J. Phys. Chem. 1995, 99, 10114-10117
10114
EPR Spectra of Partially Fluorinated Alkyl-C,jo Radicals and a Theoretical Study of Interactions on the c 6 0 Surface? J. R. Morton,*’$F. Negri? K. F. Preston,’ and G. RueF Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario, Canada K I A OR6, and Dipartimento di Chimica “G. Ciamician ”, Universita di Bologna, 40126 Bologna, Italy Received: March 17, 1995@ The EPR spectra of seven partially fluorinated derivatives of methyl-, ethyl-, and isopropyl-C60 radicals have been studied. The equilibrium configuration of each was deduced from their proton and I9F hyperfine interactions. If one or more of the hydrogens of CH3Ca are replaced by CF3, F, or CH3, there is a competition among them for the position over the pentagon adjacent to the c-c60 bond. The order CF3 > F > H > CH3 follows closely their respective inductive effect parameters 01. Quantum chemical calculations at the INDO/ UHF level have been used to rationalize this hierarchy in terms of charge separation on the c 6 0 surface.
1. Introduction In the course of our recent studies of the EPR spectra of alkyland perfluoroalkyl-C60 it became evident that there were effects at work on the Cm surface which resulted in different equilibrium configurations for RHCWand its perfluoro analog, RK60. For example, isopropyl-C60, (CH3)2CHC60, adopts the symmetric configuration for which (on the EPR time scale) there is no evidence of motion about the C-Ca bond.2 For perfluoroisopropyl-C60, however, the equilibrium configuration is asymmetric and there is exchange between its enant i o m e r ~ .The ~ converse is true for ethyl-C60 and its perfluoro analog. We rationalized these differences by noting3 that the overiding effects seemed to be (a) the preference of a CF3 group for the symmetric position over the pentagon andor (b) the tendency of a CH3 group to avoid the pentagon position. In order to further examine the interactions between the c 6 0 surface and groups attached to it, we examine in the present paper the EPR spectra of partially fluorinated derivatives of c 6 0 whose general formula is XYZC-Ca, where X, Y, and Z are H, F, CH3, or CF3. It was our intention, therefore, to establish an ordering among these ligands for pentagon-preference and to explain it with the aid of semiempirical quantum chemical computations.
2. Experimental Section The free radicals RC60 were generated by UV photolysis of the precursor iodide or bromide dissolved in a solution also containing dissolved c 6 0 (SES Research Inc., Houston, TX). CH2FBr was obtained from the Central Chemicals Co. Inc., Tokyo, Japan; CHF2Br and CF3CH2I from PCR Inc., Gainesville, FL; and CF3CHFI from the Indofine Chemical Co., Somerville, NJ. CH3CF2I was prepared by heating equimolar amounts of CF2=CH2 (PCR) and 12 in an autoclave at 185 “C for 160 h;4 CF3(CH3)CHI and (CF3)2CHI were prepared by converting the alcohol to the nonafluorobutanesulfonate (nonaflate) and treating the latter with NaLS For improved sensitivity, the Varian E-104 spectrometer was equipped with a custom-made variable temperature Dewar insert (Wilmad Glass Inc., Buena, NJ) capable of accepting Suprasil
’NRCC No. 39048.
Steacie Institute for Molecular Sciences, National Research Council
of Canada.
Universita di Bologna. @Abstractpublished in Advance ACS Abstracts, June 1, 1995.
sample tubes having an outside diameter of 6 mm. The spectrometer was also equipped with the usual accessories for measuring the magnetic field strength and the microwave frequency. The samples were photolyzed in situ using the focused light of a high-pressure HgKe arc filtered through a 5 cm column of water and an Oriel Corp. (Stratford, CT) infrared filter No. 50960. The spectrometer was operated in the “critically coupled” mode at microwave power levels of 1050 pW. The modulation frequency was 25 kHz, amplitude ca. 50 mG. 3. Results and Discussion A representative selection of the spectra of partially fluorinated alkyl-Cho radicals is shown in Figure 1. As will be seen, the individual lines are extremely sharp, the maximum slope line width being 25-50 mG. This enabled complete resolution of the hyperfine splittings of all ‘H and I9F nuclei present in the radical. Thus, the concomitant problems of analysis of the hyperfine manifold and identification of the carrier were trivial. The proton and I9F hyperfine interactions are collected in Table 1. 3.1. Partially Fluorinated MethyLC60 Radicals. We consider first the fluoromethyl-C60 radicals CH2FC6o and CHF2Cbo (Table 1). Comparing the I9F hyperfine interaction in CH2FC60 with those of CF3C6o in its “static” config~ration,~ we conclude that CH2FC60 has the symmetric configuration with its fluorine atom over the pentagon. Unfortunately, the proton hyperfine interactions of CH2FC60 cannot be compared to those of “static” CH3C60 since the latter has not yet been observed. In any event, the hyperfine interactions of lone protons (as opposed to protons of CH3 groups) do not appear to be a useful diagnostic of their position.6 780
--280 W
mG -230
IC,
Conversely, we conclude that CHF2C60 has the asymmetric configuration, a conclusion supported by quantum chemical calc~lations.~ This explains both the equivalence of the I9F hyperfine interactions and their small magnitudes. From the above diagram for CF~CW, the average I9Fhyperfine interaction
0022-3654/95/2099-10114$09.00/0 Published 1995 by the American Chemical Society
EPR Spectra of Fluorinated Alkyl-C60 Radicals A. CF3CH2Ceo ,
D. CF3CHFCeo
Figure 1. (A) EPR spectrum of C F ~ C H ~ Cin~tert-butylbenzene O at 400 K. The anomalous weakness of the low-field component is due to a CIDEP e f f e ~ t . (B) ~ EPR spectrum of CFs(CH3)CHCw at 350 K. (C) EPR spectrum of (CF3)2CHCs at 325 K. (D) EPR spectrum of CF3CHFCG0at 410 K.
