Theoretical studies on the structures, electronic spectra and ionization energies of linear isoelectronic HC2n+1P and NC2nP (n=1-10)

Extensive theoretical studies on a series of linear phosphaalkynes HC2n+1P and NC2nP 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 NC2nP 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 NC2nP and PC2nP, λ of HC2n+1P is longer than that of NC2nP, but shorter than that of PC2nP; while IE of HC2n+1P is lower than that of NC2nP, but higher than that of PC2nP. According to our study, the former may be caused by the electronic delocalization effect, and the latter may be caused by electronegativity.