263
67Zn Chemical Shifts
Effect of Complexation of Zinc( II) on Zinc-67 Chemical Shifts Gary E. Maciei,' Larry Simeral, and Joseph J. H. Ackerman Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 (Received August 2, 1976) Publication costs assisted by the National Science Foundation
Fourier transform 67ZnNMR spectra were obtained on natural abundance samples. 67Znchemical shifts are reported for a variety of aqueous Zn(I1) solutions. The dependence of the 67Znchemical shift of a Zn(C104)2 solution as a function of added Cl- or Br- is analyzed in terms of relevant complexation equilibria. Chemical shift values of ZnX+, ZnX2, ZnXS-,and ZnX:. species (X = C1- or Br-) are derived and discussed. Introduction The Zn2+ion and its complexes are not only important in inorganic chemistry, but play a role in organic chemistry and are extremely important in biological chemistry. Yet, good physical probes, e.g., spectroscopic approaches, have not been available for delineating the role of Zn(I1) species in chemical processes, including those relevant to biological systems. For example, Zn(I1) does not exhibit convenient properties for uv-visible spectroscopy, and has no EPR spectrum. A priori, the 67Znnuclide, a spin 5/2 nuclide which is 4.1% naturally abundant, could provide a spectroscopic probe via its NMR spectra. There have been few previous reports of 67Zn NMR and they were largely oriented toward measurements of physical rather than chemical interests. The small magnetogyric ratio and low natural abundance of this nuclide, together with the expectation of wide lines because of quadrupolar relaxation, yield a relatively unfavorable case for NMR study, at least in comparison with commonly studied nuclei. Nevertheless, with the availability of modern multinuclide Fourier transform (FT) NMR technique^,^-^ and considering the potential importance of having an attractive Zn(I1) probe, a serious attempt to explore the characteristics of "Zn NMR and its possible scope of applicability is warranted, as the results of Epperlein et al. demonstrate.S This paper presents some representative 67Zn NMR results we have obtained on aqueous Zn(I1) systems, and provides the first detailed information on the sensitivity of 67Zn chemical shifts to systematic variation in the structure of Zn(I1) species in solution. Together with the results published by Epperlein et al.,3this study provides some insight into the role that FT 67ZnNMR can play in studies of Zn(I1) solution chemistry. Experimental Section (a) Measurements. "Zn NMR measurements were made in natural abundance at 5.63 MHz on a Bruker HFX-90 spectrometer, interfaced to a Digilab FTS/ NMR-3 data system, using a multinuclide configuration reported Fieldlfrequency lock was based on the 'Tsignal of an external sample of c$,F,or C4Fsplaced next to the 67Znreceiver insert. Samples were run at 300 f 0.3 K in 10-mm tubes. Typically, 20000 to 60000 free induction decays were collected for each spectrum, requiring 2000 to 6000 s. The loss of signal-to-noise ratio associated with the substantial line widths encountered because of quadrupole relaxation is to some extent offset by the increased pulse repetition rate that one can use because of the corresponding short spin-lattice relaxation times. In earlier experiments, bulk susceptibility corrections were explored by measuring (on a JEOL MH-100 spectrometer) the apparent shift differences between the
TABLE I: 67ZnChemical Shifts for Various Solutions of Zinc Salts Chemical shift, PPma Solution (in H,O) 1.0 M Zn(NO,), in 11.2 M NH, 288.2 283.6 1.0 M Zn(CN), in 3 M NaCN 1.0 M ZnCl, in 12 M HCl 256.6 0.25 M ZnC1, in 12 M HCl 253.1 1.0 M ZnBr, in 9 M HBr 168.9 0.25 M ZnBr, in 9 M HBr 171.0 ~
2.0 M 1.0 M 2.0 M 2.0 M 1.0 M
ZnC1,
Zn(NO,), Zn(C10,), Zn(NO,),
