A strong asymmetric N–H–N hydrogen bond: Neutron diffraction and IR spectroscopic studies of 4-aminopyridine hemiperchlorate

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Abstract
The structure of 4‐aminopyridine hemiperchlorate, C5H6N2⋅1/2HClO4, has been determined from three‐dimensional neutron diffraction data. The crystals are monoclinic, space group P21/n, with cell dimensions a=11.224 (7) Å, b=9.325 (6) Å, c=16.631 (9) Å, β=102.72 (3) ° and Z=4. The structure was refined to an R(F02) =0.102 for 862 reflections with F02≳σF02. The structure consists of perchlorate anions and asymmetrically H‐bridged (NH2Py⋅H⋅⋅PyNH2)+ cations. The cations contain a short, asymmetric, nearly linear (N⋅H⋅⋅⋅N)+ hydrogen bond (N⋅⋅⋅N=2.698 (8) Å, N–H=1.17 (2) Å). The dihedral angle between the planes of the N–H⋅⋅N bridged pyridine rings is 85°. The IR spectra are consistent with those predicted for this type of bridge system. Strong temperature effects were found to occur both in the stretching frequency region νNH2 and in the low‐frequency broad absorption band below 800 cm−1. The νNH+band shift towards lower frequencies indicating that the bridge becomes shortened which, in turn, results in an increase of the low‐frequency absorption intensity interpreted as a ’’tunneling’’ transition.

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