276
The Journal of Physical Chemistry, Vol. 83, No.
2, 1979
,'
(a1
(b)
Flgure 6. Charge and spin densities in the 2-pentafluorophenylallyl radical. (a) Spin density in the Cl-C7 fragment in the coplanar conformation. Size of the arrows are indicative of corresponding orbital populations, cf. Table 11. (b) Pictorial representation of the a HOMO of A, symmetry for the orthogonal conformation. The INDO coefficients of the atomic orbitals lying on the yaxis are zero, those for C8, C2, F2, C3, and F3 p orbitals are 0.7046, -0.0318, 0.01 15, -0.0425 and 0.0238, the remaining ones being given by symmetry.
in the 2p, orbitals of both C1 and C7 than in the sp2 orbitals in the xy plane. Upon a 90" rotation, the spin density distribution at C7 is only slightly affected; however, the two 7r subsystems being now orthogonal, the spin densities at C1 become much less anisotropic, with actually more than twice as much density in the local 0 than in the local a system. In the orthogonal conformation the pentafluorophenyl group may be naturally considered as a sort of u radical with only a reduced unpaired charge in the dangling bond. For a C-type radical, the mechanism for the transfer of spin density is spin delocalization, which is known to fall off more rapidly than the spin polarization m e c h a n i ~ mand, , ~ therefore, can explain a fluorine ortho hyperfine splitting which is larger by one order of mag-
Roger V. Lloyd
nitude than the corresponding para splitting. With this view it is not surprising that eq 1 cannot explain t,he observed splittings since at equilibrium geometry both a and (I mechanisms are important. Furthermore, it should be pointed out that also in the orthogonal conformation 2-pentafluorophenylallyl is a true T radical. From an inspection of the INDO a HOMO, as shown in Figure 5b, xy is the actual 7r plane, and, consistently with the A2 symmetry of the electronic ground state, the orbital is antisymmetric also with respect to the yz plane of the molecular system. Efforts to obtain the ESR spectrum of the parent 2phenylallyl failed so far. Additional work has been carried out in this laboratory on the substituent effects upon the isoelectronic family consisting of 2-phenylallyl, benzoylmethyl, and benzoyloxy radicals, respectively. These results will be outlined separately in a forthcoming paper. References a n d Notes (1) M. T. S.Dewar and H. C. Longuet-Higgins, f r o c . R. SOC.London, Ser. A , 214, 482 (1952). (2) R. Hoffman, R. Bissell, and D. A. Farnum, J . Phys. Chern., 73,1789 11969). (3) k. W.Fessenden and R. H. Schuler, J. Chern. fhys., 39, 2147 (1968). (4) (a) J. R. Kochi and P. J. Krusic, J. Am. Chern. Soc., 90, 7157 (1968); (b) P. J. Krusic, P. Meakin, and B. E. Smart, ibid., 96, 621 1 (1974). (5) The dependence of the isotropic hyperfine coupling constants on the torsional angle in benzyl is given in J. A. Pople and D. L. Beveridge, J . Chern. fhys., 49, 4725 (1968). (6)J. A. Pople, D.L. Beveridge, and P.A. Dobosh, J . Am. Chert. SOC., 90, 9201 (1968). (7) P. V. Schastnev and G. M. Zhidomirov, J . Struct. Chem., 10, 885 (1969). (8) R. J. Hayward and B. R. Henry, Chern. fhys. Lett., 20, 394 (1973).
Structures of Silyl-Substituted Alkyl Radicals as Studied by EPR Spectroscopy Roger V. Lloyd Department of Chemistry, Memphis State University, Memphis, Tennessee 38 152 (Received July 28, 1978) Publication costs assisted by Memphis State University
A series of substituted methyl radicals with general formula CH2Si(CH3)3-,C1,,n = 0, 1, 2, and 3, has been prepared by UV photolysis of the corresponding chlorinated precursors in cyclopropane solvent with added di-tert-butyl peroxide and triethylsilane. Analysis of the hyperfine splitting constants in the EPR spectra shows that the radicals are planar at the radical site, with the effect of added chlorine atoms being limited to small changes in the distribution of electron density. The radicals ClCHSi(CH3)2Cland Cl&Si( CH,),H were also prepared by UV photolysis and were shown to be slightly pyramidal at the radical site. Unlike 0-chloroalkyl radicals, the present radicals did not show any tendency to undergo intramolecular rearrangements involving a chlorine-atom shift.
