Table Of ContentThe Pennsylvania State College
The Graduate School
Department of Physios
Rotational Isomerism in 1,1,2,2-tetraohloro
and tetrabromoethane
A dissertation
by
Ronald E« Kagariss
Submitted in partial fulfillment
of the requirements
for the degree of
Doctor of Philosophy
A ugust 1951
Approved*
Department of Physios
Acknowledgements
The author wishes to express his sinoere appreciation to the
following persons*
Dr. D. H. Rank, who suggested the problem and gave invaluable
aid and enoouragememt throughout the investigation*
Ur. I. R. Dagg, who did much of the experimental work entailed
in determining the energy differenoe between the rotational isomers
of s-tetrabromoethane•
The personnel of the Speetrosoopy Laboratory, who helped in
many ways to make the completion of this work possible*
J
Table of Contents
Page
Introduction . . . . . . . . . . . . . . . . . . . . . . 1
Experimental . . . ................................... . . . . . . . . . . . . 5
A. General ....................................... . . . 5
B. Raman Effect 6
C. Chlorine Isotope Effect 7
D. Infrared Effect . . . . . . . . . . . . 10
Results . . . . . . . . . . .............................. . . . . . . . . 14
Discussion 22
Conclusions 37
Bibliography 39
Appendix I . . . . . . . . . . . . . . . . . . . . . . . 41
Appendix II • • • • • • . • • • . . . • • .............................. • 44
Appendix III 46
Introduction
The problem of rotational isomerism has been investigated by many
workers from quite different points of view. The deteotion of rotation
al isomerism in organic compounds by spectroscopic measurements was
first proposed and investigated qualitatively by Kohlrausch^ and
2 4
Kohlrausoh and Koppl . Mixushima, Morino and Nakamura have investi
gated a number of hydrocarbons by obtaining Raman spectra in the liquid
and solid states. Qualitatively, the results of their researches show
that the equilibrium concentration of the rotational isomers is tempera
ture dependent. The first quantitative study of the phenomenon of rota
tional isomerism by means of the Raman effect was the work of Langseth
t
and Bernstein in which 1,1,2,2-C H Cl was investigated. Rank and his
6 w t
C.Q
coworkers have made quantitative measurements of the energy differ
ences of the observed rotational isomers in the Raman spectra of a
10
number of hydrocarbons. Recently, a paper has appeared by Bernstein
in which he has obtained quantitative measurements of the energy dif
ferences of the rotational isomers of 1,2-dichloroethane. This is the
first demonstration of the spectroscopic method of investigating these
isomers by means of infrared absorption spectra. These measurements
have been made in the vapor phase since Gerding and Uesrsan^have sug
gested that measurements made of energy differences of rotational
isomers by means of observations on spectra produced in the liquid
state many not be entirely valid, depending on the behavior of
material under investigation.
That the intensity of the lines in the Raman speotra of certain
organic molecules is temperature dependent has been an established
fact for many years. The important question to be answered is what is
the source of* 'tills temperature dependence* 0f course, it is apparent
from our previous discussion that rotational isomerism is the generally
accepted basis for changes in the relative intensity of line pairs
when the temperature is varied or when a transition is made from one
state to another, (e*g., certain lines which are present in the liquid
state, disappear in the Haman and I.R. spectra of Hie solid)*
12—15
Sirkar and Bishui have made a number of investigations on
dihalogenated ethanes obtaining the haman spectra in the liquid and
solid states. The results have been interpreted in such a manner that
association and not rotational isomerism is responsible for the tempera
ture dependence exhibited by the Haman spectra observed* In view of the
work of Bernstein with the gas phase, association can hardly be accepted
as an explanation since this phenomena would certainly not exist in the
16
gaseous state. Also the work of Hank A Axford on n-butane seems also
to invalidate an explanation on the basis of the association of molecules
Of all the substances which are thought to possess rotational
isomers, perhaps none have been studied more extensively Hi an ethane
and its halogen derivatives. Much of this work has been devoted to
17
1,2-dibromo- and dicblor©ethane. Mizushima and Morino have sum-
19
marl zed their work on the latter molecule. Neu and Gwinn have re
cently made an assignment for the corresponding bromine derivative.
In both cases the conclusion is that the trans isomer (C_, ) is the more
2n
stable form with Hie second isomer (gauche) possessing C symmetry.
The present situation concerning the energy differences between the ro
tational isomers of these two compounds has been summarized by Rank,
Kagarise and Axford®.
Consider ably less work has been done on 1,1,2,2-tetrabromo- and
3
tetrachloroethane. Langseth and Bernstein investigated the latter
molecule and have concluded that the two forms are cis and gauche,
i.e., of symmetry C^ and respectively, with the cis form being
the more stable having a potential energy of 1100 cal/mol lees than
20
that of the form. Hersberg has questioned the assumptions made
by the above authors concerning the number of vibrations of frequency
<400 cm"* without making detailed calculations, and has concluded
that the trans form is the more stable of the two isomers the second
of whioh is very likely the gauche form. These contentions are sup-
21
ported by Schomaker and Stevenson on the basis of electron diffrac-
22
tion data. However, Hassel and Viervoll have obtained an almost
identical radial distribution function which they attribute to a
molecular model consisting of the trans-form as the equilibrium
form, with an oscillation about the trans-position.
Our interest in these two molecules resulted from an earlier investi-
23
gation of the chlorine isotope effect . It was observed that the 353
cm"* line of s-tetrachloroethane has an intensity contour which is
characteristic of molecules containing only two chlorine atoms, whereas
the 366 cm"* frequency possessed the expected "four pattern”. At that
time the first of these two lines was assigned to the trans form and
the latter to the gauche form (C^ symmetry). This assignment had been
made on the basis that the trans form is the more stable one.
