Volume 46, number 2 CHEMICAL PJIYSICS LETTERS 1 hfarch 1977

DlRECTOBSERVATIONOFCHEMlCALLYlNDUCEDDYNAMIC NUCLEARPOLARlZATIONlN LOWMAGNETlCFlELDS

RZ. SACDEEV, Yu.A. GRISHIN and A.B. DUSHKIN Institute of CIwmical Kinerw and Combustion, Siberian Branch of the Soviet Academy of Sciences, Novosibusk 630090, USSR

Received 15 April 1976 Revised rnanuscrlpt received 29 November 1976

CJDNP cffccts durmg bcnzoyl peroxide thermolysis have been measured duectly by a pulsed NMR spectrometer in Jaw magnetic fields (OS-120 Oe). It IF shown tilat the traditional technique of the sample “transfer” distorts the real magnetic ticld dcpendsncc of CJDNP.

1.. Introduction

One of the remarkable features of chemically in- duced dynamic nuclear polarization (CIDNP) is that the polarization effect observed in the reaction carried out in high (lo-20 kOe) and low (5 IO0 02) magnetic fields differ essentially from one another [ 11. CIDNP studies in low fields represent a particular interest as they may supply us with umque information on the electron exchange interaction in radical pairs in solu- tions. Moreover, we note that for some reactions CIDNP studies are possible only in low fields as there is no polarization in high ones (see e.g. ref. [2] ).

Published results on CIDNP in low fields have an essential defect: all of them have been obtained by in- direct methods. In these methods the reaction and ClDNPcffect measurements are carried out with

various magnets and the sample is transferred either by hand [33 or by a flow system [4]. It is quite nat- ural that in interpreting the results obtained one meets with problems arising from the possibility that the sample “transfer” may considerably distort the real picture of polarization in low fieldst. For example, the experimental results obtained in ref. IS] by a direct CIDNP measurement at two values of the mag

t The main problems associated with the sample “transfer” are analyzed theoretically in ref. 131.

netic field (30 Oe and 47 Oe) in principle cast doubt on the polarization sign change in low fields observed in the experiments with the sample “transfer” [6].

The present report gives.results of a direct obser- vation of ‘H CIDNP in low fields in the reaction of benzoyl peroxide decomposition.

2. Experimental

2.1. Technique and device

To obtain a correct field dependence of CEDNP one should perform the reaction and measure the NMR sig- n& in the same field. However, it is very diffcu[t to create an NMR spectrometer for a wide range of mag- netic fields (1 to hundreds of Oe). Therefore WC have chosen a compromise variant for the study of the Eeld dependence of CIDNP in low magnetic fieids. The ceclc- tion was carried out in the field H, produced by the Helmholtz coils (the field intensity may be 0.5 to 350 De??). Ihe radic;-frequency part of the device work- ed with the NMR frequency in the earth field f&. For

tf l%e technicality of obtaining magnetic fields cxcecdiq 300-500 Oe by the HeJmhoJtz coils is rather complicated. Therefore the proposed technique israrely used for fields exceeding 500 Oe.

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the nuclear spin echo measuremeni the field fir was cut off. The cycle (H, + Uu + measurement) was reiter- ated every 10-20 s. Note that unhke the known sam- ple “transfer” methods the rime when the magnetic field intensity changes from Hr to HO was strictly fixed in the range of iOV3 to 1 s; the direction of Hr coin- cided with that of Ho. Under t&se conditions the spin echo technique was the simplest and most convenient methndt2.

In our device we used the CPMG pulse sequence f7f which allowed us to measure the relaxation times T, and T2 and to chinate the diffusion influence. The NMR spectrometer receiver was set up accordmg to the direct ampli~cat~on circuit and had a synchronous detector at the cxrt. The receiver band width could be set in the range of I to 500 Hz. The pulse generator producing pulses of 90” and 180° and conveying the fundamental frequency to the detector was set up ac- cording to a synthesizer circuit. The measuring ceil represented two mutually pcrpendrcular Helmho!tz systems, with a receiving cwl insrdc. The axes of the re~ci~ng coil was nornla~ to those of the Hclmho~tz cotis. The Hclmholtz coils produced the magnetic field R” and conveyed the 90’ and 180” pulses to the sam- ple. The field Zir inhomogeneity was 3% witbin a cylin- dricai specimen 3 cm in diameter and 5 cm in length.