I. Introduction There is considerable interest in the effect of substituents on the structures of alkyl radicals. In particular P-chlorine substituents can greatly affect their conform a t i o n ~ . ~Bridging, -~ defined as a distortion at the 0carbon to move a substituent toward the radical site, is believed to be important in the p-chloroalkyl radicals. Both the amount of bridging and the existence of preferred conformations, as shown by temperature-dependent effects in the EPR spectra, are determined by the degree of interaction between the p-substituents and the unpaired0022-3654/79/2083-0276$0 1.OO/O
electron orbital. In order to examine the effect of pchlorine atoms on radicals where there is no steric crowding present, we have prepared a series of carbon-centered radicals with a p-silicon substituent having the general formula .CH2Si(CH3)3_,C1,,where n = 0,1,2, and 3. From the EPR parameters of these and of several a-chloro radicals reported herein, we have been able to reach conclusions about geometries and electronic structures. The radicals were prepared by the UV photolysis of substituted silanes in cyclopropane solution, usually in the presence of di-tert-butyl peroxide and trieth~lsilane.~-' 0 1979 American Chemical Society
The Journal of Physical Chemistry, Vol. 83, No. 2, 1979 277
Silyl-Substituted Alkyl Radicals
TABLE I: EPR Parameters for Substituted Methyl Radicals line width: g precursor reactantsa radical H(a) H(CH,) 35Cl(C) 35Cl(Si) .__ G valued ----_. 0.36 2,0027 20.61 ( 2 ) 0.41 (9) ClC!H,Si(CH,),, I BOOB,TES CHzSi(CH ,), 3.42 (1) 0.40 2.00:18 ClCH,Si(CH,),Cl, I1 BOOB,TES t)H, Si(CH, )z Cle 20.68 ( 2 ) 0.68 (6,) 2.46 (2) 0.45 2.00 I 9 21.00 ( 2 ) 0.75 (3) ClCH, Si(CH, K!ln . I11 BOOB.TES CI3, Si(CH, )C1, 1.68 ( 3 ) 1.0 2.00 2 1 21.20 (2) ClCIH; Si&,, YV BOOB;TES CH,SiC1, 2.0061 3.0 (1) 3.0 (1) 1.5 ClCHSi(CH, ), C1 20.0 (1) CEH, Si(CH,),Cl, I1 3.0 (1) 3.0 (1) 1.5 2.006 1 Cl,CHSi(CH. LCl. V BOOB.TES ClCHSi(CH,), C1 20.0 (1) 1.3 2.0088 BOOB’ 9.2 (1)f 2.9 ( 2 ) Cl;CHSi(CH;j;Cl; V Cl,CSi(CH,),H Relative to Fremy’s salt, a = 13.09 G; values given to two decia BOOB = idi-tert-butyl peroxide, TES = triethylsilane. Peak-to-peak mals are k0.05 G; the others, k0.1 G. The numbers in arentheses are the number of equivalent nuclei. width of Gaussian line used in computer simulations. BRelative t o DPPH, g = 2.0036; +0.0002. e si) = 27 G. f H[fs of one proton on silicon; see text.
