Intramolecular Hydrogen Bonds in Normal and Sterically Compressed o-Hydroxy Aromatic Aldehydes. Isotope Effects on Chemical Shifts and Hydrogen Bond Strength.

Poul Erik Hansen,Bahjat A Saeed,Mark J Maclachlan,Jens Spanget-Larsen,Fadhil S Kamounah

doi:10.3390/molecules24244533

Abstract

A number of o-hydroxy aromatic aldehydes have been synthesized to illustrate the effect of steric compression and O···O distances on the intramolecular hydrogen bond and the hydrogen bond energies. Hydrogen bond energies have been calculated using the ‘hb and out’ method using either the MP2 method or the B3LYP functional with the basis set 6-311++G(d,p). However, several compounds cannot be treated this way. Hydrogen bond energies are also determined using electron densities at bond critical points and these results are in good agreement with the results of the ‘hb and out’ model. Two-bond deuterium isotope effects on 13C chemical shifts are suggested as an experimental way to obtain information on hydrogen bond energies as they easily can be measured. Isotope effects on aldehyde proton chemical shifts have also been measured. The former show very good correlation with the hydrogen bond energies and the latter are related to short O···O distances. Short O···O distances can be obtained as the result of short C=C bond lengths, conjugative effects, and steric compression of the aldehyde group. Short O···O distances are in general related to high hydrogen bond energies in these intramolecularly hydrogen-bonded systems of resonance assisted hydrogen bond (RAHB) type.

Highlights

Hydrogen bonding, hydrogen bond strength and hydrogen bond energy are still subjects that attract much attention both in chemistry and in biology [1,2,3,4,5,6,7,8,9]
Intramolecular hydrogen bonding of the resonance assisted type is important in DNA bases themselves [8], in co-factors such as pyridoxal-50 -phosphatate and the related aldimines [9], and in other small molecule such as gossypol treated in this study
Reuben [17] suggested a correlation between hydrogen bond energies and two-bond deuterium isotope effects on 13 C chemical shifts based on the work of Schaefer [18]

Summary

Introduction

Hydrogen bond strength and hydrogen bond energy are still subjects that attract much attention both in chemistry and in biology [1,2,3,4,5,6,7,8,9]. In a more quantitative fashion, the energy difference between the hydrogen-bonded and the open form (OH group turned 180◦ about the C-O axis) (‘hb and out’) has been suggested as a theoretical measure of the hydrogen bond energy [12]. This method has mainly been used in intermolecular systems and in a few cases in intramolecularly hydrogen-bonded systems [7,13]. Reuben [17] suggested a correlation between hydrogen bond energies and two-bond deuterium isotope effects on 13 C chemical shifts based on the work of Schaefer [18]. This equation was later used in proteins [19,20]

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Conclusion