J. Phys. Chem. 1983,87,3320-3327
3320
Pulse Radiolysis of Porphyrin and Ferriporphyrin Solutions in 2-Propanol-Carbon Tetrachloride Systems. Protonation and Ligand Exchange Kinetics D. Brautt" and P. Neta' Laboratoire de Biophysique, INSERM U.201, CNRS ERA 95 1, Mus6um National d'Histolre Naturelle, 75005 Paris, France; Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556; and Center for Radiation Research, National Bureau of Standards, Washington, D.C. 20234 (Received: December 14, 1982)
Pulse radiolysis of aqueous 2-propanol containing acetone and carbon tetrachloride was investigated by conductivity measurements. The primary events of solvent radiolysis and further reactions of CCll with radicals derived from 2-propanol and acetone produce HCl within 100 ns. This is followed by further production of HC1 by chain reactions, with yields drastically dependent upon oxygen concentration. CC1302radicals are the chain propagators unless the solutions are strictly deoxygenated. In the latter case, the first burst of HCl is well separated from subsequent chain processes, thus permitting direct kinetic studies of protonation reactions. In 2-propanol containing acetone and CCl,, deuteroporphyrin IX dimethyl ester (DPDME) protonates to the monocation (DPDMEH+)which is spectrophotometrically characterized by independent conventional studies. The reaction reaches equilibrium ( K = 2 X lo4 M-'); the forward and reverse rate constants are determined by pulse radiolysis. Alternatively, DPDME and its ferric complex, which do not react rapidly with CC13and CC1302radicals, are used to spectrophotometrically monitor HCl formation arising from chain processes in aerated or deaerated 2-propanol solutions. The ferric ion of ferrideuteroporphyrin IX dimethyl ester in solution in 2-propanol is associated with two axial ligands, one is a 2-propanolmolecule and the other depends on solution composition and can be a 2-propoxide ion (DPDMEFe*110CH(CH3)2 in neat 2-propanol), a chloride ion (DPDMEFeIIIClin 2-propanol containing HCl), or another 2-propanol molecule ((DPDMEFe"'HOCH(CH,),)+ in 2-propanol containing acid whose anion is not coordinating). The three forms are spectrophotometrically characterized. The fiit form is converted to the last one by protonation of the 2-propoxide ligand. Pulse radiolysis in 2-propanol solutions containing acetone and CC14showed that the protonation rate constant is (1.1 f 0.2) X lo9M-' s-'. The protonated form, (DPDMEFe1nHOCH(CH3)2)+, can also be used to monitor HC1 formation originating from chain processes. Slower optical changes observed following pulse irradiation of DPDMEFe1110CH(CH3)2 are attributed to the replacement of the 2-propoxide ligand by a chloride ion. A 2-propanol molecule of (DPDMEFe111HOCH(CH3)2)+ can also be exchanged for C1- as shown by conventional and stopped-flow spectrophotometric measurements. The equilibrium constant is 3.6 X lo4 M-' and the forwarrd and reverse rate constants for this reaction are 1.35 X lo6 M-' s-' and 35 s-', respectively. The use of pulse radiolysis to study protonation reactions is outlined.
-
In trod uction Several reports have dealt with radiolysis studies of deaerated aqueous and nonaqueous 2-propanol-acetonecarbon tetrachloride mixtures.'" In this solvent system, scavenging of the initial products of radiolysis leads to trichloromethyl radicals. In keeping with mass conservation and electrical neutrality, protons and chloride ions are also produced. Most attention has been given to the fate of the CC13 radicals, which are believed to initiate chain rea~tionsl-~ via hydrogen abstraction from 2-propanol leading to 2-hydroxy-2-propyl radicals which, in turn, react with CCll to regenerate the CC13 radicals. Chain termination occurs via CCl, self-reactions. These chain processes would account for the large yield of HC1 obtained upon y-irradiation of CC14solutions in 2-propanol.' However, with the exception of some preliminary results,' no detailed analysis was reported on the possible effect of oxygen (even in trace amounts) on the chain processes. The reactivity of trichloromethyl radicals has also important bearing on carbon tetrachloride t ~ x i c i t y . ~The formation of these radicals through reduction of CC14by a hemoprotein, cytochrome P450, appears to be a crucial step in reactions leading to lipid peroxidation and various biological damage^.^ Lipid peroxidation develops through chain reactions involving hydrogen abstraction from lipid molecules, a process which parallels the above-mentioned *Correspondence to D. Brault and P. Neta should be sent to the Paris and Washington addresses, respectively.
chain reaction. Trichloromethyl radicals were long believed to be the initiators in these chain reaction^.^ But recently it was concluded that their peroxo derivatives, the CC1,02 radicals, are much more reactive and that they are more likely responsible for most of the observed phenomena.6-8 We have recently investigated the reactions of various halogenated alkyl radicals and methyl radicals with iron porphyrins regarded as models of cytochrome P450.%" Our concern was to specify the possible importance of radical scavenging by cytochrome P450. No reaction was found between trichloromethyl radicals and ferric por(1) Radlowski, C.; Sherman, W. V. J. Phys. Chem. 1970, 74, 3043. (2) Koster, R.; Asmus, K.-D. 2.Naturforsch.E 1971,26, 1104. (3) Feldman, L.; Alfassi, 2.B. J.Phys. Chem. 1981, 85, 3060. (4) Willson, R. L.; Slater, T. F. In "Fast Processes in Radiation Chemistry and Biology";Adams, G . E.; Fielden, E. M.; Michael, B. D., Ed.; Wiley: New York, 1975; p 147. (5) Slater, T. F. 'Free Radical Mechanisms in Tissue Injury"; Pion: London, 1972. Recknagel, R. 0.; Glende, E. A., Jr.; Hruszkewycz,A. M., In "Free Radicals in Biology";Pryor, A. W., Ed.; Academic Press: New York, 1977; Vol. 3, p 97. (6) Packer, J. E.; Slater, T. F.; Willson, R. L. Life Sci. 1978,23, 2617. (7) Packer, J. E.; Mahood, J. S.; Willson, R. L.; Wolfenden, B. S. Int. J.Radiat. Biol. Relat. Stud. Phys. Chem. Med. 1981, 39, 135. (8) Packer, J. E.; Willson, R. L.; Bahnemann, D.; Asmus, K.-D. J. Chem. Soc., Perkin Trans. 2 1980, 296. (9) Brault, D.; Bizet, C.; Morliere, P.; Rougee, M.; Land, E. J.; Santus, R.; Swallow, A. J. J. Am. Chem. SOC.1980, 102, 1015. (10) Brault, D.; Neta, P. J. Am. Chem. SOC.1981, 103,2705. (11) Brault, D.; Neta, P. J.Phys. Chem. 1982, 86, 3405.
