Staggered ethane changes to eclipsed conformation upon adsorption

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1 JOURNAL OF RAMAN SPECTROSCOPY J. Raman Spectrosc. 2006; 37: Published online 9 June 2006 in Wiley InterScience ( Staggered ethane changes to eclipsed conformation upon adsorption A. Priebe, 1 A. Pucci, 1 W. Akemann, 2 H. Grabhorn 2 and A. Otto 2 1 Kirchhoff-Institut für Physik, Ruprecht-Karls-Universität, Im Neuenheimer Feld 227, D Heidelberg, Germany 2 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 C 2 H 6 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 Cbond. 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 H 3 C CH 3 (ethane). It has a moderate barrier of about 1000 cm 1 to the internal rotation around the C C axis (angle 2 between the two methyl groups CH 3 ). 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 D 3d andtheeclipsedonewith D 0andpointgroupD 3h. 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 G 36 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 G 36 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. Otto@uni-duesseldorf.de 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.

2 Conformation changes of ethane upon adsorption 1399 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 Ethane, D3d (staggered) representation of vibrations Herzberg table 104 Gas data Near normal IR reflectance of solid ethane at 25 K Raman spectra of solid ethane at 25 K Raman spectra, ice film on Ag Adsorbed ethane D3h, (eclipsed) Herzberg Table 104 SERS 10 c.d.ag SERS, 1 c.d.-ag at 40 K SERS, 1 c.d.-k SERS d 1 c.d.-in IRRAS e,1.5l on c.d.-cu at 50 K Reference a b b c c d d d e eu HbendingIR-s , 816, 813 e H bending IR-s, Rdepol (at and above 3.8 L) a1g C C Stretch, R pol , a1 H C C stretch, R pol eg Bending R depol , 1192, e H CH3 bending R depol a2u H CH3 deform. IR-p , 1368 a2 H CH3 deform IR-p 1368? 1392? No first layer signal a1g CH3 Deform. R pol a1 HCH3 deform. R pol. 1307? 1364? 1380? 1304?? 1358? 1392? eu HCH3 deform. IR-s , 1458, 1456, 1450 e H CH3 deform. 1452? 1446? 1462? 1457? 1458 (first layer signal) IR-s R depol. eg CH3 deform. R depol , 1451, 1447, e H CH3 deform. R depol 1452? 1446? ? 1457? 26 origin of Q-band, R g ? 2755? ca 2724 ca 2706 a2u HC HstretchIR-p a2 HCHstretchIR-p Nofirstlayersignal a1g C H Stretch R pol FR f a1 H CH stretch R pol eg C H Stretch R depol e H CH stretch R depol eu HC HstretchIR-s e H C H stretch (first layer signal) IR-s R depol

3 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 hyper- Raman spectroscopy 10 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 experiment 5 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 rule 13 also holds in this case. Adsorbed ethane is eclipsed The Raman selection rules of C 2 H 6 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 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 1Lc.-d.In40K IRRAS 4.9 L c.-d. Cu 50 K Torsion Torsion š š 20 Continuum Out of range Reference a b c d This work e e e f (a) (b) Figure 1. Raman spectra of ethane on silver; laser wavelength nm. (a) thick silver film deposited at room temperature, exposed to 6500 L C 2 H 6, (b) 100 nm silver film, cold-deposited, and exposed to 10 L C 2 H 6.

4 Conformation changes of ethane upon adsorption 1401 Figure 2. SERS spectra of C 2 H 6 on cold-deposited films (40 K) of potassium, silver, and indium, laser wavelength 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 ethane 19 varies from 30 to 50. Together with the electromagnetic field enhancement of about at the surface of cold-deposited silver films the overall enhancement is about For ethane, one does not need to invoke the so-called SERS-active sites. 20 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 cm 1. 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 cm 1 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

5 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, CH 3,haveC 3 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 D 3 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 ethane 21 is 36. A different access to this group, called G C 36, and more details are given in Ref. 22. Omitting many details 23 that are not important in the context of this article, the vibrational-torsion-rotational modes of the electronic ground states may be characterized by j i 4 0Jkmi. Thes arenumbers0,1,2,ððð, characterizing the ground and excited vibrational states of the 17 real vibrations i and of the torsion mode 4. 0 stands for a representation of the group G C 36, 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 H 3 C CH 3, 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, 0 upper state D 0 ground state. This corresponds to the origin of the Q-branch of the free molecule, defined by 1J D 0andJ D 0. In this case, the calculated torsional excitation wavenumbers 24 for the different 0 differ only by about 5 cm 1. 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 effect 25 of 1J D C2). In the same way we may simplify the infrared case 26 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 C 3,given by Vstaggered 3eff 1 C cos 6. For the adsorbed eclipsed species 2 V eclipsed 3eff will have the opposite pre-sign of V staggered 3eff,butthe absolute sign will be smaller because C 2 H 6 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 wavenumbers 14 are below 150 cm 1. The wavenumber of the frustrated translation of ethane perpendicular to Cu(111) is 7 only about 56 cm 1. REFERENCES 1. 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