Ab initio prediction of the spectra of carbon cumulenes Original Research Article

Ground state calculations of the optimized geometries and vibrational frequencies of the carbon cumulenes, H2Cn (n = 3–15), have been variously carried out at HF/6-31G(d), MP2/6-31G(d,p) and CASSCF/6-31G(d) levels, both with and without the constraint of linearity of the carbon chains. Substantial reductions in the A rotational constant are obtained when the constraints to linearity is removed, in agreement with the trend observed for the short cumulenes for which experimental results are available. Excited state calculations at the CIS/6-31G(d) level have focussed mainly on the intense 1A1 excited state (the third 1A1 for n > 5). Also, in these calculations the tendency for non-linearity of the carbon chain has been noted, although energy differences are smaller. CASSCF calculations have been restricted to the two smallest cumulenes. They are in good agreement with the CIS calculations for the first excited 1A1 and 1A2 states, but differ for the higher 1A1 states. Empirical corrections to the CIS-calculated transition energies for the intense state have been introduced through comparisons with the linear carbon chain molecules, Cn, for which both observed results and equivalent accuracy ab initio computations are available. Cumulenes with sizes H2C13 to about H2C34 have their first strong electronic transition in the 400–860 nm DIB range.