Decay of Methyl Radical-Bromide Ion Pairs in Acetonitrile a
b
The Journal ofPhysical Chemistry, Vol. 83, No. 7, 1979 849
benzene cation also indicate that the unpaired electron in the cation occupies neither one of the pz orbitals but rather the u orbitals of the constituent atoms (u radical) and the spin densities at the meta positions are not equivalent. These theoretical results are consistent with the characteristics of the BNB cations. The spin densities in the anion and the cation of nitrobenzene are shown in Figure 7. The molecular orbitals derived by the INDO method provide us with a good reason to conclude that the BNB anion is a K radical while the cation is a u radical.
C
NB
HA:A
0C02l
-00305
IC 0022;
(000'7)
1 -0.0044 (00082)
(C 0075)
0 000L
Figure 7. Molecular geometries of (a) anion, (b) neutral, and (c)cation of nitrosobenzene (NB) corresponding to the energy minimum determined by INDO calculation. The spin densities on hydrogen Is, nitrogen 2% 2p, and oxygen 2s and !:p atomic orbRals are shown. The dotted circles represent the nitrogen and oxygen 2p, atomic orbitals. The values in
parentheses represeni the experimental spin densities which were derived using the magnetic parameters listed by B. A. Goodman and J. B. Raynor, Adv. Inorg. Chem. Radiochem., 13, 135 (1970).
compounds originate from the difference in the orbitals occupied with an unpaired electron, and they proposed a u type radical for the cation and a K radical for the anion. Thus, it is worthwhile to try MO calculations for these ions and to compare the theoretical results with the observed one. Although INDO calculations for the BNB ion radicals are most desirable, only theoretical estimations of the coupling constants of a simple compound, a nitrosobenzene, as a model for BNB, were able to be done due to the limited capacity of the computer available to us. The details of this INDO calculation will be published elsewhere. According to this calculation, the unpaired electron of the nitrosobenzene anion radical distributes mostly on the pz orbitals of the nitrogen and oxygen, and the spin densities a t the two meta carbons are smaller but nearly equal. It is theoretically derived, therefore, that the anion is the ir radical, in which the nitrogen coupling is not so large and the two meta hydrogens couple weakly but equally. This is in good agreement with the experimental results. Theoretical calculations on the nitroso-
Acknowledgment. The authors express their cordial thanks to both Professor Inamoto, Tokyo University, and Dr. Konaka, Shionogi Pharmacy Co., for their kind supplies of the spin traps. References and Notes (1)J. A. Wagon and F. Williams, J. Am. Chem. SOC.,94,7917 (1972). (2) S. W. Mao and L. Kevan, Chem. fhys. Lett., 24, 505 (1974). (3)S. Slick and L. Kevan, Chem. fhys. Lett., 38,505 (1976). (4) F. P.Sargent, E. M. Gardy, and H. R. Falle, Chem. fhys. Lett., 24, 120 (1974). (5) F. P. Sargent and E. M. Gardy, Can. J. Chem., 52,3645 (1974). (6) F. P. Sargent and E. M. Gardy, J . fhys. Chem., 80,854 (1976). (7) M. Shiotani, S. Murabayashi, and J. Sohma, Int. J . Radiat. fhys. Chem., 8,483 (1976). (8) N. E. Zubarev, V. N. Belevaskii, and T. Bugaenko, Proceedings of the 4th Symposium on Radiation Chemistry, Keszthely, Hugary, 1976. (9)S. Murabayashi, M. Shiotani, and J. Sohma, Chem. fhys. Lett., 48, 80 (1977). (10) S. Murabayashi, M. Shiotani, and J. Sohma, Chem. fhys. Lett., 51, 568 (1977). (11) J. E. Willard, Int. J . Radiat. fhys. Chem., 6, 325 (1974). (12)D. J. Henderson and J. E. Willard, J. Am. Chem. SOC.,91,3014 (1969). (13)T. Ichlkawa and N. Ohta, J . fhys. Chem., 81,560 (1977). (14)R. Okazaki, T. Hosogai, E. Iwadare, M. Hashimoto, and N. Inamoto, Bull. Chem. SOC.Jpn., 42,3611 (1969). (15) W. H. Hamill in "Radical Ions", E. T. Kaiser and L. Kevan, Ed., Wiley, New York, 1968. (16) G. Cauquis, M. Genies, H. Lemaire, A. Rassat, and J. P. Ravet, J . Chem. fhys., 47,4642 (1967). (17) S. Murabayashi, M. Shiotani, and J. Sohma, unpublished data. (18) K. Tsuji and F. Williams, J . fhys. Chem., 72,3884 (1968). (19)W. G. French and J. E. Willard, J. fhys. Chem., 74,240 (1970). (20) G.E. Johnson and A. C. Albrecht, J. Chem. fhys., 44,3162(1966). (21) J. B. Gallivan and W. H. Hamill, J . Chem. fhys., 44, 1279 (1966). (22) M. Iwasaki and K. Toriyama, J. Chem. fhys., 46,2852 (1967). (23) P. B. Ayscough and E. P. Sargent, J. Chem. SOC.B , 907 (1966).
