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The EPR spectra of FC60 and FC70
15 April 1994
ELSEVIER
CHEMICAL PHYSICS LETTERS
Chemical Physics Letters 22 1 ( 1994) 59-64
The EPR spectra of F&, and FCTO* J.R. Morton a, K.F. Preston a, F. Negri b aSteacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario, Canada KlA OR9 b Dipartimento di Chimica ‘G. Ciamician’, Universita di Bologna, 40126 Bologna, Italy
Received 25 January 1994; in final form 2 1 February 1994
Abstract The EPR spectra of F&c and four isomers of FC,c are described and discussed. As with H&n, the four isomers of FC,u that are observed are those with fluorine atoms attached to A-, B-, C- or D-type carbon atoms of the Cm molecule. The temperature dependence ( 1/a) da/dT of the r9F hyperfme interaction in FC6c and the most abundant isomer of F&u is approximately twice that observed for the proton hyperfine interaction in HC& and HC,c, and the muon hypertke interaction in Mu&. The similarity of the temperature dependences in Co, and C,c derivatives indicates that the vibrations responsible for the temperature effect are of the same nature for the two radicals. The magnitude of the temperature-dependence coefficients of the fluoroderivatives is consistent with a smaller spin density in F( 2s) compared to that in the H ( 1s) orbital of H&u and HC,c.
1. Introduction A variety of free-radical adducts of Cbo has been studied by electron paramagnetic resonance (EPR) spectroscopy, including HCeO [ 1,2 1, alkyl-Cbo [ 3,4 1, silyl-Cm [ 5 1, thiyl-C,, [ 6 ] and pertluoroalkyl-Cho [ 7 ] radicals. In addition, a small number of tri-adducts, R3C60 [ 81, and a single penta-adduct, ( C6HSCHz) =,Ceo [ 9 1, have been examined by the same technique. Even-numbered adducts, not amenable to study by EPR spectroscopy, have been detected by NMR and mass-spectrometry [ lo], and a few examples of freeradical addition to CT0 have appeared in the literature [11,12]. From the viewpoint of the EPR spectroscopist, the most interesting species are often the simplest, and some of these are missing from the above list, for example HO& ONCeO, 02NC6,, and (halogen)&,. The simplest of all, Mu&, (Mu = muonium, a radio* NRCC No. 37226. Elsevier Science B.V. SSDI0009-2614(94)00231-E
active, light ‘isotope’ of hydrogen) is known, however. Indeed, Mu& was the first free-radical adduct of Cho to be observed, using not EPR but p.SR spectroscopy [ 13,141. In comparing the temperature dependence of the spectrum of Mu& with that of HCbO, it has been noted [ 151 that the value of ( 1/a) da/ dT, where a is the muon or proton hypertlne interaction, was the same for Mu as for H. This observation has been recently rationalized on the basis of a modulation of the hyperfine interaction by the vibrational modes of the radical [ 15 1. It was shown that, under this assumption, the contribution of each vibrational normal mode is independent of the mass of the isotope. Vibrations confined to the carbon cage were found to be responsible for the observed temperature dependence. X&, and X& (X = halogen) radicals have so far eluded detection, presumably because of their tendency to dimerize [ 4,161 or react further to form diamagnetic products. In the present Letter, we report the detection by EPR spectroscopy of C1C6o, F&,
60
J.R. Morton et al. /Chemical Physics Letters 221(1994) 59-64
and four isomers of FC7,,. The temperature dependence ( 1/a) da/dT has been measured for FC6,, and one isomer of F&,, and is compared with the corresponding parameters for HC6e and one isomer of HC,o.
2. Experimental Cso (99.9%) and CT,-, (98%) were obtained from SES Research, Inc., Houston, TX. The fluorine source was SFQ obtained from PCR Inc., Gainesville, FL. Approximately 20 umol of SF&l were added to 350 mm3 of a solution of C6e (or CTO) in tert-butylbenzene or toluene contained in a 5 mm outer diameter thin-walled SuprasilTM EPR tube fitted with a teflon stopcock. The sample was placed in the cavity of the EPR spectrometer where it could be photolyzed in situ at temperatures between 120 and 420 K. Photolyses were carried out using the full, focused light of a 1000 W high-pressure Hg/Xe arc, filtered through 5 cm of water and an Oriel Corp. (Stratford CT) infrared filter. The EPR spectrometer was a Varian El02 spectrometer equipped with a 12 inch magnet and a Lake Shore Cryotronics (Westerville OH) model 805 temperature controller. The magnetic field was continuously monitored by a Bruker ER 035M proton gaussmeter, and the microwave frequency by a SystronDonner model 6057 frequency counter. The spectrometer was operated in the critically-coupled mode, typically at a microwave power of 50 uW. The modulation frequency was 25 kHz, amplitude 50 mG. Resonant magnetic fields and microwave frequencies were converted to spectral parameters (g-factors and hyperfine interactions/MHz) by a computer program appropriate for the exact diagonalization of an isotropic Hamiltonian [ 171.
However, we have so far been unable to obtain the spectrum of SF&,,. Photolysis at 260 K of a solution of CeO in toluene or tert-butylbenzene containing dissolved SF&l yielded the spectrum of Cl&,-, described by the following parameters: gE2.00268,
a( 35C1) = 34.8 MHz .
Over the temperature range 225-325 K, the g-factor was invariant, but the 35C1hyperfme interaction (a) decreased with increasing temperature at a rate ( 1 / a) du/dT= - 11.3x 10-5/deg. We also observed a spectrum attributable to FCso in these samples (Table 1) and, when CT0was substituted for CeOin the solution, the spectra of four of the five possible isomers of F&, were observed (Fig. 1, Table 1). Spectra attributable to ClC,,, were not observed. Presumably, SF&, (if formed at all) is unstable with respect to SF4 and FC6,,, SF5 C6,, -+ SF, + FCs,, , and similarly
for SF&,.
