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abstracts
"The renaissance in Laue studies - at neutron sources - provides us with access to single crystal neutron diffraction data for synthetic compounds without requiring synthesis of prohibitively large amounts of compound or improbably large crystals. Such neutron diffraction studies provide vital data where proof of the presence or absence of hydrogen in particular locations is required and which cannot validly be proved by X-ray studies. Since the commissioning of KOALA at OPAL in 2009[1] we have obtained numerous data sets which demonstrate the vital importance of measuring data even where the extent of the diffraction pattern is at relatively low resolution - especially when compared to that obtainable for the same compound with X-rays. In the Laue experiment performed with a fixed radius detector, data reduction is only feasible for crystals in the ""goldilocks"" zone - where the unit cell is relatively large for the detector, a correspondingly low resolution diffraction pattern in which adjacent spots are less affected by overlap will yield more data against which a structure can be refined than a pattern of higher resolution - one where neighbouring spots overlap rendering both unusable (in our current methodology). Analogous application of powder neutron diffraction in such determinations is also considered. Single crystal neutron diffraction studies of several important compounds (up to 5KDa see figure below)[2] in which precise determination of hydride content by neutron diffraction was pivotal to the final formulation will be presented. The neutron data sets typically possess 20% or fewer unique data at substantially "lower resolution" than the corresponding X-ray data sets. Careful refinement clearly reveals chemical detail which is typically unexplored in related X-ray diffraction studies reporting high profile chemistry despite the synthetic route being one which hydride ought to be considered/excluded in product formulation."
abstracts
"Anisotropic parametrisation of the thermal displacements of hydrogen atoms in single-crystal X-ray structure refinement is not possible with independent atom model (IAM) scattering factors. This is due to the weak scattering contribution of hydrogen atoms. Only when aspherical scattering factors are used can carefully measured Bragg data provide such information. For conventional structure determinations parameters of ""riding"" hydrogen atoms are frequently constrained to values of their ""parent"" heavy atom. Usually values of 1.2 and 1.5 times X-U_eq are assigned to H-U_iso in these cases. Such constraints yield reasonable structural models for room-temperature data. However, todays small molecule X-Ray diffraction experiments are usually carried out at significantly lower temperatures. To further study the temperature dependence of ADPs we have evaluated several data sets of N-Acetyl-L-4-Hydroxyproline Monohydrate at temperatures ranging from 9 K to 250 K. Methods compared were HAR [1], Invariom refinement [2], time-of-flight Neutron diffraction and the TLS+ONIOM approach [3]. In the TLS+ONIOM approach non-hydrogen ADPs from Invariom refinement provided ADPs for the TLS-fit. Hydrogen atoms in all methods were grouped and analyzed according to their Invariom name. We reach a good agreement of the temperature dependence of H-U_iso/X-U_eq. At very low temperatures the ratio H-U_iso/X-U_eq can be as high as 4, e.g. for Hydrogen attached to a sp3 carbon atom with three non-Hydrogen atom neighbors. Since all methods consistently show that the H-U_iso/X-U_eq ratio is temperature dependent, this effect should be taken into account in conventional structure determinations."
research papers
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The temperature dependence of hydrogen Uiso and parent Ueq in the riding hydrogen model is investigated by neutron diffraction, aspherical-atom refinements and QM/MM and MO/MO cluster calculations. Fixed values of 1.2 or 1.5 appear to be underestimated, especially at temperatures below 100 K.
abstracts
Crystals of imidazole compounds typically organize themselves with NH---N hydrogen bonded 1D chains in the absence of other hydrogen bond donor or acceptor groups. The 'synthon' is robust, but can vary considerably in length, which is related to the pKa and the linearity of the hydrogen bond. For symmetric imidazoles with effectively perfect pKa matching of the two N atoms, this implies that many crystals may exhibit statistical disorder of the hydrogen positions. However X-ray studies alone cannot readily distinguish the true extent of this, since unlike carboxylic acid analogues, even fully ordered systems show little difference in the N(1)-C(2) and N(3)=C(2) bond lengths of the imidazole ring. Despite their fundamental nature, surprisingly few neutron studies have been carried out on such NH---N systems and only the parent compound imidazole itself has been examined in depth.[1,2] Herein we report the extension of this to a range of other imidazoles and benzimidazoles. In particular 2-methylbenzimidazole [3] and 2-ethylbenzimidazole are ideal candidates to allow study of bent and linear, ordered and disordered hydrogen bond chains in this family of compounds. Neutron data were collected at low temperature on these and other compounds, using specimens of around cubic 1mm on KOALA Laue neutron diffractometer at the Bragg Institute, Australia. The results support the general findings of related single crystal X-ray studies. However the N-H positions are obtained with good accuracy and correlations of covalent N-H to intermolecular H--N can be made. They also confirm that essentially full ordering of hydrogen bond chains in crystals such as 2-Ethylbenzimidazole (shown in Figure) is possible, with only a single 'negative peak' for the proton position found along the N---N vector. This may have positive implications for the development of such crystals as molecular ferroelectrics. The authors are grateful for an award of beam-time by ANSTO (Proposal 2765).
