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

Calculated proton affinities (PAs) and gas phase basicities (GPBs) are reported for diamantane (CH), triamantane (CH), 'globular and planar' isomers of tetramantane (CH) and pentamantane (CH), and for one 'globular' isomer of each of the larger diamondoid molecules: CH, CH, CH, and CH. 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 + H(g); the latter is slightly favored based on GPB values for diamantane through pentamantane, but less favored compared to certain CH isomers of CH, CH, and CH. Indeed, the GPB values of CH, CH, and CH 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: CH(2), CH(3), CH(3), CH(3), CH(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 CH, and compared to the published NMR spectrum for when diamantane was mixed with magic acid and H(g) was produced.