Neutral Pyrrole Nitrogen Atom as a π- and Mixed n,π-Donor in Hydrogen Bonding.

9-Dimethylaminobenzo[g]indoles 3-6 and 1-dimethylamino-8-(pyrrolyl-1)naphthalene 7 were examined as possible models for establishing the ability of the pyrrole nitrogen atom to participate in [NHN](+) hydrogen bonding as a proton acceptor. Indoles 3-5 (to a lesser extent 6) form rather stable tetrafluoroborates, with the proton mostly located on the NMe2 group but simultaneously engaged in the formation of a charged intramolecular [NHN](+) hydrogen bond (IHB) with the pyrrole N atom. The theoretically estimated energies of IHB in salts 3H(+)BF(4)(-)-6H(+)BF(4)(-) vary between 7.0-10.7 and 6.2-7.0 kcal mol(-1) in vapor and MeCN, respectively. The pyrrole N atom undergoes a perceptible pyramidalization but still remains involved in the 6 pi-electron aromatic system, suggesting that the hydrogen bonding in salts 3H(+)BF(4)(-)-6H(+)BF(4)(-) represents a previously unknown mixed NH center dot center dot center dot N(n,pi) interaction. Despite the favorable orientation of the N-H bond and the pyrrole ring in salt 7H(+)BF(4)(-), no signs of NH center dot center dot center dot N(n) bonding in it were noticed, and the existing interaction was classified as pure NH center dot center dot center dot N(pi). The results obtained may be useful in studies of secondary protein structures, especially those alpha-helix sections which contain tryptophan residues.