Stabilizing factors of the molecular structure in silicon-based peptidomimetics in gas-phase and water solution. Assessment of the correlation between different descriptors of hydrogen bond strength.

The use of DFT (B3LYP and M06L) and ab initio (MP2) computational methods allowed us to perform a thorough conformational study of N-[dihydroxy (methyl)silyl]methylformamide (DHSF) and 3-[dihydroxy (methyl) silyl] propanamide (DHSP), that could be considered simplified models of the environment of the silanediol group in silicon gem-diols that have proven efficiency as protease inhibitors. We have found a total of 13 molecular conformations that represent minima in the potential energy surfaces of DHSF (six conformers) and DHSP (seven conformers). The key feature in their molecular structure is the occurrence of intramolecular hydrogen bonding between the hydroxyl and aminocarbonyl groups. We have estimated the strength of each individual hydrogen bond in the mentioned species using the descriptors proposed by three different methodologies, i.e., the quantum theory of atoms in molecules (QTAIM), the natural bond orbitals population analysis (NBO), and the so-called empirical Rozenberg's enthalpy-distance relationship. We have found a good correlation among the calculated values for the different descriptors within the whole set of conformers in the molecular systems in this study. We have also discussed the predicted order of stabilities of the different conformers of each species in terms of the so-called ring anomeric effect (RAE) and generalized anomeric effect (GAE). Finally, we also analyzed the discrepancies found in the order of stability when going from the isolated molecule approximation to water solution (PCM).