Volume 48, number 1 CHEMICAL PHYSICS LETTERS 15 stay 1977

A MAGNETIC ISOTOPIC EFFECT IN THE TRIPLET SENSITIZED PHOTOLYSIS

OF DIBENZOYL PEROXIDE

R.Z. SAGDEEV, T.V. LESHINA, M.A. ICAMKHA, 0.1. BELCHENKO, Yu. N. MOLIN Institute of Chemical Kinetics and Combustion, Szberian Branch of the Academy of Sciences of the iISSR, Novosibirsk 630090. USSR

and

A.I. REZWKHIN Institute of Organic Chemistry, Siberian Branch of the Academy of Sciences of the USSR. Novosibusk 630090. USSR

Received 10 January 1977

A t3C isotopic effect wasdetected in the triplet sensitized photolysis of dibenzoyl peroxide. Analysis of 13C NXR spec- tra shows that the amount of 13C in position 1 of phenyl benzoate, the cage product, increases by 23 f 5% in comparison with natural abundance. The observed isotopic effect considerably exceeds the known **C/13C kinetic isotopic effects. This effect is believed to be magnetic in origin and is explaiaed in terms of the dependence of the recombination probabiIities of radic’al pairs in the cage on the electron-nuclear hyperfine interaction.

The isotopic effects observed in chemical reactions are usually associated with the influence of the iso- topic mass of nuclei on the mass, the inertia moments

and the vibrational frequencies of a molecule [l] . A novel origin of the isotopic effects connected with the difference in the isotopic magnetic properties (the so-called magnetic isotopic effect) is discussed in refs. [2-4] . Its mechanism is explained in terms of a de-

pendence of the recombination probabilities of radical pairs (RP) obtained in radical re;lctions on the elec- tron-nuclear hyperfine interaction (hi?) and, thus, on the nuclear magnetic moment. The first experimental observations of the magnetic isotopic effects were de- scribed in refs. [5,6]. In the present paper we give ex- perimental evidence on the magnetic origin of isotopic effects described in ref. [5]. We have investigated the carbon (13C/12C) isotopic effect in the triplet sensi- tized decomposition of benzoyl peroxide in carbon tetrachloride solution-

‘Ihe following facts enable us to observe the mag- . netic isotopic effect in this reaction. Firstly, the phenyl radical belongs to the group of a-radicals with

high hfi constants at l3C nuclei (= 150 Oe). Secondly_ the intermediate RP is formed in a triplet state [71 - and thus the direct formation of phenyl benzoate in cage is hardly probable *. Thus phenyl benzoate may be obtained in this reaction only after T-S conversion of triplet RP whose wte depends on the electron-

nuclear interaction. We irradiated the Ccl4 solution (1000 ml) of di-

benzoyl peroxide (0.1 M) containing acetophenone (0.3 M) as a triplet sensitizer to almost completeLy de-

compose the dibenzoyl peroxide. The solution was ir-

radiated with a high pressure mercury lamp DRSh-500 through Pyrex glass. The decrease of dibenzoyl peroxide was monitored by iodometric titration of an aiiquot.

* The infIuence of spin multiplicity on tfie ptrenyt benzoatc yield in this reaction was demonstrated in ref. [Sl_

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Volume 48, number 1 CHEMICAL PHYSICS LETTERS 15 May 1977

The phenyl benzoate was extracted with a yield of 2-3%. The analysis of the isotopic carbon composition of

the phenyl benzoate was performed using 13C FT NMR (Bruker HX-90). Double resonance with total proton decoupling was used in the measurement of amplitudes and integral intensities of 13C lines. The isotopic ef-

fect was determined by comparing the line intensities of phenyl benzoate with natural 13C abundance to that of the reaction product. The NMR I3 C spectra of phenyl benzoate with Cr(Acac), added to the solution was obtained in one series of experiments to eliminate saturation effects. In both cases similar results were ob- tained. The isotopic effect was measured as an average of 20 spectra obtaked in 3 independent experiments.

