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laser, typically 10"1 234 cm'1. The combination of diode lasers with inert rare gas waveguides then represents 4 orders of magnitude improvement in resolution over existing surface sensitive spectroscopic techniques.It is of interest to also note that in the lowest order guided modes very high field densities are attained in the slab and at the metal/dielectric interface in the case of TM modes. The total injected power in these experiments was typically between 5 and 20 µ\ . As much as 70% of this field is contained in the TM0 mode where the thickness of the slab is -3 µ . For a 50-µ beam waist, this fluence then corresponds to 6 W/cm2. The effect of this high-field density in our present experiments is that of line broadening. These high field densities, coupled with the narrow radiation bandwidth, represent a formidable spectral brightness that could be used in nonlinear spectroscopic applications.11These demonstrations were performed in a first-generation design of a waveguiding assembly. There are obvious improvements that can be implemented for enhancement of sensitivity and mode selectivity. Prism couplers19 or a grating etched in the copper substrate could be used for the injection of radiation at better defined angles and for improved efficiency in output coupling. In order to sample a larger number of molecules either in the slab or at the slab/metal interface, line focusing as opposed to the present point focusing geometry could be used. These improvements, and the incorporation of the assembly in a ultrahigh-vacuum chamber, are presently being implemented in order to apply the technique to submonolayer metal surface adsorbates.Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. The award from the Camille and Henry Dreyfus Foundation for "Newly Appointed Faculty in Chemistry" is also gratefully acknowledged.The diode lasers were obtained through a grant from the NSF Instrumentation program.

The reduction potential for the *C02" radical anion has been determined by equilibration of formate with sulfhydryl radicals of jS-mercaptoethanol, penicillamine, and lipoamide in aqueous solutions at pH 3-6. The reaction *C02" + e~+ H+ = HCQ2" yields the value E°9 = 1.49 V with an uncertainty of ±0.06 V. On the basis of this value and the known free energies of C02(aq) and HC02~(aq), E°1 9 for C02 + e~= -C02~w as found to be -1.85 V.

Standard free energy changes and redox potentials have been calculated from thermodynamic data for the following reactions: RS' + e" = RS' (1); RSSR" + e" = 2RS" (2); RSSR + e" = RSSR" (3). The values of £,°a nd E°for the H2S system (i.e., R = H) were 1.15 and 0.90 V; respectively, while for mercaptans (i.e., R = an alkyl group, such as C3H7 or HOCH2CH2) they were 0.84 and 0.65 V. These potentials are comparable to but smaller (or more negative) than those for the corresponding X' and X2" halide systems. More important is the strong negative value of E°( -1.6 V), which is significantly different from £°for the corresponding reductions of the halogens: X2 + e' = X2'" (cf. 0.11 V for I2 which is the smallest). Unlike the halide X2*' species, RSSR" is therefore potentially a strong reductant. Also it will only be formed from RSSR at very negative potentials. This fact and other predictions from the present data are shown to be in accord with recent electrochemical work and pulse radiolysis studies of the oxidation-reduction reactions between halide systems and the sulfhydryl-containing amino acid cysteine.

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The amino radicals (.AH) formed by the h°Co radiolysis of N,O-saturated 0.05 M solutions of ethylene diamine tetraacetate (EDTA) at pH 7 and 11.2 and of glycine at pH 11.2 brought about an efficient two-electron reduction of lumiflavin (FI). The spectra of the products were identical to those formed by photolysis of the same solutions and by reduction of the lumiflavin with .CO,-radicals. The products were reoxidised to flavin by oxygen. The quantum yield for flavin disappearance was 0.52 2 0.07 and 0.17 ? 0.01 in the presence of EDTA at pH 7 and 1 1.2 and 0.065 + 0.008 and 0.17 ? 0.01 for glycine at the same pHs, respectively.The overall two-electron reduction can be explained by the mechanism:The rate constants of reaction [4] were found by pulse radiolysis to be 1.8 ? 0.3 x 10' and 1.5 ? 0.3 X 10" M-' s-' for the radicals of glycine and EDTA at pH 7 and 3.6 * 0.3 x 1 0 H M S -I for glycine radicals at pH 11.2. The spectrum of .FIH formed by glycine radicals at pH 7 is similar to that produced by .CO,-, but there was some perturbation, which is apparently due to interaction with the amine. The radicals formed from the secondary amines piperazine and diethylamine at pH I 1.8 also effected reversible two-electron reduction. However, the radicals from glycine anhydride and the primary amine ethylamine yielded significant amounts of non-oxidisable products. The reaction mechanisms are discussed and effects of pH are considered. PARMINDER S . SURDHAR, DOUGLAS E. BADER et DAVID A. ARMSTRONG. Can. J. Chem. 63, 1357 (1985). Les radicaux arninks (.AH) issus de la radiolyse au "Co de solutions saturkes en N,O, contenant 0,05 M de tetraacktate d'kthylene diamine (TAED) at maintenues a des pH de 7 ou 11,2 ou contenant de la glycine a un pH 11,2, provoquent une rkduction deux Clectrons efficace de la lumiflavine (FI). Les spectres des produits sont identiques a ceux des produits obtenus par la photolyse des mCmes solutions et par la reduction de la lumiflavine par les radicaux .CO,-. L'oxygkne provoque une rkoxydation des produits en flavine. A des pH de 7 et 11,2 et en presence du TAED, les rendements quantiques pour la disparition de la flavine sont respectivement kgaux a 0,52 2 0,07 et de 0,17 ? 0,01 alors qu'en presence de glycine et aux mCmes pH ces rendements sont Cgaux a 0,065 2 0,008 et 0.17 ? 0,Ol.On peut ilustrer la reduction globale a deux Clectrons, par le mecanisme suivant:Faisant appel a la radiolyse pulsation, on a pu Ctablir que les constantes de vitesse de la reaction [4] sont Cgales a 1,8 2 0,3

