**1. Introduction**

1-oxo-3-hydroxy-2-propene, the enol of malonaldehyde in the conformation that presents an intramolecular hydrogen bond [1], has played an important role in the fields of hydrogen bonding, proton transfer [2–8], resonance assisted hydrogen bonds [6,9–12], quasi-aromaticity [13], and, in general, in non-covalent interactions. The importance of this molecule is due to the fact that it is the simplest of all 1,3-dicarbonyl compounds that were the first systems in which intramolecular hydrogen bonds had been studied [14–19].

Various aspects of 1-oxo-3-hydroxy-2-propene have been investigated, including substituent effects [20,21], solvent effects in which water molecules interact with this molecule [22], excited state properties [23], and the important problem of proton tunneling [23–27]. Both vibrational [28] and rotational [29–31] spectroscopic studies have been carried out on this molecule. In addition, interactions of 1-oxo-3-hydroxy-2-propene with Lewis acids such as methanol [32] and Li+, Na+ and FH [33], and BeX2 [34] have been reported.

A useful property for obtaining structural information about complexes linked by noncovalent interactions and, in particular by hydrogen bonds, is spin–spin coupling constants (SSCC). SSCC are related to the electronic structure of molecules and complexes through geometry, bond order, polarization, and electron densities. As Cremer and Gräfenstein wrote [35], "The analysis of NMR spin–spin coupling leads to a unique insight into the electronic structure of closed-shell molecules". This was known for molecules from the beginning of the use of NMR spectroscopy [36–38] but was extended to complexes by Limbach [39,40] and Del Bene [41,42]. Through relationships between SSCC and geometry, the problem of the localization of the hydrogen-bonded proton could be solved [43].

**Citation:** Alkorta, I.; Elguero, J.; DelBene, J.E. Perturbing the O–H ... O Hydrogen Bond in 1-oxo-3- hydroxy-2-propene. *Molecules* **2021**, *26*, 3086. https://doi.org/10.3390/ molecules26113086

Academic Editor: Mirosław Jabło ´nski

Received: 6 April 2021 Accepted: 12 May 2021 Published: 21 May 2021

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<sup>1</sup> Instituto de Química Médica, CSIC, Juan de la Cierva, 3, E-28006 Madrid, Spain; iqmbe17@iqm.csic.es

<sup>2</sup> Department of Chemistry, Youngstown State University, Youngstown, OH 44555, USA

In the present paper, we report the results of an investigation of 1-oxo-3-hydroxy-2- propene in a series of binary complexes with the acids LiH, LiF, BeH2, and BeF2. These complexes contain intramolecular O–H···O hydrogen bonds and lithium (alkali) and beryllium (alkaline earth) intermolecular bonds [44]. Specifically, we have determined the structures and binding energies of these complexes; the proton transfer barriers; the complex stabilization by charge-transfer interactions; and the spin–spin coupling constants 2hJ(O–O), 1hJ(H–O), and 1J(O–H) across the O–H **...** O hydrogen bond. It is the purpose of this paper to present and discuss the results of this study.
