Theoretical studies on structures and electronic spectra of linear free radicals CnH (n = 5–12)

The B3LYP, CAM-B3LYP, and coupled cluster CCSD(T) calculations have been utilized to determine the equilibrium structures of linear carbon radicals CnH (n = 5–12) in their ground states, as well as the CASSCF method used to optimize the ground and selected low-lying excited states. DFT-calculations show that even-n radicals C2nH have polyacetylene-like structures with significant single-triple bond length alternation, whereas the odd-numbered analogues C2n+1H exhibit a trend from polyacetylene-like characters into cumulenic-like arrangement towards C ends along the carbon chains. The stabilities of the system under study have been evaluated by analyses of the vibrational frequencies and incremental binding energies. For the whole CnH (n = 5–12) series, the vertical excitation energies and oscillator strengths have been calculated at the CASPT2/cc-pVTZ level of theory. At the B3LYP optimized geometries, the lowest 12Δ ← X2Π transitions for C5H and C7H occur at 2.36 and 2.14 eV, respectively, comparing well with the observed values of 2.33 and 2.09 eV. Moreover, the strongest 22Π ← X2Π transitions for C2nH (n = 3–6) are predicted to be at 2.39, 2.00, 1.80, and 1.64 eV, respectively, which are in agreement with the experimental observations. Additionally, the possible dissociation channels and the fragmentation energies of CnH (n = 5–12) series are discussed in the paper.

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► Properties of ground- and excited-states for CnH (n = 5–12) radicals are discussed.
► Even-n radicals are more stable than odd-n analogues with the parity alternation.
► A comparison between the CASPT2-predicted vertical excitation energies and experimental observations has been performed.
► Similarly non-linear ΔE–n relationships are found in different transition systems.