J. Phys. Chem. 1981, 85,2597-2598
sible (Figure 3) picture emerges. The presence of the p-Br substituent in the benzylidene moiety (contrast 5 and 6 on one hand with 3 and 4 on the other) means that any surplus or deficiency of charge on the carbon atoms of the benzyl moiety will contribute significantly to the electrostatic interaction involving the p-Br of the benzylidene. CH3 and C1 substitutents differ from one another electrostatically in the sense that the former is likely to provide surplus charge to and the latter to extract charge from the carbon atoms of the benzyl group. The electrostatic interaction between the charges on the carbon atoms and the Br atom of the benzylidene adjunct results in a puckering (see Figure 2) of molecules of 5, a feature which is absent in 6. We see, therefore, how minor changes in the extent of nonbonded interactions profoundly modify the photoreactivity of the solid. Moreover, by extending the arguments that have emerged from this study, other subtle
2597
substitutions should, with profit, be considered. Thus NHz and NOz groups, in view of their comparable geometricla but contrasting electronic influences, when placed in para positions in either the benzyl or benzylidene moieties of BBCP should yield patterns of photoreactivity that are predictable on the basis of this work. Acknowledgment. We acknowledge the support of the Science Research Council, and valuable discussions with Mr. Simon Kearsley, Dr. A. J. Kirby, and Drs. M. B. Hursthouse and M. Motevalli. (18) From ref 9, 20 the “incremental sizes” are NO2 (23.0 A9);NH2 (19.7 A3); C1 (19.9 l3);.Me (23.5 A3) (19) C. R. Theocharis, H. Nakanishi, W. Jones, M. B. Hursthouse, and M. Motevalli, to be submitted for publication. (20) C. R. Theocharis, H. Nakanishi, and W. Jones, Acta Crystallogr., Sect. B , 37, 756 (1981). (21) C. R. Theocharis, W. Jones, M. B. Hursthouse, and M. Motevalli, manuscript in preparation.
Metal Surface Spectroscopy. Charge Transfer and Totally Symmetric Mode Activity J. Paul Deviln’ and Keith Consanl Depafiment Of Chemistv, Oklahoma State Universm, SMwater, Oklahoma 74078 (Received May 26, 198 7; In Final Form: Ju/y 13, 1981)
For metal surface spectroscopic studies of adsorbates which participate in a charge-transfer interaction with the metal surface the interpretation of infrared and, perhaps, Raman data must recognize the possibility of strong electron density oscillations between the molecules and the surface regardless of the orientation of the adsorbate molecule. TotaUy symmetric (adsorbate)modes produce such oscillations which dominate the infrared spectra of charge-transfer systems, and which will be significant in SERS as well should the modulation of surface charge density prove to be a contributing factor. There is little question that many ions and molecules adsorbed on metals engage in a significant charge-transfer interaction and, based on information for molecular complexes, only minor molecular distortion of the adsorbate would normally be expected to occur. Just recently, one of the mechanisms advanced as explanation for surfaceenhanced Raman scattering proposed just such a chargetransfer interactions1 For the specific case of pyridine adsorbed on silver, other researchers have also presumed that the primary interaction is charge transfer.2 However, if we accept the premise that charge transfer may be an important factor in a particular adsorbate-metal interaction, a serious modification of the IR selection rules must then be recognized-a modification that has been either ignored or deliberately ruled out in the major papers on the subject of SERS. It is generally accepted by workers in the field (and by the present authors) that the image charge model for IR reflection-adsorption at a metal surface applies in the sense that only molecular modes which produce a dipole oscillating perpendicularly to the surface (2 coordinate) can be IR a ~ t i v e . However, ~ this rule has been erroneously generalized by statements which (1)E. Burstein, Y. J. Chen, C. Y. Chen, S. Lundquist, and E. Tosatti, Solid State Commun., 29, 567 (1979).
(2) F. W. King, R. P. Van D u p e , and G . C. Schatz, J. Chem. Phys.,
69. 4472 (1978). - - !
