Correlation of the hydrogen-bond acceptor properties of nitrogen with the geometry of the Nsp 2 →Nsp 3 transition in R 1(X=)C–NR 2 R 3 substructures: reaction pathway for the protonation of nitrogen

The non-bonded N...H interactions between N atoms of R 1 (X =)C-N(R 2 R 3 ) (R 2 ,R 3 = Csp 3 or H) substructures and the H atoms of N-H and O-H donors have been analysed using crystallographic data and ab initio molecular orbital calculations. A total of 946 contacts having N...H (d NH ) ≤ 2.75 A were retrieved from the Cambridge Structural Database. For the preferred all-planar (Nsp 2 ) conformation, d NH ranges upwards from ca 2.5 A and H approaches N approximately perpendicular to the plane. However, it is shown that close intramolecular steric interactions lead to major geometrical distortions [pyramidalization at N and rotation about the C-N bond : Ferretti, Bertolasi, Gilli & Gilli (1993). J. Phys. Chem. 49, 13568-13574]. The N atom undergoes a transition from sp 2 to sp 3 with gradual lone-pair formation on N. If N-pyramidality is measured by X N (the angle between the C-N vector and the NR 2 R 3 plane), then as X N increases beyond ca 35° towards the sp 3 value of ca 60° the N...H contacts tend to become significantly shorter (stronger), the N...H donor angle approaches linearity and H approaches N within a 20° cone that has the assumed N lone-pair vector as an axis. A plot of X N versus d NH is interpreted as the reaction pathway for protonation of N and data points from R 1 (X=)C-N + (R 2 R 3 )H systems (the reaction product) occur at the end of this pathway. Crystallographic evidence shows that all 153 contacts 35° and a N...H donor angle above 130° are true hydrogen bonds. The evidence also suggests that the incoming H atoms track the developing N lone-pair density as X N increases from ca 35 to 60°. Ab initio molecular orbital calculations for aniline (6-31G * basis set) give electrostatic potential maps at incremental values of X N (in the range 0-48.5°) that clearly show the developing N lone-pair density.