Bent excited valence state

Coulson has recently discussed the bond angles in these molecules in a review paper,550 using two simple models. An ionic model would predict these molecules to be linear, but in a second model, in which valence states arising from s2- sp, s2- sd excitations were permitted, the first would lead to linear, the second to bent structures. From a consideration of the excitation energies, it was shown that when the central atom is heavy and the attached halogens are as electronegative as possible, the bent situation is favoured, in agreement with the experimental results. 

The vertical electronic spectrum of acetylene shows many similarities with that of the isoelectronic molecule HCN. Both are linear molecules in their electronic ground states, but their lowest-valence excited states are bent. Photoexcitation of acetylene in the near-UV (190-240 nm) induces a electron promotion, resulting in population of the planar, trans bent A A state ( S in the linear limit) [312], The A A —transition is dominated by long progressions involving v 3 (the excited state trans bending mode), as would be expected on Franck-Condon grounds. The A—X... 

In semiconductors, which have a bandgap, recombination of the excited carriers— return of the electrons from the conduction band to vacancies in the valence band—is greatly delayed, and the lifetime of the excited state is much longer than in metals. Moreover, in n-type semiconductors with band edges bent upward, excess electrons in the conduction band will be driven away from the surface into the semiconductor by the electrostatic held, while positive holes in the valence band will be pushed against the solution boundary (Fig. 29.3). The electrons and holes in the pairs produced are thus separated in space. This leads to an additional stabihzation of the excited state, to the creation of some steady concentration of excess electrons in the conduction band inside the semiconductor, and to the creation of excess holes in the valence band at the semiconductor-solution interface. 

A similar situation arises in the ir-v excited states of ethylene and acetylene where the n interactions become antibonding the excited states are consequently twisted (11) and bent (12) respectively. Another example is provided by triatomic molecules formed by second row atoms ifthe total number of valence electrons exceeds 16, the molecules are bent 33) since there are now antibonding electrons present and bending reduces the resulting unfavourable 7r interaction by replacing p AOs by hybrid AOs (cf. ozone, 13). 

HAAH molecules containing 10 valency electrons should be linear in their ground states. Those containing 12 electrons should be bent but planar cis- and /raMs-forms). Those containing 14 electrons should be bent and non-planar. The spectra of the isoelectronic molecules HCN and C2H2 are particularly discussed the first excited state of each should be non-linear. 

The HAB Walsh diagram of Fig. 14-9 rationalizes the fact that the ten-valence electron HCN is linear in the ground state (due to Itt — 4a ) but bent in the first excited state. Such molecules as HNO, HNF, and HOCl, with 12-14 electrons, are bent in both ground and excited states because there is always at least one electron in the 5a level. 

Table 1.2 Structural parameters of bent valence excited states of CO2...

The asymmetric cation NeN-H" with ten valence electrons is linear (C ) in the electronic ground state and may be bent (CJ in excited states. The electronic ground state is represented by the MO configuration (1a) (2a) (3a) (4a) (5a) (l7t) - For the lowest electronic state A of the bent molecule, this configuration correlates with (1a ) (2a ) (3a ) (4a ) (5a ) (6a ). An examination of the ground-state MOs showed that the... 

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