Physicochemical properties of zwitterionic L- and DL-alanine crystals from their experimental and theoretical charge densities

The total experimental electron density distributions (ED) of zwitterionic L- and DL-alanine crystals, as derived from extensive sets of X-ray diffracted intensities collected at 23 and 19 K, are compared to gain an insight into the different physical properties of the two related chiral compounds in the solid state, and to explore the extent of the ED ransferability. Relevant parameters that characterize the two crystal forms are obtained, showing differences and similarities in terms of i) geometric descriptors, ii) topological indexes, iii) molecular electrostatic potential distributions, iv) atomic volumes and charges, v) molecular electric moments and vi) electrostatic interaction energies. To assess the relative stability of the racemate with respect to the pure enantiomer, the crystal lattice energies, as obtained through DFT fully periodic calculations, are also discussed and compared with the experimental sublimation enthalpies after correction for the proton transfer energies. In-crystal group charges, evaluated with the Quantum Theory of Atoms in Molecules, are found to be transferable between the racemic and the pure enantiomer, at variance with group volumes..........