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

Abstract

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 CxHy as the parent diamondoid molecule, we calculated PA and GPB values for a variety of CxHy+1+ isomers, as well as for the reaction CxHy + H+ yielding CxHy-1+ + H-2(g); the latter is slightly favored based on GPB values for diamantane through pentamantane, but less favored compared to certain CxHy+1+ 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 CxHy+1+ 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 H-2(g) was produced.