Lowest unoccupied molecular orbital bonding

The Diels-Alder reaction is believed to proceed m a single step A deeper level of understanding of the bonding changes m the transition state can be obtained by examining the nodal properties of the highest occupied molecular orbital (HOMO) of the diene and the lowest unoccupied molecular orbital (LUMO) of the dienophile... 

Chemical Properties. The chemistry of ketenes is dominated by the strongly electrophilic j/)-hybridi2ed carbon atom and alow energy lowest unoccupied molecular orbital (LUMO). Therefore, ketenes are especially prone to nucleophilic attack at Cl and to [2 + 2] cycloadditions. Less frequent reactions are the so-called ketene iasertion, a special case of addition to substances with strongly polarized or polarizable single bonds (37), and the addition of electrophiles at C2. For a review of addition reactions of ketenes see Reference 8. 

Next, examine the lowest-unoccupied molecular orbital (LUMO) for the cation. The components of the LUMO (its lobes ) identify locations where the cation might bond to a water molecule. How many lobes are associated with C 7 For each lobe, draw the alcohol that will be produced (show stereochemistry). How many alcohol enantiomers will form If more than one is expected, decide which wiU form more rapidly based on the relative sizes of the lobes. 

Compare atomic charges and electrostatic potential maps for the three cations. For each, is the charge localized or delocalized Is it associated with an empty a-type or Tt-type orbital Examine the lowest-unoccupied molecular orbital (LUMO) of each cation. Draw all of the resonance contributors needed for a complete description of each cation. Assign the hybridization of the C" atom, and describe how each orbital on this atom is utilized (o bond, n bond, empty). How do you explain the benzene ring effects that you observe ... 

One way to investigate the electrophilic properties of these molecules is to examine the orbital that each uses to accept electrons from a nucleophile. This orbital is the lowest-unoccupied molecular orbital (LUMO). Examine the LUMO for methyl acetate (Z=OCH3), acetaldehyde (Z=H), N,N-dimethylacetamide (Z=N(CH3)2) and acetyl chloride (Z=C1) (acetaldehyde is not a carboxylic acid derivative, but is included here for comparison). What is the shape of the LUMO in the region of the carbonyl group Is it a o or 7U orbital Is it bonding or antibonding What other atoms contribute to the LUMO Which bonds, if any, would be weakened when a nucleophile transfers its electrons into the LUMO ... 

Examine the lowest-unoccupied molecular orbital (LUMO) for the most stable conjugate acid of each compound (inchideprotonated acetonitrile). Which atom makes the largest contribution to this orbital Is this the site of H2O attack Will adding electrons to the LUMO strengthen or weaken die C=0 (C=N) 7U bond Explain. 

Unsaturated organic molecules, such as ethylene, can be chemisorbed on transition metal surfaces in two ways, namely in -coordination or di-o coordination. As shown in Fig. 2.24, the n type of bonding of ethylene involves donation of electron density from the doubly occupied n orbital (which is o-symmetric with respect to the normal to the surface) to the metal ds-hybrid orbitals. Electron density is also backdonated from the px and dM metal orbitals into the lowest unoccupied molecular orbital (LUMO) of the ethylene molecule, which is the empty asymmetric 71 orbital. The corresponding overall interaction is relatively weak, thus the sp2 hybridization of the carbon atoms involved in the ethylene double bond is retained. 

The method involves the irradiation of a sample with polychromatic X-rays (synchrotron radiation) which inter alia promote electrons from the innermost Is level of the sulfur atom to the lowest unoccupied molecular orbitals. In the present case these are the S-S antibonding ct -MOs. The intensity of the absorption lines resulting from these electronic excitations are proportional to the number of such bonds in the molecule. Therefore, the spectra of sulfur compounds show significant differences in the positions and/or the relative intensities of the absorption lines [215, 220, 221]. In principle, solid, liquid and gaseous samples can be measured. 

Chemical bonds are defined by their frontier orbitals. That is, by the highest molecular orbital that is occupied by electrons (HOMO), and the lowest unoccupied molecular orbital (LUMO). These are analogous with the top of the valence band and the bottom of the conduction band in electron band theory. However, since kinks are localized and non-periodic, band theory is not appropriate for this discussion. 

Table 1 Calculation of some molecular-based descriptors for BOA, DIMBOA and MBOA. Physicochemical descriptor like logP (partition coefficient between octanol and water) constitutional descriptors like the number of a specified atoms or bonds (number of carbons, hydrogens, oxygens, nitrogens, single and aromatic bonds, the total number of atoms and bonds) and molecular weight quantum-mechanical descriptors like HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital).

AMI semi-empirical and B3LYP/6-31G(d)/AMl density functional theory (DFT) computational studies were performed with the purpose of determining which variously substituted 1,3,4-oxadiazoles would participate in Diels-Alder reactions as dienes and under what conditions. Also, bond orders for 1,3,4-oxadiazole and its 2,5-diacetyl, 2,5-dimethyl, 2,5-di(trifluoromethyl), and 2,5-di(methoxycarbonyl) derivatives were calculated <1998JMT153>. The AMI method was also used to evaluate the electronic properties of 2,5-bis[5-(4,5,6,7-tetrahydrobenzo[A thien-2-yl)thien-2-yl]-l,3,4-oxadiazole 8. The experimentally determined redox potentials were compared with the calculated highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) energies. The performance of the available parameters from AMI was verified with other semi-empirical calculations (PM3, MNDO) as well as by ab initio methods <1998CEJ2211>. 

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