JOURNAL OF RAMAN SPECTROSCOPY
J. Raman Spectrosc. 2006; 37: 1398–1402
Published online 9 June 2006 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/jrs.1556
Staggered ethane changes to eclipsed conformation
upon adsorption
A. Priebe,1 A. Pucci,1 W. Akemann,2 H. Grabhorn2 and A. Otto2∗
1
2
Kirchhoff-Institut für Physik, Ruprecht-Karls-Universität, Im Neuenheimer Feld 227, D-69120 Heidelberg, Germany
Institut für Physik der kondensierten Materie, Heinrich-Heine-Universität Düsseldorf, D-40225, EU, Germany
Received 12 December 2005; Accepted 31 March 2006
Previous comparison of the infrared and Raman spectra of bulk and adsorbed C2 H6 has shown that the
C–C axis is parallel to the local surface of Cu. The conformation of ethane in thick condensed layers is
staggered. The clear appearance of a Raman band of a bending mode, not Raman active in the staggered
conformation, proves that ethane is adsorbed in eclipsed conformation on silver, indium, and potassium,
and probably on other quasi-free electron metals. A relatively low-lying Raman band is assigned to the 0
to 2 internal torsional rotation, with about half the frequency in the staggered conformation. This is to be
expected when forcing the molecule from staggered- to eclipsed conformation by adsorption. Copyright 
2006 John Wiley & Sons, Ltd.
KEYWORDS: surface enhanced Raman scattering; conformation of ethane; silver; indium; potassium
INTRODUCTION
Molecules consisting of different parts connected by a single
C–C bond can rotate better with respect to each other
than when they are connected by a double C C bond.1
Nevertheless, they will find a weakly locked configuration.
When adsorbed, the differences in bonding enthalpy may
be higher than the locking potential, and the configuration
with respect to the rotation around the C–C bond may
be changed. It is not straightforward to measure this. The
ideal molecule for these investigations is the symmetric top
molecule H3 C–CH3 (ethane). It has a ‘moderate’ barrier of
about 1000 cm1 to the internal rotation around the C–C axis
(angle 2 between the two methyl groups CH3 ).2 Considering
it as rigid and using the point group symmetry, there are
two high symmetry configurations, the staggered one with
2 D 60° and point group D3d and the eclipsed one with
D 0 and point group D3h . Both have 17 internal vibrations
approximated well by harmonic oscillators. However, the
torsional modes, described by a torsional Hamiltonian, are
properly treated only in the framework of the G36 C group
(see following text). Within the point group analysis, the
torsional modes are all collected under one representation for
both staggered and eclipsed ethane, corresponding to a silent
mode.3 Only within G36 C do the selected torsional transitions
Ł Correspondence
to: A. Otto, Institut für Physik der kondensierten
Materie, Heinrich-Heine-Universität Düsseldorf, D-40225,
EU, Germany. E-mail: Otto@uni-duesseldorf.de
Copyright  2006 John Wiley & Sons, Ltd.
become infrared- or Raman-allowed, but the intensities are
very low.
Ethane in gas form is staggered, as demonstrated by
Raman spectroscopy.4 No second harmonic generation was
observed, owing to the presence of an inversion centre in the
staggered form.5
One would expect it to settle down in the eclipsed form
with the C–C axis parallel to the surface and with four rather
than three ‘C–H legs’ down.
Methods based on creation of a core hole fail here
because hydrogen does not have one. Only the bending angle of hydrogen against the molecular plane may
be obtained in special cases.6 Electron diffraction fails
because of too small an electron density on hydrogen.
Locating hydrogen or deuterium by nuclear magnetic
resonance or neutron scattering is seldom of good use
at surfaces. Helium atom scattering did deliver the frequency and damping of the frustrated translation of ethane
at the Cu(111) surface,7 but the configuration was not
obtained. Scanning tunnelling microscopy would perhaps
be able to differentiate between staggered and eclipsed
ethane.
