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Abstract: Structures of linear valence isoelectronic carbon chain anions C4O−, C4S− and C4Se− in their ground states have been investigated by density functional theory (DFT-B3LYP) and coupled cluster with single–double substitution (CCSD) approach. Complete-active-space-self-consistent-field (CASSCF) method has been used for geometry optimization of selected low-lying states. The vertical excitation energies from the ground state to selected low-lying excited states have been carried out with complete-active-space-second-order-perturbation-theory (CASPT2). All of them have a similar set of excited states. In comparison with available experimental observations, the predicted excitation energies for the allowed transitions have an accuracy of no more than 0.12eV. Moreover, the calculations here confirm that the previous assignments of the observed electronic absorption bands of C4O− and C4S− are reliable. The excitation energies of 22Π←X2Π and 12Σ−←X2Π transitions in C4Se− are predicted to be 2.09 and 3.90eV, with oscillator strengths 0.1941 and 0.0105, respectively, which may be observed in future experiment. CASPT2 also evaluated the vertical electron detachment energies for C4O−, C4S− and C4Se− to be 3.55, 3.40 and 3.40eV, respectively. [Copyright &y& Elsevier]

Abstract: Extensive theoretical studies on a series of linear phosphaalkynes HC2n+1P and NC2n P have been reported for the first time up to n=10. Density functional theory has been used to explore their equilibrium geometries. It shows a single and triple bond alternation structure for their ground states, in good agreement with the literature values for simple chains. Using time-dependent density functional theory, the vertical excitation energies for the X1Σ+→11Σ+ transition in HC2n+1P and NC2n P have been investigated. The first adiabatic and vertical ionization energies have been obtained by B3LYP method. On the basis of present calculations, the explicit nonlinear expressions for the size dependence of the electronic absorption wavelength (λ) and the first ionization energy (IE) have been suggested, from which we can obtain the values of arbitrary n and the limits for enough large n. Compared HC2n+1P with NC2n P and PC2n P, λ of HC2n+1P is longer than that of NC2n P, but shorter than that of PC2n P; while IE of HC2n+1P is lower than that of NC2n P, but higher than that of PC2n P. According to our study, the former may be caused by the electronic delocalization effect, and the latter may be caused by electronegativity. [Copyright &y& Elsevier]

Abstract: Density functional theory has been used to explore the geometries and vibrational frequencies of the ground states for linear chains PC2n P (n=1–10). The vertical transition energies (E T) for the transition in PC2n P have been studied by time-dependent density functional theory and CASPT2 method. Based on the present calculations, the explicit exponential expression for the size dependence of E T in linear carbon chains has been suggested. The complete active space self-consistent field approach has been employed to locate both the ground state and the selected excited states of PC2n P (n=1–5) for structure comparison. Theoretical investigations on relevant excited states reveal that distinct non-linear spectroscopic feature in such polyynes can be ascribed to difference in bonding between the ground and excited states. At the B3LYP/6-311+G* level, the first adiabatic and vertical ionization energies have been calculated. The dependence relationship between the ionization energies (IE) and the size in linear PC2n P is obtained. E T and IE of PC2n P are compared with those of linear NC2n N and pure carbon chains CC2n C, and they are explained reasonably. [Copyright &y& Elsevier]

Abstract: Optimized geometries of small carbon clusters C3Cl and its ions in their ground and selected excited states have been obtained by CASSCF approach. The vertical excitation energies for the electronic excitations to low-lying excited states have been calculated by the CASPT2 method using cc-pVTZ basis set. The predicted vertical excitation energies are in good agreement with available experimental values. The spin–orbit effect on the spin-forbidden transitions is generally small, with oscillator strengths in the magnitude of 10−7–10−13. The vertical emission energies for the selected excited states were calculated at CASPT2/cc-pVTZ level of theory based on the CASSCF optimized geometries of the corresponding excited states. [Copyright &y& Elsevier]

Abstract: In this work we report the structures and stabilities of linear carbon clusters HC2 n S (n =1–5) in their ground states using the B3LYP density functional. The rotational constants at the optimized geometries give excellent agreement with the experimental and previous theoretical values. The vertical excitation energies of the 22Π←X2Π transitions at the CASPT2 level are 3.16, 2.66, 2.05, 1.78, and 1.55eV, respectively, in good agreement with the corresponding observed values of 3.01, 2.48, 2.10, 1.84, and 1.65eV. Also, the exponential-decay curves for these vertical excitation energies obtained from experiments and theoretical calculations are illuminated. [Copyright &y& Elsevier]