Electron-transfer and energy-transfer quenching of excited-state chromium(III) cyclam complexes and redox reactions of the chromium(II) product

Journal Article · Thu Nov 25 04:00:00 EST 1993 · Journal of Physical Chemistry; (United States)

DOI:https://doi.org/10.1021/j100149a026· OSTI ID:5651935

Bakac, A; Espenson, J H ^[1]

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1. Iowa State Univ., Ames, IA (United States)

The [sup 2]E excited states trans-[*Cr[14]aneN[sub 4](NH[sub 3])[sub 2]][sup 3+] and trans-[*Cr([14]aneN[sub 4])(CN)[sub 2]][sup +] oxidize methyl viologen radical cation, MV[sup +], in D[sub 2]O with rate constants k = 1.09 [times] 10[sup 9] M[sup [minus]1] s[sup [minus]1] and 3.10 [times] 10[sup 9] M[sup [minus]1] s[sup [minus]1], respectively, at 298 K and 0.050 M ionic strength. The initial chromium(II) products rapidly aquate to yield trans-[Cr([14]aneN[sub 4])(H[sub 2]O)[sub 2]][sup 2+], which reduces MV[sup 2+] back to MV[sup +]. This reaction is strongly catalyzed by coordinating anions Cl[sup [minus]], Br[sup [minus]], and SCN[sup [minus]]. For X = Cl and Br, the final chromium product is trans-[Cr([14]-aneN[sub 4])(H[sub 2]O)(X)][sup 2+]. The kinetics obey a mixed third-order rate law, k[sub obs] = K[sub x][MV[sup 2+]] [trans-Cr([14]-aneN[sub 4]) (H[sub 2]O)[sub 2][sup 2+]][X[sup [minus]]], with k[sub Cl] = 4.65 [times] 10[sup 5] M[sup [minus]2] s[sup [minus]1] and k[sub Br] = 5.22 [times] 10[sup 5] M[sup [minus]2] s[sup [minus]1] at 298 K and 0.25 M ionic strength. The SCN[sup [minus]]-catalyzed reaction has a more complex rate law and yields trans-[Cr([14]aneN[sub 4])(NCS)[sub 2]][sup +] as the chromium product implying trans-[Cr([14]aneN[sub 4])(NCS)[sub 2]] as the sole kinetically important form of chromium(II). The quenching of trans-[*Cr([14]aneN[sub 4])(CN)[sub 2]][sup +] by Cr(H[sub 2]O)[sub 6][sup 2+] (k = 5.23 [times] 10[sup 7] M[sup [minus]1] s[sup [minus]1]), V(H[sub 2]O)[sub 6][sup 2+] (k = 2.08 [times] 10[sup 7]), and VO(H[sub 2]O)[sub 5][sup 2+] (k = 4.54 [times] 10[sup 6]) takes place by energy transfer. 22 refs., 2 figs., 2 tabs.

• DOE Contract Number: W-7405-ENG-82
• OSTI ID: 5651935
• Journal Information: Journal of Physical Chemistry; (United States), Journal Name: Journal of Physical Chemistry; (United States) Vol. 97:47; ISSN JPCHAX; ISSN 0022-3654
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400102 -- Chemical & Spectral Procedures
400201* -- Chemical & Physicochemical Properties
CATALYSIS
CATIONS
CHARGED PARTICLES
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
CHROMIUM COMPLEXES
COMPLEXES
DATA
ELECTRON TRANSFER
ENERGY LEVELS
ENERGY TRANSFER
EXCITED STATES
EXPERIMENTAL DATA
HEAVY WATER
HYDROGEN COMPOUNDS
INFORMATION
IONS
KINETICS
NUMERICAL DATA
OXYGEN COMPOUNDS
RADICALS
REACTION KINETICS
REDOX REACTIONS
SPECTROPHOTOMETRY
TRANSITION ELEMENT COMPLEXES
WATER