Can the interaction density be measured? The example of the non-standard amino acid sarcosine

The experimental charge density rho(r) of the non-standard amino acid sarcosine has been determined based on an extensive and complete data set measured at 100 K to high resolution (sin theta/lambda = 1.18 A(-1)) by single-crystal X-ray diffraction. Anisotropic thermal motion of the H atoms, obtained from TLS + ONIOM cluster methods, was included in the structural model. Based on the multipole-model geometry, the theoretical Hartree-Fock interaction density of a molecule in the crystal has been calculated with CRYSTAL98. It manifests itself in local rearrangements of rho(r) and can be reproduced with a multipole projection via simulated structure factors. An attempt has also been made to obtain the interaction density from a combination of experimental and theoretical charge densities using either a whole-molecular calculation or the invariom database. Agreement with the periodic Hartree-Fock interaction density is qualitative. It is shown that invarioms reproduce the features of the theoretical multipole-projected whole-molecular electron density, and can be used to approximate it.