3.1. Identification
3. Results and discussion
In support of the above rationale for the observation of the spectra of F&, and FCO, we first consider whether the “F hyperfine interactions (Table 1) are reasonable for these species. The ground state of HCao is *A’ in C, symmetry, and it is now well established that the unpaired spin in radicals derived from it [ 4,8,14] is predominantly located in radially-directed C (2~) orbitals ortho and para to the carbon carrying the H atom. In an alkyl radical such as ethyl, CH3CH2, the magnitude of the hyperfine interaction of a p proton is a function of the angle (8) between the Cs-H, bond and the axis of the 2p orbital on the adjacent (a) carbon projected onto the plane perpendicular to the C,C, bond [ 181,
During the last few months we have been studying the effects of hindered rotation on the EPR spectra of perlluoroalkyl-C,,, radicals [ 71. As a logical extension of this work we decided to look for the EPR spectrum of SF5&, with a view to measuring the barrier to rotation about the C-S bond in that molecule.
where B,, is usually negligible, and B2 is z 150 MHz [ 191, If we assume that the same relationship holds for HC6,,, with 8% 0 for the ortho C(2p) orbitals, we can rationalize a proton hyperline interaction of only 93 MHz (62% of 150 MHz) as due to partial delo-
J.R. Morton et al. /Chemical Physics Letters 221 (1994) 59-64
61
Table 1 g-factors and r9F hypertine interactions (MHz) of FCso and FC,c in tert-butylbenzene at 325 K. The g-factors and proton hyperfine interactions (MHz) of HCso and HC,c [ 121 are included for comparison purposes. The temperature-dependence parameter ( lO’/a) da/ dT (deg) for FC& HCso and one isomer each of FC,a and HCTO are also included
F&I FC,o
No.
Isomer
Intensity
g
19F hfi
(lO’/a)
D C B A
vs vwb vs m w
2.00229 ’ 2.00148 2.00181 2.00187 2.00133
204.5 149.9 182.7 207.8 208.5
-9.5
1 2 3 4
H&o HC,c
da/dT
-11.6
Isomer
Intensity
g
‘Hhfi
(105/a)
D C B A
vs vs m m VW
2.00216 2.00255 2.00274 2.00213 2.00274
93.0 78.4 96.9 101.4 103.2
-4.6 -4.8
da/dT
’ Errors are k 1 in last digit. b Solvent: toluene at 325 K.
I
8956.50 MHZ
Fig. 1. The EPR spectra of three isomers of FC,a in tert-butylbenzene at 325 K. The numbering is the same as that used in the text and in Table 1.
calization of the unpaired spin population onto the para carbons [4,8,14]. Turning to p fluorons, it has been established for the radical CH2CH2F that the hyperfine interaction (+/MHz) of the 19F nucleus depends on 8 as [ 201 uF( @) = 297
cos*@ ,
so that, by analogy with the foregoing, one would expect a 19F hypertine interaction in FC& of z 184 MHz. Clearly, the observed hyperfine interactions of both FC6,, and FC& (Table 1) are entirely consistent with this ‘prediction’. The difference between alkyl radicals and their flu-
oro-analogues is that in the former, unpaired spin population on p hydrogens is restricted to the H ( 1s) orbital, whereas in the latter, unpaired spin can populate both F( 2s) and F( 2~). In a single crystal study of a derivative of the perfluoroethyl radical, [ 2 1] 0~ C-CF2 -CF-CO, , Rogers and Whiffen concluded from the 19FBhyperline anisotropies that the spin population in each F,( 2p) orbital was z 0.03, and that the directions of the axes of these F( 2p) orbitals were roughly parallel to that of the C,-C, bond. In the case of FChO, whose ground state is *A’ in the C, point group, spin population in F(2p) (and F( 2s) ) is allowed by symmetry provided the axis of the 2p orbital lies on the plane of symmetry of the molecule. Calculations outlined in the next paragraph indicate that in FC6,-, the F (2~) spin population is confined to an orbital roughly coincident with the C-F bond. Additional support for the assignment of the observed hyperfine interaction to i9F nuclei in FCGOor FC7,, is obtained from the MNDO estimate of the unpaired spin distribution in F&. Using the ROHF approach followed by configuration interaction (CI), as described in detail elsewhere [ 8,12,15 1, the F( 2s) spin population in FCbO is predicted to be roughly 30 times smaller than the H( 1s) spin population in HC6,,. For a given spin population in the H( 1s) or F( 2s) atomic orbital a hyperline interaction of
J.R. Morton et al. /Chemical
62
(W3kag,BN~(O) is obtained, where gN is the magnetic moment of the nucleus in question, j? the Bohr magneton, & the nuclear magneton, and @ (0) the probability of the unpaired electron being in H ( 1s) or F( 2s). For unit probability, the above factor is A(H)=1420 MHz and for F(2s),A(F)~53 GHz [ 221, i.e. about 40 times larger. Hence, the ratio of fluorine to hydrogen hyperfine interactions in F&, and HC6,, predicted from MNDO-CI spin populations is of the order of magnitude of the observed value. The underestimate of this ratio (x40/30 as compared to the experimental ratio of x 2 ) can probably be ascribed to the neglect of spin polarization effects. In summary, the above discussion confirms that (i) the hyperfine interactions observed for HC& and FC6,, are consistent with partial delocalization of the unpaired electron over the C6,, cage, and (ii) the unpaired spin distribution on the ChOcage is not affected by the nature of the substituent.
