Three-electron transitions

Optical absorption measurements give band-gap data for cubic sihcon carbide as 2.2 eV and for the a-form as 2.86 eV at 300 K (55). In the region of low absorption coefficients, optical transitions are indirect whereas direct transitions predominate for quantum energies above 6 eV. The electron affinity is about 4 eV. The electronic bonding in sihcon carbide is considered to be predominantiy covalent in nature, but with some ionic character (55). In a Raman scattering study of vahey-orbit transitions in 6H-sihcon carbide, three electron transitions were observed, one for each of the inequivalent nitrogen donor sites in the sihcon carbide lattice (56). The donor ionization energy for the three sites had values of 0.105, 0.140, and 0.143 eV (57). 

Fig. 3. Bond transition moment diagrams (Exciton model) for first three electronic transitions of 4a,4b-dihydrophenanthrene and of the iinear hexaene...

Examples of distorted square-pyramidal complexes are NiBr2(TAS) (81) and NiBr2(PNP) (82) and NiBr2(Ci5H22N4).H20 (83) ). Their spectra (83—85) exhibit a broad band in the 10—20 kK region which contains at least three electronic transitions beneath its envelope. 

List the three electronic transitions detectable by uv spectrophotometers in order of increasing A . 

The splitting of the lD term for square pyramidal complexes in the limit of symmetry is shown in Figures 7a and 7b. In this symmetry three electronic transitions are anticipated, namely Ai— 1Bi, 1Ai- 1E and 1A1 > lA2, with a stronger lA - lE band which is allowed in C4u symmetry. The three bands lie in the ranges 15000-18000, 21000-24000 and 27 000-29 000 cm-1. Typical examples of spectra are shown in Figure 20. Spectroscopic data for selected examples of low-spin five-coordinate square pyramidal complexes are shown in Table 35. 

Experimentally, S02 shows a region of absorption from ca. 2400 to 1800 A ca. 2000 A). Its maximum molecule extinction coefficient is several times that of the 3400— 2600-A system. Duchesne and Rosen (J. Chem. Phys., 1947, 15, 631) have shown, by vibrational analysis, that at least two and probably three electronic transitions arc involved in the region. The first of these has its origin at 42,170 cm.-1 (2371 A), the vibrational frequencies appearing in the upper state being 963 (symmetrical stretching)... 

Some qualitative understanding of the CICD can be gained by means of Wentzel-type theory that treats the initial and final states of the decay as single Slater determinants taking electronic repulsion responsible for the transitions as a perturbation. The collective decay of two inner-shell vacancies (see Figure 6.6) is a three-electron transition mediated by two-electron interaction. Thus, the process is forbidden in the first-order perturbation theory, and its rate cannot be calculated by the first-order expressions, such as (1). Going to the second-order perturbation theory, the expression for the collective decay width can be written as... 

Kr outer-shell vacancies present in the final state, f). Physically, this means that the three-electron transition consists of virtual two-electron recombination on krypton cations followed by ionization of X (see Figure 6.6). The corresponding recombination matrix element entering Eq. (21) is... 

The CD induced in 2-(2,4,6-cycloheptatrien-l-ylidine)-4-cyclopentene-l,3-dione upon its inclusion in 6-cyclodextrin was used to obtain full assignment of the observed absorption bands [49]. It was concluded that the low energy band system was composed of three electronic transitions, two of which were polarized perpendicular, and one parallel, with respect to the long molecular axis of the solute. The higher energy band system was found to also consist of three allowed electronic transitions, but two of these were polarized parallel, and one perpendicular, to the long axis of the solute molecule. These deductions were supported by a series of CNDO/S-CI calculations performed on the system. A similar approach was taken to assign the polarizations of the various transitions associated with 2-thioxo-l,3-benzodithiole and 2-selenoxo-1,3-benzodithiole [50]. 

The excitation energies of the three electronic transitions so far observed differ markedly. 

Fig. 25 The sign of the molar rotation (MR) and of the CD of the first three electronic transitions of benzene induced by a point charge of -O.le located 1.3 A above the plane of the ring. The distortions along the edges of the sectors (particularly evident in the center of the ring) are caused by the coarseness of the grid. MR calculated with PBEO/SVP, CD with CC2/SVP. Data to prepare the figure were taken from [155]...