Electron Spin Resonance Study of the Decay of Methyl Radical-Bromide Ion Pairs in Acetonitrile at Low Temperature Estel D. Sprague Depatfment of Chemistry, University of Cincinnati, Cincinnati, Ohio 4522 I (Received September 14, 1978)
Methyl radicals adjacent to cyanide ions are generated upon photolysis of y-irradiated, crystalline acetonitrile, and these radicals decay by hydrogen atom abstraction from neighboring matrix molecules. In this paper the kinetic consequences of a slight alteration in the immediate environment of the methyl radical are examined. It is found that replacement of the cyanide ion with a bromide ion causes the observed rate constant for abstraction to be lower by about a factor of 2 at low temperatures. Once again, the low activation energy and large deuterium kinetic isotope effect are indicative of very important tunnel effects in the abstraction reaction. Introduction It was discovered a few years ago that methyl radicalhalide ion pairs were obtained by dissociative electron capture from methyl bromide or methyl chloride acting as electron scavengers in y-irradiated, crystalline acetonitri1e.l In subsequent work, these results have been confirmed, and ESR spectra have been assigned to similar 0022-3654/79/2083-0849$0 1 .OO/O
species in a number of systems.2-12 While the presence of the halide ions led to striking modifications of the methyl radical ESR spectrum, it was believed that there was little, if any, covalent bonding between the methyl radicals and halide ions and, thus, no effect on the methyl radical geometry or reactivity. The pairs were presumed to be simply constrained to remain adjacent to each other by 0 1979 American Chemical Society
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The Journal of Physical Chemistry, Vol. 83, No. 7, 1979
the crystalline lattice in which they were located. That the methyl radicals were not distorted from planarity by interaction with the halide ions was shown clearly for CH3-.Br- by the magnitude of the 13Chyperfine ~oupling.~ It is the question of reactivity which is addressed in this paper. The geometry and reactivity of the species formed in pure acetonitrile have been the subject of detailed investigation.13J4 Acetonitrile has two crystalline phases, designated crystal I and crystal 11, with a transition temperature of 216.9 K, approximately 12 K below the fusion temperature of 229.3 K.15 Sudden cooling of liquid acetonitrile to 77 K by immersion in liquid nitrogen results in the formation of crystal I, the high-temperature phase, metastable indefinitely at 77 K. y irradiation at 77 K produces dimer radical anions, (MeCN)2-,in crystal I and monomer radical anions, MeCN-, in crystal 11. Both are photobleachable, yielding methyl radicals adjacent to cyanide ions, with which they can recombine in the reverse of the photobleaching reaction. ESR measurements on irradiated CD313CNand 13CD3CNhave shown the methyl radicals to be planar and have revealed an extremely weak interaction between the methyl radicals and cyanide ions in crystal 11, although none was detectable in crystal I.5 In either crystalline form the methyl radicals readily abstract hydrogen atoms from neighboring acetonitrile molecules.13J6 Careful examination of this reaction in crystal I has shown it to be characterized by very large quantum mechanical tunneling correction^,^' particularly as evidenced by the exceptionally large primary deuterium kinetic isotope effect.14 In the experiments to be described, the behavior of methyl radical-bromide ion pairs in crystal I of acetonitrile is compared with that of the usual species in pure acetonitrile, ie., methyl radical-cyanide ion pairs. Of interest is not only whether the analogous reactions occur, but whether there are any significant quantitative differences in the kinetic parameters arising from the presence of the bromide ions.
