Correlations of various indices of the stability and reactivity of carbon-centered radicals with ESR hyperfine splitting constants have been examined. For a large number of mono-and disubstituted radicals there is a moderately good linear correlation of a-proton hyperfine splitting constants (a(Ha)) with radical stabilization enthalpies (RSE) and with BDE(C-H), the C-H bond dissociation energies for the corresponding parent compounds determined from thermodynamic and kinetic studies of C-C homolysis reactions. There is a similarly satisfactory linear correlation of a(H a) with BDE-(C-H) determined by Bordwell's electrochemical and acidity function method. In all cases the correlations fail for nonplanar radicals. As expected, j3-proton hyperfine splitting constants (a(H,Me)) for radicals with a freely rotating methyl substituent are less sensitive to deviations from planarity and give better linear correlations with RSE and BDE(C-H). The correlations cover a range of more than 20 kcal/mol and are reliable predictors of RSE and BDE(C-H) for a variety of radicals including captodative species. However, the correlations fail for significantly nonplanar radicals and for radicals with cyclic delocalized systems, e.g., cyclopentadienyl. The ratio a(HfiMe)/ a(Ha) for suitably substituted radicals provides an index of pyramidalization and allows one to decide for which compounds values of RSE and BDE(C-H) can be confidently estimated.

An efficient and simple method for the synthesis of two uniform buckminsterfullerene hydrogenation products, C60H18 and C60H36, is offered by the title reaction. The picture on the right shows a probable structure of C60H36 obtained from MM2 calculations. These results are an important step on the way to preparative organic chemistry with C60.

The enthalpy of formation ( = 57.51 ± 0.70 kcal/mol) of triquinacene (1), newly determined by measuring its energy of combustion in a microcalorimeter, is about 4 kcal/mol higher than that previously reported and corresponds to ab initio and density functional theory computational results. As a consequence, the previously derived homoaromatic stabilization energy (claimed to be 4.5 kcal/mol) from enthalpy of hydrogenation measurements is not present in 1. The lack of homoaromaticity in 1 is supported by evaluation of geometric, energetic, and magnetic criteria. In contrast, the isomerization transition state from diademane (5) to 1 is highly aromatic on the basis of the same criteria. The enthalpy of isomerization of 5 to 1 was experimentally determined by differential scanning calorimetry (DSC) to be −29.4 ± 0.3 kcal/mol (measured at 368.2 K). The enthalpy of activation for this rearrangement as determined from the DSC measurements (28.4 ± 0.2 kcal/mol) is 2.5 kcal/mol higher than the value computed at B3LYP/6-311+G**+ZPE.

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The enthalpies of formation [Delta(g)] of tricyclo[8.2.2.2(4,7)]hexadeca-1(13),2,4(16),5,7(15),10(14),11-heptaene (2, 1,2-dehydro[2.2]paracyclophane or [2.2]paracyclophane-1-ene) and tricyclo[8.2.2.2(4,7)]hexadeca-1(13),2,4(16),5,7(15),8,10(14),11-octaene (3, 1,2,9,10-dehydro[2.2]paracyclophane or [2.2]paracyclophane-1,9-diene) have been determined by measuring their heats of combustion in a microcalorimeter and their heats of sublimation by the transpiration method. Values of the strain energies (SE) [SE(2) = 34.7 kcal mol(-)(1), SE(3) = 42.0 kcal mol(-)(1)] have been derived from the gas-phase heats of formation and are compared with those from MM3 and PM3 calculations and with the corresponding value SE(1) = 30.1 kcal mol(-)(1) for the parent tricyclo[8.2.2.2(4,7)]hexadeca-1(13),4(16),5,7(15),10(14),11-hexaene (1, [2.2]paracyclophane). The higher strain energies of 2 and 3 (by 4.6 and 11.9 kcal mol(-)(1)) are in accord with the well-known increased reactivities of their aromatic rings as a consequence of their increased bending. As revealed by an X-ray crystal structure analysis, the bending in the monoene 2 corresponds to that of 1 and 3 at one of two bridging corners.

Before the chemistry of C,, took the current fast development,"] the first two years following the landmark discovery of how to prepare macroscopic amounts of fullerenes by Kriitschmer and HuffmanI2] were characterized mainly by investigations of the physical properties of f u l l e r e n e~.~~] However, it took more than a year for a fundamental thermodynamic quantity of C,,-its heat of combustion -to be deterThis time lapse was due to problems with both the production of highly pure C,, in sufficient quantities and the accuracy of calorimetric measurements on small samples --a procedure which is still far from trivial.Both problems are reflected by large differences among the values published in the a fact that led us to reinvestigate the thermodynamic data in question. Reliable values for the heat of formation of the carbon spheres are of great importance not only because they provide an experimental criterion for the quality of theoretical calculations on fullerene structures, but also because they give an insight into the stability of C,, and of highly strained molecules in general.Even though C,, , the most strained molecule known so far, is much higher in energy than the other two carbon allotropes graphite and diamond, it exhibits a surprising kinetic stability. With regard to the higher fullerenes C,, ( n 270), the question arises whether the incorporation of additional six-membered rings into a carbon network with a fixed number (12) of fivemembered rings leads to a continuous increase in stability. asymptotically approaching the value of graphite.'*' The determination of the heat of combustion of C7(, reported here pro-0 vides the second experimental point on a curve describing the thermodynamic stability of fullerenes as a function of their size.A redetermination of the heats of formation of C,, and C,, was made possible because sufficiently large quantities of highly