The relationship between the NMR observables and the supramolecular structure of any system is not straightforward. In this work we examine the influence of the crystal packing for three purine derivatives (hypoxanthine, theobromine, and 6-(2-methoxy)benzylaminopurine) on the principal components of the NMR chemical shift tensors (CSTs). We employ density functional calculations to obtain various molecular properties (the ground-state electron density, the magnitudes and orientations of the components of NMR chemical shift tensor, and the spatial distribution of the isotropic magnetic shielding) for the isolated molecules and for the molecules embedded in supramolecular clusters modeling the crystal environment and evaluate their differences. The concept has enabled us to rationalize the effect of the crystal packing on the NMR CSTs in terms of the redistribution of the ground-state electron density induced by intermolecular interactions in the solid state.
The (13)C and (15)N chemical shift tensors are measured, calculated, and compared for three N-benzyladenine isomers with an attempt to characterize differences in electron distribution in the purine ring related to the position of the substituent. Furthermore, the aromaticity of the purine rings is evaluated on the basis of nucleus-independent chemical shifts, and variations among the isomers are discussed. Both parameters indicate significant differences between the electronic properties of the N3-substituted compound and the N7/N9 pair of structures, which can be viewed more generally as the reason for the different stabilities of the individual tautomers.
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