-a,b G
I ,
The (trimei,hylsilyl)methy17~aand the (chlorodimethylsilyl)methylg radicals have been prepared previously and our parameters are in satisfactory agreement with the literature values. There is also one report of these radicals prepared by y irradiation of the frozen silanes at 77 K, but the radicals were apparently misidentified due to the poor resolution of the spectra.1°
11. Experimental Section The precursor silanes were purchased from PCR, Inc., and used as received. Three samples were prepared from each silane; first, the silane alone, second, the silane and di-tert-butyl peroxide, and third, the silane with ditert-butyl peroxide and triethylsilane. In each case the silane or mixture was vacuum degassed by the freezepump-thaw method in a Suprasil quartz tube, cyclopropane was added as a solvent, and the tube was sealed off under vacuum. The ESR spectrometer was a Varian E-4, with a variable-temperature accessory. The portion of the flow Dewar that was in the cavity was also made of Suprasil quartz. The samples were continuously irradiated with light from a 1-kW Hanovia compact-arc Hg-Xe lamp in a Schoeffel housing. The collimated beam was focused onto the cavity with a Supraisil quartz lens. Experiments were run at -100 to -130 OC. The scan was calibrated with Fremy’s salt, and the g values were measured relative to a sample of DPPH in a capillary tube. Spectral simulations were accomplished with a first-order program running on the MSU Sigma 9 computer, with output plotted on a Houston Instruments digital. plotter. The lines were assumed to have Gaussian shapes and both chlorine isotopes were included in the calculations. Simulations were visually compared with the originals, and input parameters (hyperfine splittings and line widths) were varied until satisfactory fits were obtained.
Figure 1. First-derivative EPR spectrum of
the radical formed by UV photolysis of CICHZ8i(CH3),CI with di-tert-butyl peroxide and triethylsilane in cyclopropane at -100 O C (top)and a computer simulation based on the parameters of Table I (bottom). The broad background signal is present in the quartz Dewar.
Figure 2. First-derivative EPR spectrum of the radical formed iby UV photolysis of CICH:2Si(CH3),CIin cyclopropane at -100 O C (top) and a computer simula’tionbased on the parameters of Table I (bottom). greater intensity by direct UV photolysis of the silane (eq 3). We were not able to prepare the corresponding radicals CP 7 C1CHSi(CHB)2C1
C1CHzSi(CHJ2C1
111. Results
In this work radicals were usually formed from the chloromethylsilanes in cyclopropane (CP) by chlorine abstraction ,with di-tert-butyl peroxide (BOOB) and triethylsilane (TES). The overall reaction is shown in eq 1.
In all cases of chlorine abstraction, a chlorine atom attached to carbon rather than silicon was removed. In a few cases photolysis of the precursor with BOOB alone resulted in the expected hydrogen-atom abstraction (eq 2). The radical (eq 2) was, however, produced in much ClCH2Si(CH3)2C1
+--I
BOOB
C1CHSi(CH3)2C1
(2)
(3)
by photolysis of any other silanes. The identity of the radical product in eq 2 and 3 was confirmed when the same radical was formed by chlorine-atom abstraction according to eq 1 from ClZCHSi(CH3),C1. When the dichloromethylsilane C12CHSi(CH3)2CLwas photolyzed with BOOB, a qualitatively very different spectrum was obtained (compare Figures 1-3). It was best fitted by the aissumption that two equivalent chlorine atoms with a(C1) = 2.9 G and one proton with a(H) = 9.2 G were interacting with the unpaired electron. We assign the spectrum to the species C12CSi(CHJ2H (see Discussion). The EPR parameters for all of the radicals studied are listed in Table I. Typical first-derivative spectra with computer simulations based on the parameters of Table
278
The Journal of Physical Chemistry, Vol. 83, No. 2, 1979
t------
4G
Roger V. Lloyd
CI$SI(CH~)~H
Flgure 3. First-derivative EPR spectrum of the radical formed by UV photolysis of Ci,CHSi(CH3),CI with di- fert-butyl peroxide in cyclopropane at -1 10 OC (top) and a computer simulation based on the parameters of Table I (bottom). See text for discussion of this assignment.
