Chromyl Chloride Research Articles

Correlation of relative rates of chromyl chloride oxidation and chromic acid oxidation of acyclic alkenes versus alkene IPs and HOMOs

AbstractPlots of logarithms of relative reaction rates of chromyl chloride oxidation and of chromic acid oxidation of alkenes (log krel values) versus alkene ionization potentials (IPs) and versus their highest occupied molecular orbital energy levels (HOMOs) demonstrate excellent correlations. Each plot has a similar appearance and shows a single line with a positive slope. The results indicate that the rate‐determining step of each title reaction involves an electrophilic attack on the alkene π‐bond without significant steric effects; this supports a proposed 2 + 3 cycloaddition mechanism and disfavors a proposed stepwise 2 + 2 cycloaddition mechanism. Comparison is made with other d0 transition metal complexes that oxidize alkenes. Copyright © 2004 John Wiley & Sons, Ltd.

Mechanism of chromyl chloride alkane oxidation.

Modern electronic structure methods, including complete active space-perturbation theory to second order (CASPT2) and a range of density functional (DFT) methods, are used to characterize the intermediates and transition states associated with chromyl chloride alkane oxidation. Computed energetics for radical abstraction are found to be inconsistent with experimental activation energetics. The alcohol product is also found to be too endothermic to be viable. A novel zwitterionic direct pathway is characterized which, when coupled with a dinuclear complex, exothermically produces alcohol consistent with experiment.

Mechanism of Chromyl Chloride Epoxidation

Modern electronic structure methods including complete active space perturbation theory to second order (CASPT2), and a range of density functional (DFT) methods are used to characterize the intermediates and transition states associated with ethylene epoxidation by chromyl chloride. A novel zwitterionic direct addition pathway is characterized which, when coupled with a dinuclear complex, exothermically produces epoxide through a pathway with a small kinetic barrier.

Phase‐Selective CVD of Chromium Oxides from Chromyl Chloride

Phase selective chromium oxide films are deposited from chromyl chloride by CVD in the temperature range 320–380 °C. Phase selectivity is found to be driven, in part, by a templating effect of the substrate. Thin films of metastable CrO2 in (100), (001), and (110) orientations have been stabilized, respectively, by (100), (001), and (110) TiO2 single crystal substrates. In contrast, thin oriented films of the more stable Cr2O3 in (006) orientation are formed when depositing on (0001) Al2O3 single crystals over the same deposition temperature range. This phase selectivity appears to be unique to the chromyl chloride precursor.

Properties of epitaxial chromium dioxide films grown by chemical vapor deposition using a liquid precursor

Epitaxial chromium dioxide (CrO2) films have been grown using chemical vapor deposition on (100) TiO2 substrate with chromyl chloride (CrO2Cl2) as a liquid precursor. The films are extremely smooth (rms roughness less than 4.6 Å for a 1000-Å-thick film) and have the largest spin polarization (P=98.4%) yet observed, as determined by point contact Andreev reflection. Magnetization switching properties of the films are close to those of a single-domain particle. Preliminary results on the in situ growth of exchange-biased CrO2/Cr2O3 multilayers are also reported. Although a bias field is observed, it is much smaller in comparison with the coercivity of the CrO2 film.

Near-complete spin polarization in atomically-smooth chromium-dioxide epitaxial films prepared using a CVD liquid precursor

We have fabricated single crystal chromium dioxide $({\mathrm{CrO}}_{2})$ films using chemical vapor deposition with chromyl chloride $({\mathrm{CrO}}_{2}{\mathrm{Cl}}_{2})$ as a liquid precursor. Their electrical and magnetic properties have been investigated. These films are atomically smooth with a rms roughness of less than 5 \AA{} for 1000 \AA{} thick films. We have obtained a spin polarization of $P=98.4%,$ as determined by the technique of point-contact Andreev reflection. Magnetization and resistivity measurements on these films are in good agreement with those measured on films made with the ${\mathrm{CrO}}_{3}$ solid precursor. The process using the liquid precursor is superior to other existing techniques for the preparation of single crystal single- and multilayers containing ${\mathrm{CrO}}_{2},$ especially the structure of magnetic tunnel junctions.

