Unexpected Strengthening of the H-Bond Complexes in a Polar Solvent Due to a More Efficient Solvation of the Complex Compared to Isolated Monomers

Abstract

It is generally assumed that hydrogen-bonded complexes are less stable in solvents than in the gas phase and that their stability decreases with increasing solvent polarity. This assumption is based on the size of the area available to the solvent, which is always smaller in the complex compared to the subsystems, thereby reducing the solvation energy. This reduction prevails over the amplification of the electrostatic hydrogen bond by the polar solvent. In this work, we show, using experimental IR spectroscopy and DFT calculations, that there are hydrogen-bonded complexes whose stability becomes greater with increasing solvent polarity. The explanation for this surprising stabilization is based on the analysis of the charge redistribution in the complex leading to increase of its dipole moment and solvation energy. Constrained DFT calculations have shown a dominant role of charge transfer over polarization effects for dipole moment and solvation energy.

The Impact of the Solvent Dielectric Constant on A←NH3Dative Bond Depends on the Nature of the Lewis Electron‐Pair Systems

“…Effects of solvents were modelled with the COSMO solvation model [16] . The reliability of COSMO model is verified in our earlier works on dative and hydrogen bond complexes through other solvation models as well as explicit solvent model [1,17] …”

“…[16] The reliability of COSMO model is verified in our earlier works on dative and hydrogen bond complexes through other solvation models as well as explicit solvent model. [1,17] Solvent environments were characterized by their corresponding dielectric constants ɛ, gas phase (ɛ = 1), n carbon disulfide, CS 2 (ɛ = 2.6), acetone (ɛ = 20.7) and dimethyl sulfoxide, DMSO (ɛ = 46.8). The contribution of solvent to the overall stability was determined by the change of solvation energy (ΔE solv ) which corresponds to the difference between the solvation energies of the complex and separated subsystems, respectively.…”

Trends in the stability of covalent dative bonds with variable solvent polarity depend on the charge transfer in the Lewis electron-pair system

“…As expected, a decrease in stability with increasing solvent polarity was observed in most complexes. 13,14 However, our research has shown that certain hydrogen-bonded complexes unexpectedly stabilize with increased solvent polarity.…”

Impact of dielectric constant of solvent on the formation of transition metal‐ammine complexes

“…While the SASA consistently destabilizes the complex, the effect of charge redistribution depends on the bond type and the extent of the charge redistribution. The destabilizing effect predominates for complexes with small charge redistribution, such as those formed by sharing electrons to create a covalent bond, and most non‐covalent complexes 1–3 . The dative bond (DB) complexes, which have a dual bonding character combining covalent and ionic contributions (see Equation ), are an intriguing group in this regard 4,5 …”