**4. Conclusions**

From our G4 calculations, we can conclude that, when F, Cl, or Br atoms are introduced as substituents at the α-position of both the hydroxyl or the amino group of amino-alcohols, there is a significant change in the strength of both the O–H···N or the N–H···O IMHBs that can be formed in these compounds. Substitution at the α-position of the hydroxyl group results in a significant reinforcement of the O–H···N, which follows the trend H < F < Cl < Br, the effects on the N–H···O IMHB being the opposite. Consistently, substitution at the α-position of the amino group also results in a reinforcement of the N–H···O IMHB, but the effect is weaker than the one affecting the O–H···N IMHB. Accordingly, the global minima for the unsubstituted and the substituted amino-alcohols are stabilized by the formation of O–H···N IMHBs. In addition, for both sets, the strength of the O–H···N IMHB, as shown by the characteristics of the corresponding molecular graphs, the NBO and NCI analyses, and the stabilization energy estimated through the use of isodesmic reactions, goes through a maximum for *n* = 2 (amino-butanol).

The scenario changes completely when the HOCH2(CH2)nCH2NH2 (*n* = 0–3) aminoalcohols interact with a typical electro-deficient compound such as BeF2. The interaction of BeF2 with either the hydroxyl or the amino group of the amino-alcohol changes the strength of the IMHBs dramatically. In the first case, the O–H···N IMHB becomes much stronger, whereas, in the second case, it is the N–H···O IMHB that becomes very much reinforced, though the effect is still weaker than the one observed for the O–H···N IMHB. In this second case, interaction of BeF2 with the amino group, a new possibility is open through the formation of an additional IMHB between the O–H group of the alcoholic function and one of the F atoms of the BeF2 molecule. However, it is the possibility for the beryllium atom to interact simultaneously with the O and the N atoms of the amino-alcohol that leads to the global minimum of the potential energy surface, with the result that the IMHBs are replaced by two beryllium bonds. These complexes stabilized by the formation of two beryllium bonds, O···Be and N···Be, present very peculiar IR spectra, in which the OH stretching band appears in some cases more than 1100 cm<sup>−</sup><sup>1</sup> blue-shifted with respect to the α-substituted amino-alcohols, whereas a very intense band, around 800 cm<sup>−</sup>1, corresponding to the symmetric stretching of the BeF2 moiety, is another finger-print of these spectra.

**Supplementary Materials:** The following are available: Figure S1: Relative stability of the HOCH2(CH2)nCH2NH2 (*n* = 0–2) aminoalcohols stabilized by the formation of an O–H···N IMHB or a N-H···O IMHB, Figure S2: Molecular structures of the substituted HOCHX(CH2)nCH2NH2 (n = 0–5, X = H, F, Cl, Br) aminoalcohols, Figure S3: Molecular structures of the substituted HOCH2(CH2)nCHXNH2 (n = 0–5, X = F, Cl, Br) aminoalcohols, Table S1: Second-order orbital interactions energies between the nitrogen lone-pair and the s\*OH antibonding orbital for the HOCH2(CH2)nCH2NH2 (*n* = 0–3) amino-alcohols and their O···BeF2 complexes.

**Author Contributions:** Conceptualization, M.Y. and A.M.L.; methodology, O.M. and A.M.L.; software, A.M.L.; validation, A.M.L., O.M. and M.Y.; formal analysis, A.M.L., O.M. and M.Y.; investigation, A.M.L. and M.Y.; resources, O.M. and M.Y.; data curation, A.M.L.; writing—original draft preparation, M.Y.; writing—review and editing, A.M.L., O.M. and M.Y.; visualization, O.M.; supervision, M.Y. and A.M.L.; project administration, O.M. and M.Y.; funding acquisition, O.M., A.M.L. and M.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was carried out with financial support from the Ministerio de Ciencia, Innovación y Universidades of Spain (Project No. PGC2018-094644-B-C21 and PID2019-110091GB-I00 (MICINN), and the PRIES-CM project Ref: Y2020/EMT-6290 from the Comunidad Autónoma de Madrid.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available in the article and in the Supplementary Material.

**Acknowledgments:** This work was carried out with the financial support from the project PGC2018- 094644-B-C21 and PID2019-110091GB-I00 (MICINN) of the Ministerio de Ciencia, Innovación y Universidades of Spain and the PRIES-CM project Ref: Y2020/EMT-6290 from the Comunidad Autónoma de Madrid. The authors would also like to thank the Centro de Computación Científica of the UAM (CCC-UAM) for the generous allocation of computer time and for their continued technical support.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of the compounds are not available from the authors.
