Electron Transfer Spectra

Jørgensen, Chr. Klixbüll

doi:10.1002/9780470166130.ch2

Cited by 387 publications

(19 citation statements)

References 203 publications

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“…Figure A shows the DR UV–vis spectra of activated δ-Al 2 O 3–600 (spectrum 1), the δ-Al 2 O 3–600 /TiCl 4 precatalyst (spectrum 2), and the two catalysts (spectra 3a and 3b, respectively). The DR UV–vis spectrum of δ-Al 2 O 3–600 /TiCl 4 (spectrum 2) is dominated by three intense bands at approximately 42000, 35000, and 28000 cm –1 , which were previously assigned to ligand (either Cl or O) to metal (either 6- or 4-fold Ti 4+ sites) charge transfer transitions, as follows: Cl → Ti 4+ 6c at 28000 cm –1 , Cl → Ti 4+ 4c and O → Ti 4+ 6c at 35000 cm –1 , and O → Ti 4+ 4c at 42000 cm –1 . − A color change is observed upon activation. The δ-Al 2 O 3–600 /TiCl 4 /H 2–400 catalyst was light blue, while the δ-Al 2 O 3–600 /TiCl 4 /TEA catalyst was dark brownish, a clear indication that the reduced Ti sites experience a different local environment in the two cases.…”

Section: Resultssupporting

confidence: 77%

Tuning the Ti³⁺ and Al³⁺ Synergy in an Al₂O₃/TiCl_x Catalyst To Modulate the Grade of the Produced Polyethylene

et al. 2017

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A multitechnique approach (comprising in situ Fourier transform infrared, diffuse reflectance ultraviolet–visible, and advanced electron paramagnetic resonance spectroscopies, coupled with differential scanning calorimetry analysis) was employed to investigate the local structure and the activation of a heterogeneous ethylene polymerization catalyst obtained by grafting TiCl4 onto a transitional alumina. The activation procedure was found to affect the electronic structure and the coordinative environment of the reduced Ti sites, as well as their interaction with the Al3+ sites, measured in terms of spin density transfer from Ti3+ to nearby Al3+ ions. It was found that the extent of interaction between the two metal sites correlates with the microstructures of the obtained polyethylene. Tuning the synergy between the Ti3+ and Al3+ Lewis acid sites is proposed as an efficient way to modulate the polyethylene microstructure, switching from a high-density polyethylene to a highly branched polyethylene.

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Section: Resultssupporting

confidence: 77%

Tuning the Ti³⁺ and Al³⁺ Synergy in an Al₂O₃/TiCl_x Catalyst To Modulate the Grade of the Produced Polyethylene

et al. 2017

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“…Since many contributions overlap to each other at high energy and hence cannot be clearly identified, the analysis will focus on the absorption band at lower energy, which is more easily identifiable and straightforwardly assigned to a Cl(π) → Ti(d) CT transition. 84,85 Moreover, on this band, the Jorgensen semiempirical rule on the optical electronegativity of transition metals and their ligands can be applied. 86 The spectrum of A shows the maximum of the first absorption at ca.…”

Section: Resultsmentioning

confidence: 99%

“…While the spectrum of Mg(OEt) 2 displays no absorption in the whole UV–vis region, the spectra of the extracted samples are dominated by intense absorptions above 30000 cm –1 due to several charge-transfer (CT) transitions involving the Ti metal centers and their ligands. Since many contributions overlap to each other at high energy and hence cannot be clearly identified, the analysis will focus on the absorption band at lower energy, which is more easily identifiable and straightforwardly assigned to a Cl(π) → Ti(d) CT transition. , Moreover, on this band, the Jorgensen semi-empirical rule on the optical electronegativity of transition metals and their ligands can be applied…”

Section: Results and Discussionmentioning

confidence: 99%

Formation of Highly Active Ziegler–Natta Catalysts Clarified by a Multifaceted Characterization Approach