It was found from these measurements that the amount of 13C in position 1 of phenyl benzoate in- creased by 23 + 5%. The abundance of 13C in other positions was unchanged within the limits of the data scattering.

The isotopic effect observed considerably exceeds the known kinetic isotopic effects for 12Cj13C (<2%) [I] _ At the same time the magnitude and sign of the effect observed are readily explained by the previously mentioned nuclear dependent S-T conversion of RP.

To estimate the magnitude of the expected isotopic effect let us examine the simplest RP with one mag- netic nucleus having spin l/2. According to ref. [9] a radical pair with an arbitrary number of magnetic nuclei may reduce to a one-nucleus RP with

I l/2

A CA:k+ CAL , k ‘!

where A,, and A2n are the coupling constants of radi- cals 1 and 2 of the RI’. Let the RP at the initial instant be in a triplet electronic state. Then we have the follow- ing expression for the RP recombination probabilities (in zero magnetic field) in the RP diffusion model [lo]

P = 0.22(A#/*. (2)

Here 7 denotes the tim between the individual dif- fusion steps. Substituting A,, = 5.1 X lo8 rad/s in eq. (1) for the RP with l*C in position 1 *, A,, = 2.7 X 109 rad/s, to those containing 13C in the same posi-

tion, and T x lo-l1 s we have ,

p(‘*C) = 0.016, p(13C) = 0.036.

* In this case S-T conversion is induced only by the proton hfi coupling constan% The values of A for the phenyl radi- cal are taken from ref. [ 1 I], ana those for CsHsC=(O)-O are believed to be unimportant.

90

The above estimates show that for product (I) more than a two-fold increase in the occurrence of 13C in position 1 may be expected. A lower value of the iso- topic effect (~23%) is observed probably due to (i) a partial decomposition of peroxide from an excited sin- glet state resulting in singlet RP; (ii) the possibility of product (I) formation on encounting the radicals in the adjacent volume.

To obtain additional evidence of the magnetic origin of the isotopic effect we have studied the thermal de- composition of benzoyl peroxide proceeding through the initial singlet RP. Theory shows [9] that in this case the magnetic isotopic effect must be much lower in mag- nitude and have the opposite sign. In accordance with this we failed to observe any change in the occurrence of 13C for phenyl benzoate in position 1.

Thus the following conditions are optimum for the observation of the magnetic isotopic effects in the reac- tion products: (i) the products are generated from RF’; (ii) the initial state of the radical pair should be a triplet; (iii) the hfi constants in a radical differ appreciably for the isotopes considered. To exclude other ways for the generation of products tending to inhibit the effect, add- ing some radical traps is recommended_

The authors are grateful to A-A. Obynochny for tech- nical assistance.

References

[I] L. Melander, Isotope effects on reaction rates (Ronald Press, 1959).

[2] R.G. Lawler and G.T. Evans, Ind. Chim. Belges 36 (1971) 1087.

[ 3 ] KM Salikhov, F-S. Sarvarov, RZ. Sedeev and Yu_N_ Molin, Kinetika i Kataliz 16 (1975) 279.

[4] A.L. Buchachenko, Usp. Chim. 45 (1976) 761. [S] Yu.N. Molin and R.Z. Sagdeev, in: Proceedings of the

Chemical Kinetics Symposium, Moscow (1976). [6] A.L. Buchachenko, E-M. Galimov, V.V. Ershov, G.A.

Nikiforov and A-D. Perschin, DokL Acad. Nauk SSSR 228 (1976) 379.

[7] R. Kaptein, J. HoBander, D. Antheunis and L. Oosterhoff, Chem. Commun. (1970) 1687.

[8] K.Tokumaru. A. Oshima, T. Nakota, H. Sakuragi and T. Mishima, Chem. Letters (1974) 571.

[9] K-M. Salikhov, Thesis, Novosibirsk (1974). [LO] F.S. Sarvarov and K.M. Salikhov, Teoret. i Eksperim.

Chim. 11 (1975) 45. [ 111 J.A. Pople, D.L. Beveridge and P-A. Dobosh, J. Am. Chem.

Sot. 90 (1968) 4201.