Recognition in the Domain of Molecular Chirality: From Noncovalent Interactions to Separation of Enantiomers

It is not a coincidence that both chirality and noncovalent interactions are ubiquitous in nature and synthetic molecular systems. Noncovalent interactivity between chiral molecules underlies enantioselective recognition as a fundamental phenomenon regulating life and human activities. Thus, noncovalent interactions represent the narrative thread of a fascinating story which goes across several disciplines of medical, chemical, physical, biological, and other natural sciences. This review has been conceived with the awareness that a modern attitude toward molecular chirality and its consequences needs to be founded on multidisciplinary approaches to disclose the molecular basis of essential enantioselective phenomena in the domain of chemical, physical, and life sciences. With the primary aim of discussing this topic in an integrated way, a comprehensive pool of rational and systematic multidisciplinary information is provided, which concerns the fundamentals of chirality, a description of noncovalent interactions, and their implications in enantioselective processes occurring in different contexts. A specific focus is devoted to enantioselection in chromatography and electromigration techniques because of their unique feature as "multistep" processes. A second motivation for writing this review is to make a clear statement about the state of the art, the tools we have at our disposal, and what is still missing to fully understand the mechanisms underlying enantioselective recognition.

Spectroscopy, lifetime, and charge-displacement of the methanol-noble gas complexes: An integrated experimental-theoretical investigation

An integrated experimental-theoretical investigation was employed to determine rovibrational energies, spectroscopic constants, lifetime as a function of temperature in gas phase complexes of methanol with noble gas (NgHe, Ne, Ar, Kr, Xe, and Rn). Beside that, a parallel effort has been addressed to theoretically characterize the nature of intermolecular interactions determining the dissociation energy and equilibrium distance of the formed adducts. Dynamics and lifetime results reveal that, except for the CH₃OH-He aggregate, all other methanol-Ng compounds are sufficiently stable under thermal conditions. Their lifetimes are larger than 1 ps for the temperature of the bulk in the range between 200 and 500 K. In addition, the current lifetime results suggest that the aggregates formed by methanol and Ng are globally more stable than corresponding complexes formed by water with Ng. From the point of view of the CCSD(T)/aug-cc-pVTZ level calculation, in all compounds, the electron densities of Ng partners are weakly polarized in the presence of CH₃OH molecule. The charge-displacement curves and NBO analysis indicate that the charge transfer from Ng to methanol molecule, in general, plays a minor role, being appreciable only in the aggregate involving Ar. Finally, it was verified from the SAPT2 + (CCD)-δMP2/aug-cc-pVTZ calculations and NCI analysis that the dispersion is the essential long-range attractive contribution to the interaction energy for all studied complexes. This feature strongly suggests that these compounds are held bonded substantially by van der Waals forces. Then non-covalent intermolecular bonds are effectively formed in the gas phase, which is disturbed by small stabilizing charge-transfer contributions.

Understanding noncovalent bonds and their controlling forces

The fundamental underpinnings of noncovalent bonds are presented, focusing on the σ-hole interactions that are closely related to the H-bond. Different means of assessing their strength and the factors that control it are discussed. The establishment of a noncovalent bond is monitored as the two subunits are brought together, allowing the electrostatic, charge redistribution, and other effects to slowly take hold. Methods are discussed that permit prediction as to which site an approaching nucleophile will be drawn, and the maximum number of bonds around a central atom in its normal or hypervalent states is assessed. The manner in which a pair of anions can be held together despite an overall Coulombic repulsion is explained. The possibility that first-row atoms can participate in such bonds is discussed, along with the introduction of a tetrel analog of the dihydrogen bond.