0 1983 American Chemical Society 0022-3654/83/2087-3320$01.50/0
Pulse Radiolysis of Porphyrln and Ferriporphyrin Solutions
phyrins in buffered deaerated aqueous 2-propanol-acetone-carbon tetrachloride mixturesag On the other hand, as reported in the present study, optical changes are observed upon irradiation of deaerated or aerated ferric porphyrin solutions in 2-propanol-acetoneCCl4 mixtures. They are attributed to the protonation of the 2-Pro- axial ligand of the ferric porphyrin. Ligand exchange involving chloride ions formed by irradiation is also observed. The last reaction is also investigated by stopped flow measurements. The protonation of free base deuteroporphyrin is also studied. These results give a new insight into the physicochemical properties of porphyrins and their ferric complexes. Moreover, the HC1 formation upon pulse irrdiation of 2-propanol-acetone-CC14 systems can be conveniently monitored by optical absorption using porphyrins. The above-mentioned chain reaction is also followed by means of conductivity measurements and the effect of oxygen on the chain process is examined.
Experimental Section Materials. Deuteroporphyrin IX dimethyl ester (DPDME) was purchased from Mid-Century Chemical Co. It was found to move as a single spot on alumina plates with chloroform/petroleum ether (1.51)and was used as received. The pox0 dimer of ferrideuteroporphyrin IX dimethyl ester ((DPDMEFe1n)20)was prepared according to a procedure modified from Smith and Caughey', which involves chromatography on silica gel. 2-Propoxyferrideuteroporphyrin IX dimethyl ester (DPDMEFeqCH(CH3)2)solutions were prepared by f i s t dissolving (DPDMEFe"'),O in acetone, then adding 2propanol and letting them stand overnight. Under these conditions, the dimer is split and the ferric ion charge is balanced by a 2-propoxide ligand.13 2-Propanol, acetone, carbon tetrachloride, perchloric acid (70%), and hydrochloric acid (37%) were of the best available analytical grade and were used without further purification. Nitrogen containing less than 1ppm of O2and gaseous hydrochloric acid were purchased from Linde and Matheson, respectively. Perchloric acid solutions in 2-propanol were prepared by diluting the concentrated acid. Hydrochloric acid solutions were prepared by using either concentrated acid or by bubbling gaseous HC1 in 2-propanol. Solutions were titrated with NaOH after diluting the alcohol solution in a large excess of water. Measurements. Optical spectra were recorded by using a Cary 219 spectrophotometer equipped with a thermostated cell holder. Equilibrium studies were run at 20 f 0.2 "C. Pulse radiolysis experiments were performed by using the Notre Dame computer-controlled apparatus described previ0us1y.l~ The linear accelerator supplies 5-50-11s pulses of 7-MeV electrons. Typical doses calibrated with a thiocyanate solution ranged from 0.1 to 1.5 krd per pulse. For experiments performed under anaerobic conditions, the solution contained in an air-protected bottle was carefully bubbled with nitrogen and transferred to the cell through all-glass tubing. The solution flow through the cell was kept high enough to reduce air contamination and to ensure a fresh solution for each pulse. Cells of 2-, 5-, or 10-mm optical path length were used depending on the porphyrin concentration. Cutoff filters were used to aviod irradiation below 420 nm. The pulse radiolysis conductivity set up has been described e1~ewhere.l~ (12)Smith, M. L.; Caughey, W. S. Methods Enzymol. 1978,52,421. (13)Bizet, C.; Morliere, P.; Brault, D.; Delgado, 0.;Bazin, M.; Santus, R. Photochem. Photobiol. 1981,34, 315. (14)Patterson, L. K.;Lilie, J. Znt. J.Radiat. Phys. Chem. 1974,6,129.
The Journal of Physical Chemlstty, Vol. 87, No. 17, 1983 3321
Stopped-flow experiments were performed by using a Durrum-Gibson spectrophotometer (Type 13002)with a mixing time of about 3 ms. A 2-cm optical path length was used. Experiments were run at 20 f 0.2 "C.
Results and Discussion Basic Reactions i n the Radiolysis of 2-Propanol-Acetone-Carbon Tetrachloride Systems. The main primary products of 2-propanol radiolysis are solvated electrons, hydrogen atoms, 2-hydroxy-2-propyl radicals, and solvated protons.16J7 Within a few nanoseconds, H atoms are converted into 2-hydroxy-2-propylradicals through feaction with 2-propanol. The resulting yield of (CH3)2COH radicals is G N 4.16 The yield of solvated electrons and protons are both G N 1.2.16 Addition of acetone causes rapid scavenging of the solvated electrons according tol8 (CH3),C0 + e;
-
(CH3),CO-
(1)
with kl N 1O1O M-' s-l. Unless they undergo reaction with an added solute (see below), (CH,),CO- radicals protonate within 50-100 ns to yield 2-hydroxy-2-propyl radical^.^^^^^ Although the yield of primary species somewhat differ, the same situation prevails in water-2-propanol-acetone mixtures. Indeed, radiolysis of water present in the medium produces solvated electrons and protons (in equal yields) as well as hydrogen atoms and hydroxyl radicals.21 Both of the latter species are rapidly scavenged by 2propanol leading to 2-hydroxy-2-propyl radical^.^,,,^ Addition of carbon tetrachloride to deaerated aqueous or nonaqueous 2-propanol-acetone systems leads to the following reactions:8
- + - + + - + + +
CC14 + e;
CC13
C1-
(2)
with k2 N 3 X 1O'O M-' s-l. (CH,),CO(CH&COH
+ CC14
+ CC14
CCl,
CC13
C1-
C1-
(CH3)&0 (3)
H+
(CHJ2C0 (4)
In nonaqueous 2-propanol, the reaction rate constant k, was estimated to be k3 N 8 X lo8 M-' s-l from competition kinetics involving ferrous deuteroporphyrin IX dimethyl ester.24 Most of the experiments described in this paper were performed with solutions containing 5% (0.68 M) acetone and 2% (0.2 M) carbon tetrachloride. Under these conditions, reactions 1 and 2 compete for e; scavenging. The (CH3)2CO-radicals react with CC14partly before they undergo protonation (reaction 3) and partly after protonation (reaction 4). The rate constant for reaction 4 has been reported to be between 1 X lo8 and 7 X lo8 M-l in aqueous solution^^^^ and is expected to be somewhat similar in nonaqueous medium. The lowest value of k4 leads to a calculated half-life of reaction 4 of tl 40 ns. In conclusion, pulse radiolysis of 2-propanof-acetoneCCl, systems produces a burst of protons, one part formed
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(15)Janata, E.Radiat. Phys. Chem. 1982,19, 17. (16)Russell, J. C.; Freeman, G.R. J. Phys. Chem. 1968,73,808. (17)Freeman, G.R. Actions Chim. Biol. Radiat. 1970,14, 73. (18)Anbar, M.; Bambeneck, M.; Ross, A. B. Natl. Stand. Ref. Data Ser., Natl. Bur .Stand. 1973,No. 43. (19)Laroff, G.P.;Fessenden, R. W. J. Phys. Chem. 1973,77,1283. (20)Janata, E.;Schuler, R. H. J.Phys. Chem. 1980,84, 3351. (21)Swallow,A. J. "Radiation Chemistry"; Longmans: London, 1973. (22)Anbar, M.; Farhataziz; Ross, A. B. Natl. Stand. Ref. Data Ser., Natl. Bur. Stand. 1975,No. 51. (23)Farhataziz; Ross, A. B. Natl. Stand. Ref. Data Ser., Natl. Bur. Stand. 1977,No. 59. (24)Brault, D.; Neta, P., unpublished results.
