Molecular orbitals electronic transitions between

Intraligand (IL) excited states of coordination compounds arise from electronic transitions between molecular orbitals primarily localized on a coordinated ligand. It is difficult, a priori, to predict the reactivity of this type of state. While it is logical to expect ligand-centered reactions, the influence of the metal on such processes can be substantial and result in net photochemistry which differs from that of the free ligand. A few examples should serve to illustrate the range of IL photoreactions reported to date. 

Charger-Transfer Transitions These are electronic transitions between molecular orbitals mainly composed of metal (M) orbitals and those mainly composed of ligand (L) orbitals. Three different types of charge-transfer transitions are conceivable ... 

FIGURE 24.2 Electron transitions between molecular orbitals. Note The distance between orbitals is drawn equally for better comprehensibility. 

In this system with even number N of dimer units a orbitals HOMO and LUMO are nonbonding with zero overlap [9], Therefore, the photo-induced electron transition between these orbitals is forbidden. The first electron transition with lowest energy in optical spectrum of this system proceeds between HOMO and unoccupied molecular orbital next to LUMO [6]. Simple calculations based on formula (9) give the energy AEt of this transition at N 1 as... 

Molecular UV-vis spectroscopy is prevalent in the more advanced chemistry curriculum for undergraduates. It appears in Organic Chemistry in the analysis of organic compounds, and it can also be applied to Physical (or Quantum) Chemistry courses in discussions of molecular orbitals, electronic transitions between these orbitals, and also transition selection rules and microstates. It is also relevant to Inorganic Chemistry, as it is investigated in terms of transition metal complex color, crystal field theory, and molecular orbital diagrams and electronic transitions for a variety of inorganic compounds. 

These absorptions are ascribed to n-n transitions, that is, transitions of an electron from the highest occupied n molecular orbital (HOMO) to the lowest unoccupied n molecular orbital (LUMO). One can decide which orbitals are the HOMO and LUMO by filling electrons into the molecular energy level diagram from the bottom up, two electrons to each molecular orbital. The number of electrons is the number of sp carbon atoms contributing to the n system of a neuhal polyalkene, two for each double bond. In ethylene, there is only one occupied MO and one unoccupied MO. The occupied orbital in ethylene is p below the energy level represented by ot, and the unoccupied orbital is p above it. The separation between the only possibilities for the HOMO and LUMO is 2.00p. 

The large downfield CSA for disilenes, and indeed the large isotropic chemical shift, is caused mainly by the great deshielding of one component of the tensor, o-,. Tossell and Lazzaretti propose that this deshielding results from a low-energy electronic transition between a o--bonding orbital in the molecular plane and the Si=Si 7r -orbital.45... 

The other mechanism involves atomic-size roughness (i.e., single adatoms or small adatom clusters), and is caused by electronic transitions between the metal and the adsorbate. One of the possible mechanisms, photoassisted metal to adsorbate charge transfer, is illustrated in Fig. 15.4. It depends on the presence of a vacant, broadened adsorbate orbital above the Fermi level of the metal (cf. Chapter 3). In this process the incident photon of frequency cjq excites an electron in the metal, which subsequently undergoes a virtual transition to the adsorbate orbital, where it excites a molecular vibration of frequency lj. When the electron returns to the Fermi level of the metal, a photon of frequency (u>o — us) is emitted. The presence of the metal adatoms enhances the metal-adsorbate interaction, and hence increases the cross... 

Absorption of UV/VIS radiation in the solid state is different from UV/VIS absorption in the liquid or gaseous phase with respect to photophysical processes taking place in the crystal lattice and to the metallic, semiconductor (SC) or insulator properties of the absorbing solid (Bottcher, 1991). In crystals, multiple atomic or molecular orbitals are combined to form broad energy bands, i.e. a valence band (vb) fully occupied by electrons and a conduction band (cb) unoccupied or only partly occupied by electrons. Conduction bands and valence bands have different energetic positions relative to one another depending on the specific substrate. In a SC cluster, electronic transitions between the valence band and the conduction... 

Molecular-orbital, spectroscopic and thermochemical considerations support the assignment of the band in the visible to an electronic transition between a lone pair on the nitroso nitrogen and an anti-bonding ji orbital, and that of the near-ultraviolet band to a 7t—71 transition in a nitroso-nitro molecule with a jT-only N—N bond. 

Other spectroscopic methods can be useful in some circumstances. Visible/UV absorption spectra depend on the excitation of electrons from filled into empty orbitals. The technique has some limited use in fingerprinting but is especially suited to investigations of electronic structure, in particular the energy difference between molecular orbitals (see Topics C4-C6 and D7). Topic H8 discusses applications to transition metal complexes, as well as the use of magnetic measurements to determine the number of unpaired electrons. 

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