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RADICALS INDUCED BY ARGON ARC PLASMA IN SOME AQUEOUS AND ORGANIC SOLUTIONS: DIRECT OBSERVATION BY ESR AT 77 K

Hirotomo Hase1, Takeshi Saito1 and Kaoru Harada2


1 Research Reactor Institute, Kyoto University, Kumatori-cho, Sennnan-gun, Osaka 590-094, Japan
Fax 0724 51 2472, E-mail hase@HL.rri.kyoto-u.ac.jp
2 Kobe Shoin Women's College, Sinoharaobanoyama-cho, Nada-ku, Kobe-city, 657-0015, Japan


(Received 07 May 2001, Accepted 29 May 2001)

( Abstract ) Argon arc plasma jet was blown onto ice and some solid solutions at low temperature. ESR measurements at 77 K revealed that the plasma generates OH and related radicals in ice and silver atoms in aqueous solution containing Ag+ ion. This is direct evidence for dissociation of water molecule and induction of electron into solutions under high energy condition. CH2NH2 radical was detected in the solid solution of methylamine after exposure to the plasma.

(Key words) Argon arc plasma, OH radical, Organic radical , ESR, Liquid nitrogen temperature
(Running title) Radicals induced by argon arc plasma at low temperature

Introduction

q_q_It is supposed that the primitive Earth has no magnetic field so that high energy particles from the Sun and the outer space can reach to the surface of the Earth. Thus the chemical evolutions induced by high energy particles are expected to occur on the primitive Earth. Some simulation studies of such chemical reactions were carried out by Harada and coworkers by blowing argon arc plasma jet into aqueous solutions containing substrates at ambient temperature [1]. Under exposure to argon arc plasma, phenylglicine and phenylalanine used as substrates were hydroxylated [2], and hydrogen peroxide was formed in sulfuric acid ( pH 3 ) [3]. These reactions can be explained by the action of OH radical generated by the dissociation of water molecule under the high energy condition. Hence, it would be of great significance from the physico-chemical point of view, if one can directly detect OH radical and other radicals which might be generated under the high energy condition.
q_q_In this study, we employed argon arc plasma as a high energy source, and investigated the radical processes induced by the plasma by means of ESR at 77 K. The plasma jet was blown onto pure ice and other solid solutions which were cooled at liquid nitrogen temperature. The exposed samples were then subjected to ESR measurements at 77 K in order to get direct evidence for the formation of OH radical in pure ice and radicals in other solid solutions.


Experimental

q_q_Argon-gas saturated water or other solutions was dropped on a stainless steel dish of 40 mm in diameter and 1 mm in depth, and then soaked in liquid nitrogen so as to form in a thin, disk-like sample of polycrystalline ice or solid solutions on the dish. This sample preparation method was previously used for the Raman scattering measurements of organic solids at low temperatures [4]. Argon arc plasma torch used was a non-transferred type that consisted of a tungsten rod of 1.8 mm in diameter and 100 mm in length as the cathode and a copper nozzle of 1.8 mm in diameter as the anode. The conditions to produce plasma jet were: argon gas flow rate, 2l/min; electric current, 45 A; electric voltage, 10 V. The argon arc plasma jet of 1 mm in flame diameter was blown onto the cold surface of the solutions by rapidly changing the spot position. The bottom of the sample dish of 5 mm in thickness was always kept in liquid nitrogen. As soon as blowing was performed for about 2 cm on the surface within 1 sec, the disk sample was quickly immersed into liquid nitrogen. This procedure was repeated until the whole surface of the disk sample was exposed to the plasma. The disk sample was then cut into flakes of about 2 x 2 mm2 in liquid nitrogen and transferred into ESR Dewar.
q_q_In another experiment, pure ice and other solutions were r`-irradiated at 77 K, and their ESR spectra were measured in order to compare them with those obtained for the plasma experiment. ESR measurements were carried out at 77 K with a Varian X-band spectrometer with following settings: modulation frequency, 10 kHz; modulation amplitude, 0.5 mT; microwave output power, 0.2 mW; time constant, 0.3 sec.

