J. Phys. Chem. 1981, 85,2665-2671
where [OH] is an average value over the appropriate time interval. OH levels are highly variable, depending upon the local concentrations of 03,H20, CO, CHI, and NO,, and also the solar UV flux. Recent measurementsn suggest that the annually and diurnally averaged OH concentration in the tropical marine boundary layer is 2 X loe molecules ~m-~ Using . this value as a rough estimate for the average [OH] encountered by reduced sulfur compounds in the lower troposphere, we obtain the following lifetimes from our results for ki at 298 K: T H , ~= 27 h, TCH&H = 4.1 h, TC&CJCH = 33 h, and T C H ~ ~ C=H42 min. Midday OH levels are a tactor of -3 larger than the diurnally averaged values, so the residence times of the short-lived species CH3SH and CH3SSCH3will be a factor of 3 shorter at midday than the diurnally averaged estimates suggest. One current problem in atmospheric sulfur chemistry centers around determining the origin of the relatively high ( 100 pptv), uniformly distributed SOz levels recently measured in the upper t r o p o ~ p h e r e . ~ Because ~ of their short lifetimes, none of the compounds studied in this investigation can be transported from their ground (or ocean) sources to the upper troposphere. Therefore, only if there exist airborne sources can hydrogen-containing reduced sulfur compounds be precursors for free tropospheric SO2 (SOz itself is removed by OH and heterogeneous processes with a lifetime of -10 days; hence, the uniformly distributed SO2levels imply the existence of a nearly uniformly distributed precursor). N
(22) D. D. Davis, W. L. Chameides, D. Philen, W. Heaps, A. R. Ravishankara, and M. Rodgers, J. Geophys. Res., submitted. (23) P. J. Maroulis, A. L. Torres, A. B. Goldberg, and A. R. Bandy, J. Geophys. Res., 85, 7345 (1980).
2865
Whereas H2S is generally thought to enter the atmosphere primarily from localized sources such as swamps and marshes, recent measurements indicate that the ocean is the primary source of CH3SCH3with the diurnally averaged concentration over the ocean being 58 pptva21 Model calculations26 using the combined results of Atkinson et a1.l2 and Kurylo13 for k 3 ( T ) ,and assuming unit conversion of CH3SCH3to SOz, demonstrate that oxidation of CH3SCH3can produce -100 pptv SOz in the marine boundary layer-ca. twice the measured c o n c e n t r a t i ~ n . ~ ~ Our values for k3(T) would reduce the calculated SO2 source by a factor of -2.3. Neither GH,SH nor CH3SSCH3has been observed in the atmosphere except in the vicinity of large anthropogenic or biogenic sources. However, the very rapid rates at which these species react with OH suggest that undectectably low steady-state concentrations could be present even though reasonably large-scale sources exist. This is particularly true of CH3SSCH3. A CH3SSCH3 source equal to that for CH3SCH3 would imply an atmospheric CH3SSCH3concentration of 1pptv, a level which may not be measurable with presently available techniques. Acknowledgment. We thank D. H. Semmes and R. C. Shah for assisting with some of the experiments and Professor W. L. Chameides for helpful discussions concerning the atmospheric implications of our results. This work was supported by the National Science Foundation through grant no. ATM-80-19040.
-
(24) P. J. Maroulis and A. R. Bandy, Science, 196, 647 (1977). (25) J. A. Logan, M. B. McElroy, S. C. Wofsy, and M. J. Prather, Nature (London), 281, 185 (1979).
