Proton delocalization in short hydrogen bonds assembling HSeO₄^- anions into supramolecular adducts

Four hydrogen-bonded complexes of selenic acid with N-heterocycles (pyridine, 4,4'-bipyridil, quinoline and 2,2,6,6-tetramethylpyperidine) were studied in the crystalline state by single crystal X-ray diffraction, Fourier-transform infrared spectroscopy, and density functional theory with periodic boundary conditions. In all cases short SeO-H⋯OSe hydrogen bonds (≤2.61 Å) were found, either 'isolated' ones or within infinite chains. The coherent quasi-adiabatic proton transfer pathways were computed providing broad asymmetric single-well or (low-barrier) double-well potentials with significantly delocalized protons. The ground state vibrationally-averaged proton positions are noticeably shifted from the equilibrium ones towards hydrogen bond centres, and for asymmetric low-barrier double wells, a proton transfer in the first vibrationally excited state is established. The computed O-H stretching frequencies lie in the range of 1600-3050 cm^-1 and are in semi-quantitative agreement with experiment. Moreover, in case of low-barrier double-well potentials, a rather exotic H/D isotope effect, namely, a higher vibrational frequency for the O-D stretching than for the O-H stretching, is predicted. The mutual influence of neighboring SeO-H⋯OSe and SeO⋯H-N bonds affecting the bridging proton position in both bonds has also been addressed, indicating the vivid geometric cooperativity in complexes with 'isolated' SeO-H⋯OSe hydrogen bonds.