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Radiolytic reduction of ethylene trithiocarbonate: A pulse radiolysis study
Radiat. Phys. Chem. Vol. 40, No. 6, pp. W-521, 1992 Inr. J. R&at. Appl. Instrum., Part C Printed in Great Britain. All rights reserved
014~5724/92
$5.00 + 0.00
Copyright 0 1992Pergamon Press Ltd
RADIOLYTIC REDUCTION OF ETHYLENE TRITHIOCARBONATE: A PULSE RADIOLYSIS STUDY S. P. RAMNANI’, P. K. BHATTACHARYYA’Tand S. DHANYA’ ‘Isotope Division and *Chemistry Division, Bhabha Atomic Research Centre, Bombay 400 085, India (Received 31 May 1991; accepted 7 April 1992)
Ahatract--The reaction of e;;l with ethylene trithiocarbonate in aqueous medium has been investigated employing pulse radiolysis technique. The bimolecular rate constant for the reaction has been obtained as 3 x 10’Odm3mol-’ s-‘. A transient absorption spectrum, with &,,,,,at 315 nm, has been observed which is assigned to the neutral radical formed by the protonation of radical anion produced in the reaction of e;;l v&h ETTC. A similar absorption spectrum was also observed under the condition where the H atom instead of e; reacts with ETTC. The transient decayed by second-order kinetics with k, = (9.1 f 1.0) x 108dm3mol-’ SK’. The G(-ETTC) has been determined from MCo y-radiolysis as 3.1.
has a strong absorption at 320 nm with a molar absorbance value of 1532 m2 mol-‘. The pulse radiolysis was carried out using a linear accelerator (from Radiation Dynamics Ltd, U.K.) capable of generating 25 ns to 2 ps pulses of 7 MeV electrons. The transient species were monitored by following their optical absorption. Details of the pulse radiolysis set-up (Guha et al., 1987) are described elsewhere. Dosimetry (Fielden et al., 1982) was carried out using 0.05 mol drnm3 KSCN solution, monitoring the absorption of (SCN);’ radical anion formed at 500 mu taking G x 6 = 2152 m2 mol-‘. Taking Go, = 2.6, dose was calculated to be 9-90 Gy pulse-’ depending on the pulse width used. Due to high dose per pulse, the choice of solute concentration was so made that all the e; reacts with the solute. @Co y -source was used for steady state y -radiolysis. The dose rate of 22.4 Gy min-’ was estimated using Fricke dosimetry taking G(Fe3+) = 15.5 (Spinks and Wood, 1976). bonate
INTRODUCI’ION
Some studies on the reactions of e; with S-containing acyclic (Yamauchi et al., 1984) and cyclic (Faraggi, 1975) compounds have already been reported in the literature. In the case of S-heterocyclic compounds the rate constant for the reaction of e, is significantly higher than corresponding 0-heterocyclic compounds. The high rate constant values in S-heterocyclics are explained due to the availability of vacant d-orbitals in the S-atom and its capacity to accept electrons easily. Reaction of e& with thiones has not been investigated so far although the reaction of e; with CS2 (Roebke et al., 1973) is well known. In view of this the reaction of e, with ethylene trithiocarbonate (ETTC) which has two sulfur atoms as a part of five membered ring and one exocyclic > C=S was initiated. In our previous communication (Ramnani et al., 1990) we have reported the formation of radical cation and dication in the oxidation of ETTC by OH radical in the aqueous medium. In the present communication we report the radiolytic reduction of ETTC in the aqueous medium by e, under various conditions.
RESULTS AND DISCUSSION
EXPERIMENTAL
All the aqueous solutions were made in nanopure water (resistivity = 18.3 Mohm cm-‘). Ethylene trithiocarbonate, from Koch-Light Ltd, was recrystallized from an alcohol-water mixture before use. Solutions were deaerated by purging IOLAR grade argon or nitrogen (purity = 99.9%, from Indian Oxygen Ltd, Bombay). Absorption spectra were taken with a Hitachi-330 recording spectrophotometer. Ethylene trithiocarTAuthor to whom all correspondence should be addressed.
