Promotion, of an electron

Promotion of an electron in Hc2 from the (7 15 to a bonding orbital produces some bound states of the molecule of which several have been characterized in emission spectroscopy. For example, the configuration ((J l5 ) ((7 l5 ) ((7 25 ) gives rise to the 2i and bound states. Figure 7.24(a) shows the form of the potential curve for the state. The A-X transition is allowed and gives rise to an intense continuum in emission between 60 nm and 100 nm. This is used as a far-ultraviolet continuum source (see Section 3.4.5) as are the corresponding continua from other noble gas diatomic molecules. 

The total electron density contributed by all the electrons in any molecule is a property that can be visualized and it is possible to imagine an experiment in which it could be observed. It is when we try to break down this electron density into a contribution from each electron that problems arise. The methods employing hybrid orbitals or equivalent orbitals are useful in certain circumsfances such as in rationalizing properties of a localized part of fhe molecule. Flowever, fhe promotion of an electron from one orbifal fo anofher, in an electronic transition, or the complete removal of it, in an ionization process, both obey symmetry selection mles. For this reason the orbitals used to describe the difference befween eifher fwo electronic states of the molecule or an electronic state of the molecule and an electronic state of the positive ion must be MOs which belong to symmetry species of the point group to which the molecule belongs. Such orbitals are called symmetry orbitals and are the only type we shall consider here. 

The ground configuration of Ar is KL3s 3p, giving an inverted P /2 multiplet. The excited states involved in laser action involve promotion of an electron from the 3p orbital into excited As,5s,Ap,5p,3d,Ad,... orbitals. Similarly, excited states of Kr involved arise from promotion of an electron from the Ap orbital. In Ar the KL3s 3p configuration gives rise to 5, V, terms (see Section 7.1.2.3). Most laser transitions involve the core in one of the states and the promoted electron in the Ap orbital. 

Figure 14.12 Ultraviolet excitation of 1,3-butadiene results in the promotion of an electron from i / , the highest occupied molecular orbital (HOMO), to 1//3, the lowest unoccupied molecular orbital (LUMO).

A UV or visible absorption peak is caused by the promotion of an electron in one orbital (usually a ground-state orbital) to a higher orbital. Normally the amount of energy necessary to make this transition depends mostly on the nature of the two orbitals involved and much less on the rest of the molecule. Therefore, a simple... 

Even where the promotion is to a lower vibrational level, one that lies wholly within the 2 curve (such as Vi or V2), the molecule may still cleave. As Figure 7.2 shows, equilibrium distances are greater in excited states than in the ground state. The Franck-Condon principle states that promotion of an electron takes place much faster than a single vibration (the promotion takes... 

In the proper system, under appropriate conditions, a new intermediate which is simultaneously a carbene and a carbonium ion (i.e., a carbenonium ion) may be envisioned. A particularly attractive system would be 263 and 264, as promotion of an electron from the tr system in vinyl cation 263 a onto the... 

A CT transition which is very similar to the -> MMCT transition has been observed by Vogler et al. [55] for complexes [M(2,2 -bipyridyl)X3] with X = Cl, Br, I and M = Sb, Bi. These authors report MLCT transitions involving the promotion of an electron from the lone pair to the n orbital of the bipyridyl ligand. For example, for M = Sb and X = Br they observe an orange color for the complex due to an absorption band with a maximum at 435 nm. In the complexes considered by us the transition is to an antibonding n orbital (with pronounced d character) on the filled-shell transition-metal complex ion. 

On photochemical ring-closure, irradiation results in the promotion of an electron into the orbital of next higher energy level, i.e. jf3 and the HOMO to be considered now therefore becomes iJ/4 (14) ... 

FIGURE 5.15 Molecular orbitals for ethylene. Promotion of an electron from the ground state to the excited state is known as a n - n transition and is usually accompanied by an absorption of radiation in the ultraviolet region of the spectrum. 

The spectra of linear polyenes are modelled well as one-dimensional free-electron systems. The cyanine dyes are a classical example. They constitute a class of long chain conjugated systems with an even number n of 7r-electrons distributed over an odd number N = n — 1 of chain atoms. The cyanine absorption of longest wavelength corresponds to promotion of an electron from the highest occupied energy level, En/2 to the lowest unoccupied level, such that in terms of a free-electron model... 

The ground-state electron configuration of B suggests an ability to form one B—F bond, rather than three., JB JB -Jil I I Again, excitation, or promotion of an electron to a higher energy orbital, followed by hybridization is required to form three equivalent half-filled B orbitals. 

The F center absorption maximum for KC1 is at 565 nm and that for KF is 460 nm (Table 9.1). (a) What is the composition of a natural crystal with color centers showing an absorption peak at 500 nm (b) If the absorption peak for KF corresponds to the promotion of an electron from the F center to the conduction band, determine the energy of the color center with respect to the conduction band. (The band gap in KF is 10.7 eV.) If the relative position of the color center energy level remains the same throughout the KF-KC1 solid solution range, estimate (c) the band gap of KC1 and (d) the band gap for the natural crystal. 

The carbon atom (6C) has the electron configuration of ls22s22p2. There are 4 valence electrons, of which only two are unpaired in the ground state. During the formation of carbon compounds, one 2s and three 2p orbitals combine to give four identical sp3 orbitals by the promotion of an electron from the 2s orbital to a 2p orbital. These 4 unpaired orbitals then mix to form four identical sp3 hybrid orbitals. 

For polyenes in their ground state, the highest occupied MO (HOMO) will be symmetric with respect to m2 for 2, 6, 10. .. n electron systems and antisymmetric for 4, 8, 12. .. n electron systems. The lowest unoccupied MO (LUMO) has the symmetry opposite to that of HOMO. In the first excited state, the LUMO of the ground state will become singly occupied because of the promotion of an electron and it will thus become the new highest occupied orbital. Therefore in the circumstances the symmetry of the highest occupied orbital is opposite to that of the ground state. 

K ----> 7t Transition. In this transition there is the promotion of an electron from a... 

Photon absorption produces excited electron configurations by promotion of an electron from an occupied to a vacant MO. These electronic transitions are described as ... 

Both the Si and Ti excited states arise from the promotion of an electron from the n molecular orbital to the Jt molecular orbital. They are referred to as (njji ) and 3(n,tt ) states, respectively. The S2 and T2 states arise from the promotion of an electron from the n molecular orbital to the Jt molecular orbital and are referred to as 1(n,nx ) and 3(jt,Jt ) states, respectively. The state diagram for methanal is shown in Figure 1.11. With regard to the different spin states in molecules, the following ideas are important ... 

Figure 11.1 Promotion of an electron from the valence band to the conduction band on irradiation of a semiconductor. The electron vacancy left behind in the valence band is known as a hole (h+)...

An electronic transition consists of the promotion of an electron from an orbital of a molecule in the ground state to an unoccupied orbital by absorption of a photon. The molecule is then said to be in an excited state. Let us recall first the various types of molecular orbitals. 

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