Glycine is an essential amino acid of particular interest on prebiotic chemistry, which has already been detected in meteorites and cometary samples. According to the literature, solid structures can play a crucial role in glycine reactivity in the interstellar medium. The goal of this work is to investigate different approaches for a theoretical description of the α-, β- and γ-glycine polymorphs, validating our theoretical models by comparing the predicted lattice energy values with the experimental data. Our theoretical models were based on molecular cluster and periodic calculations. Cluster calculations were performed using the: functional and the def2-SVP basis sets were used. For BSSE correction, the geometrical counterpoise (gCP) scheme was employed. Periodic calculations were performed using the PBE exchange correlation functional and the surface k-points mesh have been sampled using a Monkhorst-Pack scheme, with the kinetic cutoff energies properly converged. In order to take the effect of the dispersive interactions, the atom-pairwise (D3BJ) correction was considered in both cluster and periodic calculations. The lattice energy value was predicted, from periodic calculations, using the equation Elatt = (Ecrystal/N) - Emolecule. In the cluster approach, the Ecrystal term is changed by the equivalent Ecluster term and a further correction, corresponding to the proton transfer energy (Uptr) from the zwitterionic glycine form, present in crystal, to neutral (gas phase) molecule, was included. The glycine unitary cell (4 molecules) and its expansions, in respect to the a, b and c axis: 2x1x2 (16 molecules), 3x1x3 (36 molecules) and 4x1x4 (64 molecules) were studied. An extrapolation to an infinite cluster, calculated at B3LYP-gCP-D3/def2-SVP level, was made for α- and β-glycine, showing lattice energy values of -39.65 and -34.75 kcal mol-1, respectively, with a deviation of 5.47 kcal mol-1 from the experimental lattice energy value for a-glycine (-34.18 kcal mol-1, 298 K). Lattice energies, predicted from periodic calculations, for α-, β- and γ-glycine are -35.95, -35.61 and -34.49 kcal mol-1, respectively, showing deviations of 3.7 kcal-mol-1 and 0.86 kcal mol-1 from the α- and β-glycine cluster calculations. It is possible to assume that the lattice energy values obtained from cluster calculations show good agreement with the periodic calculations and experimental values, showing that this model can be used for future polymorphs representation.

Geometry and lattice energy prediction for α-, β- and γ-glycine molecular crystals

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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