Etch Characteristics of Cr by Using Cl2 / O 2 Gas Mixtures with Electron Cyclotron Resonance Plasma

We investigated the etch characteristics of chromium films by using gas mixtures with electron cyclotron resonance plasma. In order to examine the chemical etch characteristics of gas plasma, we examined the etch rate with various gas mixing ratios. By X-ray photoelectron spectroscopy (XPS), the surface reaction on the chromium films during the etch was examined. At the same time, the plasma characteristics were examined by optical emission spectroscopy. From XPS analyses, it was confirmed that a chromium oxychloride layer was formed on the surface by the etch using gas mixtures. We observed a new characteristic emission line at 517 nm during the etch of chromium films with gas mixtures, using optical emission spectroscopy. It was found that the peak intensity of this emission line had a similar behavior to the etch rate. It was proposed that the emission line of 517 nm should result from chromium compounds with the oxidation state of © 2001 The Electrochemical Society. All rights reserved.

Kinetics of oxychlorination of chromite: Part II. Effect of reactive gases

The effects of Cl2/O2 ratio, P(Cl2+O2), PCl2, and PO2 on the oxychlorination rate of the chromite mineral were determined between 750 and 1000°C using isothermal TGA measurements. The effect of the gases’ composition on the oxychlorination rate of chromite were compared with those resulting from the oxychlorination of simple oxides of the chromite (Cr2O3, Fe2O3 and MgO). The apparent reaction orders with respect to Cl2+O2, Cl2, and O2 for the chromite oxychlorination at 750°C were about 0.94, 1.24, and −0.30, respectively. At 1000°C, apparent reaction orders with respect to the reactive gases changed significantly as the reaction progressed.Boat experiments were carried out to oxychlorinate a chromite concentrate between 600 and 1000°C. The reaction products were analyzed by SEM, XRD and chemical analysis. The oxychlorination of a chromite concentrate at about 800°C led to the partial elimination of iron increasing the Cr/Fe ratio in the treated concentrate. A part of chromium was also oxychlorinated and it was recovered as chromium oxychloride (CrO2Cl2).

An Ab Initio Study of the Structures and Vibrational Spectra of Chromium Oxo-Anions and Oxyhalides

The bond distance and vibrational spectrum of the chromate(VI) ion has been calculated using HF-SCF and SCF-density functional (SCF-DFT) methods in conjunction with a wide range of basis sets. It was found that the B3-LYP/LanL2DZ combination gave the best fit for the observed vibrational spectrum. The molecular geometries and vibrational spectra of chromate(V), dichromate, trichromate, chlorochromate, fluorochromate, chromyl chloride, and chromyl fluoride were also calculated using B3-LYP/LanL2DZ. Force constants and potential energy distributions were determined from the calculated spectra, after appropriate scaling, which compare favorably with the experimental IR and Raman spectra. Precise vibrational band assignments have been determined for dichromate and trichromate. Relationships have been established between the CrO bond distance and CrO principal stretching force constant.

Highly spin-polarized chromium dioxide thin films prepared by chemical vapor deposition from chromyl chloride

Highly spin-polarized chromium dioxide (CrO2) thin films were deposited on (100) TiO2 substrates by chemical vapor deposition using chromyl chloride as a precursor. The spin polarization, as measured by the point contact Andreev reflection technique, was 81±3%. X-ray diffraction θ/2θ scans indicated the films grew completely (100) oriented, in registry with the (100) oriented TiO2 substrate. X-ray diffraction φ scans on the CrO2 (110) reflection indicated the expected twofold symmetry, with no evidence of misaligned material. The resistivity at room temperature was 240 μΩ cm and decreased to 10 μΩ cm at 5 K, consistent with metallic behavior. The films were ferromagnetic with a Curie temperature of 395 K and a coercivity of ∼100 Oe at 298 K. The use of chromyl chloride as a precursor resulted in efficient and controlled CrO2 film growth.