Structure of a Ternary Complex of Copper(I1)-Poly(styrenesu1fonate)-Chelating Ligand as Revealed by Transient Electric Dichroism Aklhlko Yamaglshi Department of Chemistry. Facuky of Sclence, Hokkaido University, Sapporo 060, Japan (Recelved: July 16, 1980; In Final Form: May 18, 1961)
The structure of a ternary complex of Cu(II)-poly(styrenesulfonate)-2-(2-pyridylazo)-l-naphthol ((UPAN)or -1-(2-pyridylaz0)-2-naphthol(PPAN) has been studied by means of electric dichroism measurements. The angle 4 between the electric field direction and the transition moment of a chelating ligand is determined as a function of polymer-to-metalchelate ratio (P/M). 4 decreases from 50" to 0" with the increase of P / M from 1 to 7. The variation of 4 is rationalized in terms of the repulsive interaction between the metal chelates located on the adjacent sites. Trivalent metal ions like Ce(II1) and Ga(II1) reduce the amplitude of the dichroism remarkably. The results suggest that the bridging by a trivalent metal ion between two distant sites may deprive a polyelectrolyte chain of its flexibility. As a result, such a metal chelate as is bound somewhere between the above sites cannot orient under an electric field. Introduction Knowledge of mixed ligand complexes has provided valuable clues to understanding the structures and functions of metal enzymes.l When one proceeds one step further along this line, a ternary complex of metal ionpolyelectrolyte-simple ligand comes out as a model system closer to an enzyme-substrate complex.z In this model (1) H. Sigel, Angew. Chem., Int. Ed. Engl., 8, 167 (1969). (2) T. Tsuchida and H. Nishide, Adu. Polym. Sco., 24, 1 (1977). 0022-3654/81/2085-2665$01.25/0
a polymer chain, on which a metal complex is located, mimics the protein moiety of an enzyme. In spite of a number of works on the catalytic actions by metal-polyelectrolyte ~ y s t e m showever, ,~ there has been little direct evidence presented for the structures of metal ion-polye(3) (a) M. Hatano, T. Nozawa, S. Ikeda, and T. Yamamoto, Macromol. Chem., 141, 1 (1971); (b) A. Levitzki, I. Pecht, and B. Berger, J. A m . Chem. Soc., 94, 6844 (1972); ( c ) A. Garnier and L. Tosi, Biopolymers, 14,2247 (1975); (d) K. Honda, E. Hasegawa, and E. Tsuchida, Biochim. Biophys. Acta, 427, 520 (1976).
0 1981 American Chemical Society
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Yamagishl
A lectrolyte complexes in solution. This is particularly the case with the systems involving diamagnetic i ~ nor ~ ~ ~ , ~ randomly coiled p ~ l y m e r s ,for ~ ~which * ~ the conventional methods like EPR and CD measurements are not applicable or lead to indicative results only. The present paper reports the initial observation of CU(D(PAN)+ transient electric dichroism for a ternary system of metal ion-polyelectrolyte-chelating ligand. This method is based on the change of apparent extinction coefficients due to the electric-field-induced orientation of a chr~mophore.~ The present metal-chelate complexes do not align under an electric field, unless they are bound with a polyelectrolyte. Accordingly, for a metal ion of any kind, the appearance of dichroism provides direct evidence for the binding with a polyelectrolyte chain. Moreover, the method is applicable for a metal-coiled polymer complex if cu ( PPA N)+ the polymer segment is flexible enough to stretch out in Figure 1. Structures of CU((UPAN)+ and CU(@PAN)+. the direction of the electric field. In the present studies, we have investigated the system I I a CciPAd) of copper(II)-poly(styrenesulfonate)-2-(2-pyridylazo)-lnaphthol (aPAN) or -1-(2-pyridylaz0)-2-naphthol@PAN). A stable ternary complex is formed among those components. The large electric dichroism due to a bound chelating ligand enables us to determine the geometry of the ternary complex in solution.