I are shown in Figures 1-3. The line widths given refer to the width of the Gaussian line used in the “best fit” computer simulation in each case. It is apparent that CY or p chlorine atoms strongly increase the line widths. Because the broadening did not appear to be asymmetric, we attribute it to chlorine quadrupole interactions. IV. Discussion Formation and Stability. The formation of the ClCHSi(CH3)2C1radical by direct UV photolysis of the precursor silane was quite unexpected. It appears highly improbable that reaction 4 would occur in view of the C1CH2Si(CH3)2C1.-!% C1CHSi(CH3)2C1
(4)
relative ease of dissociation of the various bonds in the molecule, and in fact the most likely initial reaction is rupture of the C-C1 b0nd.l’ However, the CH2Si(CH3)2C1 radical was not observed and therefore was not present in any substantial steady-state concentration. We therefore propose the sequence of reactions in eq 5-8. Reaction 8 initiation C1CH2Si(CH3)&1
hu
+
propagation C1CH2Si(CH3)&1 C1-
+
-+
CH2Si(CH3)2C1+ C1
(5)
C1CHSi(CH3)2C1+ HC1 (6)
propagation termination
HC1 -kH.
+ C1.
+ R2.
RlR2
R1.
+
(7)
refers to any radical-radical recombination process. Reaction 7 is known to occur with light below 2500 A,12 which is available from our Hg-Xe lamp. The product observed in any given instance will depend on the relative efficiencies of reactions 5 and 6. In contrast to the above, for example, when the silanes C1CH2Si(CH3), and C1CH2Si(CH3)C1were similarly photolyzed, only very weak spectra of the CH2SiR3radicals were observed. The formation of radicals from BOOB and TES has been a common technique, but in many cases the 0chloroalkyl radicals that are initially produced are not observed, because of rapid chlorine migration, as shown below for the 0-chloroisobutyl radical (eq 9).2 The present CH2C(CH3)2Cl+ .C(CH3)&H2CI (9) radicals with a silicon, however, did not undergo any such intramolecular rearrangement reactions. The assignment of the spectrum shown in Figure 3 to the proposed radical is not immediately obvious. The high g value13J4and the hfs of 2.9 G for two equivalent chlorine atoms are in accord with the values for CHC12 itself (g = 2.0083, U ( ~ ~=C3.4 ~ )G, a(H) = 16.79 G), but the proton
hfs is too low for this radical and there are no other single protons in the precursor. Considering that the radical is probably nonplanar at the radical site (see below) and that hyperconjugation is less important with silicon than with carbon, we think it quite reasonable that the 0 proton on silicon has the relatively. small 9.2-G hfs. In the radicals .CH,Si(CH,),H and CH3CHSi(EtJ2H,for example, a(Hsi) decreases with increasing substitution on the radical site (14.8 and 11.8 G, respectively).16 We do not know whether the radical is produced from a small amount of C1,CHSi(CH3)2H present as an impurity (undetectable by NMR) or is a rearrangement product, but the former is much more likely in agreement with the lack of observation of silicon radical rearrangements. Structures. The results for the .CH2Si(CH3)3-,C1, radicals (Table I) show that the substitution does not greatly affect their structures, as judged by the similar hyperfine splittings throughout the series. To the extent that steric crowding is influential this is to be expected since chlorine and the methyl group are similar in size. We believe that the radicals are all substantially planar at the radical site, mostly due to substitution on the LY carbon by the less electronegative silyl groups.17 The 27-G value for the 29Sihfs in .CH2Si(CH3)&1is less than that for the 0 Si in .Si[Si(CH3)3]3(65 G)18 and similar to the 15.2 G measured in CH2Si(CH3)3,8and the latter radicals are both thought to be planar. The most striking