Physics Letters 221(1994)
59-64
---z-w (l/a)dr/dT
8 f 2
=-9.lxlO*/d/deg
190
I3
(l/a) da/dT =-11.7xlO?de#
180 L
3.2. The spectrum of FC,, 225
In Fig. 2 the temperature-dependence parameters (105/a) da/dT of aF in F&c and an in HC6,, are compared (-9.4 and -4.6/deg, respectively). We have shown elsewhere [ 15 ] that the temperature dependence of the hyperfine interaction in HCso is due to its modulation by the assembly of vibrational normal modes of individual frequency Vi, a(T)=ao+
~a~(h/8~~~v~)coth(hv~/2k~T) I
where kB is Boltzmann’s
constant
ai =O.S d’a/aQf
and (2)
is half the second derivative of the hyperfine interaction with respect to the normal coordinate Qi. At sufficiently high temperatures Eq. ( 1) approximates to a(T)=ao+ =a~+
c a:kBT/4x2vf I C biT, I
(3)
where 1, bi is independent of the mass of the isotope [ 15 1. For vibrational modes of sufficiently high frequency Eq. ( 1) approximates to
325
375
42!
Temperature/K
Fig. 2. The temperature dependence of the 19Fhyperfine interactions in FC, and one isomer of F&, (upper), and of the proton hyperfine interactions in HCso [ 151and one isomer of HCTO (lower).
U(T) =a~ + 1 hVibi/2kB = a0 + const ,
(1)
275
(4)
i.e. such vibrations merely change ao. Because of the contribution of high-frequency modes, a straight line with slope Cbi would normally only be expected at inaccessible temperatures. In fact, we have shown [ 15 ] in the case of the isotopomers MuCeO and HCeO that over the temperature range 225-400 K the high-temperature limit applies to lowfrequency modes, and that the high-frequency modes contribute essentially a temperature-invariant constant to the hypertine interaction (Eq. (4) ). Indeed, it is these temperature-invariant contributions from the high-frequency modes of MuCbO that are responsible for the anomalous hypertine ratio aMu/& [ 15 1. As a result, the temperature dependence of the ‘H hypertine interaction in HCso can be represented by a straight line (Fig. 2) whose slope is isotope-indepen-
J.R. Morton et al. /Chemical Physics Letters 221(1994) 59-64
dent (Eq. (3) ) . The vibrations responsible for the temperature-dependent &r in HChO are confined to the carbon cage, and one might expect the same vibrations to have a similar effect on uF in FC6,,. Eqs. (2) and ( 3) show that the temperature-dependence coefficient bi is proportional to the second derivative of the hyperfme interaction with respect to the normal coordinate Q, i.e. the second derivative of the spin density multiplied by the factors A(H) = 1420 MHz or A(F) = 53 GHz referred to above. The experimental results indicate that X.bi(F)/Xbi(H) is x4.5, and the ratio A(F)/A(H) is ~37, implying that the ratio of the second derivatives of the spin densities in F&c and HCdo should be z 0.1. Since we have shown above that the spin density in F(2s) is 20-30 times smaller than that in H( Is), it is not surprising that its second derivative is also smaller. Although a more complex combination of effects might be responsible for the observed temperature dependence in FCdo as compared to HC&, the above discussion shows that the magnitude of the parameter da/dT for FC6,, is compatible with that of HC6,,. It should be kept in mind, however, that the motions involving the substituent have very different frequencies in the two radicals. We have shown [ 15 ] that the C-H stretch and bending motions generate large positive temperature dependences, in contrast to most carbon-cage motions, which show negative temperature dependences. In practice, however, the positive bi coefficients associated with H motion in HC6c do not contribute to the observed temperature dependence because of their high frequencies. The situation might be different in F&,, since the corresponding vibrations will have much lower frequencies which (for CF bends in particular) will fall close to the vibrational frequencies of the carbon cage. 3.3. The spectra of FC,* C,,, differs from CsO in that not all carbons are equivalent. In C,O there are five distinct types of carbon atom labelled, from pole to equator, A, B, C, D and E [ 23 1. The numbers of each type of carbon are, respectively, 10, 10, 20, 20 and 10. In principle, therefore, one might expect to see simultaneously the spectra of five isomers of FC&,, corresponding to the five distinguishable points of attachment of the fluo-
63
rine atom. The local carbon isomer is shown below.
