Proton affinity and gas phase basicity of diamandoid molecules: diamantane to C₁₃₁H₁₁₆

Calculated proton affinities (PAs) and gas phase basicities (GPBs) are reported for diamantane (C₁₄H₂₀), triamantane (C₁₈H₂₄), 'globular and planar' isomers of tetramantane (C₂₂H₂₈) and pentamantane (C₂₆H₃₂), and for one 'globular' isomer of each of the larger diamondoid molecules: C₅₁H₅₈, C₇₈H₇₂, C₁₀₂H₉₀, and C₁₃₁H₁₁₆. Assuming CH as the parent diamondoid molecule, we calculated PA and GPB values for a variety of CH⁺ isomers, as well as for the reaction CH + H⁺ yielding CH⁺ + H₂(g); the latter is slightly favored based on GPB values for diamantane through pentamantane, but less favored compared to certain CH⁺ isomers of C₅₁H₅₈, C₁₀₂H₉₀, and C₁₃₁H₁₁₆. Indeed, the GPB values of C₅₁H₅₈, C₁₀₂H₉₀, and C₁₃₁H₁₁₆ classifiy them as 'superbases'. Calculations that had the initial location of the proton in an interstitial site inside the diamondoid molecule always showed the H having moved to the outside of the diamondoid molecule; for this reason, we focused on testing a variety of initial configurations with the proton placed in an initial position on the surface. Additional protons were added to determine the limiting number that could be, per these calculations, taken up by the diamondoid molecules and the maximum number of protons are shown in parentheses: C₁₄H₂₀(2), C₁₈H₂₄(3), C₂₂H₂₈(3), C₂₆H₃₂(3), C₅₁H₅₈(4). Bader charge distributions obtained for CH⁺ isomers (for diamantane through pentamantane) suggest that the positive charge is essentially completely delocalized over all the H atoms. NMR spectra were calculated for different isomers of C₁₄H₁₉⁺, and compared to the published NMR spectrum for when diamantane was mixed with magic acid and H₂(g) was produced.