J . Phys. Chem. 1988,92, 5097-5103
5097
Fate of Methane Radical Cations in SF,: Formation of Fluoride Adducts of Methyl Radicals and Fluorinated Methyl Radicals K. Toriyarna,* K. Nunome, and M. Iwasaki Government Industrial Research Institute, Nagoya Hirate-machi, Kita- ku, Nagoya 462, Japan (Received: November 9, 1987)
The fate of methane radical cations radiolytically produced in SF6 has been studied by electron spin resonance with CH4, CH3D, CH2D2,CHD,, and CD, together with 13CH4.Although the species formed at 4 K were not well elucidated because of their low yield and poor spectral resolution, clear evidence was obtained upon warming to 77 K for the formation of the 0 G (at 4 K) and fluoride adducts of methyl radicals, in which the I9F hyperfine coupling tensor is All = 80 G and A , the methyl proton coupling is 22 G. The 13C hyperfine coupling tensor is determined to be All = 78 G and A , = 18 G, which is nearly the same as that of free methyl radicals. It is likely that the ionization potential (IP) of CH, (IP = 12.6 eV), drastically higher than those of C2H6 (IP = 11.49eV) and other alkanes (IP 11.0eV), makes CH4+exceptionally reactive with SF6 to decompose spontaneously, in marked contrast with the other alkane radical cations, which are stable in SF6 even at 77 K. Upon further warming to 110 K, the fluoride adducts dissociate into the conventional methyl radicals. The spectra obtained from the partially deuteriated methanes indicate that both deprotonation and dedeuteriation of the methane radical cations take place with a relatively small isotope effect as is previously observed for CH3CD3+in SFs. Besides, it is found for the first time that CFD,, CF2D, and CF3 are formed from CD4 as minor products. CH4 as well as the partially deuteriated methanes also show a similar trend. These results may indicate the fragmentation of methane radical cations into CH2+,CH', and C+ in the condensed phase, although some chain reactions are not definitely excluded.
-
N
Introduction The structures and reactions of alkane radical cations may be of fundamental importance in chemistry. In 198 1, we reported the first ESR observation of C2H6+ and other prototype alkane radical cations radiolytically produced in SF6' and in perfluorocarbons., This may be the first clear and inescapable ESR although the spectroscopic evidence for alkane radical inconclusive spectrum and its intuitive interpretation had previously been reported for C2Me6+by Symons and Smith,3 followed by some controversial argument by Wang and william^,^ using Freon matrices, which was first used by Kat0 and Shida for ESR studies? SF6 SF6+ e(1) +
e-
+ SF6
-
-
+
SF6-
(2)
RH SF6 + R H + SF6+ i(3) Although in our earlier work attempts to detect'CH4+in SF6 were not successful, Knight and Steadmad have recently shown that CH4+and CH2D2+can be stabilized in a neon matrix. Their work stimulated us to elucidate the fate of methane radical cations in SF6. Since the ionization potential of SF6(15.6e v ) is higher than that of methane (12.6eV), positive hole transfer (eq 3) from SF6to a dilute solute of alkane may be possible, forming methane radical cation in our matrix, too. Unfortunately, the spectra obtained at 4 K was too poor to deduce a definite conclusion for the formation of methane radical cations. We could, however, successfully elucidate its fate after warming it up to 77 K. The methane radical cations deprotonate to form methyl radicals as is previously observed for ethane and other alkane radical cations. We have found, however, that the deprotonation begins below 77 K and that the methyl radicals are formed via the fluoride adducts of methyl radicals formed as an intermediate state. It is suggested that the fluoride adduct is a o* complex of a methyl radical with FX, which is formed from the ion-molecule reaction of CH4+with SF6, as will be discussed in a later section. Since a fluoride adduct is specific to the methyl radical and has not been detected for any ( 1 ) Iwasaki, M.; Toriyama, K.; Nunome, K. J. Am. Chem. SOC.1981,103, 3591. ( 2 ) Toriyama, K.; Nunome, K.; Iwasaki, M. J . Chem. Phys. 1982, 77, 5891. (3) (a) Smith, I. G.; Symons, M. C . R. J . Chem. Res. Symp. 1979, 382. (b) Symons, M. C. R. Chem. Phys. Lett. 1980,69, 198. (4) Wang, J. T.; Williams, F. J . Phys. Chem. 1980, 84, 3156. ( 5 ) Kato, T.;Shida, T. J . Am. Chem. SOC.1979, 101, 6869. (6) Knight, L. B., Jr.; Steadman, J. J . Am. Chem. SOC.1984, 106, 3700.