A
configuration
B
E
D
C
D
E
D
D F
D E
E C
for each
D
D F
@@ D E In Fig. 1 we show the spectrum obtained from solutions of CT0 containing dissolved SF&l. Transitions from three isomers are clearly identifiable (numbered 2,3 and 4). The spectrum of a fourth isomer (number 1) was best seen in toluene as solvent (rather than tert-butylbenzene), and was in any event transient in nature. The 19F hypertine interactions and g-factors of the four isomers of FC,,, are collected in Table 1. It is noteworthy that although there is no significant difference between the g-factors of FC6c and HC6c, both being close to the free spin value of 2.00232, this is not true for the g-factors of FC,c and HC,,,. Whereas the g-factors of FC,,, are all below the free spin value, those of H(&, are either above or very close to it. A combination of a larger spin orbit interaction constant for fluorine ( 140 cm- ’ [ 24 ] ) and a different pattern of low-lying excited states could account for the larger deviations from the free spin gfactor of the FC,c isomers. The absence of a similar effect in F&c can tentatively be ascribed to a different pattern of excited states. Only four isomers of HC,,, were observed too [ 12 1. The predicted heat of formation of isomer E (H attached to carbon E) was x 20 kcal/mol greater than that of the others [ 1, and spectrum was signed to Comparison (Table of the interactions of four isomers FC,,, with of H& FC6,, suggests irresistibly that same four of FC&, observed as observed the case HC,c. For the ratio the 19F interaction in four isomers A,
J.R. Morton et al. /Chemical Physics Letters 221(1994) 59-64
64
B, C and D of FCYOto that in FChO is 1.02, 1.02,0.89 and 0.73 respectively. For an in H&,, relative to that in HC&,, the ratios are 1.11, 1.09, 1.04 and 0.84. Thus, in both HCTO and F&, two isomers (A and B) have hyperfine interactions close to, but slightly larger than, that in HCso and F&, respectively. This is not unreasonable, given that A and B are the most C&,-like of the five kinds of carbon on the C,,, surface (see diagram). Isomer D has a smaller hyperfine interaction than its CsOanalogue in both cases, due to greater delocalization of the unpaired spin into the equatorial region [ 121. It is noteworthy that the doubling of (l/a) da/dT from HCso to FC6,, is carried over to the CT0 derivatives. This is in keeping with our interpretation of the temperature dependence in terms of modulation of the hypertine interaction by carboncage vibrations. In fact, C,,, has low-frequency modes very similar in description and frequency to those of &,, and it is reasonable to expect a similar temperature dependence for radical derivatives of Cm and Co. In the case of H addition to Cc,, one of the 2792 [26] distinct isomers of H& was also observed [ 121. With the aid of semi-empirical quantumchemical calculations it was deduced that the observed isomer was of the CCA type, i.e. hydrogen atoms attached to two C- and one A-type carbon atoms of the CT0 molecule. In the present instance no isomers of F&, have, as yet, been detected. Presumably this reflects different stability and/or reactivity for isomer C of HC& vis-a-vis isomer C of FC,,,. Indeed, this is confirmed by the relative intensities of isomer C of HC,,, and F&,: a less stable isomer C is indicated by the intensity of its signal (Fig. 1, Table 1 ), while the absence of signals from the CCA isomer of F&,, of which it is the precursor, might suggest less reactivity. Quantum-chemical calculations which we plan to carry out in the near future may shed additional light on the different characteristics of fluorides and hydrides of C6,, and Co.
References [ 1] J.R. Morton, K.F. Preston, P.J. Krusic and L.B. Knight Jr., Chem. Phys. Letters 204 ( 1993 ) 48 1.
[2] J.A. Howard, Chem. Phys. Letters 203 (1993) 540. [3]J.R. Morton, K.F. Preston, P.J. Krusic, S.A. Hill and E. Wasserman, J. Phys. Chem. 96 (1992) 3576. [4] J.R. Morton, K.F. Preston, P.J. Krusic and E. Wasserman, J. Chem. Sot. Perkin Trans. II (1992) 1425. [5] P.N. Keizer, J.R. Morton, K.F. Preston and P.J. Krusic, J. Chem. Sot. Perkin Trans. II ( 1993 ) 104 1. [6] M.A. Cremonini, L. Lunazzi, G. Placucci and P.J. Krusic, J. Org. Chem. 58 (1993) 4635. [ 71 J.R. Morton and K.F. Preston, J. Chem. Phys., to be published. [8] J.R. Morton, F. Negri and K.F. Preston, Can. J. Chem. in press. [9] P.J. Krusic, E. Wasserman, P.N. Keizer, J.R. Morton and K.F. Preston, Science 254 (1991) 1183. [lo] P.J. Fagan, P.J. Krusic, C.N. McEwan, J. Lazar, D.H. Parker, N. Herron and E. Wasserman, Science 262 (1993) 404. [ 111 P.N. Keizer, J.R. Morton and K.F. Preston, J. Chem. Sot. Chem. Commun. (1993) 1259. [ 121 J.R. Morton, F. Negri and K.F. Preston, Chem. Phys. Letters 218 (1994) 467. [ 131 E.J. Ansaldo, C. Niedermayer and C.E. Stronach, Nature 353 (1991) 121. [ 141 P.W. Percival and S. Wlodek, Chem. Phys. Letters 196 (1992) 317. 115 J.R. Morton, F. Negri and K.F. Preston, Phys. Rev. B, accepted for publication. [16‘I J.R. Morton, K.F. Preston, P.J. Krusic, S.A. Hill and E. Wasserman, J. Am. Chem. Sot. 114 ( 1992) 5454. 117‘I J.R. Morton, Quantum chemistry program exchange, Program No. 3 11, Indiana University, Bloomington ( 1976). 118 C. Heller and H.M. McConnell, J. Chem. Phys. 32 ( 1960) 1535. [ 191 R.W. Fessenden andR.H. Schuler, J. Chem. Phys. 39 (1963) 2147. [20] K.S. Chen, P.J. Krusic, P. Meakin and J.K. Kochi, J. Phys. Chem. 78 (1974) 2014. [21] M.T. Rogers and D.H. Whiffen, J. Chem. Phys. 40 (1964) 2662. [22] J.R. Morton and K.F. Preston, J. Magn. Reson. 30 (1978) 577. [23] R. Taylor, J.P. Hare, A.K. Abdul-Sada and H.W. Kroto, J. Chem. Sot. Chem. Commun. (1990) 1423. [24] J.E. Wertz, J.R. Bolton and J.A. Weil, Electron spin resonance: elementary theory and practical applications (McGraw-Hill, New York, 1993 ) App. G6. [25] I.D. Reid and E. Roduner, Hypertine Interactions, in press. [26] K. Balasubramanian, J. Phys. Chem. 97 (1993) 6990.
