Infrared, Raman, and Inelastic Neutron Scattering Spectra of Dodecahedrane: an Ih Molecule in Th Site Symmetry

[Cl@Si20H20]-: Parent Siladodecahedrane with Endohedral Chloride Ion

Fullerenes and diamondoids are at the core of nanoscience. Comparable monodisperse silicon analogues are scarce. Herein, we report the synthesis of the parent siladodecahedrane, which represents the largest Platonic solid. It shares its pattern of pentagonal faces with the smallest fullerene, C₂₀, and its saturated, H-terminated skeleton with diamondoids. Similar to endofullerenes, the silicon cage encapsulates a chloride ion ([Cl@Si₂₀H₂₀]^-); similar to diamondoids, its Si-H termini offer a wealth of opportunities for further functionalization. Mere treatment with chloromethanes leads to the perchlorinated cluster [Cl@Si₂₀Cl₂₀]^-. Both compounds were characterized by mass spectrometry, X-ray crystallography, NMR spectroscopy, and quantum-chemical calculations. The experimentally determined ³⁵Cl resonances of the endohedral chloride ions are particularly diagnostic to probe the Cl^- → Si₂₀ interaction strength as a function of the different surface substituents, as we have proven by high-level computational analyses.

Five Regioisomers of Dimethyl Dodecahedrane Derivatives: A Hybrid DFT B3LYP Study

The hybrid density functional (B3LYP/6-31G(d, p)) method was used to understand why 1,2-dimethyl dodecahedrane has not been reported yet. From the direct dimethyl substitution of the dodecahedrane cage, we could have five C20H18(CH3)2 derivatives which were geometrically optimized without constraints. The results suggest that 1,16-dimethyl dodecahedrane derivative is the most stable, whereas the 1,2-dimethyl derivative is the most unstable; this implies that the distortion due to steric hindrance interactions would be a crucial effect on the relative energies of the dimethyl dodecahedranes. It would be disadvantaged thermodynamically for 1,2-dimethyl derivative that, in the experimental results, was not synthesized yet. The LUMO of each of the derivatives was equivalently delocalized over the void within the cage, implying that dimethyl derivatives are able to encapsulate atoms at the center of the cage. The HOMO was limitedly delocalized on the cage. The characteristics of the HOMO of derivatives show three patterns, implying that each derivative might undergo one of three entirely different sets of characteristic chemical reactions with electrophilic reagents.

Bromination of Unsaturated Dodecahedranes—En Route to C20 Fullerene

As part of a study to achieve selective oligo(poly)bromination-ultimately perbromination-of the dodecahedral C(20) skeleton, the extent and direction of the ionic bromination of dodecahedrene and 1,16-dodecahedradiene were explored. Along sequences of Br(+) additions/deprotonations and allylic rearrangements, up to ten hydrogen atoms were substituted (traces of C(20)H(x)Br(10)). Tetrabromododecahedrenes obtained under defined conditions in up to 50 % total yield with three and four allylic bromine substituents protecting the extremely bent C==C bonds, proved highly unreactive even towards oxygen but reacted rapidly with CH(2)N(2). Upon electron impact ionization (MS) of the newly secured oligo(poly)bromododecahedra(e)nes, sequential loss of the substituents ended generally in polyunsaturated dodecahedranes (in the extreme C(20)H(4), "tetrahydro-C(20) fullerenes"). Only subsequently did skeletal fragmentations occur. From X-ray crystal-structure analyses, more information was obtained on the structural response of the dodecahedral skeleton to the strain induced by the voluminous substituents. As Appendix, the forcing radical bromination of 1,6-dibromododecahedrane and exploratory cis-beta-HBr/cis-beta-Br(2) eliminations in bromododecahedranes with [Fe(2)(CO)(9)], P(2)F/[FeCp(2)] and [Fe(tmeda)Cp*Cl] (in situ protection) are presented.

Structural and vibrational characterization of tetracyanoethylene-hexamethylbenzene as a function of pressure

The neutron powder diffraction and inelastic neutron scattering (INS) spectra of the electron donor-acceptor complex, tetracyanoethylene-hexamethylbenzene have been studied as a function of pressure to 0.414 GPa. Using the PW91 and PBE density functional theories, the unit cell vectors were calculated as a function of pressure and are compared to those experimentally obtained from the diffraction data. The calculated lattice vectors display large errors at low pressures but were found to be in close agreement with the experimental vectors at 0.414 GPa. Comparison of the experimental INS spectra of the TCNE-HMB enabled assignment of specific vibrational modes while providing a direct measurement of the effect of pressure on the complex. The PW91 vibrational frequency calculations reproduced both the vibrational intensities and frequencies with relative accuracy.

Inelastic Neutron Scattering Spectrum of Cs2[B12H12]: Reproduction of Its Solid-State Vibrational Spectrum by Periodic DFT

The inelastic neutron scattering (INS) spectrum of polycrystalline Cs2[B12H12] is assigned through 1200 cm(-1) on the basis of aqueous and solid-state Raman/IR measurements and normal mode analyses from solid-state density functional theory. The Cs+ cations are responsible for frequency shifts of the internal cage vibrational modes and I(h) cage mode splittings due to the crystal T(h) site symmetry. These changes to the [B12H12]2- molecular modes make isolated-molecule calculations inadequate for use in complete assignments. Solid-state calculations reveal that 30/40 cm(-1) shifts of Tg/Hg molecular modes are responsible for structure in the INS spectrum unobserved by optical methods or in aqueous solutions.

Conformational Distinguishability of Medium Cycloalkanes in Crystals via Inelastic Neutron Scattering

This paper begins an exploration of the use of the combination of DFT computations with experimental inelastic neutron scattering (INS) spectra as a method for establishing what conformation is present in a molecular crystal at low temperature. Presented here are INS spectra of a series of medium-sized cycloalkanes: C6H12, C7H14, C8H16, C10H20, C12H24, and C14H28. Optimized geometries and normal mode calculations were performed at B3LYP/6-311G(d,p) on the lowest energy conformations (i.e., those thermally accessible at the experimental temperature of 30 K). The calculated and observed spectra were analyzed for the best fit from each set of conformers, allowing a prediction of the dominant conformation in a conformationally rich system. For each cycloalkane, the calculated spectrum for the lowest energy conformer shows good agreement with experiment while the higher energy conformations have a much poorer fit. With little ambiguity, the lowest energy conformer is therefore predicted to be the dominant conformation, consistent with the diffraction data available for C6H12, C10H20, C12H24, and C14H28. These results indicate that INS spectroscopy may be a useful tool in determining the dominant conformation in a crystal lattice in cases such as this in which the intermolecular interactions are weak and the different conformers are calculated to have distinguishable spectra. Such an analysis is applied to the cases of C7H14 and C8H16 for which no low-temperature X-ray analysis is available. Clear structure predictions result, and the conformer observed is that computed to be of lowest energy for the molecule in isolation.