difference between these radicals and their carbon analogues1i2is in the size of the @-chlorinehfs. In the carbon radicals it is normally 18-20 G, in contrast to our values of 2-4 G. We attribute this to the larger Si-C bond length, so that there is much less interaction between the 0-chlorine atom and the unpaired electron. The greater bond length also allows free rotation of the silyl group, so that there are no temperature-dependent line-width effects observed in the spectra and there is no preferred orientation of the chlorine atom relative to the unpaired electron orbital. However, the small but consistent trends in the various hyperfine splittings with increasing chlorine substitution show that the chlorine does have some effect. Since these are not caused by changes in geometry or preferred orientation, they are most likely the result of the bonding interaction between chlorine lone-pair orbitals and the silicon d orbitals,lgand a hyperconjugative interaction with the methyl groups. Thus contributions from the following kinds of resonance hybrids are important. +HC H ~ CH I 3 CH23i=Ci+ I
CH3
It
C H2=Si- C i I
CH3
Structures such as these have been invoked to explain the observed dipole moments and bond lengths in silicon halides and methylsilanes.20 In their effect on the g values the 0-chlorine atoms all produce the same negative g shift relative to the nonchlorinated radical -CH2Si(CH3)3.The g values are similar to that observed in the 0-chloroethyl radical (g = 2.0021),21 in which it was attributed to interaction between the unpaired-electron orbital and the chlorine lone-pair orbital. In carbon radicals more highly substituted with methyl groups, however, the g values were higher than the freespin value. This was caused by distortion at the P carbon which moved the chlorine closer to the unpaired electron (i,e., increased “bridging”) and thus increased the g value due to the spin-orbit interacti0n.l In the present radicals the effect of the longer Si-C bond is again observed, in that
The Journal of Physlcal Chemistry, Vol. 83, No. 2, 1979 279
Q-Band EPR of Vanadyl Transferrin
\
Acknowledgment. I thank Dr. David E. Wood for helpful discussions and the MSU Computer Center for computer time.
there is not any such “bridging” to give the higher g values, and they remain low throughout. The two a-chlorine radicals have almost the same achlorine hfs (3.0 and 2.9 G, Table I) and unlike the Pchlorine hfs these are similar to those observed in alkyl radicals. In the series CCl,., CH3CC12.,and (CH3)zCCl., for example, u(,~CI)is 6.25, 4.2, and 2.3 G, respectively, and the radicals go from pyramidal to planar,ls so that the value of the cy-chlorine hfs should be a fairly sensitive probe of radical-site geometry. On this basis our radicals are both slightly nonplanar. The g values are as expected much higher than the free-spin value and approximately the same i3S in analogous alkyl radical^.^ V. Conclusions We have studied a series of silicon-substituted methyl radicals by E P R spectroscopy and have examined the effects of replacing methyl groups on the P silicon with chlorine atoms. In contrast to the corresponding alkyl radicals, intramolecular rearrangements involving chlorine-atom transfer are not observed, there is free rotation about the C-Si bond, and the P-chlorine hyperfine splittings are much smaller. These are all the result of the longer C-Si bond (compared t o the C-C bond) and the decreased interaction between the P-chlorine atoms and the unpaired-electron orbital. Thus there is no evidence in favor of chlorine bridging in these radicals. The small trends observed in the various hyperfine splittings are the result of electron transfer from chlorine onto silicon.