On the Trail of CrO2Cl2 in Its Reactions with Organic Compounds

The simple metal oxo compound chromyl chloride, CrO2Cl2, displays a very complex reactivity with respect to alkanes, olefins, alcohols, and epoxides. Synergetic investigations applying various techniques and methods have only recently led to an understanding of the key steps in the corresponding mechanisms, which are representative for more complex systems. Product formation seems to be largely determined by radical reactions initiated by this d0-metal compound.

Trapping of an Organic Radical by an O=Cr(VI) Function.

Not only capable of generating organic radicals by H abstraction, chromyl chloride can also trap them again (see scheme). This is confirmed by the crystal structure of a Cr(V) oxo alkoxide, which was formed in three successive steps when bisadamantylidene oxide was allowed to react with CrO(2)Cl(2).

Mechanisms for the formation of epoxide and chlorine-containing products in the oxidation of ethylene by chromyl chloride: a density functional study

The reaction between CrO2Cl2 and ethylene leading to the formation of epoxide and chlorohydrin precursors or directly to 1,2-dichloroethane has been studied by density functional theory. The formation of the epoxide precursor (Cl2(O)Cr-OC2H4) was found to take place via a [3+2] addition of ethylene to two Cr=O bonds followed by rearrangement of the five-membered diol to the epoxide product. The alternative mechanisms involving a direct addition of oxygen to ethylene or the [2+2] addition of the olefin to a Cr=O bond were found to have much higher activation energies. The formation of the chlorohydrin precursor (Cl(O)Cr-OCH2=CHCl) was found to take place via a [3+2] addition to one Cr—Cl and one Cr=O bond. Pathways involving initial [2+2] addition to a Cr—Cl or Cr=O bond had much higher activation barriers. The generation of 1,2-dichloroethane is highly unfavorable with an endothermicity of 44.7 kcal/mol and an even higher activation barrier. It is suggested that the formation of epoxide and chlorohydrin from the respective precursors requires the addition of H2O.Key words: reaction mechanisms, epoxide, oxidation of ethylene, chromyl chloride, DFT.

Atmospheric pressure chemical vapour deposition of chromium oxide films

Cr 2 O 3 films have been deposited on glass substrates by atmospheric pressure chemical vapour deposition reaction of chromyl chloride with either water, ethyl acetate, methanol or ethanol. Films were deposited in the temperature range 450°C - 600°C. The Cr 2 O 3 films were characterised by glancing angle XRD, SEM and X-ray photoelectron spectroscopy. These analyses showed that single-phase Cr 2 O 3 was obtained at all deposition temperatures.

Oxygen Transfer from Chromyl Chloride to Alkynes and Allene – IR-Spectroscopic Identification of η1-Ketene and Cyclopropanone Complexes of O=CrCl2

Photolytically-induced reactions of chromyl chloride with acetylene and but-2-yne (dimethylacetylene) in low-temperature argon matrices yield end-on ketene and dimethylketene complexes of O=CrCl2, respectively. The product formation probably results from an electrophilic attack of a Cr=O linkage on the C≡C triple bonds, leading to radical-like transition states or intermediates, which subsequently rearrange through 1,2-H or 1,2-methyl shifts. Consequently, allene is attacked at its central carbon and the allyl radical thus generated undergoes subsequent ring-closure to give a cyclopropanone complex of O=CrCl2.

Synthesis of Oxochromium(VI) Alkoxides via Epoxide Cleavage. Structure, Reactivity, and Mechanism

In an NMR spectroscopic study the cleavage of epoxides [ethylene, propylene, and cis/trans-butylene oxide] by chromyl chloride giving access to oxochromium(VI) alkoxides was shown to proceed via a bimolecular rate-determining step where two molecules of a complex CrO2Cl2···epoxide collide. Subsequently one Cl ligand at the first Cr center attacks the backside of an epoxide molecule complexed at the Cr center of a second CrO2Cl2···epoxide molecule and vice versa. The trans-opening of the epoxides was proved by determining the configuration of the chlorohydrins resulting from hydrolysis of the corresponding alkoxide products in the cases of cis- and trans-butylene oxide. The NMR data provide evidence that each oxochromium(VI) alkoxide adopts one preferred conformation in solution although DFT calculations did not indicate any special stabilizing effects. The product formation was rationalized by DFT calculations concerning the thermodynamics of the reactions.

