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 less than or equal to Angstrom were retrieved from the Cambridge Structural Database, For the preferred all-planar (Nsp(2)) conformation, d(NH) ranges upwards from ca 2.5 Angstrom 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 chi(N), (the angle between the C-N vector and the NR(2)R(3) plane), then as chi(N) increases beyond ca 35 degrees towards the sp(3) value of ca 60 degrees the N ... H contacts tend to become significantly shorter (stronger), the N ... H donor angle approaches linearity and H approaches N within a 20 degrees cone that has the assumed N lone-pair vector as an axis. A plot of chi(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 ( 2.75 Angstrom that have chi(N) greater than or equal to 35 degrees and a N ... H donor angle above 130 degrees are true hydrogen bonds, The evidence also suggests that the incoming H atoms track the developing N lone-pair density as lambda(N) increases from ca 35 to 60 degrees. Ab initio molecular orbital calculations for aniline (6-31G*) basis set) give electrostatic potential maps at incremental values of chi(N) (in the range 0-48.5 degrees) that clearly show the developing N lone-pair density.