3322
The Journal of Physical Chemistry, Vol. 87,
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1983
on a subnanosecond time scale, originating from primary events of solvent radiolysis, the second part originates from reaction 4. More than 80% of the total proton concentration is attained within about 100 ns. The total proton yield is equal to the sum of the yields of e; and of 2hydroxy-2-propyl radicals, which is expected to be about G = 5.2 in neat 2-propanol16and somewhat higher (5.5-6.0) in aqueous 2-propanol. Chloride ions and CCl, radicals are also formed with the same yield. In the experiments described below, the HC1 concentration produced by radiolysis does not exceed -5 X loW5 M. (The HC1 dissociation constant in 2-propanol has been estimated to be about 4 X M.16) This basic scheme may become more complicated by chain processes leading to further formation of HC1, as found in radioly~isl-~ and photolysis25experiments. The rate-limiting step of the propagation was attributed to the reaction CC1,
+ (CH,),CHOH
-
CHC1,
+ (CH,),COH
(5)
The 2-hydroxy-2-propyl radicals thus formed react with carbon tetrachloride to generate CCl, radicals (reaction 4). The termination step was assumed to be the dimerization 2Cc1,
- c2c16
(6)
There is a wide disagreement on the rate constant of reactions 5l-,vZ5and 6.26927Values in the ranges of 2.9 to 33 and 1 X lo8 to 1.8 X lo9 M-ls-l have been reported for k, and 2K6, respectively. Radiolytic studies of various organic substrates in deaerated aqueous 2-propanol containing CC14indicated that CC13radicals reacted with the substrates. However, it was later pointed out that the results were dramatically dependent upon oxygen contamination and that CC1302 radicals are much more reactive than CC13.6,7 A rate constant of k7 L lo9 M-ls-l was estimated for the reaction CC~,+ 0,
-
CCl,O,
(7)
comparable to the values of 3 X lo9 M-l s-l reported for gas-phase reaction28and to rate constants reported for reactions of oxygen with various substituted alkyl radical~.~~ Before dealing with protonation studies, it is necessary to evaluate how chain production of HC1 is affected by oxygen and if it can interfere with the first burst of protons. The buildup of HC1 following pulse irradiation of aqueous solutions of 2-propanol (50%,6.5M) containing acetone (0.68 M) and CC14 (0.2 M) was monitored by conductivity.2 The dose per pulse was in the range of 100-200 rd. Typical results presented in the form of relative conductance in arbitrary units (au) vs. time are shown in Figure 1. A large chain process is seen to develop over 1 ms in aerated solutions. Approximating the increase in conductivity as a first-order process we calculate a rate constant of about 6 X lo3 s-l. The final relative conductance was found to be -2900 au. The initial burst of protons, with amplitude -400 au, was found to be complete within 300 ns as expected (result not shown). Saturation of the solution with oxygen led to a decrease in the chain process. Final and initial relative conductance
-
(25) Van Beek, H. C. A.; Van der Stoep, H. J. Recl. Trau. Chim. Paw-Bas 1978. 97. 279. 126) Lesigne, B.f Gilles, L.;Woods, R. J. Can. J . Chem. 1974,52,1135. (27) Paul, H. Int. J. Chem. Kinet. 1979, 11, 495, and references therein. (28) Cooper, R.; Cumming, J. C. Radiat. Phys. Chem. 1980, 16, 169. (29) Rabani, J.; Pick, M.; Simic, M. J. Phys. Chem. 1974, 78, 1049. Rabani, J.; Klug-Roth, D.; Henglein, A. Ibid. 1974, 78, 2089.
Brault and Neta
-
-3000
.-
a
C
,-e.
./*-
2
........--
.A*.
?
-2
.-
n
9
-2000
01
U C
m
U 3
01
)---I
K
&
I
1
1
1
1
I
I
T i me
Flgure 1. Conductivity traces observed upon pulse irradiation of aqueous solutions containing 2-propanol (6.5 M)-acetone (0.68 M)carbon tetrachloride (0.2 M). Relative conductance (arbitrary units) vs. time: a, aerated solutions; b, deoxygenated solutions.
-
changes were 600 and 200 au, respectively. The chain (1.0-1.5) X lo4 reaction was found to be faster, with k s-l. No drastic effect of the dose on the chain process was found. The effect of oxygen traces was qualitatively checked by bubbling N2through an initially air-saturated solution. Signals were recorded every 30 s while the solution flowed through the cell. Thus, each successive measurement corresponded to decreasing oxygen concentration. First the amplitude of the chain reaction was found to increase up to 5100 au. At that time a first-order plot led to a rate constant of k N 2 X lo3 s-l. Then the signal gradually decreased as nitrogen bubbling was continued. Results obtained with nitrogen-saturated solutions are also shown in Figure 1. Only a small chain reaction is seen with an amplitude of -850 au and a rate constant of 9 X lo2 s-l. The initial proton burst (- 300 au) appears as a well-separated step. These results clearly show that CC1,02 radicals are much more efficient chain propagators than CCl, radicals. The involvement of the latter ones in chain propagation might even be questioned as oxygen contaminations are very difficult to exclude. This might explain the discrepancies between rate constants reported by various authors as mentioned above. It must be pointed out that Radlowski and Sherman1 also reported very high yields of hydrochloric acid upon y-radiolysis of aerated carbon tetrachloride solutions in 2-propanol. Thus, in the presence of oxygen, even in traces, propagation is expected to occur via CC1302+ (CHJ2CHOH CC1302H + (CH&COH (8) followed by reactions 4 and 7. The reaction rate constant k, has been estimated to be k8 7 X 103 M-' s-l by Packer et al.' Self-reaction or first-order decomposition of CC1302 radicals would account for chain termination. The decrease in the chain length observed in oxygensaturated solutions is probably the result of the reaction (CH3)2COH+ O2 (CH,),C(OH)O, (9) which is very fast, kg = (4.0-4.5)X lo9M-' s-l 30*31and can compete with the propagation reaction 4. Assuming the
-
-
(30) Adams, G. E.; Willson, R. L. Trans. Faraday SOC.1969,65,2981. Willson, R. L. Trans. Faraday SOC.1971, 67, 3008. (31) Butler, J.; Jayson, G.; Swallow, A. J. J . Chem. Soc., Faraday Trans. 1 1974, 70, 1394.
