Molecular orbitals hetero-atomics

When there is a total of four electrons, Hund s rule predicts that two will be in the lowest orbital but the other two will be unpaired, so that the system will exist as a diradical rather than as two pairs. The degeneracy can be removed if the molecule is distorted from maximum molecular symmetry to a structure of lesser symmetry. For example, if 44 assumes a rectangular rather than a square shape, one of the previously degenerate orbitals has a lower energy than the other and will be occupied by two electrons. In this case, of course, the double bonds are essentially separate and the molecule is still not aromatic. Distortions of symmetry can also occur when one or more carbons are replaced by hetero atoms or in other ways.124... 

The hetero atom parameters have been obtained in various ways, for example by striving for a best correlation of HOMO values with ionization energies, or of polarographic reduction potentials with LUMO values. The whole subject of SHM parameters and best heteroatom parameters is now of little practical importance, since much better quantitative molecular orbital methods are now readily available. 

Fig. 6. Two schematic representations of the highest occupied (HOMO) and the lowest unoccupied (LFMO) molecular orbital for compounds of the y-pyrone type. The positive parts of the MO s are indicated by a full line, the negative parts by a dotted line. Hetero atoms are denoted by heavy dots.

Grammaticakis has found that the sulfur atom in the ring has a considerable bathochromic effect compared to oxygen. In a series of simple 1,2-diazoles, the bathochromic effect of the third hetero atom was found to increase in the order nitrogen < oxygen < sulfur, in agreement with the results of molecular orbital calculations. 

The photochemical behavior of StPs is determined by number, position, and substitution of the hetero atoms [42, 78, 109, 128, 130, 183, 281, 302]. The Of and 0, c values of the 3-aza compounds do not differ remarkably from those of the parent hydrocarbons. Presumably, low nonfluorescent n, n states are operative in the 2- and 4-isomers but are not available for the 3-isomer. On the basis of molecular orbital (MO) calculations trans-azastilbenes are assumed to possess low-lying nonfluorescent i(n,n ) states [129, 133, 137] which should be easily accessible by internal conversion from the state with the 2- and 4-aza compounds but not with the... 

Electronic Digital Computers to Molecular Orbital Problems. II. A New Approximation for Hetero-atom Systems. 

In the case of pyrrole, furan and thiophen, the six jr-electrons derive four from the carbon atoms and two (the lone pair ) from the hetero atom. For this reason Albert has termed these heterocyclic compounds Ti-exccssive, in contrast to pyridine, and related compounds, which are referred to as 7r-dejS.cient, because the hetero atom tends to withdraw jr-electrons from, the molecular orbitals and thus reduce the resonance energy. 

FIGURE 1.5 Illustration of hetero-atomic combination in chemical bonding (H-0 bonding here) at the level of s-s- and s-p atomic orbitals overlapping in resulted bonding and anti-bonding molecular orbitals in bottom and upper parts of the central horizontal axis, respectively. 

---