Exploring short strong hydrogen bonds engineered in organic acid molecular crystals for temperature dependent proton migration behaviour using single crystal synchrotron X-ray diffraction (SCSXRD)

Seven multi-component molecular crystals containing O-HMIDLINE HORIZONTAL ELLIPSISO/O+-HMIDLINE HORIZONTAL ELLIPSISO- and N+-HMIDLINE HORIZONTAL ELLIPSISO- short strong hydrogen bonds (SSHBs) have been engineered by combining substituted organic acids with hydrogen bond acceptor molecules N,N-dimethylurea and isonicotinamide. In these materials, the shortest of the SSHBs are formed in the N,N-dimethylurea set for the ortho/para nitro-substituted organic acids whilst a twisted molecular approach favours the shorter SSHBs N+-HMIDLINE HORIZONTAL ELLIPSISO- in the isonicotinamide set. Temperature dependent proton migration behaviour has been explored in these systems using single crystal synchrotron X-ray diffraction (SCSXRD). By using a protocol which considers a combination of structural information when assessing the hydrogen atom (H-atom) behaviour, including refined H-atom positions alongside heavy atom geometry and Fourier difference maps, temperature dependent proton migration is indicated in two complexes (2: N,N-dimethylurea 2,4-dinitrobenzoic acid 1 : 1 and 5: isonicotinamide phthalic acid 2 : 1). We also implement Hirshfeld atom refinement for further confidence in this observation; this highlights the importance of having corroborating trends when applying the SCSXRD technique in these studies. Further insights into the SSHB donor-acceptor distance limit for temperature dependent proton migration are also revealed. For the O-HMIDLINE HORIZONTAL ELLIPSISO/O+-HMIDLINE HORIZONTAL ELLIPSISO- SSHBs, the systems here support the previously proposed maximum limit of 2.45 angstrom whilst for the charge assisted N+-HMIDLINE HORIZONTAL ELLIPSISO- SSHBs, a limit in the region of 2.55 angstrom may be suggested.