534
The Journal of Physical Chemistry, Vol. 83, No. 4, 7979
J. R. Morton, K. F. Preston, and S. J. Strach
TABLE I: Isotropic EPR Parameters f a r Radicals Observed in ?-Irradiated N,O, and N,O, in SF, at 1 1 0 K hyperfine interactions, MHz radical
g value
NO2 NO 3 X(N,O, ')
2.0000( 1y 2.0143( 1) 2 . 0 0 0 l l ( 1)
I4N
I5N
170
l9
F
152.0(6) (-)&214.0(8) 62.2(3) 10.26(3) 199.70(5 ) (- I b 279.5( 6 ) 21.74( 5 ) (-jb30.7(6) F,NO 2.00578( I) 262.86( 1) - 368.43( 8) 39.6(3) 402.1 1( 2 ) c a Numbers in parentheses are one standard deviation in the last significant figure. Negative signs correspond to an assumed positive spin density in N 2s. 19Finteraction for F,l4NO; for F,"NO, ulgF = 402.33(5) MHz.
spectral analysis of NO and NO2 in condensed phases have caused unforseen difficultie~.~ In the case of NO, detection problems undoubtedly stem from incomplete quenching of the orbital electronic motion; for NO,, efficient spin relaxation caused by spin-rotation coupling3n4may account for the absence of hyperfine structure in fluid solution spectra. In recent work in our laboratory5 we have made extensive use of the plastic phase6 of SF, for observing isotropic E P R spectra of inorganic free radicals trapped therein. The matrix is inert, and between 94 K and the melting point behaves very much as a true liquid. By y irradiating a solid solution of nitrogen dioxide in sulfur hexafluoride we have been able to generate isotropic spectra of NO2, NO3, F 2 N 0 , and a hitherto unknown species which is probably a radical ion of nitrosyl nitrate, an isomer of dinitrogen tetroxide. The spectra are reported here and discussed in detail. Experimental Section Cylinder SF,, NO, and NO2, obtained from Matheson of Canada, Ltd., were degassed and vacuum distilled before use. 15N0,99% enriched in 15N,was a product of Bio-Rad 50% enriched in I7O, was Laboratories, N.Y., and 1702, purchased from the Yeda Research Co., Israel. 15N02and N1702were prepared from these materials by the gas-phase reaction of nitric oxide with oxygen. Small amounts of N203were generated by the co-condensation a t -196 "C of equimolar quantities of NO and NO2. Sealed quartz sample tubes containing -0.1 9'0 solute in solid SF6 were y irradiated for 2 h a t -196 "C in a 9000-Ci 6oCo source. E P R spectra were recorded and measured at'llO K using a Varian E-12 spectrometer and the accessories described e l ~ e w h e r e . ~ Results The spectrum of y-irradiated 14N02in SF, a t 110 K showed (Figure 1) strong isotropic features in the g = 2 region which were not present prior to irradiation nor in an irradiated sample of pure SF6. The intense 1:1:1 50-G triplet is clearly due to the interaction of the unpaired electron with a single 14N ( I = 1)nucleus, and the g value and hyperfine interactions (Table I) show beyond all reasonable doubt8 that the species is NO2. A weaker 3.6-G 1:1:1 triplet, centred a t g = 2.014 and partially overlapped by the central lines5 of SF6,was assigned to a symmetric NO3 radical on the basis of many previous measurements.8 Immediately to low and to high field of the outer components of the NO2 spectrum a second spectrum X was detected consisting of a pair of 1:l:ltriplets (Figure l),with some suggestion of a further such triplet a t the center of spectrum. Unfortunately, the strong central line from '*NO2 and lines from SF5 prevented adequate resolution in that region of field. In order to circumvent this difficulty, the spectra of radicals enriched to 99% in 15N ( I = 1/2) were generated and examined. It was then im-
I
Figure 1. First-derivative EPR spectrum of y-irradiated NOp in SF, at 110 K.
