An investigation into the usefulness of a flash photolytic method of studying redox reactions of the benzyl radical

2.1 The Flash Photolysis System

i) General Remarks

The flash photolysis apparatus is shown schematically in Figs. 2.1 and 2.2. A bank of ten 1μF rapid-discharge capacitors connected in parallel was charged to a high voltage (10-20Kv.) by means of a 100W 25KV D.C. power supply. They were then discharged through two quartz lamps by shorting the solenoid-activated switch, S. This produced a pulse of light which decayed to 1/e of its initial value in a time of ca. 25μs. (Fig. 2.3), and with total energy of 500-2000J. The lamps and reaction vessel were placed in a polished aluminium housing.

Fig. 2.1

Fig. 2.1 Physical Arrangement Of Flash Photolysis System

Fig. 2.2

Fig. 2.2 Electrical Arrangement Of Flash

Fig. 2.3

Fig. 2.3 Oscilloscope Trace Of Photoflash

ii) Flash Lamps

The two flash lamps were constructed from high-purity quartz tubing. Electrodes were sealed into both ends of the tubing and the interior of the lamps washed in aqueous hydrogen fluoride solution to remove all adsorbed substances from the quartz surface. Each lamp was evacuated and heated to dull red heat, the whole system being kept under vacuum, until the entire length of the lamp had been heated. After cooling, the lamp was filled with krypton to a pressure of 6 Torr, and connected to the capacitor bank to be fired about fifty times at 10KV. The electrode connections were reversed and the lamp was fired another fifty times to ensure both electrodes were degassed to the same extent. The whole process from evacuation was then repeated. Finally, the lamps were filled with 6 Torr of krypton and sealed off.

iii) The Monitoring Source

The background source was a 100W quartz-iodine lamp operating at 12V. The wavelength characteristics of the source show maximum intensity at about 900nm, falling off rapidly in the near ultra-violet but with sufficient output in the region of the benzyl transition at 318nm for the purposes of this investigation.

iv) Optics

The monitoring beam leaving the reaction vessel was bent through an angle of 90° with a plane silvered mirror. This horizontal beam was then focused on the slit of the monochromator by means of a quartz lens whose focal length was adapted to the aperture of the monochromator (i.e. with a focal length of about 20cm).

v) The Monochromator

The monitoring beam was dispersed by means of a Hilger and Watts D292 monochromator having a dispersion of 7nm/mm. This was calibrated against the known wavelengths of the emission lines from a low-pressure mercury lamp viz. the lines at 253.7, 296.7, 313, 334, 366, 405, and 426nm. The monochromator reading corresponding to each line (shown by maximum deflection on the oscilloscope) was compared with the true wavelength of the particular line and the correction plotted as a function of wavelength. The average correction to be applied to the monochromator reading is then the area enclosed by the graph divided by the wavelength spread of the measurements. Results are shown in Table 2.1.

Table 2.1 Corrections to Monochromator Reading
Monochromator Reading (nm) True (nm) Correction (nm)
251.5252.5252 253.7 +1.7
311311311313 +2.0
332.5332.5332.5 334+1.5
363.5363.5363.5 366+2.5
403403403405 +2.0
434434434436 +2.0

The alignment of the optical system is checked by passing the light beam from a 1mW Spectra Physics helium/neon laser through the monochromator in the reverse direction and then tracing its path back through each component to the centre of the monitoring lamp.

vi) The Photomultiplier

Dispersed light from the monochromator was passed to a side-windowed nine-stage S5 phototube (EMI 9660B) which was operated at 1KV from a Farnell E2 EHT unit. The last two dynode stages of the photomultiplier were shorted out to reduce the noise level. The output of the photomultiplier was adjusted, where possible, to 1V.

The photomultiplier response was determined by placing a series of filters of different optical densities in the light beam and monitoring the changes in output voltage as measured on the oscilloscope. The voltage was plotted as a function of the relative incident light intensity as shown in Fig. 2.4.