Pulse Radiolysis of Porphyrin and Ferriporphyrin Solutions
I------'''
Figure 2. Optical spectra of deuteroporphyrin dimethyl ester (1 X M) sdutions in 2propand containing acetone (0.68 M). 1 cm cell: (-) no acid added (free base form); (---) 1.8 X M perchloric acid (essentially monocationic form): (. e .) 3.1 X lo-* M hydrochloric acid (dcationic form).
The Journal of Physical Chemistry, Vol. 87, No. 17, 1983 2.10 5
1
'
I
3323 ,2.104
'
Figure 3; Optical spectra of ferrldeuteroporphyrin dimethyl ester derivatives in 2-propanol containing acetone (0.68 M): (-) DPDMEFe11'OCH(CH3),(neat 2propanol); (---) [DPDMEFe1''HOCH(CH3)2]+;(.-.) DPDMEFe"'C1 (solution bubbled with gaseous hydrochloric acid, or 1.25 X lo-* M hydrochloric acM for a porphyrin concentration of 5 X
MI.
in two distinct steps. The ratio of the equilibrium conKlo and Kll was found to depend strongly on the stants solubility of oxygen in 2-propanol-water mixtures is intermediate between the solubilities in the pure s o l v e n t ~ , ~ ~ nature of the acid as observed before. By using perchloric acid, K,, and Kll were found to be 2 X lo4 and 30 M-l, we estimate an oxygen concentration of (0.5-1.0) X respectively.a Upon stepwise titration, the monocation M. This leads to a rate of reaction 9 of (2-5) X lo7 s-l as concentration reached more than 90% of the total porcompared with (2-14) X lo7 s-l for reaction 4. Thus, phyrin concentration. The monocation spectrum (Ama = competition between reactions 4 and 9 could explain the 589.5,552-557,524-529, and 388 nm) reported in Figure effect of oxygen saturation, since (CH3)2C(OH)Oz does not 2 agrees with that obtained by using detergent^.^^ The react with 2-propanol as rapidly as does CC1302,and it is spectrum of the dicationic species obtained with an excess not expected to react with CClk of perchloric acid was characterized by two visible bands In conclusion, irradiation of 2-propanol-CC1, systems at 589 and 548 nm and a Soret band at 401 nm. Titration allows production of a pulse of HC1, which is now applied with hydrochloric acid led to the same basic results except to the study of porphyrin and iron porphyrin protonation. that the two protonation steps somewhat overlapped. This Solution Structures of Deuteroporphyrin I X Dimethyl was due to an increase of the Kll value while Klo was Ester and Ferrideuteroporphyrin I X Dimethyl Ester. independent of the acid used. The monocation concenProtonation Equilibria. Free base porphyrins (P) can protonate according to the following e q ~ i l i b r i a : ~ ~ ! ~ ~ tration reached only 70% of the total porphyrin concentration. The monocation spectrum deduced from comkio posite spectra was found to be very similar to that reported P + H+?PH+ k-lo in Figure 2. On the other hand, the dication spectrum kll obtained with an excess of hydrochloric acid (Ama = 595, PH+ H+?PH:+ (11) 553, and 413 nm, see Figure 2) was slightly shifted comk-11 pared to that obtained with perchloric acid. This suggested Addition of acids to aqueous solutions of porphyrins is some specific interactions between the dicationic species generally accompanied by changes from the characteristic and anions contrary to what happens in the case of monfour-band "free base spectrum- to the two-band "dication ocations. spectrum" with little or no formation of intermediate Coordination of one ferric ion by porphyrins leaves one Only in rare cases, of porphyrin in ~ a t e r , ~ ~ positive , ~ ~ charge unbalanced. In the solid state neutrality in or in some organic s01venta,~~-~~ did acid is generally achieved through bonding of a counteranion titration lead to the appearance of a well-defined threeby iron or formation of p-oxo dimers.41 In neat alcohols, band spectrum which was attributed to the porphyrin the p-oxo dimer of ferrideuteroporphyrin M dimethyl ester monocation. Similarly, we found that deuteroporphyrin is split into monomers and an alkoxide ion is bound to the M dimethyl ester is titrated by strong acids in 2-propanola iron to achieve neutrality.13 In 2-propanol the reaction is described by42
+
(32) Stephen, H.; Stephen, T. "Solubilities of Inorganic and Organic Compounds"; Pergamon Press: New York, 1963; Vol. 1. (33) Falk, J. E."Porphyrinsand Metalloporphyrins";Elsevier: Amsterdam, 1964. (34) Smith, K. M.In 'Porphyrins and Metalloporphyrins";Smith, K. M., Ed.; Elsevier: Amsterdam, 1975. (35) Baker., H.:, Hambrieht. P.: Waener. L. J. Am. Chem. SOC.1973, I
I
1
I
.
95,'5942.
(36) Pasternack. R. F.: Sutin.. N.:. Turner. D. H. J. Am. Chem. SOC. 1976, k , 1908. ' (37) Aronoff, S.J. Phys. Chem. 1958,62,428. Corwin, A. H.; Chiwis, A. B.; Poor, R. W.; Whitten, D. G.; Baker, E. W. J.Am. Chem. SOC.1968, 90,6577. (38) Caughey, W.S.;Fujimoto, W. Y.; Johnson, B. Biochemistry 1966, 5, 3830. (39) Ogcahi, H.;Watanabe, E.; Yoshida, Z. Tetrahedron 1973,29,3241. Austin, E.;Gouterman, M. Bioinorg. Chem. 1978, 9, 281. (40) Brault, D.,unpublished results.
+
-
(DPDMEFe111)20 2(CH3)2CHOH 2DPDMEFe1110CH(CH3)2 + HzO (12) The spectrum of DPDMEFe1110CH(CH3)2 (Ama (nm), (M-l cm-l) = 580, 7900; 462, 11400; 393, 83300) is given in Figure 3. Addition of strong acids whose anions are not, or poorly, coordinating (perchloric, sulfuric acids) led to the typical "acid hemin spectrum".43 This spectrum was (41) Buchler, J. W., in ref 34. (42) A 2-propanol molecule is likely to be bound as a sixth ligand to the ferric ion (see ref 13). But, as it is not directly involved in the processes to be described, it will be disregarded in writing the formulas. The Same consideration applies to the "acid form".
