Orbitals in covalent bonding

Examine the eleetrostatie potential map for ketene. Which (non-hydrogen) atom is most eleetron poor, and which regions around this atom are most electron poor After oxygen, which atom is most electron rich, and which regions are most electron rich Account for these data with a diagram that shows the orbitals on each atom, their orientation and electron occupancy, and whether or not they participate in covalent bonds (assume that oxygen is sp hybridized). 

We saw in the last chapter how covalent bonds between atoms are described, and we looked at the valence bond model, which uses hybrid orbitals to account for the observed shapes of organic molecules. Before going on to a systematic study of organic chemistry, however, we still need to review a few fundamental topics. In particular, we need to look more closely at how electrons are distributed in covalent bonds and at some of the consequences that arise when the electrons in a bond are not shared equally between atoms. 

Table 5.8. The NBO descriptors ofH-bonded water complexes E O- -HA, showing the net intermodular charge transfer from Lewis base to Lewis acid (Qcr), change in covalent-bond polarity ( Az ah). and (P)NBO overlaps (Sno, SniJ) of n0 with antibond ctah and ctah orbitals...

Table 5.8 summarizes the NBO descriptors of the net charge transfer from Lewis base to Lewis acid (Qcf), change in covalent-bond polarization (A/Ah), and (P)NBO overlap of n0 with bond (Sna) and antibond (Sna>) orbitals of the Lewis acid. The entries in Table 5.8 show the unfavorable diminution of l/w /5nal and reduced charge transfer as the Lewis acid changes from polar HF to apo-lar CH4. These NBO descriptors can also be closely correlated with quantities in Table 5.7, showing their mutual dependence on the strength of n-a donor-acceptor interaction. 

The greater the difference in electronegativity between two atoms, the more polar the bond is that forms between them. Imagine the electrons in the bond as being spread out into a cloud within the molecular orbital. In polar bonds, the cloud is denser in the vicinity of the more electronegative atom. In nonpolar bonds, like those formed between atoms of the Scime element, the cloud is evenly distributed between both atoms. Polar bonds have more ionic chciracter, whereas nonpolar bonds have more covalent character. Here s how to distinguish the chciracter of a bond ... 

Oncogenic viruses 248 One-start helix 334 Oparin, I. V. 9 Open systems 289 Operator sequence of DNA iH NMR spectrum 269 Operons 240 Opsin(s) 553 Optical rotation 42 Optimum rate for enzymes 469 d Orbitals, participation in covalent bond formation 311 Orcinol 251 Ordered binding 464 Ordered sequential mechanism 475 Organelle(s) 11... 

Two electrons with opposite spins in the same orbital are described as paired. When extended to molecules, the exclusion principle allows us to understand the pairing of electrons in covalent bonds. The net spin angular momentum of a pair of electrons is zero. If not all electrons are paired in a molecule or solid, magnetic properties arise, as happens with many compounds of transition metals. 

These restrictions embody what is known as the aufbau approach to the description of atomic orbitals. Thus hydrogen is represented in its ground state by (Is), helium by (Is)2, lithium by (ls)2(2s) etc. The similarity between the atomic orbital and covalent bonding lies in the two electrons and only two electrons per orbital or per bonding. 

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