On the equilibrium structures and the IR active bending vibrations of linear C13 and C15: results of large-scale coupled cluster calculations

Accurate equilibrium (r_e) structures (ca. 0.0005 Å accuracy in bond lengths) have been established for linear C₁₃ and C₁₅ by applying a uniform correction to the results of CCSD(T) calculations with the cc-pVQZ basis set. The equilibrium bond lengths cover a small range between 1.2690 and 1.2928 Å and are indicative of strong carbon–carbon double bonds. Equilibrium structures of still longer chains may be obtained by taking the recommended r_e structure for C₁₅ and inserting one or more C₂ links with R_e = 1.277 Å in the middle of the molecule. Both linear C₁₃ and C₁₅ exhibit no sign of floppiness and appear to behave like fairly normal semi-rigid molecules. Diagonal potentials for the IR active bending vibrations of both molecules have large correlation contributions between -33 and 47%, with MP2 strongly overshooting with respect to CCSD(T). Harmonic cis-bending vibrational wavenumbers and their absolute IR intensities are predicted. Since the latter are rather small, the chances of an interstellar detection of linear C₁₃ or C₁₅ in the far IR may be poor.