NOTES
2671
c H-
changes in configuration mixing in tetrahedral environments. Thus the results obtained here for copper in a distorted tetrahedron of nitrogens are not in conflict with the postulate of such an environment in copper oxidases, although we do not feel that this study provides an unambiguous proof of this structure. l8
Acknowledgment. We wish to thank Dr. J. M. Fritsch for much assistance during the course of this work.
Figure 1. The epr spectrum for (Cu,Znl-,)Hg(SCN)p for x = 0.57Yc is shown. The transition denoted by a n asterisk has been assigned to the impurity CuHg( SCN)4. The insert is a magnification of the low-field component of the parallel copper hyperfine structure for n = 0.30%. The nitrogen hyperfine structure can be readily discerned.
We were able to observe both ligands as well as copper hyperfine structure. The copper hyperfine interaction constants were found to be = 78 f 2 G and jAICUl = n(12 f 2) where n = 1, 2, or 3. The nitrogen hyperfine interaction constants were found to be lAllNl = 11 & 1 G and lAINl = 12 f 1 G. A typical spectrum which corresponds to 0.57% of the Zn sites occupied by Cu is shown in Figure 1. A magnification of the ligand hyperfine structure of the parallel component is also given. This was taken from the sample with 0.30% of the Zn sites occupied by Cu as shown in Figure 1. The observed hyperfine parameters have several points of interest: (1) the copper hyperfine interaction constants are appreciably smaller than those usually observed for copper in distorted octahedral or square planar complexes; ( 2 ) the finding of both copper and ligand hyperfine structure contrasts markedly with the study of copper dipyrromethene,6 where no fine structure could be resolved even at 1.40K;16 (3) these values are similar to those reported6 for oxidative copper enzymes ; (4)however, the copper hyperfine interaction constants observed in this study are considerably different from those observed for copper in other types of tetrahedral e n ~ i r o n m e n t . ~ ~It” appears that the copper hyperfine splitting parameters are sensitive to small (16) Bates, et al., attributed this effect to strong admixture of the 4p state into the 3d orbitals of the copper. The possibility of strong exchange narrowing was excluded because i t was thought that the copper atoms would be a t least 8 A apart owing to the size of the ligands. This may be true in this instance, but reasoning based upon steric requirements of ligands should be used with caution, since it was suggested that the palladium in bis(a,a-dipyridyliminato) palladium(I1) (which is closely related to the copper complex) should be tetrahedrally coordinated because of the steric requirements of the ligands, whereas a complete X-ray structural determination [H. C. Freeman and M. R. Snow, Acta Crystallogr., 18, 845 (1965)l showed the palladium to be surrounded by a square plane of nitrogens and that the ligands suffered gross distortion. (17) R. E. Dietz, H. Kamimura, M. D. Sturge, and A. Yariv, Phys. Rev., 132, 1559 (1963).
(18) It should be pointed out that an additional similarity exists between the blue cuproproteins and (Cu,Zn)Hg(SCN)c. The cuproproteins display an intense electronic transition ( e N 103-104 M-1 cm-1) a t about 16,000-18,000 cm-1 which has been attributed to a charge-transfer band of anomalously low energy. The (CuZn)Hg(SCN)4 system has12 a charge-transfer band a t 18,300 cm-1 and the intensity12 of this transition in the related Cu(NCS)42- ion is of a similar magnitude to that reported for blue cuproproteins.
