Lone pair dipole moment

Since phenol has an appreciable dipole moment, and no low energy acceptor orbitals, it should interact best with the donors that have the largest lone pair dipole moment — the oxygen compounds. Iodine has no dipole moment and the interaction with iodine is expected to be essentially covalent. Iodine should interact best with the donors that have the lowest ionization potential, i.e., the ones whose charge clouds are most easily polarized. Similar considerations have been employed to explain the donor strengths of primary, secondary and tertiary amines 35a) and the acid strengths of (35b) ICl, Bt2, I2. CeHsOH and SO2. 

It should be mentioned that one would not expect the Eb, Cb, Ea and Ca parameters to be related to the ground state properties of the donor and acceptor. For example, although BF3 is a planar molecule with no dipole moment, there will be considerable contributions to the bonding in a BF3 adduct from dipole-dipole interactions. In the adduct, the geometry of the BF3 part is such that this fragment is very polar. Consequently, its Ea parameter will be appreciable. One must also consider that the lone pair dipole moment of the donor is modified upon adduct formation by the amount of charge transfer that takes place and somehow the parameters and Eq. (13) take this into account. 

As a starting point, Drago and Wayland chose an indirect method to simplify the calculation of equation (12.42). Iodine was chosen as a reference acid and assigned the parameters Ea = 1 (kcal/mol) and Ca = 1 (kcal/mol). The Eb term was then related to the lone-pair dipole moment and the Cb term to the lone-pair polarizability. This allowed a quantification of the add-base interactions for a number of small molecules. 

Phosphine is a colourless gas at room temperature, boiling point 183K. with an unpleasant odour it is extremely poisonous. Like ammonia, phosphine has an essentially tetrahedral structure with one position occupied by a lone pair of electrons. Phosphorus, however, is a larger atom than nitrogen and the lone pair of electrons on the phosphorus are much less concentrated in space. Thus phosphine has a very much smaller dipole moment than ammonia. Hence phosphine is not associated (like ammonia) in the liquid state (see data in Table 9.2) and it is only sparingly soluble in water. 

Because of the presence of the lone pairs of electrons, the molecule has a dipole moment (and the liquid a high permittivity or dielectric constant). 

Moleeular properties sueh as dipole moment and polarizability, although in eertain fully empirieal models, bond dipoles and lone-pair eontributions have been ineorporated (although again only for eonventional ehemieal bonding situations). 

In H2O and NH3, shown in Figures 4.18(a) and 4.18(b), the direction of the dipole moment is along the C2 or C3 axis, respectively. In both molecules there are lone pairs of electrons directed away from the 0-FI or N-FI bonds so that the negative end of the dipole is as shown in each case. 

The method has also been applied to partially saturated systems. For instance, the dipole moments of a series of 1-acylpyrazolines (42) with R = H, Me, Et and Ph have been measured (72CHE445) they range from 3.46 to 4.81 D. When compared with values computed by the fragmentary calculation method, the conclusion was reached that here also the E form predominates. In all these examples the lone pair-lone pair repulsions determine the most stable conformation. 

NF3 is a colourless, odourless, thermodynamically stable gas (mp —206.8°, bp —129.0°, AG29g — 83.3kJmol ). The molecule is pyramidal with an F-N-F angle of 102.5°, but the dipole moment (0.234 D) is only one-sixth of that of NH3 (1.47 D) presumably because the N-F bond moments act in the opposite direction to that of the lone-pair moment ... 

The exact expression for the dipole moment does n( consider atoms as point charges, but rather as nuclei (eat with a positive charge equal to the atomic number) ar electrons (each with unit negative charge). Atoms wii lone pairs may contribute to the dipole moment, even the atom is neutral, as long as the lone pair electrons a not symmetrically placed around the nucleus. 

In contrast with water, methanol, ammonia, and other substances in Table 2.1, carbon dioxide, methane, ethane, and benzene have zero dipole moments. Because of the symmetrical structures of these molecules, the individual bond polarities and lone-pair contributions exactly cancel. 

Methylamine contains an electronegative nitrogen atom with two lone-pair electrons. The dipole moment thus points generally from -CH3 toward NH2. 

If the four atoms attached to the central atom in a tetrahedral molecule are the same, as in tetrachloromethane (carbon tetrachloride), CCI4 (30), the dipole moments cancel and the molecule is nonpolar. However, if one or more of the atoms are replaced by different atoms, as in trichloromethane (chloroform), Cl ICI, or by lone pairs, as in NH3, then the dipole moments associated with the bonds are not all the same, so they do not cancel. Thus, the CHCI, molecule is polar (31). 

Each molecule has four fluorine atoms at the comers of a square. The Xe—E bond polarities cancel in pairs, leaving XeFq with no dipole moment. Four bond polarities also cancel in CIF5, but the fifth Cl—F bond has no counterpart in the opposing direction, so CIF5 has a dipole moment that points along the axis containing the lone pair and the fifth Cl—F bond. 

XeFq, like I3, is a species that has lone pairs but zero dipole moment. Symmetry explains this. We can see why by examining how the lone pairs are placed. In I3, the three lone pairs form a symmetrical trigonal plane. In XeFq, the two lone pairs oppose each other. 

The dipole moment of tributylpliosphine varies from 1.49 to 2.4 D according to the solvent used. Inductive effects in phosphines have been estimated by comparing the calculated and observed dipole moments, and the apparent dipole moment due to the lone electron pair on phosphorus has been estimated. A method of calculating the hybridization of the phosphorus atom in terms of bond angles is suggested which leads to a linear relationship between hybridization ratio and lone electron pair moment. The difference between experimental and calculated dipole moments for para-substitued arylphosphines, phosphine sulphides, and phosphinimines has been used to estimate mesomeric transfer of electrons to phosphorus. 

That way, the Distributed Electrostatic Moments based on the ELF Partition (DE-MEP) allows computing of local moments located at non-atomic centres such as lone pairs, a bonds and n systems. Local dipole contributions have been shown to be useful to rationalize inductive polarization effects and typical hydrogen bond interactions. Moreover, bond quadrupole polarization moments being related to a n character enable to discuss bond multiplicities, and to sort families of molecules according to their bond order. 

Another example of the importance of atomic dipoles appeared in Chapter 2, where we attributed the small dipole moment of NF3 to the moment produced by the lone pair on nitrogen, which makes an important contribution to the atomic dipole on nitrogen and opposes the charge transfer moment due to the electronegativity difference between nitrogen and fluorine. 

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

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