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⁺BF₄^––6H⁺BF₄^– 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 6π-electron aromatic system, suggesting that the hydrogen bonding in salts 3H⁺BF₄^––6H⁺BF₄^– represents a previously unknown mixed NH···N­(n,π) interaction. Despite the favorable orientation of the N–H bond and the pyrrole ring in salt 7H⁺BF₄^–, no signs of NH···N­(n) bonding in it were noticed, and the existing interaction was classified as pure NH···N­(π). The results obtained may be useful in studies of secondary protein structures, especially those α-helix sections which contain tryptophan residues.