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Quantum Chemical Interpretation of the Photoinitiated Autoxidation of Sulphite Catalysed by Ferric Ions

A. M. El-Wakil, M. S. Soliman*, and A. B. Farag Chemistry Department, Faculty of Science, Mansoura University, Mansoura, Egypt

Z. Naturforsch. 38b, 858-860 (1983); received March 11, 1983

Photoinitiation, Autoxidation, Sulphite Ferric Ion, Sensitization, Quantum Chemical Interpretation

Autoxidation of sulphite is the first well known chain reaction proceeding through free radicals in solutions. Dark and photochemical autoxidation of sulphite were stated to take place by the same mechanism except in the initial steps. It was found that the formation of sulphite radicals in photoinitiated autoxidation of sulphite which is a metal catalysed reaction as well as the classical thermal reaction is due to the absorption of photons by ferrisulphite complexes rather than sulphite itself. Confirmation of this finding was achieved spectrophotometrically as well as by the fact that such complexes are specified by very high stability constants. The present work is a further evidence for the presence of such complexes and to cover the gap present before in the literature data. Quantum mechanical calculations using the extended Hückel molecular orbital method has proved that the range of absorption by sulphite lies in a region at which sulphite alone does absorb at all, and the absorption process is thus due to the formation of ferrisulphite complexes.

Introduction

The autoxidation of sulphite, both thermal (which is one of the best known trace metal cata-lyzed reaction) and photochemical, has long been considered to proceed by a free radical chain reac-tion where the sulphite radical acts as chain carrier [1-5]. The reaction course proposed is similar for both the thermal and photochemical autoxidation of sulphite except in the initiation step:

Cu2+ + SO§- -> SO3 + CU+ (thermal) (1)

SO§- — • SOg + e-q (photoinitiated) (2)

The indoubitable success of the chain reaction mechanism, in explaining sulphite autoxidation in solution, had compelled other authors [6] apply the same mechanism to enzymatic oxidations. Serious objections were raised by Laidler [7, 8] who be-lieved that the most probable intermediates in enzymatic oxidations wrere ternary complexes of the type (02-enzyme-substrate).

It has been found by Siska and Lunak [9], that reaction (1) cannot proceed in this way due to the high stability constants of sulphotocuprous com-plexes formed during the copper catalysed autoxida-tion of sulphite. The reaction intermediates of the thermal reaction are probably ternary complexes

* Reprint requests to Dr. M. S. Soliman. 0340-5087/83/0700-0858/$ 01.00/0

of the type (02Cu(S03)w)-2n+1 . The fact that all data on the autoxidation of sulphite suggested that both thermal and photoinitiated reactions proceed by the same mechanism, made us investigate whether the course of the photochemical reaction wTas influenced by metal ions similarly as the course of the thermal reaction, and whether ternary com-plexes of the same type were also intermediates of the photochemical reaction.

Experimental Unless otherwise stated, all the chemicals used

were of analytical grade. Bi-distilled water from a silica apparatus was used during this investigation. The reaction course of sulphite autoxidation was carried out in a silica flow-cell irradiated by UV light. On one side, the flow-cell is connected to a thermostated vessel flushed with oxygen, and on the other side it is connected to a flow-cell of a spectro-photometer (Unicum SP 800). The circulating reac-tion mixture was irradiated with UV light emitted from a high pressure mercury arc of 90 W output (Philips Spectral Lamp No. 93136). The concentra-tion of sulphite was followed spectrophotometrically at x = 245nm. The electronic structure calculation were performed with the acid of IBM 370/145 com-puter system of Al-Ahram Management and Com-puter Center (AMAC).

Results and Discussion The influence of iron concentration was tested

both on the thermal and photochemical autoxida-tion of sulphite. In case of thermal reaction, iron

A. M. El-Wakil et al. • Photoinitiated Autoxidation of Sulphite Catalysed by Ferric Ions 859

(III) has an immeasurable effect on the rate of sulphite oxidation with the instantaneous forma-tion of an intense red colouration. The red colour is due to the formation of ferrisulphite complexes. On the other hand, immediately after addition of iron(II) a very quick decrease of sulphite occurred which ceased after a few seconds and the reaction then continued exactly as if the reaction had been started after adding iron(III). In accordance with this kinetic fact it was spectrophotometrically found (Fig. 1) that immediately after the addition of Fe|+ into a reaction mixture Fe(II)-sulphite complexes were formed (curve 2 in Fig. 1), which

m 0.8

- \ 1

> 1 1

Vi

t

\ 2

V \ V

Fig. 1. 4.30 - Absorption spectra. 1) 2.91 • 10~4 M FeS04 ; 2) 2.78 • 10~4 M F e S 0 4 + 10~2 M S032~ under

atmosphere; 3) The same concentrations as in No. 2, measured immediately after letting O2 bubble through. Path length = 2 cm, 20 °C; 4) glass (Simax, 1 mm).

