Evidence for a bond homolysis pathway for O/sub 2/ release in the decomposition of the superoxochromium(III) ion CrO/sub 2//sup 2 +/

Journal Article · Wed Jul 22 04:00:00 EDT 1987 · J. Am. Chem. Soc.; (United States)

DOI:https://doi.org/10.1021/ja00249a021· OSTI ID:6335909

Brynildson, M E; Bakac, A; Espenson, J H

The cation (H/sub 2/O)/sub 5/CrO/sub 2//sup 2 +/ has a considerable lifetime in aqueous solution in the absence of Cr/sup 2 +/. One pathway for decomposition consists of bond homolysis, CrO/sub 2//sup 2 +/ ..-->.. Cr/sup 2 +/ + O/sub 2/, with k = 2.5 x 10/sup -4/ s/sup -1/ at 25.0/sup 0/C (..delta..H/sup double dagger/ = 25.1 +/- 1.1 kcal mol/sup -1/, ..delta..S/sub double dagger/ = 9.7 +/- 3.6 cal mol/sup -1/ K/sup -1/). The resulting Cr/sup 2 +/ then reacts very rapidly with CrO/sub 2//sup 2 +/, with k = 8 x 10/sup 8/ M/sup -1/ s/sup -1/, estimated by kinetic modeling of the effect of (O/sub 2/). A second pathway, which yields HCrO/sub 4//sup -/, proceeds by a bimolecular reaction between two CrO/sub 2//sup 2 +/ ions. It is characterized by a rate constant 6.0 +/- 0.9 M/sup -1/ s/sup -1/ at 25/sup 0/C (..delta..H/sup double dagger/ = 12.0 +/- 1.7 kcal mol/sup -1/, ..delta..S/sub double dagger/ = -14 +/- 6 cal mol/sup -1/ K/sup -1/). A structure of the activated complex for this pathway is proposed. The equilibrium constant for homolysis of CrO/sub 2//sup 2 +/ is K/sub hom/ = 1.6 x 10/sup -12/ M and that calculated from the thermodynamic data for heterolysis (CrO/sub 2//sup 2 +/ in equilibrium Cr/sup 3 +/ + O/sub 2//sup -/) is K/sub het/ = 3 x 10/sup -8/ M. The latter, although kinetically unimportant, is suitable for thermodynamic analysis. The Lewis basicity of O/sub 2//sup -/ for Cr/sup 3 +/ is discussed on the basis of comparisons to cobalt(III)-ammine complexes, other (H/sub 2/O)/sub 5/CrX/sup 2 +/ complexes, and acid ionization constants. The data do not require a description of CrO/sub 2//sup 2 +/ as other than a superoxochromium(III)ion.

• Research Organization: Ames Lab., IA
• DOE Contract Number: W-7405-ENG-82
• OSTI ID: 6335909
• Journal Information: J. Am. Chem. Soc.; (United States), Journal Name: J. Am. Chem. Soc.; (United States) Vol. 109:15; ISSN JACSA
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201* -- Chemical & Physicochemical Properties
BASES
CATIONS
CHARGED PARTICLES
CHEMICAL BONDS
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
CHROMIUM COMPOUNDS
DATA
DECOMPOSITION
ELEMENTS
EXPERIMENTAL DATA
INFORMATION
IONS
KINETICS
LEWIS BASES
NONMETALS
NUMERICAL DATA
OXYGEN
OXYGEN COMPOUNDS
PHYSICAL PROPERTIES
REACTION KINETICS
THERMODYNAMIC PROPERTIES
TRANSITION ELEMENT COMPOUNDS