Experimental Section All metal ions except Ce(II1) were used as perchlorate salts. Ce(II1) was added as Ce(N03)2.6H20. The concentration of a metal ion was determined by EDTA titration. 2-(2-Pyridylazo)-l-naphthol(aPANH) was synthesized according to the method by Anderson and Nicklema6 1-(2-Pyridylaz0)-2-naphthol@PAN ) was purchased from Dojin Chemical Lab. Poly(styrenesu1fonic acid) (HPSS) was kindly donated by Dr. S. Harada. The polymer had been synthesized by sulfonating polystyrene (M,= 3.5 X lo5) in the established methoda6 The concentration of styrenesulfonate residue (ss-) was determined by titration with KOH. The resultant potassium poly(styrenesulfonate) (KPSS) solution was used as a stock solution. Other materials were all of reagent grade. The solutions contained 5 % dioxane, since aPANH and PPANH were stored in dioxane. Electronic spectra were recorded with a Hitachi EPS-3T spectrophotometer at 20 "C. The pHs of the solutions were measured with a Radiometer 4d. Dialysis experiments were made with a seamless cellulose tubing (Visking Co. Type 18/32). The electric conductivity was measured with a Yanagimoto Model MY-7 conductivity outfit. The transient electric dichroism was obtained with a conventional temperature-jump apparatus, a Union Giken T-jump apparatus.' In order to detect the optical anisotropy of a signal, a rotatory polarizer (Nikon Kogaku) was installed in between a monochromator and a T-jump A coaxial cable of 70 or 157 m (characteristic impedance 50 Q ) was charged up to 13-30 kV. The electric discharge was performed on a sample solution (0.5 cm2 X 0.4) with electric resistance of 10-80 k0. Accordingly the electric field of 30-75 kV cm-l was imposed on the solution within the first reflection of an electromagnetic wave (0.5-1.0 ps for a 70-157-m ab le).^ The electric field (4) F. S. Allen and K. E. Van Holde, Reu. Sci. Instrum., 41,211 (1970). (5) (a) R. G. Anderson and G. Nickless, Analyst (London),93, 13 (1968); (b) D. Betteridge, D. John, and F. Snape, ibid., 98, 512 (1973). (6) M. Kanda, T. Komatsu, and T. Nakagawa, Polym. Phys. Jpn., 16, 61 (1973). (7) A. Yamagishi, T. Masui, and F. Watanabe, J. Phys. Chem., 84,34 (1980). (8) M. Dourlent, J. F. Hogrel, and C. Helene, J. Am. Chem. SOC.,96, 3398 (1974).
1
Wavelengt h/nm
Figure 2. Electronic spectra of CuL' and CuL,; e is per one ligand: (a) aPANH (-), Cu(aPAN)+ (- - -), and Cu(aPAN), (. -). (b) PPANH (-), CU(PPAN)+, (- - -), and Cu(@PAN), (. .).
-
-
decayed with a half-lifetime of 100-800 p s , depending on the conductivity of the solution.1° A simultaneous rise of temperature occurs along with the decay of the electric fielde8The temperature perturbation was, however, found to have no effect on the equilibrium of these systems, unless the concentration of the polyelectrolyte was very low. This was concluded because the observed change of transmittance recovered to the original level within 10 ms, where a sample solution was still maintained at the elevated temperature.s The fact that the amplitude of the signal did not depend on the length of a coaxial cable also confirmed that the rise of temperature was not the cause of the transient change of transmittance. Results Electronic Spectrum of Cu"(aPAN) or CU'~(PPAN) (Figure 1 ) in the Presence of KPSS. Both aPANH and BPANH form a 1:l or 1:2 complex with Cu2+ at an appropriate pH according to Cu2+ LH * CuL+ H+ (K,K,) (1)
+ CUL' + LH
+
C U L+ ~ H+ (K2Ka) (2) Here Ki and K, are the ith formation constant of CuLii and the acid dissociation constant of LH, respectively. Among the literature value^,^ K1, K2,and K , are chosen as 1013*6 (9) G. W. Hoffman, Reu. Sci. Instrum., 42, 1643 (1971). (10) The half-lifetime of decay of the electric field (tip) was found to be given by tl12 = 0.01R ps for a 157-m coaxial cable with R = the resistance of a solution in 0.
Copper(I1)-Poly(styrenesu1fonate)-Chelating Ligand
The Journal of Physical Chemistv, Vol. 85,No. 18, 198 1 2667
i
04
t 06 4
04 A /nm
3
Flgure 4. Spectrum of the Cu(PPAN)+/KPSS system. [Cu(@PAN)+] = 3.1 X M. P/M = (a) 0, (b) 0.33, (c) 0.66, (d) 1.00-1.30, (e) 2.46, and (f) 5.65. pH 5.00.
