PULSED-LASER EVAPORATION OF BORON CARBON PELLETS - INFRARED-SPECTRA AND QUANTUM-CHEMICAL STRUCTURES AND FREQUENCIES FOR BC2

Pulsed laser evaporation of pellets pressed from boron and graphite po wder gave a new 1:4 doublet at 1232.5 and 1194.6 cm-1 in addition to t he carbon cluster absorptions reported previously. The 1232.5 cm-1 ban d dominated boron-10 experiments. The new bands increased as carbon cl uster bands decreased with increasing B/C ratio in the pellet and with increasing laser power. Augmented coupled cluster and full-valence co mplete active space SCF (CASSCF) calculations predict the global minim um BC2 Structure to be an asymmetric triangle: however, the vibrationa lly averaged structure will be an isosceles triangle with a strong sym metric B-C2 stretching frequency near 1200 cm-1. The calculated boron- 10/boron-11 frequency ratio (1.0323) is in excellent agreement with th e observed ratio (1.0317), and confirms assignment of the 1194.6 cm-1 band to the BC2 ring. Calculations predict linear BCC to be less stabl e by 6.2 +/- 2 kcal/mol and to absorb in the 2000-2050 cm-1 range: the barrier towards rearrangement to the cyclic structure is very low (1. 1 kcal/mol). Linear BCC was not detected in these experiments. Compute d energetics explain why BC2 is abundant in B/C experiments, but absen t in B + C2H2 experiments.