et al. 2021

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Although the formation of nanosized and defective δ-MgCl2 is essential for the performance of Ziegler–Natta catalysts, the process has not sufficiently been elucidated due to certain limitations in characterization. Here, the formation of nanostructures and active surfaces of Ziegler–Natta catalysts was investigated in detail based on a multifaceted set of characterization techniques represented by X-ray total scattering and various spectroscopies in correlation with chemical composition analysis and polymerization tests. Solid samples were extracted in the course of catalyst preparation from Mg(OEt)2 and subjected to the analysis. Several interesting results were found. The addition of TiCl4 almost spontaneously converts Mg(OEt)2 into MgCl2 seeds mainly exposing the {001} basal surface, whose dimensions are below 2 nm; a large Ti amount remains on the material as physisorbed 4-fold-coordinated TiCl x (OEt)4–x species. The heating treatment removes the physisorbed TiCl x (OEt)4–x and/or convert them into chemisorbed 6-fold-coordinated TiCl x (OEt)4–x , while the subsequent addition of an internal donor (here dibutyl phthalate, DBP) promotes a substantial reconstruction and growth of MgCl2 seeds to almost the same size as the final catalyst (ca. 6 nm), with the exposure of the more catalytically relevant lateral surfaces. DBP is in one part adsorbed on MgCl2 surfaces and in the other part complexed with Ti sites. This complex is only partially removed in the following steps of the synthesis. The second TiCl4 addition replaces the chemisorbed TiCl x (OEt)4–x with 6-fold-coordinated TiCl4 species, but it also causes side reactions with DBP, as testified by the formation of phthaloyl chloride. After activation by triethylaluminum (TEAl), the activity per Ti for ethylene was almost constant throughout the whole preparation process after the initial TiCl4 addition, whereas the activity for propylene was negligible before the addition of the donor and increased dramatically in the subsequent steps of the preparation. This was further investigated based on spectroscopies for TEAl-activated samples in order to individuate the active Ti species responsible for the catalysis and to monitor the fate of DBP upon TEAl reaction. The multifaceted characterization approach allowed us to integrate information on the formation of δ-MgCl2, their surfaces, and adsorbed species, providing us with deep insights into the meaning of each step within an industrial catalyst preparation method that has been empirically refined over a long history.

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“…In this view, interactions involving the four equatorial nitrogen and two axial carbon donor centers effectively determine the d-orbital splitting. Charge transfer could involve promotion of an electron into a dσ or a dπ orbital; however, introducing another electron into one of the dπ orbitals would require electron pairing, so injection of an electron into an empty dσ level is likely to be a lower energy process . Whether to the a 1g d z 2 or b 1g d x 2 – y 2 orbital, the excitation originates from one of two degenerate e u (L) orbitals, which are C–C bonding.…”

Section: Ligand To Metal Charge Transfer Absorptionmentioning

confidence: 99%

“…Charge transfer could involve promotion of an electron into a dσ or a dπ orbital; however, introducing another electron into one of the dπ orbitals would require electron pairing, so injection of an electron into an empty dσ level is likely to be a lower energy process. 17 Whether to the a 1g d z 2 or b 1g d x 2 −y 2 orbital, the excitation originates from one of two degenerate e u (L) orbitals, which are C−C bonding. The excited state is therefore orbitally degenerate, and the full term symbol for the excited state is 4 E u .…”

Section: Journal Of Chemical Educationmentioning

confidence: 99%

To Be or Not To Be Symmetric: That Is the Question for Potentially Active Vibronic Modes

et al. 2017

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Electronic spectra often exhibit vibronic structure when vibrational and electronic transitions occur in concert. Theory reveals (1) that orbital symmetry considerations determine specific roles played by the nuclear degrees of freedom and (2) that the vibrational excitation is often highly regiospecific, that is, attributable to an identifiable subset of atoms within the molecule. Spectra obtained from a chromium(III) complex involving a macrocyclic ligand and two axially disposed butadiynide groups nicely illustrate many of the concepts involved.

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Electron Transfer Spectra

Cited by 387 publications

References 203 publications

Tuning the Ti³⁺ and Al³⁺ Synergy in an Al₂O₃/TiCl_x Catalyst To Modulate the Grade of the Produced Polyethylene

Tuning the Ti³⁺ and Al³⁺ Synergy in an Al₂O₃/TiCl_x Catalyst To Modulate the Grade of the Produced Polyethylene

Formation of Highly Active Ziegler–Natta Catalysts Clarified by a Multifaceted Characterization Approach

To Be or Not To Be Symmetric: That Is the Question for Potentially Active Vibronic Modes

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Electron Transfer Spectra

Cited by 387 publications

References 203 publications

Tuning the Ti3+ and Al3+ Synergy in an Al2O3/TiClx Catalyst To Modulate the Grade of the Produced Polyethylene

Tuning the Ti3+ and Al3+ Synergy in an Al2O3/TiClx Catalyst To Modulate the Grade of the Produced Polyethylene

Formation of Highly Active Ziegler–Natta Catalysts Clarified by a Multifaceted Characterization Approach

To Be or Not To Be Symmetric: That Is the Question for Potentially Active Vibronic Modes

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Tuning the Ti³⁺ and Al³⁺ Synergy in an Al₂O₃/TiCl_x Catalyst To Modulate the Grade of the Produced Polyethylene

Tuning the Ti³⁺ and Al³⁺ Synergy in an Al₂O₃/TiCl_x Catalyst To Modulate the Grade of the Produced Polyethylene