Stereoelectronic Interactions and Molecular Properties. An NBO-Based Study of Uracil

The contributions of stereoelectronic interactions to several molecular properties are used to analyze the propagation of information between different parts of the uracil molecule with a method that is based on the natural bond orbital (NBO) deletion technique. The emphasis is not on the effect of the contributions of localized orbitals to selected properties but rather on their interactions, giving information that is complementary to that resulting from a standard localized molecular orbital contribution. The analysis of how information between orbitals is transmitted throughout the molecule allows interpretation of the ways in which an interaction can affect a molecular property localized in the same region or in a different region of the molecule. A network of stereoelectronic interactions was identified in the uracil molecule, and the relative influences of the interactions that transmit information between different parts of the molecule were evaluated. An analysis was performed over localized properties on atoms and bonds of the two carbonyl groups, namely bond orders, distribution of electronic charge, and NMR shielding tensors. Interactions n(N₃) → π*(C₄O) and n(N₁) → π*(C₅C₆) were the most important delocalizations that carry information concerning the change of substituents at C-5, whereas interactions n(N₁) → π*(C₂O) and n(N₃) → π*(C₂O) were those mainly responsible for transmitting this information to the C-2 uracil carbonyl (C₂O) properties.