alkyl radicals formed from other alkane radical cations in SFs,, the formation of this adduct is supposedly related to the instability of methane radical cations in SF,. The exceptional instability of CH4+ in SF6 may be due to the relatively high ionization potential of CH, as compared with those of other alkanes, and it must be a cause of our unsuccessful observation of a clear ESR spectrum of CH4+,which can be stabilized in neon (IP = 21.56 eV).6 A series of partially deuteriated methanes as well as 13CH4 have been studied to obtain further spectroscopic evidence for the interpretation together with the additional information about the deuterium isotope effect on the reaction of methane radical cations. The present result may provide the first ESR evidence for the ion-molecule reaction of methane radical cations with SF6 and its fate. Besides of these findings, it has been unexpectedly found that a considerable amount of CFD,, CF2D, and CF3 are formed from CD, together with CD3. This may provide the ESR evidence for the fragmentation of methane radical cations into CH2+,CH', . and C+ in the condensed phase, which is well-known in the gas-phase electron impact.
Experimental Section The samples of CHI and SF6were obtained from Takachiho Kogyo Co., and those of deuteriated and I3C-enriched homologues from MSD Canada. The frozen samples were prepared by quenching rapidly a liquid SF6solution containing a dilute solute of methane (0.05-1.8 mol 3' %) into liquid nitrogen. The greaseand mercury-free vacuum line for preparing the gaseous mixtures as well as the experimental setups for low-temperature irradiation and for computer-assisted ESR measurements are described in a previous paper., Irradiation at 4 K was carried out by X-rays and that at 77 K by y-rays. The ESR spectra were measured with a Varian E-I2and a Bruker ER-2OOD spectrometer equipped with a homemade liquid helium cryostat. Results Spectra Obtainedfrom CH, and CD4. The samples irradiated at 4.2 K exhibit poorly resolved weak spectra overlapping with the matrix background signal. This may indicate that the solute cation or its decomposed species is not efficiently formed at 4.2 K. In such cases, however, we often experience that the migration of the matrix positive hole is thermally activated upon warming to 77 K or to a higher temperature and the positive hole transfer to the solutes takes place to give clear spectra of solute cations or their decomposed Shown in Figures l a and 2a are
0022-365418812092-5097$01.50/0 0 1988 American Chemical Societv
5098
Toriyama et al.
The Journal of Physical Chemistry, Vol. 92, No. 18, 1988 a)
/\ Jll
fi
Figure 1. ESR spectra obtained from the dilute solution of CH4 in SF6: (a) observed at 77 K after X-irradiation at 4 K; (b) simulated by using the spin Hamiltonian parameters listed in Table 11; (c) observed at 77
K after annealing at 110 K for 10 min.
A,.
a)
CO, C)
Figure 3. ESR spectra obtained from the dilute solution of CH3Din SF6: (a) observed at 77 K after X-irradiation at 4 K; (b) simulated by using the parameters listed in Table I1 and the relative amount of CH2D-.FX and eH3.-FX for 3:l;(c) observed at 77 K after annealing at 110 K for 10 min; (d) simulated spectrum for c by assuming the relative amount of CH,D and CH, to be 3:l.
... FX
n
1I
PI1 _d
256
Figure 2. ESR spectra obtained from the dilute solution of CDd in SF6: (a) observed at 77 K after X-irradiation at 4 K; (b) simulated by using the spin Hamiltonian parameters listed in Table 11; (c) observed at 110 K.
the ESR spectra obtained from a dilute solute of CHI and CD4 in SF6, respectively, at 77 K after irradiation at 4 K. The two-line spectrum with a splitting of ca. 36 G obtained from CD4 cannot be explained in terms of a deuterium ( I = 1) coupling of CD4+ nor CD3, so that the origin of this splitting must be 19F with I (7) Iwasaki, M.; Toriyama, K.; Nunome, K. Faraday Discuss.Chem. SOC. 1984, 78, 19. (8) (a) Iwasaki, M.; Muto, H.; Toriyama, K.; Nunome, K. Chem. Phys. Lett. 1984, 105, 586. (b) Muto, H.; Toriyama, K.; Nunome, K.; Iwasaki, M. Chem. Phys. Lett. 1984, 105, 592.