932 3 0.0 -1.4 -1.9 -35.5
65 + 5 40 + 5 37 + 5 462 5 46* 5 42+ 5 2 0 0 2 20 41 + 5 37 + 5 8 9 2 10 70+ 5
ZnI, in 12 M HI More positive values refer to lower shielding. Bulk susceptibility corrections were not applied, since they were found to be less than the experimental uncertainty. Experimental error * 0.5 ppm unless noted otherwise. Line width at half-height, Hz. a
methyl-" resonances of 3-(trimethylsilyl)propanesulfonate (sodium salt) dissolved in the sample and the 'H resonance of tetramethylsilane contained in a 1-mm capillary centered concentrically within the 5-mm NMR tube used for the measurements. All susceptibility corrections determined in this manner were found to fall within the experimental uncertainties of the 67Znchemical shifts (f0.5 ppm), and were not applied to the 67Zn data. In later experiments the 'Hresonances of water were observed on the same samples immediately before or after the 67Zn measurements were made. A total range of about 0.2 ppm was observed for the 'H chemical shifts measured, supporting our neglect of bulk susceptibility effects in these systems. (b) Materials. All chemicals were obtained from commercial sources and used without further purification. Zinc chloride, sodium bromide, and zinc nitrate were Baker, reagent grade. Zinc bromide was obtained from Matheson Coleman and Bell, reagent grade. Zinc perchlorate was from Alfa/Ventron (98.9%). Sodium perchlorate was from G. Frederich Smith Chemical Co., reagent grade. Lithium chloride, sodium chloride, concentrated HCl, and concentrated HBr were Fisher, reagent grade. Lithium bromide was Fisher, purified. Solutions of 0.5 M Zn(I1) at constant ionic strength (4.5) were prepared volumetrically from solutions of zinc perchlorate (standardized by titration with EDTA), sodium perchlorate (standardized by evaporation to constant weight at 150 "C),and the sodium salt of whichever halide was being studied (C1- or Br-). Results a n d Discussion Table I presents chemical shift and line width data for a number of 67Zn(II)resonances in solution. The total The Journal of Physical Chemistry, Vol. 81, No. 3, 1977
264
0 . E. Maciel, L. Simeral, and J. J. H. Ackerman
Scheme I
\L
1.04
I
0.78 0.48
1.86
,r
0.89 0.50
t
Zn2’ + Ln- = ZnL2-n ZnL2-n + Ln- = ZnL, 2-2n ~ ~ ~ +2 Ln- =mz n ~ , 2 - 2 n ~
~
~
+2 Ln2=
z n ~ ”
1.40
T
T
I
0.71
range of chemical shifts is in exces of 300 ppm. Such large variations in shielding are attributed to domination of the shielding constant by local paramagnetic contribution^.^*^ The solutions whose chemical shifts are summarized in Table I were chosen so as to examine qualitatively the sensitivity of the 67Znchemical shift and line width of Zn2+ to common ligands known to coordinate with zinc(I1). The high concentrations of zinc(I1) were necessitated by the low natural abundance and low NMR sensitivity of 67Zn. Concentrations of about 0.25-0.50 M solutions of zinc salts represent the lowest concentration for which a large number of spectra could conveniently be gathered in a reasonable amount of time with the instrumental configuration employed. With the use of 67Zn enriched samples, higher magnetic fields, larger sample tubes, and available electronic improvements (e.g., crystal filter,lo quadrature detection,” etc.) one should be able to extend such studies to much lower concentrations. For example, with enrichment to the 40% level, a factor of 10 in effective chemical sensitivity would be gained. A maximum improvement of between 2 and 3 orders of magnitude can be anticipated on the basis of current technology. A set of experiments was carried out in which the 67Zn resonances were observed on a series of aqueous 0.5 M Zn(C104)zsolutions containing varied concentrations of Cland with the formal ionic strength