H-bond network in amino acid cocrystals with H ₂O or H ₂O ₂. the DFT study of serine - H ₂O and Serine - H ₂O ₂

The structure, IR spectrum, and H-bond network in the serine - H ₂O and serine - H ₂O ₂ crystals were studied using DFT computations with periodic boundary conditions. Two different basis sets were used: the all-electron Gaussian-type orbital basis set and the plane wave basis set. Computed frequencies of the IR-active vibrations of the titled crystals are quite different in the range of 10 - 100 cm ^-1. Harmonic approximation fails to reproduce IR active bands in the 2500 - 2800 frequency region of serine - H ₂O and serine - H ₂O ₂. The bands around 2500 and 2700 cm ^-1 do exist in the anharmonic IR spectra and are caused by the first overtone of the OH bending vibrations of H ₂O and a combination vibration of the symmetric and asymmetric bendings of H ₂O ₂. The quantum-topological analysis of the crystalline electron density enables us to describe quantitatively the H-bond network. It is much more complex in the title crystals than in a serine crystal. Appearance of water leads to an increase of the energy of the amino acid - amino acid interactions, up to ∼50 kJ/mol. The energy of the amino acid - water H-bonds is ∼30 kJ/mol. The H ₂O/H ₂O ₂ substitution does not change the H-bond network; however, the energy of the amino acid - H ₂O ₂ contacts increases up to 60 kJ/mol. This is caused by the fact that H ₂O ₂ is a much better proton donor than H ₂O in the title crystals.