Radical Adducts to Di-tert-butyl Selenoketone Westhof and Van Rooten have suggested that T4 is actually the radical whose structure is that of a radical produced by loss of a hydrogen atom from the methyl group of TMP.22In our previous work with thymidine in 5 M K2C03we reported a similar spectrum to T4 (ref 9, Figure 2E) and suggested it was due to such a radical (ref 9, structure VI). However we cautioned that it had sufficient similarity to the a cation to warrent only a tentative assignment. At present it seems that the n cation is not T4 but the precursor to Tq. Future work is necessary to test this hypothesis.
References and Notes (1) This research was supported by the United States Energy Research and Development Administration and was performed in part at Lawrence Berkeley Laboratory, University of California, Berkeley. (2) The term P catlon is used for radicals produced by loss of an electron from the n-electron system. It does not refer to the total formal charge on the radical. The formal charge on the radicals In thls study Is zero for the ?r cations of thymine, thymidine, 1-methylthymlne, and 1-ethylthymine and -2 for thymidine 5'-monophosphate. (3) A. Dulcic and J. N. Herak, J. Chem. Phys., 57, 2537 (1972). (4) G. Hartig and H. Dertinger, lnt. J. Radiat. Biol., 20, 577 (1971). (5) J. N. Herak and V. Galogaza, J. Chem. Phys., 50, 3101 (1969). (6) A. Graslund, A. Ehrenberg, A. Rupprecht, G. Strom, and H. Crespl, Int. J.
1901 Radiat. BIOI., 26, 313 (1975). (7) M. D. Sevilla, J. Phys. Chem., 75, 626 (1971). (8)M. D. Sevilla, C. Van Paemel, and C. Nichols, J. Phys. Chem., 76, 3571 (1972). (9) M. D. Sevilla, C. Van Paemel, and G. Zorman, J. Phys. Chem., 76, 3477 (1972). (10) M.D. Sevilla and P. A. Mohan, Int. J. Radiat. Biol., 25, 635 (1974). (1 1) L. Kevan in "Radiation Chemistry of Aqueous Systems", G. Stein, Ed., Wiley-lnterscience, New York, N.Y., 1968, p 78. (12) The g values measured for 0-in 8 M NaC104 are g~ N 2.054 and Q I ~ 2.003. The value for g~ Is significantly less than that found for 0- in 10 M NaOH (gl = 2.07). (L. Kevan in "Radiatlon Chemistry of Aqueous Systems", G. Stein, Ed., The Weizman Science Press, Jerusalem, Israel, 1972). (13) M. D. Sevilla, C. Clark, and R. Failor, Radlat. Res., 65, 29 (1976). (14) S. M. Adams and H. C. Box, J. Chem. Phys., 63, 1185 (1975). (15) 0. Vincow In "Radical Ions", E. T. Kaiser and L. Kevan. Ed., Intersclence, New York, N.Y., 1968, p 165. (16) Reference 15, p 170. (17) R. Drews, D. Cadena, Jr., and J. Howland, Can. J. Chem., 43, 2439 (1965). (18) (a) W. Flossmann, J. Huttermann, A. Mtiller, and E. Westhof, 2.Naturforsch. C,28, 523 (1973); (b) W. Flossmann, A. Mtiller, and E. Westhof, Ml. phys., 29, 703 (1975); (c) W. Flossmann, E. Westhof, and A. Muller, lnt. J. Radiat. Biol., 26, 105 (1975). (19) J. N. Herak and C. A. McDowell, J. Chem. Phys., 61, 1129 (1974). (20) J. Schmidt, J. Chem. Phys., 62, 370 (1975). (21) S. Gregoli, M. Olast, and A. Bertinchamps, Radiat. Res., 65, 202 (1976). (22) E. Westhof and M. Van Rooten, Z.Naturforsch., in press.
