David Ferro-Costas

A QTAIM‐based energy partitioning for understanding the physical origin of conformational preferences: Application to the Z effect in O=C‐X‐R and related units

A quantum theory of atoms in molecules‐based energy partitioning was carried out for Z and E conformers of a series of O=C‐X‐R containing compounds. The results obtained for the simplest compound (formic acid) indicate that the attraction of the electron density within carbonyl oxygen by the nucleus of the acid hydrogen is the most important energy term for Z preference. This conclusion can be extended (mutatis mutandis) to larger carboxylic acids, esters, sulfur derivatives, secondary amides, and carbonyl isocyanates, and even explains the sequence of relative conformational energies in the HCXOH series (X = O, S, Se). In contrast, although the hyperconjugative model has been traditionally employed to explain this preference, we observe it is incompatible with: (i) relative values of diverse QTAIM atomic populations for the Z/E conformational equilibrium; (ii) conformational energies in the HCXOH series.

Anomeric effect in halogenated methanols: a quantum theory of atoms in molecules study.

The quantum theory of atoms in molecules (QTAIM) has been used to analyze the gauche conformational preference of fluoromethanol and chloromethanol. The analysis of the total atomic population and localization and delocalization indices show trends that are not in line with the hyperconjugative explanation. Energy terms arising from the QTAIM partitioning have been obtained for fluoromethanol, revealing that (i) C-O interaction plays the most significant role in stabilizing the gauche rotamer and (ii) the summation of exchange terms (the only ones that could be related to hyperconjugation) has a smaller weight than electrostatic ones in the energy balance among gauche, anti, and syn conformations; however, they are far from being negligible.

Complementarity of QTAIM and ELF Partitions: Deeper Understanding of the Anomeric Effect.

In this work, fragments with chemical significance are defined inside QTAIM basins through the use of the ELF partition. In an ideal situation, core and monosynaptic ELF basins for an atom A (CA and VA, respectively) should belong exclusively to its atomic basin (CA, VA ∈ ΩA), while disynaptic ones ought to be divided between the atoms of the corresponding bond (for an A-B bond, VA-B ∈ ΩA ∪ ΩB). Several examples here analyzed verify this situation (within 0.01 au). This combined partitioning is also applied to the analysis of the conformational preference in diverse anomeric compounds. Results lead to an alternative interpretation, independent of hyperconjugative effects.

Electronegativity estimator built on QTAIM-based domains of the bond electron density.

The electron localization function, natural localized molecular orbitals, and the quantum theory of atoms in molecules have been used all together to analyze the bond electron density (BED) distribution of different hydrogen‐containing compounds through the definition of atomic contributions to the bonding regions. A function, gAH, obtained from those contributions is analyzed along the second and third periods of the periodic table. It exhibits periodic trends typically assigned to the electronegativity (χ), and it is also sensitive to hybridization variations. This function also shows an interesting S shape with different χ‐scales, Allred–Rochows being the one exhibiting the best monotonical increase with regard to the BED taken by each atom of the bond. Therefore, we think this χ can be actually related to the BED distribution.

Influence of the O-Protonation in the O═C–O-Me Z Preference. A QTAIM Study

One of the three O-protonations of methyl formate (MF) gives rise to a system where the Z preference is switched off and the E conformer becomes the most stable arrangement. The quantum theory of atoms in molecules scheme for splitting the physical space of a molecule into atomic subspaces has been employed to analyze this trend, as well as the effect of the protonation in MF. The most important changes in energy and electron density upon protonation do not take place when MF reorganizes its nuclei, but when the proton is introduced explicitly. The same trend is observed when the H attached to the carbonyl C is replaced by electron donating and withdrawing groups (CN, F, OH, CH(3), and CF(3)). The origin of the inversion in the conformational preference relies in the changes experienced by two interactions: (i) the methyl group with the proton and (ii) that between the atoms of the ether C-O bond.

How Electronic Excitation Can be Used to Inhibit Some Mechanisms Associated to Substituent Effects.

Despite the fact that transferability and chemistry go hand in hand, transferability studies in electronically excited states (EESs) are normally omitted, although these states are becoming extremely important in modern processes and applications. In this work, it is shown that this kind of studies can be used to understand how substituent effects can be modified in EESs. Thus, for example, the analysis of the carbonyl oxygen transferability in different HCO-R molecules allowed us to find that the nO→πCO* excitation can be used to break the π conjugation associated to the resonance substituent effect. Moreover, as a direct consequence, the oxygen transferability is enhanced in the first electronically excited state.