The Journal of Physical Chemistry, Vol. 87,
3324
a
-2
i
W
500
-
i
:i I
600
700
Brault and Neta
No. 17, 1983
X(nm)
4
Figure 4. Pulse radiolysis of N,-bubbled solutions of DPDME (lo4 M) in 2-propanol containing acetone (0.68M) and carbon tetrachloride (0.2 M). Dose was 445 rd. (a) Absorbance changes vs. time, A = 430 nm; 1, 10 ps/division; 2, 200 ps/division; 3, 20 msldivision. (b) (0) Difference spectrum taken 100 ps after the pulse (end of the first step). (-) Difference spectrum, DPDMEH' minus DPDME, derived from conventional spectra (see Figure 2) by assuming that the porphyrin concentration is 1.65 X lo-' M.
attributed to the ferric porphyrin cation in which the iron is coordinated by two solvent molecules and the counterion is either associated in the outer coordination sphere or completely dissociated. Conversion to the acid form was found to depend linearly on the acid concentration as shown by spectrophotometric measurements on DPDMEFe1110CH(CH3)2 solutions of various concentrations to M). Thus, even for the lowest concentrations, the e q u i l i b r i ~ m ~ ~ DPDMEFe"'OCH(CH,), (CH3),CHOH2+F? [DPDMEFe"'HOCH(CH,),]+ + (CH,),CHOH (13) is always driven to the right. A lower limit of the equilibrium constant can be estimated as K L 5 X lo5 M-l. The spectrum of [DPDMEFe111HOCH(CH3)2]+C104in 2propanol (Am= (nm), E (M-l cm-') = 614,3300; 494,6350; 388,166000) is given in Figure 3. Addition of hydrochloric acid to ferrideuteroporpyrin IX dimethyl ester solutions in 2-propanol leads to a new spectrum (A,,= (nm), E (M-' cm-') = (629,4250; 530, 8500; 504, 7800; 372, 76800; see Figure 3) characteristic of coordination of the ferric ion by chloride ion.44145The process may involve reaction of chloride ions with either DPDMEFe"'OCH(CH,), or [DPDMEFe*'HOCH(CH,),]+. The latter reaction can be easily investigated by first acidifying a solution of ferriporphyrin. In a typical experiment, a 5 X lo+ M DPDMEFe11'OCH(CH3)2 solution in 2-propanol was converted to the acid form by using perchloric acid (2.4 x M). Further addition of hydrochloric acid (1 X to 5 X lo4 M) led to a set of spectra with well-defined isosbestic points at 614,592,399, and 372 nm. Plotting absorbance changes according to classical r e l a t i o n s h i p ~indicated ~!~~ that one chloride ion was bound by the ferric ion according to the equilibrium [DPDMEFe"'HOCH(CHJ2]+ + C1- G DPDMEFe"'C1 + (CH,),CHOH (14)
+
(43) Maehly, A. C. Acta Chem. Scand. 1958,12,1247. Maehly, A. C.; Akeson, A. Zbid. 1958,12, 1259. Davies, T. H. Biochem. Biophys. Acta 1973, 329, 108. (44) Erdman,G.; Corwin, A. H. J. Am. Chem. SOC.1947, 69, 750. (45) Alben, J. 0.;Fuchsman, W. H.; Beaudreau, C. A.; Caughey, W. S. Biochemistry 1968, 7, 624. (46) Brault, D.; Rougee, M. Biochemistry 1974, 13, 4591. (47) In this case the chloride ion concentration w a ~calculated by taking into account incomplete dissociation of hydrochloric acid (Kdise= 4 X lo-* M, see ref 16).
The equilibrium constant was found to be K14 = (3.6 f 0.4) X lo4 M-' at 20 "C. Pulse Radiolysis of Deuteroporphyrin I X Dimethyl Ester Solutions i n 2-Propanol-Acetone-Carbon Tetrachloride S y s t e m . As shown in Figure 4a, pulse irradiation of nitrogen-bubbled solutions of DPDME M) in 2propanol containing acetone and C C 4leads to two steps of absorbance changes. The first step was found to obey pseudo-first-order kinetics with a rate constant of (9.1 f 0.6) X lo4s-l. The differential spectrum (Figure 4b) taken 100 I.LS after corresponds pulse &e., at the end of the first step) is in very good agreement with the differential spectrum of the monocation porphyrin minus the free base form, indicating that this step corresponds to the addition of one proton. The spectra match if the concentration of monocation produced by irradiation is 1.65 X 10+ M. If we assume the proton yield in 2-propanol is GH+N 5.2,16 the total concentration of protons produced by pulses of 445 rd is [H+], = 2.41 X lo4 M, a value higher than the monocation concentration. This is accounted for by the fact that equilibrium 10 is not fully drawn to the right. The monocation concentration at equilibrium can be calculated by using the law of mass action and the value of Klo previously determined: [PH+l/([Pl[H+l)= [PH+l/[[Pl([H+lt- [PH+l)I = K~~= 2 x 104 M-' where [H+]stands for the concentration of free protons. This gives [PH+] = 1.61 X lo4 M in very good agreement with the value deduced from spectral data. Under the present experimental conditions, i.e., [PI >> [H+], it can be shown48that equilibrium 10 is attained via apparent first-order kinetics with a rate constant, k , related to the forward and reverse rate constants by k = klo[P] + k-lo. This relationship was difficult to test by varying the porphyrin concentration, since the solubility of DPDME in 2-propanol is low and the use of lower porphyrin concentrations led to absorbance changes overlapping with the slower step described below, thus precluding accurate measurements of rate constants. Assuming KIO= kl~/.lz-lo we calculate k,, = 6 X lo8 M-' s-l and kl0= 3 X lo4 s-l. However, the reaction mechanism might be more complicated. Thus, Pasternack et al.36suggested that some free base porphyrin might exist in two forms in equilibrium, a planar form and a buckled form, the latter being more reactive toward protons. Such an equilibrium has been neglected in the above calculation but its existence cannot be ruled out. As shown in Figure 4a, further absorbance changes occur over 1-2 ms. First-order analysis yields rate constants ranging between 1 and 2 X lo3 s-l. The lowest values, obtained under anaerobic conditions, were the best. The differential spectrum recorded at the end of this step was found to be very similar to that shown in Figure 4b and was indicative of an approximate two-fold increase in the monocation concentration. Therefore, these changes are attributed to the reaction of protons formed via the rate-limiting chain propagation in this solvent system as described above. No further changes were observed over -200 ms (Figure 4a). Similar results were obtained with aerated solutions of DPDME (lo4 M) in 2-propanol containing acetone (0.68 M) and carbon tetrachloride (0.2 M). But, in this case, the second step was faster ( k N 2 X lo4 s-') and overlapped with the first one. Differential spectra recorded at 14 ps
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(48)Capellos, C.; Bielski, B. H. J. "Kinetic Systems"; Wiley: New York, 1972.
