Role of C?H���O hydrogen bonds in the ionic complexes of 1,8-bis(dimethylamino)naphthalene

On the basis of experimental charge density distributions in a series of ionic complexes of 1,8-bis(dimethylamino)naphthalene (DMAN) with four different acids, 1,2,4,5-benzenetetracarboxylic acid (pyromellitic acid), 4,5-dichlorophthalic acid, dicyanoimidazole and o-benzoic sulfimide dihydrate (saccharin), the role of minor CH···O hydrogen bonds was analysed. We propose a multicentre model of hydrogen bonding in proton sponges {[Me2NH···.NMe2]+ ··· Xδ−}. It appears that weak interactions of electronegative atoms with the nearest methyl hydrogen atoms can influence the location of the proton in the intramolecular {NH···N}+ hydrogen bridge. In the case of CH···O hydrogen bonds, the amount of electron density in the donor CH bonds involved in hydrogen bonding is slightly larger than that in those CH from the same methyl group which are not involved in hydrogen bonding. Numerical values of the Laplacian follow a reverse relation (they are smaller for the donor CH bonds). Both of these critical point parameters (rho and Laplacian) are linearly dependent on the length of interaction line. An increase in the amount of electron density in the H···O hydrogen bonding is associated with an increase in electron density in the donor C(methyl)H donor bond, which confirms the mainly ionic nature of such weak hydrogen bonds. Copyright © 2003 John Wiley & Sons, Ltd.