J. Phys. Chem. 1994,98, 4993-4997
4993
Hindered Internal Rotation in Perfluoroalkyl-Ca Radicals? J. R. Morton' and K. F. Preston Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, ON, Canada K I A OR6 Received: November 9, 1993; I n Final Form: February 24, 1994'
The EPR spectra of a series of perfluoroalkyl-C:ao radicals F ~ , C ( C F ~ ) , C ~ O(3 In I0) are described and discussed. It is concluded that the enthalpy barrier hindering rotation about the C-C60 bond in CF& is ca. 7 kcal/mol and that in the nonrotating configuration its 19Fnuclei have hyperfine interactions of opposite sign. On the EPR time scale, C F ~ C F ~ Cis ~locked O into the symmetric configuration. The equilibrium configuration of ( C F ~ ) & F C ~isOasymmetric, and the barrier between its two enantiomeric forms is ca. 3.5 kcal/mol. In the case of (CF3)3CCao, the CF3 groups liberate in synchrony with rotation about the c-c60 bond.
1. Introduction It now appears that virtually any free radical will attack the double bonds of Cm forming a multitude of products, most of them the result of multiple addition.l** The number of isomers of RnC60 (R = the attacking radical) increases dramatically with n. It has been calculated that for n = 1,2, and 3 the number of isomers of RnCmis 1,37, and 577, respectively, with the numbers for higher n reaching astronomical proportions.3 Clearly, discriminatory techniques are needed which can focus on a particular n or a particular isomer. EPR spectroscopy is one such technique, being blind to products having even n, while for odd n, no examples have yet been reported with n > 5. EPR spectroscopy has proven particularly felicitous for n = 1, i.e., RC60 radicals. Several EPR studies of such species have been carried out in recent years and have yielded information on the unpaired spin distribution on the Cm surface,Ib the barrier hindering rotation about the R-C.50 bond,ld the equilibrium conformation of R with respect to the c 6 0 surface,ld.eand the enthalpy of formation of certain R C ~ C ~ dimers.Id OR In the more complex situation of RC70 radicals, EPR has been used to identify and measure the proton hyperfine interaction of four of the five isomers of HC704and three of the five isomers of tert-butyl-C70.5 In the present article we present EPR data on F~,C(CF~),,C~O (3 1n 1 0) radicals and draw conclusions concerning the dynamics of their C-CSO and C-CF3 bonds. While this manuscript was in preparation, an article appeared which nicely complements the present study, exploring the above-mentioned problem of multiple addition of RF radicals to c 6 0 by means of NMR and mass spectrometry.6
in the range 85-450 K. The EPR spectrometer had a Varian E-102 microwave bridge and a 12-in. magnet. It was equipped with the usual devices for continuous monitoring, of the temperature of the sample, the microwave frequency, and magnetic field of the spectrometer. The spectrometer was operated in the critically-coupled mode, typically at a power level of 0.01 mW and a modulation of 0.05 G, 25 kHz. Perfluoroalkyl radicals were generated in the solutions by photolysis with the focused output of a Schoeffel 1000-W Hg/ Xe arc filtered through a 5-cm column of water and an Oriel Corp. (Stratford, CT) IRfilter. These precautions werenecessary to prevent drift of the microwave frequency during the course of an experiment.
3. Results and Discussion At low temperatures (1 50-200 K) the hyperfine pattern in the spectrum of CF3Cao is that of two equivalent 19F nuclei [a&) = 0.28 GI plus a unique I9F nucleus [aF(1) = 0.63 GI, i.e., a 1:2:1:1:2:1 sextet (Table 1, Figure la, 185 K). Clearly there is no rotation about the C-c60 bond on the EPR time scale. The CF3 group has taken up a symmetric configuration with one of the fluorine atoms lying on the symmetry plane (e = Oo) and the other twosymmetrically disposedon either sideof it (0 = *120°). On the basis of ROHF/MNDO calculation^,^ we assign a positive sign to the hyperfine interaction of the unique 19Fnucleus which is, of course, at 8 = Oo (Figure 2a, top left).
