DEVELOPMENT OF A PULSED-ACCELERATED-FLOW SPECTROMETER AND A RAPID-SCANNING EPR STOPPED-FLOW SYSTEM FOR THE STUDY OF REACTIONS IN SOLUTION

A pulsed-accelerated-flow instrument, which uses a high resolution digital positioning ram, has been assembled. An accelerated-flow profile (velocities in the observation tube of 2 to 13 m/s) in conjunction with the method of integrating observation was used to measure rapid rates of reaction. Rates of mixing and chemical reaction were treated as sequential first-order processes. Absorbance versus velocity measurements permit the resolution of the two steps. First-order reaction rate constants in the range of 500 to 12,000 s(\u27-1) were determined in a single push which requires 6 mL of each reagent. The system capabilities were demonstrated by measuring electron-transfer rate constants for four calibration reactions: Ru(bipy)(,3)(\u273+)+Fe(,aq)(\u272+), Ce(IV)+Fe(CN)(,6)(\u274-), IrCl(,6)(\u272-)+Fe(,aq)(\u272+), and IrCl(,6)(\u272-)+Fe(CN)(,6)(\u274-). The rate constant for the reduction of the nickel(III)-deprotonated-peptide complex of tri-(alpha)-aminoisobutyric acid by Fe(CN)(,6)(\u274-) was determined to be (1.40 (+OR-) 0.08) x 10(\u277) M(\u27-1) s(\u27-1). A rapid-scanning EPR spectrometer with stopped-flow mixing which is capable of obtaining up to 64 spectra (100 gauss per scan) with scan times of 0.010 to 0.900 s and with 0.020 to 900 s delay times between scans, has been constructed. A flow system delivered reagents into a mixing/observation EPR cell and triggered the generation of a microprocessor-generated waveform which controlled a rapid-scan unit. The spectra were digitalized and processed to determine rates of reaction. The system was applied to the kinetics and mechanism of rearrangement of bis(dipeptide)nickelate(III) complexes. Oxidation of blue bis(dipeptide)nickelate(II) complexes yields violet-black nickel(III) complexes (A), which are long-lived species in neutral solution. Dipeptides with glycyl, alanyl and isobutryryl residues were examined. The EPR spectra of the frozen aqueous glasses have g(,(PARLL)) greater than g(,(PERP)) for A. The unpaired electron appears to be in the d(,x)2(,-y)2 orbital of a tetragonally-compressed octahedral environment. Acid catalyzes the rearrangement of A to a yellow nickel(III) complex (B), which has properties more typical of nickel(III) peptide complexes. This reaction is not reversible. Below pH 2 sequential reactions are observed. The conversion of A to B is followed by an acid-independent decomposition of B to nickel(II). The first step is first-order in {H(\u27+)} and {Ni(III)} with rate constants (25.0(DEGREES)C) which vary from 5.6 to 160 M(\u27-1) s(\u27-1) for different dipeptides. The rate constants for the second step also vary with the dipeptide and range from 0.016 to 0.15 s(\u27-1)