Pulse Radiolysis of Porphyrin and Ferriporphyrin Solutions
The Journal of Physical Chemistry, Vol.
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500
600
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500
600
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Time
Flgure 5. Absorbance changes following pulse irradiation of DPDMEFe%CH(CH,), solutions in 2-propanol containing acetone (0.68 M) and carbon tetrachiorlde (0.2 M). (a) DPDMEFe"'OCH(CH,)Z = lo4 M, 1-cm cell, dose = 640 rd, X = 580 nm, N2-bubbledsolution, absorbance changes are negative. Full line: pseudo-firstorder plot w h k = 9.8 X l o 4 s-'. (b) DPDMEFe1''OCH(CH3), = 2 X lo-' M, 5-mm cell, dose = 1.31 krd, h = 580 nm, aerated solution, absorbance changes are negative. Full line: pseudo-firstorder plot with k = 3.7 X l o 4 s-'. Times corresponding to spectra shown in Figure 6a are indicated by arrows. (c) DPDMEFe"'OCH(CH3), = lo4 M, 1-cm cell, dose = 580 rd, X = 525 nm, aerated solution. Times corresponding to spectra shown in Figure 6b are indicated by arrows.
(Le., when essentially the first step has taken place) and 290 ps (end of the second step) were indicative of stepwise monocation formation with little or no side reactions. The final monocation yield was G N 8.3, which corresponds to a total yield of protons (calculated by applying the law of mass action as described above) of GH+N 12.5. No further absorbance changes were recorded after the second step. It should be pointed out that, in aerated or deaerated solutions, chain reactions develop whether porphyrin is present or not. This indicates that CC13and CC1302radicals are not efficiently scavenged by the porphyrin. Puke Radiolysis of Ferrideuteroporphyrin I X Dimethyl Ester Solutions in 2-Propanol-Acetone-Carbon Tetrachloride Mixtures. (a)Protonation Reactions. Irradiation of DPDMEFe1110CH(CH3)2solutions in deoxygenated 2-propanol containing acetone and CC14 with pulses of about 400 rd led to two steps of absorbance changes over several hundred microseconds. The first step was found to obey pseudo-first-order kinetics (Figure 5a). The apparent rate constant was found to be directly proportional to the porphyrin concentration in the range of 1-5 X lo4 M." The differential spectrum recorded at the end of the first step compared well with that of [DPDMEFe111HOCH(CH3)2]+ minus DPDMEFe1110CH(CH3)2as obtained from conventional measurements (these results are very similar to that obtained with aerated solutions which are presented in Figure 6a). Thus, this step was attributed to the protonation of DPDMEFe1110CH(CH3)2 according to reaction 13. The yield of [DPDMEFellrHOCH(CH,),]+ was found to be G N 5, close to the expected proton yield (GH+N 5.2). This agreement and the proportionality between the observed rate constant and the porphyrin concentration both indicate that equilibrium 13 was almost totally shifted to the right49under these experimental conditions (i.e., proton concentration = 2 x lo+ M). The protonation reaction rate constant k13 was found to be (1.1 f 0.2) X lo9 M-ls-l. A second step took place with a half-life of 1000-1500 ps leading to further [DPDMEFe11rHOCH(CH3)2]+ formation, and was attributed to reaction of protons formed via chain processes as explained above. It was independent of the porphyrin concentration. The two successive steps (49) A plot of k,,, vs. porphyrin concentration gave a straight line with intercept near zero. Thus, the value of the reverse reaction rate constant k-13is too low to be measured.
Flgure 6. Difference spectra recorded after pulse lrradatlon of aerated DPDMEFe"'OCH(CH,), solutions in P-propanoi containing acetone (0.68 M) and carbon tetrachloride (0.2 M). (a) DPDMEFe'"OCH(CH,), =2 X M, 5-mm cell, dose = 1.31 krd, time elapsed after the pulse: 0 , 9 ps, . , 72 ps. (-), (---) conventional difference spectra [DPDMEFe'"HOCH(CH,),]+ minus DPDMEFe"'OCH(CH,), by assuming M, the porphyrin concentrations are 6.23 X lo-' and 1.33 X which corresponds to yields of 4.6 and 9.8, respectively. (b) M, 1-cm cell, dose = 580 rd, time DPDMEFe"'OCH(CH3), = elapsed after the pulse: m, 1.5 ms, V, 60 ms. (---) conventlonal difference spectrum [DPDMEFe"'HOCH(CH,)2]+ minus DPDMEFe*''OCH(CH,), by assuming the porphyrin concentration is 5.91 X 10" M which corresponds to a yield of 9.8 (this spectrum and the one shown in Figure 6a (72 ps) both correspond to the end of protonation steps). (-) Calculated difference spectrum ([DPDMEFe"'HOCH(CH,)J+ DPDMEFe"'Ci) minus DPDMEFe"'OCH(CH3), by assuming concentrations in [DPDMEFe'1'HOCH(CH3)Z]+and DPDMEFe"'C1 are 2.26 X lo-* and 5.28 X lo-' M, whlch corresponds to yields of 3.75 and 8.75, respectively.
+
appeared well separated in the whole range of porphyrin concentrations. As expected, similar results were obtained with aerated solutions, but the second step was faster. Increasing the porphyrin concentration from 5 X to M led to progressive separation of the successive steps. Indeed, the rate constant of the second step was only slightly dependent on the porphyrin concentration (first-order rates increased from 2.5 X lo4to 6 X lo4 s-l when the porphyrin concentration was increased by a factor of 20) while the rate constant of the first step was found to be proportional to the porphyrin concentration. A typical time profile is given in Figure 5b. As shown in Figure 6a, these steps correspond to successive formation of (DPDMEFe111HOCH(CH3)2)+. The highest porphyrin concentrations (5 X to 1 X M) permitted monitoring of reaction 13 without interference with the subsequent chain reaction. Then, a rate constant k13 = (1.0 f 0.2) X lo9 M-' s-l was obtained, in good agreement with the previous results. The total yield of (DPDMEFe111HOCH(CH3)2)+ (i.e., including protons formed via the chain reaction) was 9.8.