maintained at 4.5 M with NaC104. The dependence of the 67Znchemical shift on added chloride is shown in Figure 1. It must be kept in mind that the shifts of the systems reported in Table I and in Figure 1 represent weighted averages of the shifts for the various species in equilibrium in the solution, The formation constants for zinc(I1)chloride complexes are low enough that, even though the tetrahalo complex may predominate in very concentrated chloride solutions, the mono-, di-, and trihalo complexes may have appreciable concentrations for intermediate chloride concentrations. Considering zinc(I1) as being in rapid equilibrium between a hydrated state and a state in which it is bound in various complexes to a coordinating ligand, L”., the following equations can be written:
~
\L
0.56
2-4n
K,
(1)
Kl
(2)
K,
(3)
K4
(4)
where n may be 0,1,2, etc. (the magnitude of the charge of the ligand), and K , is the equilibrium constant for the indicated association reaction. Although transient species with higher coordination numbers of ligand L”- (e.g., C1-) can be envisioned, current evidence indicates that the species indicated in eq 1-4 are the most The extent to which water is involved in Zn(I1) coordination is not known unequivocally for all species of the types represented in eq 1-4, and is not represented explicitly in the symbolism. In the rapid exchange limit, the The Journal of Physical Chemistry, Vol. 81, No. 3, 1977
0
A 0
m 0
A 0
I
OA
2 .o
1.0
3.0
Ci M Figure 1. The dependence of the 67Zn chemical shift of a 0.50 M Zn(CIO& solution on added sodium chloride, with .the total Zn(I1) concentration maintained at 0.50 M and the ionic strength maintained at 4.5 M. Experimental data are represented by circles, while triangles represents values that were calculated using eq 5, and mole fractions of Zn(II)-&loride species calculated from known equilibrium constants.12 The solvent is H20.
observed shift is given by the “weighted average” expression, 6obsd = 60x0
+ 61x1 + 62X*+ 6 3 x 3 + 6 4 x 4
(5)
where X o ,X1,etc., are the respective mole fractions (with respect to total Zn(I1)) of hydrated and halide-bound zinc species as indicated in eq 1-4, and a0, etc., are the respective chemical shifts of the species indicated in eq 1-4. If, for a given total Zn(I1) concentration, one knows the time-average 67Znchemical shift and the mole fractions of the individual zinc species in solution as functions of added ligand concentration, the corresponding set of linear, weighted-average shift equations based on eq 5 can be solved for the shifts of the individual species in solution. For ionic strength 4.5 M, the stepwise and overall formation constants shown in Scheme I have been reported for the Zn(I1)-chloride system.lZ Based upon these constants and a computer program (HALTAFALL”) kindly provided by Dr. J. Christie, the curves shown in Figure 2 were generated, showing the values of Xo, XI, Xz,X3, and X4of eq 5 for the range of “total added C1- concentrations” employed in this study. From the curves of this figure, it is clear that appreciable concentrations of all five species exist in the added C1- concentration range of 1-3 M. Using X values taken from these calculations, a least-squares treatment was applied to eq 5 to determine values for bo, ?j1, b2, a3, and 64. The values obtained are: a. = 0 (arbitrary reference being Zn2+),a1 = 30 ppm (for ZnCl’), 6 2 = 295 ppm (for ZnClz), 63 = 119 ppm (for ZnCl3J, and 6 4 = 253
265
”Zn Chemical Shifts
Scheme I1
0.22 0.08
0.52 0.26
0.42
0.19
\L
2.15
0.020 0.055 1.0
7 2 . 2*
.2
1.0
-
Br M
Figure 2. The calculated dependence of the fractions of Zn(I1) present as various Zn(I1) species upon the total added CI- concentration, with the total Zn(I1) concentration maintained at 0.50 M. The solvent is H20.
