Quinone-sensitized Steady-state Photolysis of Acetophenone Oximes Under Aerobic Conditions: Kinetics and Product Studies[dagger]

Photochemistry and Photobiology, Jan/Feb 2006 by Park, Adriana, Kosareff, Nicole M, Kim, Jason S, de Lijser, H J Peter

Instrumentation. The gas chromatography analyses were performed on a Perkin-Elmer (Boston, MA) Autosystem equipped with a flame ionization detector (FID) and on a Hewlett-Packard (Agilent, Palo Alto, CA) 5890 series II coupled to a Hewlett-Packard mass selective detector (MSD) 5917 series. The Perkin-Elmer GC/FID was equipped with a Chrompack CP-Si-5-Cb capillary column (30 m × 0.32 mm i.d., film thickness 0.25 µm). The Hewlett-Packard GC/MS was equipped with an HP-5 capillary column (30 m × 0.25 mm i.d.; film thickness: 0.25 µm).

Photochemical reactors. The irradiations were carried out in a Rayonet RPR-100 photochemical reactor equipped with 16 RPR-3500A (black-phosphor) bulbs (Southern New England Ultraviolet Company, Branford, CT). The output wavelength range of these bulbs is between 300 and 400 nm, with the maximum output (~90%) at 350 nm.

Oxidation potentials. All electrochemical experiments were performed on a Bioanalytical Systems (BAS, West Lafayette, IN) C3 Electrochemistry Cell Stand with the use of a CV-50 Voltammetric Analyzer. Oxidation potentials were recorded with the use of a platinum working electrode, a platinum wire counter/auxiliary electrode, and a Ag/AgCl reference electrode. Tetraethyl ammonium perchlorate (0.1 M) served as an electrolyte salt in acetonitrile. The redox potential of the electrolyte solution was recorded first, followed by that of the oxime (~5 mmol). The scan rate was 100 mV/s. The system was calibrated by recording the oxidation potential of ferrocene.

Steady-state photolysis experiments and sample analysis. Appropriate amounts of the meta- or para-substituted oxime (0.015M), acetophenone oxime (0.015M), and chloranil (CA, 0.005M) were weighed out and dissolved in 5 mL acetonitrile. The solution was placed in a Pyrex tube (8 mL volume, cutoff wavelength = 300 nm) and photolyzed in the Rayonet reactor for 60 min. The solutions were analyzed by GC/MS and GC/FID and showed the corresponding ketones as the major products. Other (minor) products, tentatively identified as (1-chloroethyl)arenes, were observed as well. Quantification of the products was accomplished by calibrated GC/ FID with the use of authentic materials; the relative conversions over time were used as a measure of the rates of the reactions.

RESULTS AND DISCUSSION

Oxidation potentials of meta- and para-substituted acetophenone oximes

The measured peak potentials for the meta- and para-substituted acetophenone oximes are listed in Table 1. The oxidative scans are broad and irreversible in every case. For certain compounds (m-Cl, m-F), no reliable data could be obtained. Plotting the measured peak potentials against the measured rate for quenching triplet chloranil (k^sub q^) gives an excellent correlation (r^sup 2^ = 0.93; Fig. 3A).

The ionization potentials (IP) of these oximes were determined by theoretical methods. All syn and anti isomers were considered: the lowest energy conformers were found by a conformational search (MMFF) and were further optimized by AM1 calculations (34). The geometries of all the oxime isomers were then optimized by DFT calculations with the use of Becke's three-parameter functional and the nonlocal correlation of Lee, Yang and Parr (B3LYP) (35,36) with the 6-31G* basis set (37) implemented within Spartan 2002 (38). The geometry of the optimized neutral compound was used as a starting point for a geometry optimization of the corresponding radical cation species.