= 1/2. The spectrum obtained from CH4 consists of 2 X four lines with a ca. 36- and 22-G splitting, respectively, so that the species must have three protons other than one fluorine. The splitting of aH = 22 G, which is slightly smaller than that of CH3, suggests that the species is a weak g* complex of the methyl radical with a fluorine-containing ligand. As shown in Figure IC, when the sample is warmed to 110 K, the 2 X four-line spectrum irreversibly changed into the conventional 1:3:3:1 four-line spectrum with aH = 23 G, supporting this interpretation together with the spectrum of CD3 (aD= 3.6 G) obtained from the doublet spectrum (Figure 2c). Spectra Obtained from CH3D,CHzDz,and CHD3. As shown in Figure 3a, the spectrum obtained from a dilute solution of CH3D in SF6at 77 K after irradiation at 4 K consists of 2 X three-line hyperfine structures with ca. 36 (1F) and 22 G (2H), which are consistent with the fluoride adduct of CHzD formed from deprotonation of the cation CH3D+. The shoulders on the outermost two lines indicate that the 2 X four-line spectrum with ca. 36 (1F) and 22 G (3H) due to the fluoride adduct of CH3 formed from dedeuteriation of CH3D+ is overlapping as shown by the stick diagram. As will be shown in a later section, the spectrum can be simulated by the statistically expected abundance ratio of 3:l by using the fluorine coupling tensor determined from the fluoride adduct of I3CH3. The apparent fluorine splitting (36 G) is close to the intermediate principal value (34 G) of the 19F hyperfine tensor as expected from the line-shape theory.9 When the sample is warmed to 110 K, the spectra of.the fluoride adducts irreversibly changed into those of CHzD and CH3 as shown in Figure 3c. The deuterium coupling constant of 3.6 G of CH2D is equivalent to aH = 23 G, supporting the spectral interpretation of the spectrum in Figure 3a as a fluoride adduct. Likewise, the spectrum obtained from CHzDz(Figure 4a) is Fmposed of the 2 X two-line spectrum of the fluoride adduct of CHDz with ca. 36 (1F) and 22 (1H) and the 2 X three-line spectrum with ca. 36 (1F) and 22 G (2H) (9) Lefebvre, R. J . Chem. Phys. 1960, 33, 1826.
Fate of Methane Radical Cations
The Journal of Physical Chemistry, Vol. 92, No. 18, 1988 5099
n
bD3
... FX
V
~ H D ... * FX
Figure 4. ESR spectra obtained from the dilute solution of CHzDzin SF6: (a) observed at 77 K after X-irradiation at 4 K (b) simulated by using the parameters listed in Table I1 and the relative amount of CHDz and CHzD to be 1:l; (c) observed at 110 K.
of the fluoride adduct of CH2D. The overlapping spectrum can be simulated by the statistically expected abundance ratio 1:l of the two adducts as shown in Figure 4b; a detailed description of them will be given in a later section. The spectrum observed at 110 K (Figure 4c) is composed of those of CHDl and CHZD of equivalent amounts as is expected from the adducts. Similarly, CHD3 in SFs gives the fluoride adducts of CD3 and CHDz as shown in Figure 5 together with the irreversible change into the isolated CD3 and CHD2. Spectra Obtained from 13CH4.As shown in Figure l a , the 2 X four-line spectrum of the fluoride adduct of CH3 exhibits the apparent six-line feature because of insufficient resolution at 77 K. This six-line feature becomes eight-line due to the additional 13C splitting in the fluoride adduct of 13CH3as shown in Figure 6a. From the increase of the overall splitting of the spectra, the approximate 13C splitting is estimated to be 34 G , which is close to the isotropic 13Ccoupling in the free I3CH3radicals.1° Upon further warming of the sample to 110 K, the typical spectrum of 13CH3composed of 4 X two lines appeared as shown in Figure 6c. Now the spectrum of the fluoride adduct of 13CH3observed at 77 K cannot be reproduced by the isotropic simulation. So, the spectrum was observed at 4.2 K to find the anisotropy of the I9F and I3C hyperfine couplings. As shown in Figure 7a, the overall spectral width is extremely increased a t 4.2 K by the contribution from the anisotropic hyperfine couplings, which are motionally averaged in part at 77 K. The 3 X four-line spectral feature, with the ca. 80- and 22-G splittings respectively, suggests that the parallel components of both the 19F and 13C coupling tensors are ca. 80 G and the two tensors are coaxial as is expected for the m* complex as shown in Figure 8a, where X denotes the rest of the molecule of the fluoride ligand. The four-line substructure with 22 G is obviously due to the three methyl proton couplings. The strong central four-line feature with ca. 22 G suggests that the perpendicular components of both the I9F and (10) Fessenden, R. J . Phys. Chem. 1967, 71, 74.