? This method, in partrcular, makes It pos$rbfe to use Fauauricr ~~nsfoI~tlon_

* Since the chemical shift dirpcrslon ie lost III a low magnets field, rhrs method is applicable to CISCS where there is only one strongly polarized line.

2.2. l7re benzoy! peroxide thennolysis. Results and discussion

It is known 111 that in the benzoyl peroxide ther- molysis the CIDNP effects occur in the singlet pair of benzoyloxy and phcnyl radicals,

l?Gs system is convenient because during the reaction one can observe a single polarized benzene signal (the main product).

The experiment was performed as foIlows. 1 M benzoyl peroxide solution in 30 ml of cyclohexanone heated up to 13O’C was placed in 3 receiving cod of the probe in which the reaction takes place. The reac- tion Tasted for about two minutes. This enabled us to study the intensity of the bcndenc spin echo signal versus the magnetic field. The device was calibrated and adjusted by 3 water sample (400 ml, polarized by a 50 Oc field).

The benzene signal intensity plotted versus the field (the result of averaging four experiments) and CIDNP as a function of the field obtained by the cStransfer” method i6I are given in fig. 1. The absorption signal is seen to occur in low fields in both cases+, the values of& associated with the signal maxima being nearly tbc same. At the same time one can see a change in the

+ The polarization sign for low fields is fully consistent wi:h the theory [ 81.

3

Fig. 1. CIDNP versus the magnetic &Id in the bcnzoyt peroxide thermolysis. Curve (A) IS ob&ined m tic present work; curve [B) is taken from ref. [Sl.

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Volume 46. number 2 CHEMICAL PHYSICS LEITERS 1 Mirth 1977

trends of the curves in the cross-over region*. This might possibly result either from nuclear relaxation in the reaction products or from CIDNP in the intermediate fields during the sample “transfer” from the low field HI to the NMR spectrometer field [I]. It is clear that the greatest distortions caused by the sample “transfer” are observed in that H, region where the polarized sig- nal intensities are close to zero (i.e. in the cross-over region). Note that it is thus wrong to use the cross- over region as a critlcal parameter (in comparisons of calculated and experimental field dependence of CIDNP), as in ref. [9) -

Thus, our experiments have shown that to describe the field dependence of CIDNP in low fields m detail one should use direct observations. An essential advan- tage of direct CIDNP observations m low fields is the absence of the solvent signal obscuring the general sig- nal in the “transfer” method. It is very interesting that in low fields very high polarlzatlon coefficients, k = (I, - fo)/fo, are observed. Here 1, and f, are the signal intensities during and after the reaction, respecti-jely .

++ The result obtamcd is in agrccmcnt with ref. [SI.

For example, in the benzoyl peroxide thermolysis dis- cussed k = 104-2 X IO4 when H, = 10 Oe. Such values exceed considerably those observed in high fields (=lOO [l]).

References

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131

I41 (51 I61

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PI

A-L. Buchachenko. Chemical polarization of cIectrons ,md nuclei (“Nauka”, Moscow. 1974). J. Garst, F. Bargon and J. Morris. J. Am. Chcm. Sot. 93 (1971)4310. R. Kaptein and J.A. den Iiollandcr, I. Am. Chcm. Sue. 94 (1972) 6269. R.G. Lawler and hf. Ilalfon. Rev. Sci. lnstr. 4.5 (t974) 84. S. Rykov and V. Baldin, Zh. Struk. Chim. 10 (1969) 928. J. Slonim, Ya. Urman and A. Konovalov, DokIzdy Akad. Nauk SSSR 195 (1970) 1153. T.C. Parrar and E.D. Becker, Pulse and Fourier transform NMR. Introduction to theory and methods (Accadcmic Press, New York, 1971). F.S. Sarvarov, K.hf. SaMchov and R.Z. Sq&ev, Chctn. Phys. 16 (1976) 41. K.Y. Choo and J.K.S. Wan, J. Am. Chem. Sot. 97 (:975) 7127.

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