ELSEVIER
CHEMICAL PHYSICS LETTERS
Chemical Physics Letters 22 1 ( 1994) 59-64
The EPR spectra of F&, and FCTO* J.R. Morton a, K.F. Preston a, F. Negri b aSteacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario, Canada KlA OR9 b Dipartimento di Chimica ‘G. Ciamician’, Universita di Bologna, 40126 Bologna, Italy
Received 25 January 1994; in final form 2 1 February 1994
Abstract The EPR spectra of F&c and four isomers of FC,c are described and discussed. As with H&n, the four isomers of FC,u that are observed are those with fluorine atoms attached to A-, B-, C- or D-type carbon atoms of the Cm molecule. The temperature dependence ( 1/a) da/dT of the r9F hyperfme interaction in FC6c and the most abundant isomer of F&u is approximately twice that observed for the proton hyperfine interaction in HC& and HC,c, and the muon hypertke interaction in Mu&. The similarity of the temperature dependences in Co, and C,c derivatives indicates that the vibrations responsible for the temperature effect are of the same nature for the two radicals. The magnitude of the temperature-dependence coefficients of the fluoroderivatives is consistent with a smaller spin density in F( 2s) compared to that in the H ( 1s) orbital of H&u and HC,c.
1. Introduction A variety of free-radical adducts of Cbo has been studied by electron paramagnetic resonance (EPR) spectroscopy, including HCeO [ 1,2 1, alkyl-Cbo [ 3,4 1, silyl-Cm [ 5 1, thiyl-C,, [ 6 ] and pertluoroalkyl-Cho [ 7 ] radicals. In addition, a small number of tri-adducts, R3C60 [ 81, and a single penta-adduct, ( C6HSCHz) =,Ceo [ 9 1, have been examined by the same technique. Even-numbered adducts, not amenable to study by EPR spectroscopy, have been detected by NMR and mass-spectrometry [ lo], and a few examples of freeradical addition to CT0 have appeared in the literature [11,12]. From the viewpoint of the EPR spectroscopist, the most interesting species are often the simplest, and some of these are missing from the above list, for example HO& ONCeO, 02NC6,, and (halogen)&,. The simplest of all, Mu&, (Mu = muonium, a radio* NRCC No. 37226. Elsevier Science B.V. SSDI0009-2614(94)00231-E
active, light ‘isotope’ of hydrogen) is known, however. Indeed, Mu& was the first free-radical adduct of Cho to be observed, using not EPR but p.SR spectroscopy [ 13,141. In comparing the temperature dependence of the spectrum of Mu& with that of HCbO, it has been noted [ 151 that the value of ( 1/a) da/ dT, where a is the muon or proton hypertlne interaction, was the same for Mu as for H. This observation has been recently rationalized on the basis of a modulation of the hyperfine interaction by the vibrational modes of the radical [ 15 1. It was shown that, under this assumption, the contribution of each vibrational normal mode is independent of the mass of the isotope. Vibrations confined to the carbon cage were found to be responsible for the observed temperature dependence. X&, and X& (X = halogen) radicals have so far eluded detection, presumably because of their tendency to dimerize [ 4,161 or react further to form diamagnetic products. In the present Letter, we report the detection by EPR spectroscopy of C1C6o, F&,
60
J.R. Morton et al. /Chemical Physics Letters 221(1994) 59-64
and four isomers of FC7,,. The temperature dependence ( 1/a) da/dT has been measured for FC6,, and one isomer of F&,, and is compared with the corresponding parameters for HC6e and one isomer of HC,o.
2. Experimental Cso (99.9%) and CT,-, (98%) were obtained from SES Research, Inc., Houston, TX. The fluorine source was SFQ obtained from PCR Inc., Gainesville, FL. Approximately 20 umol of SF&l were added to 350 mm3 of a solution of C6e (or CTO) in tert-butylbenzene or toluene contained in a 5 mm outer diameter thin-walled SuprasilTM EPR tube fitted with a teflon stopcock. The sample was placed in the cavity of the EPR spectrometer where it could be photolyzed in situ at temperatures between 120 and 420 K. Photolyses were carried out using the full, focused light of a 1000 W high-pressure Hg/Xe arc, filtered through 5 cm of water and an Oriel Corp. (Stratford CT) infrared filter. The EPR spectrometer was a Varian El02 spectrometer equipped with a 12 inch magnet and a Lake Shore Cryotronics (Westerville OH) model 805 temperature controller. The magnetic field was continuously monitored by a Bruker ER 035M proton gaussmeter, and the microwave frequency by a SystronDonner model 6057 frequency counter. The spectrometer was operated in the critically-coupled mode, typically at a microwave power of 50 uW. The modulation frequency was 25 kHz, amplitude 50 mG. Resonant magnetic fields and microwave frequencies were converted to spectral parameters (g-factors and hyperfine interactions/MHz) by a computer program appropriate for the exact diagonalization of an isotropic Hamiltonian [ 171.
However, we have so far been unable to obtain the spectrum of SF&,,. Photolysis at 260 K of a solution of CeO in toluene or tert-butylbenzene containing dissolved SF&l yielded the spectrum of Cl&,-, described by the following parameters: gE2.00268,
a( 35C1) = 34.8 MHz .
Over the temperature range 225-325 K, the g-factor was invariant, but the 35C1hyperfme interaction (a) decreased with increasing temperature at a rate ( 1 / a) du/dT= - 11.3x 10-5/deg. We also observed a spectrum attributable to FCso in these samples (Table 1) and, when CT0was substituted for CeOin the solution, the spectra of four of the five possible isomers of F&, were observed (Fig. 1, Table 1). Spectra attributable to ClC,,, were not observed. Presumably, SF&, (if formed at all) is unstable with respect to SF4 and FC6,,, SF5 C6,, -+ SF, + FCs,, , and similarly
for SF&,.
