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J. Phys. Chem. 1980, 84, 3483-3485
(4) L. Guczi and H. J. V. Tyrrell, J. Chem. Soc., 6576 (1965). (5) S. Chapman and T. G. Cowling, "The Mathematical Theory of Non-Uniform Gases", Cambridge University Press, New York, 1960. (6) C. C. Wei and H. T. Davis, J. Chem. Phys., 45, 2533 (1966). (7) I. Prigogine, L. De Brouckere, and I?. Amand, Physica, 16, 577 (1950).
(8) J. L. Lebowitz, Phys. Rev. A , 133, 895 (1964). (9) D. Longree, Ph.D. Thesis, University of Brussels (U.L.B.), Brussels, Belglum, 1979; D. Longree, J. C. Legros, and G. Thomaes, Bull. Acad. R. Be&., in press. (10) D. Longfee, J. C. Legros, and G. Thomaes, Z.Naturforsch. A, 32, 1061 (1977).
Reanalysis of the ESR Spectrum of the Triethylarsine Dimer Radical Cation (Et3As'AsEt3+) Reggle L.
Hudson*
Department oi Chemistry, Eckerd College, St. Petersburg, Florkfa 33733
and Ffrancon Wllllams" Department of Chemistty, Universky of Tennessee, Knoxvlle, Tennessee 379 16 (Received: August 1, 1980)
The dimer radical cation As2Et6+has been generated by y irradiation of triethylamine,and its ESR spectrum interpreted as the full 16-line75Aspattern expected when higher-order effects are important. The ESR parameters (All = 546 G, A, = 413 G, gl = 2.004, g, = 2.027) result in spin densities which establish that the unpaired electron is largely localized Letween the two arsenic atoms in As2Etg+.
Introduction Some time ago, the anisotropic ESR spectra of the dimer radical cations P2(OMe1)6+and A S ~ ( E ~(Me ) ~ += methyl and Et = ethyl) were presented and analyzed by first-order ESR theory to give the hyperfine parameters of their 31P and 75Asnuclei, respe~ctively.'-~However, a later study4 demonstrated the necessity and benefits of carrying out a higher-order analysis of the P2(OM&+ spectrum and suggested that a similar analysis for the &,(Et),+ spectrum was needed. It was noted in particular that large second-order ~plittings,~ ithe largest on the order of 200 G, should give rise to marked deviations from a first-order seven-line hyperfine splitting pattern for A S ~ ( E ~ )L9' ~ince +. arsenic dimer radical cations remain virtually unknown, we report here a higher-orderanalysis of the ESR spectrum of A S ~ ( E ~along ) ~ + with new experimental results. This information gives new rnagnetic resonance parameters for this radical and shows that the previous ESR parameters for A S ~ ( E ~are ) ~ in + considerable error. Experimental Section Triethylarsine was purchased from Strem Chemicals Inc. (Newburyport, MA) and used without purification. Samples were prepared in Suprasil ESR tubes according to standard high-vacuum techniques. Samples which were cooled quickly in liquid nitrogen gave glmses, while careful warming of such glasses gave polycrystalline solids which were subsequently cooled to 77 K. The equipment used for y irradiation, photobleaching, and ESR measurements has been described previouslya6 Results Figure 1 shows the ESR spectrum of glassy AsEt3 at 85 K after y irradiation of the sample at 77 IC. While for the most part this spectrum strongly resembles that shown in it also Figure 3 of a previous study by Lyons and Sy~nons,~ reveals the presence of two additional wing features which are now identified as the outermost lineo of the parallel components for A S ~ ( E ~ )Stick ~ + . diagrams in our Figure 0022-36541aoi2oa4-34a3~01.OOIO
TABLE I : ESR Spectroscopic Transitions for As,(Et)l
-
1 line
(3, 3) (3, 2) (2, 2) (3,1)
(1,1) (3,O) (290) (1,O)
(090)
(39-1) (%--I)
(1,-1)
(3, - 2) (2, - 2) (3, -3)
exptl position: G 1876 2100 23 20 2403 2629 (-2756C d d d d 3443 3588 3743 3965 4353
calcd position,b G
difference, G
1876 2100 2319 2386 261 5 2764 2746 2984 31 36 3210 3193 3434 3585 3738 3963 4353
0 0 1 17 14
9 3 5 2 0
a Taken from the ESR spectrum of a polycrystalline sample (Figure 2). The uncertainty in the line position is t 4 G. Calculated line position using A 1 1 = 559 G, .A1 = 409 G, gii = 1.994, and g1= 2.029. The microwave frequency was 9116.5 MHz. Overlap too great t o resolve both lines. d Lines masked by other signals in the center of the spectrurn.
