*2.2. Computational Details*

We carried out electronic structure calculations for a series of derivatives of malondialdehyde in their open and closed configurations (Figure 1), where one of the hydrogens in the three carbon atoms of the conjugated skeleton is replaced by an EWG or an EDG, namely −F, −Cl, −Br, −CF3, −OCH3, −N(CH3)2, −NHCOCH3 or −NO2 (Figure 1). Thus, we computed the open and closed conformations for eight substitutions in three different positions resulting in 48 different structures wherein the RAHB is present in

24 of them. The conformers were chosen to minimise the differences between open and closed configurations and also to avoid secondary interactions, e.g., contacts between carbonyl and C-H groups. All the geometries were optimised with the aid of the B3LYP functional [63,64], along with the aug-cc-pVTZ basis set [65–68], as implemented in the GAUSSIAN09 package [69]. This combination of exchange-correlation functional and basis set has yielded good results concerning the study of intramolecular hydrogen-bonded systems [40]. Harmonic frequency calculations were done in order to confirm that the optimised structures are indeed local minima. The QTAIM analyses were carried out with the help of the AIMALL program [70]. The IQA energy partitions were carried out with our in-house PROMOLDEN code [71] using *β*-spheres with radii between 0.1 and 0.3 Bohr along with restricted angular Lebedev quadratures. We partitioned the exchange-correlation energy in accordance with Equation (1) via scaling techniques [72] previously used in conjunction with QTAIM.

**Figure 1.** Malondialdehyde structure in its open (**left**) and closed (**right**) conformations. The Rn (n = 1, 2, 3) symbols indicate the different positions available for substitution by the −F, −Cl, −Br, −CF3, −OCH3, −N(CH3)2, −NHCOCH3, and −NO2groups.
