Molecular geometry lone pairs effect

This molecule (type AB3U) has a tetrahedral electronic geometry and a pyramidal molecular geometry. Cl (EN = 3.0) is more electronegative than As (EN = 2.1). The polar As-Cl bond dipoles oppose the effect of the lone pair. The molecule is only slightly polar (Section 8-8). 

This molecule (type AB2U2) has a tetrahedral electronic geometry and an angular molecular geometry. Oxygen (EN = 3.5) is less electronegative than F (EN = 4.0). The O-F bond dipole opposes the effect of the two lone pairs of electrons and so, OF2 is polar (Section 8-9). 

The most recent theoretical study, by Alhrichs and co-workers, deals with the di(phosphino)carbene Id and (phosphino) (phosphonio)carbenes Ie,f.16 The optimized geometry of the di(phosphino)carbene Id is weakly bent (PCP angle 160.5°) and highly unsymmetrical Only one of the phosphorus centers (P1) is in a planar environment, and it is much more closely bonded to the carbenic center than the other one (P1C 1.533 and P2C 1.765 A). The atomic charges (P1 +1.0, C -0.8, P2 +0.6) indicate that the short P bond is a double bond reinforced by Coulombic attraction, while the nature of the molecular orbitals revealed a slight delocalization of the carbene lone pair into the low-lying a (P-N) orbitals of the two phosphino substituents. The distortion from the symmetrical structure can be viewed as a second-order Jahn-Teller effect. 

There are a few attributes of molecules that must be included in a discussion of molecular geometry. These include the three-dimensional arrangement of the atoms, the bond angles, the bond lengths, and the locations and effects of lone pairs of electrons. These issues are explained in the VSEPR model. 

Preferred geometry of the benzene oxide-oxepin system can be predicted by molecular orbital methods. Thus benzene oxide la is predicted to be markedly non-planar (with the epoxide ring at an angle of 73° to the benzene ring), while the oxepin lb has been predicted to prefer a shallow boat structure (MINDO/3) or a planar structure ab initio) As previously mentioned, the proportion of each tautomer present at equilibrium is both temperature and solvent-dependent. Molecular orbital calculations have been used to rationalize the solvent effects, both in terms of the more polar character of the arene oxide that is favored in polar solvents and the strengthening of the oxirane C-C bond upon coordination of the oxygen atom lone pair in polar solvents. Thus values in the range 1.5-2.0 D and 0.76-1.36 D for the dipole moments of arene oxide la and oxepin lb have been calculated. 

Note the pair of nonbonding electrons on the nitrogen atom. These are often called a lone pair, or unshared pair, of electrons. As we will see later in this section, lone pair electrons have a profound effect on molecular geometry. The geometry, in turn, affects the reactivity of the molecule. 

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