Orbitals perpendicular

Since the boron atom has a ls22s22p configuration, we describe the B2 > molecule by considering how p atomic orbitals combine to form MOs. Recall that p orbitals have two lobes and that they occur in sets of three mutually j perpendicular orbitals [Fig. 14.35(a)]. When two B atoms approach each other, two pairs of p orbitals can overlap in a parallel manner [Figs. 14.35(b) and (c)] and one pair can overlap head-on [Fig. 14.35(d)],... 

A triple bond is composed of one sigma bond and two pi bonds. Since the two pi bonds are perpendicular to each other, they are treated as two separate, noninteracting pi bonds. Perpendicular orbitals do not interact. The triple bond is linear, and atoms bonded to it lie in a straight line (Fig. 1.13). Organic chemists tend to draw skinny p orbitals (see Fig. 1.8) the triple bond is overall cylindrically symmetrical. 

The orbital alignment requirements have some slack an error of 10° off the proper angle appears to have little effect. Pi-bond-forming reactions that involve deformations from coplanarity of up to 30° can occur in rare cases. Pi overlap falls off with the cosine of the twist angle. The extreme, a 90° twist, would have no pi bonding at all, a loss of about 63 kcal/mol (264 kJ/mol) bonding energy stabilization because perpendicular orbitals do not interact. 

How much angular slop there is in a transition state is currently a debated topic. There is agreement that perpendicular orbitals do not interact. However, most calculations of the actual shape of orbitals show them to be much fatter than they tend to be drawn in beginning textbooks. It now appears that a 10 deviation from the optimum has a negligible effect. 

The total affinity of the ligands for an electron pair in such an orbital is taken as 2 x 2 (for 2 CO groups) plus 2x1 (for two phosphines), so each phosphorus has a 5 i-bond order . Because there are two mutually perpendicular orbitals of this type, the 7t-bond order of each P—bond is taken to be 7 (70/210). The 71-bond order in the cis isomer is calculated to be somewhat smaller (65/210), and the results for these and other complexes are given in Table VIII. 

Disubstituted allenes are chiral, and when resolved, can be racemized thermally in a first-order reaction. Roth determined log 13.61 — 46 170/2.3Rr for the racemization of 2,3-pentadiene at 1-2 in a pressure-independent reaction. The pathway most likely involves a 90° rotation around one rr-bond to give an allylic biradical where the electrons are in mutually perpendicular orbitals (Scheme 4.1). 

Consider the set of perpendicular -orbitals in the poly aromatic planar molecule coronene shown below. This set gives rise to a symmetry of molecular orbitals of a u and b g. Can you draw both these orbitals (Use the standard orientation of the central benzene frame, as shown in Fig. 3.10)... 

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