Structure of molybdenum and tungsten sulfide M(x)S(y)+ clusters: experiment and DFT calculations

A combination of experiment and density functional theory was used to investigate the energetics of CO adsorption onto several small M(x)S(y)(+) (M = Mo, W; x/y = 2/6, 3/7, 5/7, 6/8) clusters as a probe of their atomic and electronic structure. Experimentally, tandem mass spectrometry was used to measure the relative yields of M(x)S(y)(+)(CO)(n) cluster adducts formed by collisions between a beam of mass-selected M(x)S(y)(+) cluster ions and CO molecules in a high-pressure collision cell (hexapole ion guide). The most probable M(x)S(y)(+)(CO)(n) adducts observed are those with n < or = x, that is, only one CO molecule bound to each metal site. The notable exception is the M(5)S(7)(+) cluster, for which the n = 6 adduct is found to have nearly the same intensity as the n = x = 5 adduct. Density functional calculations were used to search for the lowest energy structures of the bare M(x)S(y)(+) clusters and to obtain their relative stability for sequential CO binding. The calculated trends in CO binding energies were then compared to the experimental adduct distributions for assigning the ground-state structures. In this way, it was possible to distinguish between two nearly isoenergetic ground-state isomers for the M(2)S(6)(+) and M(3)S(7)(+) clusters, as only one isomer gave a calculated CO stabilization energy trend that was consistent with the experimental data. Similar comparisons of predicted and observed CO adsorption trends also provide evidence for assigning the ground-state structures of the M(5)S(7)(+) and M(6)S(8)(+) clusters. The latter contain metallic cores with most of the sulfur atoms bonded along the edges or in the faces of the metal core structure. The n = 6 and 7 adducts of M(5)S(7)(+) are predicted to be more stable than the n = x = 5 adduct, but only the n = 6 adduct is observed experimentally. The DFT calculations show that the n = 7 adduct undergoes internal bond breaking whereas the n = 6 framework is stable, albeit highly distorted. For the M(6)S(8)(+) cluster, the calculations predict that the two lowest energy isomers can bind more than six CO molecules without fragmentation; however, the apparent binding energy drops significantly for adducts with n > 6. In general, the ability of these small M(x)S(y)(+) clusters to bind more CO molecules than the number of metal atoms is a balance between the gain in CO adsorption energy versus the strain introduced into the cluster structure caused by CO crowding, the consequences of which can be fragmentation of the M(x)S(y)(+)(CO)(n) cluster adduct (n > x).