On the use of crystal vibrational modes in the estimation of the anisotropic displacement parameters of hydrogen atoms in molecular crystals: para-Nitroaniline as a test case

The use of crystal vibrational modes and frequencies calculated for the periodic system to complement a Translation Libration Screw (TLS) fit in the estimation of anisotropic displacement parameters (ADPs) of hydrogen atoms in molecular crystals is proposed. As a test case we have used the para-nitroaniline crystal, for which a reference set of ADPs has been obtained by performing a single crystal neutron diffraction study at 100K. Although the largest difference between estimated and experimental reference values amounts to 0.06Å2, this value being about six times the experimental uncertainty, the agreement is better than three experimental uncertainties for 33 out of the total of 36 ADPs. The performance of the suggested method, particularly for the amino atoms, is thoroughly analyzed.