Diatomic molecules comparison with metals

More recently the application of sub-picosecond, time-resolved pump-probe methods revealed the timescale for vibrational relaxation of a diatomic molecule at a metal surface directly. See for example Refs. 19-21. In comparison to vibrational relaxation on NaCl salts,22 which occurs on the millisecond timescale, another relaxation mechanism is clearly at play. Theory of vibrational relaxation based on excitation of electron-hole pairs gave agreement with observed ps timescales for CO on copper.23... 

It is this resonance energy that would be in the main responsible for the difference in energy of the crystal and the gas of diatomic molecules Li2. But the heat of formation of Li2 molecules from atoms is only 6-6 kcal./g.-atom, whereas that of the metal is 39kcal./g.-atom. It seems unlikely, by comparison for example with the analogous case of Kekule-like resonance in aromatic molecules, that the great difference, 32-4 kcal./g.-atom, could result from the synchronized resonance, of type f Li—Li Li Li)... 

A comparison of EH and CNDO with experimental data has been made by Baetzold (30) for other metal homonuclear diatomic molecules. This work has employed the orbital exponents of Clementi et al. (10,11) and experimental atomic data for ionization potentials. Table III lists representative data for transition metal molecules calculated by CNDO and EH. No one procedure is universally superior to another. 

A comparison of data calculated by EH and CNDO with experimental data for metal homonuclear diatomic molecules has been made by Baetzold (30). Employing the input data of Table I leads to the data compiled in Table III. Calculated binding energies, excitation energies, and ionization potentials generally agree better with experiment than calculated bond lengths or vibration frequency. The observation of lower ionization potential for Ag2 than for Ag (also Cu2, Au2) is predicted by CNDO but not by EH. 

The issue of reliability of approximate MO theory is complex. The work presented here tends to support its validity as a means of explaining and predicting phenomena. For example, the calculated electronic properties of diatomic molecules and CuCl clusters in Section II.F.2 agree well with experiment. The properties of metal clusters are reasonable in light of the small amount of experimental data available on them. The chemisorption calculations in Section IV all support the available experimental evidence. More critical comparisons of experiment and theory are necessary to establish the degree of usefulness of approximate MO theory. 

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