mediately apparent that the spectrum X was due to hyperfine interactions with two inequiualent nitrogen nuclei (Table I). The spectra of NOz and of X were both observed in y-irradiated N203in SF,, but the intensity of X relative to NO2 was less than half the value measured in N02/SF6 samples. An additional, isotropic spectrum spanning almost 500 G was detected in NZo3/SF6samples. The hyperfine pattern was characteristic of one spin I = 1 together with two equivalent spins I = 112, and the spectrum was attributed to the radical F2N0. An intense spectrum of this species (along with SF,) was obtained by y irradiation of NO in SF6 and presumably arises through the stepwise addition of fluorine atoms to nitric oxide. This radical has been detected p r e v i o u ~ l yin~ UV ~ ~ ~and y-irradiated F3N0. Discussion
The absence of detectable EPR signals in N02/SF6 samples prior to y irradiation indicated that nitrogen dioxide was trapped in the host lattice entirely as the dimer N20+ Infrared studies have shownl1-l3that the latter when trapped alone or in inert matrices a t low temperatures contains significant amounts of nitrosyl nitrate ONONO? in addition to the more stable and symmetric isomer 02NN02. The paramagnetic centers detected in this study may result either from the direct decomposition of these isomers by y rays or from secondary reactions with transient species, e.g., e-, F, 0, generated within the host matrix.
EPR Spectra
01
Paramagnetic Nitrogen Oxides
Rupture of the very weak N-N and N--0 bonds in the dimers is, of course, expected and the appearance of an NO2 spectrurn (Figure 1)is no surprise. What is surprising is that this spectrum in SF6is isotropic with a peak-to-peak line width of only -5 G. The EPR spectrum of NO2 in liquid solution consists of a single line approximately 150 G wide, whereas in solid matrices it is almost invariably observed as an anisotropic s p e ~ t r u m .Only ~ when trapped in the intracrystalline voids of zeolites has it been seen as a well-resolved, isotropic t r i ~ 1 e t . lIt~ appears likely that the abnormally large spectral line width for freely tumbling NOz in liquids is due to the exceptional efficiency of spin-rotational relaxation for the radical4 This being the case, the considerable reduction in line width observed in SF, (and possibly in zeolites) may be associated with a much slower rotation of NO2 in that matrix. Between the transition temperature of 94 K and the melting point solid SF6exists a8 a plastic phase characterized by the orientational disorder and rapid molecular reorientation more typical of a liquid.6 The rotational correlation time in this phase of SF, is, however, several orders of magnitude longer than the value for true liquids.6 The simplest route to NO, (Figure 1) formation is the decomposition of nitrosyl nitrate, although an alternative pathway, via 0 atom addition to NOz, cannot be ruled out. Since the isotropic parameters measured for this species in SF, correspond very closely to those reported in various irradiated ni~trates,~ there can be little doubt as to its identity. Measurements of the 170hyperfine interaction tensor for NO3 in y-irradiated NaN03 have established15 that the radical has a threefold symmetry axis and thus is not the peroxy radical O N 0 0 postulated as an intermediate in certain gas-phase reactions of the nitrogen oxides I6 Nitric oxide, the remaining simple paramagnetic nitrogen oxide expected as a decomposition product in this study, was not expected to exhibit an observable isotropic EPR spectrum. Quenching of the electronic orbital motion in this 2~ radiical would not be appreciable in an SF6 lattice and the remaining large g anisotropy would broaden the absorption beyond detection. The similarity between the g values of radical X and of NOz and the proximity of the larger of the two 14N hyperfine interactions of X to that of NO2 suggest that radical X is a derivative of NO2. 170labeling experiments were, unfortunately, not helpful in elucidating the structure of this species since much of the pertinent region of magnetic field was obscured by lines from 170-labeled NO2. It was thus not possible to determine the number of oxygen atoms in radical X. The complete absence of the spectrum in y-irradiated N 2 0 in SF, eliminates radical ions or other derivatives of nitrous oxide as possibilities. Furthermore, it is unlikely that the species is a radical ion of N203since y irradiation of the latter in SF6 did not enhance the yield of x. A cationic formulation ONONO,' for X, in which the unpaired spin density largely resides on the dicoordinated N atom, would certainly account for the observed EPR parameters. This species can be regarded as a derivative of ONOH" (unknown), the conjugate acid of NO2, whose 14Nhyperfine interaction would be expected to exceed that of NO2 itself This is because, in converting the radical to its conjugate acid an oxygen atom is replaced by the more electronegative OH+ ligand, with consequent displacement of spin density toward the central atom. The alternative formulation ONON02- can be regarded as a derivative of oither NO;- or depending on whether the spin density is predominantly on the dicoordinated or