Electron Paramagnetic Resonance Spectra of Radical Adducts to Di-terf-butyl Selenoketone' J. C. Scaiano2 and K. U. Ingold' Division of Chemistry, National Research Council of Canada, Ottawa, Ontario K I A OR9, Canada (Received February 3, 1976) Publication costs assisted by the National Research Council of Canada
EPR parameters are reported for the first selenoalkyl radicals. They have been prepared by addition of a variety of transient R,M. radicals (M = C, 0, S, Si, Sn, etc.) to di-tert-butyl selenoketone. They have the R,MSec( CMe& structure and, despite the large atomic radius of selenium, they all adopt a conformation in which the R,M group eclipses the C, 2p, orbital. The EPR parameters are remarkably sensitive to substituent and to temperature. At -50 OC, g values range from 2.0005 [R,M = F3C] to 2.0051 [RnM = (n-Bu)sSi], 13C, hyperfine splittings range from 38.4 [(CH&CO] to 53.9 G [(n-Bu)3Si], and 77Se splittings from 1 1 0 [(n-Bu)3Si]to 66.3 G [(CH3)3CO].These variations are discussed in terms of the polar effect of the R,M group and in terms of possible changes in the geometry of the radical. The sensitivity of the parameters to R,M suggests that di-tert-butyl selenoketone may have some value as a spin trap.
Introduction The chemical, physical, and spectroscopic properties of alkylselenoalkyl radicals, RSecRlRZ, are virtually unknown. Apart from a preliminary communication from this laboratory,3and a cryptic mention in another communication,4 the EPR spectra of such species have not been previously reported. Indeed, only a very limited number of selenium containing organic radicals have ever been observed by EPR ~pectroscopy.~ The obvious assumption would be that the notorious odor of volatile selenides has prompted a profound lack of interest in these radicals. However, even if this assumption is correct, we have found that simple alkylselenoalkyls are not readily generated from their parent dialkyl selenides, a t least in so far as their observation by EPR
spectroscopy is concerned. That is, alkylselenoalkyl radicals are not detected when the usual hydrogen atom abstracting radicals (tert-butoxy or phenyl) are generated photochemically in solution in the cavity of an EPR spectrometer in the presence of dialkyl selenides.7 Whether this result is due to difficulties in the formation or to difficulties in the detection of these radicals remains an open question. However, it is worth noting that the large spin-orbit coupling constant for selenium (1688 cm-1 vs. 382 cm-I for sulfur and 151 cm-1 for oxygen) will produce not only a large shift in the g factor but will also cause the EPR lines to be greatly broadened when there is appreciable spin density on the selenium.638 That is, as Gilbert has suggested,6 the scarcity of EPR data on organoselenium radicals may reflect, in part, the difficulty in achieving high resolution. The Journal of Physical Chemistry, VoI. 80, No. 17, 1976
1902
J. C. Scaiano and K. U. lngold
Our interest in alkylselenoalkyl radicals was sparked by Barton's et al.9 recent preparation of di-tert -butyl selenoketone. From our earlier work with di-tert -butyl thioketonelo we expected that the selenoketone would be a good spin trap yielding persistentll adducts, 1, with a variety of transient R,M. radicals.
R,M.
+ Se=C(CMe&
--+
R,MSeC(CMe,), 1
Our expectations were fulfilled and the adduct radicals were found to give well-resolved EPR spectra with lines that were, in general, as sharp as those obtained from adducts to ditert- butyl thioketonelO and to 1,l-di-tert-butylethylene.12J3 This alone indicated that there could be relatively little spin density a t selenium and this, in turn, implied that 1 adopt the eclipsed conformation, 2, rather than the staggered conformation, 3. That is, 1 prefer the same conformation as do all other R,,MXC(CMe3)2 radicals, X = S,l0X = CH2. 12,13 However, despite this "normal" conformation, 1 show several novel features in their EPR spectra which are described in the present paper. MRi7
I
2, eclipsed
3, staggered (never observed)
