Linear Tetraatomics and Their Deformation

For the tetraatomic system HXXH, representing both the linear acetylene and the non-linear hydrogen peroxide, we expect to be able to construct twelve symmetry coordinates. Three of them are translational, whereas two of the remaining nine in the linear conformation and three in the non-linear one are reserved for rotations. Linear tetraatomics thus have seven vibrational coordinates, motion along which changes the potential energy, whereas their nonlinear counterparts have six. Those of the linear HXXH molecule are shown in Fig. 4.4 with the subgroup into which each is taken, if only momentarily, by the displacement. 

the more highly destabilized is its antibonding counterpart. The symmetric member of any pair of symmetry-adapted combinations is placed below its antisymmetric partner, which is bisected by a nodal plane. As usual, the MOs are labelled according to their irreps in both D2/1 and Doo/i- 

The most important effect of bending the XH bonds away from linearity is to convert one tt orbital and its antibonding partner into combinations of lone-pair hybrid orbitals, the former going up in energy and the latter going down. In addition, the consequent desymmetrization allows each of these two 

One 7T -orbital goes down strongly in each bending mode, as expected, but its bonding partner goes up very little - if at all. 

The excited states of both spin states are bent, as expected, but take up different conformations the singlet is indeed trans but the triplet is cis. 

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Linear Tetraatomics and Their Deformation Axis Convention... 

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