Figure 5. ESR spectra obtained from the dilute solution of CHD3in SF,: (a) observed at 77 K after X-irradiation at 4 K; (b) simulated.by using the parameters listed in Table I1 and the relative amount of CD3.-FX and CHDZ-FX for 1:3; (c) observed at 77 K after annealing at 110 K for 10 min; (d) simulated for c by assuming the relative amount of CD3 and CHzD as 1:3. TABLE I: Hyperfine Coupling Tensors of 13C, 19F, and 'H in the CH3-FX Complex Obtained from the Simulation of the Spectrum Observed at 4.2 K obsd values/G
"C 19F IH
Ai,
A,
U
74.0
14.5
34.3
80.0
0
27.0 22.0
.L
.IBI. 20.0 26.7
free methyl radicals' u / G 38.0
24.0
'Observed at 110 K. TABLE 11: Partially Averaged Hyperfine Coupling Tensors of I3C and 19F at 77 K and Those Estimated from the Coupling Tensors Observed at 4 K calcd from A obsd/G obsd at 4 K/G
"C I9F
-,
Ai
A,
A,
u
Allro'
A
41.0
31.0 0
31.0
34.3 27.0
44.0
29.0
0
40.0
46.0
34.0
Io'
I3C coupling tensors are unresolvably small. From the anisotropic spectral simulation shown in Figure 7b, the observed spectrum at 4.2 K is found to be reproduced by the coaxial I9Ftensor with All = 80 G and A , = 0 G (aF = 26.7 G ) and the 13Ctensor with All = 74 G and A , = 14.5 G (ac = 34.3 G) together with u H = 22 G (see Figure 8a and Table I). The isotropic term of 13Ccoupling tensor, 34 G, indicates that some 10% of the unpaired electron is transferred to the I9F nucleus. From the dipolar term of 19Fcoupling tensor, spin density on the I9F is expected to be at least 0.06 or more. Taking the effect of spin polarization into considerations, it must be higher than 0.06, consistent with the delocalized spin estimated from the I3Chyperfine coupling. Taking the partial motional average of the I9F and 13Ctensors into consideration, the spectrum observed at 77 K is found to be
5100 The Journal of Physical Chemistry, Vol. 92, No. 18, 1988
Toriyama et al.
Figure 8. Hyperfine tensor axes of "C and I9F of the '3CH3-.'9FX complex: (a) for the rigid state observed at 4 K and (b) for the partially averaging state at 77 K.
Figure 6. ESR spectra obtained from the dilute solution of "CH4 in SF6: (a) observed at 77 K after X-irradiation at 4 K; (b) simulated by assuming the partially averaged IgFand "C hyperfine couplings listed in Table 11; (c) observed at 77 K after annealing at 110 K for 5 min.
Figure 7. ESR spectra obtained from the dilute solution of "CH4 in SF6: (a) observed at 4 K after X-irradiation at 4 K and successive annealing at 77 K for 10 min; (b) simulated by using the parameters listed in Table I.