Experimental Section All EPR spectroscopic measurements were carried out using a Varian E-4 EPR spectrometer. The g values were measured using a l H NMR frequency marker. They are accurate to f0.0001 and reproducible to better than fO.OOO 02. When measuring the temperature dependence of g, special care was taken to ensure that any changes in g were not due to instrumental factors. Materials. Di-tert-butyl selenoketone was prepared by a modification of Barton's p r o ~ e d u r eDi-tert .~ -butyl ketone, triphenylphosph~ranylidenehydrazone~~~ and selenium metal were heated to 120-130 "C in the presence of a trace (2-3%) of tri-n-amylamine as a catalyst. The reaction was carried out under argon in a reaction vessel fitted with grease-free stopcocks. Every hour, during the first 4 h, the argon supply was cutoff, and the selenoketone was pumped out of the reaction vessel and into a trap held at liquid nitrogen temperature. The reaction was continued for 20 h, with the pumping interval being gradually increased. The accumulation of the deep-blue selenoketone is readily observed. The overall yield is 75-80%, bp 83-85 "C a t 30 Torr, 6(CCl4) 1.53 s, identical with an authentic ~ a m p 1 e . l ~ All other compounds were commercial materials, or had been prepared previously.1°J2 Most of these compounds were used without purification. R,M- Radicals for Addition to the Selenoketone. Transient R,M. were generated photochemically, using a 500-W highpressure mercury lamp, in the presence of the selenoketone. The reaction was carried out in solution directly in the cavity of the EPR spectrometer. Most R,M. were produced by conventional routes. Thus, (CH3)3Sn., (n-Bu)sSn., n-Bus. The Journal of Physical Chemistry, Vol. 80,No. 17, 1976
(CH3)3CS., CF3S., (CH3)3CO., (CD3)3CO., and CgH5(CH&CO. were produced by photolysis of the corresponding dimers. The C H r radical was generated from azomethane and by the reaction of (n-Bu)3Sn- with CH31. The former method is more convenient, but the latter technique allowed us to generate the 13CH3.(90 atom % 13C)and CD3. (>99% D) adducts without difficulty or great expense. Other alkyl radicals were generated from the appropriate azo compound ((CH3)&. and C&&(CH3)2C.) and/or by reaction of (n-Bu)& with the appropriate halide (CF31, CC13Br, C6H5I, C6H5(CH&CBr, and C6FbBr). The ( M ~ ~ C H C H Z ) ~ ( C H ~radical ) C O - was generated from its hyponitrite. We also used di-tert-butyl hyponitrite as a thermal source of (CH3)3CO., but a t the temperatures required for its decomposition the lifetime of the adduct was so short that only the main line in its EPR spectrum could be observed. Trifluoromethyl peroxide reacted thermally with the selenoketone, even a t -80 "C. Red selenium was deposited and a radical could be observed for as long as the reaction continued. At -40 "C in neat CF~OOCFB, the EPR parameters of the radical were g = 2.0025, uF (CF3) = 1.55 G, a H (multiplet) = 0.31 G, (1C) = 65.0 G, (6C) = 9.5 G. Any other 13C hfsc due to a single carbon must be 1 2 2 G (or >400 G ) and any 77Sehfsc must be 17 G. We believe this radical does not contain selenium and is, in fact, the CF30C(CMe3)2. To judge from the I3C, hfsc this is the least planar R,MXC(CMe& radical that has, as yet, been prepared. In previous W O ~ ~ , ~ O J ~ J we ~ J have ~ J ~ generated many R,M. by hydrogen abstractions from R,MH using (CH3)3CO. radicals generated photochemically from di-tert -butyl peroxide. This procedure was satisfactory because (CH3)&0. radicals either do not add, or else add very slowly, to thioketones and to olefins. In contrast, (CH3)3CO. radicals add readily to the selenoketone and this produced problems even with good hydrogen donors. For example, even with a large excess of (EtO)2P(0)H,18 both (Et0)2P=O and (CH3)3CO. adducts were produced simultaneously, while the SH2 reaction of (CH3)sCO. with (EtO)zPOP(OEt)2 [- (EtO)zP=O + (CH3)3COP(O)(OEt)2]was so slow that only the (CH3)3CO. adduct was produced. Similarly, a large excess of (CH3)3SiH or (n-Bu)sSiH gave the R3Si. adducts together with the (CH3)3CO. adduct, while SiH4 gave only the (CH3)3CO- adduct. Fortunately, the (CH3)3Si-and (n-Bu)sSi. adducts could be prepared uncontaminated by other adducts when triplet benzophenone was used as the hydrogen abstracting agent. However, in order to avoid extensive triplet quenching it was necessary to use high [R3SiH]/[Se=C(CMe3)2] ratios. In addition, relatively high concentrations of the selenoketone were required in order to trap as many of the R3Si. radicals that were generated as possible. The overall reaction scheme can be represented as follows: hu
Ph2CQ --+ 'PhzCO
+ + 3PhzC0
3PhzCQ
+
+ SeC(CMe3)z
3Ph2C0 R3Si.