Pulse radiolysis of a N,-bubbled aqueous solution of lO-4 mol drnm3 ETTC containing 0.1 mol drne3 t-butanol at pH 7 was carried out using a 50 ns electron pulse. Under these conditions OH is scavenged by t-butanol to produce an unreactive t-butyl radical and only e, is allowed to react with ETIC. The reaction between e, and ETTC was monitored by following the optical absorption of e,;l at 700 nm. The decay of absorption at 700 nm followed pseudo first-order kinetics depending on the concentration of ETTC. From the linear plot of the observed firstorder rate constant against ETTC concentration a bimolecular rate constant for the reaction (k,,,,~) is evaluated as 3 x 10” dm3 mol-’ s-‘.
519
S.
520
P. RAMNANI et al. Table I S No.
I 2 3 4 5
240
280
320
380
400
Wavelength (nm)
Fig. 1.Absorption spectra of the transient species obtained on pulse radiolysis of a N,-bubbled 100 x 10W6moldm’ BTTC solution in water. (A) Observed transient absorption spectrum and (B) trapsient absorption spectrum after correcting for ETTC absorption.
In order to identify the transient species generated due to the reaction of e;, with ETTC, a transient absorption growth was searched in the time scale in which e& was found to decay. A growth signal was obtained in the region of 350-400nm. It was also observed that at wavelengths below 350 nm bleaching of absorption occurred. This is because of the consumption of ETTC which absorbs below 350 nm with A_ = 320 nm. The observed extent of bleaching was found to be less than the expected on the basis of the yield of e;, reported at neutral pH, i.e. G,,, = 2.6 (Buxton et al., 1988) and the radiation dose used per pulse. This discrepancy on the bleaching of absorption may arise due to the absorption of transient generated at the cost of ETTC reacting with e&. Thus, it was possible to construct the transient absorption spectrum as shown in Fig. 1. On pulse radiolysis of a N,-bubbled aqueous solution of ETTC containing 0.1 mol dm-’ t-butanol at pH N 1 and after correcting for the bleaching, a transient absorption spectrum was obtained with & = 315 nm. This is found to be similar to the one obtained at neutral pH. However, at pH m 1 the reacting species is mostly H atom (as e;, is scavenged by H+ to produce H atom). This observation suggested that transient species produced from the reaction of e;, and H atom with ETTC are the same or they have similar absorption characteristics. The rate constant for the reaction of H atom with ETTC was obtained from the growth of the transient species at 350 nm as 1.6 x 1O’Odm3 mol-’ s-l. Transient observed was found to decay on a millisecond time scale. Since the transient absorption has maximum at 315 nm where ETTC also absorbs, its decay kinetics was studied at 350 nm where
Ionic strength (mM)
0.0 60 120 240 3al
k/c(s-‘) 4.8 4.5 3.6 3.8 4.0
x x x x x
IO’ 10s 10’ 10’ 105
absorption from ETTC is negligible. The decay of the transient at neutral pH and at pH 1 followed secondorder kinetics with k/E = 4.8 x lo5 s-l. This again suggests that the transient species generated by the reaction of e; and H atom with ETTC are same. The molar absorbance value of the transient at 350 mn was evaluated as 190 + 30 m* mol-’ considering G, + n) = 3.2 (Buxton et al., 1988) at neutral pH. Using this value the second-order rate constant was estimated to be (9.1 f 1) x 108dm3 mol-’ s-l. In electrochemical (Goodman et al., 1976) reduction of ET’IC in N-N dimethyl formamide at a platinum electrode intermediate formation of radical anion was reported which decomposes into ethylene and trithiocarbonate radical anion (CS;-). However, in our system on pulse radiolysis we have no evidence of such a reaction as the decay of transient following second-order kinetics. Moreover, we did not obtain any evidence for trithiocarbonate radical anion either. Since CO;- has an absorption maximum at 600nm (Behar et al., 1970) it is logical to assume that its sulfur analogue CS;- should be absorbing at a higher wavelength than 600mn. However, we have not observed any absorption growth above 600 nm in the time scale where transient decay was observed. Hence we rule out the possibility of CS;formation on decomposition of transient radical in our system. The second-order rate constant for the decay of the transient was found to be independent of the ionic strength of the solution at pH N 7 which