Cooperativity Assisted Shortening of Hydrogen Bonds in Crystalline Oxalic Acid Dihydrate: DFT and NBO Model Studies

Abstract

The distance of ∼2.49 Å separating the carboxylic OH oxygen from the water oxygen atom in the α-polymorph of crystalline oxalic acid dihydrate is by ∼0.1 Å shorter than the average distance in carboxylic acid monohydrates. It is also by ∼0.2 Å shorter than the corresponding distance presently calculated for the heterotrimer consisting of one acid and two water molecules. The large difference between RO···O in the heterotrimer and in the crystal is attributed to the cooperative effect in the latter; this is supported by calculations carried out on clusters constituted of an increasing number of acid and water molecules. The present DFT calculations with geometry optimization include seven isolated model clusters, the largest of which contains five acid and eight water molecules. The RO···O of the short hydrogen bond shortens progressively with increasing the number of cluster constituents; in the largest cluster, it reaches 2.50 Å. This is remarkably close to both the experimental distance as well as to the distance obtained by the periodic DFT calculation. The electronic effects were studied by Natural Bond Orbital analysis, revealing an enhancement of hydrogen bonding on extending the network by increased polarization of the carbonyl group and by the increased delocalization interaction between the lone electron pair on the acceptor oxygen atom and the OH antibond orbital. The formation of circular motifs appears to be the most important factor in the cooperative shortening of the hydrogen bonds. In agreement with the measured hydrogen bond distances, inspection of the electron density reveals a notable difference in hydrogen bond shrinking tendency between the two known polymorphs of the title system.

Structure Determination by Joint Effort of X-ray Powder Diffraction and Quantum Calculations: Crystal Structure and Short Hydrogen Bonding in Pentadecafluorooctanoic Acid Hydrate

“…[23] One of the most interesting features of PDFO·H2O and related systems is that the shortness of its H-bonds in the crystal is governed by the strong cooperative effect. Namely, as already demonstrated for oxalic acid dihydrate, [24] infrared spectroscopy and cluster calculations suggest that in aqueous solution the H-bonds are much weaker and longer (RO...O ≈ 2.65 Å). In the crystal, however, the ordered H-bonded motifs cause strong additional polarization which provides substantial enhancement of the H-bonds, resulting in sizable shrinking (by ≈0.15 Å).…”

Effect of Hydrogen Bonding to Water on the31P Chemical Shift Tensor of Phenyl- and Trialkylphosphine Oxides and α-Amino Phosphonates

“…In CDCl 3 the first effect is absent, and the second is smaller. Experimental manifestations of these two phenomena were reported in the past. − A detailed theoretical study of these effects in the solid state was published recently . Thus, the isotropic 31 P NMR chemical shift of 3 in D 2 O will be larger as compared to δ iso ( 31 P) in the 1:1 complex of 3 and water in CDCl 3 .…”

The applicability of the dimeric heterosynthon concept to molecules with equivalent binding sites. A DFT study of crystalline urea–H2O2

“…This approximation yields a reasonable description of different properties of two-component crystals. [41][42][43][44][45] The atomic positions determined by experiment are used as the starting point in the solid-state DFT computations. Details of the solid-state DFT calculations are given in the supplementary materials.…”