EXPERIMENTAL AND THEORETICAL CHARGE DENSITIES IN A MODEL "PROTON SPONGE" COMPLEX. Paul R. Mallinson, Kirsty L. McCormack, Chemistry Department, University of Glasgow, UK, and Krzysztof Wozniak, Chemistry Department, University of Warsaw, Poland

Substituted 1,8-diaminonaphthalenes show a wide range of basic strengths, with observed pKa values up to 16.3 [1]. On methylation of 1-dimethyl,8-methylaminonaphthalene to form 1,8-bis(dimethylamino)naphthalene (DMAN) there is a decrease of six orders of magnitude in the acidity constant Ka of the conjugate acid: the pKa value changes from 6.4 to 12.1. The ammonium ion, for comparison, has pKa = 9.25.

The acid salt formed by DMAN with 1,2-dichloromaleic acid contains two strong, asymmetric hydrogen bonds [N-H...N]+ and [O-H...O]- [2]. For the cation, ab initio MO calculations [3] predict an asymmetric H bond to be 25kJ/mol more stable than a symmetric one. Directionality of the H-bond acceptor lone pairs towards the hydrogen nuclear position is observed in Laplacian distributions of the charge density in both cation and anion, obtained from X-ray and neutron diffraction data. On protonation of DMAN, density in the bonds to the carbon at the 9-position increases, while the density in the C-N bonds decreases.

[1] H. A. Staab and T. Saupe, Angew. Chem. 27 (1988), 885-879.

[2] K. Wozniak, H. He, J. Klinowski, W. Jones, and T.L. Barr, J. Phys. Chem. 99 (1995), 14667-14677.

[3] J.A. Platts and S.T. Howard, J. Org. Chem. 59 (1994), 4647-4651.