GUNTERCASPARIAND ALRRECHTGRANZOW
836
The Flash Photolysis of Mercaptans in Aqueous Solution
by Giinter Gaspari and Albrecht Granzow Department of Chemistry, Boston University, Boston, Massachusetts
02216
(Received J u n e 17, 1969)
The flash photolysis of 2-mercaptoethanol, benzenethiol, and cysteine hydrochloride in deoxygenated aqueous solutions of various pH values was studied. Transient spectra (A, -420 nm) were observed which were identified as arising from the RSSR radical anion. The transient decay was first order with approximately equal rate constants for cysteine and 2-mercaptoethanol; the benzenethiol decayed about ten times faster, It is proposed that the formation of RSSR from flash photolysis is via the fast reaction of the primary RS radical with an RS- anion. The validity of the mechanism is supported by the effect of added allyl alcohol on the decay rates of the transient. Introduction
age oscilloscope. A Vycor filter restricted the flash to
The primary reaction in the photolysis of mercaptans in the and liquid4 phase is the formation of RS. radicals to which has been attributed the strong absorp-420 nm) observed when alkylmercaptans are tion (A, photolyzed at 2537 .& in a rigid EPA matrix.6 Similar absorptions have resulted from the y radiolysis of mercaptans in rigid matrices at 77’K.‘j However, pulse radiolytic studies of cysteine7,*and 2-mercaptoethanole have indicated that the absorbing species is not the RS radical but rather the RSSR radical anion. There is support for this proposal from the analysis of the esr spectra of such trapped radicals.Io I n the radiolysis cases, the primary process is the generation of transients fyom the attack of solvated electrons or 013 radicals on the mercaptan. The question must be raised concerning the nature of the transient produced in the photolysis and toward this end the flash photolysis study was directed.
h >230 nm and a Pyrex filter in front of the analyzing
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Experimental Section 2-MercaptoethanoI (K & E( Laboratories), benzenethiol (Matheson Coleman and Bell), and allyl alcohol (Fisher) were purified by distillation. Cysteine hydrochloride (Fisher, research grade) was used without further purification. Solutions of the mercaptans were prepared with deionized water that had been deoxygenated by a stream of purified nitrogen for at least 1 hr. The mercaptan was then added with a syringe. The pH value was adjusted using perchloric acid and sodium hydroxide and measured with a Leeds and Northrup pH meter. A fresh solution was used for each flash delivered. The flash apparatus (Xenon Corp., Medford, Mass.) delivered a flash with an energy of up to 250 J from xenon-filled flash lamps. The flash exhibited a rise time of 5 psec, a half-peak duration of 20 psec, and a total duration of 50 psec. The decay of the transients was monitored using a Hilger-Engis 0.6-m combination spectrograph-monochromator, an RCA 1P28 photomultiplier and dynode chain, and a Tektronix 564 storT h e Journal of Physical Chemistry
lamp was used to prevent unwanted photolysis. The quartz irradiation vessel was 22 cm long with a 2-cm diameter and had optically ff at quartz windows at both ends. The absorption spectra of the mercaptan solutions were determined using a Cary 14 recording spectrophotometer. Results and Discussion In the spectral region X >230 nm, mercaptans show absorption bands which are attributed to n 4 CT* or n --t. n* transitions of the free electrons at the sulfur atom. The ultraviolet absorption spectra of the mercaptan solutions show that, whereas these bands are quite strong in basic solution, no absorption occurs in neutral or acid medium except for benzenethiol. The spectrum of 2-mercaptoethanol in aqueous solution at various pH values is shown in Figure 1. The spectrum of cysteine is reported to show a similar pH dependence-l‘ In both cases the extinction coegcient at the band maximum is directly proportional to Ihe anion concentration as is seen by a comparison with the titration curves of these compounds. l 1 For benzenethiol this anion (1) 31.JIeissner and H. W. Thompson, Trans. Faraday Soc., 34, 1238 (1938). (2) N. P. Skerret arid M. W. Thompson, ibid., 37, 81 (1941). (3) T. Inaba and B. deB. Darwent, J . Phus. Chem., 64, 1431 (1960). (4) W. E. FIaines, G. L. Cook, and J. S. Ball, J. Amer. Chem. Soc., 78, 5213 (1956). (5) K. J. Rosengren, Acta Chem. Scand., 16, 1418 (1962). (6) J. Wendenburg, W. Moeckel, A. Granzow, and A. Henglein, 2. Naturforsch., B, 21,633 (1966). (7) G. E, Adams, G. 8 . McNaughton, and B. D. Michael, “The Chemistry of Ionization and Excitation,” Taylor and Francis Ltd., London, 1967, p 281. (8) G. E. Adams, “Symposium on Radiation Research,” NorthHolland Publishing Co., Amsterdam, 1967, p 195. (9) G. Meissner, A. Granzow, and A. Henglein, unpublished results. (10) F. K. Truby, J. Chem. Phys., 40,2768 (1964). (11) It. E. Benesch and R. Benesch, J. Amer. Chsm. Soc., 77, 5877 (1955).
FLASH PHOTOLYSIS OF MERCAPTANS IN AQUEOUSSOLUTION
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d 0
x Cnml
Cnml Figure 1. The uv absorption spectra of aqueous solutions of 2-mercaptoethanol: [mercaptan] = 2.0 X 10-4 M ; cell path length = 1 cm; pH of solutions: 1, 13.0; 2, 10.5; 3, 9.7; 4, 9.0; 5, 6.0.
Figure 2. The uv absorption spectra of aqueous solutions of benzenethiol: [mercaptan] = 1.25 X 10-4 M ; cell path length = 1 cm; pH of solutions: 1, 5.9; 2, 6.6; 3, 8.9; 4, 12.5.
Table I: Rate Constants for the Decay of R$SR Radical
band is shifted to about 260 nm, but at low pH values another absorption at 235 nm is observed which is due to the undissociated form (Figure 23. In all three systems strongly absorbing transients are observed after the flash. The absorption maxima are a t 420 nm for 2-mercaptoethanol and cysteine and a t 470 nm for benzenethiol. The spectra of the transients in Figure 3 are measured a t those pH values where the highest intensity of the transient absorption is obtained. If the solutions are flash photolyzed without the Vycor filter so that X
