Bonds and structure

Planar projections of molecular graphs of hydrocarbon molecules generated from theoretical charge distributions. Bond critical points are denoted by black dots. Structures 1 to 4 are normal hydrocarbons from methane to butane, 5 is isobutane, 6 is pentane, 7 is neopentane, and 8 is hexane. The remaining structures are identified in Table 3.2. The structures depicted in these diagrams are determined entirely by information contained in the electronic charge density. 

In addition to the fundamental difference in the form of the bridgehead charge density between the two sets of molecules, there are also substantial quantitative differences in the values of p. Thus, its value at the bond critical point in [l.l.l]propellane is 0.203 au, four-fifths of the value for a normal C-C bond, while the value of p at the corresponding cage critical point in the bicyclic molecule is 0.098 au. It is also clear from Fig. 3.8(d) that the values of p at the bond critical point and the three adjacent ring critical points are almost equal, giving rise to a very broad bonded maximum in p in the bridgehead interatomic surface in [l.l.ljpropellane. 

The value of the charge density at a bond critical point can be used to define a bond order (Bader et al. 1983 Cremer and Kraka 1984). The molecular graphs for ethane, ethylene, and acetylene are shown in Fig. 2.8. In each case the unique pair of trajectories associated with a single (3, — 1) critical point is found to link the carbon nuclei to one another. Multiple bonds do not appear as such in the topology of the charge density. Instead, one finds that the extent of charge accumulation between the nuclei increases with the assumed number of electron pair bonds and this increase is faithfully monitored by the value of p at the bond critical point, a value labelled p, . For carbon-carbon bonds, one can define a bond order n in terms of the values of Ph using a relationship of the form 

The bond paths are noticeably curved in those structures in Fig. 3.7 where the geometry of the molecule precludes the possibility of tetrahedrally directed bonds between the carbon nuclei. The differences between the bond path angle a., the limiting value of the angle subtended at a nucleus by two bond paths, and the geometrical or so-called bond angle is important in 

Change in p at bond critical point for vertical electron ionization and addition  

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Lewis s interest in chemical bonding and structure dated from 1902. In attempting to explain "valence" to a class at Harvard, he devised an atomic model to rationalize the octet rule. His model was deficient in many respects for one thing, Lewis visualized cubic atoms with electrons located at the corners. Perhaps this explains why his ideas of atomic structure were not published until 1916. In that year, Lewis conceived of the... 

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