ppm (for ZnC142-). From Table I it is seen that the “Zn chemical shift of a 0.25 M solution of ZnC12in concentrated HCl(l2 M), in which the dominant zinc species is believed to be ZnC142-,is also 253 ppm. Hence, the least-squares analysis appears to give reasonable results, in terms of consistency with the other data. The quality of the least-squares fit is shown in Figure 1, where both experimental points and points calculated by eq 5 are shown. A similar set of experiments was carried out for the Zn(I1)-bromide system, examining a series of solutions in which the total Zn(I1) concentration was 0.5 M, the ionic strength was maintained a t 4.5 M with NaC104, and the “total added Br- concentration” was varied up to 3 M. For this system, the previously reported formation constants are12 shown in Scheme 11. Based upon these constants, the curves shown in Figure 3 were generated, showing the Zn(I1) mole fractions of the five pertinent species as the “total added Br- concentration” is varied. It is seen from these curves that, because of small formation constants of ZnBr3- from Zn2+, ZnBr+, or ZnBr2, and because of a relatively large equilibrium constant for the formation of ZnBr42-from ZnBr3-, the concentration of the later species is small over the entire range of the figure. Using the same kind of least-squares treatment employed for the Zn(Wchloride case, the assumption of eq 5 led to the following results: a0 = 0, bl = 10 ppm (for ZnBr+),a2 = 511 ppm (for ZnBr2),a3 = -230 ppm (for ZnBr3-),and a4 = 136 ppm (for ZnBrd2-). The level of agreement between the experimental chemical shifts and those computed by eq 5 is shown in Figure 4. As the concentration of ZnBr3is so small throughout most of the range of these experiments, the quality of the fit is not sensitive to the value of 63. Hence, the value -230 ppm is probably not significant. The 67Znchemical shift given in Table I for a 0.25 M solution of ZnBrz in concentrated HBr (9 M) is 171 ppm,
2.0
30
Figure 3. The calculated dependence of the fractions of Zn(I1) present as the various Zn(I1) species upon the total added Br- concentration, with the total Zn(I1) concentration maintained at 0.50 M. The solvent is H20.
loo
75
c
0 A
I
0
A
> A
3A 0
6,50-
A 0
25
A 0
0
I
A
Flgure 4. The dependence of the 67Zn chemical shift of a 0.50 M Zn(CIO4)2 solution on added sodium bromide, with the total Zn(1I) concentration maintained at 0.50 M and the ionic strength maintained at 4.5 M. Experimental data are represented by circles, while triangles represents values that were calculated using eq 5 and mole fractions of Zn(II)-bromide species cakuhted from known equlibrium constants.12 The solvent is H20.
appreciably different from the value derived for ZnBr42from the least-squares analysis based on eq 5. There are at least two reasons that could account for this difference. First, one could question the reliability of utilizing formation constants for the calculation of concentrations in solutions of such high electrolyte concentrations, even though the ionic strength of our solutions was the same as those upon which the experiments for determining the formation constants were based. Second, it is possible that The Journal of Physical Chernisrw, Vol. 81. No. 3, 1977
266
the time-average structure of Zn(I1) in concentrated HBr, presumably a ZnBr2- species, is not precisely the same as that of a ZnBr2- species in aqueous solution without excess acid. The results described above for the Zn(I1)-chloride system, however, provide no direct support for either of these rationalizations. If, for the sake of discussion, one assumes that the 171 ppm value obtained in the concentrated HBr system should apply to the data represented in Figure 4, a least-squares analysis based on eq 5 can be carried out with a4 constrained a t the value 171 ppm. In this case, the following results are obtained: a0 = 0, a1 = 0.03 ppm, 15, = 585 ppm, a3 = -590 ppm (not meaningful, for the reasons discussed above) and, of course, 84.