Structural and energetic mapping of HC₄N and HC₄N^-: a new anion geometry

We present a systematic theoretical exploration of the isomeric landscape of neutral HC₄N and its anion HC₄N^-, motivated by their astrochemical relevance and by the limited structural information available so far for HC₄N^-. An initial stochastic DFT structural search identified more than 170 distinct minima across the singlet and triplet neutral surfaces and the doublet and quartet anionic surfaces. The energetics of the key low-lying structures were refined using RCCSD(T)-F12 and multireference AQCC and MRCI(Q) calculations. For neutral HC₄N, the bent singlet n.S.1 is confirmed as the lowest isomer, with the linear triplet n.T.1 lying 13.74 kJ mol^-1 higher at the RCCSD(T)-F12 optimized-geometry level. For HC₄N^-, we identify a previously unreported folded-chain doublet anion with a central hydrogen atom, a.D.1, as the lowest-energy anionic form. The terminal-hydrogen isomer a.D.2, corresponding to the structure used in the previous interpretation of the photoelectron spectrum, remains the second-lowest anion, 10.61 kJ mol^-1 above a.D.1. AQCC/CBS calculations reproduce the experimental electron affinity for the a.D.2 → n.T.1 detachment channel, giving 2.06 eV, while the adiabatic electron affinity between the lowest neutral and anion, n.S.1/a.D.1, is 1.93 eV. Finite-field DLPNO-CCSD(T)-F12D calculations give permanent dipole moments of 4.42 D for a.D.1 and 3.51 D for a.D.2, indicating that both low-lying anions, if formed, should have appreciable rotational intensities. No bound valence excited states of anions were found, although very weak dipole-bound states were predicted near the detachment threshold.