References and Notes (1) K. A. Chen, I. H. Elson, and J. K. Kochi, J . Am. Chem. Soc., 95, 5341 (1973). (2) K. A. Chen, D. Y. H. Tang, L. K. Montgomery, and J. K. Koohl, J . Am. Chem. Soc., 96, 2201 (1974). (3) F?.V. Lloyd and D. E. Wood, J. Am. Chem. Soc., 97, 5988 (1975). (4) J. K. Kochi, Adv. Free-Radical Chem., 5, 189 (1975). (5) A. Hudson and R. A. Jackson, Chem. Commun., 1323 (‘1969). (6) P. J. Krusic and J. K. Kochi, J . Am. Chem. SOC.,91, 3938 (1969). (7) P. J. Krusic and J. K. Kochi, J . Am. Chem. SOC.,91, 6161 (1989). (8) J. H. Mackey and D. E. Wood, Mol. Phys., 18, 783 (1970). (9) J. Cooper, A. Hudson, and R. A. Jackson, Tetrahedron Left., 831 (1973). (10) J. Roncin, Mol. Cryst., 3, 117 (1967). (11) R. A. Jackson, Adv. Free-Radical Chem., 3, 231 (1969). (12) J. H. Raley, F. F. Rust, and W. E. Vaughan, J . Am. Chem. SOC., 70, 2767 (1948). (13) A. W. Bennett, C . Eaborn, A. Hudson, R. A. Jackson, and K. D. J. Root, J. Chem. SOC.A , 348 (1970). (14) M. Lehnig and H. Fischer, Z. Naturforsch., A , 27, 1300 (‘1972). (15) A. Lurid, T. Gillbro, D. Feng, and L. Kevan, Chem. phys., 7, 414 (1975). (16) A. J. Bowles, A. Hudson, and R. A. Jackson, J. Chem. SOC.B, 1947 (1971). (17) L. Pauling, J . Chem. Phys., 51, 2767 (1969). (18) J. Cooper, A. Hudson, and R. A. Jackson, Mol. phys., 23,209 (1972). (19) D. C . Frost, F. G. Herring, A. Katrib, R. A. N. McLean, J. E. [)rake, and N. P. C. Westwood, Chem. Phys. Lett., 10, 347 (1971). (20) C. Eaborn, “Organosilicon Compounds”, Butterworths, London, 1960, p 04. (21) T. Kawarnura, 0.J. Edge, and J. K. Kochi, J . Am. Chem. Soo., 94, 1752 (1972).
A Q-Band Electron Paramagnetic Resonance Study of Vanadyl(1V)-Labeled Human Serotransferrin Lawrence Keith White and N. Dennis Chasteen” Department of Chemistry, University of New Hampshire, Durham, New Hampshire 03824 (Received March 29, 1978; Revised Manuscript Received August 11, 1978) Publication costs assisted by the National Institute of General Medical Sciences
X-band (9.2 GHz) and &-band (35.0 GHz) electron paramagnetic resonance (EPR) spectra of frozen solutions of vanadyl(1V) serotransferrin are reported. The high-frequency &-band measurements reveal features otf the metal sites not previously observed. At least three metal site environments are apparent, A, R1, and B2,instead of only two, A and B, as previously reported. In earlier reports B1and B2 were referred to collectively 8s B. An increase in the pH to 10.0 brings about a conformational change A B1 + B2. The metal ion binding sites acid buffer (HEPES), DzO, are sensitive to the presence of N-(2-hydroxyethyl)piperazine-N’-2-ethanesulfonic limited bicarbonate, and sodium perchlorate and to pH. The g values and vanadium nuclear hyperfine coupling constants indicate that most of the ligands about the metal are oxygen donors. Noncoincident magnetic axes for the g and hyperfine tensors are observed for the B1 and Bzenvironments. Computer simulations of the &-band spectrum indicate, when noncoincidence is taken into account, that the directions of the A,, and g,, axes differ by 7 f 2 and 12 f 2’ for B1 and Bz, respectively. Presumably the A,, direction corresponds to the V02+ bond axis. The noncoincidence, if any, is small for the A einvironment. These results suggest that in the B1and Bz environments there may be significant distortions about the V02+ion from its normally preferred square-pyramidal or square-bipyramidal geometry in which the vanadyl oxygen occupies an apical position. This could explain the observation that at pH 6.0, V02+ binding is lost in B1 and B2 but retained in A.
-
Introduction The structural and functional heterogeneity of the two iron binding 13ites of human serum transferrin has been of considerable interest in recent years.l Recently, we labeled the two sites of transferrin with the electron paramagnetic resonance probe vanadyl ion, V02+. We observed two sets of resonances, A and B, in the X-band (9.2 GHz) spectrum 0022-3654/79/2083-0279$01 .OO/O
of frozen-solution samples a t 77 K.2-5 Originally we attributed these signals to the two chemically distinct binding sites of t r a n ~ f e r r i n . ~However, ,~ subsequent experiments, including a detailed p H dependence study of the intensities of the A and B signals, indicated that the EPR spectrum i!3 detecting conformational differences in the metal sites.4 The ionization of a functional group with 0 1979 American Chemical Society
