J . Chem. SOC., Faraday Trans. 1, 1988, 84(7), 2305-2309

Electron Spin Resonance of a y-Irradiated Single Crystal of Carbamylcholine Chloride

Fevzi Koksal* and Fahri Celik Faculty of Arts and Sciences, Ondokuz Mayis University, Samsun, Turkey

Electron spin resonance spectra of a y-irradiated carbamylcholine chloride, H,NCOOCH,CH,N(CH,),Cl, single crystal has been investigated at room temperature, and the spectra were found to be independent of temperature down to 130 K. The radical was proved to be -CHCH,-, and the g factor and the hyperfine coupling constant of P-protons were found to be isotropic, with the values of 2.0029 and 27.6 G, respectively. The principal values of the hyperfine tensor of the a-proton were found to be A, , = 12.0 G, A,, = 27.6 G and A,, = 3.9 G. The results were found to be in agreement with the a-and P-proton coupling and are discussed.

The radiation sensitivity of choline chloride [OHCH,CH,N(CH,),]Cl has long been recognized,', and it was observed that crystalline choline chloride rapidly decomposes trimethylamine hydrochloride and acetaldehyde. Selective deuteration of choline chloride has been employed to establish the localization of the unpaired electron in the radical using e.s.r. techniques., Although the spectra were not well resolved it was concluded that the observed radical is 'CH,CH,OH. Furthermore, the e.s.r. spectra of radiation-damage centres in choline chloride were interpreted* as a biradical, (CH,),N+. . .CH,CH,OH. In addition, e.s.r. spectra of choline iodide, choline sulphate, [(CH,),NCH,CH,Cl]Cl, [(CH,),NCH,CH,CH,OH]Cl and [(C,H,),NCH,CH,OH]Cl, were examined, and it was found that the radicals formed in all anologues except choline chloride and choline bromide were not ethanol radicals ; however, detailed inter- pretations were not a t t em~ted .~ To our knowledge no further e.s.r. studies have been carried out on choline derivatives, and it is the purpose of this study to investigate an analogue of the abovementioned compounds, carbamylcholine chloride, in the hope of determining its radical structure after y-irradiation.

Experiment a1 The carbamylcholine chloride single crystals were grown in the laboratory from concentrated ethanol solutions. In its single-crystal form carbamylcholine chloride, H,NCOOCH,CH,N(CH,),~1, is orthorhopbic6 with spaceogroup P,,,, and with unit- cell dimensions a = 10.248 A, b = 12.850 A and c = 6.809 A.

The crystals were irradiated at room temperature by a 6oCo y-ray source of 0.3 Mrad h-' for 5 h. The e.s.r. spectra were recorded with a Varian model E-109 C e.s.r. spectrometer using 1 mW microwave power. The low-temperature measurements were carried out using a Varian temperature-control unit. The crystals were rotated on a Lucite pillar about the three axes shown in fig. 1, and the angle of rotation were read on a scale graduated in degrees. Because the crystals were hygroscopic they were coated with a thin film of collodion to protect them against moisture during the observations. The g factor was found by comparison with a DPPH sample ( g = 2.0036).

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2306

I

E.S. R. of y-lrradiated Carbamylcholine Chloride

Fig. 1. Crystal habit and the axes of carbamylcholine chloride.

20 G &----+

Fig. 2. E.s.r. spectrum of y-irradiated carbamylcholine chloride obtained along the H , l a and H, I1 b axes.

Results and Discussion The e.s.r. spectra of carbamylcholine chloride exhibit mainly three different patterns as shown in fig. 2-4. The spectra shown in fig. 2,3 and 4 appear when the magnetic field is parallel to the b, c and a axes, respectively. The spectra are independent of temperature between 300 and 130 K. The g value does not change when the crystal is rotated about the a, b and c axes, and therefore is very nearly isotropic within experimental error; its measured value is g = 2.0029+0.0005.

Thespectrashowninfig. 2, 3and4exhibit 1:3:3:1,nearly 1:2:1 and 1:1:2:2:1:1 patterns, respectively, and therefore indicate the hyp$rfine interaction of three protons with the electron spin; the radical must thus be -CHCH,-. Site splitting does not

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F. Koksal and F. celik 2307

20 G -

Fig. 3. E.s.r. spectrum of y-irradiated carbamylcholine chloride obtained along the H , l a and H,, 11 c axes.

