Some Electrical and Optical Aspects of Molecular Behaviour

197 Pages·1965·3.114 MB·English

by  Mansel Davies, Robert Robinson, H. M. N. H. Irving and L. A. K. Staveley (Auth.)

This book is sold subject to the
condition that it shall not, by
way of trade, be lent, re-sold, hired
out, or otherwise disposed of with-
out the publisher's consent, in
any form of binding or cover other
than that in which it is published.
Peter j. W. Debye. Pioneer in the study of the electrical and optical
properties of molecules.
Some Electrical
and Optical Aspects
of Molecular Behaviour
Mansel Davies
University Reader in Chemistry
The Edward Davies Chemical Laboratories
Aberystwyth
PERGAMON PRESS
OXFORD · LONDON · EDINBURGH · NEW YORK
PARIS · FRANKFURT
Pergamon Press Ltd., Headington Hill Hall, Oxford
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Federal Publications Ltd., Times House, River Valley Rd., Singapore
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Copyright © 1965
Pergamon Press Ltd.
Library of Congress Catalog Card No. 65-18188
First edition 1965
Set in 10-12 pt. Times and Printed in Great Britain by
Bell and Bain Limited, Glasgow
Some Numerical Constants
(To be used in the calculations)
c « velocity of electromagnetic radiation in vacuo
= 3-00 χ lO^cmsec"1
Ν = Avogadro's number = 6Ό2 χ 1023
A = Planck's constant = 6-63 χ 10""27 erg sec
e = electronic charge = 4-80 χ 10"10 e.s.u.
k = Boltzmann molecular gas constant
= 1-38 χ HT16 erg "K"1
R = molar gas constant = 1-987 cal °K""1
1 calorie = 4-18 χ 107 erg
1 e.s.u. (c.g.s.) unit of potential = 300 V(olt)
1 erg molecule""1 = 1-44 χ 1016 cal mole"*1
1 electron-volt = 1-60 χ 10"12 erg molecule""1
vii
Preface
THIS volume is intended—as its appearance in the Pergamon series
testifies—for students taking up the initial study of dipole
moments and molecular polarizability. Moreover, it is intended
for students of chemistry or, as we are now learning to call it,
molecular science.
A dictum of Lord Rutherford's, already old enough to have
grown a considerable beard, tells us: " Science can be divided
into two branches: physics and stamp-collecting." Not all
scientists appreciate this view but it embodies an essential element
of scientific (i.e. organized pragmatic) truth. The physical chemist
is fully committed to Rutherford's position and this small volume,
were it successful, would illustrate his thesis.
A more recent view of molecular science stems from Dirac
(1932), and forms the opening sentence of Kauzmann's excellent
volume Quantum Chemistry: " There is every reason to
believe that all of chemistry should be deducible from the laws
of quantum mechanics." This also is an interesting thesis. Those
who have heard Professor R. B. Woodward of Harvard Uni-
versity describe the chemical synthesis of natural products will
realize that the present relation of quantum mechanics to some
aspects of chemistry partly resembles that of the theory of sound
to the Jupiter Symphony. It is hoped that this volume will be of
service to organic chemists.
An attempt has been made to present those electrical and optical
features of molecules which are of particular significance in
studying their structure and interactions. In addition to the
immediate value of molecular polarizabilities and dipole moments
in this respect, it may be recalled that optical activity was the
ix
χ Preface
starting point from which the whole of structural chemistry has
developed: its molecular origin is described. Of molecular inter-
actions, the van der Waals forces are the most general expression:
their origin and magnitudes are considered.
The treatment aims at expounding only the simpler features
and then illustrates these with a variety of experimental results.
Little is assumed which is not familiar to first-year university
students who have taken physics and chemistry at school.
Specific indications are given of experimental methods. These,
it is hoped, will serve to remind the student that some apprecia-
tion of the measuring technique, and a lively regard for the
significance attained in the available data, are always needed for
the intelligent discussion of physico-chemical topics.
Each chapter has references to texts where further details can
be found and also some examples for typical calculations.
Aberystwyth,
May, 1964
CHAPTER 1
An Introduction
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 electro-
chemistry. 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)]4"". This is because the
6
motion is dominated by the net electrical charge present on the
ion. We are not directly concerned with such electrical conduc-
tions 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, @HQ), 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 hydro-
gen and chlorine atom together to form the hydrogen chloride
1
2 Some Aspects of Molecular Behaviour
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, CP, 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 con-
sider the molecule being formed from the H+ and CI" ions; the
+ + 4- ±_
Ε
'ΐ
(α) (b)
FIG. 1.
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 e separated by a
distance d within the molecule, the force on each charge will be
eE where Ε 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 {eE) χ d. Thus, in unit field
An Introduction 3
(Ε = 1) the maximum moment of the couple is e χ d and this
is, by definition, the electrical moment of the dipole.
The molecule will turn under the action of the field and, if no
other factors interfere, the dipole will align itself along the direc-
tion of the electrical field, Fig. 1(b). The turning couple is then
zero as the two forces (eE) 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.
Electric dipole moment = μ = (charge) χ (distance) = e χ d
Magnetic dipole moment = M = (pole strength) χ (distance)
= m χ d
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 (e) and distance (rf), 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 e or d 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 e will be
of the same general magnitude as that of an electron (4*80 χ 10"10
electrostatic or c.g.s. units), whilst the effective distance d 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
e χ d = I χ 10"10 χ 1 χ 10"8 c.g.s. units
= 1 χ 10"18 c.g.s. units
= 1 Debye
This unit of dipole moment is called " the Debye " after Peter