~~
~
~~~
-I
(3) See, for example, R. M. Hexter and M. G . Albrecht, Spectrochim. Acta, Part A , 35, 233 (1978). 0022-3654/81/2085-2597$01.25/0
emphasize that only modes for which there is a significant motion by the atoms of the absorbate along Z can be strongly infrared a ~ t i v e . ~ This - ~ generalization is contradictory to a well-established general principle for the infrared activation of totally symmetric molecular modes through vibronic coupling with charge-transfer excited states. Specifically, it is a unique characteristic of the infrared spectroscopy of CT complexes that the dominant infrared activity is often the result of dipole oscillations which can be perpendicular to the atomic displacements. These dipole oscillations have their origin in an electron density oscillation produced via a vibronic coupling of the molecular modes (particularly the totally symmetric modes) with the CT excitation. This situation has been thoroughly described in t h e ~ r y , ~ which J indicates that the effect is dependent upon a variation in the vertical ionization potential (or electron affinity) with respect to a displacement in a normal coordinate of the donor (or acceptor). The theory has been confirmed by experiments on oriented single crystals of both and dative CT complexes.& (4) M. Moskovits and D. P. DiLella, J. Chem. Phys., 73,6068 (1980). (5) N. Sheppard and T. T. Nguyen in “Advances in Infrared and Raman Spectroscopy”, Vol. 5, R. J. H. Clark and R. E. Hester, Ed., Heyden, London, 1978, Chapter 2. (6) (a) E. E. Ferguson and F. A. Matson, J. Am. Chem. SOC.,82,3268 (1960); (b) E. E. Ferguson, J. Chim. Phys., 61, 257 (1964). (7) H. B. Friedrich and W. B. Person, J.Chem. Phys., 44, 2161 (1966).
0 1981 American Chemical Society
2598
The Journal of Physical Chemistty, Vol. 85,No. 18, 1981
In the latter case, the oscillator strengths for modes activated by the CT electron oscillation mechanism may exceed that of the other IR-active modes by an order of magnitude.sd In a recent paper in which the argument is presented that the optical selection rules for metal surfaces require recognition of the effect of the large electric field gradient at the adsorbate, Moskovits et al. have derived selection rules that include, for example, IR activation of the ag modes for D,,,, molecules? This proposal only strengthens the need to recognize that such modes may also be activated for flatly (or otherwise) adsorbed Dnh molecules through the CT (perpendicular) charge-oscillation mechanism. It cannot be stressed too strongly that, for a wide range of known cases for which charge transfer is important, it is not possible to reasonably interpret the infrared spectra without appreciating that, regardless of molecular distortion or orientation, charge oscillation between the donor and the acceptor, oftentimes perpendicularly to the direction of atomic displacement, strongly activates the totally symmetric normal modes. Since there is no reason to expect the same mechanism to be unimportant for CT interactions between adsorbates and metal surfaces, the possibility should be considered in any analysis. This has not been the case so, without a more universal appreciation of this fact, the systematic and logical testing of models will be incomplete. Though the emphasis here has been on reflection-absorption measurements, the principle applies as well to expectations for electron energy loss (ELS) and inelastic electron tunneling spectroscopic measurements. For ex(8)(a) This area is reviewed by R. E. Hester in “Advances in Infrared and Raman Spectroscopy”, Vol. 4, R. J. H. Clark and R. E. Hester, Ed., Heyden, London, 1978,Chapter 1. See also (b) B. Hall and J. P. Devlin, J. Phys. Chern., 71,465(1967);B.Moszynska and A. Tramer, J. Chern. Phys., 46,820 (1967); (c) G. R. Anderson and J. P. Devlin, J. Phys. Chern., 79,1100(1975);(d) J. J. Hinkel and J. P. Devlin, J. Chern. Phys., 58, 4750 (1973). (9)J. K. Sass, H. Neff, M. Moskovits, and S. Holloway, J. Phys. Chern., 85, 621 (1981).
Letters
ample, the bands observed by ELS at 1428 and 1275 cm-l for C2H2and C2D2adsorbed on Pt(1ll)’O could be the multiple carbon-carbon bond mode activated by the CT charge oscillation mechanism after a weakening of the triple bond via metal reduction. Any complete analysis must recognize that possibility. Finally, with respect to SERS itself, charge transfer has been included as a part of a modulated reflectance theory suggested by McCall and P1atzman.l’ However, those authors again stressed that the important atomic displacement is that of the bound atom(s) along the normal to the surface. Such a displacement would be totally unnecessary for a sizeable modulation of the surface charge density by the molecular vibrations of an adsorbate molecule, should charge transfer be important. Any mode that changes the vertical ionization potential (or electron affmity) of the adsorbate moiety will produce an oscillating dipole along 2,by actually transferring electron density to and from the surface in unison with an adsorbate vibration. This will result in a surface charge density modulation irrespective of the orientation of the molecule with respect to the surface. This surface charge density oscillation could then couple with laser radiation, as in other mechanisms of SERS.
Note Added in Proof. Since journal acceptance of this Letter, a paper by A. G. Mal‘shukov (Solid State Commun. 1981, 38, 907) has appeared, which emphasizes, from a different perspective, the importance of electron density oscillations between absorbate and metal surfaces, and, similarly, Dilella and Moskovits have referred (ref 13 of J . Phys. Chem. 1981, 85, 2042) to a paper, in press, in which the possible SERS role of absorbate-surface charge oscillation is described. Acknowledgment. This research was supported by NSF Grant CHE 79-25567. (10)H. Ibach, M.Hopster, and B. Sexton, Appl. Surf. Sci., 1,l (1977). (11)S.L.McCall and P. M. Platzman, Phys. Rev. B , 22,1660(1980).
