09 - Chemistry 2000 Lecture 9: Intermolecular forces Marc...

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Unformatted text preview: Chemistry 2000 Lecture 9: Intermolecular forces Marc R. Roussel Bond polarity I When the two atoms at each end of a bond are different, they will not share electrons equally. I This results in bond polarity , i.e. a separation of charge between the atoms at either end of the bond. I The bond polarity will increase the greater the difference in electronegativity between the atoms. Example: N 2 is nonpolar. Example: CO should be more polar than NO. Molecular polarity in diatomic molecules I In diatomic molecules, if the bond is polar, then the molecule is automatically polar. I Molecular polarity can be calculated or measured. Dipole moment: vector measure of molecular polarity = X atoms q i r i q i = partial charge on atom i r i = position vector of atom i I For a neutral diatomic molecule, the size of the dipole moment vector (or just the dipole moment) is | | = = | q | R | q | = absolute value of the partial charge carried by each atom R = bond length I The dipole moment vector calculated from the formula points toward the positive end of the molecule. I However, chemists draw their dipole moment vectors pointing the other way, using an arrow with a cross bar at the positive end of the dipole: + :N O .. .. . - Measurement of dipole moments Dipole moments can be measured in two ways: Temperature dependence of the relative permittivity: I The force between two charges in any medium is F = q 1 q 2 4 r r 2 where r is the relative permittivity of the medium. I For solids and liquids, the relative permittivity is easily measured by measuring the capacitance of a pair of parallel plates separated first by vacuum and then by the material under study. I r depends on temperature in a way which allows us to recover both the dipole moment and the polarizability of the medium. Stark effect in rotational spectroscopy: I Molecules in the gas phase have discrete rotational energy levels. I Molecules with a permanent dipole moment can undergo pure rotational transitions. I These are studied by microwave spectroscopy. I In the presence of an added electric field, the rotational levels shift (Stark effect). I The size of the shift tells us the size of the molecular dipole moment. A mess of units... I The SI unit for the dipole moment is the C m. I Dipole moments in C m are small ( 10- 30 C m). I Example: The dipole moment of HCl is 3 . 60 10- 30 C m. I Although its use is discouraged, many people like to use the debye (D), a non-SI unit, as a unit for dipole moments because these values are of order unity. I 1 D = 3 . 335 64 10- 30 C m I Example: The dipole moment of HCl is 1.08 D....
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09 - Chemistry 2000 Lecture 9: Intermolecular forces Marc...

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