ESTIMATING THERMAL PARAMETERS OF HYDROGEN NUCLEI IN NITROANILINES

C. G. POZZI; A. C. FANTONI; G. PUNTE

Arica Chile

Congreso; X Seminario Latinoamericano de Análisis por Técnicas de Rayos X; 2006

Much effort has been devoted in the last decade to the study, design and understanding of the aggregation of organic molecules. The considerable interest in the field has been based on the fact that crystal engineering, the ability to predict and/or direct the three-dimensional arrangement, can be used to tailor materials of biological or technological importance. Intermolecular hydrogen bonds are an effective tool for organising molecules, and in the case of non-linear optical compounds this is very well typified by nitroaniline compounds, which associate via intermolecular H-bonds. Nitroanilines have received special attention due to their recognized second harmonic generation capability and their large microscopic hyperpolarizabilities. To go deeper into the relationship between molecular and macroscopic properties a detailed knowledge of the charge density, the only true observable, is crucial. It contains valuable information to understand the intra and intermolecular bonds and contacts and can be obtained from accurate X ray diffraction single crystal data. The interpretation is not straight forward as the way in which molecule´s charge density is partitioned in atoms is not unique and accurate atomic displacement parameters, which are essential for obtaining reliable results on physical properties, are not easily available for H atoms. Most charge density studies on molecular crystals assume isotropic motion of hydrogen nuclei, in large part due to the difficulty of performing accurate neutron diffraction studies on all the systems of interest. Failure to model the motion correctly not only alters the topology of the electron density close to the nuclei where the thermal motion has been simplified, but also in the vicinity of neighboring nuclei . Methods for approximating hydrogen atom ADP’s based on a combination of rigid-body analysis and allowances for internal modes have been shown to be quite successful.2,. Spackman et al have recently developed a new method for estimating hydrogen atom ADP´s from first principles, using two layer "ONIOM" calculations, which mimic the effects of the crystal field and yield internal (high frequency) and external (low frequency) contributions to the nuclear motion. The authors claimed that the results obtained for small molecules are consistent with neutron experiments at a variety of temperatures. Taking account of these results we decided to applied the same approximation to three non substituted nitronilines: 4-nitroaniline, 3-nitroaniline and 2-methyl-5-nitroaniline. In this work the ADP´s predicted by the model are compared with neutron diffraction data obtained at 100K. The capability of the method for adequate ADP´S prediction is discussed.