The Journal of Physical Chemistry, Vol. 83, No. 4,
1979 535
the tricoordinated nitrogen atom. The larger 14Nhyperfine interaction in radical X is appreciably greater than that of N032- (typically8130 MHz) and very much larger than that of NOZ2-(typically8 40 MHz). We are thereforte inclined to favor the identification of radical X as ONON02+. A remarkable feature of our experiments with y-irradiated NOz in SF, was the total absence of EPR spectra attributable to radical ions of the stable symmetric isomer of N204. It would appear that such species are unstable or undetectable under our conditions. A spectrum displaying small hyperfine interactions with two equivalent 14Nnuclei in irradiated NaN02 crystals has been ascribed by Tatano and Gesi17to N204-. For the sign choice all = 8 G, a, = -2 G, the isotropic absorption of that species in SF6 would possibly consist of a single broad line near g = 2.008, a region of our experimental spectrum (Figure 1)where it might easily lie undetected. More recently, Brown and Symonsl* have assigned certain features observed in irradiated polycrystalline N204 to N204+and Nz04-. Their analysis suggests isotropic hyperfine interactions for two equivalent 14Nnuclei of ca. 30 and 50 G for the cation and anion, respectively. This analysis must be accepted with caution, however, since many of the features of their Nz04powder spectrum could be due to NOz radical pairs.lg The intense spectrum observed in y-irradiated NO in SF6, which is undoubtedly due to the radical ONF2, was originally reported by Fessenden and Schuler.20 Due to considerable overlapping with the spectra of other radicals, they were able to discern only eight lines of the spectrum and concluded that it arose through hyperfine interactions with two equivalent 19Fnuclei and a further, unique 19F nucleus. Subsequently, we interpreted21their analysis in terms of the radical ONSF4, a derivative of SF5. This identification is now known to be erroneous. In this situdy we were able to measure with high precision 10 of the 12 possible lines and show that to well within the experimental error they all belonged to a single spectrum of one spin 1 and two equivalent spins 1/2. The least-squares data for ONFz are given in Table I. The choice of relative signs shown for the hyperfine interactions of 14N,15N,and 19Fled to the smallest standard deviations in the best-fit mean values of those parameters.22 Application of the statistical F test to the variances indicated a confidence level of better than 99.9% in that choice of relative signs. The parameters for ONF2 agree well with those reported earliergJOfor the radical observed in UV and y-irradiated F3N0. The magnitude and signs of the coupling constants are consistent with a pyramidal structure for the radical and a semioccupied molecular orbital consisting largely of a nitrogen sp3 hybrid orbital. References and Notes (1) NRCC No. 17179. (2) NRCC Research Associate 1977-1979. (3) P. W. Atkins and M. C. R. Symons, "The Structure of Inorganic Radicals", Elsevier, New York, 1967, Chapters 6 and 7. (4) G. Nybery, Mol. Pbys., 12, 69 (1967). (5) J. R. Morton and K. F. Preston, ACS Symp. Ser., No. 66, 386 (1978). (6) S. K. Gary, J. Cbem. Phys., 66, 2517 (1977). (7) A. R. Boate, J. R. Morton, and K. F. Preston, J . fhys. Chem., 80, 2954 (1976). (8) For a summary of pertinent parameters see J. R. Morton and K. F. Preston, Landolt-Bornstein New Series, Group 11, Vol. 9, Part a, "Inorganic Radicals", Springer-Verlag, Berlin, 1977. (9) K. Nishikida and F. Williams, J . Am. Cbem. Soc., 97, 7166 (1975). (IO) N. Vanderkooi, J. S. Mackenzie, and W. B. Fox, J . Nuorhe Chem., 7, 415 (1976). (11) W. G. Fately, H. A. Bent, and B. Crawford, Jr., J. Cbem. Pbys., 31, 204 (1959). (12) I. C. Hisatsune, J. P. Devlin, and Y. Wada, J. Cbem. Pbys., 33, 714 (1960). (13) R. V. St. Louis and B. Crawford, Jr., J . Cbem. fbys., 42, 857 (1'965).
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The Journal of Physical Chemistry, Vol. 83,
No. 4, 1979
6.Brocklehurst
(14) C. 8. Colburn, R. Ettinger, and F. A. Johnson, Inorg. Chem., 2, 1305 (1963). (15) A. Reuveni and 2 . Luz, J . Magn. Reson., 23, 271 (1976). (16) S. W. Benson, “The Foundations of Chemical Kinetics”, McGraw-Hill, New York, 1960, p 416. (17) J. Tatano and K. Gesi, J . Chem. Phys., 40, 1317 (1964).
(18) D. R. Brown and M. C. R. Symons, J . Chem. Soc., Dalton Trans., 1389 (1977). T. J. Schaafsma and J. Kornmandeur, Mol. Phys., 14, 525 (1968). R. W. Fessenden and R. H. Schuler, J. Chem. Phys., 45, 1845 (1966). J. R. Morton and K. F. Preston, Chem. Phys. Lett., 18, 98 (1973). R. W. Fessenden, J. Magn. Reson., 1, 277 (1969).
