Mapping of N-C Bond Formation from a Series of Crystalline Peri-Substituted Naphthalenes by Charge Density and Solid-State NMR Methodologies

A combination of charge density studies and solid state nuclear magnetic resonance (NMR) ¹ J_NC coupling measurements supported by periodic density functional theory (DFT) calculations is used to characterise the transition from an n-π* interaction to bond formation between a nucleophilic nitrogen atom and an electrophilic sp² carbon atom in a series of crystalline peri-substituted naphthalenes. As the N⋅⋅⋅C distance reduces there is a sharp decrease in the Laplacian derived from increasing charge density between the two groups at ca. N⋅⋅⋅C = 1.8 Å, with the periodic DFT calculations predicting, and heteronuclear spin-echo NMR measurements confirming, the ¹ J_NC couplings of ≈3-6 Hz for long C-N bonds (1.60-1.65 Å), and ¹ J_NC couplings of <1 Hz for N⋅⋅⋅C >2.1 Å.