Crystal-field effects in l-homoserine: multipoles versus quantum chemistry

In contrast to an isolated molecule with identical geometry, the electron density of a molecule in the crystalline solid state is influenced by the field of surrounding molecules and by intermolecular hydrogen bonding. These influences have not yet found wide study on the level of the molecular electron-density distribution, which can be obtained both from high-resolution X-ray single-crystal diffraction as well as from ab initio quantum chemistry. To investigate this `crystal-field effect' on the non-standard amino acid l-homoserine, three approaches were taken: (i) an ab initio point-charge isolated-molecule model; (ii) structure refinement with Hirshfeld atoms with and without surrounding point charges and dipoles; (iii) benchmark periodic calculations using density functional theory. For (i) and (iii) multipole models were fitted to static structure factors. The difference between the electron density obtained from the respective in-crystal model and from the isolated-molecular calculations yields detailed information on the crystal-field effect and dipole-moment enhancements. The point-charge model produces features of interaction density which are in good agreement with those from periodic ab initio quantum chemistry. On the other hand the multipole model is unable to reproduce fine details of the interaction density for zwitterionic homoserine.