Theoretical study of low-lying states in phosphorus-containing carbon chains PC2nP

Density functional theory has been used to explore the geometries and vibrational frequencies of the ground states for linear chains PC2nP (n=1-10). The vertical transition energies (ET) for the 1Σu+1←XΣg+1 transition in PC2nP 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 ET 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 PC2nP (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 PC2nP is obtained. ET and IE of PC2nP are compared with those of linear NC2nN and pure carbon chains CC2nC, and they are explained reasonably.