Chain reactions take place in aerated or deaerated solutions even if ferric porphyrin is present, since CCl, and CC1302radicals are not expected to react efficiently with the porphyrin. Direct measurements of the reactivity of CCl, or CC1,02 radicals toward DPDMEFe"'OCH(CH3!, is possible if the protonation reactions are impeded. This
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The Journal of Physical Chemistry, Vol. 87, No.
was achieved by using either buffered aqueous 2-propanol solutions (pH 7,lO mM phosphate buffer) or 2-propanol solutions containing sodium 2-propoxide (2 mM). No transients were recorded over 1ms with the latter solutions whether they were bubbled with nitrogen or with air. A very weak signal was observed with aerated or deaerated buffered solutions but it seemed to be due to either a relatively poor efficiency of the buffer in this solvent mixture or reaction of chlorjde ions (see below). The reaction of CCl, and CCl,02 radicals with [DPDMEFe"'HOCH(CH,),]+ can be easily investigated by using 2-propanol solutions containing perchloric acid (1.2 x lo-, M). In this case, the amount of acid originating from irradiation is negligible. No transients were seen on pulse irradiation of such aerated or deaerated solutions. Thus, if any reactions occur between ferric deuteroporphyrin and CCl, or CC1302radicals (the very small dependence of the chain reaction rate upon the porphyrin concentration in aerated solutions might suggest some scavenging of CC1302 radicals by porphyrins which is currently studied under more favorable conditions), it appears that they interfere very little with the protonation reaction as described above. Furthermore, the final yield of (DPDMEFe11'HOCH(CH3)2)+ does not depend significantly on the porphyrin concentration (see following paragraph). ( b ) Chloride Ion Reactions. Contrary to what was observed with the iron free porphyrin DPDME, further slow optical changes were recorded on a millisecond time scale upon pulse irradiation of aerated or deaerated DPDMEFe'"OCH(CH,), solutions in 2-propanol-acetone-carbon tetrachloride mixtures. A typical transient is reported in Figure 5c for an aerated solution. The initial optical change corresponds to the protonation steps. As shown in Figure 6b, the differential spectrum taken 1.5 ms after the pulse, i.e., at the end of the protonation steps, is almost identical with the one reported in Figure 6a (in particular, the yields are the same) while the porphyrin concentrations differ by a factor of two. The slower positive changes seen in Figure 5c were found to be independent of the dose in the range 0.2-1.1 krd. They obeyed pseudo-first-order kinetics with a reaction rate constant of k = 55 f 15 s-l (the quite large uncertainty comes from the noisy character of the signal). The differential spectrum recorded 60 ms after the pulse (Figure 6b) is characterized by typical absorbance growths at 630 and 530 nm which are indicative of DPDMEFe"'C1 formation. However, detailed spectral analysis showed that this differential spectrum actually corresponded to a mixture of [DPDMEFemHOCH(CH3),]+ and DPDMEFe"'C1. The starting DPDMEFe'I'OCH(CH,), solution being taken as a reference, the best fit with conventional spectra was obtained by assuming that the yields of [DPDMEFemHOCH(CH3),]+and DPDMEFemCl were 3.75 and 8.75, respectively. The formation of DPDMEFe"'C1 can arise from exchange of C1- with either a 2-propanol ligand according to reaction 14 or an alkoxide ligand according to DPDMEFe"'OCH(CH,), + C1- s DPDMEFe"'C1
Brault and Neta
17, 1983
+ (CH3),CHO- (15)
The alkoxide ion formed is expected to react with the ferriporphyrin acid form according to
+ (CH3),CHO- s [DPDMEFe11'HOCH(CH3)2]+ DPDMEFe"'OCH(CH,), + (CH,),CHOH (16) Reaction 14 cannot be the major process leading to the observed changes. Firstly, in this case, second-order kinetics should be expected. Secondly, under the experi-
mental conditions of Figure 6b (i.e., dose = 580 rd), the hydrochloric acid concentration produced is expected to be (5.9-7.5) X lo4 M.50 By using the law of mass action with K14 = 3.6 X lo4 M-l, one can calculate that the yield of DPDMEFe"'C1 should range between G = 1.5 and G = 2.3, which is 17-265'0 of what is actually observed (GDPDMmeql= 8.75). Thus, the slow process depicted in Figure 5c appears to be mainly due to reaction 15. The yield of DPDMEFe"'C1 is lower than the yield of chloride ions. However as reaction 16 is expected to draw equilibrium 15 to the right, equilibrium conditions are not fulfilled. Thus it is not possible to determine either an equilibrium constant or forward and reverse rate constants for reaction 15 accurately. A value of k15 lo5 M-l s-l can be estimated from Figure 5c. Reaction 14 was easily followed by means of stoppedflow experiments. A [DPDMEFe11'HOCH(CH3)2]+ solution in 2-propanol containing acetone (0.68 M) and perM) was mixed with various hychloric acid (1.2 X drochloric acid solutions in 2-propanol containing acetone (0.68 M). The porphyrin concentration after mixing was equal to 2 X lo4 M and was kept constant. Hydrochloric acid concentration after mixing ranged from 3 X to 1.25 X loT4M. Absorbance changes were followed at 388 nm, i.e., the peak of the ferriporphyrin acid form. As expected from spectra reported in Figure 3, absorbance was found to decay. Absorbance changes were found to obey pseudo-first-order kinetics with apparent constant ranging from 74 to 169 s-l. Plotting the apparent rate constant vs. the chloride ion c ~ n c e n t r a t i o nyielded ~~ very good straight line with a slope of (1.35 f 0.06) X lo6 M-' s-l, an intercept of 35 f 4 s-l, and correlation coefficient of 0.99987, as expected from the relationship, k, = k14[Cl-] + k-14 which holds in our experimental conditions (i.e., [C1-] >> [ [DPDMEFeInHOCH(CH3)2] +]).47,48 Thus, k14 and k-14 are (1.35 f 0.06) X lo6 M-' s-l and 35 f 4 s-l, respectively. The equilibrium constant derived from these data, K14 = k14/k-14 = (3.9 f 0.6) X lo4 M-l, is in very good agreement with the value determined via conventional means. The contribution of reaction 14 to the pulse radiolysis experiment depicted in Figure 5c can now be estimated. To simplify, reaction 14 is considered independently of other process and [DPDMEFe11'HOCH(CH3)2]+ and C1concentrations are assumed to be equal to [A,]. Under these conditions, the kinetic law has an integrated form48
-
.