02
h /nm
Flgure 3. Spectrum of the Cu(aPAN)+/KPSS system. [Cu(aPAN)+] M. P/M = (a) 0,(b) 0.24, (c) 0.48, (d) 0.72, (e) 0.98, = 4.7 X (f) 1.2, (9) 2.0, (h) 4.0, and (i) 5.80. pH 4.50.
M-' 108.0M-l, and 10-lO.O M for aPANH, and 1017.0M-l 108.dM-l, and 10-l2.OM for PPANH, respectively. From these values, it is concluded that, at low pH (4-5), an equimolar solution of Cu2+and LH gives a CuL+ complex exclusively. For example, the fractions of LH and CuL, are below 1% for [Cu2+]= [LH] = 5 X lo" M at pH 4.5. On the other hand, a 1:2 solution of Cu2+and LH gives a CuL, complex exclusively at pH higher than 8. These are ascertained in the present experiments, and the electronic spectra of these complexes are shown in Figure 2, a and b, for aPANH and PPANH, respectively. When KPSS is added to a Cu(aPAN)+ solution at pH 4.5, the absorption peak at 565 nm decreases with the simultaneous increase of absorbance above 650 nm (Figure 3). The isosbestic points are seen at 420,485, and 650 nm. The absorbance at 565, Am, decreases almost linearly with [KPSS] until the polymer-to-metal chelate ratio (P/M) reaches 1. The results are most simply interpreted by the quantitative addition of Cu(aPAN)+on a styrenesulfonate residue (ss-):
centration, while the results in Figure 3 are unaffected by the change of pH from 4 to 5. Under the mechanism of reaction 5, the absorbance at 547 nm should not decrease on the addition of KPSS, since the extinction coefficient of CuL, per one L is almost the same as that of CuL+ at this wavelength. Even if one may accept the formation of CuL, by reaction 5, the complex is not stable at this pH and decomposes to CuL+ according to
+
C U L+ ~ H+ + CUL+ LH
(6)
where one Cu2+ion is assumed to bind with two ss- residues. These possibilities are, however, discarded on the following grounds. If reaction 4 takes place, the absorbance at 485 nm (Ams of aPANH) should increase with the addition of KPSS, which is not realized in Figure 3. Another difficulty is that reaction 4 is dependent on the H+ con-
This again contradicts the low absorbarce at 485 nm. As will be stated later, the dialysis experiments show that neither LH nor CuL, is generated in a bulk medium during the spectral change in Figure 3. In addition, the electric dichroism measurements will show that the whole spectral shape of curve i in Figure 3 is reproduced in the transient dichroism spectrum (Figure 9), implying that the species responsible for that spectrum, whether one or more compounds are involved, are all bound with the polyelectrolytes. Supported by the above facts, we conclude that reaction 3 is the most probable to rationalize the results in Figure 3. The binding constant of reaction 3, Kb, is estimated to be larger than 2 X lo5 M-l. This value is extremely large compared with the binding of monocationic ions like K+ and H+ with KPSS.12 In addition, one may raise the question of why the resultant complex, [CuL+-ss-],does not precipitate from solution in spite of its neutrality. Similar circumstances, however, have been observed for the binding equilibria between poly(styrenesu1fonate) (PSS-) and various kinds of organic dye cations.ll For instance, Kbfor the reaction of acridine orange cation with NaPSS is reported to be larger than lo7 M-l (ref l l ) , and the 1:l complex, [(acridine orange cation)-ss-],is stable in water at least at the concentration of M. Similar spectral results are obtained for the Cu(PPAN)+/KPSS system (Figure 4). The interaction of a Cu(aPAN)+ with sodium p toluenesulfonate is investigated spectrophotometrically. The compound is regarded as a monomer unit of PSS-. The spectrum of a free Cu(aPAN)+undergoes no change on addition of M sodium p-toluenesulfonate. Thus Cu(aPAN)+ does not form a stable complex with the compound. The interaction of a ligand, LH, with KPSS is investigated spectrophotometrically. As shown in Figure 5, both aPANH and PPANH show a decrease of absorbance at their peak positions with an increase of KPSS. We conclude that these ligands themselves bind with PSS-, although they carry no charge at the investigated pH.
