More Magic Numbers in Titanium-Carbon Anion Clusters: Can This Be Explained by Any Structural Growth Pattern?

Titanitum-carbon anion clusters with a mass range up to 1800 amu have been generated in a Smalley-type cluster source involving a target rod composed of titanium and graphite powders, and have been analysed by time-of-flight mass spectrometry. In the lower mass range (up to 1200 amu), our mass spectrum reproduces all the magic numbers, which have been reported previously, i.e. Ti_(3)C_(8)^(-), Ti_(6)C_(13)^(-), Ti_(7)C_(13)^(-), Ti_(9)C_(15)^(-), Ti_(13)C_(22)^(-), and Ti_(14)C_(24)^(-), in spite of many differences in experimental conditions. In the higher mass range (1200-1800 amu), we observe new magic numbers at Ti_(15)C_(26)^(-), Ti_(20)C_(33)^(-)/Ti_(20)C_(34)^(-) and Ti_(25)C_(42)^(-), which cannot be accounted for by the previously proposed structural growth patterns. All the magic and local maximum Ti_(x)C_(y) peaks have a metal-to-carbon ratio of approximately 1:1.7. The present results suggest a further complexity to the structural growth patterns of the transition metal-carbon mixed cluster systems.