reproduced by the anisotropic spectral simulation as shown in Figure 6b by using the coupling tensors given in Table 11. The I9F coupling tensor at 77 K is approximately axially symmetric with respect to the axis perpendicular to the F2, spin orbital. This indicates that the CH,-.FX complex is rapidly oscillating or rotating around the axis perpendicular to the weak C-F bond resulting in the partial average of the two tensor components of A l l = 80 G and A , = 0 G to give A', = 0, A12 = 34.0, and A'3 = 46.0 G. They are expected to reach an axially symmetric tensor
with A,,"' = 0 and Almt = 40 G in the rapid motional limit (Figure 8b). On the other hand, the I3C coupling tensor at 77 K is axially symmetric with respect to the axis parallel to the ClP spin orbital giving A,lro'= 41 and Alrot = 31 G. This indicates that 13CH3 is undergoing rapid tumbling, keeping the loose u* bonding with the F,, orbital, in addition to the oscillation or rotation accompanied by the F-X ligand around the axis perpendicular to the C--F bond. It must be mentioned here that the oscillation or the rotation of the complex as a hole is necessary to reduce the anisotropy of the I9F hyperfine coupling. The rotation of the ligand F-X independently of the methyl radical does not change the magnitude or the direction of the maximum coupling of I9F, because either of the two equivalent FZplone-pair orbitals comes in to overlap with the C2, unpaired electron orbital, alternatively. Contrary to this, the off-axis tumbling of the C2, orbital from the C-F bond must reduce the overlapping of the C?, and F2, lone-pair orbitals, resulting in the reduction of the bonding energy. However, the trace of the I9F coupling tensor at 77 K is the same as that at 4.2 K, so the spin density on the fluorine atom and thus the energy of the loose bonding is kept constant during the molecular motion at 77 K. This may suggest that the dynamically averaged C-F bond distance is reduced at 77 K to keep the same bonding energy as that at 4.2 K. If the potential curve is highly nonharmonic, the increase of the C-F stretching vibrational amplitude may result in the shortening of the dynamically averaged C.-F bond distance. More detailed study of such loosely bonded molecular complex may be interesting. Anisotropic Simulation of the Temperature Changes of the I2CSpectra. With the 19Fcoupling tensors at 4.2 K and 77 K determined from the fluoride adduct of I3CH3,the anisotropic simulations have been performed for the temperature changes of the spectra of a series of fluoride adducts of the I2C methyl radicals. For the cases of the partially deuteriated methyl radicals, the abundance ratios statistically expected were assumed for the deprotonated and dedeuteriated products. The spectra observed at 4.2 K and their anisotropic simulations are given in Figures 9 (for CH,, CD,) and 10 (for CHD,, CH2D2,CHD3). The anisotropic simulations for 77 K spectra are given in parts b of Figures 1-5. The satisfactory agreement warrants the 19F coup!ing tensors at 4.2 and 77 K determined from the spectrum of I3CH3.-FX as well as the interpretation of the molecular motions at 77 K. Formation of Fluorinated Methyl Radicals. Besides the methyl and deuteriated methyl radicals, a considerable amount of mono-, di-, and trifluoro methyl radicals are unexpectedly found, to be formed. As shown in Figure 11 the formation of CFD2, CF2D, and CF, from CD, are especially evident because of less interference with the spectrum of the main product CD,. The hyperfine coupling constants of these radicals are essentially the same as those reported by FessendenlO as tabulated in Table 111. The stick diagrams in Figure 11 illustrate the second-order hyperfine splittings due to the large hyperfine coupling of a-fluorine nuclei." ( 1 1) (a) Fessenden, R. W.; Schuler, R. H. J . Chem. fhys. 1965,43, 2704. (b) Iwasaki. M . J . Magn. Reson. 1974, 16, 417.
The Journal of Physical Chemistry, Vol. 92, No. 18, 1988 5101
Fate of Methane Radical Cations
10
c-i-
50 G
Figure 11. ESR spectrum of the CD4/SF6 system X-irradiated at 4 K and observed at 110 K with the sweep width of 2600 G. The receiver gain of the spectrometer for the outer part of the spectrum 1s 10 ,times higher than that of the central part. The stick diagrams for CF,, CF2D, and CFD,, respectively, are shown beneath the spectrum.
-
\
I
\VI
25G
Figure 9. ESR spectra of FX adducts of (a) CH, and (c) CD3 radicals observed at 4 K; b and d are the simulated ones for a and c, respectively, by using the parameters obtained from the adducts of ”CH,.-FX (Table I). The lines marked by an asterisk in a are due to the free methyl radicals formed in the coagulated methane region as a minor product. The dotted line in b is the color center formed in the sample tube. n
TABLE III: Hyperfine Coupling Constants of I9F and *D of Fluorinated Methyl Radicals Formed in CD4/SF6 Observed at 110 K obsd from ref 11 aF uD (#)a UF UH CF3 144.8 142.4 CF2D 85.0 3.4 (22.5) 84.2 21.1 CFD, 63.0 3.7 (24.0) 64.3 22.2 CD3 3.7 (24.0) 23.0 “Calculated from uD by using aH/uD= 6.513.
