Lone pairs, non-bonding

The UV absorption of pyrimidine occurs in two bands centred at 243 and 298 nm in cyclohexane. The second band is ascribed to the electronic transition from a nitrogen lone pair non-bonding orbital to an empty ring tt-orbital, in short an n transition, on... 

C, N, and O atoms hybridize their sp orbitals upon reacting with atoms disregarding the structure phases to form the CH4, NH3, and H20-like tetrahedron structures. The reaction not only causes bond and non-bond reformation but also modifies the valence band and the surface potential barrier with generation of lone-pair non-bonding, electron-hole pair, H-like bonding, and dipole antibonding. 

That way, the Distributed Electrostatic Moments based on the ELF Partition (DE-MEP) allows computing of local moments located at non-atomic centres such as lone pairs, a bonds and n systems. Local dipole contributions have been shown to be useful to rationalize inductive polarization effects and typical hydrogen bond interactions. Moreover, bond quadrupole polarization moments being related to a n character enable to discuss bond multiplicities, and to sort families of molecules according to their bond order. 

A drawing of a two-dimensional, electron-domain model of a conventional Lewis lone pair is shown in Fig. 23. The lone pair and bonding pairs are structurally equivalent they have identical van der Waals envelopes. Such seems to be nearly the case for lone pairs in the valence-shells of small-core, non-octet-expanding atoms (carbon, nitrogen, oxygen and fluorine). 

A single bond contains two electrons, a double bond contains four electrons and a triple bond contains six electrons. A lone (or non-bonding) pair of electrons is represented by two dots. 

T Antibonding (double bonds) n Non-bonding (e.g. lone pair) 7T Bonding (double bonds)... 

The chemist s idealized Lewis structure picture describes the N/2 electron pairs as localized in one-center ( lone pair ) or two-center ( bond ) regions of the molecule. The natural bond orbital (NBO) algorithm leads to an optimal set of one- and two-center orbitals that are in close correspondence with this picture. In effect, the algorithm searches the density matrix for the set of N/2 localized Lewis-type lone pair and bond orbitals of near-double occupancy that best describe the given wave-function, with the residual weakly occupied non-Lewis-type... 

Fig. 2.2 The possible bond configurations and their consequences on the electron clouds of surrounding atoms (shaded areas stand fin dipoles), a and b are the well-known bonding events, c Ionic bond formation alters atomic sizes (broken circles) and valences, d Non-bonding lone-pair formation (represented by induces B. e H-Uke bond fcmns if B" replaces the f O-M bonds involve non-bonding lone pairs and bonding electron pairs, g Antibonding dipoles, h Hydrocarbon-like bonds can form by replacing H with B, which also induces antibonding dipoles (reprinted with permission from [3])...

The events of 0 formation, 0 sp-hybrid bonding, non-bonding lone pair, anti-bonding dipole and the H-like bonding are essential in the electronic process of oxidation, and in the reactions involving other electronegative... 

The representation of non-bonding orbitals on an atom again uses the concept of. T-systems, though they may have any kind of hybridization (p, sp etc.), In Figure 2-56 the three possibilities arc shown lone pairs, radicals, and orbitals without electrons can be accommodated by this eoneept. 

In some force fields the interaction sites are not all situated on the atomic nuclei. For example, in the MM2, MM3 and MM4 programs, the van der Waals centres of hydrogen atoms bonded to carbon are placed not at the nuclei but are approximately 10% along the bond towards the attached atom. The rationale for this is that the electron distribution about small atoms such as oxygen, fluorine and particularly hydrogen is distinctly non-spherical. The single electron from the hydrogen is involved in the bond to the adjacent atom and there are no other electrons that can contribute to the van der Waals interactions. Some force fields also require lone pairs to be defined on particular atoms these have their own van der Waals and electrostatic parameters. 

The O atom uses one of its sp or sp hybrids to form the CO a bond and antibond. When sp hybrids are used in conceptualizing the bonding, the other sp hybrid forms a lone pair orbital directed away from the CO bond axis one of the atomic p orbitals is involved in the CO n and 71 orbitals, while the other forms an in-plane non-bonding orbital. Alternatively, when sp hybrids are used, the two sp hybrids that do not interact with the C-atom sp2 orbital form the two non-bonding orbitals. Hence, the final picture of bonding, non-bonding, and antibonding orbitals does not depend on which hybrids one uses as intermediates. 

For example, the three NH bonding and three NH antibonding orbitals in NH3, when symmetry adapted within the C3V point group, cluster into ai and e mos as shown in the Figure below. The N-atom localized non-bonding lone pair orbital and the N-atom Is core orbital also belong to ai symmetry. 

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