= 171 ppm. Both this set of values and the ones obtained directly from an unconstrained least-squares analysis (neglecting the results for ZnBr3-) are qualitatively consistent with the shielding order derived for the Zn(I1)-chloride system, i.e., Zn2+> ZnL2-n> ZnL32-3n> ZnL42-4n> ZnL2-2n. Based upon previous studies of the coordination geometries of Zn(I1)-halide species,13J4it would appear that this pattern reflects primarily differences in coordination geometry and/or coordination number. Raman studies have been interpreted in terms of an octahedral arrangement of water ligands about Zn2+ in aqueous solution, and tetrahedral structures for ZnC12and ZnBr,2-. It has been suggested, on the basis of analogy to X-ray studies of [HgC1.5H20]+,that the complexes ZnCP and ZnBr+ involve the replacement of one water molecule from octahedrally coordinated [Zn(H20)6]2' by halide The Raman results on ZnCla and ZnBr, have been interpreted in terms of linear C1-Zn-C1 and BrZn-Br arrangements, with four Zn-OH2 contacts for octahedral c00rdination.l~For ZnClC and ZnBr3-, based on Raman and solvent extraction data, trigonal bipyramidal structures with two apical water ligands and planar ZnC1, and ZnBr, systems have been proposed.14 If these assessments of structure are correct, then the octahedral Zn(I1) systems of this Zn(I1)-halide study include both the most shielded, [Zn.6H20I2+, and least shielded, [ZnC12.4H20]and [ZnBr204H20],cases, with the tetrahedral ZnC1:- and ZnBr2- species having intermediate shielding values. The values of the chemical shifts derived for the halide complexes are, of course, only as good as the equilibrium constants from which they are derived by the least-squares fitting scheme described above. It is noteworthy that the agreement cited above for the ZnC142-species is much better than that for the ZnBrd2-species. This is consistent with the fact that the uncertainties reported for the pertinent K values for the Br- complexes are generally much larger than those reported for the corresponding chloride complexes. As the equilibrium constant for the formation of ZnBrt- from the simple ions has the largest reported uncertainty of any of the eight formation constants,12 the source of the ZnBrd2-discrepancy may well be such uncertainties. When the formation constants for ZnCl+, ZnCl,, ZnC13-, and ZnCld2-were individually increased arbitrarily by 10% (the general magnitude of the reported uncertainties for the chloride complexes12),the corresponding al, d2, 8,, and a4 values changed appreciably (up to 20% for a3), but not enough to alter the order of the shifts given above. It is interesting to note that the shielding of the ZnCP species is found to be lower than that of the ZnBr+ species. This behavior, which can be viewed as manifestations of interactions of [Ar] 3d1° ions (Zn2+),is opposite to what has been attributed to the effect of ion pairing between C1- or Br- and 39K+,16an ion with the [Ar] electronic The Journal of Physlcal Chemistv, Vol. 81, No. 3, 1977
G. E. Maciel, L. Simeral, and J. J. H. Ackerman
configuration. While the bromide-chloride order is reversed for these two cases, for both Zn2+and K+ the interaction with C1- or Br- reduces the shielding of the metal nucleus, relative to its value in octahedrally hydrated Zn2+. It is also interesting that, just as the shielding of 13Cin CC14 is lower than that in CBr4,17the 67Znshielding of ZnC12is lower than that of ZnBr?-. While this correspondence may not be especially significant, it should be noted that all four of the species being compared have tetrahedral geometries. It was not possible to study the 67Znshift of Zn(NO& as a function of added cyanide or ammonia due to the low solubilities of zinc cyanide and zinc hydroxide. Zinc (at 1 M) is soluble in cyanide or ammonia only in large excess of these ligands, in which cases primarily tetracoordinate species are present.12 At the concentrations employed in the solutions shqwn in Table I, the Zn(I1) exists nearly quantitatively as the tetracoordinate, tetrahedral,l* cyanide, and ammonia species. Thus, the data given in Table I show that, as was the case with the tetrahedral tetrachloride and tetrabromide complexes, the tetracyano and tetraammine complexes have lower shieldings than the octahedral Zn2+species. Line widths were also measured for the species on which chemical shift data are given in Table I and Figures 1and 4. Large experimental uncertainties in much of the data preclude attempting a detailed