Can. J. Chem. 62, 580 (1984). Spectral changes and rates of reaction of flavins and several radical species have been investigated at pH 7, 9, and 10 in the presence and absence of P-mercaptoethanol. The radicals .C02-, e,,-, and LS;-reacted with FAD at pH 10 to give a spectrum of FAD .FI-with rate constants of 7 ? 1 x 10' and 4 ? 1 x IOX M ' s ' for . C O i and LS;-respectively. At pH 7 only .FIH was observed and at pH 9 a mixture of .FIH and .FI-.Interactions between flavin radicals and sulphydryl at M concentration did not cause perturbations in the uv-visible spectra until either the radical and/or the sulphydryl were ionized. With FAD at pH 9 or 10 and LFI at pH 10 the 370 nm peak of .FI-was enhanced by about 15% and a second larger growth occurred near 450 nm in the presence of to M sulphydryl. We attribute this to the formation of labile intermediate RSHFl .-, which must also be involved in the reduction of F1 by RSSR-at pH 9 or 10. The second order rate constant k13 for reaction of RSSR-with FAD at pH 9 and 10 was found to be 4.2 2 0.5 x lo%-' s-' and 2.0 +-0.4 X 10' M -' s-' respectively. The rate constant for the reaction between RSSR-and LFI at pH 10 was slightly faster, 7 * 1 X

, 1874 (1984).The one electron oxidant .Brz-reacts with 5-methyl-1-thia-5-azacyclooctane (4) in aqueous solution at high pH with an overall rate constant of -2 x 10% s-'. The radical intermediate produced has a broad maximum at 500 nm with E = 2400 M-' cm-' and at pH 10 decays with a first order rate constant of 2.3 k 0.3 x 10' s-I, first half-life of 30 k 5 ps. Its Mechanisms of oxidation and reduction of sulphurcontaining molecules are of importance in many areas of chemistry (1, 2) and biochemistry (3). Previous studies have indicated that intramolecular nucleophilic assistance can facilitate both one (4, 5) and two electron oxidations ( I , 6 , 7) of compounds, such as disulphides and thioethers. In recent studies of mesocyclic dithia compounds in nonaqueous solvents oxidation has been shown to produce cation radicals ( I ) and dications (2), which are stabilized by transannular interactions between the sulphur atoms (8, 9). A relatively long-lived N-S bonded cation radical (3) and the corresponding dication have also been prepared in nonaqueous media from the mesocyclic aminothioether, 5-methyl-I-thia-5-azacyclooctane (4) ( 10). A N-S interaction similar to that shown in 3 has been considered as a possible step in the mechanism of the one electron oxidation of methionine (4). However, although species similar ' Author to whom correspondence may be addressed. . This species has a strong peak (E = 9900 * 600 M -' cm-') at 360 nm (18). In the absence of a reactant it undergoes second order decay with 2k = 3.6 x lo9 M-I s-' (19). However, in the presence of 3 to 8 x 10-% 4 at pH 10, the 360-nm peak decayed in a pseudo first order process, with a lifetime which varied inversely with the concentration of 4 . An example of that decay is seen in Figs. l a and b. In contrast, the absorbance at and above 500 nm, where .Br,-absorbs only weakly, exhibited growth over the initiaI -10 FS and then decay (Fig. Ic) For personal use only.