Oxygen Transfer from Chromyl Chloride to Alkynes and Allene – IR-Spectroscopic Identification of η1-Ketene and Cyclopropanone Complexes of O=CrCl2

Photolytically-induced reactions of chromyl chloride with acetylene and but-2-yne (dimethylacetylene) in low-temperature argon matrices yield end-on ketene and dimethylketene complexes of O=CrCl2, respectively. The product formation probably results from an electrophilic attack of a Cr=O linkage on the C≡C triple bonds, leading to radical-like transition states or intermediates, which subsequently rearrange through 1,2-H or 1,2-methyl shifts. Consequently, allene is attacked at its central carbon and the allyl radical thus generated undergoes subsequent ring-closure to give a cyclopropanone complex of O=CrCl2.

Reactive Intermediates in Olefin Oxidations with Chromyl Chloride. IR-Spectroscopic Proof for O=CrCl(2)-Epoxide Complexes.

After photolytic induction (411 nm) of the reaction between chromyl chloride and olefins in low-temperature matrixes, for the first time primary products of this system have been isolated and identified: the reaction of propylene yields O=CrCl(2).O=CHCH(2)CH(3) and while in the case of trans- and cis-2-butylene, OCrCl(2).O=C(CH(3))(CH(2)CH(3)) as well as OCrCl(2).(trans-2,3-butylene oxide) and OCrCl(2).(cis-2,3-butylene oxide) are obtained, respectively. Epoxide formation proceeds under complete retention of stereochemistry, and the regioselective reaction route leading to the carbonyl products can be suppressed if all hydrogen atoms at the olefinic bond are substituted: tetramethylethylene is selectively oxidized to tetramethylethylene oxide, which forms a complex with OCrCl(2). All products were characterized by IR spectroscopy, which was further supported by DFT calculations in certain cases. The mechanism leading to the formation of these products, their electronic states, and the implications of the matrix results on the thermal reactions of chromyl chloride are discussed.

Mechanisms for the formation of epoxide and chlorine-containing products in the oxidation of ethylene by chromyl chloride: a density functional study

The reaction between CrO2Cl2 and ethylene leading to the formation of epoxide and chlorohydrin precursors or directly to 1,2-dichloroethane has been studied by density functional theory. The formation of the epoxide precursor (Cl2(O)Cr-OC2H4) was found to take place via a [3+2] addition of ethylene to two Cr=O bonds followed by rearrangement of the five-membered diol to the epoxide product. The alternative mechanisms involving a direct addition of oxygen to ethylene or the [2+2] addition of the olefin to a Cr=O bond were found to have much higher activation energies. The formation of the chlorohydrin precursor (Cl(O)Cr-OCH2=CHCl) was found to take place via a [3+2] addition to one Cr—Cl and one Cr=O bond. Pathways involving initial [2+2] addition to a Cr—Cl or Cr=O bond had much higher activation barriers. The generation of 1,2-dichloroethane is highly unfavorable with an endothermicity of 44.7 kcal/mol and an even higher activation barrier. It is suggested that the formation of epoxide and chlorohydrin from the respective precursors requires the addition of H2O.Key words: reaction mechanisms, epoxide, oxidation of ethylene, chromyl chloride, DFT.

Characterization of chromium (III) oxide powders prepared by laser-induced pyrolysis of chromyl chloride

Ultrafine chromium (III) oxide powders have been produced by laser-induced pyrolysis of chromyl chloride vapour. The crystalline particles are characterized by their mean diameter, which is found to be between 10 and 300 nm. This is determined as a function of the parameters of the reaction. The pressure in the reaction chamber, focusing the laser beam or using different carrier gases has a significant influence on the mean particle size. Variation of the laser power and of the flow rate of the carrier gas does not influence the particle size.