Antibonding molecular electronic transitions from

Fig. 31. Schematic potential energy diagram for interaction between absorbate A and a surface M. G is the ground state of the molecular complex, M" + A is an ionic state, (M + A) is an antibonding state, M + A is a state where the adsorbate is excited and the substrate is in its ground state, M + A is a state where the substrate is excited and the adsorbate is in its ground state. Possible electronic transitions from the ground state G to the various excited states are indicated by the shaded Franck-Condon region. Electron bombardment can presumably excite any of these states. (From Ref. )...

Electronic transitions from bonding to antibonding molecular orbitals are often encountered. In this case the potential energy curve for the ground state will be quite different from that of the excited state because less bonding electron density is found in the excited state. An example is the transition from the ground state of NO+ ion to the first excited state, as shown in Fig 14.55. 

In an electronic transition, an electron from one molecular orbital moves to another orbital, with a concomitant increase or decrease in the energy of the molecule. The lowest-energy electronic transition of formaldehyde promotes a nonbonding (ri) electron to the antibonding pi orbital (it ).1 1 There are two possible transitions, depending on the spin quantum numbers in the excited state (Figure 18-11). The state in which the spins are opposed is called a singlet state. If the spins are parallel, we have a triplet state. 

The third step hypothesized [eq. (9)] is electron demotion from the singly occupied antibonding molecular orbital of XIII to the low energy, singly occupied oxygen p orbital which originally lost the electron in the n-ir excitation process. It is important and of interest to note that this demotion process is an electronic transition thus XIII and XIV are... 

In a molecular system which clearly possess a ct LUMO the photoexcitation process may involve promotion of an electron from a n HOMO to the ct LUMO. This process may or may not produce bond cleavage. Direct observation of a 7t — ct electronic transition is often difficult due to the localized nature of the ct molecular orbitals resulting in a low probability for the transition [91]. More likely ait->it electronic transition takes place initially and ET, i.e., a - ct, is required to eventually populate the lower energy ct antibonding molecular orbital. Onium salts are examples of chemical structures that possess a ct LUMO and are expected to behave in this manner (see in Sect. 3.3). 

From spectroscopic data it seems likely that the excited states involved in alkyne photochemistry are either (tt, — ) states, in which an electron from a bonding tt molecular orbital has been promoted to an antibonding - molecular orbital, or Rydberg stales, in which a tt electron has been promoted to an extended a-type orbital covering more than one nucleus. The spectra of acetylene, propyne and but-l-yne all show features characteristic of both types of electronic transition. In... 

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