varied from p = 0.0 to p = 0.3 as shown in Table 1. This observation strongly suggested that the transient species absorbing at 320 mn is indeed a neutral radical. Thus, the radical anion which is initially produced in the reaction of e, with ETTC gets protonated very rapidly. The second-order rate constant was also found to be independent of the pH of the solution, ETTC concentration and radiation dose. Thus, it appears that the transient decays by a simple second-order process, i.e. either by dimerization or disproportionation. In the former case the G value for the consumption of ETTC will be equal to the Gceiil+nj and in the latter case it will be half that of G(_+n) produced in the radiolysis of water. The G value for the consumption of ETTC was obtained experimentally from steady state radiolysis as 3.1. DETERMINATION
OF G(-El-K)
A N,-bubbled aqueous solution of ETTC containing 0.1 mol dm-’ t-butanol was radiolyzed by @‘Co
Radiolytic reduction of ethylene trithiocarbonate
y-source for different intervals of time. Consumption of ETTC was monitored spectrophotometrically at 320 mu. The plot of change in the absorption of the solution before and after the radiolysis against the time of irradiation gave a straight line and from the slope of the linear plot G( -ETTC) was estimated as 3.1. This value agrees well with the Gceq+aj = 3.2 produced in the radiolysis of water, thus confhming that the decay of the transient radical srxcies occurs through dimerization. An attempt was made to determine the pK, of the transient radical for which absorption of the transient species at 350 mu was recorded at different pH (from pH 4 to 11). The transient absorption remained nearly constant within this pH range. This indicated that protonated and deprotonated species have similar absorption characteristics or p& of the transient is above 11. However we could not go beyond pH = 11 as ETTC reacts with OH- at pH above 11. Transient species generated from the reaction of e;;l with ETTC may be attributed to the radical species produced by either of the following two reactions:
REFEIWNCES
Bebar D., Crap&i G. and Duchovny I. (1970) Carbonate radical in Flash Photolysis and Pulse Radiolysis of aqueous carbonate system. J. Phys. Chem. 14,2206. Buxton G. V., Greenstock C. L., Helman P. W. and Ross B.A.(1988)Criticalreviewofrateconstantsforreactionsof hydrated electron, hydrogen atoms and hydroxyl radicals in aqueous solutions. J. Phys. Chem. Ref. Data 17, 513. Faraaai M. (1975) In The Fast Processes in Radiation C&%.rtry 6nd Biology (Edited by Adams G. E., Fielden E. M. and Michael B. D.), p. 285. Wiley, New York. Fielden E. M. (1982) In The Study of Fast Processes and Transient Species in Puke Radiolysis (Edited by Baxendle J. H. and Busi F.), p. 59. Reidel, Boston, Mass. Friedman M. (1973) In The Chemistry and Biochemistry of Suljhydryl Group in Amino Aci& and Proteins, p. 16. Pergamon Press, Braunschweig, Germany. Goodman F. J. and Chambers J. Q. (1976) Electrochemical reduction of ethylene trithiocarbonate. Reactions of trithiocarbonyl radical anions. J. Org. Chem. 41, 626. Guba S. N., Moorthv P. N.. Kishore K.. Naik D. B. and Rao K. N. (1987) One electron red&ion of thionine studied by pulse radiolysis. Proc. Indian Acad. Sci. (Chem. Sci.) 99, 26 1. Ramnani S. P., Dhanya S. and Bhattacharyya P. K. (1990) Pulse radiolytic studies on the oxidation of I,3 dithiolane Zthione by OH radicals in aqueous medium. Radiat. Phys. Chem. 36,409.
(
s\
C=S
S’
+ es -
(
S’
r H+
.-
s\c-s
521
1
L(
s\.
C-SH
(1)
S'
s\
CH-;
(2)
H+
In reaction (1) the product is a carbon centered radical which has a - SH group. Usually such species have a p& below 11 (Friedman, 1973). Thus, it appears that reaction (2) is responsible for the transient absorption at 315 mu. However, the possibility of reaction (1) cannot be ruled out as the protonated and deprotonated species might have similar absorption characteristics.
S’
Roebke W., SchoneshofferM. and Strahlenchemie B. (1973) The radiolysis and pulse radiolysis of carbon disulfide in aaueous solution. Z. Naturforsch. 28B. 12. Spiis J. W. T. and Wood k. J. (1976) In An Introduction to Radiation Chemistry, 2nd edn, p. 96. Wiley-
Interscience, New York. Yamauchi K., Nakamura J., Masuda J. and Kondo M. (1984) Reactivity of organic sulfur compounds towards solvated electron. Radiat. Phys. Chem. 23, 619.
