New Predictions for Singlet-Trlplet Gaps of Substituted Carbenes

of niobium clusters with cyclohexene and 1,3-~yclohexadiene makes both the energy and the entropy contribution to the free energy even more favorable for the dehydrogenation reaction. These two factors, the lack of aromatic stability and the increased entropy change, appear to lessen the effect of the relative stability of the Nbs and Nb,,, structures on their reactivity with cyclohexene and 1,3-~yclohexadiene. There is no distinct minimum reactivity for these cluster sizes with cyclohexene and 1,3-~yclohexadiene as was observed for benzene. Before closing, we would like to point out some obvious difficulties with the interpretations used in this field in general: (1) As we pointed out p r e v i o ~ s l y , ~ ~ ~ it is difficult to differentiate between neutral thermal reactions (which is generally assumed) in the reactor and a one-photon photochemical reaction induced during the detection process (or a combination). (2) The internal temperature of these clusters before, during, or after the reaction is not known. (3) The ions formed in the plasma by the vaporization laser are not completely neutralized prior to their arrival at the reactor. Due to the large ion-molecule reaction cross section, small amounts of unneutralized ions could greatly contribute to the neutral distribution of the organometallic species, which upon ionization with the excimer laser can be detected. (4) Finally, the name “cluster” probably does not apply to these compounds, in particular when the number of atoms in a cluster is as small as it is in our system. All these compounds have relatively strong chemical bonds so they are actually molecules; for the pure metal clusters they are homonuclear polyatomics, and for their reaction products with organic compounds they are organometallic compounds. However, the word cluster here is useful in differentiating this type of work from other kinds in the field of chemical reactivity.