2. Experimental Section C a (99.9%) was obtained from SES Research Inc., Houston, TX. Saturated solutions of it in various carefully dried solvents were stored in a glovebox continuously flushed with dry argon. Perfluoroalkyl iodides were obtained from PCR Inc., Gainesville, FL. These were degassed by a freezepumpthaw cycle but were otherwise used as received. A typical sample consisted of 350 mm3 of a solution of c 6 0 in a suitable solvent plus 30 pmol of the perfluoroalkyl iodide. The latter was added by means of a microsyringe unless (CF31, CF3CF2I) it was a gas at ambient temperatures, in which case standard vacuum techniques were used to add it to the sample. The samples were contained in a 5-mm-o.d., thin-walled Suprasil tube equipped with a Teflon stopcock. The sample was transferred to the microwave cavity of the spectrometer, in which it could be examined at temperatures t NRCC No. 31231. *Abstract published in Advance ACS Abstracts. April 15, 1994.
0022-3654/94/2098-4993$04.50/0
As the temperature is raised above 200 K, the outer lines broaden gradually, but the inner 1:l doublet remains sharp. Eventually, new lines appear between them, and the spectrum becomes a 1:3:3:1 quartet as the three I9F nuclei become indistinguishable on the EPR time scale (Figure la, 285 K). This behavior is diagnostic of opposite signs for aF(1) and aF(2), and since a positive sign has been assigned to aF(l), a negative sign must be assigned to a ~ ( 2 )the , hyperfine interactions of fluorines at 8 = f120°. The spectrum of CF3Cm was simulated over the critical temperature range 175-225 K, using the program ESREX* and
Published 1994 by the American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 19, 1994
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Morton and Preston
TABLE 1: 19F Hyperfine Interactions (G).of Perfluoroakyl Adducts of C a radical 19Fhyperfine interactions temp/K 150-200* ChC6o (IF(1) 0.63, aF(2) 0.28 aF(3)
0.074 aF(3) = 0.18 C F ~ C F ~ C ~ O aF(2) = 0.33, aF(3) = 2.43 aF(2) = 0.53, aF(3) E 2.30 (CF3)zCFCm aF(1) = 0.33, a ~ ( 6 ) ‘ UF(1) E 0.83, @(6) 2.00 a ~ ( 1 )= 1.02, @(a) 2.05 (CFd3cC60 aF(1) 2.92, aF(2) = 1.93, aF(2) = 4.49,0.27,0,13
285e 42Y 225d 45od 150-2OOb 30Oe 45Oe 225d
As measured. For inferred sign information see text and Figure 2. All g factors were 2.00240 f 0.00003. Solvent: methylcyclohexane. Solvent: toluene. Solvent: tert-butylbenzene. See text. 0
4 -
m
8 i:
o
2
-200
5 W
-400
@
-600
&
o
1
2
i:
(a)
’\
200
u
d
-
L-
-0.87 mG/deg a
-800
3
-1000
-1200
’
I
I
I
1
I
100
200
300
400
500
TEMPERATURE / K Figure 2. Variation with temperature of the I9F hyperfine interactions
(mG) of (a) CFlCm (triangles), (b) C F ~ C F ~ C(open M circles), and (c) (CF3)zCFCm (filled circles). Circles around the fluorine atoms indicate which 19Fnuclei are relevant to each set of data. Asterisks indicate the major centers of unpaired spin population.
Figure 1. EPR spectra of (a) CF3C60 at 185 and 285 K, (b) CFaCF2Cm at 400 K, and (c) (CF,)zCFC6o at 190 and 300 K.
