Coupled cluster calculations of the vertical excitation energies of tetracyanoethylene

Abstract

Coupled cluster linear-response formalism has been used to compute the vertical spectrum of ethylene and tetracyanoethylene ͑TCNE͒. We show that for both molecules the * excitation 1 A 1g → 1 B 1u of the experimental spectrum is not vertical nor the 0-0 transition. For TCNE this excitation is the only experimentally observed band. We have computed vertical excitations of 5.2 eV in gas phase and 5.1 eV in acetonitrile and estimated a lower bound for the 0-0 transition in the gas phase of 4.3 eV.

Near-edge x-ray absorption studies of Na-doped tetracyanoethylene films: A model system for theV(TCNE)xroom-temperature molecular magnet

“…10 Though the exact structure is not known, it is likely that the strong local coordination between the vanadium and the cyano groups will introduce both some twisting and stretching to the vinyl units to compensate, which as previously has been shown can introduce strong modification to the transition energies of TCNE. 20 Despite the failure of the C K-edge spectrum of NaTCNE to precisely model the V͑TCNE͒ x spectrum, we speculate that the carbon-based frontier unoccupied molecular orbital of V͑TCNE͒ x also has no significant hybridization with vanadium. We base the speculation on the nature of the TCNE anion SOMO that is delocalized over the whole TCNE molecule, i.e., nitrogen and carbon atoms share the same lowest unoccupied molecular orbital.…”

“…lations that the transition energies of TCNE can be strongly modified by very small changes in the ground state geometry, 20 and it may be that the calculated bond lengths and angles for the vinyl bridge are slightly off whereas the cyano groups are better described. Comparing our vinyl bond length for the Na + -compensated TCNE anion, 1.448 Å, with results from literature on TCNE anions, 1.420-1.429 Å ͑Ref.…”

“…For TCNE, the lowest singlet excited state involves promotion from the highest occupied molecular orbital ͑HOMO͒ into the lowest unoccupied molecular orbital ͑LUMO͒. 20,21 The TCNE is reduced into its anion TCNE − , the former LUMO becomes singly occupied forming a socalled singly occupied molecular orbital ͑SOMO͒. 11,19,21 This affects the excitation energies and for TCNE − the lowest energy excitation is formed by promotion of an electron from the previous HOMO into the singly occupied molecular orbital.…”

“…The molecular structure of TCNE and TCNE − , 19 as well as the vertical excitation energies of TCNE and TCNE − for low energy transition ͑Ͻ10 eV͒, 20,21 have been studied theoretically. For TCNE, the lowest singlet excited state involves promotion from the highest occupied molecular orbital ͑HOMO͒ into the lowest unoccupied molecular orbital ͑LUMO͒.…”

The V state of ethylene: valence bond theory takes up the challenge

“…V transition energy in the MO-CI framework [12,22,[33][34][35]. Still, the corresponding wave functions, which are constructed with large perturbative expansions, lack compactness and lucid interpretability, whereas the compact CASSCF(2,2) wave function is quite inaccurate.…”