Theoretical study of low-lying states in phosphorus-containing carbon chains PC 2 nP

Abstract

Density functional theory has been used to explore the geometries and vibrational frequencies of the ground states for linear chains PC 2 n P ( n=1–10). The vertical transition energies ( E T) for the 1 Σ u + 1 ← X Σ g + 1 transition in PC 2 n 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 PC 2 n 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 PC 2 n P is obtained. E T and IE of PC 2 n P are compared with those of linear NC 2 n N and pure carbon chains CC 2 n C, and they are explained reasonably.