014~5724/92
$5.00 + 0.00
Copyright 0 1992Pergamon Press Ltd
RADIOLYTIC REDUCTION OF ETHYLENE TRITHIOCARBONATE: A PULSE RADIOLYSIS STUDY S. P. RAMNANI’, P. K. BHATTACHARYYA’Tand S. DHANYA’ ‘Isotope Division and *Chemistry Division, Bhabha Atomic Research Centre, Bombay 400 085, India (Received 31 May 1991; accepted 7 April 1992)
Ahatract--The reaction of e;;l with ethylene trithiocarbonate in aqueous medium has been investigated employing pulse radiolysis technique. The bimolecular rate constant for the reaction has been obtained as 3 x 10’Odm3mol-’ s-‘. A transient absorption spectrum, with &,,,,,at 315 nm, has been observed which is assigned to the neutral radical formed by the protonation of radical anion produced in the reaction of e;;l v&h ETTC. A similar absorption spectrum was also observed under the condition where the H atom instead of e; reacts with ETTC. The transient decayed by second-order kinetics with k, = (9.1 f 1.0) x 108dm3mol-’ SK’. The G(-ETTC) has been determined from MCo y-radiolysis as 3.1.
has a strong absorption at 320 nm with a molar absorbance value of 1532 m2 mol-‘. The pulse radiolysis was carried out using a linear accelerator (from Radiation Dynamics Ltd, U.K.) capable of generating 25 ns to 2 ps pulses of 7 MeV electrons. The transient species were monitored by following their optical absorption. Details of the pulse radiolysis set-up (Guha et al., 1987) are described elsewhere. Dosimetry (Fielden et al., 1982) was carried out using 0.05 mol drnm3 KSCN solution, monitoring the absorption of (SCN);’ radical anion formed at 500 mu taking G x 6 = 2152 m2 mol-‘. Taking Go, = 2.6, dose was calculated to be 9-90 Gy pulse-’ depending on the pulse width used. Due to high dose per pulse, the choice of solute concentration was so made that all the e; reacts with the solute. @Co y -source was used for steady state y -radiolysis. The dose rate of 22.4 Gy min-’ was estimated using Fricke dosimetry taking G(Fe3+) = 15.5 (Spinks and Wood, 1976). bonate
INTRODUCI’ION
Some studies on the reactions of e; with S-containing acyclic (Yamauchi et al., 1984) and cyclic (Faraggi, 1975) compounds have already been reported in the literature. In the case of S-heterocyclic compounds the rate constant for the reaction of e, is significantly higher than corresponding 0-heterocyclic compounds. The high rate constant values in S-heterocyclics are explained due to the availability of vacant d-orbitals in the S-atom and its capacity to accept electrons easily. Reaction of e& with thiones has not been investigated so far although the reaction of e; with CS2 (Roebke et al., 1973) is well known. In view of this the reaction of e, with ethylene trithiocarbonate (ETTC) which has two sulfur atoms as a part of five membered ring and one exocyclic > C=S was initiated. In our previous communication (Ramnani et al., 1990) we have reported the formation of radical cation and dication in the oxidation of ETTC by OH radical in the aqueous medium. In the present communication we report the radiolytic reduction of ETTC in the aqueous medium by e, under various conditions.
RESULTS AND DISCUSSION
EXPERIMENTAL
All the aqueous solutions were made in nanopure water (resistivity = 18.3 Mohm cm-‘). Ethylene trithiocarbonate, from Koch-Light Ltd, was recrystallized from an alcohol-water mixture before use. Solutions were deaerated by purging IOLAR grade argon or nitrogen (purity = 99.9%, from Indian Oxygen Ltd, Bombay). Absorption spectra were taken with a Hitachi-330 recording spectrophotometer. Ethylene trithiocarTAuthor to whom all correspondence should be addressed.
