Davies / Robinson / Irving Some Electrical and Optical Aspects of Molecular Behaviour

The Commonwealth and International Library: Chemistry Division
1. Auflage 2014
ISBN: 978-1-4831-9614-5
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, 200 Seiten

Some Electrical and Optical Aspects of Molecular Behaviour deals with dipole moments, molecular science, and molecular polarizability. The author explains the refractive index and the electric permittivity including polarization, polarizability, the Mossotti-Clausius equation, molar polarization, and molar refraction. The author also addresses the results of dipole moment measurements, the significance of the values obtained, and how these can determine molecular structural features. By using the Debye equation, one can determine the molecular moments of polar solutes in non-polar solvents. The book discusses how molecular polarizability and molecular polarization are measured and how these factors are associated with molecular structure. He also explains light scattering and the molecular weight of solutes by employing the research made by Debye which determines the weights, shapes, and interactions of molecules in solution. The author also addresses the form and order of the magnitude of interaction energies and cites typical examples of these occurrences. These energies often represent an electrostatic nature; in some ways, they can be understood as being quantum-mechanical in nature. This book can be helpful for students taking up dipole moments, molecular polarizability or molecular science.

Autoren/Hrsg.

Davies, Mansel

Robinson, Robert

Irving, H. M. N. H.

Staveley, L. A. K.

Leseproben

CHAPTER 1

An Introduction

Publisher Summary

This chapter discusses electrical conduction in chemical compounds. All atoms are composed of particles whose most obvious properties are of electrical character. It follows that material substances and the molecules, which are the smallest chemical units in their structure, will show a variety of behavior when subjected to electrical forces. If a material has atoms or molecules, which themselves carry electrical charges, the particles (or ions) of opposite charge move under the action of an electric field. The movement or mobility of the oppositely charged ions, however, does not often directly tell about the structure of the charged molecular species. This is because the motion is dominated by the net electrical charge present on the ion. While the simplest form of electric field is the stationary or static one formed by the charged plates of an electrical condenser, such a field is very inconvenient for making measurements. All electrical frequencies are a part of the electromagnetic spectrum and one cannot generate the oscillating electric field without simultaneously producing the oscillating magnetic field of the same frequency.

ALL matter is composed of atoms; all atoms are composed of particles whose most obvious properties are of an electrical character; it follows that material substances, and the molecules which are the smallest chemical units in their structure, will show a variety of behaviour when subjected to electrical forces. If a material has atoms or molecules which themselves carry electrical charges the particles (or ions) of opposite charge move under the action of an electric field, i.e. when a voltage is applied. This is so for salts whether in the solid, liquid, solution or gaseous state, and it is most obvious in metals where the free electrons can move very readily. This process of electrical conduction is of much interest and, in chemical compounds, forms the subject of electrochemistry. The movement or mobility of the oppositely charged ions, however, does not often directly tell us much about the structure of the charged molecular species—the motion of the fluoride ion (F-) is only quantitatively different from that of the complex ferrocyanide ion [Fe(CN)6]4-. This is because the motion is dominated by the net electrical charge present on the ion. We are not directly concerned with such electrical conductions in this volume, but rather with what happens to uncharged molecules in the presence of electric fields.

Two separated chlorine atoms will each be electrically neutral and the electrical centre of the electron distribution will coincide with the positive nucleus. When two such atoms come together to form the chlorine molecule, , the dumb-bell-like structure is electrically balanced about its centre, and the two ends of the molecule are identical. If, however, we bring a hydrogen and chlorine atom together to form the hydrogen chloride molecule, a marked electrical dissymmetry can be anticipated. The hydrogen atom, we know, has a tendency to form the H+ ion, i.e. to give up its electron. The chlorine atom, on the other hand, fairly readily forms the chloride ion, Cl-, i.e. picks up an electron. The net effect when these atoms come together in hydrogen chloride, , is for the chlorine to get the greater share of the pair of electrons which go to form the bond between them. This is anticipated even more clearly if we consider the molecule being formed from the H+ and CI- ions; the latter gives only part of its net negative charge to the hydrogen.

The molecule is thus a typical electric dipole, , with the hydrogen end appreciably positive with respect to the negative chlorine; note that there is no resultant charge on the molecule.