Experimental Section Acetonitrile (Fisher Certified ACS grade; >9970) and acetonitrile-d, (Merck Sharp and Dohme, >99 atom 70 deuterium) were dried in vacuo over anhydrous magnesium sulfate (Matheson Coleman and Bell, reagent grade). Methyl bromide (Matheson, 99.5%) was obtained in a lecture bottle and used as received. Samples were prepared by standard high vacuum techniques in Suprasil tubes of -3 mm i.d. Mixtures containing from 7 to 25 mol % methyl bromide in acetonitrile were prepared by condensing portions of each substance at measured PVT into the sample tubes. Irradiation was carried out in a cobalt-60 y source at 77 K at a dose rate of 0.20 Mrd/h. The total dose in each case was 0.40 Mrd. ESR measurements were made at X-band on a Varian E-4 spectrometer, with the samples under liquid nitrogen in a standard quartz dewar or in the standard Varian variable temperature accessory. In the latter case, the temperature was monitored with a Doric Model 410 digital thermometer. The copper-constantan thermocouple probe was situated alongside the sample, just at the top edge of the sensitive portion of the ESR cavity. The thermometer was calibrated a t 0 “C and the boiling point of nitrogen, corrected for the prevailing barometric pressure. The temperature never varied during a kinetic run by more than a few tenths of a degree, and usually less. Sample illumination was provided, when needed, by focusing the unfiltered light from the tungsten filament of a microscope illuminator (American Optical Model 653) on the ESR
Estel D. Sprague
d
tl
Flgure 1. ESR spectrum of y-irradiated samples of 25 mol % CH,Br in CH3CN at 77 K. Irradiation dose = 0.40 Mrd. The arrows indicate features due to CH,-CN-.
cavity grid. All measurements were made with a nominal microwave power level of 0.5 mW to avoid power saturation of the signals of interest.
Results The ESR spectrum of an irradiated sample of CH3Br in CH3CN at 77 K is shown in Figure 1. The central portion of the spectrum is due mainly to CHzCN radicals. A small amount of CH3-.CN-, produced by photobleaching the few dimer anion radicals also formed, is responsible for the superposition of the four relatively sharp lines on the CH&N spectrum, marked with arrows in Figure 1. The features in the wings of the spectrum, shown at much higher amplification, arise from methyl radical-bromide ion pairs. As observed earlier,l there was often some preferential alignment in these samples, as evidenced by a small degree of orientation dependence of the spectrum. In Figure 1, the portions of the CH,-Br- spectrum seen are highly characteristic of the parallel features in polycrystalline spectra.’ In the kinetic measurements described below, time was not taken to find the “best” orientation of the sample, i.e., the one giving the spectrum most closely resembling Figure 1. In every case, nevertheless, the features belonging to CH3--Br- were easily recognizable, and these were observed as a function of time without disturbing the sample orientation. The CH3-Br- concentration was taken to be proportional to the intensity of any of these outermost features in the spectrum. That for CHzCN was determined from the strong line in the center of the spectrum, and that for CH3-CN- from the line marked with an arrow just to the right of the center. A typical kinetic experiment was carried out as follows. The sample was irradiated at 77 K in the dark for 2 h, and was then transferred in the dark as soon as possible to the ESR cavity at 77 K or some selected higher temperature. Measurements were begun 10-15 min after the end of the irradiation and continued until the reaction was substantially