-
M show how the spectirum may be analyzed in terms of two overlapping 16-line patterns, one for parallel and the other for perpendicular hyperfine features. The positions of the lines in the stick diagrams were given by a matrix diagonalization calculations using All = 546 G, A, = 413 G, gil = 2.004, and g, = 2.027. While 7-line patterns would be expected for As2(Et)B+from first-order ESR theory, the 16-line patterns of Figure 1 are exactly what is predicted for hyperfine interaction involving two 75As( I = 3/2) nuclei with higher-order effects being important. These patterns are thus assigned to h2(Et)6+. Our analysis is corroborated by the ESR spectrum of the dimer radical cation in a sample of y-irradiated poly@ 1980 American Chemical Society
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The Journal of Physical Chemistry, Vol. 84, No. 25, 1980
(I/)
(1)
Flgure 1. First-derivative ESR spectrum of y-irradiated (dose, 2 Mrd) glassy As(Et), at 85 K. The two extremely sharp lines flanking the center of the spectrum and -500 G apart are due to hydrogen atoms produced from water adsorbed on the walls of the sample tube.
(1) Flgure 2. First-derivative ESR spectrum of y-irradiated (dose, 2 Mrd) polycrystalline As(Et)a at 85 K.
crystalline As(Et),, as shown in Figure 2. This sample displayed a very strong preferred orientation effect7which was used to simplify the complex pattern of overlapping parallel and perpendicular features seen in the glassy samples. The spectrum shown was taken with the sample oriented to give maximum intensity for the perpendicular features. The clarity of the spectrum further establishes the analysis in terms of two I = 3/2 nuclei with higher-order effects being strikingly evident. As demonstrated by the results given in Table I, excellent agreement was obtained between the calculated and experimental line positions. Both figures show that the ASz(Et)6+lines are broadest in the wings of the spectrum and narrower toward the center. We have not explored this observation of an MI line-width dependence but simply note that it accounts for the variation in line height among the components of each 16-line pattern.
Discussion The 75Ashyperfine couplings obtained from our analysis lead to spin densities of pk = 0.088 and pg = 0.390 for each of the arsenic atoms in A S ~ ( E ~ )and ~ + thus , a total spin density on arsenic of 0.956.8a Therefore, the unpaired electron in A S ~ ( E ~occupies ) ~ + a u* molecular orbital and
Hudson and Williams
can be considered part of a three-electron bond between the arsenic atoms, similar to the case of the related radical P~(oMe)6+.~ Also, from Coulson’s equations: the ratio P ~ , , / Pwas ~ ~ used to calculate LASASC= 107O, which can be compared with the corresponding bond angle of 1 1 2 O in P,(OMe)6+.10 A few comments on the previous treatment of the ESR spectrum of A S ~ ( E ~are ) ~needed. + The shortcomings of that analysis3resulted from the assumption of first-order, seven-line parallel and perpendicular hyperfine patterns for A S ~ ( E ~ ) The ~ + . features we have referred to in our Figure 1 are present for the most part in the original spectrum but in some cases were left unassigned, in some cases apparently unobserved, and in other cases misassigned. The lowest-field perpendicular line we observe is easily seen in the original spectrum but was left unassigned; the highest- and lowest-field parallel lines of Aszare clear in Figure 1but were “not resolved” in the original work; three of the four lines tentatively assigned to an AsR4 species are now correctly assigned as lines 5 , 12, and 14 (measured from low field) of the perpendicular pattern for A S ~ ( E ~ )In ~ +the . second-order notation these lines would be designated (2,+1), (2,-1), and (3,-2), res p e ~ t i v e l y . The ~ ~ ~ fourth line assigned by Lyons and Symons to AsR4 is seen in our Figure 1 as a large line immediately to the left of the low-field hydrogen atom line. We assign this feature to the lowest-field parallel line of an 75Asquartet due to an arsinyl radical, with AV E 220 G.ll Finally we note that use of Lyons and Symons original g and A values in a matrix diagonalization calculation gives a calculated spectrum which is quite different from our Figure 1 (e.g., total spectral width in error by several hundred gauss) and that the scale in their figure is incorrect.