Results and discussion

Pure ice q_q_ESR spectrum of polycrystalline ice obtained after exposure to the plasma is shown in Fig.1(A), while the spectrum obtained after r`-irradiation at 77 K is shown in Fig.1(B). The spectrum for r`-irradiation is exclusively identified as OH radical [5]. Comparison of both spectra reveals that the spectrum for the plasma consists of three components: first, a deformed doublet with a wide hump at low magnetic field; second, a sharp singlet overlapped with the first component at around ge ( g = 2.0023 ); third, an asymmetric singlet appearing at g = 1.9670. We tentatively ascribe the first spectrum to OH radical from the spectral profile.


Fig.1 ESR spectra of pure ice observed at 77 K after exposure to argon arc plasma at low temperature (A), and after r`- irradiation at 77 K (B). In spectrum (A), an asymmetric singlet, which is denoted by asterisk, overlaps the spectrum due to OH radical at around ge. Spectrum (B) is ascribed to OH radical.

q_q_It was previously reported that in r`-irradiated ice, the singlet spectrum appeared at around ge afterOH radical decays between 110 - 140 K. The new radical was identified as HO2 radical [6]. In this study, it is plausible that some of OH radicals are converted to HO2 radical in ice under exposure to the plasma since the temperature of ice at the spot of the plasma jet must be raised. Thus we may ascribe the second component to HO2 radical. Since the r`-irradiated ice at 77 K did not show any spectrum at g = 1.9670; nor was the sample wormed to ca. 140 K after r`-irradiation at 77 K, the third component remains unidentified.
q_q_In order to get further evidence for the production of OH radical, polycrystalline D2O ice was exposed to the plasma, and the ESR spectrum obtained is shown in Fig.2(A). In the spectrum of r`-irradiated D2O ice, as shown in Fig.2(B), the triplet lines with a splitting of ca.0.7 mT well are resolved, being characteristic of OD radical [7]. The spectrum induced by the plasma consists of poorly resolved triplet at ge ,which seems to overlap with another spectrum, and a spectrum at g = 1.9670 similar in profile to that for H20 ice. The appearance of triplet lines due to OD radical in the plasma-induced spectrum confirms the conclusion that the plasma induces dissociation of water molecule to give rise to OH radical. The signal overlapped to OD lines may be due to DO2 radical, but still remains uncertain. The asymmetric singlet appeared at g q" ge seems to be common to both H2O and D2O. We infer that the singlet is due to some radical consisting of oxygen atom and/or its anion.
q_q_It is well established that H radical is only observed for r`-irradiated H2O ice at 4 K, but not at 77 K[6]; H radical decays at temperatures lower than 77 K. This is the reason why we don't detect any spectrum due to H radical in ice under the high energy condition.

Fig.2 ESR spectra of D2O ice observed at 77 K after exposure to argon arc plasma at low temperature (A), and after r`- irradiation at 77 K (B). In spectrum (A), another spectrum seems to overlap the spectrum due to OD radical at around ge. Spectrum (B) is ascribed to OD radical.

Aqueous solution of AgNO3
q_q_A solid aqueous solution containing 0.5 M AgNO3 was exposed to the plasma jet in liquid nitrogen. Another solution was r`-irradiated at 77 K for spectral comparison. The ESR spectra obtained are shown in Fig.3(A) and (B), respectively. The two couples of lines observed for r`-irradiated solution, which are indicated by the stick diagrams under spectrum (B), have already been identified as 107Ag and 109Ag atoms [7]. These atoms are produced by reaction:

q_q_q_q_q_ Ag+q_+ q_e- q_qGq_Ag0 q_q_q_q_q_q_(1),

where e- represents slow electrons generated in r`-irradiated solutions.

