Hydrogen atoms in bridging positions from quantum crystallographic refinements: influence of hydrogen atom displacement parameters on geometry and electron density

Resonance-assisted hydrogen bonds (RAHBs) are exploited in chemical synthesis or serve as models for biologically relevant proton-transfer reactions [K. T. Mahmudov and A. J. L. Pombeiro, Chem. - Eur. J., 2016, 22, 16356-16398]. Their properties depend on the position of the hydrogen atom between donor and acceptor atoms. In the intramolecular RAHB of hydrogen maleate salts, this position is strongly influenced by the counter cation and varies from highly asymmetric to perfectly symmetric. This compound class is therefore ideally suited to investigate the factors determining geometry and electron density of intramolecular RAHBs. Here, it is shown that Hirshfeld atom refinements (HARs) of X-ray diffraction data of different hydrogen maleate salts can accurately and precisely match the hydrogen atom positions obtained from neutron diffraction and are independent of the model used for the hydrogen atom displacement parameters. Thus, the simplest, i.e. isotropic, HAR model can be used to locate hydrogen atoms also in bridging positions. In contrast, the determination of electron-density parameters with X-ray constrained wavefunction (XCW) fitting requires anisotropic, highly accurate hydrogen atom displacement parameters.