This average hyperfine interaction was measured between room temperature and 425 K and was found to increase in magnitude from 0.074 G at 285 K to 0.18 G a t 425 K. By fitting a straight line to the data by the method of least squares, a slope of f0.73 f 0.04 mG/deg was obtained. Before this graph could be plotted in Figure 2, however, a decision had to be made regarding the sign of d ( a ~ ( 3 ) ) l d T .As we shall see later, a ~ ( 2 of ) CF~CF$&O must be negative in sign, since its two equivalent fluorines occupy the same asymmetric positions as those of CF3CWbelow 200 K. Their hyperfine interactions also increase in magnitude (become more negative) with increasing temperature, with a slope d ( a r (2))/dT = -0.87 f 0.03 mG/deg (Figure 2b). Differentiating eq 1, we obtain
the following parameters for the low-temperature regime: aF(1) = +0.63 G
aF(2) = -0.28 G
The simulation for a preselected exchange rate was matched against the observed spectra in order to obtain the best correlation of temperature (T/K) and exchange rate (k/Hz). By plotting ln(k/T) against lOOO/T, a straight line was obtained of slope -3.5 0.25O. Multiplying by the gas constant R yielded an enthalpy barrier to CF3 rotation in CF3C60 of 7.0 f 0.5 kcal/mol. In the case of CH&, a similar procedure (albeit without the benefit of hyperfine parameters for the low-temperature regime) yielded an enthalpy barrier of 3.3 f 0.2 kcal/mol for rotation about the CH3-Cso bond.l‘J A similar value (3.4 kcal/mol) has been reported for the barrier to internal rotation in CH3CF3.9 It is perhaps not unreasonable to find a slightly higher barrier in the present instance. The value of a ~ ( 3 observed ) at any temperature above 250 K can be regarded as the average of the instantaneous values of a ~ ( 1 and ) a~(2):
*
If we assume that in CF3C60 d ( a ~ ( Z ) ) / d Tis also -0.87 mG/deg and that this term dominates the right-hand side of eq 2 (Le., imposes a negative sign on the left-hand side), then d(aF(l))/dT = -0.45 mG/deg. Bearing in mind that the unique fluorine of CF3C60 lies on the symmetry plane, a location where its 2s spin population is maximum? it is clear that its hyperfine interaction must have a negative temperature coefficient. The alternative sign choice for the left-hand side of eq 2 leads to d(aF(l))/dT = +3.93 mG/deg, a value unreasonable in both sign and magnitude. For these reasons, we have drawn the temperature variation of aF(3) for CF3C60 with a negative slope in Figure 2a. We turn now to perfluoroethyl-C60. This spectrum, discussed briefly by Fagan et a1.,6 consists of a quartet of triplets over the temperature range 200-450 K (Figure lb). At 225 K, the I9F hyperfine interactions are aF(2) = 0.33 G and aF(3) = 2.43 G (Table 1). There is no suggestion of any broadening of the central components of the triplets at the lower end of the temperature range (as was observedldTcfor C H ~ C H ~ C ~and O )we , therefore conclude that CF3CF2C60 has the symmetric configuration on the EPR time scale. Its CF3 group is freely rotating at all
The Journal of Physical Chemistry, Vol. 98, No. 19, 1994 4995
EPR Spectra of Perfluoroalkyl-C60 Radicals accessible temperatures.