interpretation. Because the 67Znline width of an individual Zn(IJ) species is expected to be dominated by the quadrupolar relaxation mechanism, which in turn depends upon the symmetry of the electric field gradient a t the nucleus (as well as the correlation time for reorientation of the species),lg one expects the narrowest lines to be associated with species that approach cubic symmetry most closely. In the rapid exchange limit, the line width of the single signal observed is a weighted average of the line widths of all the species involved in the exchange. Assuming this limit is valid for the systems for which data are summarized in Figures 1 and 4, one might expect increasing 67Znline widths as the halide ion concentration is increased, driving the Zn(I1) distribution from Zn2+(aq)to include contributions from species of lower symmetry (such as ZnX+, ZnX2, and ZnX3-). A rough trend of this type was observed (data not shown here) most definitively for the chloride case. The fact that the 67Znline width of a 2 M ZII(NO,)~solution is roughly twice that of a 2 M Zn(C104)2solution or a 1M Zn(N03)2 solution may indicate interactions, albeit transient and weak, between Zn2+and nitrate ion. Summary and Conclusions The 67Znexperiments reported here demonstrate the potential utility of 6'Zn NMR for studying the nature of Zn(I1) species in solution. Both the chemical shift and line width are sensitive to details of the environment of the 67Zn nucleus. Using available equilibrium constants for complexation, and the assumption of a weighted-average shift for a rapidly exchanging system, approximate values of the shifts of individual Zn(I1) species have been obtained, reflecting differences in coordination number and geometry. When a large number of studies of this general type on a variety of metal nuclides are described in the literature (hopefully during the next couple of years), it should be possible to develop empirical relationships that will be useful for structural studies of dynamic systems in solution and to develop theoretical guidelines to account for them.
Acknowledgment. The authors are grateful to the National Science Foundation for support of this work
Excess Electron Reactions in Liquid Hydrocarbons
under Grant No. MPS74-23980 and for grants contributing to the purchase of the spectrometer and data system. They also wish to thank Dr. Christie for his assistance with the equilibrium calculations and Dr. V. J. Bartuska for helpful discussions.
References and Notes (1) P. W. Spence and M. N. McDermott, Phys. Lett., 24A, 430 (1967). (2) B. W. Epperlein, H. Kruger, 0. Lutz, and A. Schwenk, Phys. Lett., 45A. 255 (1973). (3) B. W. Epperlein, H. Kruger, 0. Lutz, and A. Schwenk, Z Naturforsch., A, 29,660, 1553 (1974). (4)D. D. Traficante, J. A. Simms, and M. Mulcay, J. Man. Reson., 15, 484 (1974). (5) P. D. Ellis, H. C. Walsh, and C. S. Peters, J. Magn. Reson., 11, 426 (1973). (6) H. C. Dorn, L. Simeral, J. J. Natterstad, and G. E. Maciel, J Magn. Reson., 18, l(1975). (7) G. E. Maciel and J. J. H. Ackerman, J. Magn. Reson., 23, 67 (1976).
267 (8) C. J. Jameson and H. S. Gutowsky, J Chem. Phys., 40, 1714 (1964) (9)J. H. Letcher and J. R. Van Wazer, J. Chem. Phys., 44, 815 (1966). (10) A. Allerhand, R. F. Childers, and E. Oldfield, J. Magn. Reson., 11, 272 (1973). (11)E. 0.Stejskal and J. Schaefer, J. Magn. Reson., 14, 160 (1974). (12) (a) L. G. Sillen and A. E. Martell, Chem. Soc., Spec. Pub/., No. 17 (1964);(b) S.A. Shchukarev, L. S. Lilich, and V. A. Latysheve, J. Inorg. Chem. (USSR), 1, 36 (1956). (13) C. 0.Quicksall and T. G. Spiro, Inorg. Chem., 5, 2233 (1966). (14) D. F. C. Morris, E. L. Short, and D. N. Waters, J Inorg. Nucl Chem., 25, 975 (1963). (15)N. Ingri, W. Kakolowicz, L. G. Sillen, and B. Narnqvist, Talanta, 14, 1261 (1967). (16)(a) C. Deverell and R. E. Richards, MOL Phys., 10, 551 (1966);(b) E. G. Bloor and R. G. Kidd, Can. J. Chem., 50,3926 (1972). (17)J. B. Stothers, “Carbon-I3 NMR Spectroscopy”, Academic Press, New York, N.Y., 1972,Chapter 5. (18) F. A. Cotton and G. Wilkinson, “Advanced Inorganic Chemistry”, 2d ed, Interscience, New York, N.Y., 1966,p 610. (19) A. Abraham, “The Principles of Nuclear Magnetism”, Clarendon Press, Oxford, 1961,Chapter 8.