3.1. Identification
3. Results and discussion
In support of the above rationale for the observation of the spectra of F&, and FCO, we first consider whether the “F hyperfine interactions (Table 1) are reasonable for these species. The ground state of HCao is *A’ in C, symmetry, and it is now well established that the unpaired spin in radicals derived from it [ 4,8,14] is predominantly located in radially-directed C (2~) orbitals ortho and para to the carbon carrying the H atom. In an alkyl radical such as ethyl, CH3CH2, the magnitude of the hyperfine interaction of a p proton is a function of the angle (8) between the Cs-H, bond and the axis of the 2p orbital on the adjacent (a) carbon projected onto the plane perpendicular to the C,C, bond [ 181,
During the last few months we have been studying the effects of hindered rotation on the EPR spectra of perlluoroalkyl-C,,, radicals [ 71. As a logical extension of this work we decided to look for the EPR spectrum of SF5&, with a view to measuring the barrier to rotation about the C-S bond in that molecule.
where B,, is usually negligible, and B2 is z 150 MHz [ 191, If we assume that the same relationship holds for HC6,,, with 8% 0 for the ortho C(2p) orbitals, we can rationalize a proton hyperline interaction of only 93 MHz (62% of 150 MHz) as due to partial delo-
J.R. Morton et al. /Chemical Physics Letters 221 (1994) 59-64
61
Table 1 g-factors and r9F hypertine interactions (MHz) of FCso and FC,c in tert-butylbenzene at 325 K. The g-factors and proton hyperfine interactions (MHz) of HCso and HC,c [ 121 are included for comparison purposes. The temperature-dependence parameter ( lO’/a) da/ dT (deg) for FC& HCso and one isomer each of FC,a and HCTO are also included
F&I FC,o
No.
Isomer
Intensity
g
19F hfi
(lO’/a)
D C B A
vs vwb vs m w
2.00229 ’ 2.00148 2.00181 2.00187 2.00133
204.5 149.9 182.7 207.8 208.5
-9.5
1 2 3 4
H&o HC,c
da/dT
-11.6
Isomer
Intensity
g
‘Hhfi
(105/a)
D C B A
vs vs m m VW
2.00216 2.00255 2.00274 2.00213 2.00274
93.0 78.4 96.9 101.4 103.2
-4.6 -4.8
da/dT
’ Errors are k 1 in last digit. b Solvent: toluene at 325 K.
I
8956.50 MHZ
Fig. 1. The EPR spectra of three isomers of FC,a in tert-butylbenzene at 325 K. The numbering is the same as that used in the text and in Table 1.
calization of the unpaired spin population onto the para carbons [4,8,14]. Turning to p fluorons, it has been established for the radical CH2CH2F that the hyperfine interaction (+/MHz) of the 19F nucleus depends on 8 as [ 201 uF( @) = 297
cos*@ ,
so that, by analogy with the foregoing, one would expect a 19F hypertine interaction in FC& of z 184 MHz. Clearly, the observed hyperfine interactions of both FC6,, and FC& (Table 1) are entirely consistent with this ‘prediction’. The difference between alkyl radicals and their flu-
oro-analogues is that in the former, unpaired spin population on p hydrogens is restricted to the H ( 1s) orbital, whereas in the latter, unpaired spin can populate both F( 2s) and F( 2~). In a single crystal study of a derivative of the perfluoroethyl radical, [ 2 1] 0~ C-CF2 -CF-CO, , Rogers and Whiffen concluded from the 19FBhyperline anisotropies that the spin population in each F,( 2p) orbital was z 0.03, and that the directions of the axes of these F( 2p) orbitals were roughly parallel to that of the C,-C, bond. In the case of FChO, whose ground state is *A’ in the C, point group, spin population in F(2p) (and F( 2s) ) is allowed by symmetry provided the axis of the 2p orbital lies on the plane of symmetry of the molecule. Calculations outlined in the next paragraph indicate that in FC6,-, the F (2~) spin population is confined to an orbital roughly coincident with the C-F bond. Additional support for the assignment of the observed hyperfine interaction to i9F nuclei in FCGOor FC7,, is obtained from the MNDO estimate of the unpaired spin distribution in F&. Using the ROHF approach followed by configuration interaction (CI), as described in detail elsewhere [ 8,12,15 1, the F( 2s) spin population in FCbO is predicted to be roughly 30 times smaller than the H( 1s) spin population in HC6,,. For a given spin population in the H( 1s) or F( 2s) atomic orbital a hyperline interaction of
J.R. Morton et al. /Chemical
62
(W3kag,BN~(O) is obtained, where gN is the magnetic moment of the nucleus in question, j? the Bohr magneton, & the nuclear magneton, and @ (0) the probability of the unpaired electron being in H ( 1s) or F( 2s). For unit probability, the above factor is A(H)=1420 MHz and for F(2s),A(F)~53 GHz [ 221, i.e. about 40 times larger. Hence, the ratio of fluorine to hydrogen hyperfine interactions in F&, and HC6,, predicted from MNDO-CI spin populations is of the order of magnitude of the observed value. The underestimate of this ratio (x40/30 as compared to the experimental ratio of x 2 ) can probably be ascribed to the neglect of spin polarization effects. In summary, the above discussion confirms that (i) the hyperfine interactions observed for HC& and FC6,, are consistent with partial delocalization of the unpaired electron over the C6,, cage, and (ii) the unpaired spin distribution on the ChOcage is not affected by the nature of the substituent.
