H-Bond Network in Amino Acid Cocrystals with H2O or H2O2. The DFT Study of Serine鈥揌2O and Serine鈥揌2O2
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The structure, IR spectrum, and H-bond network in the serine鈥揌2O and serine鈥揌2O2 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鈥?00 cm鈥?. Harmonic approximation fails to reproduce IR active bands in the 2500鈥?800 frequency region of serine鈥揌2O and serine鈥揌2O2. The bands around 2500 and 2700 cm鈥? do exist in the anharmonic IR spectra and are caused by the first overtone of the OH bending vibrations of H2O and a combination vibration of the symmetric and asymmetric bendings of H2O2. 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鈥揳mino acid interactions, up to 50 kJ/mol. The energy of the amino acid鈥搘ater H-bonds is 30 kJ/mol. The H2O/H2O2 substitution does not change the H-bond network; however, the energy of the amino acid鈥揌2O2 contacts increases up to 60 kJ/mol. This is caused by the fact that H2O2 is a much better proton donor than H2O in the title crystals.

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