Abstract: | Density functional theory has been used to investigate the geometries, bonding, and vibrational frequencies of HC2nH (n = 1–13) and HC2n+1H (n = 2–12). Vertical excitation energies for the X1Σ → 11Σ transition of HC2nH (n = 1–5) and for the X3Σ → 13Σ transition of HC2n+1H (n = 2–5) have been calculated by the time‐dependent density functional theory and ab initio second‐order multiconfiguration perturbation method, respectively. On the basis of the present calculations, explicit expressions for the size dependence of excitation energy in linear polyynes HC2n+1H and HC2n+1H are suggested. Such analytical λ ? n relationships show good agreement with experimental observations. Theoretical investigations of relevant excited states demonstrate that distinct linear and nonlinear spectroscopic features in such polyynes can be ascribed to similarity and difference in bonding between the ground and excited states in HC2n+1H and HC2nH. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004 |