Physics Letters 221(1994)
59-64
---z-w (l/a)dr/dT
8 f 2
=-9.lxlO*/d/deg
190
I3
(l/a) da/dT =-11.7xlO?de#
180 L
3.2. The spectrum of FC,, 225
In Fig. 2 the temperature-dependence parameters (105/a) da/dT of aF in F&c and an in HC6,, are compared (-9.4 and -4.6/deg, respectively). We have shown elsewhere [ 15 ] that the temperature dependence of the hyperfine interaction in HCso is due to its modulation by the assembly of vibrational normal modes of individual frequency Vi, a(T)=ao+
~a~(h/8~~~v~)coth(hv~/2k~T) I
where kB is Boltzmann’s
constant
ai =O.S d’a/aQf
and (2)
is half the second derivative of the hyperfine interaction with respect to the normal coordinate Qi. At sufficiently high temperatures Eq. ( 1) approximates to a(T)=ao+ =a~+
c a:kBT/4x2vf I C biT, I
(3)
where 1, bi is independent of the mass of the isotope [ 15 1. For vibrational modes of sufficiently high frequency Eq. ( 1) approximates to
325
375
42!
Temperature/K
Fig. 2. The temperature dependence of the 19Fhyperfine interactions in FC, and one isomer of F&, (upper), and of the proton hyperfine interactions in HCso [ 151and one isomer of HCTO (lower).
U(T) =a~ + 1 hVibi/2kB = a0 + const ,
(1)
275
(4)
i.e. such vibrations merely change ao. Because of the contribution of high-frequency modes, a straight line with slope Cbi would normally only be expected at inaccessible temperatures. In fact, we have shown [ 15 ] in the case of the isotopomers MuCeO and HCeO that over the temperature range 225-400 K the high-temperature limit applies to lowfrequency modes, and that the high-frequency modes contribute essentially a temperature-invariant constant to the hypertine interaction (Eq. (4) ). Indeed, it is these temperature-invariant contributions from the high-frequency modes of MuCbO that are responsible for the anomalous hypertine ratio aMu/& [ 15 1. As a result, the temperature dependence of the ‘H hypertine interaction in HCso can be represented by a straight line (Fig. 2) whose slope is isotope-indepen-
J.R. Morton et al. /Chemical Physics Letters 221(1994) 59-64
dent (Eq. (3) ) . The vibrations responsible for the temperature-dependent &r in HChO are confined to the carbon cage, and one might expect the same vibrations to have a similar effect on uF in FC6,,. Eqs. (2) and ( 3) show that the temperature-dependence coefficient bi is proportional to the second derivative of the hyperfme interaction with respect to the normal coordinate Q, i.e. the second derivative of the spin density multiplied by the factors A(H) = 1420 MHz or A(F) = 53 GHz referred to above. The experimental results indicate that X.bi(F)/Xbi(H) is x4.5, and the ratio A(F)/A(H) is ~37, implying that the ratio of the second derivatives of the spin densities in F&c and HCdo should be z 0.1. Since we have shown above that the spin density in F(2s) is 20-30 times smaller than that in H( Is), it is not surprising that its second derivative is also smaller. Although a more complex combination of effects might be responsible for the observed temperature dependence in FCdo as compared to HC&, the above discussion shows that the magnitude of the parameter da/dT for FC6,, is compatible with that of HC6,,. It should be kept in mind, however, that the motions involving the substituent have very different frequencies in the two radicals. We have shown [ 15 ] that the C-H stretch and bending motions generate large positive temperature dependences, in contrast to most carbon-cage motions, which show negative temperature dependences. In practice, however, the positive bi coefficients associated with H motion in HC6c do not contribute to the observed temperature dependence because of their high frequencies. The situation might be different in F&,, since the corresponding vibrations will have much lower frequencies which (for CF bends in particular) will fall close to the vibrational frequencies of the carbon cage. 3.3. The spectra of FC,* C,,, differs from CsO in that not all carbons are equivalent. In C,O there are five distinct types of carbon atom labelled, from pole to equator, A, B, C, D and E [ 23 1. The numbers of each type of carbon are, respectively, 10, 10, 20, 20 and 10. In principle, therefore, one might expect to see simultaneously the spectra of five isomers of FC&,, corresponding to the five distinguishable points of attachment of the fluo-
63
rine atom. The local carbon isomer is shown below.