Formation of such fluorinated alkyl radicals has not been reported previously in the solid state. The reaction mechanism of forming fluorinated methyl radicals will be discussed in a later section.
h
Discussion Origin of the Adducts and the Mechanism of Methyl Radical Formation via Its Fluoride Adducts. The present results may exclude the possibility that methyl radical is first formed from the methane radical cation and then it complexes with a fluoride ligand, because the methyl radical is formed via the fluoride adduct. We have carried out the INDO calculations for a number of candidates of the fluoride ligands,12 such as F, HF, SFs, etc. For any of these, with the C--F distances assumed to be 2.7-2.8 A, the isotropic coupling of 19Fof 24-28 G is obtained, in good agreement with the observed value. However, among these, H F and FSFs ligands having reasonable F-H or F-S distances showed the Fapspin density to be very small. This may be caused by the cancelation of the small spin delocalization to the FZplone-pair orbital by the spin polarization. So, the observed large anisotropy of the fluorine hyperfine coupling cannot be expected from these ligands. On the other hand, the F ligand shows that the spin delocalization to the Fzqlone pair orbital is fairly large as compared with the H F and SF6 ligands. One of the reasons may be in the following situation:
25G
‘I‘L.J, i
....._,‘ Figure 10. ESR spectra obtained from the dilute solutions of (a) CH3D, (b) CH2D2,and (c) CHD, at 4 K after X-irradiation at 4 K and subsequent annealing at 77 K for 10-30 min. The central singlet signals superimposed on the observed spectra are attributed to the background signal, which could not be subtracted completely. The superposed spectra drawn by dotted lines are the simulated ones by using the spin Hamiltonian parameters in Table I and the statistically expected abundance ratios for CH,-FX, CH2D--FX, CHD2--FX, and CD,.-FX.
v
0
OF- X W
n W
(12) QCPE Program 485, “GEOMO” by D. Rinaldi, is used.
5102
The Journal of Physical Chemistry, Vol. 92, No. 18, 1988
one can estimate energetics of the following ion-molecule reaction (eq 5 ) between an alkane radical cation with SF6, as illustrated in Figure 12. Combining eq 4 with eq 6 and 7 , the heat of reaction
SF,++e'+F+RH+
I
4
Toriyama et al.
I
RH+ + e--+ R H
I,(RH)
,
RH
I
14.68eV
1
I
+F
-
R
-
IP(RH)
+ H F - [D(HF) - D(RH)]
AH = +14.68 eV - IP(RH) - [D(HF) - D(RH)] SF,++HF+R-FX
Figure 12. Energy diagram of the ion-molecule reaction of alkane radical cations with SF6.
The F-atom structure I would be energetically more stable than the X-F+ structure I1 with the formal positive charge on fluorine atom as is expected from its high electronegativity. On the other hand, the spin polarization effect on the F2, lone pair orbital may not be largely different in the F and the X-F ligand, as is schematically shown in I11 and IV.
n
cm,
cm,
Thus, it seems that the fluoride anion, F, might.be the best candidate as the ligand of the CT* complex of the CH, radical:
A
X-
i
@
However, if you consider that the calculated F2, spin density in the case of FH is the difference of two small values with opposite signs, the possibility of F H being the ligand of the complex may not be excluded. The tensor orientations of I3C and I9F with the different parallel axes observed at 77 K seem to be somewhat peculiar. One loosely bonded methyl radical undergoes a tumbling motion in a relatively independent fashion with that of the solvated fluoride ligand keeping the energy of the loose bonding essentially constant. The mode of tumbling may be determined by a potential surface of loosely bonded CH3.-FX in the matrix cavity with a considerable interaction. In the present study, however, we do not get into the detail of this tumbling motion. It may be interesting to observe the detailed temperature change of the spectra from 4 to above 77 K. The exceptional instability of CH4+ in SF6 upon warming to 77 K, at which ethane and other alkane radical cations are stable, may be related to a drastically higher ionization potential of CH4 (IP = 12.6 eV) as compared with those of ethane (1 1.5 eV) and other alkanes. Using the mass spectroscopic data given by eq 4,13 SF,5
-+
SFs+
+ i' 4- e- + 1.468 eV
RH+ + SF6
-
R
+ SFS++ H F
(13) Babcock, L. M.; Steit, G. E J . Chem Phys. 1981, 7 4 , 5700.
