In this research it was found that the formation of HBs is largely influenced by the OH bond rather than by the CH 3 group. 15 found that the resonance excitation of the σ*(O–H) orbital exhibited blueshifts of 0.25 eV and 1.20 eV in the C and O K-edge regions, respectively. 14 Similar results were found for methanol clusters where a further understanding of the experimental spectra was reached after comparison with the calculated spectra of the ring and chain methanol trimers taking into account different OH sites. 14 The major differences on the orbital structure are originated from the proton donor character, as was concluded from the comparison of the calculated spectra of the water monomer and both water molecules in the dimer. These latter analyses are based on the observation of the inner-shell excitation of those clusters at oxygen K-edge. 13 As a rule (with exception of inverse HBs 12), the hydrogen of the HBs loses electronic charge and undergoes changes in the electronic structure as has been observed in several systems such as water, 14 methanol, 15 and ethanol 16 by the analysis of both experimental and calculated x-ray absorption spectra. Therefore, there is a need to continue with the investigations where new tools would be evaluated to complement available information about HBs as well as the parameters commonly used from different perspectives.įor example, the analysis of either molecular or atomic orbitals is an interesting and valid tool because the HBs formation implies a redistribution of the atomic orbitals (AO) occupancy and energy shifts of certain molecular orbitals (MO). 11 In the specific case of the HBs, recently, the IUPAC Physical and Biophysical Chemistry Division has reported a definition for the HBs 12 which is very general because it takes into consideration from very weak to very strong HBs. 2,4–10 Many parameters (energetic, geometric, spectroscopic, and topologic) have been used to describe the existence of weak interactions. The computational study of small clusters is one of the most suitable approaches to understand theses elusive interactions. 1–3 Therefore, much effort has been dedicated to search for methodologies to determine, characterize, and classify weak interactions. Weak interactions such as those of the hydrogen bond (HB) type are of great interest in different scientific fields such as chemistry and biology because many important phenomena are explained due to their existence. However, this analysis cannot be applied to the study of H-H interactions observed in the molecular graphs. In summary, the current MO and AO analysis provides alternative ways to characterize HBs. Changes of the occupancy and energy of the AO are correlated with the strength of O–H-O and C–H-O HBs as well as with the proton donor and/or acceptor character of the involved molecules. The HOMO of the hetero(clusters) is less stable than the HOMO of the isolated alcohol monomer as the hetero(cluster) size increases, that destabilization is higher for linear geometries than for cyclic geometries. These systems showed the same trends observed in the (ethanol) n-water, n = 1 to 5 heteroclusters such as an O-O distance of 5 Å to which the O–H-O hydrogen bonds (HBs) can have significant influence on the constituent monomers. Results were compared with those obtained for (ethanol) n, (methanol) n, n = 1 to 6 clusters and (methanol) n-water, n = 1 to 5 heteroclusters. The molecular (MO) and atomic (AO) orbital analysis and the topological study of the electron density provided results that were successfully correlated. A computational study of (ethanol) n-water, n = 1 to 5 heteroclusters was carried out employing the B3LYP/6-31+G(d) approach.
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