Conclusions While the original suggestion of the dimer cation in the glasses by Lyons and ESR spectrum of y-irradiated AS(E~)~ Symons was correct? the present study shows that their analysis of the spectrum was in error, having failed to consider higher-order effects. The coupling constants and g values presented here are thought to be far more accurate than those previously reported. This study illustrates very well the point made by Fessenden and Schuler12that, with large hyperfine coupling constants and a well-resolved spectrum such that the line width is less than =A2/2H, it is imperative that higher-order effects be considered in the spectral analysis. Acknowledgment, This work was carried out at the University of Tennessee and was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, U. S. Department of Energy (Document No. ORO-2968-125). References and Notes (1) A. R. Lyons, G. W. Neilson, and M. C. R. Symons, J . Chem. Soc., Chem. Commufl., 507 (1972). (2) M. C. R. Symons, Mol. Phys., 24, 885 (1972). (3) A. R. Lyons and M. C. R. Symons, J . Cbem. Soc., Faraday Trans. 2, 68, 1589 (1972). (4) T. Gillbro, C. M. L. Kerr, and F. Wllams, Mol. phys., 28, 1225 (1974). (5) R. W. Fessenden, J . Chem. Phys., 37, 747 (1962). (6) A. Hasegawa, M. Shlotani, and F. Wllllams, Faraday D&cuss. Chem. Soc., 63, 157 (1977). (7) (a) P. H. Kasai, W. Weltner, Jr., and E. B. Whipple, J . Chem. phys., 42, 1120 (1965); (b) C. M. L. Kerr and F. Williams, J. Am. Chem. Soc., 94, 5212 (1972); (c) R. L. Hudson and F. Williams, ibM., 9% 7714 (1977); (d) R. I. McNell, F. Wllllams, and M. B. Ylm, Chem. Phys. Lett., 61, 293 (1979). (8) (a) J. R. Morton and K. F. Reston, J . Magfl. Reson., 30, 577 (1978); (b) J. R. Morton, J. R. Rowlands, and D. H. Whiffen, Nat. phys. Lab. (U.K.), Rep., No. BPR 13 (1962). (9) C. A. Coulson, ”Volume CommBmoratlf Victor Henrl: Contrlbutlon 1I’Etude de la Structure MolBculalre”, Desoer, LIBge, 1948, p 15.
J. Phys. Chem. 1980, 84, 3485-3486 (10) The spin densities were calculated by uslng the atomic parameters of ref 8a. If the older parameters of ref 8b are employed, the total spin density on arsenic in As#@+ increases to 1.26. I t should be noted, however, that the choice of atomic parameters affects the derived bond anglles by less than 1'.
3485
(11) (a) J. R. Preeir, F. D. Tsay, and H. 8. Gray, J. Am. Chem. Soc., 94, 1875 (1972); (b) A. R. Lyons and M. C. R. Symons, /bid., 95, 3483 (1973). (12) R. W. Fessentden and R. H. Schuler, A&. Radlat. Chem., 2, 1 (1970).
