Evaluation of Spartan semi-empirical molecular modeling software for calculations of molecules on surfaces: CO...

This paper reports the use of the PM3(tm) semi-empirical method in the Spartan molecular modeling software to optimize geometries and calculate vibrational frequencies for increasingly complex transition metal- and carbon monoxide (CO)-containing systems, culminating in calculations of CO adsorbed on a Ni(111) surface. Mononuclear and dinuclear transition metal carbonyl molecular species were used to establish the level of accuracy that could be expected for vibrational frequencies to provide a context for the results from the adsorbed molecule calculations. One to four CO molecules adsorbed on the (111) face of a 22-atom-nickel crystal were then modeled, and the accuracy of the adsorption geometry and vibrational frequency was evaluated. The calculated CO stretching vibrational frequencies were within 8% larger than the gas phase experimental values for the molecular species and were approximately 10% larger than the range of experimental values for CO on the nickel surface. The geometry optimization predicted that the CO molecules on the Ni(111) surface occupy three-fold hollow sites with no preference for sites over Ni atoms, in agreement with recent structural data and other theoretical calculations. The software was less successful in calculating the CO bond angle to the surface and the distance of the CO molecules from the surface, but the calculation did produce a reasonable distance between CO molecules on the surface. In general, the PM3(tm) method in Spartan shows promise for predicting adsorption sites and vibrational frequencies of molecules on metal surfaces.