R3SiH
SeC(CMe3)2
R3Si.
-
3Ph2C0
PhzCO
Ph2CO
(1)
+ 3SeC(CMe3)2
(2)
+ Ph2COH
(3)
R&iSeC(CMe&
(4)
R3Si.
loss by other reactionslg
(5)
The quantum yield for the production of &SiSeC(CMe3)2 will be given by
Radical Adducts to Di-terf-butyl Selenoketone
1903
TABLE I: EPR Parameters for R, MSeC(CMe& Radicals" Rn M
(n-Bu)3Sn
g
a13Ca
a77Se
2.0021b 2.001gb 2.0005 2.0020 2.0030 2.0026O 2.0048 2.0051 2.0030 2.0045 2.0043'
19.0b.e 49LiC 23.7d 48.5 21.3 g 46.5 g 36.6 g 45.Eih*j 4 6 ~ 5 ~ 49hb g 18.Yrn g g g IlOgJ' 53.6 18.5 53.9 IlogJJ 18.0 g g (51) (10)h,q 52.06 ll.Oh,q
2.0045'
E12.3~
2.0016w~" 2.00242 2.0024 2.0014dd 2.001Bhh 2.0024 2.0022 2.0026
g 38.4"" g
44.1ee g
g g g
a
aM
aother
Temp range, "C
9.9 (9)h 9.5 9.6
30 to -100 20 to -150 30 to -130 -20 to -70 30 to -50 -20 only -30 only -50 only +40 only -50 only 40 to -130
8.8
g 9.9 9.1
-50 to -100
(10)h,q g 66.3bb 64.2 59.4ff 65.8" g 39.22 g
g
0.51
3.8(CF3)
0.52h
60 to -20 0 to -100 -70 only 20 to -80 -110 only -10 to -80 -110 only -120 only
a Unless otherwise noted, parameters are reported in n-pentane or isopentane at -50 "C and they are essentially invariant over the range of temperatures studied. Positions in the radicals are designated as follows: RaM,Se&a[Cp(C,Hg)]. Hyperfine splittings are given in gauss. b Constant over the temperature range. daldT = 0.002 GPC. d daldT = 0.014 GPC. e Using 90 atom % 13C labeled CH3 from CH31. f Determined by comparison of CH3 and CD3 adduct spectra, see Figure 1. g Not resolved: Poorly resolved. daldT = 0.0015 GPC. I f1.0 G. Overlaps with the V e lines. Measured at -70 "C. daldT = 0.050 G/"C. Probably overlap of 35Cland Hg. dalaT = 0.023 GPC. Probably overlap of some C6H5 protons and Ha. fO.OOO 25. An unidentified radical with a higherg is present as an "impurity" and more accurate EPR parameters cannot be obtained. p Would have been resolved if >10 G. q i0.5 G. The 77Se and l3Ca lines overlap. r dg/dT = 8 X 10-7/OC. S daldT = -0.006 GPC. l19Sn. daldT = 0.050 GPC. " l17Sn. At 40 "C. Corrected using the Breit-Rabi equation. Y dalaT = 0.002 GPC. dg/dT = -4 X 10-6/OC. daldT = 0.014 GPC. bb daldT = 0.04 GPC. c c See text and footnotes 35 and 36. dd dg/dT = -7 X 10-6PC. e e aaldT = 0.018 GPC. ff daldT = -0.025 G/"C. gg See footnote 37. hh Calculated from the 77Selines using the Breit-Rabi equation because of an impurity with g 2.0000. The lines are unusually broad, AHpp N
-
5.5 G.
where aiscis the quantum efficiency for intersystem crossing. The C13Si- and (CH3)sGe. radicals were produced from R,MH using (CH3)3CO. radicals. Although this procedure was not very satisfactory with C13Si-, the (CH&Ge. adduct was so much longer lived (at -40 "C) than the (CH&CO- adduct that a "clean" EPR spectrum could be easily obtained.