COMMUNICATIONS TO THE EDITOR Quenching of Methylene Blue (S,) by Fe(1II)
Sir: We recently reported evidence for reductive quenching of singlet excited methylene blue, MB+(SI),by Fe(II).l In tho course of an investigation of the mechanism of quenching of 3ME!H2+and 3MB+by oxidative quenchers, we have found that quenching of MB+(S1) by Fe"'(H20),3+in 0.01 M aqueous nitric acid, 1.5 M in KNOB, occurs with the formation of the half-oxidized product, MB2+,. The efficiency of net electron transfer per quenching event has been estimated to be less than 1% . Cqtalysis of intersystem crossing to the triplet manifold is not significant. The Q-switched ruby laser (1.0 J per flash) and monitoring system are ddescribed Absorbance by 3MBH2+and MB2+vwere monitored at 7102and 5203nm, respectively. Fluorescence was measured with a PerkinElmer MPF 44A spectrofluorimeter with excitation a t 635 nm and emission monitored at 710 nm. Methylene blue chloride trihydrate was Fluka puriss. Ferric nitrate, potassium nitrate, and nitric acid were reagent grade. Laboratory distilled water was passed through a Millipore deionizer and filter. Deaeration was by purging with deoxygenated nitrogen. Figure l a shows adherence of the data to the form of the Stern-Volmer equation with a slope of 6.6 M-l. Together with the intrinsic lifetime of MEP(Sl),365 i 21 ps: this corresponds to kp,SL= 1.7 X 1O1O M-l s-l, a value about 5 times the specific rate of encounter of large molecules of all charge types in water at 23 "C with p 1 1 M, 3.2 X loBM-l s- la5Accordingly, static quenching is the principal process. The data do not discriminate between rapid Forster transfer of excitation energy among molecules of MB+ to statistical M13+-Fe1"(H20)63+pairs and groundstate association.6 If clrlly the latter is involved, the slope of Figure 1A corresponds to K, = 6.6 M-l. Flash photolysis of I O pM MB+ gave the following: (1) The pseudo-first-order rate of decay of absorbance by 3MBH2+at 710 nm increased linearly with increasing [Fe111(Hz0)63+], k q ~=l 1.4 X lo6 M-' s-' . (2) The absorbance intensity by bMBH2+extrapolated to the time of the laser flash decreased with increasing [I~eeI"(H20)63+]. (3) Absorbance by MB2+.a t 520 nm was almost fully developed by the end of the flash and increased very little during decay of 3MBH2+. (4) The intensity of prompt absorbance by MB2+-increased less than linearly with increase in [Fe1n(H20),3+]from 0.01 to 0.12 M. As shown in Figure lB, 1/[MB2+.]( c ~ ~ * +=' 58000 M-l cm-l)" varied linearly with 1/ [Fe111(]H20)63+], consistent with formal association of ground-slate dye with qulencher and corresponding to 1/[MB2+.] = (l/[MB+]$l')(l + l/K,[Fe"'(H20)63+]0),where [XIorepresents the initial concentration of X and F,' is the apparent efficiency of electron transfer (see below). The resulting values of K, and [MB+]$,' are 0022-3654/80/2084-3405$0 1.OOf 0
Figure 1. (A) Stern-Volmer plot of data for quenching of fluorescence of MB+(S,) by FeyH20):+ in 0.01 M aqueous nitric acld, 1.5 M in KNO,. (8)Double reciprocal plot of dependence of yields of ME2+-on [Fe(III)] under same conditions as for A.
12 M-l and 6.9 pM,respectively, and Flf is 0.69. The intrinsic lifetime of MB+(S1),0.365 ns: is very short compared to the flash length, -40 ns at half-height for a 1.0-J flash. Thus, many excitation-decay cycles will occur during a single flash unless the long-lived (intrinsic 7 = 4.5 ps2) and slowly quenched triplet is generated. Since kq,T1is less than lo9 times the specific rate of encounter and prompt yields of 3MBH2+vary inversely with [FelI1(HzO)63+], efficient catalysis of intersystem crossing from S to T is ruled out. The lifetime of MB+(S1)presumably decreases with increasing [Fen1(H20),3+]so that the number of excitation-decay cycles per flash and the value of Fl' increase. This increase in F,' may account for the value of K, from flash data (12 M-l) being greater than ik, value from fluorescence quenching (6.6 M-l). The number of excitation-decay cycles during a flash is >lo2. Since F,' = Fl(Con(l - F,)") != Fl(l + n),where Fl is the average efficiency of net electron transfer per decay and n is the number of excitation-decay cycles per flash, Fl is