Linevsky (4) determined the concentration of CH radicals in equilibrium with 1 atm. of hydrogen gas, by application of a high resolution spectro-photographic technique to three electronic transitions in absorption. A third law analysis, using the present functions, of 27 determinations in the temperature range 3065 - 3155 K yielded AjH (CH, g, 298.15 K) = 142.01 1.28 kcal raol with a negligible drift. This value is in excellent agreement with the spectroscopic predissociation value. 

The low-resolution spectra of Ca and Sr monoformate and monoacetate were first reported in 1985 [42], A more extensive study of the formates, acetates, propionates, and butanoates [92] appeared in 1990 (Fig. 19). Calcium monoformate is believed to have C2v symmetry with bidentate bonding. The spectra show three electronic transitions similar to CaNH2... 

Isoelectronic analogies are very helpful in understanding the geometry and electronic structure of the monovalent alkaline earth derivatives. For example, formamide, HC(0)NH2, differs from formic acid, HC(0)0H, by the substitution of an NH2 group for OH. The use of formamide as an oxidant in a Broida oven gives rise to a spectrum [93] with three electronic transitions A X, B-X, and C-X (Fig. 20) very similar to the corresponding monoformate derivatives. 

The electronic spectra of CaSH and SrSH were first published in 1991 by Fernando et al. [111]. The molecules were made by the reaction of the electronically excited Ca or Sr atoms with H2S. From the three electronic transitions A2A -X2A B2A"-X2A and C2A -X2A (Fig. 26) in the spectra, it was evident that CaSH and SrSH were bent molecules. Unlike the very ionic Ca+OH molecule that prefers to keep the H(<5+) atom as far as possible from Ca+, the bonding in CaSH and SrSH is more covalent. The electronic spectra and orbitals in CaSH are very similar to the states of CaNH2 but the lower symmetry (Cs) of CaSH changes the labels of the states (A2B2 to A2A, B2Bl to B2A" and C2A1 to C2A ). The ab initio calculations of Ortiz [57] on CaSH confirm the bent geometry and this picture of the electronic structure. 

Fig. 42. Energy levels in [CrOCls], showing the three electronic transitions. The 61 molecular orbital is bonding and the unpaired electron is in 62-...

Figure 7.17a shows the molecular orbitals of benzene. If you were to attempt a simple explanation for the electronic transitions in benzene, you would conclude that there are four possible transitions but each transition has the same energy. You would predict that the ultraviolet spectrum of benzene consists of one absorption peak. However, owing to electron-electron repulsions and symmetry considerations, the actual energy states from which electronic transitions occur are somewhat modified. Figure 7.17b shows the energy-state levels of benzene. Three electronic transitions take... 

The splitting of the term for square pyramidal complexes in the limit of symmetry is shown in Figures 7a and 7b. In this symmetry three electronic transitions are anticipated, namely and Ai > A2, with a stronger E band which is allowed in... 

As an example of the above, let me consider the one-photon, three-electron transitions from the n = 2 to the n = 3 shell in Carbon,... 

This process is spin allowed, but it is technically a three-electron transition. In a corresponding ion pair, it would be strongly forbidden, and k i should be much less than 1. It is important to address the general behavior of outer-sphere electron transfer systems before this issue is considered in greater detail. 

Vo is the (0,0) band origin in each of the three electronic transitions given in the fourth line of transitions listed above. 

In protein molecules the main source of optical activity is the peptide bonds. Consequently CD spectra are taken at wavelengths shorter than 240 nm where the predominant absorption comes from peptide bonds. This arises from three electronic transitions, namely 0) sn n- Jt transition at -210-220 nm, in which an electron in the non-bonding molecular orbital of the carbonyl oxygen is promoted to an antibonding n molecular orbital, (ii) a it r transition at -190 nm, and (iii) a probable transi-... 

In many ways the electronic coupling in both the neutral and the mixed-valence compounds can be reduced to the simple orbital diagram depicted in Fig. 7 [26]. In the neutral complex only the MLCT transition is observed, but for the singly oxidized radical cations three electronic transitions are possible the LMCT, the IVCT (in a fully delocalized compound this is better described as charge resonance transition), and an LMCT. For carboxylate linkers the LMCT is not observed because the CO2 Tt orbitals are too low in energy, but substitution of O by S or NR raises the energy of this filled n orbital. 

Figure 2.16 gives an example of the nomenclature for the absorbance spectrum of PO, observed in a flame (see Section 7.2.14). In the analytically important wavelength range between 200 nm and 350 nm, three electronic transitions to the excited states D, A, and B can be observed. Also visible is the striking substructure of these transitions, which will be discussed in the next sections. 

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