Walsh triatomic molecule

Figure 9.4. Walsh diagram showing correlation of orhitals between bent and hnear triatomic molecules, and suggesting variation in orbital energy with bond angle.

Calculations on triatomic molecules, because of their theoretical and experimental significance, will be mentioned below in some detail. Herzberg s Polyatomics [6] gives diagrams of the correlation of orbitals between large and small internuclear distances in linear AH2 molecules and in linear ABa molecules [6]. Walsh, in a classic series of papers [97], gave correlations of molecular orbitals between linear and bent AH2 molecules and linear and... 

This approach was first applied to relatively simple cases many years ago by Walsh4 and led to certain generalizations, called Walsh s rules, relating the shapes of triatomic molecules to their electronic structures. The basic approach is to calculate, or estimate, the energies of molecular orbitals for two limiting structures, say, Unear and bent (to 90°) for an AB2 molecule,... 

Diatomic molecules are necessarily linear, but a triatomic molecule can be either linear like CO2 and HCN or bent like SO2 and H2O. Mulliken s correlation diagram procedure was extended to tri- and tetraatomic molecules by Walsh [1], who promulgated a set of simple but remarkably viable [2, 3, 4] rules for predicting whether or not a molecule will remain linear, from the effect of the departure from linearity on the energy of its occupied molecular orbitals. 

Figure 7.3 Nodal patterns and qualitative energies of MOs in linear and bent triatomic molecules according to Walsh [4]. The lowest three MOs, composed of 1s AOs on the constituent atoms (Fig. 7.2), participate little in the chemical bonding and are excluded. Horizontal coordinate is the bending angle 6. Orbital occupancies are shown for linear CO (0 = 180°) and for bent NO2 (0 = 134°). Irreducible representations are given for the MOs in and C v point groups at left and right, respectively.

Orbital correlations alone cannot rationalize the electronic structure in triatomic molecules one needs to know how the MO energies are ordered and how they are influenced by the bond angle (f>. In a remarkably prescient series of papers published in the early 1950s (long before accurate wave functions became available for polyatomic molecules), Walsh [4] developed semiempirical rules... 

We begin by considering the symmetric triatomic class of molecules HAH, where A is any atom. Such molecules can be linear or bent. Walsh s approach predicts which are linear, which are bent, and sometimes which of two bent molecules is more bent. 

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