Impact of dihydrogen bonding on lattice energies and sublimation enthalpies of crystalline [H2GaNH2]3, [H2BNH2]3 and [H2GeCH2]3

The lattice energies of [H₂GaNH₂]₃, [H₂BNH₂]₃ and [H₂GeCH₂]₃ in their experimentally determined space groups, P2₁/m, Pmn2₁ and Pbcm, respectively, were calculated using density functional methods for periodic structures with the ab initio periodic code CRYSTAL17. Using the basis set pob-TZVP for all calculations, B3LYP including Grimme''s D3 dispersion correction was found to reproduce experimental bond distances and angles most accurately. CRYSTAL17 was also used to optimize geometries and calculate energies of the molecular structures in the gas phase. While the chair conformation of the six-membered rings is found in all of the crystals, only [H₂GeCH₂]₃ retains this as the preferred conformation in the gas phase. By contrast, a twist-boat conformation is preferred for both [H₂GaNH₂]₃ and [H₂BNH₂]₃ in the gas phase, and thus a correction for this change in conformation must be included in corresponding sublimation enthalpy calculations. In addition to the D3 dispersion correction, all lattice energies included a correction for basis set superposition error. The lattice energies for [H₂GaNH₂]₃, [H₂BNH₂]₃ and [H₂GeCH₂]₃ were 153.5, 120.8 and 84.9 kJ mol⁻¹, respectively. These values were used to calculate the sublimation enthalpies, which exhibited good agreement for the single case where an experimental measurement is available, namely [H₂BNH₂]₃ (exp ΔH_sub(298), 119 ± 12 kJ mol⁻¹; calcd, 119.4 kJ mol⁻¹). The energetic impact of the crystal structure was assessed by minimizing the structures of each molecule in each of the three space groups spanned by them experimentally and calculating their respective lattice energies. In every case, the experimentally observed space group was the one computed to be the most stable.

Calculated lattice energies and sublimation enthalpies provide quantitative measures of the importance of intermolecular dihydrogen bonds.