Pulse radiolysis of a N,-bubbled aqueous solution of lO-4 mol drnm3 ETTC containing 0.1 mol drne3 t-butanol at pH 7 was carried out using a 50 ns electron pulse. Under these conditions OH is scavenged by t-butanol to produce an unreactive t-butyl radical and only e, is allowed to react with ETIC. The reaction between e, and ETTC was monitored by following the optical absorption of e,;l at 700 nm. The decay of absorption at 700 nm followed pseudo first-order kinetics depending on the concentration of ETTC. From the linear plot of the observed firstorder rate constant against ETTC concentration a bimolecular rate constant for the reaction (k,,,,~) is evaluated as 3 x 10” dm3 mol-’ s-‘.
519
S.
520
P. RAMNANI et al. Table I S No.
I 2 3 4 5
240
280
320
380
400
Wavelength (nm)
Fig. 1.Absorption spectra of the transient species obtained on pulse radiolysis of a N,-bubbled 100 x 10W6moldm’ BTTC solution in water. (A) Observed transient absorption spectrum and (B) trapsient absorption spectrum after correcting for ETTC absorption.
In order to identify the transient species generated due to the reaction of e;, with ETTC, a transient absorption growth was searched in the time scale in which e& was found to decay. A growth signal was obtained in the region of 350-400nm. It was also observed that at wavelengths below 350 nm bleaching of absorption occurred. This is because of the consumption of ETTC which absorbs below 350 nm with A_ = 320 nm. The observed extent of bleaching was found to be less than the expected on the basis of the yield of e;, reported at neutral pH, i.e. G,,, = 2.6 (Buxton et al., 1988) and the radiation dose used per pulse. This discrepancy on the bleaching of absorption may arise due to the absorption of transient generated at the cost of ETTC reacting with e&. Thus, it was possible to construct the transient absorption spectrum as shown in Fig. 1. On pulse radiolysis of a N,-bubbled aqueous solution of ETTC containing 0.1 mol dm-’ t-butanol at pH N 1 and after correcting for the bleaching, a transient absorption spectrum was obtained with & = 315 nm. This is found to be similar to the one obtained at neutral pH. However, at pH m 1 the reacting species is mostly H atom (as e;, is scavenged by H+ to produce H atom). This observation suggested that transient species produced from the reaction of e;, and H atom with ETTC are the same or they have similar absorption characteristics. The rate constant for the reaction of H atom with ETTC was obtained from the growth of the transient species at 350 nm as 1.6 x 1O’Odm3 mol-’ s-l. Transient observed was found to decay on a millisecond time scale. Since the transient absorption has maximum at 315 nm where ETTC also absorbs, its decay kinetics was studied at 350 nm where
Ionic strength (mM)
0.0 60 120 240 3al
k/c(s-‘) 4.8 4.5 3.6 3.8 4.0
x x x x x
IO’ 10s 10’ 10’ 105
absorption from ETTC is negligible. The decay of the transient at neutral pH and at pH 1 followed secondorder kinetics with k/E = 4.8 x lo5 s-l. This again suggests that the transient species generated by the reaction of e; and H atom with ETTC are same. The molar absorbance value of the transient at 350 mn was evaluated as 190 + 30 m* mol-’ considering G, + n) = 3.2 (Buxton et al., 1988) at neutral pH. Using this value the second-order rate constant was estimated to be (9.1 f 1) x 108dm3 mol-’ s-l. In electrochemical (Goodman et al., 1976) reduction of ET’IC in N-N dimethyl formamide at a platinum electrode intermediate formation of radical anion was reported which decomposes into ethylene and trithiocarbonate radical anion (CS;-). However, in our system on pulse radiolysis we have no evidence of such a reaction as the decay of transient following second-order kinetics. Moreover, we did not obtain any evidence for trithiocarbonate radical anion either. Since CO;- has an absorption maximum at 600nm (Behar et al., 1970) it is logical to assume that its sulfur analogue CS;- should be absorbing at a higher wavelength than 600mn. However, we have not observed any absorption growth above 600 nm in the time scale where transient decay was observed. Hence we rule out the possibility of CS;formation on decomposition of transient radical in our system. The second-order rate constant for the decay of the transient was found to be independent of the ionic strength of the solution at pH N 7 