(7)
5 is obtained to be
SF5++F+RH
n
(6)
(8)
Now, with IP(C2H6) = 11.65 eV and the bond dissociation energy difference 1.6 eV between D(HF) = 135 kcal/mol and D(C2Hs-H) = 98 kcal/mol? AH is obtained to be endothermic by +1.43 eV. This is why the radical cations of C2H6 and other alkanes with a lower IP can be stabilized in SF6without dissociation. However, in the case of CH4+,an extremely higher IP (12.6 eV) and a relatively small difference between D(CH,-H) = 104 kcal/mo16 and D(C2Hs-H) result in AH as small as +0.63 eV Although reaction 5 is still endothermic for CH4+,energy lowering by complexing of CH, with FX may be sufficient to make the reaction exothermic, exceptionallyfor CH4+as shown in Figure 12. Mechanism of the Deprotonation of Alkane Radical Cations to Form Alkyl Radicals. Thus, the exceptional instability of CH4+ in SF6 can be understood in terms of a relatively high ionization potential of CH4. Furthermore, the stability observed for C2H6+ and other alkane radical cations except CH4+ in SF6at 77 K is consistent with these energetia2 This suggests that the mechanism of alkyl radical formation above 100 K in SF6 from n-alkane radicai cations other than CH4+ is different from that of methyl radical'formation from CH4+. The lack of an observation of the fluoride adduct of alkyl radicals other than methyl radicals may also provide other evidence for the discrimination of the reaction of CH4+with SF6, although it is not conclusive because the reactions of other alkane radical cations to form alkyl radicals in SF6 proceed at higher temperature, at which the dissociation of the fluoride adduct of the methyl radical occurs. Another reason for the discrimination of methane radict.1 cation is the fact that the formation of the fluoride adduct of CH, and its dissociation into free CH, is independent of solute concentration, whereas the deprotonation reactions of other alkane radical cations is dependent upon the solute c o n c e n t r a t i ~ n .This ~ ~ may provide more conclusive evidence for the discrimination of the reaction of CH4+. If solvated F is present below 77 K, one may have to seriously consider the following charge-recombination reaction for the mechanism of deprotonation of alkane radical cations in SF,: SF6RH+
f
+F
SFs +
R
F
(9)
+ HF
(10)
The dependence of R formation on the solute concentration observed in our previous work may be interpreted in terms of that of the averaged separation of SF6- and RH+, that is, the encounter probability of the positive hole with the solute molecule becomes higher with increasing solute concentration, resulting in a shorter separation of RH+ from the place of ionization of SF6. On the other hand, the travel distance of ejected electron to from SF6may not be much affected by the solute concentration. So, the averaged separation of SF6- and RH+ and thus the probability of charge neutralization of RH+ and F may become shorter and higher, respectively, with increasing solute concentration, as is briefly described in our previous p a p e r ~ . ~ J ~ The site preference of deprotonation may be reasonably expected because F may attack a site with high positive charge density. If this is the case, the fluorine addition radicals formed from CH2==CH2+ and CH=CH+ may be ascribable to the charge
(4) (5)
(14) Vedeneyev, V. 1.; Gurvich, L. V.; Kondrat'yev, V. N.; Medvedev, V.
A.; Framkevich, Ye. L. Bond Energies Ionization Potentials and Electron Affinities; Edward Arnold: London, 1966. (15) Iwasaki, M.; Toriyama, K.; Nunome, K. Radiar. Phys. Chem. 1983, 21, 147.
