Non‐Covalent Interactions in Molecular Systems: Thermodynamic Evaluation of the Hydrogen‐Bond Strength in Amino‐Ethers and Amino‐Alcohols

Abstract

The intramolecular hydrogen bond (intra-HB) is one of the best-known examples of non-covalent interactions in molecules. Among the different types of intramolecular hydrogen bonding, the NH•••O hydrogen bond in aminoalcohols and amino-ethers is one of the weakest. In contrast to the strong OH•••N intramolecular hydrogen bond, the strength of the NH•••O bond can hardly be measured with conventional spectroscopic methods, even for simple aminoalcohols, since the band belonging to the NH•••O conformer merges with the free OH band. In this work, we developed a combination of G4 calculations, and a method based on experimental vaporization enthalpies to determine the NH•••O hydrogen bonding strength. The archetypal compounds for this study are 2-amino-1-ethanol and 3-amino-1-propanol as well as their respective methoxy analogs. Based on these molecules, different series were studied to investigate various factors influencing NH•••O intra-HB strength.In the first series, the influence of alkylation near the hydroxy or methoxy group and the amino group in sterically hindered aminoalcohols was examined. In the second series, the influence of alkylation of the amino-group was investigated. In the third series, the effect of extending the alkyl chain between functional groups was studied.

Molecular Liquids versus Ionic Liquids: The Interplay between Inter-Molecular and Intra-Molecular Hydrogen Bonding as Seen by Vaporisation Thermodynamics

“…In terms of vaporisation thermodynamics, the main challenge is to find a suitable reference system that mimics the desired molecule without the intra-HB. In our earlier work, we proposed the reference system based on conventional group additivity [ 2 , 3 ]. In this work, we improve this concept of group additivity (GA) and propose an alternative method based on the well-balanced equations (WBE).…”

“…Nevertheless, the vaporisation enthalpies, , are reliably measured by various experimental techniques [ 1 ], challenging the task of separating and understanding the energetics of molecular interactions using these thermodynamic data. Indeed, in a number of recent publications, we have developed a few thermodynamically based methods to quantify hydrogen bond strengths (inter- and intra-HB) for molecular [ 2 , 3 , 4 ] and ionic liquids [ 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 ]. In this work, we develop and apply these methods to a series of molecular liquids (MLs) containing a six-membered aliphatic ring (alkyl-piperidines) and a hydroxyl group as a special feature (see Figure 1 left); we also apply these methods to corresponding ionic liquids (ILs) that also contain a six-membered aromatic ring (alkyl-pyridinium) with the hydroxyl attached to the alkyl chain (see Figure 1 right).…”

“…The association of the hydroxyl-containing molecules (e.g., alcohols, diols, amino alcohols, etc.) in the liquid phase via the OH···O and OH···N hydrogen bond types (see Figure 2 ) is a well-established phenomenon [ 2 , 3 , 4 , 9 ].…”

“…The formation of both the inter- and intra-types of association is theoretically feasible, but in practice, most molecules (even in amino alcohols and diols) in the liquid are connected via the OH···O and OH···N inter-molecular hydrogen bonds, which are more thermodynamically favourable [ 2 , 3 , 4 ]. In contrast, the amino alcohol molecule, which has left the liquid phase and freely flies in the gas phase (or in vacuum) without adjacency, has only one option—to build the intra-molecular H-bond, which brings the molecule into the energetic lowest conformations.…”

“…In contrast, the amino alcohol molecule, which has left the liquid phase and freely flies in the gas phase (or in vacuum) without adjacency, has only one option—to build the intra-molecular H-bond, which brings the molecule into the energetic lowest conformations. Depending on the molecular flexibility and temperature, the number of coexisting intra-H-bonded conformations can be significant, making the quantification of the intra-HB strength in the gas phase a difficult task [ 2 , 3 , 4 ]. Similar complications apply to the hydroxyl-containing ionic liquids.…”