Theoretical calculations of hypersurfaces of the 13C chemical shift anisotropy in the C [dbnd]O⋯H [sbnd]N hydrogen bond and the benefit for the ab initio structure determination

Abstract

We investigated the influence of structural changes on the anisotropic part of the carbonyl 13C chemical shift tensor in a model complex containing hydrogen bonded cyanuric acid and pyrrole. The model was chosen for its chemical resemblance to cyameluric acid. In the solid state this compound comprises three different hydrogen bonds which are well distinguishable based on the anisotropy parameters δ aniso and η of the carbonyl 13C atoms. The variation of six relevant structural variables in the model system produced hypersurfaces for the isotropic shift, δ aniso and η. Our goal was to investigate whether such surfaces can be used for the ab initio structure determination of hydrogen bonds. With a medium size basis set it could be shown that although the absolute values differ DFT describes the relative change in δ aniso and η close to the quality of MP2 calculations. Due to the high dimensionality of the hypersurface we had to reduce the number of variables in our study. We systematically created subsurfaces each described by three of the six variables and investigated their isolated influence on the NMR observables. We identified the most important structure parameters and on this base built a minimal model. For a fixed NO distance the hydrogen bond arrangement was altered by two angular variations and one dihedral distortion. In this model evidence was found that the η surfaces for different NO distances exhibit a uniform shape and can be transformed into one another by a simple shift and multiplication by a mean factor. Furthermore, the experimental parameters δ aniso and η of cyameluric acid were taken as a base for the extraction of structures from the hypersurfaces. δ aniso and η unequivocally selected ensembles of similar structure and the COHN arrangement in two of the three cyameluric hydrogen bonds could be predicted with good quality from the theoretical model. Our results show that it is possible to predict the distance and at least qualitatively the orientation in a hydrogen bond environment from an analysis of the anisotropic part of the 13C chemical shift tensor.