Types of Electronic Transitions

If a compound is to absorb in die region above 200 nm, the energy separation between its HOMO and LUMO must be smaller than in the case of alkanes. The bonding and antibonding MOs for pi bonds, which are weaker than sigma bonds, are closer 

5-hexatriene. As would be expected based on this figure, Amax for the 77- tj  

In the case of an aldehyde or a ketone, the chromophore is the carbonyl group. The 

77--- 77 transition for acetone, like that for ethene, occurs below 200 nm (Amax = 

188 nm) because there is no conjugation. However, acetone has electrons in nonbonding orbitals that are higher in energy than the pi electrons. The longest wavelength absorption for acetone is due to an n-- 77 transition (Amax = 279 nm, emax = 15). The 

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The +, —, e, and/labels attached to the levels in Figure 7.25 have the same meaning as those in Figure 6.24 showing rotational levels associated with and Ig vibrational levels of a linear polyatomic molecule. Flowever, just as in that case, they can be ignored for a Z — I, type of electronic transition. 

The electronic spectra of actinide compounds arise from three types of electronic transition ... 

Figure 1.8 Molecular orbital diagram for an octahedral d-block metal complex ML6. The vertical arrows indicate different types of electron transition that may be brought about by photon absorption...

Let us consider a semiconductor electrode, at which a redox reaction of type (1) occurs. Electrons of both the conduction band and valence band may take part in the electrode process. As a result, the reversible reaction considered is characterized by four different types of electron transitions (see Fig. 6a). Transitions in which electrons leave the semiconductor and holes come in contribute to the cathodic current, and those where electrons come in and holes escape contribute to the anodic current. Thus, the resultant current is a sum of four currents i p, i >p (when referring to currents we shall always mean current densities). 

The electronic configuration of formaldehyde under different types of electronic transitions can be represented in terms of electron occupancy (Table 3.2). To obtain the molecular state symmetry the symmetry of singly occupied orbitals need only be considered for the direct product (Section 2.9). 

The spin selection rule breaks down somewhat in complexes that exhibit spin-orbit coupling. This behavior is particularly common for complexes of the heavier transition elements with the result that bands associated with formally spin forbidden transitions (generally limited to AS — s ) gain enough intensity to be observed. Table 11.16 summarizes band intensities for various types of electronic transitions, including fully allowed charge transfer absorptions, which will be discussed later in the chapter. 

There are two types of electronic transition commonly responsible for photochemically induced reactions in organic molecules. The first of these is the n—>ir transition in which an electron in a nonbonding atomic orbital is excited to an antibonding ir orbital, the excited state being referred to as n, it. This occurs in nitrogen-, oxygen-, and sulfur-containing molecules, and the nature of the n, 77 state of the carbonyl function has been the subject of considerable study.5,6 Excitation to the n, tt state in aldehydes and ketones occurs at approximately 290 nm. 

Three types of electronic transition can be distinguished among compounds of the d block transition elements d-d bands, charge transfer (or electron-transfer) bands and intra-ligand bands. Configuration interaction may make the distinctions rather hazy, however. 

Fig. 2.2. Schematic illustration of the types of electronic transition found in coordination compounds of the transition elements.

Calculations were performed at TD/6-31+G(d) level. Geometries were optimized at the UB3LYP/6-311G level. Calculated values of oscillator forces ( ) and types of electron excitation (A - electron , B - hole types of electron transitions) are presented in parenthesis. 

Fig. 1. Schematic orbital energy diagram representing various types of electronic transitions in octahedral complexes. A line connects an atomic orbital to that molecular orbital in which it has the greatest participation. 1 metal centered (MC) transitions 2 ligand centered (LC) transitions 3a ligand-to-metal charge transfer (LMCT) transitions 3b metal-to-ligand charge transfer (MLCT) transitions...

Another important type of electronic transition is the charge transfer to solvent (CTTS) transition. Recently, it has also been found5 that electronic transitions may occur between MO s which are localized on different ligands of the same complex. 

Optical Spectra. The optical properties of smectites have been studied by various workers (32,37-40), and involve several different types of electronic transitions. One important type of transition is the intervalence charge transfer (IT), which is observed in the optical spectra of minerals containing both Fe2 and Fe3 in their... 

In this section, we use the square-planar complex ML4 as an example to discuss the various types of electronic transitions observed in coordination compounds. 

Three types of electronic transition can occur for lanthanide compounds. These are f->-f transition, nf - (n + l)d transition and ligand -> metal f charge-transfer transition. 

Type of electronic transition Molar extinction coefficient, e Mineral examples Reference... 

For this study we have used methylen-cyclopropene (MCP) and acrolein (ACRO) in two solvents, an apolar (dioxane) and a polar one (acetonitrile). The selected transitions can be seen as representative examples of different types of electronic transitions for which different solvent responses can be studied for MCP the first 77 - 77 transition for MCP, and the first n -> 77 and 77 -> 77 transitions for ACRO. We note that in MCP the resulting excited state is characterized by a dipole moment which has an opposite direction with respect to that of the ground state, whereas in ACRO, the n -> 77 and 77 -> 77 transitions are characterized by a decrease and an increase in the dipole moment passing from ground to excited state, respectively. 

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