when contacting oxygen, immediately oxidized to Fe(III)-sulphite complexes, disregarding the sur-plus sulphite (curve 3 in Fig. 1). On the contrary, Fe|+ revealed a very pronounced effect on the photoinitiated reaction using light of X > 270 nm (where sulphite itself absorbs at X > 260 nm). As a simple filter which would "cut o f f " the far UV spectrum of an arc we used the glass cuvette (see curve 4 in Fig. 1). Evidently, the absorbance is due to another species other than sulphite alone which is best explained and confirmed by the existence of ferrisulphite complexes. Their considerably high stability constants are well known [10, 11] but no details have so far been reported on their photo-chemistry. The results presented about the photo-chemical behaviour of ferrisulphite complexes ex-hibits similar behaviour as some other ferri-com-plexes with reducing ligands (e.g. ferrioxalate ac-

tinometer). We can presume that there occurs a reduction (on exposure to light) from Fe(III) to Fe(II) shich, as proved above that Fe(II), is a pro-nounced catalyst in thermal sulphite autoxidation.

Since iron(III) forms octahedral complexes [12, 13] and similar to the well known ferrioxalate actinometer, the sulphite ion is expected to form an intermediate complex with the ferric ion of the type [Fe(S03)3]3- in which sulphite ions acts as bidentate oxygen bonded ligands to the metal ion [12]. Such complex is seen to be very sensitive and is imme-diately photodecomposed when subjected to the appropriate radiation to generate the sulphite ion radical, which is the initiator of the autoxidation process. This proposed mechanism may be sum-marized as follows:

,0 0-I

0 — F e — 0 / 0 0—S^

hv I / 0 — Fe 0

V . I 0—s>

0—s 0 — S Fe S — 0 V V

2 -

therally

The released sulphite radical left behind the active species responsible for thermal reaction was replaced either by a H2O ligand taking away the free electron (forming a solvated electron) or through the interaction with the diffused oxygen leading to the proposed oxygen reaction sequence [14]. Of course, the left species in which iron is again trivalent prefers adding sulphite ion accomplishing octahedral configuration which is the active photo-chemical intermediate.

The quantum mechanical molecular orbital me-thod at the Extended Hiickel level was applied to calculate the electronic structure of the formed ferrisulphite intermediate complex. Such calcula-tions were successful in interpreting a similar photo-chemical reaction [15]. Since there are no available structural data for this intermediate in the litera-ture, intuitive estimates for bond lengths and angles to be used in the calculation were made, Fe -0 :1 .96 Ä ; S -0 :1 .70 A ; < Fe-O-S : 80.4°; < 0 - F e - 0 : 9 0 ° and < O-S-0:109,2° . The energies of atomic or-bitals and their slater exponents considered in the

860 A. M. El-Wakil et al. • Photoinitiated Autoxidation of Sulphite Catalysed by Ferric Ions 860

calculation are given in Table I. The off-diagonal terms of Hückel matrix were evaluated using the Wolfsberg-Helmholtz formula [17]. A basis set of 57 atomic orbitals was used in the calculation.

Tab. I. The energies of atomic orbitals and their slater exponents [16].

Atom Ionization potential Slater exponents [eV]

Fe (3d) — 7.07 2.6 Fe (4s) — 8.68 1.36 Fe (4p) — 3.72 1.36 S (3s) — 21.13 2.12 S (3p) — 13.30 2.12 O (2s) — 35.57 2.246 O (2p) — 18.30 2.246

A set of 57 molecular orbitals was calculated. The 77 valence electrons are accomodated in the lowest 39 molecular orbitals. The highest occupied three, are found to be a triply degenerate belonging to the Tiu symmetry species in the Oh point group. These Tiu levels are accupied by 5 electrons. Their cal-culated eigenfunctions indicated that these highest occupied levels have their major contributions from the oxygen atoms, of the sulphite ions, which are directly bonded to the Fe metal ion. This arises from the relatively negligible coefficients of atomic orbitals located on the Fe-metallic ion in compari-

son with those coefficients on the oxygen atoms. The lowest two unoccupied molecular orbitals are doubly degenerate and belong to the Eg symmetry species. They receive their major contributions from the d-functions of the (Fe) metallic ion. This arises from the relatively large coefficients for the d-orbitals located on the (Fe) metallic ion when compared with the negligible coefficients for the atomic orbitals located on the O-atoms of the ligand sulphite ions.

Interaction of electromagnetic radiation with the intermediate ferrisulphite complex gives rise to the absorption of energy quanta and the promotion of an electron from the highest occupied orbitals to the lowest unoccupied orbitals. As indicated above, this electron promotion is a charge-transfer from the ligand to the (Fe) metallic ion. On other words, this charge-transfer is accompanied by an increase in electron density on the Fe ion at the cost of a decrease of electron density on the O-atoms of the sulphite ion ligands, and consequently the probable cleavage of the F e - 0 linkage as presented above.

The energy difference between the highest oc-cupied and the lowest unoccupied level is found to be 4.4925 eV, which corresponds to the energy of the electromagnetic radiation of wavelength 275nm. This calculated wavelength is considered a reason-able result, in good agreement with the experiment-ally applied light.

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