Using density functional theory, geometries and vibrational frequencies of linear chains NC2nN and HC2n＋1N （n = 1 - 10） have been investigated. Time-dependent density functional theory （TD-DFF） has been used to calculate the vertical transition energies and oscillator strengths for the x^1∑g^＋→I^1∑u^＋ transition in NC2,N （n = 1 -10） and X^1∑ → I^1∑^＋ transition in HC2n＋1N （n =1 -7）. On the basis of present calculations, the explicit expressions for the size dependence of the excitation energy and the first adiabatic ionization energy in both carbon chains have been suggested.