(11) V. Vitagliano, L. Costantino, and 2. Zagari, J.Phys. Chem., 77, 204 (1973).
79, 2381 (1975).
+
CuL' ss- F= cuL+'ss(Kb) (3) When P/M exceeds 1, the absorbance at 565 nm increases again with the new isosbestic points at 430, 510, and 650 nm. If one accepts reaction 3, the latter process is assigned as the dilution of a bound metal chelate along the empty site of a polymer chain. The situations are similar to the methachromasy observed for the binding of an ionic dye on a polyelectrolyte chain.ll Another possible interpretation of the spectral changes in Figure 3 is to take into consideration the decomposition or disproportionation of a metal chelate due to the complexation of a Cu2+ion with ss-. For example, the following reactions are conceivable: CUL++ 29s- H+ + LH + Cu2+*2ss(4) or 2CuL+ + 29s- F= CUL, + cu2+-2ss(5)
+
(12) J. C. T. Kwak,M. C. O'Brien, and D. A. Maclean, J.Phys. Chem.,
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The Journal of Physical Chemlstry, Vol. 85, No. 18, 1981
Yamaglshl
b BPAN
i I
500
400
600
h/nm
Figure 5. Spectrum of the LH/KPSS system: (a) [aPANH] = 2.7 X lo-' M (-), [aPANH] = 2.7 X lo-' M, and [KPSS] = 6.7 X lo-' M (---). (b) [PPANH] = 3.3 X lo-' M (-), [PPANH] = 3.3 X lo-' M, and [KPSS] = 7.6 X M (--- I.
78
;$.
a
oi 2
902
0
,
'ri,
,
0 ;a3
5 lMZ'l/
10
165M
M KPSS is packed into dialysis tubing. The tubing is soaked in 8 mL of water and left for -30 h at room temperature. The blank test in the absence of KPSS assures that dialysis equilibrium is attained within this interval. The electronic spectra of outer or inner solutions almost coincide with those of free or bound Cu(PPAN)+ complexes. Neither a free ligand nor Cu(PPAN), exists in the outer solvent. The equilibrium constant, Kb, is calculated by
1
Kb
15
Figure 0. (a) Dependence of the fraction of a bound Cu(PPAN)+ ( X ) on the concentration of trivalent metal ion: (0) Ce3+ and (0) Ga3+. [KPSS] = 1.5 X lo-' M. [Cu(PPAN)+] = 3.4 X IO-' M. [KCI] = 1.0 X lom3M. The solid curve is the theoretical one calculated for the infinite binding strength of M3+ with PSS-. K, for CU(PPAN)+ is taken as 4 X lo5 M-'. (b) Dependence of the amplitude of the electric dichroism of a bound CU(,BPAN)+on the concentration of divalent or trivalent metal ion: (A) Zn2+, ( 0 )Cu2+, (0) Ce3+, and (0) Ga3+.Other conditions are the same as In (a).
When a trivalent metal ion (M3+)like Ce3+and Ga3+is added to a solution of a bound Cu(PPAN)+,the absorbance at 550 nm (A- of Cu(PPAN)+)is found to increase. Under the present conditions, M3+ does not complex with PPANH, because Cu2+is by far a stronger agent toward PPANH than M3+. Since M3+competes with Cu(PPAN)+ for the site on PSS-, the observed absorbance increase is due to the dissociation of a bound Cu(PPAN)+according to ~CU(PPAN)+*SS+ M3++ 3Cu(PPAN)++M3+.3ss- ( 7 ) where one M3+ion is assumed to bind with three ss- residues. The fraction of bound copper chelate ( x ) is calculated by the equation x = (A0- A)/(A
