trimetal cluster,Ru(3)(CO)12, was investigated on the ultrafast timescale using UV-vis pump, infrared probe spectroscopy in order to study the transient intermediates formed upon photoexcitation.
Date: September 18, 2005
Creator: Glascoe, Elizabeth A.; Kling, Matthias F.; Shanoski, Jennifer E. & Harris, Charles B.
Partner: UNT Libraries Government Documents Department

19-electron intermediates in the Ligand Substitution of CpW(CO)3with a Lewis Base

Description: Odd electron species are important intermediates in organometallic chemistry, participating in a variety of catalytic and electron-transfer reactions which produce stable even-electron products. While electron deficient 17-electron (17e) radicals have been well characterized, the possible existence of short-lived 19-electron (19e) radicals has been a subject of continuing investigation. 19e radicals have been postulated as intermediates in the photochemical ligand substitution and disproportionation reactions of organometallic dimers containing a single metal-metal bond, yet the reactions of these intermediates on diffusion-limited time scales (ns-{micro}s) have never been directly observed. This study resolves the 19e dynamics in the ligand substitution of 17e radicals CpW(CO){sub 3}{sup {sm_bullet}} (Cp = C{sub 5}H{sub 5}) with the Lewis base P(OMe){sub 3}, providing the first complete description 19e reactivity.
Date: December 14, 2005
Creator: Cahoon, James F.; Kling, Matthias F.; Sawyer, Karma R.; Frei,Heinz & Harris, Charles B.
Partner: UNT Libraries Government Documents Department