A symmetric hydrogen bond revisited: potassium hydrogen maleate by variable temperature, variable pressure neutron diffraction and plane-wave DFT methods

Abstract

The symmetric hydrogen bond in potassium hydrogen maleate has been re-investigated by diffraction and computational methods. Single crystal neutron diffraction under varying temperature (30-295 K) and pressure (ambient-4 kbar) confirmed the crystallographically-constrained symmetric disposition of the hydrogen atom in the short, strong hydrogen bond for each of eight sets of (p; T ) conditions. Plane-wave DFT calculations show the hydrogen atom still to occupy this symmetric position even when crystallographic symmetry is removed, in stark contrast to the results from isolated molecule ab initio calculations. These results demonstrate that the symmetric (or close to symmetric) nature of this hydrogen bond is intrinsic.

Predicting the Position of the Hydrogen Atom in the Short Intramolecular Hydrogen Bond of the Hydrogen Maleate Anion from Geometric Correlations

“…As an independent check of this statement, the position of the hydrogen atom H1 for the reported eight neutron-diffraction-derived structures was calculated using our correlation (eq , only based on 12 K data), and the values are still in agreement with the refined positions (Table S7). This confirms the results by Wilson et al, who state that the temperature has no effect on the H1 position. Figure shows that the linear fit including all data is slightly less precise so that we will use the original one determined in Figure for further application.…”

“…There is a large number of crystal structures of hydrogen maleate salts in the Cambridge Structural Database (CSD) − which show that the O···O distance varies from 2.361 to 2.540 Å with a large variety of intermediate distances. Neutron diffraction studies establish that the O–H distances vary from 1.079 up to 1.215 Å. , Dayananda et al, Vanhouteghem et al, and Olovsson et al report that a symmetric hydrogen bond is often related to crystallographic mirror symmetry, but not necessarily. It can also be observed in compounds without any symmetry constraints.…”

“…The compounds measured at the Bragg Institute are (in the order from symmetric to asymmetric, together with the abbreviation used throughout the manuscript, the chemical formula, the reference to the figures where the neutron-derived structures are depicted, and the references to previous X-ray and neutron structure determinations): potassium hydrogen maleate ( K , K­[C 4 H 3 O 4 ], Figure a ,,,− ); l -phenylalaninium hydrogen maleate ( PhAla , [C 9 H 12 NO 2 ]­[C 4 H 3 O 4 ], Figure b , ); barium bis­(hydrogen maleate) tetrahydrate ( Ba , Ba­[C 4 H 3 O 4 ] 2 ·4H 2 O), four independent hydrogen maleate anions in the asymmetric unit labeled 1 Ba to 4 Ba with different degrees of symmetry/asymmetry, Figure c); magnesium bis­(hydrogen maleate) hexahydrate ( Mg , Mg­[C 4 H 3 O 4 ] 2 ·6H 2 O, Figure d , ); 4-aminopyridinium hydrogen maleate ( 4AP , [C 5 H 7 N 2 ]­[C 4 H 3 O 4 ], Figure e); calcium bis­(hydrogen maleate) pentahydrate ( Ca , Ca­[C 4 H 3 O 4 ] 2 ·5H 2 O, Figure f , ); lithium hydrogen maleate dihydrate ( Li , Li­[C 4 H 3 O 4 ]·2H 2 O, Figure g , ); sodium hydrogen maleate trihydrate ( Na , Na­[C 4 H 3 O 4 ]·3H 2 O, Figure h ,− ); 8-hydroxyquinolinium hydrogen maleate ( 8HQ , [C 9 H 8 NO]­[C 4 H 3 O 4 ], Figure i). These hydrogen maleate salts span the whole range from perfectly symmetric (K) to highly asymmetric intramolecular O1–H1···O2 hydrogen bonds. The asymmetry in the 8HQ structure is the highest ever reported (Δ = 0.306 Å; Δ = d (O2···H1)– d (O1–H1)).…”

Structural Variety of Alkali Hydrogen Maleates at High Pressure

“…As far as we know, there is only one study of KHMAL under a rather moderate pressure of 4 kbar (0.4 GPa) that does not observe any visible structural changes to the applied load . KHMAL crystallizes in the orthorhombic space group Pbcm .…”

“…As far as we know, there is only one study of KHMAL under a rather moderate pressure of 4 kbar (0.4 GPa) that does not observe any visible structural changes to the applied load. 58 KHMAL crystallizes in the orthorhombic space group Pbcm. Both apical carbonyl oxygens (O2) in HMAL are surrounded by three potassium cations, which form a regular but not symmetry-constrained triangle (Figures 5a and 6a and b), with a mean K−O distance of 2.861 Å.…”