over. In all cases, the reactions obeyed pseudo-first-order kinetics. The intensity vs. time data were analyzed by means of simple least-squares technique@ with the Guggenheim method.lg This made it unnecessary to determine the intensities of the spectral features at infinite time, thus eliminating the need for detailed background signal corrections. The pseudo-first-order rate constants for CH,-.Br- decay obtained from a number of experiments are summarized in Table I and plotted in Figure 2. Also plotted in Figure 2 are rate constants for CH,-.CN- decay in crystal I of acetonitrile from earlier measurements.16J7 It should be pointed out that the rate constants for CH3-Br- decay cannot be determined so precisely as those for CHS-CNdecay, because the former are based on rather broad, weak spectral features measured at high amplification with
Decay of Methyl Radical-Bromide Ion Pairs
in Acetonitrile
The Journal of Physical Chemistry, Vol. 83, No. 7, 1979 851
TABLE I : Pseudo-First-Order Rate Constants for CH,. . .Br- Decay
k , minT, K
in CH,CN
77.2 89.6 95.0 100.2 113.4 124.1
0.013 ( 6 ) 0.042 0.042 0.22
___-
In CH3CN in CD,CN
0.0020 0.015 ( 2 )
considerable noise present. The value a t 77.2 K is the mean of six separate determinations, and the standard deviation calculated from this set is 0.005, about 40% of the mean value. The other table entries represent one experiment each, except that for 124.1 K, which is the mean of two values differing by about 8%. The methyl bromide concentrations used in the experiments a t 77.2 K ranged from 7 to 25 mol % , usually near 10 mol %, but no correlation with the observed rate constants was found. It was verified that crystal I of acetonitrile had been formed in these samples by observing the decay rate of the few methyl radical-cyanide ion pairs present in the dark after irradiation and/or that of those produced by subsequent photobleaching. In general, these decay rate constants agreed well with the earlier values, plotted in Figure 2, although some obtained during photobleaching were a little high. This was due to a noticeable heating in some cases where the bleaching lamp had been set up to provide the maximum possible light intensity on the sample. This did not affect the CH3.-Br- decay, which was observed in such cases in the dark before photobleaching. The CH3...CN- pairs are known to decay by hydrogen atom abstraction from adjacent acetonitrile molecules.16 This was verified by comparing the rate constant for the increase of CHzCN concentration with that for CH,...CNdecay. Similar comparisons were made in the present study, and it was found that the CH 3...Br- pairs indeed decay by hydrogen atom abstraction in CH3CN. It should be further noted that photobleaching the sample following CH3.-Br- decay ditl not regenerate any of these species. Nothing was observed except a simple decay by hydrogen atom abstraction. In CD3CN, on the other hand, deuterium atom abstraction was not observed. The CH3...Brdisappeared a t somewhat higher temperatures by an unidentified process. This is very similar to the results for CH34!N-', where an extremely large primary deuterium isotope effect was found.14 The half-life for the reaction a t 113.4 K was about 6 h, so the reaction could not be conveniently studied a t lower temperatures. At higher temperatures, on the other hand, essentially all species present began to decay, so measurements were limited to the relatively narrow range reported.
C H j - B r - In CD3CN
-*;
6 Ib
1'1 I/T x
;1 103,
I3
1' 4
15
K-I
Figure 2. Arrhenius plot of pseudo-first-order rate constants for CH3-CN- and CH,-Br- decay in acetonitrile.