TABLE 1: Hyperfine Interactions (mG)of Certain Partially Fluorinated Alkyl-Cm Radical@ a/mG at 295 Kb
radical
a All g-factors lie in the range 2.0021-2.0023. * A s measured. For inferred signs see text. Errors are ca. 1 2 . 5 mG. Photolysis of RBr in ter?-butylbenzene/C60. Photolysis of RI in tert-butylbenzenelcs.
for the 8 = 0 and 120" positions is calculated to be 175 mG, since the value for 8 = 120" is negative. Since there is no requirement in CHF2Cm that the angles are the same as in CF3Cm, this is in reasonable agreement with the experimental value of 240 mG. 3.2. Partially Fluorinated Ethyl- and Isopropyl-Ca Radicals. In order to assign equilibrium configurations to these species, we refer to data from our earlier study of perfluoroalkylCm radical^.^ The data for (CF3)3CCm were particularly infor-
(
F
e
3 -1630
mG
mative since (at 225 K) there was no rotation about the C-Cm bond on the EPR time scale. The three 19Fnuclei of the CF3 group in the 8 = 0" position had average hyperfine interactions of 2260 mG, whereas the six I9F nuclei of the equivalent CF3
J. Pkys. Chem., Vol. 99,No. 25, 1995 10115 groups in the 8 = f120" positions had average hyperfine interactions of 1630 mG. The validity of these numbers as a diagnostic of the position of CF3 groups on the Cm surface was c o n f i i e d by the data for CF3CF2Cm (0 = O", 2430 mG).398 We conclude, therefore, from the hyperfine interactions of the three '9F nuclei of CF3CHzCm (2720 mG, Figure lA)9 and CF3(CH3)CHCao (2620 mG, Figure 1B) that their CF3 groups are also located at or near the 8 = 0" position, i.e. over the pentagon. For CF3(CH3)CHCm, there is additional confirmation from the hyperfine interaction of the protons of the CH3 group (140 mG) that it lies over one of the hexagons. These hyperfine interactions are identical to those of the two methyl groups of (CH3)zCHCm (140 mG), which is known to have the symmetric configuration (both methyls over the hexagons).IO Turning to (CF&CHCm (Figure lC), we note that the six equivalent 19Fnuclei have hyperfine interactions of only 2010 mG, reminiscent of the 1960 mG hyperfine interactions for the six equivalent 19Fnuclei in ( C F ~ ) ~ C F C MWe . ~ conclude that, like (CF3)2CFCm, the equilibrium configuration of (CF3)zCHCm is asymmetric, i.e. the hydrogen lies over one of the hexagons (OH = zk120"). This configuration is enantiomeric, and rapid exchange between the enantiomers interchanges the CF3 groups and results in a hyperfine manifold of six equivalent 19Fnuclei. As with ( C F ~ ) Z C F Cit~did , ~ not prove possible to freeze out this exchange and obtain the spectrum of the individual enantiomers. 3.3. Access to the Pentagon Position in RC60 Radicals. We can establish from these and earlier results a hierarchy for pentagon-preference in radicals of the type xYzCc60, where X, Y, and Z are CF3, F, H, or CH3. Thus, since CF3CF2Cm has the symmetric configuration, we conclude that pentagonpreference is stronger for CF3 than for F. Similarly, since CH2FC60 has the symmetric configuration, pentagon-preference is stronger for F than for H. Finally, because (CH~)ZCHC~O is symmetric, H has stronger pentagon-preference than CH3. In other words, we have established the hierarchy for pentagonpreference CF3 > F > H > CH3, which can be tested against the remaining entries in Table 1. Take, for example, the partially fluorinated ethyl-Cm radicals CF3CH2Cm and CH3CFzCm. The 2720 mG 19Fhyperfine interactions of C F ~ C H Z Care ~ Ocharacteristic of a CF3 group over the pentagon, and it therefore has the symmetric configuration predictable from the above hierarchy. For C H ~ C F ~ Con ~ Othe , other hand, the 19Fhyperfine interactions of 150 mG are close to the average of those of CF& (175 mG, see above), and the proton hyperfine interactions (80 mG) are similar to those of the methyl groups over the hexagons in (CH3)3CC60 (88 mG).'O We have, therefore, no hesitation in concluding that CH3CFzCm has the asymmetric configuration and that it is undergoing rapid exchange between its enantiomers." Since the fluorine atoms have a stronger preference for the pentagon position than a CH3 group, this is exactly what would be expected. The third partially fluorinated ethyl-Cm radical, CF3CHFCm (Figure lD), also clearly has its CF3 group over the pentagon, as indicated by the 19Fhyperfine interactions of its CF3 group (2770 mG). This conclusion is also in accord with the above hierarchy. Then, consider the partially fluorinated isopropyl-C60 radicals. Since CF3 has a greater preference for the pentagon position than either CH3 or H, it will tend to occupy a position near 8 = 0" in CF3(CH3)CHCm. The large I9F hyperfine interactions of the CF3 group (2720 mG) demonstrate that the CF3 group is indeed located over the pentagon, although the lack of symmetry imposes no requirement that 8 = 0" exactly. Finally, CF3 being to the left of H in the above sequence, (CF3)2CHC60 would be expected to have the asymmetric configuration, as concluded above. Thus,
10116 J. Phys. Chem., Vol. 99, No. 25, 1995
Morton et al. -0.333 I
0.060
0.009
I
\
M - 0 . 0 0 3
A Figure 2. INDONHF charge distribution in (A) CH&, CHjCF2C6o.
B
C
(B) the symmetric conformer of CH3CF2Cao. and (C) the asymmetric conformer of
the equilibrium configurations, not only of the seven radicals in Table 1 but also of their perfluoro3 and perhydro2 analogs, can be rationalized by the simple hierarchy CF3 > F > H > CH3 for occupation of the pentagon position.
C1, and (2) atoms C2, C2’, C4, C4’, C8, and C8’ bear positive charge, the largest of amounts being on C8, C8’, C2, and C2’. The various substituents experience interactions of different sign and induce changes in the charge distribution in order to minimize repulsions and maximize attractions. 4. Conformational Analysis It is clear from the charge distribution of Figure 2A that replacing a hydrogen atom by a fluorine subjects the latter to The above discussion establishes the order of preference for smaller repulsions when it is over the pentagon than when it is the 0 = 0” or pentagon position, CF3 > F > H > CH3, a over the hexagon. This is because in the latter situation the sequence which closely parallels their respective inductive effect fluorine is closer to atom C1, which bears a negative charge parameters (01): 0.42, 0.52, 0.00, -O.05.l2 This, in turn, larger than that on C5 and C5’. In addition, a fluorine over the suggests that the equilibrium configuration of a given radical pentagon will be subject to stronger attractions to atoms C8 is mainly determined by the charge distribution on the c 6 0 and C8’ than to atoms C4 and C2 when it is over a hexagon. surface for that radical. Conversely, any positively charged group, such as a methyl In a recent study6 we investigated the stability of the two group, will prefer a location over one of the hexagons. conformers of CH2FC60 and CF2HC60 and found that, among As an example of the effect of a more complicated substituent, the various semiempirical Hamiltonians, only INDO’ successwe present in Figure 2B,C the atomic charge distribution for fully predicted their correct relative energies. In the present the symmetric and asymmetric conformers of C H ~ C F Z CIt~ ~ . article, we have extended this approach to more complex c 6 0 is seen that the presence of the fluorine atoms induces a derivatives. Optimized geometrical structures were obtained considerable rearrangement of the charge on the c 6 0 cage. by the MNDONHF methodi4 for all possible conformers of the five ethyl derivatives CH3CH2Cm, C H ~ C F ~ CC WF,~ C H ~ C M , However, for the more stable asymmetric conformer (Figure 2C), the charge rearrangement is less dramatic (cf. Figure 2A) CF3CFHC60, and CF3CF2C60 and the four isopropyl derivatives and the computed energy is lower. (CH3)2CHCm,CF3(CH3)CHCm,(CF3)2CHCm, and (CF3)2CFCm. From the above discussion it is clear that, in terms of pointEnergies and atomic charge distributions were computed with charge interactions, the conformational position of a CF3 group the INDO Hamiltonian. will be determined by a delicate balance between the interactions For five of the nine compounds examined, namely, isopropylof the large positive charge on the carbon atom (which would c 6 0 and all the ethyl-(& derivatives except perflUOrOethyl-C60, lead to a preference for the hexagon position) and the negative INDO calculations predicted the observed carrier of the EPR charges on the fluorines (which would lead to