Fig. 2.4

Fig. 2.4 Linearity Of Photomultiplier Response

vii) DC Bias Unit

When measuring small absorptions it was desirable to operate the oscilloscope at high vertical sensitivities (<50 mV/div.). To achieve this, the total signal was offset using a DC bias unit consisting of a stabilized power supply and a potentiometer.

viii) The Filter Unit

High frequency noise from the quartz-iodine lamp was reduced by introducing p-type filters between the photomultiplier output and the oscilloscope input. By using various filters with varying cut-off frequencies it was possible to reduce the noise but still to obtain a faithful reproduction of the transient signal. Since most transients can be represented by low-frequency fundamentals (100KHz-100Hz) the filter unit can be designed to pass these frequencies unattenuated but to attenuate the high-frequency content. The time constant of the filter circuit was made at least five times less than the transient half-life for all decays.

ix) The Oscilloscope

The signal was then passed to an oscilloscope. Two types of oscilloscope were used - a Tektronix Type 453 and a Telequipment Type D53, both having a high sensitivity range (down to 5mV/div.). Light from the photoflash was allowed to fall on a photodiode and the output from this was used to externally trigger the oscilloscope. Traces were recorded on Polaroid film and then measured with an x-y travelling microscope.

x) Reaction Cells

The cylindrical reaction cells were constructed entirely of silica with dimensions 20cm × 1.6cm diameter. There was an outlet arm at each end of the cells to allow them to be filled with degassed solution.

2.2 Materials and Purifications

i) Cleansing Procedure

To minimize contamination by impurities all glassware used was subject to a strict cleansing procedure. Generally, this consisted of first washing with permanganic acid (concentrated sulphuric acid + potassium permanganate), rinsing, and then washing with a mixture of hydrogen peroxide and nitric acid. Further washing was with water of increasing viz. tap water, distilled water and, finally, triply-distilled water, the apparatus then being dried in an oven. Any water used as a solvent was also triply-distilled.

ii) Methanol

Purification of methanol was attempted by two methods. The first method consisted of heating 50cm3 of Fisons AR grade methanol under reflux with 5g. of magnesium and 0.5g. of iodine until all reaction had ceased. The remainder of the methanol was added (usually about 500cm3) and refluxed with a slow bleed of oxygen-free nitrogen for about 4 to 5 hours. The methanol was then distilled and the middle fraction of the distillate collected at a reflux ratio of 5:1.

Purification by the second method involved dissolving two lumps of sodium (cleaned under Fisons AR petroleum ether) and 7g. of BDH sodium borohydride in a winchester volume of AR methanol. The solution was refluxed for an hour in an atmosphere of oxygen-free nitrogen and distilled under the same conditions as the above method.

iii) Cyclohexane

The required volume of BDH AR cyclohexane was initially washed several times in a separating funnel with concentrated sulphuric acid until no further discolouration of the acid was observed. After further washing with distilled water the cyclohexane was allowed to dry overnight over anhydrous calcium chloride. The purification was then completed by refluxing the solvent for an hour in the presence of a few grams of sodium borohydride and an atmosphere of nitrogen, distilling and collecting the middle fraction of the distillate at a reflux ratio of 5:1.

iv) Benzyl Phenylacetate

Benzyl phenyl acetate (Emmanuel) was found (from Thin Layer Chromatography on a silica plate) to contain at least two impurities. Distillation was attempted as a means of purification but found to be unsatisfactory because the ester was extensively decomposed on heating, even under vacuum. It was therefore chromatographed on an alumina column using a mixture of petrol and ether (3:1) as elutant. A Thin Layer Chromatography test on several of the fractions showed that separation of the constituents was successful and the solvent was then removed with a rotary evaporator.

v) Degassing of Solutions

The solutions were thoroughly degassed to remove any dissolved oxygen by bubbling with nitrogen (BOC white spot) or argon (BOC). The apparatus used is shown in Fig. 2.5.

Initially, the whole system (minus the solution) was purged of air by opening taps A and C. The cell was then capped at the open end, the solution introduced and tap C closed and B opened. Degassing of the solution thus occurs and was allowed to continue for at least 30 minutes. Tap B was then closed and C opened, the cap was removed from the cell and the degassed solution was forced into the cell which was sealed off when full of solution.

Some solutions were degassed using the freeze-pump-thaw technique with the apparatus shown in Fig. 2.6. The solution was frozen in the dry ice/acetone bath and the whole vessel evacuated to remove any dissolved gas liberated. The tap was closed and the solution allowed to warm up again and the whole cycle repeated five or six times. Finally, the degassed solution was tipped into the cell which was then sealed off whilst still under vacuum.

Fig. 2.5 Apparatus For Solution Degassing By Bubbling

Fig. 2.6 Apparatus For Freeze-Pump-Thaw Degassing