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Chemical Bond II: Molecular Orbitals
Published in Franco Battaglia, Thomas F. George, Understanding Molecules, 2018
Franco Battaglia, Thomas F. George
Diagrams representing molecular-orbital energy changes as a function of angle bonds are called Walsh diagrams. The left portion in Figure 7.11 is a representative example for molecules of the type
One-colour (∼220 nm) resonance-enhanced (S1 − S0) multi-photon dissociation of acetylene: probe of the C2 A 1 Π u − X 1 Σ+ g band by frequency-modulation spectroscopy
Published in Molecular Physics, 2020
Jun Jiang, Zhenhui Du, Jacques Liévin, Robert W. Field
Given our current experimental knowledge of the CH state, we believe that this second-lowest energy, doubly-excited state is the only reasonable candidate for the super-excited acetylene dissociation precursor state in the two-photon region. We further hypothesise that its PES supports both trans- and cis-bent equilibrium structures. While the existence of a cis-bent equilibrium geometry was not reported in Ref. [40], the new calculation on the acetylene valence states indicates that the second doubly-excited PES supports a cis-bent conformer ( eV, , Å, Å), in addition to the trans-bent conformer ( eV, , Å, Å). According to the Walsh diagram for the CC-H bending coordinate of the H-CC-H molecule, the doubly-occupied, anti-bonding orbital correlates to the and orbitals for the and states, respectively. Similar to the state, the bent geometries for this state are due to stabilisation of the (trans-bent) and (cis-bent) orbitals with respect to the orbital along the bending coordinate, because bending reduces the anti-bonding interaction in the orbital. The larger values for the - and -conformers than their S counterparts (1.347 Å and 1.379 Å, respectively) [8, 12] can be explained by the presence of an additional electron in an anti-bonding orbital. Detailed results from the new calculation will be reported in a future paper [44].