Results EPR Spectra of R,MSeC(CMe& Radicals. The EPR parameters for the R,M. adducts to di-tert -butyl selenoketone are listed in Table I. Data are, when possible, given a t -50 "C and the solvent is n-pentane or isopentane. In most cases the g values and hyperfine splitting constants (hfsc) are essentially temperature independent. When this was observed not to be the case, it is so indicated. The spectra were usually sufficiently intense to resolve 13C, and 77Sehfsc in natural abundance (77Sehas I = 1/2, natural abundance 7.6%). For some 1 the hfsc due to M and to certain other nuclei could also be resolved. The central lines in the spectra obtained with R,M = H3C and D3C are shown at high resolution in Figure 1. Radicals Containing Two Selenium Atoms. Photolysis of a pentane solution of dicumyl peroxide and the selenoketone did not give the expected cumyloxy adduct in appreciable
yield even at low temperatures. The major radical (a second radical appears to be present in much lower concentrations) has a short lifetime and the sample must be irradiated continuously if it is to be visualized. The EPR parameters for this radical indicate that it is identical with the radical first discovered by de Mayo and co-workers20 which has two nonequivalent selenium atoms (g = 2.0022 a t -110 "C, a77Se = 39.22 and 65.06 G). This radical has been assigned structure 4.20
(MejC)2CHSeSeC(CMe3)z 4
The same radical was also produced by reaction of the selenoketone with cumyl radicals formed by photolysis of cumyl bromide and hexa-n-butylditin or by photolysis of azocumene even, in the latter case, when the light was filtered through Pyrex. With the azocumene, the lines due to 77Se-77Secoupling could be detected (overall splitting 104 G, intensity relative to the central line = (0.076)2/4 = 0.14%). However, no lines assignable to I3C, could be observed in this spectrum, presumably because they underlie one of the 77Se lines. The fact that 4 is produced from cumyl and from cumyloxy radicals implies that the latter are deoxygenated by the selenoketone. Radical 4 was also produced during an attempt to add benzyl radicals by photolysis of dibenzyl ketone and the selenoketone in n-pentane at -110 "C. Photolysis of MeSeSeMe in the presence of the selenoketone a t -120 "C in isopentane gave a single line EPR signal (g = 2.0026) which may be due to a similar radical but was, unfortunately, too weak for any 77Sehfsc to be detected. The Journal of Physical Chemistry, Vol. 80, No. 17, 1976
J. C. Scaiano and K. U. lngold
1904
TABLE 11: Comparison of M H f s c (Gauss) in Rn MSeC(CMe3)z and RnMSC(CMe3)z Radicals and Spin Density in the s Orbital of M in R,MSeC(CMe&
H313C (CH3)3llgSn (n-Bu)3119Sn (Et0)231P=0 a
19.0 18.5 198.2 163.8 89.3
23.5 23.5 254.5 209.9 101.0
0.81 0.79
0.78 0.78 0.88
1110 1209 7554 7554 3638
0.017 0.015 0.026 0.022 0.025
See ref 22.
ably reflects the larger size of selenium which causes the M to be held further away from the a carbon.loJ6 Although some of the M hfsc are quite large the unpaired spin density a t M or, a t least, the unpaired spin in the valence-shell s atomic orbital of M, ps,22 is rather small (1.52.6%). This quantity is given by ps = a M / a o M
2G Figure 1. (A) Central line in the EPR spectrum of H3CSeC(CMe3)2recorded at low modulation amplitude in isopentane at -30 O C . (B) The
same for D3CSeC(CMe3)2.
