Isomers Of C60H36 Research Articles

Thermodynamic Properties and Hydrogen Accumulation Ability of Fullerene Hydride C60H36

Thermodynamic properties of fullerene hydride C60H36 in the ideal‐gas and crystal states were studied by theoretical methods. Molecular structures and vibration frequencies were calculated for 9 isomers of C60H36 by the density functional theory (DFT) by use of a combination of the B3LYP functional with 6‐31G* basis sets. Ideal‐gas thermodynamic properties were calculated based on those parameters. Enthalpies of formation of C60H36 isomers in the ideal‐gas state were derived from homodesmic reactions involving adamantane, cyclohexane, and C60 fullerene. Using the standard methods of statistical mechanics, heat capacity and derived thermodynamic properties of crystalline C60H36 were calculated at 340–1000 K that extended the range of experimental measurements. With a crystal‐gas heat capacity difference, the experimental value of sublimation enthalpy was extrapolated to room temperature as Δsub H m o (298.15 K)=(193±10) kJ · mol−1. Combining this value with the known experimental enthalpy of formation in the crystalline state, the ideal‐gas enthalpy of formation of C60H36 at the synthesized sample isomer composition was obtained: Δf H m o (298.15 K)=(1206±28) kJ · mol−1. Equilibrium constants and compositions were calculated for the reactions of hydrogenation of C60 fullerene in different states. It was shown that C60 can act as a hydrogen accumulator.

Hydrogen storage in hydrofullerides

Highly reduced fullerenes containing nearly 5 wt. % of dihydrogen can be synthesized using readily available reagents. Hydrides prepared from C60 by dissolving metal reductions give complex mixtures of unstable isomers of C60H36. High temperature syntheses using transfer hydrogenation reagents such as dihydroanthracene provide mainly two stable isomers of C60H36 that have been characterized spectroscopically. Dihydrogen can be removed thermally from these materials at 500 °C. The experimentally determined activation energy for the thermal removal of dihydrogen from C60H2 is 61.4 kcal/mol. This value is in good agreement with the value determined computationally for a multi step radical reaction. The 92 kcal/mol barrier determined computationally for the concerted loss of dihydrogen reflects the symmetry-imposed barrier to dehydrogenation.

Thermodynamic Rearrangement Synthesis and NMR Structures of C1, C3 and T Isomers of C60H36.

The structures of three C60H36 isomers, produced by high-temperature transfer hydrogenation of C(60) in a 9,10-dihydroanthracene melt, was accomplished by 2D (1)H-detected NMR experiments, recorded at 800 MHz. The unsymmetrical C(1) isomer is found to be the most abundant one (60-70%), followed by the C(3) isomer (25-30%) and the least abundant T isomer (2-5%). All three isomers are closely related in structure and have three vicinal hydrogens located on each of the 12 pentagons. Facile hydrogen migration on the fullerene surface during annealing at elevated temperatures is believed to be responsible for the preferential formation of these thermodynamically most stable C60H36 isomers. This hypothesis was further supported by thermal conversion of C60H36 isomers to a single C(3v) isomer of C60H18.

Thermodynamic rearrangement synthesis and NMR structures of C1, C3, and T isomers of C60H36.

The structures of three C60H36 isomers, produced by high-temperature transfer hydrogenation of C(60) in a 9,10-dihydroanthracene melt, was accomplished by 2D (1)H-detected NMR experiments, recorded at 800 MHz. The unsymmetrical C(1) isomer is found to be the most abundant one (60-70%), followed by the C(3) isomer (25-30%) and the least abundant T isomer (2-5%). All three isomers are closely related in structure and have three vicinal hydrogens located on each of the 12 pentagons. Facile hydrogen migration on the fullerene surface during annealing at elevated temperatures is believed to be responsible for the preferential formation of these thermodynamically most stable C60H36 isomers. This hypothesis was further supported by thermal conversion of C60H36 isomers to a single C(3v) isomer of C60H18.

Formation, isolation, spectroscopic properties, and calculated properties of some isomers of C(60)H(36).