fact, as was demonstrated in the initial report,' suddenly raising the temperature has the opposite effect. The interaction disappears, showing that the methyl radicals and bromide ions have completely separated. The rate constants reported here refer to hydrogen atom abstraction in CH3CN and some other, unidentified decay Process in CD3CN, and the results for CH3.-Br- are completely analogous to those for CH3-.CN-. As indicated by the slightly curved line drawn for CH3-Br- in CH&N Seems to in Figure 2, the decay behavior of CH3***Brsimply parallel that of CH3***CN-.The precision of the CH3-Br- data alone, of course, would not suffice to establish any nonlinearity. For the comparisons to be made below, it was necessary to have estimates of the rate constants, and their uncertainties, a t particular temperatures. This was accomplished, ignoring the probable slight nonlinearity in CHSCN, by finding the best straight lines for the Arrhenius plots for CH3.-Br- decay in CH,CN and in CD3CN by weighted least-squares techniques.20y21 All rate constants were used (nine in CH3CN and three in CD3CN) and were assumed to have standard deviations equal to 40% of their value, as found from the data at 77.2 K. (All rate constants a t 77.2 K were assigned the same standard deviation, equal to 40% of the mean value.) The temperatures were assumed to have standard deviations of 0.3 K. For each line the program returned values for the slope and the intercept, along with their respective variances and covariance. These were used to determine values for the rate constants and, by means of straightforward propagation of errors,18,20 their standard deviations a t 77.2 and 100 K. The results are listed in Table 11. Since the rate constant for decay of CH3-CN- in CH3CN at 77.2 K is known14 to be 0.025 f 0.003 min-l, the rate constant for CH3.-Br- decay is different by a factor of 0.47 f 0.11, On the other hand, from the least-squares results, the apparent activation energy for CH3-Br- decay in CH3CN between 77 and 100 K is 1.56 f 0.26 kcal/mol,
Discussion The most obvious difference in reactivity between CH3.-Br- and C H 3 4 W in acetonitrile lies, of course, in the ability of the latter to recombine to form genuine radical anions. Such a process was not observed for C H p B r - , in accord1 with the view that the radical anion of methyl bromide would have no intrinsic stability. In
TABLE 11: Kinetic Isotope Effect on CH,.. .Br- Decay in Acetonitrilea
T,K 100 77.2
L(CH,CN), min-' 0.12(+0.04) 0.0118( t 0 . 0 0 2 3 )
k(CD,CN),b min-' 8.4(+0.4) 3.0(+0.4)
x x
lo-*
L(CH,CN)/k(CD,CN)
kH/kDC
x lo3 3.9(+0.9)x l o 5
x lo2 1.5 x 103
1.4(+0.5)
3.1
The uncertainties are standard deviations, determined as described in the text. These values were determined by extrapolation from higher temperatures. Maximum isotope effect values calculated from eq 1 in the text.
052
The Journal of Physical Chemistry, Vol. 83, No. 7, 1979
Estel D. Sprague
which compares very favorably with the value of 1.4 kcal/mol determinedl‘j for CH3.-CN- decay in CH3CN between 77 and 87 K. Thus the abstraction rate constant for CH3-.Br- is only about half that for CH34!N-, but the temperature variation is the same, once again yielding a remarkably low apparent activation energy.16J7 Two possible explanations for the decrease in rate constant might be offered. First, there could be, after all, a very small degree of covalent bonding between the methyl radical and the bromide ion. All else being equal, an increase in activation energy of about 0.1 kcal/mol would be sufficient to halve the rate constant at these temperatures, so extensive bonding would not be necessary for this to occur. Unfortunately, the evaluation of this hypothesis would require much more precise kinetic results than are attainable in this system. Second, the bromide ion might alter the rate constant for the methyl radical reaction indirectly by slightly increasing the thickness of the potential barrier for the abstraction reaction, either by sterically crowding the methyl radical slightly less than the cyanide ion does or by slightly distorting the remainder of the lattice around the methyl radical. It has been shown that the CH,-CN- abstraction reaction occurs essentially via quantum mechanical tunneling,”J4 so the rate constant is very sensitive to the barrier thickness. In fact, an increase in barrier thickness of only about 2% would result in halving the rate ~ 0 n s t a n t . lThe ~ crystal structure is not known, however, and calculations based on this would be required to determine whether such effects might be expected upon substitution of Br- for CN-. This latter hypothesis is attractive, but a firm conclusion must await the detailed theoretical treatment mentioned. Although deuterium atom abstraction was not actually observed, the rate constants in Table I1 were used to estimate a lower limit for the primary deuterium isotope effect on the abstraction reaction, since deuterium atom abstraction must be at least as slow as the reaction which was observed. It was assumed that the line in Figure 2 for decay in CD3CN could be extrapolated linearly to the temperature range measured in CH3CN. The results are also presented in Table 11, where the observed rate constant ratios at 77.2 and 100 K are compared with the maximum ratios excluding the tunnel corrections, calculated from eq l, where kH and kD are the rate constants kH/kD =
exp(AEdRT)
(1)
for hydrogen and deuterium atom abstraction, respectively,
and aEois the difference in zero-point energy between the C-H and C-D bonds being b r ~ k e n . aEo ~ ~ for ? ~ acetonitrile ~ is about 375 cm-l (1.07 kcal/mol), as determined from measured values of the asymmetric stretching frequencies in solid CH,CN and CD3CNaZ3 The estimated lower limits for the isotope effect given in Table I1 are seen to exceed the maximum effects in the absence of tunnel corrections by factors of 4.5 at 100 K and 260 at 77.2 K. This is completely analogous to the earlier results on CH34!N- at 77 K.14 The hypothesis that these reactions occur essentially via quantum mechanical tunneling is thus supported.