To measure this electric dissymmetry, consider the molecule placed in an electric field: for instance, between the parallel plates of a simple charged electrical condenser, Fig. 1. If this electrical distribution is represented by equal charges separated by a distance within the molecule, the force on each charge will be where is the field strength; this follows from the definition of the field strength. The two forces will produce a turning couple which is a maximum when the dipole lies at right angles to the field direction, and is of moment () × d. Thus, in unit field ( = 1) the maximum moment of the couple is × and this is, by definition, the electrical moment of the dipole.

FIG. 1

The molecule will turn under the action of the field and, if no other factors interfere, the dipole will align itself along the direction of the electrical field, Fig. 1(b). The turning couple is then zero as the two forces () act along the one straight line and merely cancel out.

Exactly the same conditions as for an electric dipole are found for a simple bar magnet whose magnetic moment is similarly given by the product of the individual pole strengths and their effective distance apart.

As in the magnetic, so also in the electrical case, it is readily possible to determine the dipole moment but the separation of the latter into the components, the charge () and distance (), is far more arbitrary. Basically this is the result of the non-localized form of the electric charge (or electron cloud distribution). In many cases it is quite impossible to provide any definite estimate of the or values even when the product, for which the symbol µ is used in the electrical case, is known.

However, the order of magnitude of the electric dipole moments to be found in molecules can be established. The charge will be of the same general magnitude as that of an electron (4–80 × 10-10 electrostatic or c.g.s. units), whilst the effective distance will be of atomic or molecular dimensions, i.e. a distance of the order of 10- 8 cm. Accordingly, an appropriate unit for a molecular electrical dipole moment will be

This unit of dipole moment is called “the Debye” after Peter Debye who was born at Maastricht in the Netherlands in 1884. He pioneered the theory and study of dipole moments: and with his many outstanding contributions it can be said of him that he has added more to physical chemistry than any two other living scientists.

A great deal of what follows in other chapters is concerned with the interaction of dipoles with electric fields. Whilst the simplest form of electric field is the stationary or static one formed by the charged plates of an electrical condenser, such a field is very inconvenient for making measurements. Far more convenient for measurements and far more interesting in the variety of behaviour encountered are alternating electrical fields.

An alternating electric current, such as the town mains which swings from positive to negative to positive fifty times a second, i.e. has a frequency of 50 cycles per second (c/s), produces an alternating electric field of the same frequency. In the neighbourhood of this oscillating current or voltage the oscillating electric field will be accompanied by a similarly oscillating magnetic field; in fact, the surrounding space has an alternating electromagnetic field of 50 c/s frequency. Not only does the field oscillate at any one point but it is transmitted through space with an intensity which falls as the square of the distance from the source. Physical space can be defined as a medium having volume and the ability to transmit electromagnetic fields.

From an oscillating field, the transmission will be in the form of a wave of the same frequency, Fig. 2. At one point the electric potential (voltage) will oscillate through one complete cycle in a fiftieth of a second, i.e. = 0·02 sec for 50 c/s. At any one instant, the potential in any one direction will show the same wave pattern, usually with the amplitude of the wave () decreasing with distance from the source. If the line represents distance, then the length or represents the wavelength (?). Using the symbol v for the frequency of the wave motion and considering the motion of the wave past a point (, say) we see that

FIG. 2

is a constant for all electromagnetic frequencies, i.e. for 5 × 103 c/s = 5 K(ilo)c/s; 5 × 106 c/s = 5 M(ega)c/s; 5 × 109 c/s = 5 G(iga)c/s; 5 × 1012 c/s = 5 T(era)c/s, the velocity is the same as for 50 c/s. It is the velocity of light, = 3 × 1010 cm/sec.

All electrical frequencies are part of the electromagnetic spectrum and one cannot generate the oscillating electric field without simultaneously producing the oscillating magnetic field of the same frequency. The magnetic field will not concern us but we must briefly examine the different regions found in the electromagnetic spectrum as the frequency increases.

Thanks to the basic quantum relation, energy per quantum of radiation = × where = Planck’s constant, the frequency () is, throughout the spectrum (Fig. 3), a measure of the energy.

FIG. 3 The electromagnetic spectrum.

A = direct current: differentiation of...