© Copyright 2002 American Chemical Society
MAY 14, 2002 VOLUME 18, NUMBER 10
Letters The Vibrational Spectra of Methyl Groups in Methylthiolate and Methoxy Adsorbed on Cu(100) M. P. Andersson and P. Uvdal* Chemical Physics, Department of Chemistry, Lund University, PO Box 124, S-221 00 Lund, Sweden Received August 6, 2001. In Final Form: February 20, 2002 We show that the difference in the infrared vibrational spectrum of C-H stretch modes arising from the methyl group of two surface intermediates, methylthiolate and methoxy, originates from the difference in the methyl deformation modes frequencies of the two intermediates. Importantly, while the vibrational fingerprint of the two methyl groups are not transferable between the intermediates, the Fermi resonance coupling constants of the methyl group are.
Introduction The notion of spectroscopic fingerprinting of vibrational modes from different groups in molecules is essential in vibrational spectroscopy. The fingerprint or signature, that is the frequencies and the intensities, of a set of modes arising from a specific molecular group, CH3, CH2, NH2, etc is assumed to be transferable between molecules.1 As a consequence the fingerprint allows for an identification of various molecular groups of an unknown species. Assuming that the fingerprint notion is correct for surface adsorbates as well, it allows, for instance, for a rather straightforward determination of adsorbate orientation with respect to a metal surface using techniques as surface infrared and electron energy loss spectroscopies. The reason is the dipole selection rule for metal surfaces, which states that only modes having a component of their dynamic dipole moment perpendicular to the surface are dipole active. In addition the dynamic dipole moment component perpendicular to the surface has to be large enough to be observable. In terms of group theory this is the same as saying that only totally symmetric modes can * Corresponding author, e-mail:
[email protected]. Fax: + 46 46 222 41 19. Phone: +46 46 222 96 66 or +46 46 222 81 21. (1) Atkins, P. W. Physical Chemistry, 6th ed.; Oxford University Press: Oxford, Melbourne, Tokyo, 1998.
be observed. Some additional precautions are necessary in the analysis of Cs symmetric species. A′ (in plane) modes, in a Cs symmetric species, if oriented along the surface are unobservable even when exhibiting a sizable dynamic dipole moment. In the following we will demonstrate that this fingerprint notion is not always applicable to surface adsorbates. Indeed the vibrational spectrum of the C-H stretches of the methyl group of two such similar surface intermediates as methoxy and methylthiolate are quite different when adsorbed on Cu(100). The difference could, in principle, be caused by different coordination or orientation with respect to the copper surface. We will, however, show that this difference in the vibrational fingerprint in the C-H stretch region can be traced back to the different mode frequencies of the methyl deformation modes. The physical explanation is that the Fermi resonance coupling2 between the overtones of the deformation modes, δ(CH3), and the fundamental C-H stretch mode, νs(CH3), are quite different for the two species. Interestingly, this difference can be entirely understood by the difference in frequency of the fundamental deformation modes, δs(CH3) and δas(CH3), of the two species. In contrast and most importantly we will show that the Fermi resonance coupling constants between the overtones (2) Herzberg, G. Infrared and Raman Spectra of Polyatomic Molecules; Van Nostrand Reinhold Company, Inc.: New York, 1945; Vol. II.
10.1021/la011249c CCC: $22.00 © 2002 American Chemical Society Published on Web 04/18/2002
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Langmuir, Vol. 18, No. 10, 2002
Letters
of the deformation modes and the fundamental stretch mode of the methyl group can be assumed to be preserved for the two species. Hence, an important physical characteristic of the vibrational properties of the methyl group is indeed transferable between the two different species. Our approach is to combine experimental data, when available, with ab initio calculations of an appropriate complex representing the methylthiolate surface intermediate. On the basis of these data the Fermi resonance coupling is calculated, using previously determined coupling constants,3 and the resulting vibrational spectrum is determined. Experimental Section Experiments were conducted using a Bruker IFS 66v/S FTIR spectrometer connected to an ultrahigh vacuum chamber (base pressure 7 × 10-11 Torr) in which the Cu(100) sample was mounted. The crystal was oriented with an accuracy of