1
where xe and x stand for the concentrations of DPDMEFe"'C1 at equilibrium and at time t , respectively. Taking x = xe/2, the half-reaction time tlj2 is estimated to be 11-14 ms. Thus, reaction 14 does contribute to the optical changes shown in Figure 5c, but this contribution is small as outlined above. Conclusion Pulse radiolysis of aqueous and nonaqueous 2propanol-acetone-carbon tetrachloride mixtures has been investigated by means of conductivity measurements and spectrophotometric detection. In these solvent mixtures, a pulse of hydrochloric acid originating from either primary (50) These extreme values are calculated by assuming the hydrochloric acid yield in aerated Solutions is 9.8 or 12.5, respectively. The former value corresponds to the experiments performed with DPDMEFe"'OCH(CH,), solutions by assuming that all protons are scavenged. The latter value corresponds to the experiments performed with DPDME solutions.
J. Phys. Chem. 1983, 87, 3327-3333
3327
events of solvent radiolysis or further reactions of carbon tetrachloride with radicals derived from 2-propanol and acetone is produced within about 100 ns. Further production of hydrochloric acid also occurs owing to chain reactions involving hydrogen abstraction by CC13or CC1302 radicals. The latter ones appear to be much more reactive and are likely to be involved unless the solutions are thoroughly deoxygenated. Protonation reactions of porphyrins and ferriporphyrins, which do not react rapidly with CC13and CC1302radicals, were followed by means of spectrophotometricmeasurements. In turn, these reactions can be used to monitor hydrochloric acid formation. In this respect, spectrophotometric measurements on porphyrins corroborate conductivity results, although a rigorous quantitative comparison is not possible owing to differences in solvents.51 Thus, fast protonation reactions can be easily followed by means of pulse radiolysis in 2-propanol-acetonecarbon tetrachloride mixtures provided that the solutes do not react with CC13 or CC1302radicals. In particular, in rigorously deoxygenated solutions, the first proton burst is very well separated from further chain reactions, allowing straightforward kinetic analysis. This method has already
been used fruitfully at the Radiation labor at or^.^^ It favorably compares to the laser-generated pH jump described elsewhere.53 Following pulse irradiation, ligand exchange involving chloride ions has been observed in the case of ferric porphyrins, and this reaction was further investigated by means of stopped-flow measurements. This also emphasizes that, although most attention is generally given to the most reactive species produced by solvent radiolysis (e;, OH, H, and other radicals), pulse radiolysis allows the investigation of reactions of nonradical species such as H+ and OH- ions.
(51) Owing to a greater solubility of oxygen in 2-propanol, the organic solutions used to solubilize porphyrins might be more difficult to deoxygenate than aqueous solutions used for conductivity measurements.
(52) Bobrowski, K.; Das, P. K. J. Am. Chem. SOC. 1982, 104, 1704. 1981,103, (53) Gutman, M.; Huppert, D.; Pines, E. J. Am. Chem. SOC. 3709.
Acknowledgment. The authors thank Dr. E. Janata for helpful discussions and for his help with the conductivity experiments. The Radiation Laboratory of the University of Notre Dame is supported by the Office of Basic Energy Sciences of the U.S.Department of Energy. (This is document No. NDRL-2404.) Registry No. DPDME, 10589-94-3; DPDMEH’, 86163-35-1; DPDMEFem,63455-44-7; DPDMEFe*110CH(CH3)2, 80665-61-8; DPDMEFeInC1, 19442-32-1; DPDMEFemHOCH(CH3)2+, 8616336-2; 2-propanol,67-63-0; acetone, 67-64-1; carbon tetrachloride, 56-23-5.
Aqueous Solutions. 1. Structural Thermodynamic Internal Pressure of Water J. V. Leyendekkers School of Biological Sciences, Building A 12, University of Sydney#Sydney 2006, New South Wales, Australia (Recelved: September 22, 1982; In Final Form: January 3 1, 1983)
The 3-Dhydrogen bonding gives water its unique structural properties (e.g., the configuration can vary on a molecular level even at constant volume) and gives rise to an internal structural pressure, p s . This pressure provides a link between macroscopic and microscopic properties. p s has been analyzed in terms of dielectric correlations and relaxations and compared with related quantities from an infrared orientation-defect (OD) model. In the limit, as p s approaches zero, the water structure appears to approach that of ”glassy water”.
I. Introduction The success of theories like the RISM (reference interaction site model-an approximate statistical mechanical theory) suggests that our understanding of nonassociated liquids is nearly complete.’ This is far from the case with liquid water, for which a successful theory must be able to describe the effects of repulsive and attractive forces equally well. Nevertheless, over the last decade a considerable effort has been made to understand the structure of aqueous solt~ions.’-~A recent revie$ outlines (1) Chandler, D. Annu. Rev.Phys. Chem. 1978, 29, 441. (2) Enderby, J. E.; Neilson, G. W. Ado. Phys. 1980, 29, 323. (3) Luck, W. A. P. “Structure of Water and Aqueous Solutions, Proceedings of the International Symposium held at Marburg in July 1973”; Verlag Chemie: Weinheim, West Germany, 1974. (4) Franks, F., Ed. “Water: A Comprehensive Treatise”; Plenum Press: New York, 1972-1979; Vols. 1-6. (5) Luck, W. A. P. Schriftenr. Dtsch. Wollforschungsinst. (Tech. Hochsch. Aachen) 1981, 84, 215. 0022-3654183l 2087-3327$0l.50/0
the main achievements, especially those from spectroscopic studies so that only a brief assessment of the current situation is given here. Simulation methods, both mathematical and numerical, have continued to be used but are now much refined compared to when they were initiated in the 1950~.”~* Even so, the achievements have been disappointing in (6) Wertheim, M. S. Annu. Rev. Phys. Chem. 1979, 30, 471. (7) Friedman, H. L. “Thermodynamicsof Aqueous Systems with In-
dustrial Applications”;Newman, s. A., Ed.; American Chemical Society: Washington, DC, 1980;ACS Symp. Ser. No. 133; Annu. Rev. Phys. Chem. 1981, 32, 179. (8) Enderby, J. E.; Neilson, G. W. Rep. Prog. Phys. 1981, 44, 594. (9) Richards, W. G. “Water: A Comprehensive Treatise”;Franks, F., Ed.; Plenum Press: New York, 1979; Vol. 6, Chapter 3. (10) Wood, D. W. ’Water: A Comprehensive Treatise”; Franks, F., Ed.; Plenum Press: New York, 1979; Vol. 6, Chapter 6. (11) Stillinger, F. H. Science 1980, 209, 451. (12) Okazaki, Keiji; Nose, Shuichi; Kataoka, Yosuke; Yamamoto, Tsunenobu, J. Chem. Phys. 1981, 75, 5864.
0 1983 American Chemical Society
