Structural changes in the series of boron-carbon mixed clusters C(x)B(10-x)- (x = 3-10) upon substitution of boron by carbon.

We report a theoretical investigation on the ten-atom boron-carbon mixed clusters C(x)B(10-x)(-) (x = 3-10), revealing a molecular wheel to monocyclic ring and linear species structural change as a function of x upon increasing the number of carbon atoms in the studied series. The unbiased searches for the global minimum structures of the clusters with x ranging from 3 to 9 were conducted using the Coalescence Kick program for different spin multiplicities. Subsequent geometry optimizations with follow-up frequency calculations at the hybrid density functional B3LYP∕6-311+G(d) level of theory along with the single point coupled-cluster calculations (UCCSD(T)/aug-cc-pVTZ//B3LYP/6-311+G(d) and RCCSD(T)/aug-cc-pVTZ//B3LYP/6-311+G(d)) revealed that the C3B7(-) and C4B6(-) clusters possess planar distorted wheel-type structures with a single inner boron atom, similar to the recently reported CB9(-) and C2B8(-). Going from C5B5(-) to C9B(-) inclusive, monocyclic and ring-like structures are observed as the most stable ones on the PES. The first linear species in the presented series is found for the C10(-) cluster, which is almost isoenergetic with the one possessing a monocyclic geometry. The classical 2c-2e σ bonds are responsible for the peripheral bonding in both carbon- and boron-rich clusters, whereas multicenter σ bonding (nc-2e bonds with n > 2) on the inner fragments in boron-rich clusters is found to be the effective tool to describe their chemical bonding nature. It was shown that the structural transitions in the C(x)B(10-x)(-) series occur in part due to the preference of carbon to form localized bonds, which are found on the periphery of the clusters. Chemical bonding picture of C10(-) is explained on the basis of the geometrical structures of the C10 and C10(2-) clusters and their chemical bonding analyses.