Unshared pair dipole moment

Both water and ammonia have four groups attached to their central atom and therefore both possess a tetrahedral electronic (or VSF PR) geometry. However, H20 has two unshared electron pairs while NH3 only has one, producing a larger dipole moment for H,0. 

Unshared pairs (lone pairs) of electrons make large contributions to the dipole moment. (The O-H and N-H moments are also appreciable.)... 

The dipole moments of C1F3 and BrF3 provide another interesting illustration of the effects of unshared pairs of electrons. The molecules can be shown as follows ... 

Contribution of Unshared Electron Pairs to the Electric Dipole Moments of Molecules.—In the preceding chapter we discussed the dipole moments of molecules, in relation to the partial ionic character of bonds, without considering the possible contribution of unshared electron pairs. A simple treatment based on hybrid orbitals provides some justification of this procedure. 

In the foregoing treatment of the water molecule, which we shall use as an example, each of the two bond orbitals of the oxygen atom has been calculated to have 6 percent s character and 94 percent p character. Each of the two unshared-pair orbitals then has 44 percent s character and 56 percent p character. The maxima for the unshared-pair orbitals lie in directions making an angle of 142° with one another and such that their resultant is opposed to that for the two bond orbitals, which have their maxima at 93.5° with one another. The component for the four unshared-pair electrons is determined by the direction cosine —0.34, and that of the two bonding electrons of the oxygen atom by the direction cosine 0.68 hence the contribution of the four unshared-pair electrons to the dipole moment is just balanced by that of the two bonding electrons.18... 

Sulfur hexafluoride sublimes at -64 °C to produce a dense gas (6.14 g L-1). Under a pressure of 2 atm, the melting point is -51 °C. The molecule has the expected octahedral structure and a dipole moment of zero. The compound is so inert that it is used as a gaseous insulator, and rats allowed to breathe a mixture of SF6 and oxygen show no ill effects after several hours of exposure. This inertness is a result of the molecule having no vacant bonding site or unshared electron pairs on sulfur to initiate a reaction and the fact that six fluorine atoms shield the sulfur atom from attack. Consequently, there is no low-energy pathway for reactions to occur, and the compound is inert even though many reactions are thermodynamically favored. 

How are we to account for this We have forgotten the unshared pair of electrons. In NF3 (as in NH3) this pair occupies an sp orbital and must contribute a dipole moment in the direction opposite to that of the net moment of the N—F bonds (Fig. 1.15) these opposing moments are evidently of about the same size. 

Figure 1.15. Dipole moments of some molecules. Contribution from unshared pairs. In NF3, moment due to unshared pair opposes vector sum of bond moments.

The dipole moments of most compounds have never been measured. For these substances we must predict polarity from structure. From our knowledge of electronegativity, we can estimate the polarity of bonds from our knowledge of bond angles, we can then estimate the polarity of molecules, taking into account any unshared pairs of electrons. 

Unfortunately, the dipole moments associated with individual bonds can be measured only in simple diatomic molecules. Entire molecules rather than selected pairs of atoms must be subjected to measurement. Measured values of dipole moments reflect the overall polarities of molecules. For polyatomic molecules they are the result of all the bond dipoles in the molecules. In Chapter 8, we will see that structural features, such as molecular geometry and the presence of lone (unshared) pairs of electrons, also affect the polarity of a molecule. 

The electronegativity difference is large (1.4 units), and so the bonds are quite polar. Additionally, the bond dipoles reinforce the effect of the two unshared pairs, so the H2O molecule is very polar. Its dipole moment is 1.8 D. Water has unusual properties, which can be explained in large part by its high polarity. 

Unshared pairs of electrons make large contributions to the dipole moments of water and ammonia. Because an unshared pair has no other atom attached to it to partially neutralize its negative charge, an unshared electron pair contributes a large moment directed away from the central atom (Fig. 2.5). (The O — H and N—moments are also appreciable.)... 

The adsorption capacities of large pore zeolites for benzene and pyridine vapors are practically equal. The adsorption rate of pyridine is substantially lower than the adsorption rate of benzene due to its electronic structure. A comparison of the adsorption heats of pyridine and benzene makes it possible to conclude that the unshared pair of electrons and the large dipole moment of pyridine both make an important contribution to the energy of reaction of pyridine with zeolites [26]. Most linear and branched amines have also been used to titrate the acid sites in zeolites. 

The effect of an unshared pair on the dipole moment depends on the orbital in which it resides. An unshared pair in an s orbital, as in HjS, has no effect on the molecular moment since the s orbital is symmetrically disposed around the nucleus. An unshared pair in a p orbital has no effect since the two lobes are of equal size and in opposite direction. An unshared pair in an s-p type hybrid atomic orbital, as in - NH3, contributes to the molecular moment, since the two lobes are of unequal... 

Dipole moment. Account for the fact that the dipole moment of NCI3 is greater than that of NF3. Hint Consider the effect of the unshared pair.)... 

Dipole moment and unshared pairs. After comparing experimental and calculated dipole moments, Charles A. Coulson suggested that in HCl the chlorine atom is sp hybridized, (a) Give the orbital electronic structure for an sp hybridized Cl atom, (b) Which HCl molecule would have a larger dipole moment—the one in which the chlorine uses pure p, or uses sp hybrid orbitals for bonding with the H atom ... 

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