Fig.3 ESR spectra of 0.5 M AgNO3 aqueous solutions observed at 77 K after exposure to argon arc plasma at low temperature (A), and after r`- irradiation at 77 K (B).Two isotopes of silver atoms are denoted by 107Ag0 and 109Ag0 in the stick diagrams.

q_q_It is seen in Fig.3(A) that the coupled lines are also distinguished in the spectrum for the plasma, although the whole spectrum skews owing to the application of a high modulation amplitude. The appearance of the ESR lines due to Ag atoms indicates that reaction (1) occurs in the solution under exposure to the plasma. It is plausible that electrons as the counterpart of argon ion in the plasma escape in some extent from the torch nozzle of anode, undertaking reaction (1) on the surface of the solid solution. It is also possible that water molecule is not only dissociated, but also ionized by the argon arc plasma, generating electrons. The second possibility is to be further investigated in order to elucidate chemical reactions induced by high energy conditions.

Methylamine solution
q_q_As a typical, simple organic compound, methlamine was examined. Aqueous methylamine solution (40 % v/v) was frozen into a disk sample and exposed to the plasma in liquid nitrogen. Fig.4 shows ESR spectra obtained at 77 K for the solutions after
Fig.4 ESR spectra of methyl amine ( 40 % v/v) observed at 77 K after exposure to argon arc plasma at low temperature (A), and after r`- irradiation at 77 K (B). Both spectra are ascribed to CH2NH2 radical.
exposure to the plasma (A) and after r`-irradiation at 77 K(B). The spectrum for r`-irradiation is ascribed to the alkylamine radical, CH2NH2 [9]. Note that the spectrum for the plasma is in good accordance with that for r`-irradiation. This leads us to conclude that the same alkyl radical is produced and remains stable in the solution under exposure of the plasma.

Conclusion

q_q_ESR evidence for the formation of OH radical was obtained for pure ice which was exposed to the argon arc plasma at low temperature. Ag atom was generated in aqueous solution containing Ag+ ion after exposure to the plasma, indicating the participation of electron in reactions under high energy conditions.
q_q_The alkylamine radical was observed for aqueous solution of methyamine after exposure of the plasma, and the radical was observed for the r`-irradiated solution at 77 K.

References

[1] See review by Harada, K. Aqueous organic reactions by using high energy conditions, J. Synth. Org. Chem. 48, 522 - 535 (1990). Harada,K Experimental evidence for the participation of hydroxy radicals in the high energy conditions induce by argon arc plasma and contact glow discharge: A review, Akaboshi,M. etal Ed., The role of radiation in the origin and evolution of life, pp 185, Kyoto university press, 2000.
[2] Takasaki, M, and Harada, K. Hydroxylation of aromatic rings in aqueous solution induced by argon arc plasma, Tetrahedron Lett., 25, 885 - 888 (1983).
[3] Takasaki, M, and Harada, K. Oxidation reaction of aliphatic amines and aminoalcohols in aqueous solution induced by argon arc plasma, Tetrahedron 41, 4463 - 4473 (1985).
[4] Hase, H., Ishioka, K., Kobayashi, M. and Kabayashi, M. Raman study of ethanol and ethanolic solutions of LiCl at low temperatures, J. Phys. Chem., 95, 8541 - 8546 (1991).
[5] Hill. M. J. and Wyard, S. J. Low frequency electron spin resonance of irradiated ice and frozen solutions of hydrogen peroxide, J. Phys. B. 1, 289 - 294 (1968).
[6] Kevan, L. Radiation chemistry of frozen polar systems. Haissinsky et al Ed, The chemical and biological actions of radiations, pp 57, Masson & CIE Editeurs, Paris, 1969. [7] Siegel, S., Flournoy, J. M. and Baum, L. H. Irradiation yields of radicals in gamma-irradiated ice at 4.2 and 77 K, J. Chem. Phys. 34, 1782 - 1788 (1961).
[8] Kevan, L., Hase, H. and Kawabata, K. Silver atom solvation and desolvation in ice matrices: ESR studies of radiation-produced silver atoms formed at 4 K, J. Chem. Phys., 66, 3834 - 3835 (1977).
[9] Pukhal'skaya, G. V., Kotov, A. G. and Pshezhetskii, S. Ya., Photoinduced changes for the free radicals produced in r`-irradiated methylamine, Doklady AN SSSR 171, 1380 - 1383 (1966).

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