The oscillating chemiluminescent luminol Reaction

This work was performed from September 1991 to August 1992 as the experimental part to the diploma exam at the university of Würzburg
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Introduction

The luminol system was discovered in 1988 [1]. It bases on the inorganic Orban oscillator [2]. Luminol (3-aminophtalhydrazide) is oxidized by hydrogen peroxide to 3-Amino phtalic acid. This is produced in an electronically excited state which emits blue light. The emission of light occurs in periodic burts in the oscillatory region. The reaction can therefore be followed with a photomultiplier. Luminol as a light emitting molecule has been known since 1928 [3] and has since then be the object of extensive mechanistical studies.

I. Oscillation of luminol derivatives

Experiments using derivatives of luminol were important for two reasons. The first reason is the special mode of luminescence of luminol. The chemiluminescence is mediated. That means that it is not the primarily formed excited singulett or triplett state of the phatalate dianion which emits the photon, but a phtalic hydrazide monoanion via energy tranfer from the phtalate dianion. Other phtalic hydrazides do not seem to follow this pathway. If this additional reaction step plays a role in the oscillatory mechanism, a change in behaviour is to be expected for derivatives. The second reason is the assumed role played by copper-II/luminol complexes. Does the complexing involve solely the hydrazide nitrogens or also the amino group (this would form a sterically favourable 6-ring)? If the latter is the case, derivatives whithout side groups would exhibit a different behaviour.
Three derivatives were investigated, naphtalene dicarboxylic acid hydrazide, phtalic hydrazide and 3-nitro phtalic hydrazide. Naphtalene dicarboxylic acid was synthesized from the acid through dehydration with acetic anhydride followed by amination with urea, methylation of the nitrogen with methyl iodide an finally formation of the hydrazide with hydrazine. 3-Nitrophtalhydrazide was obtained from 3-nitro-phtalic acid in a similar way. An attempt to synthesize N-substituted 3-aminophtalhydrazides, which would have helped to clarify the role of the amino group as a complexation site, had to be abandoned as the 3-aminophtalic amide obtained from 3-nitrophtalhydrazide could be substituted with neither MeI nor iPrI.
Both naphtalene dicarboxylic hydrazide and phtalic hydrazide showed oscillations in shape and period similar to luminol (

_osc=138 s, 150 s, 133 s in that order). The peak intensity is only 2.67% respectively 1.2% for naphtalene dicarboxylic acid and phtalic hydrazide compared to luminol. It thus seems that the amino group is not essential to the mechanism but to the efficiency of chemiluminescence of energy transfer. No chemiluminescence could be detected for 3-nitrophtalic hydrazide albeit it is possible that the intensity ot the peaks was simply too low to be detected or that the wavelength of the emitted light exceeded the range of the photomultiplier.

II. Qenching of oscillations with Cyclopentadiene

A commonly postulated intermediate in the chemiluminescent reaction of luminol with H₂O₂ is an azachinone. If this was present at low concentrations in the CSTR, it could be trapped by cyclopentadiene (cp) in a [4+2] Diels-Alder cycloaddition. It is indeed observed that addition of freshly cracked cyclopentadiene suppresses the oscillations for a period of time depending on the amount of cp added. This is not simply a dilution effect as adding 100 µl while varying the cp:water ratio gives the same result. This proves the existence of the azachinone intermediate.

III. Stirring effects

Incomplete mixing in nolinear chamical reactions can have profound effects on the behaviour of this system. Steady states values as well as period and amplitude of oscillations are altered and bifurcation points shift.
Three effects were analysed:

the effect of stirring rate
the effect of inflow configuration/stirring sense
the effect of premixing and selected premixing

on the position of Hopf bifurcations with the flow rate as bifurcation parameter.
The three feedstreams can be permuted over the three inflow ports at the reactor top. This can affect the behaviour of the system because the feestreams are not instantly mixed but retain their spatial integrity in a small zone near the inlet ports. Through permutation of the inlet configurations and of the stirring sense, the strength of interactions between these subvolumes and the reactor bulk are altered, which may change the behaviour of the system, as was shown experimentally and computationally for the Belouzhov-Zhabotinsky reaction [4]. In the luminol system, however, no change in the critical bifurcation flow rates could be observed upon varying the inlet configuration or the stirring sense.
Decreasing the stirring rate has two effects. One is to shift the upper Hopf bifurcation to lower flow rates and the lower Hopf bifurcation to higher bifurcations thereby shrinking the oscillatory region. The other effect is an increase of oscillation periods from 133 seconds at 1000 rpm to 250 seconds at 500 rpm. This increase in oscillation periods is contrary to the predominantly observed and computationally verified decrease at lower stirring. This finding strongly suggests the implication of heterogensous effects.
Premixing experiments in this system yield informations about the copper complexes involved in the catalysis. Premixing of luminol and Cu²⁺ increase the oscillatory domain, premixing of all reactants even more whereas premixing of peroxide and Cu²⁺ decrease the oscillatory domain. It seems that a catalytically active conplex is formed by Cu²⁺, luminol, HO₂^- and OH^-. Hydroxide and peroxide are largely present in the bulk so that the limiting interaction is between luminol and Cu^{2+.

[1] J. Amrehn, P. Resch, F. W. Schneider,J. Phys. Chem. 92, 3318, (1988)

[2] M. Orbán, J. Am. Chem. Soc.108, 6893, (1986)

[3] H. O. Albrecht, Z. Phys. Chemie 136, 321, (1928)

[4] M. Hauser, D. Lebender, F. W. Schneider, to be published, (1992)

sebastian weissenberger

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