J. Phys. Chem. 1993,97,1736-1738
1736
EPR Study of Hindered Internal Rotation in Alkyl-Ca Radicalst P.J. Krusic,' D. C. Roe, and E. Johnston Central Research and Development, E. I. du Pont de Nemours & Company, Wilmington, Delaware 19880-0328
J. R. Morton' and I(. F. Preston' Steacie Institute for Molecular Sciences, National Research Council of Canada, 100 Sussex Drive, Ottawa, Ontario, Canada KIA OR6 Received: November 12, 1992;In Final Form: January 19,I993
Electron paramagnetic resonance (EPR) spectra of several simple monoalkyl radical adducts of Cm, studied in solution over a broad temperature range, reveal substantial barriers to internal rotation about the bond connecting the alkyl substituent to the Ca framework. An analysis of the temperature-dependent shape of the spectrum of tert-butyl-C60 afforded an activation energy for internal rotation of 8.2 kcal/mol. Observation of temperature-dependent line-shape effects and comparisons of 'Hand I3Chyperfine interactions indicate the existence of similar barriers for other alkyl adducts (except methyl), which adopt symmetric (e.g., isopropyl) or asymmetric (e.g., ethyl) equilibrium conformations. Reactive free radicals add readily to Cm to yield paramagnetic adducts detectable and identifiable by EPR spectroscopy.l-3 The spectra of the monoalkyl radical adducts, R-Cm. were particularly informative because of their exceptionally narrow line widths (-30 mG).ICsCHyperfine interactions between the unpaired electron and magnetic nuclei in R provided unequivocal identification of the radical adducts. The spectra of the tert-butyl adduct, (CH3)3C-Cm, and of its perdeuterio analog were sufficientlyintense to allow thedetection of hyperfine interactions with framework 13C nuclei in natural abundance. From the number and relative intensitiesof the l3C satellites and associated hyperfine interactions, we concluded that the unpaired electron is essentially confined close to the site of radical addition. The resulting radical structure of C, symmetry and 2A' ground state has most of the unpaired spin density on C1 with lesser amounts on C3, C3' and CS, CY. An analysis of the temperature dependenceofthespectral intensitiesindicated that R-Cmradicals exist in equilibrium with their dimers, R C ~ ~ Oand R allowed , the determination of the dimer bond strengths.1d-c
I\
I.
We now show that in these monoalkyl radical adducts there exist substantial barriers to rotation about the R-Cm bond (C6C9), so that the alkyl substituents adopt preferred equilibrium conformationsrelative to the Cm framework. This conclusion is based on (a) analysis of the temperature-dependent line-shape changes in the spectrum of rert-butyl-&, (b) the observation of distinct rotamers for ( ~ ~ C H ~ ) ( C H ~ ) ~atClow - Ctemperatures, W and (c) a comparison of the magnitudes of proton and I3C hyperfine interactions in the series R = methyl, ethyl, isopropyl, and tert-butyl.
' du Pont Contribution No.6423, NRCC No.35240. 0022-3654/93/2097-1736$04.00/0
The spectrum of tert-butyl-& can be generated in a variety of ways, includingthe photolysis of equimolardi-tert-butylmcrcury and Cm in toluene (-0.003 M ) . ' G 4 When all of the dialkylmercury has decomposed, the sample contains the stable tertbutyl-Cm dimer which can be thermally or photochemically dissociated. As reported earlier, the spectrum at 340 K consists of a binomial multiplet of 10lines appropriate for nine equivalent protons (0.17 G) in fast exchange. As the sample is slowly cooled under continuous illumination to 225 K,the spectrum gradually changes (Figure 1) into that appropriate for six protons of one kind (0.085 G) and three of another kind (0.34 G). These two sets of &proton hyperfine interactions must be of the same sign since their weighted average (0.17 G) equals that of the nine equivalent protons at 340 K. We conclude that the rotation of the rert-butyl group about C b C 9 is strongly hindered at low temperatures and that the potential function governing rotation has minima for the staggered conformationsand maxima for the eclipsed conformations, as in substituted ethanes.5~6 Throughoutthe changes illustrated in Figure 1,two lines remain sharp, and at 275 K they are the only lines detectable. They are thecomponentsoftheMl= f1.5 transitionsofthe340Kspectrum which correspond in total MI (*1.5) and magnetic field location to two transitions in the 225 K spectrum. The outside lines of the spectrum (MI = *4.5) also remain sharp for a similar reason, but they are too weak to detect. The spectral shapes at each temperature were simulated by a modification of the density matrix method for cyclical nonmutual exchange within groups of spins as a function of the rate of exchange k (Figure l).' The activation parameters for the exchange were obtained from a plot of In ( k / T )vs 1/T: AG*= 8.2 kcal/mol, AH* = 7.3 kcal/ mol, and AS*= -2.9 eu. The barrier height is thus commensurate with that hindering rotation of the rerr-butyl group in substituted ethanes (-10 k~al/mol)~ as well as in the recently reported t-BuCm anion (9.3 kcal/mol).8 Confirmation of hindered rotation in terr-butyl-Cm was obtained by labeling one of the methyl groups with I3C (Table I). At 325 K,the 13Chyperfine interactions of the methyl (6) carbon atoms is 0.40 G. At 225 K, the spectrum of ('3CH3)(CH3)2C-Cm is a quintet of septets (Figure 2A), indicating a hyperfine interaction for the 8 13C equal to that of the three protons attached to C11 (0.34 G). Assuming that rotation about C 6 4 9 has ceased on the EPR time scale, the observed spectrum will be a superposition of two spectra in 2:l intensity ratio, one with l3C at C10 or ClV, the other with 13C at C11. Two Q 1993 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 97, No. 9, 1993 1737 Experiment
A
I
K
0.05 G
295+
n
B
h
A
Figure 2. First-derivative EPR s w t r u m of (13CH3)(CH3)2C-C~ at 225 K in toluene (A) and two simdations based on differentbrameters (B and C).
A
B
rl
OK
0.15 G
Figure 1. Second-derivative EPR spectra of (CH,)lC-Ca in toluene at various temperatures and simulations as a function of the proton-exchange rate.
n n
TABLE I: Proton and l3C Hyperfine Interactions (gauss) of R in R-Ca R ~ d i c a l s 4 ~ R
'H
3H, = 0.035 2H, = 0.28 3Hd = 0.12 (CHihCH lH, = 0.49 6Hs 0.13 CHiCHzCHz 2H; = 0.29 2Ha 0.09 3H, = 0.25 C H ~ C H Z C H ~ C H2H, ~ 0.29 2Ha = 0.05 2H, = 0.18 (CHi)zCHCHf 2H, 0.34 1Ha < 0.03 6H, = 0.23 (CHi)iC 9Ha 0.17 (CHM 6Ha = 0.088 3Ha 0.34
CHI CHiCHz -
rJ3C
8J3C
A
h
260K
temp, K
16.5c 15.lC
0.17