which varied from p = 0.0 to p = 0.3 as shown in Table 1. This observation strongly suggested that the transient species absorbing at 320 mn is indeed a neutral radical. Thus, the radical anion which is initially produced in the reaction of e, with ETTC gets protonated very rapidly. The second-order rate constant was also found to be independent of the pH of the solution, ETTC concentration and radiation dose. Thus, it appears that the transient decays by a simple second-order process, i.e. either by dimerization or disproportionation. In the former case the G value for the consumption of ETTC will be equal to the Gceiil+nj and in the latter case it will be half that of G(_+n) produced in the radiolysis of water. The G value for the consumption of ETTC was obtained experimentally from steady state radiolysis as 3.1. DETERMINATION
OF G(-El-K)
A N,-bubbled aqueous solution of ETTC containing 0.1 mol dm-’ t-butanol was radiolyzed by @‘Co
Radiolytic reduction of ethylene trithiocarbonate
y-source for different intervals of time. Consumption of ETTC was monitored spectrophotometrically at 320 mu. The plot of change in the absorption of the solution before and after the radiolysis against the time of irradiation gave a straight line and from the slope of the linear plot G( -ETTC) was estimated as 3.1. This value agrees well with the Gceq+aj = 3.2 produced in the radiolysis of water, thus confhming that the decay of the transient radical srxcies occurs through dimerization. An attempt was made to determine the pK, of the transient radical for which absorption of the transient species at 350 mu was recorded at different pH (from pH 4 to 11). The transient absorption remained nearly constant within this pH range. This indicated that protonated and deprotonated species have similar absorption characteristics or p& of the transient is above 11. However we could not go beyond pH = 11 as ETTC reacts with OH- at pH above 11. Transient species generated from the reaction of e;;l with ETTC may be attributed to the radical species produced by either of the following two reactions:
REFEIWNCES
Bebar D., Crap&i G. and Duchovny I. (1970) Carbonate radical in Flash Photolysis and Pulse Radiolysis of aqueous carbonate system. J. Phys. Chem. 14,2206. Buxton G. V., Greenstock C. L., Helman P. W. and Ross B.A.(1988)Criticalreviewofrateconstantsforreactionsof hydrated electron, hydrogen atoms and hydroxyl radicals in aqueous solutions. J. Phys. Chem. Ref. Data 17, 513. Faraaai M. (1975) In The Fast Processes in Radiation C&%.rtry 6nd Biology (Edited by Adams G. E., Fielden E. M. and Michael B. D.), p. 285. Wiley, New York. Fielden E. M. (1982) In The Study of Fast Processes and Transient Species in Puke Radiolysis (Edited by Baxendle J. H. and Busi F.), p. 59. Reidel, Boston, Mass. Friedman M. (1973) In The Chemistry and Biochemistry of Suljhydryl Group in Amino Aci& and Proteins, p. 16. Pergamon Press, Braunschweig, Germany. Goodman F. J. and Chambers J. Q. (1976) Electrochemical reduction of ethylene trithiocarbonate. Reactions of trithiocarbonyl radical anions. J. Org. Chem. 41, 626. Guba S. N., Moorthv P. N.. Kishore K.. Naik D. B. and Rao K. N. (1987) One electron red&ion of thionine studied by pulse radiolysis. Proc. Indian Acad. Sci. (Chem. Sci.) 99, 26 1. Ramnani S. P., Dhanya S. and Bhattacharyya P. K. (1990) Pulse radiolytic studies on the oxidation of I,3 dithiolane Zthione by OH radicals in aqueous medium. Radiat. Phys. Chem. 36,409.
(
s\
C=S
S’
+ es -
(
S’
r H+
.-
s\c-s
521
1
L(
s\.
C-SH
(1)
S'
s\
CH-;
(2)
H+
In reaction (1) the product is a carbon centered radical which has a - SH group. Usually such species have a p& below 11 (Friedman, 1973). Thus, it appears that reaction (2) is responsible for the transient absorption at 315 mu. However, the possibility of reaction (1) cannot be ruled out as the protonated and deprotonated species might have similar absorption characteristics.
S’
Roebke W., SchoneshofferM. and Strahlenchemie B. (1973) The radiolysis and pulse radiolysis of carbon disulfide in aaueous solution. Z. Naturforsch. 28B. 12. Spiis J. W. T. and Wood k. J. (1976) In An Introduction to Radiation Chemistry, 2nd edn, p. 96. Wiley-
Interscience, New York. Yamauchi K., Nakamura J., Masuda J. and Kondo M. (1984) Reactivity of organic sulfur compounds towards solvated electron. Radiat. Phys. Chem. 23, 619.