20 G - n

V

Fig. 4. E.s.r. spectrum of y-irradiated carbamylcholine chloride obtained along the H,, I b and H,, 11 a axes.

occur, and therefore the molecules in the unit cell of the crystal are magnetically equivalent. The spectra can be fitted to the spin Hamiltonian

Z = g/3Ho S, + S. ZiAi* Ii (1) where /3 is the Bohr magneton and H,, is the applied magnetic field. The first term represents the interaction of the electron spin with the proton spins (Ii = :). The

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2308 E.S.R. of y-Irradiated Carbamylcholine Chloride

I I I 1 I I

180 < -

160 -

140 ' -

120 -

?. 100 - 80 - s

60 - 40 -

20 - 0

- M

5 10 15 2 0 2 5 30 A /G

Fig. 5. Variation of the a-proton coupling constant in -CHCHz- radical when the crystal is rotated about the a(O), b(O) and c(e) axes.

Table 1. Principal values of the a-proton coupling in the -CHCHz- radical produced in carbamylcholine chloride by

y-irradiation

principal values anisotropic values direction /G /G cosines

A m 12.0 - 2.5 1 0 0

A cc 3.9 - 10.6 0 0 1 27.6 13.1 0 1 0

variations of the a-proton coupling about the a, b and c axes are shown in fig. 5. The principal values of the hyperfine tensor of the a-proton are given in table 1. The isotropic value of the hyperfine coupling of the a-proton is a, = - 14.5 G, and the anisotropic values are given in table 1. McConnell and Strathdee' have calculated the pure dipole-dipole coupling of a CH proton with an electron entirely in a p orbital on the carbon atom. They obtain the values 15.6 G along the CH bond, - 1.7 G along the p orbital and - 13.9 G for the direction perpendicular to both the CH bond and the p orbital. The anisotropic values in table 1 are in agreement with these values and the a- proton coupling constants given in the literature.8

The variations in the spectra indicate that the two a-protons are equivalent, and their hyperfine coupling constant is isotropic with a value of 27.6 G. The /?-proton

(2) coupling constant is given by9

where 0 is the angle between the C,-H, bond and the p orbital direction, projected perpendicular to the C,-C, bond. Bo is a constant and includes the contributions from spin density which arise from conformation-independent mechanisms, in particular spin

A, = Bo+ B , C O S ~ ~

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F. Koksal and F. celik 2309

polarisation, and B, includes the hyperconjugative contributions. In the case of rapid free rotation about the C,-C, bond the average value of A, becomes

a, = B,+iB, . (3) Replacing the very pronounced value^,^.^* B, = (r3.5 G and B, = 50 G in eqn (3), we obtain a, = 28.5 Gs; and this is in good agreement with our experimental result, 27.6 G. Hence we conclude that the /?-protons in the -CHCH2- radical rotate about the C,-C, bond. Since we observed the equivalence of the P-protons down to 130 K, we can state that this rotation of 8-protons exists at these temperatures.

As as a result this study shows that the ethanol radical in choline chloride and choline bIomide4y5 does not occur in carbamylcholine choloride; this is in agreement with the conclusion by Nath et aL5 for choline iodide, choline sulphate, [(CH,),NCH,CH,CI]Cl, [(CH,),NCH,CH,OH]Cl and [(C,H,),NCH,CH,OH]Cl. Furthermore, at moderate microwave power, 1-20 mW, there is no trace of the biradical or (CH,),N+ as in choline ~h lo r ide .~ However, when the microwave power is raised above 50 mW the spectra are less clear, probably because of the unknown species. Therefore, further e.s.r. work on y-irradiated choline derivatives will be carried out at various microwave powers and temperatures in this laboratory.

This work was supported partly by the Research Fund of Ondokuz Mayis University.

References 1 B. M. Tolbert, J . Am. Chem. Soc., 1953, 75, 1867. 2 R. M. Lemmon, M. A. Parsons and D. M. Chin, J. Am. Chem. SOC., 1955, 77,4139. 3 R. 0. Lindblom, R. M. Lemmon and M. Calvin, J . Am. Chem. SOC., 1961, 83, 2484. 4 Y. Tomkiewicz, R. Agarwal and R. M. Lemmon, J. Am. Chem. SOC., 1973, 95, 3144. 5 A. Nath, R. Agarwal and R. M. Lemmon, J. Chem. Phys., 1974, 61, 1542. 6 B. Jensev, Acta Chem. Scand., Part B, 1975, 29, 891. 7 H. M. McConnell and J. Strathdee, Mol. Phys., 1959, 2, 129. 8 D. V. G. L. Narasimha Rao and W. Gordy, J. Chem. Phys., 1961, 35, 362. 9 J. R. Morton, Chem. Rev., 1964, 64, 453.

10 N. M. Atherton, Electron Spin Resonance (John Wiley, New York, 1973).

Paper 711260; Received 14th July, 1987

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