As there is no second harmonic generation (SHG)
from the free molecules with inversion centre, maybe
the adsorption of these molecules does not change the
SHG intensity from a metal surface. However, we would
only know the difference if it were possible to adsorb
ethane in both ways. Sum-frequency generation delivers
intensity only for those vibrations that are both infrared
Copyright  2006 John Wiley & Sons, Ltd.
2746.9
2896
2897 2954 FRf
26 origin of Q-band, Rg
a2u H C–H stretch IR-p 5
a1g C–H Stretch R pol 1
2985
1468
eg CH3 deform. R depol 11
eu H C–H stretch IR-s 7
–
1471
eu H CH3 deform. IR-s 8
2969
1397
eg C–H Stretch R depol 10
2880
1379
a2u H CH3 deform. IR-p 6
a1g CH3 Deform. R pol. 2
–
2972
–
–
–
1465, 1458, 1456, 1450
–
1371, 1368
–
994
1196
eg Bending R depol. 12
825, 816, 813
b
Near normal
IR reflectance of
solid ethane at 25 K
a1g C–C Stretch, R pol. 3
822
a
Gas
data
eu H bending IR-s 9
Reference
Ethane, D3d (staggered)
representation of vibrations
Herzberg table 104
–
–
2877
–
–
1462, 1451, 1447, 1402
–
1376
–
1200, 1192, 1185
996, 993
–
b
Raman spectra
of solid
ethane at 25 K
–
2963
2883
–
2733
1466
–
1405
–
1194
999
–
c
e’ H C–H stretch
IR-s R depol
e’’ H CH stretch R depol
a1 ’ H CH stretch R pol
a2 ’’ H CH stretch IR-p
–
e’’ H CH3 deform. R
depol
e’ H CH3 deform.
IR-s R depol.
a1 ’ H CH3 deform. R pol.
a2 ’ H CH3 deform IR-p
e’’ H CH3 bending R
depol.
a1 ’ H C–C stretch, R pol.
e’ H bending IR-s, Rdepol
–
Raman
spectra, Adsorbed ethane
ice film D3h , (eclipsed)
on Ag Herzberg Table 104
–
–
978
813
d
SERS, 1
L on
c.d.-Ag at
40 K
2964
2909
2859
–
2718? 2755?
1452?
1452?
–
2900
2843
–
–
1446?
1446?
1307? 1364? 1380? 1304?? 1358?
–
1188
987
816
c
SERS 10
L on
c.d.Ag
at 40 K
–
2934
2876
–
ca 2724
14 562?
1462?
1392?
1368? 1392?
1191
993
821
d
SERS, 1
L on
c.d.-K
at 40 K
–
2923
2868
–
ca 2706
1457?
1457?
–
–
–
992
818
d
SERSd
1 L on
c.d.-In
at 40 K
2958 (first layer signal)
–
–
No first layer signal
–
–
1458 (first layer signal)
–
No first layer signal
–
–
818 (at and above 3.8 L)
e
IRRASe , 1.5 L
on c.d.-Cu at 50 K
Table 1. Assignment of the 17 proper vibrations of ethane, without the torsional mode. (According to Herzberg, torsions a1u and a1 ’ are inactive.) Mode numbering
according to Table 104 and Fig. 49 of Herzberg.3 FR: Fermi resonance. a D Ref. 2, b D Ref. 14, c D this work, d D Ref. 15, e D Ref. 12, f D Ref. 16, g D Ref. 17, h D Ref. 18.
H: hyper-Raman-active
Conformation changes of ethane upon adsorption
J. Raman Spectrosc. 2006; 37: 1398–1402
DOI: 10.1002/jrs
1399
1400
A. Priebe et al.
and Raman active,8,9 and hence only for two bands of
eclipsed ethane. According to the selection rules of hyperRaman spectroscopy10 only six modes (inclusive of the
torsion) of the staggered conformation, indicated by H in
Tables 1 and 2, are hyper-Raman-active, in agreement with
the experiment5 and ab initio calculations.11 For eclipsed
ethane, all vibrations should be hyper-Raman-active, with
the exception of the torsion. Observation of the C–C
stretch mode would indicate the presence of the eclipsed
conformation.
