Structure,Stability and Spectra of C_(2n)H_3 & C_(2n)H_3~+(n=4,5,6,7)

Density functional theory(DFT) has been used to investigate the geometries,vebrational frequencies of C_(2n)H_3 and C_(2n)H~+_3(n=4,5,6,7).Vertical electronic transition energies of C_(2n)H_3 and C_(2n)H~+_3(n=4,5,6,7) are calculated by the time-dependent density functional theory(TD-DFT).In addition,the adiabatic ionization potential and vertical ionization potential of C_(2n)H_3(n=4,5,6,7) are obtained.The results show that the ground state structures of C_(2n)H_3 and C_(2n)H~+_3(n=4,5,6,7) are same.The structure is open chain with C_2_v symmetry showing polyacetylenic structure,the number of carbon atom of which is 8 and 10.The structure is single ring with Cs symmetry,the number of carbon atom of which is 12 and 14.The stability of C_(2n)H_3 and C_(2n)H~+_3(n=4,5,6,7) increase as the number of carbon atoms increase,respectively.The lowest frequency origins in the 30cm~(-1)～60cm~(-1) of C_(2n)H_3 and C_(2n)H~+_3(n=4,5,6,7) which show polyacetylenic structure.Theoretical investigations of vertical transition energies and ionization potentials found that C_(2n)H_3(n=4,5,6,7) has the large ionization potential(IP).It is opposite of the report for C_(2n+1)H_3.And the electron of C_(2n)H_3(n=4,5,6,7) is difficult to transfer and lose an electron.So the ground state isomer of C_(2n)H_3(n=4,5,6,7) radical is stable than that of C_(2n)H~+_3(n=4,5,6,7) cation.