Orbital filling process

Another aspect of qualitative application of MO theory is the analysis of interactions of the orbitals in reacting molecules. As molecules approach one another and reaction proceeds, there is a mutual perturbation of the orbitals. This process continues until the reaction is complete and the new product (or intermediate in a multistep reaction) is formed. PMO theory incorporates the concept of frontier orbital control. This concept proposes that the most important interactions will be between a particular pair of orbitals. These orbitals are the highest filled oihital of one reactant (the HOMO, highest occupied molecular oihital) and the lowest unfilled (LUMO, lowest unoccupied molecular oihital) orbital of the other reactant. The basis for concentrating attention on these two orbitals is that they will be the closest in energy of the interacting orbitals. A basic postulate of PMO... 

Logic suggests that an electron will occupy the lowest energy level available, and electrons will successively fill these levels as they are added to an atom or molecule. "Quantum mechanics" restricts all orbitals to a maximum of two electrons (these two have opposite "spins" and do not strongly repel one another), and hence a filling process occurs. The filling pattern for the sodium atom (sodium is atomic number 11 - therefore there will be 11 electrons in the neutral atom) is shown in Figure 2.7). 

The theory for this intermolecular electron transfer reaction can be approached on a microscopic quantum mechanical level, as suggested above, based on a molecular orbital (filled and virtual) approach for both donor (solute) and acceptor (solvent) molecules. If the two sets of molecular orbitals can be in resonance and can physically overlap for a given cluster geometry, then the electron transfer is relatively efficient. In the cases discussed above, a barrier to electron transfer clearly exists, but the overall reaction in certainly exothermic. The barrier must be coupled to a nuclear motion and, thus, Franck-Condon factors for the electron transfer process must be small. This interaction should be modeled by Marcus inverted region electron transfer theory and is well described in the literature (Closs and Miller 1988 Kang et al. 1990 Kim and Hynes 1990a,b Marcus and Sutin 1985 McLendon 1988 Minaga et al. 1991 Sutin 1986). 

In some cases, a further parallel can be drawn between the order of filling of electron orbitals and the order of filling a dendrimer shell. In atomic Aufbau, electrons are placed in each p or d orbital until each orbital contains one electron before any pairing of electrons within the orbitals takes place. In the preparation of poly(amidoamine) dendrimers, there is a significant difference in the rates of the first and second alkylation of a given amine such that most of the primary amines are converted to secondary amines before the conversion to tertiary amines becomes the predominant process. This can be viewed as analogous to the orbital filling and can, within the limits of the selectivity of the alkylation, be used in the control of dendrimer valence . 

The restrictions associated with four-coordinate complexes are reversed when the band of d orbitals is filled with metal valence electrons (e.g., d systems). In these situations, ligand field restrictions are encountered from the tetrahedral complex and not the square planar. This departure from the quaUtative picture based on pure d orbitals is primarily due to hybridization factors in these systems. The square planar complex requires an empty d orbital (in the plane) to construct the four hybrid orbitals in the square plane. The [2-f2] transformation from the square planar complex thus returns two valence electrons (formally from a p orbital) to this orbital generating a filled d band in the process. The process proceeds without an orbital crossing. The tetrahedral system, in contrast, starts with a filled d band. The [2- -2] process formally moves a pair of d electrons into a p orbital. This process thus involves an orbital crossing and therefore encounters ligand field restrictions. 

Anomalies (such as V) come from irregularities in the shell filling process around the element. For the spin-orbit splitting (lower right), maxima do not appear at P, As, Sb, or Mn and Tc (half filled shells), but do appear at Sc, Ar, Zn, Kr, Cd (full shells) while minima appear at Sr and Xe. The variations of this property present more anomalies because its values result from small differences between large values, which cumulates experimental errors and anomalies due to the finite-difference procedure. 

In certain cases, before relaxation occurs, an excited molecule can transfer an electron from some potential donor to fill its low-lying empty orbital. This process has been studied well because of its major role in photosynthesis" and is called photoinduced electron transfer (PeT). PeT is a signaling event that relies on emission quenching or enhancement. 

Covalent bonds form as a result of an orbital on one atom merging with an orbital on another atom such that each shares a pair of electrons with the other. With the slot-filling process, each of these orbitals has one electron before bonding occurs. However, having one electron before bonding is not a requirement of these orbitals. It is possible for one orbital to have two electrons while the other has none. A bond formed in such a manner is called a coordinate covalent bond. 

The d block refers to that section of the periodic table in which the process of orbital filling (aufbau process) involves a d subshell. 

Fats are triglycerides in which satiuated fatty acid components predominate. The/block is that portion of the periodic table where the process of filling of electron orbitals (aufbau process) involves / subshells. These are the lanthanide and actinide elements, fee (See face-centered cubic.) Ferromagnetism is a property that permits certain materials (notably Fe,... 

Although the reduction process is not always a reversible one, oxidation and reduction potential values can be sometimes related to the Hiickel energies of the highest and lowest filled molecular orbital of the dye (108). 

Figure 8.21 shows schematically a set of lx, 2s, 2p and 3s core orbitals of an atom lower down the periodic table. The absorption of an X-ray photon produces a vacancy in, say, the lx orbital to give A and a resulting photoelectron which is of no further interest. The figure then shows that subsequent relaxation of A may be by either of two processes. X-ray fluorescence (XRF) involves an elecfron dropping down from, say, fhe 2p orbifal fo fill fhe lx... 

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