301
Journal of Molecular Structure, 22 (1974) 301-303
~ Elsevier Scientific Publishing Company, Amsterdam - Printed in The NetherIands
Short communications
ON THE MICROWAVE SPECTRUM OF ETHYLENE GLYCOL
K.-M. MARSTOKK AND HAR1D
M0LLENDAL
Department of Chemistry, University of Oslo, Blindern, Oslo 3 (Norway)
(Received 10 October 1973)
As a part of our interest in intramolecular hydrogen-bonding problems the
microwave spectrum of ethylene glycol was investigated. Previously, electron
diffraction investigations [1, 2] have shown that the preferred conformation of
the free molecule has the two C-O bonds gauche to one another, and the dihedral
angle between the two OCC planes is about 65°. Moreover, the infrared spectrum
[3] of gaseous ethylene glycol presents strong evidence for the presenee of intramolecular hydrogen bonding stabilizing the gauche rotamer.
In an attempt to assign the microwave spectrum the following procedure
was used. A plausible structure of the gauche form was first assumed and th~
rotational constants calculated to be about A = 16.4 GHz, B = 5.1 GHz, and
C = 4.4 GHz, respectively. A rigid rotor spectrum was the n predicted with these
rotational constants and a very thorough search was made for low J a-, b-, and
c-type transitions in the 8-38 GHz spectral region. However, no lines with app~opriate Stark effects were found in the predicted frequency ranges, and we therefore
feel that it is very unlikely that all three dipole moment components follow rigid
rotor selection rulcs.
lnstead of the expected rigid rotor spectral features, a very unusual spectrum
was revealed. Of about 600 transitions* of medium and strong intensities
occurring in the examined spectral range, more than 200 of the strongest lines fall
in the 16-19 GHz range with the majority centered around 17.1 GHz. Study of
the unresolved Star k effects of these as well as of the great majority of the other
intense lines strongly indicated that they are high J transitions. Only the 12 lines
ofTable 1 were found to possess resolved Stark lobes with relative intensity features
characteristic for R-branch transitions [4]. Attempts to fit these lines to a rigid
rotor spectrum of the gauche rotamer proved impossible.
* The complete list of measured frequencies is available from the authors upon nquest or from
Microwave Data Center, National Bureau of Standards, Washington, D.C., wher~ it has been
deposited.
302
TABLE 1
MICROWAVE
TRANSITIONS
OF ETHYLENE GLYCOL EXHIBITING
RESOLVED STARK EFFECTS
Tentative
assignment
Alternative J
assignment
Number of Stark
lobes observed
Frequency'
(MHz)
00.0 --+10.1+b
1 --+2+
1 --+2+
2--+3
2--+3
2--+3
3--+4
3--+4
3--+4
3--+4
4 --+54 --+5-
1--+1
none
none
none
none
none
none
none
4 --+54 --+55--+6
5--+6
1
2
2
3
3
3
4
4
4
4
5
5
26381.65
37187.78
37830.78
25027.70
36061.57
37557.36
31612.80
33272.94
33994.64
34465.50
33806.40
33977.01
.
)
::1::0.10MHz.
bLIis approximately equal to the assumed tunneling frequency of about 17.1 GHz, but is not
expected to be exactly a constant.
H
o
c
o
c
H
H
c
c
o
Fig. 1. Two identical mirror image forms of gauche ethylene glycol which are interchangeable
by rotation about the two C-O bonds. Hydrogen atoms attached to carbon atoms are omitted
for simplicity.
To account for the unusual microwave spectrum of ethylene glycol the
following qualitative explanation is tentatively suggested. There are two identical
mirror image forms of the hydrogen-bonded gauche conformation. This is
illustrated in Fig. l. Tunneling between these two mirror images is brought abou1
by rotation about the two C-O bonds. The region around 17.1 GHz is suggested as
the tunneling frequency and the numerous lines in this range are probably almos1
pure vibrational high J transitions. Similar features have been observed in the
microwave spectra of propargyl mercaptan [5] and hydroxyacetonitrile [6].
303
lf the model of Fig. l were correct, the a- and c-type lines should follow
the selection rules for "inverting"
dipole transitions [5-7], whereas the b-type
lines should follow rigid rotor selection rules [5,6,8]. The latter transitions
should probably consist of dosely spaced doublets of equal intensities, corresponding to the symmetrical (+) ground state and antisymmetrical (-) first
excited vibrational state, respectively, of a double minimum potential. The a- and
c-type spectra, on the other hand, should in~
transitions between the ( + ) and
the ( - ) vibrational states in addition to the ordinary rotational transitions. Moreover, most intense low l b- and c-type lines are Q-branch transitions, whereas
the intense low l a-type lines belong to the R-branch. The predicted features for
the b- and c-type spectra are missing, possibly because the corresponding dipole
moment components are small. Rowever, the transitions listed in Table l are
probably compatible with a strongly perturbed a-type spectrum since they are
apparantly /)"l = + l transitions. This is further supp:>rted by bond moment
calculations which predict a large /la for the Fig. l conformation. Unsuccessful
attempts were made to fit these lines to the simple picture of a rigid rotor plus or
minus a fixed valuo;:of about 17.1 GRz for the tunnel ing frequency. This might
perhaps be explained by extensive coupling betwe~n vibration and rotation
because the tunnel ing frequency is dose to typical low l rotational trall3itions.
The above tentative assignments were made on the basis of the Stark elfect
only. The powerful method of double resonance [9] c;mld be med to check m3.ny
of the attributions of Table l but, unfortunately, this technique has not been
available to the authors.
Much of the present work was made during Harald Møllendal's stay at
the University of Texas at Austin. Re would like to thank Professor James E.
Boggs for his kind hospitality and Dr. R. E. Penn for numerous discussions and
suggestions. The Robert A. Welch Foundation, the University of Texas, the Fulbright Foundation, and the Norwegian Research Council for Science and the
Humanities are thanked for financial support.
l O. Bastiansen,Acta Chem. Scand., 3 (1949) 415.
2
3
4
5
6
7
L. Fernholt, personal communication, 1972.
P. Buckley and P. A. Giguere, Can. l. Chem., 45 (1967) 397.
S. Golden and E. B. Wilson Jr., J. Chem. Phys., 16 (1948) 669.
K. Bolton and J. Sheridan, Spectrochim. Acta, 26A (1970) 1001.
G. Cazzoli, D. G. Lister and A. M. Mirri, lCS Faraday Trans. Il, 69 (1973) 569.
W. Gordy and R. L. Cook, Microwave Molecular Spectra, Interseienee, New York,
p.149.
8 E. Hirota, J. Mol. Speetrase.,26 (1968) 335.
9 R. C. Woods, V. M. Ronn and E. B. Wilson Jr., Rev. Sei. Instrum., 37 (1966) 927.
1970,