Cooperative hydrogen bond between piperidine-ethanol and 2,6-dichloro-4- nitrophenol

2-(1-Piperidine)ethanol (PEt) and 2,6-dichloro-4-nitrophenol (DCNP) form 1:1 and 1:2 hydrogen-bonded complexes. Their molecular structures and spectroscopic properties have been characterized by X-ray diffraction, the B3LYP/6-311++G(d,p) calculations, Raman, FTIR and UV–vis spectroscopy. In the 1:1 complex, piperidinium-ethanol cation and DCNP− anion form a centrosymmetric dimer through two NH⋯O and OH⋯O hydrogen bonds of 2.735(1) and 2.588(1) Å, respectively. Two independent PEt·(DCNP)2 molecules (A and B) exist in the 1:2 complex. In each of them the PEt+H cation, neutral DCNP and DCNP− anion molecules interact through the cooperative N+H⋯O−⋯HO⋯HO hydrogen bond. The experimental FTIR spectra of the 1:1 and 1:2 complexes show a broad absorption in the 3570-2200 cm−1 region, while the computed IR spectra of these complexes show significant differences in the above range. However, the experimental Raman and FTIR, and computed IR spectra of the 1:1 and 1:2 complexes show a great similarity in the fingerprint region. The UV–vis spectra of both complexes in ethanol show the absorbance band at ca. 395 nm, suggesting the presence of the DCNP− anion. Diagrams of the HOMO and LUMO of molecules are presented. The Mulliken atomic charges for atoms participated in the cooperative hydrogen bonds were given.