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CS The hyperfine interactions aF(2) of C F ~ C F ~ C are, ~ Oas one would expect, similar in magnitude to those of the two equivalent l9F nuclei of CF& in its low-temperature regime. As discussed above, the latter are known experimentally (Figure 2a) to be of opposite sign to aF(1) and therefore negative.' For this reason, we conclude that the hyperfine interactions a ~ ( 2 of ) CF3CF2Cm are negative and, as shown in Figure 2b, become increasingly negative with increasing temperature. The temperature coefficient of -0.87 f 0.05 mG/deg was determined by fitting a straight line to the data by the method of least squares. In the case of C F ~ C F Z Cwhich ~ , is essentially confined to the symmetric configuration on the EPR time scale, it is reasonable to assume that the negative temperaturecoefficient of&) isdue to torsional oscillation about the C-Ca bond and that this motion is harmonic in nature. The temperature variation of aF(2) can then be described by the hypercotangent function for a single mode model10
changes in hyperfine interaction with increasing temperature are anticipated theoretically12 when geometric isomers which differ in energy equilibrate rapidly. To explain these changes in aF( 1) with increasing temperature, we conclude that there is exchange between the two equivalent, enantiomeric configurations, even at 150 K. Each enantiomer occupies a potential well, the locations of which are defined by the angle 8; the system has very high side walls at 8 = Oo, somewhat similar to the ammonia inver~ion.~Because of the lack of symmetry in the asymmetric configuration, there is no requirement that theminima beat 8 = f120'. Exchange between theenantiomers also interchanges the CF3 groups, rendering their I9Fnuclei equivalent in pairs. At 190 K, the hyperfine manifold of the six fluorines of the two CF3 groups is a triplet of doublets,
thedoublet splitting (0.33 G) being due to the hyperfine interaction of the unique fluorine (Figure IC, 190 K). We conclude that this is a spectrum in the intermediate temperature regime for CF3 rotation and that the only other measurable information in the spectrum is the average hyperfine interaction of the six 19F a ( T ) = a, (aza/aQ2)[h/16~'v] coth(hv/2KBT) (3) nuclei: 1.96 G. In the absence of hyperfine data for the static state, any calculation of the barrier hindering the exchange of where KBis Boltzmann's constant, v the frequency of the torsional enantiomers will be approximate a t best. With this caveat in oscillation, and a2a/aQ2 the second derivative of the hyperfine mind, however, an estimate that AG* I3.5 kcal/mol can be interaction with respect to the normal coordinate Q. At sufficiently high temperatures, eq 3 becomes made under the following assumptions: (a) that the coalescence temperature T i s 1 1 5 0 K,I3 (b) that the change in the hyperfine interactions of the CF3 groups as a result of the exchange is ca. a ( T ) = a, KBT(aZa/aQ2)/87r2v2 = a, bT (4) 10 MHz, and (c) that the hyperfine interaction of the unique fluorine is unaffected by the exchange. Le., the temperature variation of the hyperfine interaction becomes As the temperature is raised, CF3 rotation sets in, and other a straight line (as observed)." From the value of da/dT and the intercept of the fitted straight line on the T = 0 axis, the frequency doublets appear between those shown in Figure IC, 190 K. These of the torsional oscillation was calculated from eq 4 to be 100 new doublets becomesharper and more intenseas the temperature is raised, until at 300 K a fully developed hyperfine manifold of cm-1 or less, not unreasonable for such motion. six equivalent l9F nuclei plus a unique l9F nucleus is observed In Contrast to U F ( ~ )Of C F ~ C F ~ C W UF(1) , Of (CF3)zCFC60 (Figure IC, 300 K). Why then does the hyperfine interaction of undergoes dramatic changes with temperature. Below ca. 200 K, the magnitude of aF(1) is 0.33 G (Table 1, Figure 2c). As the unique I9F nucleus change so dramatically? We think it is the temperature is increased, the magnitude of aF( 1) increases because of a different equilibrium value of 8 when the CF3 groups sharply near 225 K, thereafter continuing to increase but at a are (a) stationary or (b) rotating on the EPR time scale. As was reduced rate. At 450 K the magnitude of aF( 1) is 1.02 G. As mentioned earlier, there is no requirement in the asymmetric