As described below, the combination of the linear optical
methods of infrared and Raman spectroscopy can solve the
problem relatively easily.
RESULTS AND DISCUSSION
Orientation of the C–C axis
The last column in Table 1 shows that the infrared active
modes 6 and 5 , which have the dipole moment parallel
to the C–C axis, are not observed in infrared reflection
absorption. This is extensively discussed in Ref. 12, with the
conclusion that the C–C axis is parallel to the local surface
of the cold-deposited Cu films and that Greenler’s infrared
surface selection rule13 also holds in this case.
Adsorbed ethane is eclipsed
The Raman selection rules of C2 H6 are different for the
staggered and the eclipsed conformation (see Table 1). Only
Table 2. Position of origins of the Q-bands of the pure torsion of free ethane (IR 0 ! 1, Raman 0 ! 2), and tentative assignment of
SERS bands of adsorbed ethane to the Raman 0 ! 2 torsional band of eclipsed ethane. a D Ref. 26, b D Ref. 18, c D Ref. 27,
d D Ref. 24, e D Ref. 15, f D Ref. 12
Ethane
staggered
IR
IR
R
Torsion 4
Reference
287
a
289
b
545
c
R
Adsorbed
ethane
eclipsed
SERS
10 L c.-d.
Ag 40 K
SERS 1 L
c.-d.
Ag 40 K
SERS 1 L
c.d.K
40 K
SERS
1 L c.-d.In 40 K
IRRAS
4.9 L c.-d.
Cu 50 K
545
d
Torsion 4
–
303
This work
270 š 20
e
220 š 20
e
Continuum
e
Out of range
f
(a)
(b)
Figure 1. Raman spectra of ethane on silver; laser wavelength 514.5 nm. (a) thick silver film deposited at room temperature,
exposed at 40 K to 6500 L C2 H6 , (b) 100 nm silver film, cold-deposited at 40 K, and exposed to 10 L C2 H6 .
Copyright  2006 John Wiley & Sons, Ltd.
J. Raman Spectrosc. 2006; 37: 1398–1402
DOI: 10.1002/jrs
Conformation changes of ethane upon adsorption
Figure 2. SERS spectra of C2 H6 on cold-deposited films (40 K) of potassium, silver, and indium, laser wavelength 514.5 nm. (From
Ref. 15).
the staggered conformer has a strict exclusion between IR
and Raman activity.
For comparison, Fig. 1 shows the Raman spectrum of bulk
ice of ethane on top of a non-SERS-active silver film. There are
no bands at the position of the IR modes, which corroborates
that the molecules in crystalline ethane are staggered.
Owing to the so-called first layer effect of surface
enhanced Raman scattering (SERS) by electron impact
scattering from molecules without low-lying Ł orbitals,
the average enhancement of the Raman signal of directly
adsorbed ethane19 varies from 30 to 50. Together with
the electromagnetic field enhancement of about 200–300
at the surface of cold-deposited silver films the overall
enhancement is about 104 . For ethane, one does not need
to invoke the so-called SERS-active sites.20
Copyright  2006 John Wiley & Sons, Ltd.
The first layer Raman spectra of ethane on silver (Fig. 1),
potassium, and indium (Fig. 2) clearly show a band of the
infrared active mode 9 at about 820 cm1 . Hence, according
to the selection rules in Table 1, ethane is adsorbed in eclipsed
conformation on the three metals. Also, the IR active mode
8 will become Raman active. This mode probably shows up
at 1452 cm1 in Fig. 1(b); however, owing to the proximity of
the 11 mode this is not certain. All the other IR modes do not
become Raman active. Probably, the adsorption of ethane in
the eclipsed form will hold for all metals with exclusively
s-electrons at the Fermi level.
