Theoretical spectroscopic characterization of the ArBeO complex

Reassessing the Role of σ Holes in Noncovalent Interactions: It is Pauli Repulsion that Counts

A number of prototypical weak electron donor-electron acceptor complexes are investigated by the Symmetry Adapted Perturbation Theory, some of which belong to novel classes of weak bonds such as halogen and chalcogen bonds. Also included are complexes involving strong Lewis acids such as BeO and AuF. The common view in the literature is to associate these novel bonds with a variety of "holes", σ, π, δ, or positive areas in their electrostatic potential maps. The presumption is that these positive areas of the electrostatic potential are indicative of the electrostatic nature of these noncovalent bonds. The electrostatic view extends to the explanations of the directionality of approaches between the subsystems forming these bonds. This work demonstrates that one common feature of these electrostatic potential "holes" is the local depletion of electron density of which the best detector is the first-order Pauli repulsion. The minimization of this repulsion determines the bond directionality and its relative angular rigidity. In relatively strong complexes of BeO with rare gases, where BeO shows a clear cavity in electron density-an ultimate "σ hole"-the electrostatic effect does not control the bending potential-the exchange repulsion does. In halogen bonds, the halogen atom is nonspherical, displaying an axial "σ hole" in its electrostatic potential. However, in no examined case, from rare gas acting as an electron donor to a polar donor to an anionic donor, is the electrostatic energy responsible for the directionality of the halogen bond. In fact, it is not even maximized in the direction of the σ hole in N2-ClF and NH3-ClF. Yet, in all the cases, the exchange repulsion is minimized in the direction of the σ hole. The minimized exchange repulsion associated with the subtle and less subtle depletions of the electron density occur on the nodal planes or on the intersections thereof in the highest occupied molecular orbitals of Lewis acids, provided that the systems are closed-shell. The role of nodal planes in covalent and coordinate covalent bonds is well recognized. This work points to their similarly equal importance in certain types of donor-acceptor noncovalent interactions.

Complex rovibrational dynamics of the Ar·NO+ complex

Rotational-vibrational states of the Ar·NO⁺ cationic complex are computed, below, above, and well above the complex's first dissociation energy, using variational nuclear motion and close-coupling scattering computations. The HSLH potential energy surface used in this study (J. Chem. Phys., 2011, 135, 044312) is characterized by a first dissociation energy of D₀ = 887.0 cm^-1 and supports 200 bound vibrational states. The bound-state vibrational energies and the corresponding wave functions allow the interpretation of the scarcely available experimental results about the intermonomer vibrational motion of the complex. A very large number of long-lived quasibound combination states of the three vibrational modes, exhibiting a very similar energy-level structure as that of the bound states, are found embedded in the continuum. Additional short-lived resonance states are also identified and their properties are analyzed.