Discussion Conformation of R,MSeC(CMe& Radicals. A rather wide variety of transient R,M. radicals add readily to di-tert-butyl selenoketone (see Table I). Because of the large atomic radius of selenium, steric factors and hyperconjugative effects21are much less likely to dominate the conformation of these adduct radicals than is the case with the analogous adducts derived from 1,l-di-tert-butyle$hylene12J3 and di-tert-butyl thioketone.l0 Nevertheless, there is strong evidence that l adopt the same conformation as these other adducts. The favored conformation has the R, M group in the eclipsed position relative to the C, 2p, orbital. That is, R,MSeC(CMe3)2 radicals adopt conformation 2, or the analogous conformation 5 MRn
I
5
if the a carbon is nonplanar. The evidence favoring the eclipsed (2 or 5) conformation over the staggered (3) conformation comes from the magnitude of the M hfsc and from the fact that the g values are centered around the free spin value. Comparison of the M hfsc for adducts to the thioketone and selenoketone (Table 11)indicates that both classes of radical adopt similar conformations. Since we have shown previouslylo that the R,M group in R,MSC(CMe& radicals is in the eclipsed position, the same must be true in the R,MSeC(CMe& radicals. The fact that the M hfsc are ca. 15-20% smaller in 1than in the thioketone adducts presumThe Journal of Physical Chemistry, Vol. 80, No. 17, 1976
where aoMis the estimated coupling constant of M with unit odd electron density in its valence-shell s orbital. Values of aoM 21 and p s are included in Table 11. The presence of only a small spin density on the selenium valence7shell s atomic orbital is indicated by the relatively23 small 77Sehfsc. A small spin density on the selenium atom's lone pair of electrons is indicated by the fact that the g values for 1, although showing an unusually large variation for a series of structurally related radicals, are centered around the free-spin value (2.0023). If there were appreciable delocalization of the unpaired electron via the selenium atom's lone pair, i.e.
then the high spin-orbit coupling constant of selenium would produce a large shift in the g factor (which in turn would cause the EPR spectral lines to be greatly broadened, see Introduction). The g values therefore support an eclipsed conformation since in this conformation the selenium lone pair lies in the nodal plane of the radical. 13C, and 77Se Hfsc. While it is clear that 1, like other R,MXC(CMe& radicals, adopt an eclipsed conformation, their EPR parameters exhibit a number of unusual features. Most of these appear to be connected mainly with the polar effect of the R,M group, rather than with its size as was generally the case in R,MCH&(CMe& radi~a1s.l~ It now (with hindsight) is possible to see that the EPR parameters for R,MSc(CMe3)2 radicals show effects that are similar, though less pronounced, than the effects found in 1. The 13C, hfsc for 1 increase from a low of 37.6 G to a high of 53.9 G along the series R,M = (CH3)3CO < (CH3)3CS < F3C < (CH3)sC < H3C C6Hb < (CH3)3Sn < (n-Bu)sSn < (CH3)3Si < (n-Bu)sSi. Obviously this order is not determined primarily by the size of R,M, but it does appear that the 13C, hfsc increases as the R,M group becomes less electronegative. The 77Sehfsc behave in just the opposite manner, decreasing as the 13C, hfsc increase, that is, the 7%e hfsc decrease in magnitude as the R,M group becomes less electronegative.
-
1905
Radical Adducts to Di-tert-butyl Selenoketone
away from C, and toward the selenium as indicated below.29
70-
50 -
60
In
s
40-
Y
\ O \
IO
0 OO
I 40
I 45 (Gauss)
I 50
\ \ J 55
8
Figure 2. Correlation of "Se hfsc with 13C, hfsc for R,,MSeC(CMe& radicals at -50 O C .
