On the basis of a density-based quantification of the steric effect [Liu, S. B. J. Chem. Phys.2007, 126, 244103], the origin of the internal rotation barrier between the eclipsed and staggered conformers of ethane is systematically investigated in this work from an information-theoretical point of view by using the Fisher information measure in conjugated spaces. Two kinds of computational approaches are considered in this work: adiabatic (with optimal structure) and vertical (with fixed geometry). The analyses are performed systematically by following, in each case, the conformeric path by changing the dihedral angle from 0 to 180° . This is calculated at the HF, MP2, B3LYP, and CCSD(T) levels of theory and with several basis sets. Selected descriptors of the densities are utilized to support the observations. Our results show that in the adiabatic case the eclipsed conformer possesses a larger steric repulsion than the staggered conformer, but in the vertical cases the staggered conformer retains a larger steric repulsion. Our results verify the plausibility for defining and computing the steric effect in the post-Hartree-Fock level of theory according to the scheme proposed by Liu.

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In this work, we investigate quantum entanglement-related aspects of the dissociation process of some selected, representative homo-and heteronuclear diatomic molecules. This study is based upon high-quality ab initio calculations of the (correlated) molecular wavefunctions involved in the dissociation processes. The values of the electronic entanglement characterizing the system in the limit cases corresponding to (i) the united-atom representation and (ii) the asymptotic region when atoms dissociate are discussed in detail. It is also shown that the behaviour of the electronic entanglement as a function of the reaction coordinate R exhibits remarkable correspondences with the phenomenological description of the physically meaningful regimes comprising the processes under study. In particular, the extrema of the total energies and the electronic entanglement are shown to be associated with the main physical changes experienced by the molecular spatial electronic density, such as charge depletion and accumulation or bond cleavage regions. These structural changes are characterized by several selected descriptors of the density, such as the Laplacian of the electronic molecular distributions (LAP), the molecular electrostatic potential (MEP) and the atomic electric potentials fitted to the MEP.

We investigate the complexity of the hydrogenic identity S N 2 exchange reaction by means of information-theoretic functionals such as disequilibrium (D), exponential entropy (L), Fisher information (I), power entropy (J) and joint information-theoretic measures, i.e., the I-D, D-L and I-J planes and the Fisher-Shannon (FS) and López-Mancini-Calbet (LMC) shape complexities. The several information-theoretic measures of the one-particle density were computed in position (r) and momentum (p) spaces. The analysis revealed that the chemically significant regions of this reaction can be identified through most of the information-theoretic functionals or planes, not only the ones which are commonly revealed by the energy, such as the reactant/product (R/P) and the transition state (TS), but also those that are not present in the energy profile such as the bond cleavage energy region (BCER), the bond breaking/forming regions (B-B/F) and the charge transfer process (CT). The analysis of the complexities shows that the energy profile of the identity S N 2 exchange reaction bears no simple behavior with respect to the LMC and FS measures. Most of the chemical features of interest (BCER, B-B/F and CT) are only revealed when particular information-theoretic aspects of localizability (L or J), uniformity (D) and disorder (I) are considered.

In this work, we have investigated in position (r) and momentum (p) spaces the concurrent phenomena occurring at the vicinity of the transition state (TS) (the so-called transition region) of selected chemical reactions (such as the hydrogenic abstraction and the exchange hydrogenic reactions) by means of a broad set of single informationtheoretic functionals of the one-particle density (such as the disequilibrium (D), exponential entropy (L), Fisher information (I) and the power entropy (J)) and composite informationtheoretic measures which includes various information planes (such as the I-D, D-L, and I-J planes) and complexities of the Fisher-Shannon and L opez-Mancini-Calbet (LMC) types. The analysis of the single functionals and the information planes revealed that these information-theoretical elements can identify all the chemically significant regions, not only the reactant/product regions (R/P) and the TS but also those that are not present in the energy profile, such as the bond cleavage energy region (BCER), the bond breaking/forming regions (B-B/F) and the charge transfer complex. Moreover, the analysis of the complexities shows that in position as well as in the joint (r-p) spaces, the energy profile of the abstraction reaction bears the same information-theoretical features of the LMC and FS measures. Finally, it is shown why most of the chemical features of interest (such as e.g., BCER and B-B/F) are lost in the energy profile, being only revealed when particular information-theoretical aspects of localizability (L or J), uniformity (D) and disorder (I) are considered.