Highest occupied molecular orbital modeling

Thermodynamic properties such as heats of reaction and heats of formation can be computed mote rehably by ab initio theory than by semiempirical MO methods (55). However, the Hterature of the method appropriate to the study should be carefully checked before a technique is selected. Finally, the role of computer graphics in evaluating quantum mechanical properties should not be overlooked. As seen in Figures 2—6, significant information can be conveyed with stick models or various surfaces with charge properties mapped onto them. Additionally, information about orbitals, such as the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), which ate important sites of reactivity in electrophilic and nucleophilic reactions, can be plotted readily. Figure 7 shows representations of the HOMO and LUMO, respectively, for the antiulcer dmg Zantac. 

In addition to electrophilic attack on the pyrrole ring in indole, there is the possibility for additions to the fused benzene ring. First examine the highest-occupied molecular orbital (HOMO) of indole. Which atoms contribute the most What should be the favored position for electrophilic attack Next, compare the energies of the various protonated forms of indole (C protonated only). These serve as models for adducts formed upon electrophilic addition. Which carbon on the pyrrole ring (C2 or C3) is favored for protonation Is this the same as the preference in pyrrole itself (see Chapter 15, Problem 2)1 If not, try to explain why not. Which of the carbons on the benzene ring is most susceptible to protonation Rationalize your result based on what you know about the reactivity of substituted benzenes toward electrophiles. Are any of the benzene carbons as reactive as the most reactive pyrrole carbon Explain. 

The model clusters were chosen in accordance with the structure of PANI proposed in Refs. [1, 3], The present model of PANI also takes into account that under the influence of dopants (in this particular case, protons and anions (chlorine ions) which form bonds to PANI nitrogen), the spins of the highest occupied molecular orbital (HOMO) become unpaired, and PANI changes to triplet state. It should be noted that only in this state there is a considerable increase in PANI conductivity. 

Fig. 9 OMT bands for NiOEP, associated with transient reduction (1.78 V) and transient oxidation (—1.18 V). Data obtained from a single molecule in a UHV STM. The ultraviolet photoelectron spectrum is also shown, with the energy origin shifted (by the work function of the sample, as discussed in [25]) in order to allow direct comparison. The highest occupied molecular orbital, n, and the lowest unoccupied molecular orbital, %, are shown at their correct energy, relative to the Fermi level of the substrate. As in previous diagrams,

TPP (45, 46), as well as data for octaalkyl and octathioalkyl porphyrazines. As with the phthalocyanines and porphyrins, the electronic spectra of porphyrazines can be rationalized using Gouterman s four-orbital model, shown in Fig. 5 (47, 48). All of these macrocycles, when symmetrically substituted and with a metal ion incorporated in the central hole, for example, the M[pz(A4)] or B4, have D4h symmetry, with a doubly degenerate lowest unoccupied molecular orbital (LUMO) (eg) and two highest lying highest occupied molecular orbitals (HOMOs) that complete the four Gouterman orbitals with alM and a2u symmetry. 

In the four-orbital model (1 ), low-lying ir-ir states of free-base porphyrins (symmetry D2h) are considered as resulting from single electron excitation from a pair of nondegenerate highest occupied molecular orbitals (bi, bo) to a pair of nondegenerate lowest unoccupied molecular orbitals (ci, cg). In the case of symmetry D2h mutually perpendicular electric transition dipoles X and Y are not equivalent and, therefore, in the visible absorption spectra of free-base porphyrins two different electronic bands Qx(0>0) and Qy(0,0) are observed (Table 1 and Fig. 10). 

Table XII summarizes HMO quantities referring to models of sulfur compounds total 7r-electronic energy (W7), energies of the two highest occupied molecular orbitals (k2 and ky kx is referred to as HOMO in the text), energies of the two lowest free molecular orbitals (k-x and k 2 k-i is referred to as LFMO in the text), and the energy...

Figure 2-2. Spatial representation (ball-and-stick model) of benzene, with C-atoms in grey and H-atoms in white. The dotted lines between the C-atoms represent the delocalized electrons. The image on the right shows the surface area of the highest occupied molecular orbital (HOMO). Note how the 71-electrons are above and below the benzene ring.

Commonly used descriptor variables for QSARs involving redox reactions include substituent constants (o), ionization potential, electron affinity, energy of the highest occupied molecular orbital (EHOMO)or lowest unoccupied molecular orbital (ELUMO), one-electron reduction or oxidation potential (E1), and half-wave potential (E1/2)- One descriptor variable (D), fit to a log-linear model, is usually sufficient to describe a redox property of P. Such a QSAR will have the form... 

MLCT metal to ligand charge transfer LMCT ligand to metal charge transfer HOMO highest occupied molecular orbital AOM angular overlap model RHF restricted Hartree-Fock... 

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