A
configuration
B
E
D
C
D
E
D
D F
D E
E C
for each
D
D F
@@ D E In Fig. 1 we show the spectrum obtained from solutions of CT0 containing dissolved SF&l. Transitions from three isomers are clearly identifiable (numbered 2,3 and 4). The spectrum of a fourth isomer (number 1) was best seen in toluene as solvent (rather than tert-butylbenzene), and was in any event transient in nature. The 19F hypertine interactions and g-factors of the four isomers of FC,,, are collected in Table 1. It is noteworthy that although there is no significant difference between the g-factors of FC6c and HC6c, both being close to the free spin value of 2.00232, this is not true for the g-factors of FC,c and HC,,,. Whereas the g-factors of FC,,, are all below the free spin value, those of H(&, are either above or very close to it. A combination of a larger spin orbit interaction constant for fluorine ( 140 cm- ’ [ 24 ] ) and a different pattern of low-lying excited states could account for the larger deviations from the free spin gfactor of the FC,c isomers. The absence of a similar effect in F&c can tentatively be ascribed to a different pattern of excited states. Only four isomers of HC,,, were observed too [ 12 1. The predicted heat of formation of isomer E (H attached to carbon E) was x 20 kcal/mol greater than that of the others [ 1, and spectrum was signed to Comparison (Table of the interactions of four isomers FC,,, with of H& FC6,, suggests irresistibly that same four of FC&, observed as observed the case HC,c. For the ratio the 19F interaction in four isomers A,
J.R. Morton et al. /Chemical Physics Letters 221(1994) 59-64
64
B, C and D of FCYOto that in FChO is 1.02, 1.02,0.89 and 0.73 respectively. For an in H&,, relative to that in HC&,, the ratios are 1.11, 1.09, 1.04 and 0.84. Thus, in both HCTO and F&, two isomers (A and B) have hyperfine interactions close to, but slightly larger than, that in HCso and F&, respectively. This is not unreasonable, given that A and B are the most C&,-like of the five kinds of carbon on the C,,, surface (see diagram). Isomer D has a smaller hyperfine interaction than its CsOanalogue in both cases, due to greater delocalization of the unpaired spin into the equatorial region [ 121. It is noteworthy that the doubling of (l/a) da/dT from HCso to FC6,, is carried over to the CT0 derivatives. This is in keeping with our interpretation of the temperature dependence in terms of modulation of the hypertine interaction by carboncage vibrations. In fact, C,,, has low-frequency modes very similar in description and frequency to those of &,, and it is reasonable to expect a similar temperature dependence for radical derivatives of Cm and Co. In the case of H addition to Cc,, one of the 2792 [26] distinct isomers of H& was also observed [ 121. With the aid of semi-empirical quantumchemical calculations it was deduced that the observed isomer was of the CCA type, i.e. hydrogen atoms attached to two C- and one A-type carbon atoms of the CT0 molecule. In the present instance no isomers of F&, have, as yet, been detected. Presumably this reflects different stability and/or reactivity for isomer C of HC& vis-a-vis isomer C of FC,,,. Indeed, this is confirmed by the relative intensities of isomer C of HC,,, and F&,: a less stable isomer C is indicated by the intensity of its signal (Fig. 1, Table 1 ), while the absence of signals from the CCA isomer of F&,, of which it is the precursor, might suggest less reactivity. Quantum-chemical calculations which we plan to carry out in the near future may shed additional light on the different characteristics of fluorides and hydrides of C6,, and Co.
References [ 1] J.R. Morton, K.F. Preston, P.J. Krusic and L.B. Knight Jr., Chem. Phys. Letters 204 ( 1993 ) 48 1.
[2] J.A. Howard, Chem. Phys. Letters 203 (1993) 540. [3]J.R. Morton, K.F. Preston, P.J. Krusic, S.A. Hill and E. Wasserman, J. Phys. Chem. 96 (1992) 3576. [4] J.R. Morton, K.F. Preston, P.J. Krusic and E. Wasserman, J. Chem. Sot. Perkin Trans. II (1992) 1425. [5] P.N. Keizer, J.R. Morton, K.F. Preston and P.J. Krusic, J. Chem. Sot. Perkin Trans. II ( 1993 ) 104 1. [6] M.A. Cremonini, L. Lunazzi, G. Placucci and P.J. Krusic, J. Org. Chem. 58 (1993) 4635. [ 71 J.R. Morton and K.F. Preston, J. Chem. Phys., to be published. [8] J.R. Morton, F. Negri and K.F. Preston, Can. J. Chem. in press. [9] P.J. Krusic, E. Wasserman, P.N. Keizer, J.R. Morton and K.F. Preston, Science 254 (1991) 1183. [lo] P.J. Fagan, P.J. Krusic, C.N. McEwan, J. Lazar, D.H. Parker, N. Herron and E. Wasserman, Science 262 (1993) 404. [ 111 P.N. Keizer, J.R. Morton and K.F. Preston, J. Chem. Sot. Chem. Commun. (1993) 1259. [ 121 J.R. Morton, F. Negri and K.F. Preston, Chem. Phys. Letters 218 (1994) 467. [ 131 E.J. Ansaldo, C. Niedermayer and C.E. Stronach, Nature 353 (1991) 121. [ 141 P.W. Percival and S. Wlodek, Chem. Phys. Letters 196 (1992) 317. 115 J.R. Morton, F. Negri and K.F. Preston, Phys. Rev. B, accepted for publication. [16‘I J.R. Morton, K.F. Preston, P.J. Krusic, S.A. Hill and E. Wasserman, J. Am. Chem. Sot. 114 ( 1992) 5454. 117‘I J.R. Morton, Quantum chemistry program exchange, Program No. 3 11, Indiana University, Bloomington ( 1976). 118 C. Heller and H.M. McConnell, J. Chem. Phys. 32 ( 1960) 1535. [ 191 R.W. Fessenden andR.H. Schuler, J. Chem. Phys. 39 (1963) 2147. [20] K.S. Chen, P.J. Krusic, P. Meakin and J.K. Kochi, J. Phys. Chem. 78 (1974) 2014. [21] M.T. Rogers and D.H. Whiffen, J. Chem. Phys. 40 (1964) 2662. [22] J.R. Morton and K.F. Preston, J. Magn. Reson. 30 (1978) 577. [23] R. Taylor, J.P. Hare, A.K. Abdul-Sada and H.W. Kroto, J. Chem. Sot. Chem. Commun. (1990) 1423. [24] J.E. Wertz, J.R. Bolton and J.A. Weil, Electron spin resonance: elementary theory and practical applications (McGraw-Hill, New York, 1993 ) App. G6. [25] I.D. Reid and E. Roduner, Hypertine Interactions, in press. [26] K. Balasubramanian, J. Phys. Chem. 97 (1993) 6990.