Phenomenology of Excess Electron Reactions in Liquid Hydrocarbons Robert Schiller* and Lajos Nyikos Central Research Institute for Physics, H 1525 Budapest, Hungary (Received December 18, 1975) Publication costs assisted by the Central Research Institute for Physics
The aim of the present calculation is to give a phenomenological description of the reactions between excess electrons and solutes in liquid hydrocarbons. Reaction rates and their temperature dependence are quantitatively described in terms of the Noyes theory of diffusion-controlledreactions by making use of the theory of partial electron localization and by adding a conjecture to the established picture, i.e., that the reactivity of the electrons depends on their states.
Introduction Excess electrons in liquid hydrocarbons are highly reactive upon a number of solutes. The second-order rate constants were measured as falling between 10l1 and several times 1014M-I s-l, values which were much higher than those usually observed in liquids and for ionic or atomic reactants. This seems to harmonize well with the fact that electrons in saturated hydrocarbons move much faster than do heavy ions. A closer investigation, however, reveals that the electrons behave paradoxically. Apparent chemical rate constants, k,, are usually found to be either independent of reactant mobility, p, or to change monotonically (most often proportionally) with it. The independence of k, and p corresponds to a rate of chemical transformation much slower than that of diffusion, while their proportionality indicates diffusion to be much slower than chemical transformation. This latter is the limiting case of diffusion-controlled chemical reactions. The apparent diffusion-controlled rate constant is given by the general expression of Noyesl as k
k, =
1+
ek 47rkTRp
Here k is the “genuine” rate constant, i.e., the value one would observe if the reactants moved extremely fast; e the elementary charge; k the Boltzmann constant, -T the temperature, and R the reaction radius. The diffusivity, D, has been replaced by the mobility by virtue of the
relationship D = kTp/e, p denoting the sum of the reactant mobilities. Electron reactions, in spite of their high rate, do not seem to obey eq 1, neither in its general form nor as an approximate proportionality between k, and 1.1. In these cases p is equal, to a fair degree of accuracy, to electron mobility since the heavy electron-acceptor molecules move with a velocity negligible in comparison to that of the electrons. Beck and Thomas2 and Allen, Gangwer, and Holroyd3 measured the rates in different hydrocarbons. The rate constant with a given solute was found to vary proportionally to a noninteger power of electron mobility measured in the liquids, e.g., k, appeared to be proportional to po.8or p0.5depending on the solute. In several cases the k , vs. p function4 exhibited a maximum a t a certain mobility, pm. The activation energy of a given reaction was observed to be positive in solvents of low electron mobility, i.e., where p was lower than pm, and to be negative in the opposite case. The aim of the present work is to attempt a rationalization of these phenomena unexpected of conventional reactants and reactions. We intend to demonstrate the Noyes expression as holding for the electron reactions if one only considers the electrons to be in different states in different liquids. The real nature of interaction between electron and solute will not be tackled here, the treatment being given in terms of phenomenological reaction kinetics. Electrons in liquids are now generally acknowledged to be either in the quasi-free or in the localized state. The first of these two is described as a plane-wave scattered by the molecules, hence in this state the electron exhibits high mobility and does not distort the medium. The The Journal of Physical Chemistry, Vol. SI,No. 3, 1977