a preference for spectrum to be the more stable conformer. However, for the the pentagon position). The larger polarizability of the fluorine remaining radicals (all of which contain CF3 groups), INDO electron cloud is probably responsible for a stronger interaction predicted as more stable the isomer in which the CF3 group(s) with the C ~ surface O in the case of CF3 groups. This interaction lay over the hexagon(s), although the energy differences were cannot be correctly reproduced by the INDO Hamiltonian very small (‘1 kcallmol). These results are at variance with because of an imperfect description of the long-range interacthe evidence of the EPR spectra. The INDO calculations clearly tions of electronic clouds. To get a more conclusive answer to indicated that a reliable description of a CF3 group is more the problem of the conformational order of fluoroalkyl-Cho difficult than that of a lone fluorine atom. radicals, particularly those containing CF3 groups, higher level To obtain a deeper understanding of the factors determining calculations which account properly for the polarizability of the conformational ordering, we analyzed the atomic charge fluorine will be required. However, the present analysis has distribution computed for the radical CH3Cm (Figure 2A). We shown that considerations based on atomic charge distributions take this radical as a reference point and compare its charge and the changes induced by the substituent account for the distribution with those of the more complex radicals in which conformational preference of most of the compounds examined. one or more of the hydrogens have been replaced by F, CH3, or CF3 groups. The charge distribution presented in Figure 2A shows two main features: (1) atoms C1, C5, and C5’ bear a Acknowledgment. The authors thank Drs. J. Lusztyk and considerable amount of negative charge, the largest being on W. Siebrand for stimulating discussions, and Mr. R. Dutrisac
EPR Spectra of Fluorinated Alkyl-Cm Radicals for painstaking technical help. This work was supported in part by NATO Collaborative Research Grant No. 940108.
References and Notes (1) Morton, J. R.; Preston, K. F.; Krusic, P. J.; Wasserman, E. J. Chem. Soc., Perkin Trans. 2 1992, 1425. (2) Keizer, P. N.; Morton, J. R.; Preston, K. F.; Krusic, P. J. J. Chem. SOC., Perkin Trans. 2 1993, 1041. (3) Morton, J. R.; Preston, K. F. J. Phys. Chem. 1994, 98, 4993. (4) Hauptschein, M.; Fainberg, A. H.; Braid, M. J. Org. Chem. 1958, 23, 322. (5) Hanack, M.; Ullmann, J. J. Org. Chem. 1989, 54, 1432. (6) For example, in (CH3)zCHCm the unique roton is over the pentagon and has a hyperfine interaction of 470 mG, not very different from those of CF3CHzCw (420 mG) which are over the hexagons. It appears that the proton hyperfine interaction is strongly perturbed by the presence of CF3 groups or F atoms in the radical. (7) Morton, J. R.; Negri, F.; Preston, K. F. Chem. Phys. Lett. 1995, 232, 16. (8) Fagan, P. J.; Krusic, P. J.; McEwan, C. N.; Lazar, J.; Parker, D. H.; Herron, N.; Wasserman, E. Science 1993, 262, 404.
9
J. Phys. Chem., Vol. 99, No. 25, 1995 10117 (9) The anomalous weakness of the low-field components is due to a chemically induced dynamic electron polarization (CIDEP) effect related to the mechanism of formation of the radicals. See: Wan, J. K. S . ; Elliot, A. J. Acc. Chem. Res. 1977, I O , 161. Adrian, F. J. Chem. Intermed. 1979, 3, 3. (10) Methyl groups over the hexagons have proton hyperfine interactions which vary from 140 mG in the case of (CH3)zCHCmto 88 mG for (CH3)3cc60 at 225 K.2 In the latter, the third methyl group is over the pentagon, in which location its protons have the distinctly larger hyperfine interaction of 340 mG. (11) Furthermore, at 250 K the central component of the 1:2:1 I9F hyperfine splitting broadens to the point of undetectability, an absolute diagnostic of fluorine atom exchange (cf? C H ~ C H ~ C ~ O ) . (12) Charton, M. J. Org. Chem. 1964, 29, 1222. (13) Pople, J. A.; Beveridge, D; Dobosh, P. J. Chem. Phys. 1967, 47, 2026. (14) Dewar, M. J. S . ; Thiel, W. J.Am. Chem. SOC. 1977,99,4899,4907. Dewar, M. J. S . ; McKee, M. L.; Rzepa, H. S . J. Am. Chem. SOC. 1978, 100, 3607.
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