Acknowledgment. The author is indebted to Professor James Anno of the University of Cincinnati, Nuclear Engineering Department for use of the y radiation source. Financial support from the Research Corporation is gratefully acknowledged. References and Notes (1) E. D. Sprague and F. Williams, J. Chem. Phys., 54, 5425 (1971). (2) Y. J. Chung and F. Williams, J. Phys. Chem., 76, 1792 (1972). (3) S. P. Mishra and M. C. R. Symons, J . Chem. Soc., Perkin Trans. 2 , 391 (1973). (4) M. C. R. Symons, J. Chem. Soc., Perkin Trans. 2 , 797 (1973). (5) E. D. Sprague, K. Takeda, J. T. Wang, arid F. Williams, Can. J. Chem., 52, 2840 (1974). (6) A. R. Lyons, M. C. R. Symons, and S. P. Mishra, Nafure(London), 249, 341 (1974). (7) Y. J. Chung, K. Nishikida, and F. Williams, J. Phys. Chem., 78, 1882 (1974). (8) Y. Fujita, T. Katsu, M. Sato, and K. Takahashi, J . Chem. Phys., 61, 4307 (1974). (9) T. A. Claxton, S. A. Fieldhouse, R. E. Overill, and M. C. R. Symons, Mol. Phys., 29, 1453 (1975). (10) D. Nelson and M. C. R. Symons, TetrahedronLeff., 34, 2953 (1975). (11) M. C. R. Symons, J. Chem. Soc., Perkin Trans. 2, 908 (1976). (12) K. Toriyama and M. Iwasaki, J . Chem. Phys., 65, 2883 (1976). (13) See ref 5 and the references contained therein. (14) E. D. Sprague, J . Phys. Chem., 81, 516 (1977). (15) W. E. Putnam, D. M. McEachern, and J. E. Kilpatrick, J. Chem. Phys., 42, 749 (1965). (16) E. D. Sprague and F. Williams, J . Am. Chem. Soc., 93, 787 (1971). (17) R. J. LeRoy, E. D. Sprague, and F. Williams, J t Phys. Chem., 76, 546 (1972). (18) P. R. Bevington, “Data Reduction and Error Analysis for the Physical Sciences”, McGraw-Hill, New York, 1969. (19) A. A. Frost and R. G. Pearson, “Kinetics and Mechanism”, 2nd ed, Wiley, New York, 1961, pp 49-50. (20) W. E. Wentworth, J. Chem. Educ., 42, 96, 162 (1965). (21) J. L. Dye and V. A. Nicely, J. Chem. Educ., 48, 443 (1971); an updated version of the program described in this article was used. (22) See, for example, R. P. Bell, “The Proton in Chemistty”, 2nd ed,Cornell University Press, Ithaca, N.Y., 1973, Chapter 12. (23) (a) D. E. Milligan and M. E. Jacox, J . Mol. Spectrosc., 8, 126 (1962); (b) E. L. Pace and L. J. Noe, J . Chem. Phys., 49, 5317 (1968).
