Surface-adsorbed carbon monoxide, stretching frequency

The red shift of the CO stretch vibrational frequency due to the static interaction between the neighboring adsorbed COs seems to be abnormal, as their repulsive, static dipole-dipole interaction would push electron charge away from the negative charged CO, mostly from the anti-bonding 27t orbitals, into the metal surface, leading to a strengthen CO bond and, therefore, a blue frequency shift. It is found that this red shift of the CO stretch vibrational frequency is caused mainly by the interaction between the Cu 4sp band electrons and the 27t orbitals of the adsorbed carbon monoxides, as discussed below. 

What is interesting in our calculated results is that a negative chemical shift of the stretch vibrational frequency of carbon monoxide adsorbed on the Cu(lOO) surface is obtained from the first principle DFT calculation. It comes from the abnormal negative frequency shift due to the static interaction between the adsorbed carbon monoxides. 

One important catalytic reaction cycle which starts from a primary gas mixture of carbon monoxide and hydrogen is the Eischer—Tropsch synthesis. Depending on the reaction parameters (temperature of the catalytic surface, gas pressure and composition of the gas mixture) a great variety of aliphatic, aromatic and even oxygen-containing compounds can be obtained. The understanding of reaction mechanisms in terms of the appearance of intermediates on the surface, their structure and symmetry, is of fundamental interest for the development of well-defined reaction pathways. The frequency of the C—H stretching Raman band is a measure of the state of hybridization of the adsorbed molecule. 

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