Molecular clusters charge-transfer interactions

At shorter distances, particularly those characteristic of H-bonded and other charge-transfer complexes, the concepts of partial covalency, resonance, and chemical forces must be extended to intramolecular species. In such cases the distinction between, e.g., the covalent bond and the H-bond may become completely arbitrary. The concept of supramolecular clusters as fundamental chemical units presents challenges both to theory and to standard methods of structural characterization. Fortunately, the quantal theory of donor-acceptor interactions follows parallel lines for intramolecular and intermolecular cases, allowing seamless description of molecular and supramolecular bonding in a unified conceptual framework. In this sense, supramolecular aggregation under ambient thermal conditions should be considered a true chemical phenomenon. 

The other mechanism involves atomic-size roughness (i.e., single adatoms or small adatom clusters), and is caused by electronic transitions between the metal and the adsorbate. One of the possible mechanisms, photoassisted metal to adsorbate charge transfer, is illustrated in Fig. 15.4. It depends on the presence of a vacant, broadened adsorbate orbital above the Fermi level of the metal (cf. Chapter 3). In this process the incident photon of frequency cjq excites an electron in the metal, which subsequently undergoes a virtual transition to the adsorbate orbital, where it excites a molecular vibration of frequency lj. When the electron returns to the Fermi level of the metal, a photon of frequency (u>o — us) is emitted. The presence of the metal adatoms enhances the metal-adsorbate interaction, and hence increases the cross... 

It should be noted that broad emission spectra are also observed in some cases for molecular dimers and larger aggregates, e.g., in the case of naphthalene [11]. The excited state in these homogeneous clusters is characterized as an excimer, and the interaction between the two components is mainly due to exchange interactions, although the contribution from charge-transfer states such as A A " is also important [12, 13]. 

In the case of Ni2P(001), the Ni—>P charge transfer is not large (<0.1 e) [15] and the surface has a substantial number of Ni atoms. Clusters of three Ni atoms are present (Fig. 6.8), and the separation between these clusters is not large enough to prevent effective bonding interactions with a relatively big molecule like thiophene. At 100 K molecular adsorption of C H S on Ni2P(001) occurs, but at temperatures above 200 K the surface is able to crack the C-S bonds of the adsorbate [15]. Similar results have been found for the interaction of thiophene with Ni P/SiOj catalysts [32]. 

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