J . Phys. Chem. 1988, 92, 5 103-5 110 recombination reaction of olefin radical cation and F . Nevertheless, the possibility of an ion-molecule reaction of unrelaxed RH+ with the neighboring R H to form an alkyl radical may not be disregarded when the dimer and higher aggregates are formed at high solute concentration, because the alkyl radicals are formed from the beginning during irradiation at 4 K in the case of high solute concentration, as is described in our previous paper: I s (RH+)*
+ RH
-
R
+ RH2+
+F CH, + F-
-
-
CH,
+ HF
CH,-F
is deprotonation (dedeuteriation) to form methyl radicals. However, if the methane radical cations exhibits C,, distortion as in the neon matrix,6 H, elimination to form carbene cations can be expected from their singly occupied molecular orbital (SOMO) as shown in eq 14. H
(11)
Evidence for reaction 11 was recently obtained from the alkane radical cations formed in the zeolite channels.16 The absence of alkane radical cations as a major product in the ESR study of the radiolysis of neat crystalline alkanes may be due to reaction 11. I 7 This is supported by the selective information of chain-end alkyl radicals observed in the linear alkanes at 4 K, since the deprotonation from the chain-end CH, is expected for the alkane radical cations with an extended Contrary to this, if the ligand is F,formation of CH, by the charge neutralization of CH4+with F may be less possible since two fluoride anions are required for the formation of CH,-.F: CH4+
5103
(12) (13)
Mechanism of Formation of Fluorinated Methyl Radicals. From the discussion described in the previous section it is likely that the methane radical cations undergo ion-molecule reactions with SF6. The main reaction channel of methane radical cations (16) Toriyama, K.; Nunome, K.; Iwasaki, M. J . Am. Chem. SOC.1987, 109, 4496. (17) Iwasaki, M.; Toriyama, K.; Fukaya, M.; Muto, H.; Nunome, K. J . Phys. Chem. 1985, 89, 5278. (18) Toriyama, K.; Nunome, K.; Iwasaki, M. J . Phys. Chem. 1986, 90, 6836.
If this is the case, F transfer from SF6may produce CH2F, as is observed:
-
CH2+ + SF6 C H 2 F + SFS+ (15) So, the formation of CH2F may not be surprising. However, formations of CHF, and CF3 in such low-temperature matrices are somewhat surprising. In the gas-phase electron impact experiment~,'~ however, stripping of three and four H atoms to form CH+ and C+ fragment ions is observed as a minor process. The formation of CHF, and CF, in our experiment suggests that such fragmentations of CH4+ may take place in the SF6 matrix, although the details of reactions of these fragment cations with SF6 to form CHF, and CF, are not clear. In any event the products detected by ESR provide evidence for all the possible fragments of the methane radical cations. Since fluorinated alkyl radicals have not been detected in other alkane/SF6 systems, such a fragmentation of the alkane radical cations in SF6seems to be characteristic of only methane radical cations.
Acknowledgment. discussions.
We thank Dr. M. Okazaki for helpful
(19) Derwish, G. A.; Galli, A,; Giardini-Guidoni, A.; Volpi, G. G. J . Chem. Phys. 1964, 40, 5.
Analysis of Electron Paramagnetic Resonance Pulse Saturation Recovery Kinetics of Myoglobin Solutions P. D. Levin? and A. S. Brill*,* Biophysics Program and Department of Physics, University of Virginia, Charlottesville, Virginia 22901 (Received: November 9, 1987; In Final Form: February 17, 1988)
Recovery from microwave saturation of the EPR low-field resonance from aquo ferric myoglobins does not obey simple single-exponentialkinetics. The need, definition, and merits of a well-defined experimental timing protocol, and the generation of the residual curve, for characterizing this process are described. From analysis of data obtained in this way, spin-Hamiltonian parameters and their distributions are calculated, and saturation recovery (including residual curves) is simulated. The model for these computations is based upon independent Gaussian distributions in the energies of the two lowest excited electronic states of a four-level system. The relaxation mechanism involves the Orbach process (with a distribution in zero-field splitting energies) in parallel with the direct process. The use of measured and simulated EPR spectra and saturation recovery to quantify the parameters of the model and the dimensionality of the vibrational space effective in the Orbach process is demonstrated.
Introduction The recovery kinetics of a system subjected to saturation levels of power at resonance frequencies can provide important information regarding the energetics of that system. Often, theoretical considerations suggest that the recovery process should show 'Biophysics Program. Current address: Biology Department, Brookhaven National Laboratory, Upton, NY 11973. *Biophysics Program and Department of Physics.
0022-3654/88/2092-5103$01.50/0
single-exponential kinetics characterized by a constant, T I . Should the experimental saturation recovery behavior not conform with the expected single-exponential kinetics, the possibility of multiple species must be considered. In such an instance, the sample comprises a number of subpopulations, each being characterized by a distinct T I . The overall macroscopic kinetics represents a convolution of the rate process or processes with the distribution of values of the rate parameters. The characterization of this distribution can reveal much information concerning the under-
0 1988 American Chemical Society
