STRUCTURE AND STABILITY OF LANTHANUM-CARBON CATIONS

A theoretical study has been carried out to determine the stability of various isomers of the lanthanum(-) carbon clusters LaCx+. Included i n this study are linear isomers and several classes of cyclic and grap hitic structures. Geometries and energies of the proposed structures a re calculated using the B3LYP density functional method. Cyclic isomer s are clearly established as more stable than linear isomers over a wi de range of cluster sizes. The results reveal that two classes of cycl ic isomers, which we denote ''open rings'' and ''closed rings'', have a pattern of relative thermodynamic stability that correlates to the p attern of cluster intensities in the ion mobility observations of the LaCx+ clustering process. The competitive factors that provide stabili ty to lanthanum-carbon clusters are elucidated, such as ring strain an d variations in the lanthanum bonding environment. Graphitic LaCx+ wit h 18 or 20 carbon atoms are shown to be more stable than their cyclic counterparts, and B3LYP is shown to underestimate the stability of gra phitic LaCx+.