Conformational Mobility and Proton Transfer in Hydrogen-Bonded Dimers and Trimers of Phosphinic and Phosphoric Acids

Abstract

The monomers, H-bonded cyclic dimers, and trimers of five acids were studied by density functional theory calculations, such as hypophosphorous acid (H2POOH, 1), dimethylphosphinic acid (Me2POOH, 2), phenylphosphinic acid (PhHPOOH, 3), dimethylphosphoric acid ((MeO)2POOH, 4), and diphenylphosphoric acid ((PhO)2POOH, 5). Particular attention was paid to the conformational manifold existing due to the internal degrees of freedom: proton transfer (PT), puckering (“twist”) within the ring of H-bonds, and mobility of the substituents (namely, −Ph, −OMe, and −OPh rotations). For acid 3, the number of conformers is additionally increased because of the varying relative orientation of nonequivalent substituents in cyclic complexes. We show that 31P NMR chemical shifts (δP) are very sensitive to the details of the conformation, spanning ranges from ca. 1 ppm (for trimers of acids 1 and 2) to ca. 12 ppm (for trimers of 4). The energy barriers for the transitions between conformers are rather low (<6 kcal/mol for PTs, <2.5 kcal/mol for puckerings, and ca. <3 kcal/mol for rotations of substituents), such that the fast exchange regime in the NMR timescale and subsequent δP averaging are expected. Correlations are proposed linking the change of average δP with the H-bond energy, showing the slope of ca. 4 ppm per kcal/mol. The sensitivity of δP to the OPO angle and the OPOH dihedral angle and the geometries of both H-bonds formed by the POOH moiety are analyzed.

Phosphine Oxides as Spectroscopic Halogen Bond Descriptors: IR and NMR Correlations with Interatomic Distances and Complexation Energy

“…Again, the average coefficient refers to the fitting of the entire data set shown in Figure 8. It should be noted that despite the generally high sensitivity of δP to the molecular structure and to non-covalent interactions [71], in the literature there are only few attempts to use δP for the solution of the reverse spectroscopic problem for non-covalent complexes, i.e., for the finding of the complex's energy and structure based on the phosphorous chemical shift value [72][73][74][75]. Partially the reason for this might be in the high sensitivity itself, because contributions to δP from various weak secondary non-covalent interactions might "smudge" the effect of the halogen bonding, thus strongly reducing the diagnostic value of the spectroscopic marker.…”

“…value [72][73][74][75]. Partially the reason for this might be in the high sensitivity itself, because contributions to P from various weak secondary non-covalent interactions might "smudge" the effect of the halogen bonding, thus strongly reducing the diagnostic value of the spectroscopic marker.…”

ATMP derived cobalt-metaphosphate complex as highly active catalyst for oxygen reduction reaction

“…Moreover, theoretical studies and experiments also indicate that the phosphate/metaphosphate group make good contributions in proton transfer, which is of great significance for catalysis. [8,9] Many methods have been developed to synthesize cobalt phosphate/metaphosphate nanostructures, such as hydrothermal route [10][11][12], thermolytic molecular precursor (TMP) method [13], sol-gel process [14], freeze-drying method [15] and combustion assisted argon-annealed route [16].…”

H/D Isotope Effects on 1H-NMR Chemical Shifts in Cyclic Heterodimers and Heterotrimers of Phosphinic and Phosphoric Acids

“…We believed that the combination of the three abovementioned arguments builds a strong case for the formation of heterotetramers of acids 1 and 4 (1-4-1-4) as well as acids 1 and 3 (1-3-3-1). Such tetramerization, being an even more rare type of complexation of small molecules than trimerization, most likely is governed by the same factors as trimerization: the non-planarity of the ring of hydrogen bonds (the directions of proton-accepting and proton-donating abilities lie out of the POO plane) and the high residual mobility of the tetramer due to various possible puckering motions [18]. Finally, there might be two reasons why homotetramers are not formed, while heterotetramers are: the larger dipole moment of heterotetramers could stabilize them in the polar aprotic medium and the molecules of phosphinic and phosphoric acids might be packing more conveniently in the mixed complex than in a self-associate, due to steric effects.…”

“…It has been speculated that the trimers are formed because the OPOH group is not planar and the lone pairs of the P=O group lie out of the OPO plane, thus making the formation of a planar dimer less preferable. In turn, the formation of a cyclic trimer allows molecules to better align their proton-donating and proton-accepting groups to form a non-planar ring of three almost linear hydrogen bonds, in which each OH group points along one of the P=O lone pairs [18]. The non-planarity of the ring of three hydrogen bonds leads to various possible puckering motions in the complex.…”