Molecular orbitals stabilization

In both molecules and crystals, the delocalization of the molecular orbitals stabilizes the system. Nevertheless, in metals, the values of the cohesive energies per atom cover a wide range from 15 kcal in mercury to 200 kcal in tungsten. Consequently, the melting temperatures are also very different from -39°C for mercury to > 5000°C for osmiun. The broad range of the values of the cohesive energy is due to the variety of orbitals that intervene in the lattice. Consequently, in each case, the direct calculation of the energy is necessary. 

Each of the three s s overlapping systems produces a bonding <5 = Ir) and an antibonding (o = sp — Ir) molecular orbital. Stabilization is maximized if we form two-electron bonds, as the stabilized bonding molecular orbital will be filled and the antibonding, high-energy molecular orbital is empty (Fig. 3.6). 

Notice the power of the molecular orbital approach. Every electron that enters a bonding molecular orbital stabilizes the molecule or polyatomic ion, and every electron that enters an antibonding molecular orbital destabilizes it. The emphasis on electron pairs has been removed. One electron in a bonding molecular orbital stabilizes half as much as two, so a bond order of one-half is nothing mysterious. 

Ultraviolet photoelectron spectroscopy (UPS) results have provided detailed infomiation about CO adsorption on many surfaces. Figure A3.10.24 shows UPS results for CO adsorption on Pd(l 10) [58] that are representative of molecular CO adsorption on platinum surfaces. The difference result in (c) between the clean surface and the CO-covered surface shows a strong negative feature just below the Femii level ( p), and two positive features at 8 and 11 eV below E. The negative feature is due to suppression of emission from the metal d states as a result of an anti-resonance phenomenon. The positive features can be attributed to the 4a molecular orbital of CO and the overlap of tire 5a and 1 k molecular orbitals. The observation of features due to CO molecular orbitals clearly indicates that CO molecularly adsorbs. The overlap of the 5a and 1 ti levels is caused by a stabilization of the 5 a molecular orbital as a consequence of fomiing the surface-CO chemisorption bond. 

HMO theory is named after its developer, Erich Huckel (1896-1980), who published his theory in 1930 [9] partly in order to explain the unusual stability of benzene and other aromatic compounds. Given that digital computers had not yet been invented and that all Hiickel s calculations had to be done by hand, HMO theory necessarily includes many approximations. The first is that only the jr-molecular orbitals of the molecule are considered. This implies that the entire molecular structure is planar (because then a plane of symmetry separates the r-orbitals, which are antisymmetric with respect to this plane, from all others). It also means that only one atomic orbital must be considered for each atom in the r-system (the p-orbital that is antisymmetric with respect to the plane of the molecule) and none at all for atoms (such as hydrogen) that are not involved in the r-system. Huckel then used the technique known as linear combination of atomic orbitals (LCAO) to build these atomic orbitals up into molecular orbitals. This is illustrated in Figure 7-18 for ethylene. 

The thermal stability can be correlated with the energy of the highest occupied molecular orbital of the molecule (HMO approximation) (300). 

Cyclic conjugation although necessary for aromaticity is not sufficient for it Some other factor or factors must contribute to the special stability of benzene and compounds based on the benzene ring To understand these factors let s return to the molecular orbital description of benzene... 

Ab initio molecular orbital calculations for the model systems RCN3S2 (R = H, NH2) show that these dithiatriazines are predicted to be ground state singlets with low-lying triplet excited states (Section 4.4). The singlet state is stabilized by a Jahn-Teller distortion from C2v to Cj symmetry. In this context the observed dimerization of these antiaromatic (eight r-electron) systems is readily understood. 

Intramolecular heteroatom coordination may also influence the stabilities or structures of catenated tellurium compounds. For example, a rare example of a tritelluride, bis[2-(2-pyridyl)phenyl]tritelluride, is stabilized by a Te N contact of 2.55 The ditelluride (2-MeOCelFtCOTe) has an unusual planar structure. Although the C=0 Te interaction is longer (3.11 A) than the Me 0 contact (2.76 A), ab initio molecular orbital calculations indicate that the planarity results predominantly from the former intramolecular connection. 

Methylcyclohexanone, pK 20, is typical of a weak acid that undergo H/D exchange. Identify the acidic protons of 2-methylcyclohexanone, i.e., those most susceptible to attack by base, as positions for which the value of the lowest-unoccupied molecular orbital (LUMO) is large. Use a LUMO map (the value of the LUMO mapped onto the electron density surface). Does this analysis correctly anticipate which of the anions obtained by deprotonation of 2-methylcyclohexanone is actually most stable Are any of the other ions of comparable stability, or are they aU much less stable ... 

Two independent molecular orbital calculations (HMO method) of delocalization energies for isoindole and isoindolenine tautomers agree that the isoindole form should possess the more resonance stabilization. The actual difference calculated for isoindole-isoindolenine is about 8 kcal/mole, but increases in favor of the isoindole with phenyl substitution at position 1 (Table VI).Since isoindole and isoindolenine tautomers have roughly comparable thermodynamic stabilities, the tautomeric proce.ss is readily obser-... 

Molecular orbital theory indicates that there is little difference between the stability of the two tautomers of purine, 42 and 43. Molecular orbital calculations indicate that purine forms a monocation by protonation at N-3 or at A precise X-ray crystal-... 

The stability order of alkenes is due to a combination of two factors. One is a stabilizing interaction between the C=C tr bond and adjacent C-H a bonds on substituents. In valence-bond language, the interaction is called hyperconjugation. In a molecular orbital description, there is a bonding MO that extends over the four-atom C=C—< -H grouping, as shown in Figure 6.6. The more substituents that are present on the double bond, the more hyperconjugation there is and the more stable the alkene. 

Figure 6.12 Stabilization of the ethyl carbocation, CH3CH2+, through hyperconjugation. Interaction of neighboring C H

In molecular orbital terms, the stability of the allyl radical is due to the fact that the unpaired electron is delocalized, or spread out, over an extended 7T orbital network rather than localized at only one site, as shown by the computer-generated MO in Fig 10.3. This delocalization is particularly apparent in the so-called spin density surface in Figure 10.4, which shows the calculated location, of the unpaired electron. The two terminal carbons share the unpaired electron equally. 

Stability of Conjugated Dienes Molecular Orbital Theory 4133... 

Structure and Stability of Benzene Molecular Orbital Theory 521... 

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