Pi orbital

Sigma and pi orbitals, like s and p, differ in shape each orbital can hold 2e. ... 

Valence bond theory (Chapter 7) explains the fact that the three N—O bonds are identical by invoking the idea of resonance, with three contributing structures. MO theory, on the other hand, considers that the skeleton of the nitrate ion is established by the three sigma bonds while the electron pair in the pi orbital is delocalized, shared by all of the atoms in the molecule. According to MO theory, a similar interpretation applies with all of the resonance hybrids described in Chapter 7, including SO S03, and C032-. 

Another species in which delocalized pi orbitals play an important role is benzene, QHg. There are 30 valence electrons in the molecule, 24 of which are required to form the sigma H H bond framework ... 

In this calculation the values qc = 0, qn = 2d and q0 = 4/3 were used for the Coulomb energies of the pi orbitals of carbon, nitrogen and oxygen, respectively. 

The data sets (above) with the large values of X are physical property measurements which presumably involve predominantly the pi electrons. For example, the F - and C -nmr shifts are dominated by paramagnetic contributions resulting from unbalanced pi orbitals (24). Ionization equilibria, on the other hand, are characterized by a much higher blend of polar effects (a blend which traditionally has been defined as X = 1.00 for the ionization of benzoic acid, H2O, 25 ). 

Fig. 13. Change in the metal Pi orbital structure in square- planar substitution.

Exercise Find the Hiickel pi-orbital energies e, for 1,3-butadiene. 

The authors are particularly grateful to Kenneth Sundberg for plotting the pi-orbital contours. 

We shall first consider the effect of atomic replacement on sigma or pi orbital... 

We shall next consider the effect of substitution on sigma and pi orbital energies. 

Fig. 6. Pi orbital interactions in the A + (B + Q union to form cis and tram 1, 3, 5 hexatriene. See Fig. 5 for the construction of the group MO s spanning B and C. The symmetry labels are assigned with respect to a mirror plane (cis isomer) or a rotational axis (tram isomer)...

Fig. 8. Pi orbital interactions obtaining in the union of two allyl radical fragments to form 1,3,5-hexatriene...

Fig. 21. Stabilizing pi orbital interactions in propene The key stabilizing orbital interactions are sketched below...

Fig. 25. Stabilizing pi orbital interactions in cis and trims l,2-difluoro-l,2-dicyanoethylene. Symmetry labels are with respect to a rotational axis (trans isomer) and mirror plane (cis isomer)...

Fig. 30. Dominant stabilizing pi orbital interactions in the Cs and Ts conformers of methyl vinyl ether...

Fig. 31. Dominant stabilizing pi orbital interaction in the Cee and Css conformers of 2-propyl cation. The symmetry labels are assigned with respect to a mirror plane...

Fig. 2 (top) Planar regioregular polymer with directly overlapping pi orbitals vs. twisted backbone with 7i orbitals out-of-plane [1]... 

CO is an excellent probe molecule for probing the electronic environment of metals atoms either supported or exchanged in zeolites. Hadjiivanov and Vayssilov have published an extensive review of the characteristics and use of CO as a probe molecule for infrared spectroscopy [80]. The oxidation and coordination state of the metal atoms can be determined by the spectral features, stability and other characteristics of the metal-carbonyls that are formed. Depending on the electronic environment of the metal atoms, the vibrational frequency of the C-O bond can shift. When a CO molecule reacts with a metal atom, the metal can back-donate electron density into the anti-bonding pi-orbital. This weakens the C-O bond which results in a shift to lower vibrational frequencies (bathochromic) compared to the unperturbed gas phase CO value (2143 cm ) [62]. These carbonyls form and are stable at room temperature and low CO partial pressures, so low temperature capabilities are not necessary to make these measurements. 

In an electronic transition, an electron from one molecular orbital moves to another orbital, with a concomitant increase or decrease in the energy of the molecule. The lowest-energy electronic transition of formaldehyde promotes a nonbonding (ri) electron to the antibonding pi orbital (it ).1 1 There are two possible transitions, depending on the spin quantum numbers in the excited state (Figure 18-11). The state in which the spins are opposed is called a singlet state. If the spins are parallel, we have a triplet state. 

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