Competing intramolecular vs. intermolecular hydrogen bonds in solution.

Peter I Nagy

doi:10.3390/ijms151119562

Abstract

A hydrogen bond for a local-minimum-energy structure can be identified according to the definition of the International Union of Pure and Applied Chemistry (IUPAC recommendation 2011) or by finding a special bond critical point on the density map of the structure in the framework of the atoms-in-molecules theory. Nonetheless, a given structural conformation may be simply favored by electrostatic interactions. The present review surveys the in-solution competition of the conformations with intramolecular vs. intermolecular hydrogen bonds for different types of small organic molecules. In their most stable gas-phase structure, an intramolecular hydrogen bond is possible. In a protic solution, the intramolecular hydrogen bond may disrupt in favor of two solute-solvent intermolecular hydrogen bonds. The balance of the increased internal energy and the stabilizing effect of the solute-solvent interactions regulates the new conformer composition in the liquid phase. The review additionally considers the solvent effects on the stability of simple dimeric systems as revealed from molecular dynamics simulations or on the basis of the calculated potential of mean force curves. Finally, studies of the solvent effects on the type of the intermolecular hydrogen bond (neutral or ionic) in acid-base complexes have been surveyed.

Highlights

Conformational preferences can cause non-contiguous atoms within an isolated molecule to become close neighbors
A hydrogen bond for a local-minimum-energy structure can be identified according to the definition of the International Union of Pure and Applied Chemistry (IUPAC recommendation 2011) or by finding a special bond critical point on the density map of the structure in the framework of the atoms-in-molecules theory
The intramolecular hydrogen bond may disrupt in favor of two solute-solvent intermolecular hydrogen bonds

Summary

Introduction

Conformational preferences can cause non-contiguous atoms within an isolated molecule to become close neighbors. These spatial arrangements may be driven by favorable electrostatic interactions or by the special case where three of such atoms form a so-called “hydrogen bond” (H-bond). The situation becomes more complicated when the molecular structure is considered within a solution environment, these same two factors remain important to drive additional intermolecular interactions possible between solute molecules themselves and with the solvent molecules as partners. The 2011 IUPAC recommendations provide a definition [1] that can be used as the basis for critical evaluation of a number of structural that will be examined in Section 3 of this review.

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