Excited States of a Single Bond

Let us take H2 as an example, and construct the molecular states from the familiar minimal VB configuration set shown in Fig. 7.1a these are the covalent and two ionic structures. Since the molecule has left-right symmetry we can instantly make two symmetry-adapted configurations from the two 

FIGURE 7.1 (a) The YB structures and their mixing diagram leading to the states of the H—H bond, (b) A Schematic MO representation of the first singlet excited state and its correspondence to the YB representation. 

Another question one may have is Where do I see the antibonding character in the ) VB state Here too, use of the semiempirical theory in Chapter 3 and the rules for taking matrix elements between VB determinants, will show that the matrix element between the two ionic determinants is 2(35, where (3 and 5 are the corresponding resonance and overlap integrals between the two AOs of the H atoms. As such, the negative combination of the ionic configurations 

USING VALENCE BOND THEORY TO COMPUTE AND CONCEPTUALIZE 

FIGURE 7.2 The VB structures and their mixing diagram leading to the states of an A-X bond (A = H, R3C, etc. X = an electronegative group, for example., Cl, OH). 

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