The correlation between these two hfsc is shown in Figure 2. It can be represented by the equation a13Ce = 54 + In this equation, and in Figure 2, it is assumed that the selenium carries negative spin since, in the eclipsed conformation 2, spin will reach the selenium principally by spin polarization. There are two possible explanations for the correlation between the 13C, and the 77Se hfsc, which are not mutually exclusive. In the first place, if the 13C, hfsc depends only on the planarity, or deviation therefrom, at C, then it is clear that the radicals become less planar as the electronegativity of R,M decreases. That is, Me3COSeC(CMe3)2with a 13C, hfsc a t low temperatures (see below) of 537.6 G must be planar (the l3C, hfsc for CH3 is 38.3 G)24but the radicals with larger 13C, hfsc may adopt conformation 5 with a nonplanar C,. One consequence of increased bending at C, would be that increased amounts of positive spin would reach the selenium, thereby offsetting the negative spin from spin polarization. Hence, as bending increased, the l3C, hfsc would rise and the 77Se hfsc would fall. It must be added that our data require a change in C, geometry with R,M electronegativity that is in the opposite direction from that which occurs when electronegative substituents, such as fluorine, are directly bonded to the a carbon.25~26Unfortunately, there is little information regarding the influence (if any) on C, geometry of electronegative substituents bonded to the p atoms. However, we may note that in the series CH~CHZCHZ, CC13CHzCH2, and C F ~ C H ~ C Hthe Z , H, hfsc are 22.08,22.46, and 22.92 G, resp e c t i ~ e l y ,while ~ ~ ~ in the series CH&H2, CHzFCH2, CHF2CH2, and CF&H2, the H, hfsc are 22.37,22.15, 23.16, and 23.77 G, r e s p e c t i ~ e l y .Provided ~~ the H, hfsc reflect changes in the C, geometry, rather than changes in the spin density a t C,, both sets of data can be taken to indicate that electron-withdrawing groups attached to the p carbon do indeed increase the planarity at C,. Similarly, the 13C, hfsc for (CF3)& and for (CH3)& are 44.3 and 49.5 G, respectively,2sb which has been taken to imply that the fluorine containing radical is the more planar. The dependence of the 13C, and 77Sehfsc of 1 on the polarity of R,M can, alternatively, be attributed to the high polarizability of selenium. Thus, an electronegative R,M will increase the positive charge on selenium, one consequence of which is that the C,-Se u-bonding electrons will be polarized
Thus, an electronegative R,M will increase the 77Sehfsc and decrease the 13C, hfsc since, for a planar C,, the magnitudes of both hfsc are dependent on the spin polarization of these u electrons. An electron-donating R, M wodd, of course, have the opposite effect, decreasing the 77Sehfsc and increasing the l3C, hfsc. In 1the 13C, hfsc are more sensitive to R,M polarity than is the case in R,MSC(CMe3)2 radicals (in which, for example, the 13C, hfsc = 43.9 and 49.7 G for R,M = CF3 and (CH3)3Si, respective1y)lO and they are much more sensitive than is the case in R,MCH&(CMe3)2 radicals (in which, for example, the 13C, hfsc = 45.5 and 46.4 G for R,M = CF3 and (CH&Si, respectively).12 We therefore suggest that polarization of the C,-Se bond by R,M probably has the greatest influence on the EPR parameters of 1. g Values. The g values for 1 cover a wider range (2.00052.0051) than do the g values for R,MSC(CMe& (2.00242.0033)1° or those for RnMCH2C(CMe& (2.0023-2.0032).12 The general trend of the effect of R,M substituents on g is similar for the adducts to the selenoketone and thioketone, but quite different trends are found in the RnMCH2C(CMe3)2 radicals (see Table 111). Very few carbon-centered n radicals have g values below the free electron value (2.0023).Those that do contain an atom with a high spin-orbit coupling constant in the position (e.g., (CH3)3SncH2, 2.0008; (CH&PbCHz, 1.9968).30In this respect, therefore, CF3SeC(CMe3)2 and those other R,MSeC(CMe3)2 radicals with low g values are not unusual. Deviations of g from the free spin value occur when the odd electron acquires some orbital angular momentum through the effects of spin-orbit c o ~ p l i n g . Low ~ ~ ~g~values l (2.0023), on the other hand, arise when the ground state singly occupied orbital mixes with filled (i.e., doubly occupied) states of lower energy. An increase in the spin-orbit coupling constant of M tends to increase the g values of R,MSeC(CMe3)2 radicals. Since M is favorably situated for interaction with the unpaired electron in these eclipsed radicals, it is not too surprising to find that some radicals with M = S and all radicals with M = Si, Ge, and Sn have g > 2.0023, even though these M all have smaller spin-orbit coupling constants than selenium. That is, the enhancement of g by these M overrides its reduction by the selenium. This picture is, of course, oversimplified, otherwise the g values for the radicals containing M atoms from group 4 would increase along the series M = C < Si < Ge < Sn, rather than in the observed order, C