discussed above, CF3C60 and C F ~ C F Z Ceach ~ O have two fluorines configuration that 8 be f 120'. Indeed, fast exchange between in the asymmetric position at/near 8 I= f120' whose hyperfine the enantiomers at 150 K would be facilitated by a larger 8. We interactions are (-)0.28 and (-)0.33 G, respectively (Table 1, conclude that, concomitant with the onset of CF3 rotation near 225 K, the equilibrium value of 8 increases, resulting in a larger Figure 2). We therefore conclude that at 150-200 K the 0.33-G (more negative) hyperfine interaction for the unique fluorine hyperfine interaction of the unique fluorine in ( C F ~ ) Z C F Cis~ O nucleus. The reason for this is as follows: a spin population in also negative and diagnostic of an asymmetric equilibrium configuration for this molecule. the F(2s) atomic orbital, which has its maximum value at 8 = Oo, is least at 8 = 180O.' Indeed, if reliable calculations at the We consider briefly the alternative possibility that the equiINDO level were possible for CF3C60, we are confident that they librium configuration of ( C F ~ ) ~ C F Cis~symmetric ,J and that the would confirm our conclusion that there is net spin population hyperfine interaction of its unique 19F nucleus is +0.33 G at 200 in F(2s) for the hypothetical configuration 8 = 180O. As the K. It was shown earlier that d(uF( l ) ) / d T for CF3C60 was -0.45 mG/deg. This means that d2a/aQZ for a 19F nucleus in this equilibrium value of 8 increases, the hyperfine interaction of the position (e = ) ' 0 is negative, and its hyperfine interaction must unique fluorine becomes larger (more negative). The following questions are of interest: why is the equilibrium decrease with increasing temperature. However, it can be seen configuration of CF3CF2Cm symmetric but that of CH3CH2Ca from Figure 2 that plotting a ~ ( 1 of ) (CF3)2CFCm as a positive asymmetric? Also, why is the equilibrium configuration of (CF3)znumber would result in a hyperfine interaction which increases CFC60 asymmetric but that of (CH3)2CHC60 symmetric? We with temperature. For this reason we reject the proposition that believe that the answer to both questions is as follows: a CF3 the equilibrium configuration of ( C F ~ ) ~ C F C is ~symmetric. O group is more comfortable in the symmetric position (over the Nonetheless, the temperature dependence of aF( 1) of (CF3)zCFC60 is extraordinary compared to that of aF(2) of C F ~ C F ~ C ~ Opentagon), but a CH3 group is more comfortable over one of the hexagons. In other words, the total energy of the molecule is a or, indeed, to any other plot of hyperfine interaction us temperature minimum for the symmetric configuration in the case of CF3that we are aware of. We note, however, that such dramatic
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Morton and Preston
The Journal of Physical Chemistry, Vol, 98, No. 19, 1994
experiment senses only their average. The observed spectrum was simulated by the computer program ESREX,8 using the 19F hyperfine interactions mentioned above and in Table 1, except that instead of a ~ ( 2 = ) 4.49 G, separate hyperfine interactions of 3.49 and 5.49 G were used for the two configurations. The other 19Fhyperfine interactions were assumed invariant over the exchange. The I9Fhyperfine interactions in the absence of motion
b.
2.929
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Figure 3. EPR spectra of (CF3)3CCm (a) observed in tert-butylbenzene at 225 K and (b) simulated8 using the parameters discussed in the text (maximum-slope line width = 0.05 G, exchange rate = 10 MHz).
CF& and (CH3)2CHC60 and a minimum for the asymmetric configuration in the case of CH3CH2C60 and ( C F ~ ) ~ C F CFor ~O. (CH3)2CHCm the symmetric configuration, having both CH3 groups over hexagons, is stronger preferred. In the case of (CF3)2CFCao, the asymmetric configuration is preferred because one CF3 group is then located over the pentagon, whereas the symmetric configuration locates neither CF3 group over the pentagon and is therefore strongly disfavored. Computational chemists should eventually be able to provide a quantitative basis for these conclusions. The spectrum of perflUOrO-tert-bUtyl-c6owas also interesting. The spectrum shown in Figure 3a was obtained after 2-min UV photolysis at 225 K and persisted even with the UV off. The hyperfine manifold was analyzed as follows: on each side of the spectrum there are six groups such