Isomers of C(60)H(36) and He@C(60)H(36) have been synthesized by the Birch or dihydroanthracene reduction of C(60) and isolated by preparative high-pressure liquid chromatography. (3)He, (13)C, and (1)H NMR spectroscopic properties were then determined. A comparison of experimental chemical shifts against those computed using density functional theory (B3LYP) with polarized triple- and double-zeta basis sets for He and C,H, respectively, allowed provisional assignment of structure for several isomers to be made. Theoretical calculations have also been carried out to identify low-energy structures. The transfer hydrogenation method using dihydroanthracene gives a major C(60)H(36) isomer and a minor C(60)H(36) isomer with C(3) symmetry as determined by the (13)C NMR spectrum of C(60)H(36) and the (3)He NMR spectrum of the corresponding sample of (3)He@C(60)H(36). In view of the HPLC retention times and the (3)He chemical shifts observed for the Birch and dihydroanthracene reduction products, the two isomers generated by the latter procedure can be only minor isomers of the Birch reduction. A significant energy barrier apparently exists in the dihydroanthracene reduction of C(60) for the conversion of the C(3) and C(1) symmetry isomers of C(60)H(36) to the T symmetry isomer previously predicted by many calculations to be among the most stable C(60)H(36) isomers. Many of the (1)H NMR signals exhibited by C(60)H(36) (and C(60)H(18), previously reported) are unusually deshielded compared to "ordinary" organic compounds, presumably because the unusual structures of C(60)H(36) and C(60)H(18) result in chemical shift tensors with one or more unusual principal values. Calculations clearly show a relationship between exceptionally deshielded protons beta to a benzene ring in C(60)H(18) and C(60)H(36) and relatively long C-C bonds associated with these protons. The additional information obtained from 1D and 2D (1)H NMR spectra obtained at ultrahigh field strengths (up to 900 MHz) will serve as a critical test of chemical shifts to be obtained from future calculations on different C(60)H(36) isomers.

The 3He NMR spectra of C60F18 and C60F36; the parallel between hydrogenation and fluorination

Both i 3HeC60F18 and i 3HeC60F36 have been prepared by fluorinating i 3HeC60. The 3He NMR spectrum of i 3HeC60F18 shows a single line at –16.66 ppm, very close to the value of –16.45 ppm, recorded previously for the isostructural i 3HeC60H18. Density functional calculations afford values of –15.0 and –16.2 ppm for the hydrogenated and fluorinated compounds, respectively. The 3He NMR spectrum of i 3HeC60F36 consists of two almost coincident lines (intensity ratio of ca. 3∶1), at –10.49 and –10.52 ppm, attributable respectively to the C3 and T isomers shown previously to be the components (also in ca. 3∶1 ratio) of C60F36. The spectrum is similar to that (lines at –7.8 and –7.9 ppm in a similar ratio) recorded previously for i 3HeC60H36, and provides compelling evidence that C60H36 also consists of a mixture of C3 (major) and T (minor) isomers. The observed ca. 2–3 ppm upfield shift of the spectral lines for the C60F36 isomers compared to those for the C60H36 isomers is reproduced by both density functional and SCF calculations. The C3 isomer involved has been identified by comparison of its 2D 19F NMR spectrum with that for the T isomer. It is not the isomer calculated to be the most stable one, and its formation is believed to be favoured by contiguous activation of double bonds adjacent to those that have already undergone addition.

Structural parallels in hydrogenated and fluorinated [60]- and [70]-fullerenes

P. W. Fowler, J. P. B. Sandall and R. Taylor, J. Chem. Soc., Perkin Trans. 2, 1997, 419 DOI: 10.1039/A607408A

Possidle Isomers and Electronic Structure of C60H36

ABSTRACTNewly developed empirical hydrocarbon potentials and self-consistent first-principles local density functional methods are used to investigate possible isomers and the electronic structure of C60H36. Within the high symmetry Th structure conjectured by the groups at Rice University there are two inequivalent sets of hydrogen atoms containing twelve and twenty-four atoms respectively. Binding each set either inside or outside of the C60 cage leads to four isomers of C60H36 with inequivalent strain energies. Although we find that placing twelve hydrogens inside the cage can lead to a metastable structure, our calculated total energies suggest that the isomer with all the hydrogens on the outside of the cage is the energetically most stable.