Is there any fundamental difference between ionic, covalent, and others types of bond? A canonical perspective on the question

Abstract

The concept of chemical bonding is normally presented and simplified through two models: the covalent bond and the ionic bond. Expansion of the ideal covalent and ionic models leads chemists to the concepts of electronegativity and polarizability, and thus to the classification of polar and non-polar bonds. In addition, the intermolecular interactions are normally viewed as physical phenomena without direct correlation to the chemical bond in any simplistic model. Contrary to these traditional concepts of chemical bonding, recently developed canonical approaches demonstrate a unified perspective on the nature of binding in pairwise interatomic interactions. This new canonical model, which is a force-based approach with a basis in fundamental molecular quantum mechanics, confirms much earlier assertions that in fact there are no fundamental distinctions among covalent bonds, ionic bonds, and intermolecular interactions including the hydrogen bond, the halogen bond, and van der Waals interactions.

Quadruple Bonding in the Ground and Low-Lying Excited States of the Diatomic Molecules TcN, RuC, RhB, and PdBe

“…At first we calculated 51 states i.e., singlet, triplet, quintet, septet, and nonet Σ + , Σ − , Π, Δ, Φ, Γ, and Η states at CASSCF/aug-cc-pV5Z N -(PP) Tc . The ground states of Tc ( 6 S, 5s 2 4d 5 ) and N ( 4 S) give rise to a total of four molecular states, i.e., 3,5,7,9 Σ − . The first excited state of Tc ( 6 D, 5s 1 4d 6 ), the second ( 4 D, 5s 1 4d 6 ), the third ( 4 P, 5s 1 4d 6 ), and the forth ( 4 F, 4d 7 ) combined with the ground state of N ( 4 S) give rise to a total of 12 ( 3,5,7,9 Δ, Π, Σ − ), 12 ( 1,3,5,7 Δ, Π, Σ − ), 8 ( 1,3,5,7 Π, Σ + ) and 16 ( 1,3,5,7 Φ, Δ, Π, Σ + ) molecular states.…”

“…Finally, the relative energy levels of the 1 Σ + and 3 Δ states are not very well described by the DFT/TPSSh; for TcN the energy gap is significantly overestimated by 74% while, in RhB, is underestimated by 72%. Finally, in RuC, 3 Δ is calculated as the ground state instead of 1 Σ + . To sum up, for the correct study of diatomic molecules containing second-row transition metals, the use of the multireference configuration interaction or coupled cluster methodology is necessary.…”

π covalency in the halogen bond

“…Other descriptions focus on the covalent charge-transfer component, where the lone pair of the nucleophile donates electron density into the hydrogen or halogen σ* antibonding orbital of the HB or XB-donor 4,8,[21][22][23][24][25][26][27][28][29] . While both of these models have advantages and disadvantages, and an expansive view of these interactions will consider both as contributing factors 7,21,24,[30][31][32][33] , they both describe hydrogen bonding and halogen bonding as the same type of interaction.…”

Canonical Approach To Generate Multidimensional Potential Energy Surfaces

“…The first observation is that the radial slices of the full H 2 O PES corresponding to fixing the two angular variables (Q,F) possess the same canonical shape seen in the wide array of diatomic molecules studied in Refs. [17][18][19][20][21][22][23][24][25][26]. The direct interpolation method described above (Section III.A) exploits the canonical nature of these radial slices to construct an approximation of the full H 2 O PES (for 44° < Q < 180° and 5° < F < 45°) that utilizes nine values for F and ten values for Q along with fourteen of the special radial values defined by the associated (radial) force.…”

“…This aspect of the canonical transformation methodology was recently extensively explored in the setting of diatomic molecules (see Refs. [17][18][19][20][21][22][23][24][25][26]. In particular, it was shown that through consideration of force (through which the key radial separation distances Rj are defined) rather than just potential energy the canonical structure of diatomic potential curves is revealed.…”