as that designated A in Figure 3a. The groups have relative intensities 1:2:1:1:2:1, i.e., two I9F nuclei with 1.93-G interactions plus a third with a 2.92-G interaction. Two more 19F hyperfine interactions of 4.49 G connect the high- and low-field sides of the spectrum, except that only their M I = f l . O components are sharp. The central MI = 0 component can be discerned as base-line deflections on either side of the center of the spectrum. The positions of the centers of the 12 groups were measured and analyzed by a least-squares method14 to yield the hyperfine interactions mentioned above, whose errors are less than 0.005 G. Finally, each group, which consists of seven lines, was analyzed as two 19Finteractions of 0.13 G and two more of 0.27 G to yield a substructure of relative intensities 1:2:3:4:3:2:1. Thus, the three CF3groups of perflUOrO-terf-bUtyl-c6oare not freely rotating, and their 19Fhyperfine interactions are those of four pairs of fluorines (on opposite sides of the symmetry plane) and a single fluorine (lying on the symmetry plane). It will also be evident that rotation about the C-c60 bond has ceased on the EPR time scale. We have already established that the unique CF3 group has one 19Finteraction of 2.92 G. The assignment of other 19F nuclei to this group is arbitrary, but two 19F nuclei of 1.93 G seem most likely. This would give an average of 2.26 G, very similar to that of the CF3 group in C F ~ C F ~ C (2.30 ~ O G) where, as we have seen, the CF3 group also lies on the symmetry plane. This leaves the other CF3 groups with l9F hyperfine interactions of 4.49, 0.13, and 0.27 G (average 1.63 G). Loss of the central components of an EPR spectrum is not unusual in a system undergoing exchange. In (CF3)3CC60the central ( M I = 0) transitions are broadened because at 225 K there is exchange between two configurations in each of which there is a pair of fluorine nuclei of unequal hyperfine interaction, whose average is 4.49 G. The hyperfine interactions of these 19F nuclei are interchanged by the exchange process, and the EPR
P
could not be experimentally determined because of line broadening at lower temperatures.15 The best simulation is shown in Figure 3b, which was obtained at an exchange rate of 10 MHz. The observed behavior is probably the result of cooperative interaction between C-CF3 and C w C hindered motions. Using the Eyring rate equation," a net AG' of ca. 5.7 kcal/mol was obtained for the process illustrated above. Finally, we compare the two averages (2.26 and 1.63 G) for CF3 groups of (CF3)3CC60 on and off the plane of symmetry with theaverage 19Fhyperfine interaction of the CF3 groups in (CF3)zCFCm at low temperature (1.96 G). Since there is exchange between the enantiomers of (CF3)2CFC60, we might expect the average l9F hyperfine interaction to be ca. 0.5( 1.63 2.26) = 1.94 G, in almost exact agreement with experiment (1.96 G) and incidentally confirming that the 19Fhyperfine interactions of CF3 groups in the two positions have the same sign.16 This excellent correlation with independent data from I9Fhyperfine interactions in (CF3)3CC60confirms the postulated fast exchange between enantiomers of ( C F ~ ) ~ C F C in~the O low-temperature regime. There is a rich vein of computational chemistry which will have to be explored in order to answer some of the questions raised here. First, thesomewhat surprising indication fromROHF calculations7 that the unique I9F nucleus in CF3Cm carries the largest positive spin population and its inevitable consequence (theseexperiments) that the equivalent fluorines carry net negative spin population. As will be seen from the diagrams in this paper, it is the latter fluorines that are closest to the spin-bearing carbons on the c 6 0 surface (asterisks) and which might therefore have been expected to have a positive hyperfine interaction. Absolute confirmation of the signs of the hyperfine interactions shown in Figure 2 can only be obtained by ENDOR experiments. However, an indication could, perhaps, eventually be obtained from computations capable of reliably predicting negative spin populations in CF3Cao. Second, in CF3CF2C60r the equilibrium configuration is symmetric, but in CH3CH2C60 it is asymmetric and enantiomericid The reverse is true for (CF3)2C&o and (CH3)2CHC60. Are these purely steric effects, or do electronic interactions determine the geometry of the equilibrium configuration?
