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The mechanism of the oxalate/hydrogen peroxide chemiluminescence reaction has been examined by magnetic resonance techniques. Investigation of the reactive intermediates involved in chemiluminescence was carried out with bis(2,6-difluorophenyl)oxalate (DFPO) using 19F NMR to probe its reactions with aqueous hydrogen peroxide. Formation and reactions of the intermediate hydroperoxy oxalate ester B along with the formation of the half ester product C and difluorophenol D were monitored by 19F NMR. When the reaction of DFPO and aqueous hydrogen peroxide was carried out in the presence of dansylphenylalanine, a typical fluorescent analyte, the intensity of the resonance due to the intermediate B was diminished in direct proportion to the concentration of the analyte. Comparison of the time/intensity profile of the chemiluminescence emission with that of the 19F NMR transient suggests that the hydroperoxy oxalate ester B is the likely 'reactive' intermediate, capable of participating in a chemically initiated electron exchange luminescence mechanism.

a strong case cannot be made for S,N,0 chelation by cysteine and penicillamine.A substantial contribution of S,N chelation to kinetic Cu(II)-S bond stability is readily apparent from the fc2((Me6tren)Cu-SR, (tmpa)Cu-S-pen) and A:3((tmpa)Cu-S-cys and -S-cme) rate constants. The presence of an optimal, five-membered S,N chelate unit certainly would stabilize the Cu(II) oxidation state and retard Cu(II)-S bond breaking in a reductive-elimination pathway leading to Cu(I) and a monodentate, N-bonded thiyl radical. Oxidation rate constants of S,N-bonded cysteine and its methyl ester are not greatly different in complexes with both (tmpa)Cu11 and (Me6tren)Cun, but the reactivity of penicillamine is lower by more than 1 order of magnitude in both systems. This observation lends further support to a reductive-elimination mechanism requiring Cu(II)-S bond cleavage in the rate-determining step. Considering the surprisingly small rate of ring closure within (20) Voltammograms generated from 1 mM Cu(Me6tren)2+ in pH 6.0, / = 0.1 M (MES) buffer showed a cathodic current maximum at -217 mV vs. NHE (50 mV/s sweep rate). E6(Cu(tmpa)2+/+) is -147 mV under these conditions. MES = morpholinoethanesulfonic acid. (21) Coates,

2.0 x 10-4 k2, M-' 6' 5.9 k2k3/k-2, M-2 s-I 1.3 x 103 1.3 X 10, K,, M-I 17 not obsdbReferences 39 and 40. This is not observed even though [CN-] is a factor of 1.5-36 higher than in the G3 study. strength occurs, which results in the shift of the d-d band to higher energy.The crystal field stabilization energy (CFSE) for the squareplanar ds nickel(I1) complex should be greater than that for the d9 copper(I1) complex. This is reflected in shorter bonds for the nickel complex. Recent crystal structures17 show that the average equatorial bond lengths are 1.83 A for Ni"(H-,Aib3)-compared to 1.92 A for Cd1(H-,Aib3)-. The stability constant for the copper(I1) complex is larger because electron-pairing energy is required to form the low-spin square-planar nickel(I1) complex. The d8 nickel(I1) system shows a greater increase in stability constant for the Aib,, relative to the G3 complex, as compared to d9 copper(I1) system. This can be attributed to the larger gain in CFSE for the nickel(I1) system as the donor strength of the ligand increases.The nickel(II1,II) and copper(II1,II) reduction potentials have been measured for the complexes of Aibj6,16 and G3.33,34 The ratios of the stabilities of the complexes of divalent nickel and copper with Aib, and G3 are now known. This information can be used to calculate the ratios ( R , eq 20-22) of the stabilities of the 81-21 81-21-c -2H' trivalent metal complexes of these two ligands. The R values follow the sequence Cu" ( << Cu"' (lo",). In other words, Ni111(H-zAib3) is over 5000 times more stable than Ni111(H-2G3), and Cu"'(H_,Aib3) is almost 1.6 million times more stable than Cu11'(H-,G3). Thus, the inductive effect of the six methyl groups on the copper(II1) complex is enormous. The enhanced stability is consistent with previous observations of the increased thermal stability of the nickel(II1) and copper(II1) complexes of Aib,, relative to that of the corresponding complexes of G3.6 and the large CFSE, which is expectedfor a d8 trivalent metal ion with a square-planar geometry.32The reaction of Nin(HzAib3)-with cyanide ion is also markedly slower than the corresponding reaction of Ni"(H,G,)-. The values of the resolved rate constants for the reactions of both of these complexes with cyanide ion are given in Table VII. These complexes both rapidly form stable 1 : 1, cyanide ion mixed-ligand intermediates. The complexes react by a similar mechanism, with a rate step that depends on [CN-],, which carries the major portion of the reaction. The third-order rate constant for this process, kZk3/k+ as defined in eq 19, is -100 times smaller for the Aib, complex than the corresponding value for Ni"(H-,G,)-. In the case of the G3 complex, there is kinetic evidence for the formation of significant concentrations of a second intermediate, Ni"-(H-,G3)(CN-)>-. In the present study, there is no evidence for the formation of such a species, despite the use of cyanide concentrations up to 36 times that used in the G3 study. The failure to observe a second intermediate is presumably due to...

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