Determination of Electron Affinities of Radicals and Bond Dissociation Energies
by Electron-Attachment Studies a t Thermal Energies-Electron
Affinity of
Acetate Radical by W. E. Wentworth, Edward Chen, and Joe C. Steelhammer Department of Chemistry, University of Houston, Houston, Texas 77004 (Received January 86, 1967)
In previous publications,l-a three mechanisms for electron attachment to molecules at thermal energies have been proposed. These mechanisms involve (I) formation of a stable molecular negative ion, (11) a single bimolecular electron-attachment step followed by immediate dissociation into a negative halide ion and an organic radical by way of a dissociative potential energy curve, and (111) a two-step dissociative process which first involves the formation of a molecular negative ion followed by a dissociative step giving generally a negative halide ion and an organic radical by way of a dissociative potential energy curve. The distinct difference of these three mechanisms can best be described in terms of potential energy diagrams as shown in an earlier publication.8 In this paper we wish to present data supporting a fourth mechanism (IV) which is identical with the third (1) W. E. Wentworth and R. 8. Becker, J . Amer. Chem. SOC.,84, 4263 (1962). (2) W. E. Wentworth, E. Chen, and J. E. Lovelock, J. Phys. Chem., 70, 445 (1966). (3) W. E. Wentworth, R. S. Becker, and R. Tung, ibid., 71, 1652 (1967).
Volume 78, Number 7 J u l y 1968
NOTES
2672
C
>-
c3 02 W
Z W -I
a: -I Z
w
IO
a.
re
INTERNUCLEAR DISTANCE Figure 1. Potential energy diagrams representing electron athchment according to mechanism IV.
mechanism above except that dissociation does not occur along a dissociative potential energy curve. This mechanism is shown in Figure 1. This fourth mechanism is important since the activation energy (E*) is a direct measure of the difference in bond dissociation energy (DAB)and the electron affinity of the radical ( E A ) . If the compound being studied is sufficiently volatile, the activation energy can generally be determined with a standard error of h0.2-0.4 kcal/mol. Therefore, if one knows the bond energy rather precisely, this method would permit a precise determination of electron affinities of radicals. On the other hand, if the electron affinity of the radical were previously known, the bond dissociation energy could be determined. In another paper,4 a technique for the determination of bond dissociation energies of some aliphatic halides is presented. However, that technique involves an empirical linear relationship between E* and (DAB-EAB),where B refers to the halide, which necessarily is an indirect method. Of the existing experimental methods for the determination of the electron affinity of radicals probably the two most important are the electron impact and the magnetron methods. The magnetron method was first developed by Sutton and Mayer6 and refined more recently by Page.6 The precision of the electron impact The Journal of Physical Chemistry
method is generally on the order of i 2 - 5 kcal/mol. The magnetron method is generally more precise than the electron impact method; however, interpretation of the results is often complicated by a complex mechanism which must be established and hence is always subject to error. If the correct mechanism is selected, then the magnetron method can give excellent results. The technique proposed in this paper should yield results which are both simple to interpret and have the desired precision. In this paper the results of three compounds leading to the electron affinity of the acetate radical will be presented. This radical was selected since a rather reliable estimate of the electron affinity has been established.’ Presently, compounds leading to the electron affinity of trifluoroacetate and nitro radicals are being investigated. Experimental Section The experimental procedure was essentially that (4) W. E. Wentworth, H. Keith, and R. George, in preparation. (5) P. P. Sutton and J. E. Mayer, J . Chem. Phys., 2, 145 (1934); 3, 20 (1935). (6) F. M.Page, Trans. Faraday Soc., 56, 1742 (1960). (7) S. Tsuda and W. H. Hamill, “Advances in Mass Spectrometry,” Vol. 111, W. L. Mead, Ed., The Institute of Petroleum, London, 1966, pp 249-257.