(19) (20) (21) (22)
Electron Tunneling in Molecular Solids. An Orbital Overlap Model Brian Brocklehurst Department of Chemistv, The University, Sheffield, S3 7HF, United Kingdom (Received August 2 1, 1978)
-
The long-range transfer processes (range, R 20-50 A) of excess electrons produced by radiolysis of molecular solids are analyzed in terms of an orbital overlap model. A united atom approximation makes possible the separation of angular and radial factors. The angular dependence is used to treat the effect of the relative orientation of donor and acceptor molecules on the transfer rate; variations of up to a factor of -1000 are predicted. A distinction is drawn between reactions of electrons with scavengers (or transfer between additives), where the significant overlap is localized around donor and acceptor sites, and recombination with a cation produced by the radiolysis, where the overlap is largely spread out over the intervening volume. In the former case, interference due to nodes in the wave functions is significant, while in the latter, various powers of R appear in the preexponential factor. The role of the Franck-Condon principle in determining the effective barrier height is discussed. Though Franck-Condon factors play a major role, electronic interaction is also important in determining transfer rates for different donors and acceptors.
Introduction Electron scavenging and electron transfer are major processes in radiolysis; their rates determine the mechanism of many reactions important in radiation chemistry and radiation biology. A considerable amount of information about them has been obtained from the study of molecular solids, especially glassy solutions, in which the electron must tunnel through the m a t r i ~ . l -Electrons ~ are localized in such systems on a molecule with greater electron affinity than the matrix, or in a “physical trap” among the matrix molecules. Descriptions in terms of delocalized states, conduction bands, etc., are not appropriate here. The processes may be classified in terms of the excess charges produced by the radiolysis on the donor, D, and acceptor, A (reactions 1-4). Reaction 1 represents electron D-+ A D + A(1) D- A+ D A(A*)
-
+
+ D- + A+ D + A+
+
D
+
+ A + hv
( 3)
+
D+ A (4) scavenging (D is a physical trap) or electron transfer between two different molecules, Reaction 2 is a recombination, usually a geminate process; i.e., the electron is thermalized and trapped fairly close to the parent cation. A large amount of energy is released in some cases and excited states are formed, or there is simultaneous emission of a photon, reaction 3. Reaction 4 represents transport of a positive hole. Reaction 3 has not been identified with certainty in molecular solids;6 reaction 4 occurs, leading to scavenging of positive charge, but there is no evidence that it involves electron jumps over large distance^.^ On the other hand, reactions 1 and 2 can take place in solids even though the donor and acceptor are separated by up to 50 A.1-4 Electron scavenging, reaction 1,has been studied extensively, mainly in polar solvents (frozen aqueous solutions, +
0022-3654/79/2083-0536$01 .OO/O
alcohols, etc.) where A may be an ion (e.g., CrO:-, Cu2+). Its occurrence in dilute solutions provided evidence for long-range tunneling and this has been confirmed by kinetic studies over a wide range of times (104-103 s) using pulse radiolysis. Reaction 2 is also important in such systems but it has been studied mainly in alkanes, especially with aromatic additives which can luminesce. The distances involved are not easily measured because the process is geminate and the distribution of trapping distances is not known, but the logarithmic time dependence of the luminescence and its occurrence a t temperatures down to 4 K provide strong evidence for electron tunneling. Some authors8agconsider that electrons do not tunnel through large distance in a single step; when D is a physical trap, this is debatable in some cases though the evidence for long-range tunneling is strong,lobut when D is a solute molecule providing a deep trap,11J2a series of short jumps can be ruled out. The simplest tunneling model involves penetration through a one-dimensional rectangular barrier. For an electron with energy E < V, the barrier height, the transmission rate, W, is given by W = v exp(-2bR) b2 = 2 m ( V - E ) / h 2 (5) If the electron collides with the barrier at a rate, u, of 1015 s-l, V - E is 1 eV, R the barrier width is 40 A, then W = 1.58 X s-l; for R = 20 A, W = 1.26 X lo6 s-l. Longrange tunneling is readily accounted for, but this would
suggest that there will be no differences between acceptors if their electron affinity is great enough, and that the only relevant property of the donor is the value of E. In practice, huge variations are found in scavenging r a t e ~ . ~ J ~One J ~ Jexplanation ~ is that Franck-Condon factors are involved. The author15 compared electron transfer to energy transfer and energy conversion processes in large molecules, applying Fermi’s golden rule in the form
w = 27T/32pq/h 0 1979 American Chemical Society
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