Electron Paramagnetic Resonance (EPR) Spectroscopy

1. Autoxidation of Butyraldehyde

1. Prepare a solution of butyraldehyde (100 mg) and CoCl₂·6H₂O (1 mg) in 1,2-dichloroethane (DCE) (4 mL) in a 20 mL scintillation vial. Add a magnetic stir bar and fit the vial with a rubber septum.
2. Attach the barrel of a 1 mL plastic syringe to a short piece of rubber tubing. Insert the rubber tubing into a latex balloon and secure the balloon to the tube with a rubber band and electrical tape. Inflate a latex balloon with O₂.
3. Insert the needle of the O₂ balloon into the reaction vial. Insert a second needle into the septum and purge the head-space of the reaction vessel with O₂.
4. Using a stir plate, stir the reaction at room temperature for 4 h under an O₂ atmosphere.
5. Concentrate the reaction mixture using a rotary evaporator and take an ¹H NMR spectrum of the resulting oily residue in CDCl₃.

2. Using BHT as an Antioxidant for the Autoxidation of Butyraldehyde

Set up two vials as described below. One will be used to analyze the product distribution and one will be used in step 3 for EPR spectroscopy.

1. Prepare a solution of butyraldehyde (100 mg) and CoCl₂·6H₂O (1 mg) in DCE (4 mL) in a 20 mL scintillation vial. Add BHT (10 mg) to the solution. Add a magnetic stir bar and fit the vial with a rubber septum.
2. Attach the barrel of a 1 mL plastic syringe to a short piece of rubber tubing. Insert the rubber tubing into a latex balloon and secure the balloon to the tube with a rubber band and electrical tape. Inflate a latex balloon with O₂.
3. Insert the needle of the O₂ balloon into the reaction vial. Insert a second needle into the septum and purge the head-space of the reaction vessel with O₂.
4. Using a stir plate, stir the reaction at room temperature for 4 h under an O₂ atmosphere.
5. Concentrate the reaction mixture using a rotary evaporator and take an ¹H NMR spectrum of the resulting oily residue in CDCl₃.

3. Measuring EPR Spectra

1. Turn the EPR spectrometer on and let the instrument warm up for 30 min. Set up an EPR acquisition with the following parameters: center field 3,345 G, sweep width 100 G, sweep time 55 s, time constant 10 ms, MW power 5 mW, modulation 100 kHz, and modulation amplitude 1 G.
2. Measure an EPR spectrum of an empty EPR tube to ensure that there are no background signals from either the EPR tube or the instrument resonator.
3. Prepare a solution of BHT in DCE in an N₂-filled glove box. Transfer 0.5 mL of the solution to an EPR tube and measure the EPR spectrum of BHT using the acquisition parameters set up in step 3.1.
4. Transfer 0.5 mL of the BHT-added reaction solution from Step 2 to an EPR tube and acquire and EPR spectrum using the acquisition parameters set up in Step 3.1.