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Acknowledgment. The authors are grateful to Dr. F. Negri for communicating the results of some preliminary R O H F calculations prior to publication.7 We also thank Dr. Negri and Drs. C. I. Ratcliffe, W. Siebrand, and J. S. Tse for many stimulating discussions and Mr. R. Dutrisac for technical assistance. We also thank Dr. P. J. Krusic for a preprint of ref 6. References and Notes (1) (a) Krusic,P. J.; Wasserman, E.;Keizer, P. N.; Morton, J. R.; Preston, K. F. Science 1991, 254, 1183. (b) Morton, J. R.;Preston, K. F.; Krusic, P. J.; Hill, S.A.; Wasserman, E. J . Phys. Chem. 1992, 96, 3576. (c) Morton, J. R.; Preston, K. F.; Krusic, P. J.; Wasserman, E. J. Chem. SOC.,Perkin
EPR Spectra of Perfluoroalkyl-C60Radicals Trans. 2 1992,1425. (d) Keizer, P. N.; Morton, J. R.; Preston, K. F.; Krusic, P. J. J. Chem. Soc., Perkin Trans. 2 1993, 1041. (e) Krusic, P. J.; Roe, D. C.; Johnston, E.; Morton, J. R.; Preston, K. F.J. Phys. Chem. 1993,97,1736. (2) Tumanskii, B. L.; Bashilov, V. V.; Solodovnikov, S.P.; Sokolov, V. I. Izv. Akad. Nauk Chem. Ser. 1992, 6, 1457; 1992, 8, 240. (3) (a) Balasubramanian, K. Chem. Phys. Lett. 1991, 182, 257. (b) Balasubramanian, K. J . Phys. Chem. 1993, 97, 6990. (4) Morton, J. R.;Negri, F~Prcst0n.K.F. Chem. Phys. Lett. 1994,218, 467. ( 5 ) Keizer, P. N.; Morton, J. R.; Preston, K. F. J . Chem. Soc., Chem. Commun. 1992, 1259. (6) Fagan, P. J.; Krusic, P. J.; McEwen, C. N.; Lazar, J.; Parker, D. H.; Herron. N.: Wasserman. E. Science 1993. 262. 404. (7)'This sign choice, which would otherwise have been completely arbitrary, was made as a result of preliminary ROHF/MNDO calculations on the molecules CH3Ca and C F & , by Dr. F. Negri. In each case, there was more positive spin population in the H( 1s) or F(2s) orbital when the atom was in the staggered symmetric position (0 = Oo) than when it was in the asymmetric position (0 = &120°). In turn, an H or F atom in the latter position carried more positive spin population than one in the ecliped symmetric position (0 = 180'). This calculation cannot generate negative spin populations,however. Calculationson theseradicals at the INDO level (which can, in principle, predict negative spin populations) are suspect because of massive spin contamination.
The Journal of Physical Chemistry, Vol. 98, No. 19, 1994 4997 (8) Heinzer, J.; Norris, J. R. Quantum Chemistry Program Exchange; Indiana University: Bloomington, IN, 1972; Program No. 209 (ESREX). (9) Townes, C. H.; Schawlow,A. L. MicrowaueSpecrroscopy;McGrawHill Book Company Inc.: New York, 1955; Chapter 12. (10) Moss, R. E. Mol. Phys. 1966, 10, 339. (1 1) Morton, J. R.; Negri, F.; Preston, K. F. Phys. Rev. E, in press. (12) Chen, K. S.;Krusic, P. J.; Meakin, P.; Kochi, J. K. J . Phys. Chem. 1974, 78, 2014. (13) At this temperature, the exchange rate is r A a / d 2 , where Aa is the difference in the hyperfine interactions (Hz) averaged by the exchange, and AGt can be estimated from the Eyring rate equation [rate = ( K s T / h ) e x p (-AG'IRT)]. (14) Preston, K. F. Quantum Chemistry Program Exchange; Indiana University: Bloomington, IN, 1976; Program No. 31 1 (ESRLSQ). (15) A referee has pointed out that the above analysis does not eliminate the possibility of two different *9Fnuclei,one with an 8 . 9 8 4 and another with a 0.00-Ghyperfineinteraction, rather than two'9Fnuclei with4.49-G hyperfine interactions. While agreeing in principle, we regard this suggestion as improbable. (1 6) An interesting fact in itself, since we have shown that in CF~CF.I9F hyperfine interactions in the symmetric (0= Oo) and asymmetric positions (0 = *l2Oo) have opposite signs (Figures la and 2).
