Calculation of the Energy of Intramolecular Hydrogen Bonds Using the Clipping Method: The Case of ω-X-1-Alkanols, X(CH2)nOH

Abstract

Although the energies of intermolecular hydrogen bonds, E_HB, can be ascertained by a variety of approaches, there is not a general method to accurately determine the energies for intramolecular hydrogen bonds, E_IMHB. Structures for compounds X(CH₂)_nOH {X = F, OH, NH₂, Cl, Br, SH; n = 4–5} are calculated and then “clipped” to form complexes CH₃CH₂X•CH₃CH₂OH such that the critical geometric, spectroscopic, and electron density features are preserved. The E_IMHB of the parent molecule is assumed to equal the E_HB of the complex. Of the previous methods of determining E_IMHB, the molecular tailoring approach (MTA) comes closest to the values from this work with the differences due to incomplete cancellation of conformational effects in the MTA. In general, parametric methods fare poorly, only being effective for groups of similar molecules. The cis–trans and isodesmic approaches are of limited value for longer carbon chains due to conformational strain.