24
Plyler , however, has investigated the infrared spectrum of this
molecule in the KRS-5 region and reports bands at 290, 326 and 352 cms"*.
The first and last of these bands apparently have corresponding Raman
lines at 287 and 353 wave numbers. Furthermore, these two Raman
frequencies have previously been observed bo belong bo the more sbable
form. Obviously, if these are true correspondences the sbable form
cannob be bhe brans form since the rule of mutual exolusion must be
satisfied for bhe point group C_, •
dh.
lb should be pointed out that the conclusions of previous in
vestigators concerning bhe symmetry of bhe rotational isomers of s-
tetrachloroethane were made without the benefit of any infrared data.
We feel that complete spectral data is essential if one is to make
reliable conclusions regarding the structure of even the simpler
polyatomic molecules. For this reason we have attempted in this
investigation to obtain as muoh of the spectral data physically
possible. With -this additional information one should be able to
clarify some of the discrepencies which exist at the present time with
regard to the problem of rotational isomerism in s-tetrahalogenated
ethanes.
Exper imental
A. General
The Raman spectrum of s-tetrabromoethane has been previously
investigated by several workers^®”^^. Of these, Ananthakrishnan made
the most exhaustive study and has reported the frequencies, relative
intensities and qualitative depolarisation factors of twenty-four Raman
lines. From an investigation of the literature it appears that no
intensity-temperature dependence measurements have been made and with
the exception of P lyler's^ work in the KRS-5 region the infrared
spectrum has not been studied.
Somewhat more work has been done on the structure of s-
tetrachloroethane. The Raman spectrum has been obtained by numerous
3 27-30
investigators * • The most complete work is -that of Langseth and
Bernstein which, as we have previously mentioned, was the first quanti
tative study of the phenomenon of rotational isomerism by means of the
Raman effect. They have reported the frequencies, relative intensities
and in most instances qualitative depolarization factors for twenty-
three Raman lines. In addition they have investigated the corresponding
deuterium derivative and have made intensity-temperature measurements
for fourteen of the stronger lines. Mizushima et. a l.3* have made
similar qualitative measurements from the Raman spectra of the liquid
and solid phases. So far as we know Plyler^4 has been the only person
to report infrared data.
The s-tetrachloroethane used was a sample of practical quality
which was purified by fractional distillation in a 25-plate column
(b.p 145.0-145.5°C). The bromine compound was an Eastman Kodak ohemical
(D2.965/20°C) vacuum distilled into the sample tube to remove any dust
particles.
B. Raman Effect
The general features of the two spe otrographs used in this
investigation have already been adequately described*^*®^*®^.
The frequencies of the Raman lines were determined from photographic
plates obtained with the 3-prism instrument and f:8.0 camera. Reproduc
tions of these plates are shown in Fig. 1. The comparison spectrum used
was an iron-chromium (stainless steelj arc. In addition the region be
low 850 om“^- was observed using the 15 ft. oonoave grating instrument
by replacing the exit slit normally employed in the photoelectrio method
of detection by a plate holder. Since the field is not flat, it was
either necessary to bend the photographic plate to f it the field curva
ture, or to observe the spectrum by segments over which the field is
approximately flat. The latter method was used since we were primarily
interested in the region -<850 cm"^ where one would expect to find the
chlorine isotope effect appearing. Fig. 2 shows the Raman spectrum of
the bromine compound obtained by the above described method. With
both instruments Eastman Kodak Type 103-0 plates were used.
Qualitative depolarization measurements were made using the method
3 S
of Douglas and Rank • In addition quantitative measurements were ob
tained for most of the lines using the photoelectric instrument and the
w j
method of empirical convergence correction previously described •
Relative intensities were determined by comparing the peak inten
sity of the various lines to that of the most intense line. These peak:
intensities were measured directly from recorder traoes of the spectra
7
obtained with the photoeleetrio instrument. It was observed that in
general peak intensities and integrated intensities, i.e ., area of the
Raman lines, gave comparable results and for simplicity the former were
used. This has also been pointed out by Bernstein^ in the case of
intensity measurements in the infrared.
Intensity-temperature dependence of the Raman lines was deter
mined from observations of the Raman spectra at two temperatures; (0°C
and 150°C for the chlorine derivative, 5°C and 110° for
s-tetrabromoethane)• For these measurements the photoelectric spec
trometer and. the apparatus previously desoribed® for oooling the liquid
sample tube were employed.
C. Chlorine Isotope Effect
One of the more interesting features of the Raman spectrum of
1,1,2,2-tetraohloroethane is the fine structure exhibited by the 353-
366 om"*^ line pair, due to the chlorine isotope effeot. The work of
Rank, Sheppard and Szasi^ indicated, as previously mentioned, that the
353 cm”* frequency showed an "anomolous" intensity oontour pattern. We
have undertaken to improve on the experimental results, since we now
have a light source of considerably more intensity than that previously
used but s till possessing all the desirable speotral characteristics.
For this work the grating spectrometer was employed. Speotra were re
corded both photographically and photoeleotrically. The results are
shown in Fig. 3 and Fig. 4. Fig. 3 shows the fine structure patterns
obtained photographically. In all cases the s lit width was .05 mm
and the time of exposure about 16 hovrs. For the purpose of comparing
the fine structure patterns expected of molecules containing two, three
