Parametrizing the Spatial Dependence of H NMR Chemical Shifts in π-Stacked Molecular Fragments.

Most recently a renewed interest in several areas has arisen in factors governing the H NMR chemical shift (H CS) of protons in aromatic systems. Therefore, it is important to describe how H CS values are affected by π-stacking intermolecular interactions. The parametrization of radial and angular dependences of the H CS is proposed, which is based on conventional gauge-independent atomic orbital (GIAO) calculations of explicit molecular fragments. Such a parametrization is exemplified for a benzene dimer with intermonomer vertical and horizontal distances which are in the range of values often found in crystals of organic compounds. Results obtained by the GIAO calculations combined with B3LYP and MP2 methods were compared, and revealed qualitatively the same trends in the H CS data. The parametrization was found to be quantitatively correct for the T-shaped benzene dimers, and its limitations were discussed. Parametrized H CS surfaces should become useful for providing additional restraints in the search of site-specific information through an analysis of structurally induced H CS changes.