An introductory overview of intermolecular forces

Let us first seek to give a more rigorous and operational ab initio characterization of such units. The important physical idea underlying the above definitions is that of the connecting covalent bonds that link the nuclei. One can therefore recognize that a molecular unit is equivalently defined by the covalent-bond network that contiguously links the nuclei included in the unit. We can re-state the definition of a molecular unit in a way that emphasizes the electronic origin of molecular connectivity. 

Because the electronic distribution of a system is determined by I2 for a specific solution of Schrodinger s equation, definition (D1) allows us to determine molecular character directly from the form of the system s wavefunction f, corresponding to some definite point on the Born-Oppenheimer potential-energy surface.3 

If we now understand covalent bond to correspond to a specific form of two-center (or three-center) NBO in the optimal localized description of the electron 

Already in the early twentieth century it was realized that definitions such as (D1) do not adequately cover all units of interest in chemistry. Thus, by 1902 Werner had demonstrated (Section 4.5.1) that numerous covalently saturated ligand (L) species (L = CO, NH3, H20, etc.) could exist both as free molecular species and in coordinated form as components of transition-metal complexes ML with open-shell metals M, 

By 1920 it had also become apparent that weak associations existed between closed-shell hydride-bearing molecules (AH) and lone-pair-bearing bases ( B), leading to hydrogen-bonded 5 species of the form 

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