The spectra of ethane on K, Ag, and In show big
differences of intensity and position in the range of the
‘internal’ modes (see also the entries with question marks
in Table 1), whereas the C–C and C–H stretching modes
J. Raman Spectrosc. 2006; 37: 1398–1402
DOI: 10.1002/jrs
1401
1402
A. Priebe et al.
and the bending mode vary less. These ‘internal modes’ will
change more in the transition from staggered to eclipsed
conformation, but how this depends on the metal substrate
is an open question.
Assignment of the bands near 300 cm−1 to the
torsional Raman wavenumber of eclipsed ethane
The rotation of one methyl group with respect to the other
by 120 or 240° is not a point group symmetry operation of
the ethane molecule, which is regarded as a rigid figure,
but it is a symmetry operation of the isometric group of the
molecule.21 The two methyl groups, CH3 , have C3 symmetry.
If they are rotated around the single bond C–C axis with
the relative angle 2 between them being neither 0 nor 60° ,
the symmetry group is D3 with six symmetry operations.
If rotations around the C–C axis of one methyl group with
respect to the other one by 120° and 240° are included, the
number of symmetry operations increase to 18. Since the
initial arrangement is chiral, there exists another set of 18
enantiomorphic structures. Thus, the order of the isometric
group of ethane21 is 36. A different access to this group,
called G36 C , and more details are given in Ref. 22.
Omitting many details23 that are not important in the context of this article, the vibrational-torsion-rotational modes
of the electronic ground states may be characterized by
ji 4 Jkmi. The s are numbers 0, 1, 2, Ð Ð Ð, characterizing
the ground and excited vibrational states of the 17 real vibrations i and of the torsion mode 4.  stands for a representation
of the group G36 C , characterizing the symmetry of the torsion
state. J is the quantum number of the total angular momentum, k is the quantum number of the angular momentum
parallel to the axis of the symmetric top molecule H3 C–CH3 ,
and m is the component of total angular momentum parallel
to an axis defined in the laboratory frame.24
In our case of low temperatures and adsorbed molecules,
with rotations quenched, transitions start at the ground state,
given by i D 0, 4 D 0, J D k D m D 0. The Raman selection
rules yield for the Raman excited state: i D 0, 4 D 2, k D 0,
upper state D ground state . This corresponds to the origin of the
Q-branch of the free molecule, defined by J D 0 and J D 0. In
this case, the calculated torsional excitation wavenumbers24
for the different  differ only by about 5 cm1 . Because this
fine structure is not resolved, we describe the pure torsional
Raman band by 4 : 0 ! 2. (This corresponds to diatomic
molecules in the electronic ground state with the selection
rule of the pure rotational Raman effect25 of J D C2).
In the same way we may simplify the infrared case26 to
4 : 0 ! 1. The experimental values of free staggered ethane
in Table 2 show that the Raman wavenumber is much less
than double the infrared wavenumber, reflecting tunnelling
in the non-harmonic 3-well potential of symmetry C3 , given
staggered
V
1 C cos 6. For the adsorbed eclipsed species
by 3eff
2
eclipsed
staggered
will have the opposite pre-sign of V3eff
, but the
V3eff
Copyright  2006 John Wiley & Sons, Ltd.
absolute sign will be smaller because C2 H6 is forced by
adsorption into the eclipsed conformation. Accordingly, the
wavenumber of the torsional Raman band should be lower,
and might depend on the substrate metal (see Table 2).
Adsorption might also put more intensity into this band by
symmetry breaking.
An assignment to intermolecular vibrations is highly
unlikely because the observed Raman crystal lattice mode
wavenumbers14 are below 150 cm1 . The wavenumber of the
frustrated translation of ethane perpendicular to Cu(111) is7
only about 56 cm1 .
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J. Raman Spectrosc. 2006; 37: 1398–1402
DOI: 10.1002/jrs