Biphenyl molecular orbitals

Tuppurainen K, Ruuskanen J. 2000. Electronic eigenvalue (LEVA) a new QSAR/QSPR descriptor for electronic substituent effects based on molecular orbital energies. A QSAR approach to the Ah receptor binding affinity of polychlorinated biphenyls (PCBs), dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs). Chemosphere 41 843 48... 

Desulfurization of polynuclear thiophenes was performed by continuous stirring with a Raney nickel catalyst in ethanol at 0-78.8 °C. The major products were biphenyl in the desulfurization of dibenzothiophene, a-phenylnaphthalene in the desulfurization of benzo[b]naphtho[l,2-d]thiophene, and -phenylnaphthalene in the desulfurization of benzo[b]naphtho[2,3-d]thiophene and benzo[b]naphtho[2,l-d]thiophene. Observation of these products shows that the main reaction pathway is the extrusion of a sulfur atom to give the corresponding hydrocarbon. The tt-electron densities of the sulfur atoms, which were calculated by using simple Huckel molecular orbital theory, are considered to be related to the adsorption of the sulfur compounds to the surface and consequently to the C-S bond-breaking rate. 

If the two rings happen to be perpendicular in biphenyl, the pi molecular orbitals are less delocalized. In naphthalene the pi molecular orbital is always delocalized over the entire molecule. What do you think is the most stable structure for biphenyl both rings in the same plane or both rings perpendicular ... 

Hole-transporting materials (HTM) have relatively low ionization potentials (1P). 2 The IP is defined as the energy required to remove an electron from the highest occupied molecular orbital (HOMO) of a substance. It can be measured, for instance, by photoelectron spectroscopy or obtained from electrochemical oxidation potentials in solution. It is also preferable that the HTM have sufficiently high hole drift mobilities. Various classes of materials have been used in the HTL, for example, starburst amorphous molecules, spirocompounds, triarylamines, and tetraarylbenzidines are representative classes of well-known HTMs. The structure of the most commonly used HTM, 4,4 -bis[N-(l-naphthyl)-N-phenylamino]biphenyl (NPB), is shown in Figure 14.2. 

In molecules, the conjugation of unsaturated systems, i.e., possessing molecular orbitals built up from Ti-atomic orbitals (2p-orbitals in hydrocarbons benzene), is, in general, a planarity factor (e.g., butadiene). Nevertheless, steric hindering can oppose this tendency to make a planar system (e.g., biphenyl, cyclooctatetraen ). The same phenomena are encountered in crystals. 

We now turn to a discussion of the low-energy spectrum of biphenyl, again starting from the noninteracting limit. It is convenient to regard biphenyl as two benzene molecules (stripped of one hydrogen atom each) bonded together, as illustrated in Fig. 11.5. Since biphenyl possesses D h symmetry it is convenient to use the D2h symmetry-adapted molecular orbitals of benzene (shown in Fig. 11.4) to construct its molecular orbitals. 

In the molecular orbital description the lowest-lying excitations of biphenyl are... 

Fig. 11.6. Molecular orbitals of biphenyl represented by their predominant component left and right molecular orbitals of benzene (denoted by X L) and X E) and illustrated in Fig. 11.4). Also shown are the molecular orbital energies (using ts =t), their electronic occupation in the ground state and the low-lying electronic transitions, labelled 1,2, and 3.

Stable mesophases are produced by substituted p-terphenyls and trans-stilbenes, whereas mesophases of lower thermal stabihty are formed by Schilf bases, esters and biphenyls. Stereochemical considerations are important and affect the position in the order, e.g. stibenes are planar, azoxybenzenes are shghtly twisted and Schiff bases are considerably twisted. The relatively low position of the tolanes (Z= =C ) in the order is surprising since crystalline diphenyl-acetylene contains planar molecules and possibly in the mesophase the rings become non-coplanar. The consequences, however, could be less serious than those in Schilf bases, since the cylinder of electron density associated with the molecular orbital of the =C linkage may still allow conjugation to occur even when the rings are non-coplanar. 

Figure 4-9. Schematic energy level diagrams, (a) The radical anion of biphenyl (PP2 ) (b) the dianion of biphenyl (PP2 ). , electron arrow, electronic transition. The molecular orbital levels are taken from [77],...

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