NOTES
2673
used in previous studies of this A Rlicrotek 2000-R gas chromatograph utilizing a 250-ft, 0.03-in. i.d. stainless steel capillary column coated with polyphenyl ether (6 rings, 250" limit) was used for all compounds. It was operated isothermally at the following temperatures : acetic anhydride, 100" ; benzyl acetate, 130"; ethyl acetate, 50". Argon was used as the carrier gas with methane added prior to the electron capture cell to make up a 10% methane mixture. A pulse width of 0.5 psec a t an interval of 1000 psec was employed in the electron capture cell to collect the free electrons. The acetic anhydride was Baker reagent grade and the ethyl and benzyl acetate were Eastman reagent grade. The polyphenyl ether was obtained from Applied Science Laboratories. It was not necessary to carry out a purification since this is accomplished satisfactorily in the gas chromatograph prior to measurement in the electron capture cell. Solutions of appropriate concentrations to inject 1-5-p1 samples were prepared using Mallinckrodt nanograde benzene. The temperature of the electron capture cell was generally elevated to the highest temperature ( = 230") and measurements were made at successive intervals as the temperature was allowed to drop. In the case of acetic anhydride, the data at lower temperatures were obtained at a later date and the data were adjusted to the high-temperature data by four data points which were common to the two regions. In all cases the span correction bo/b was used;2 however, the correction was generally on the order of 1.5 or less. The areas under the chromatographic peaks were obtained by using a Leeds and Northrup analog computer which integrates the response J ( b - [e-]/[e-l) dx.8 Results and Discussion The kinetic expression for this mechanism of electron capture is identical with the mechanism identified earlier in the paper as mechanism 111. This expression has been developed in a previous publication3 where it was referred to as the klkz/k-l mechanism. A more general model was considered at that time and various steps eliminated where they were shown to be negligible. These approximations and simplifications will be made in the present discussion. For mechanism IV the products will be designated as A- and B'. The electron attachment and negative ion reaction steps for this mechanism are ee-
+ P+ --+ IC"
neutrals
+ R' -+ R- *neutrals e- + AB ABAB- 2A- + B'
(1)
P+
kR'
(2)
B1
IC-
(3)
+ P+ ICN!: neutrals P+ + R' k"l: AB + R- + neutrals AB-
AB-
(5) (6)
Assuming steady state for the [e-] and [AB-] and an excess concentration of P+, R', and capturing species [AB] = a, the following expression can be derived
where b is the concentration of electrons when no capturing species is present. k~ = k ~ '[Pf], k R = k ~ [R], ' etc., where ICN, k ~k , ~ and 1 k ~ are 1 pseudo-firstorder rate constants. The first term in the brackets is the dissociative mechanism I11 or IV whereas the second term corresponds to the stable negative ion mechanism I. The compounds acetic anhydride, benzyl acetate, and ethyl acetate were run in this study and the temperature dependence results are shown in Figure 2. The data for acetic anhydride a t higher temperatures define a linear In KT"I" vs. 1/T plot with a negative slope corresponding to the dissociative mechanism. At lower temperature there is an upward trend of the data away from the negative slope. The data for benzyl and ethyl acetate likewise show a negative slope at higher temperatures; however, the departure at lower temperatures leads to a positive slope in both cases. The positive slope is associated with nondissociative electron capture where the second term in eq 7 has now become predominant. Since only the acetate radical has a sufficient electron affinity relative to the bond dissociation energy in these compounds, only a single negative ion potential energy curve is expected in the vicinity of the ground state of the neutral molecule. We have elected to classify this electron attachment phenomenon involving a single negative ion potential energy curve as mechanism IV. A similar temperature dependence was observed for 1-~hloronaphthalene;~ however, two negative ion potential energy curves are apparently involved, and it has been classified as mechanism 111. The two regions of temperature dependence shown in Figure 2 are important in distinguishing this mechanism from mechanism 11. In mechanism I1 the activation energy for electron attachment, El*, is greater than AE = (DAB- EAB) and dissociation should occur almost immediately upon electron attachment to the vibrationally active molecule. For mechanism I1 only a single negative slope should be observed for the temperature dependence. The
1
(4)
(8) W. E. Wentworth and E. Chen, J. Gas Chromatogr., 5, 170
(1967). Volume 72, Number 7 July 1968
NOTES
2674 -
Table I : Evaluation of Electron Affinity of Acetate Radical (2)
(4)
(6)
(6)
Molecular E A , eV
E*,
E*pssa,
kcal
kcal
Compound
Intercept
(3) -(negative slope X E), koa1
Acetic anhydride
34.86 i 0.14
7.82
