A comparison of neutral and charged species of one- and two-dimensional models of graphene nanoribbons using multireference theory

Abstract

This study examines the dependence of the polyradical character of charged quasi-linear n-acenes and two-dimensional periacenes used as models for graphene nanoribbons in comparison to the corresponding neutral compounds. For this purpose, high-level ab initio calculations have been performed using the multireference averaged quadratic coupled cluster theory. Vertical ionization energies and electron affinities have been computed. Systematic tests show that the dependence on chain length of these quantities can be obtained from a consideration of the π system only and that remaining contributions coming from the σ orbitals or extended basis sets remain fairly constant. Using best estimate values, the experimental values for the ionization energy of the acene series can be reproduced within 0.1 eV and the experimental electron affinities within 0.4 V. The analysis of the natural orbital occupations and related unpaired electron densities shows that the ionic species exhibit a significant decrease in polyradical character and thus an increased chemical stability as compared to the neutral state.

Measuring multi-configurational character by orbital entanglement

“…We further analyzed the multi-configurational character of the electronic ground state of several polyacenes in Table I. In contrast to the series of alkane molecules, we observe a progressively strong multi-configurational character in agreement with previous studies [56][57][58][59][60][61][62][63][64] . However, we see a saturation because our diagnostic condenses information from all orbitals into one number while previous studies report natural orbital occupation numbers (NOONs) of all π-orbitals.…”

The Antiferromagnetic Spin Coupling in Non‐Kekulé Acenes—Impressive Polyradical Character Revealed by High‐Level Multireference Methods

“…These methods will be designated as p-MR-CISD and p-MR-AQCC. It has been shown previously in calculations on oligoacenes and periacenes [39,42] that the polyradical character of these systems and the singlet-triplet gap can be well described in this way. First, CASSCF calculations were performed with CAS(10,10), except for system 1 for which CAS(6,6) was used.…”

“…In particular, the MR‐AQCC method allows for a reliable calculation of size‐extensivity effects in the MR case. It has been successfully applied in a number of notoriously difficult quantum chemical problems, such as determination of the automerization barrier in CBD, investigation of valence bond isomers, singlet–triplet energy gap calculations, and most recently in the study of the polyradical character of graphene nanoribbons . The experience with these multireference methods will be used herein to study the remarkably high polyradical character of non‐Kekulé acenes and to provide a theoretically well‐founded picture of the evolution of different spin states for DMBCDs with increasing chain length.…”

Multireference Approaches for Excited States of Molecules

“…It, furthermore, showed that the computational description of this state was significantly improved when moving from the 6-31G** basis set to a more sophisticated atomic natural orbital basis set even though both basis sets have the same computational cost in the CASSCF and CASPT2 steps. The ionization potentials and characters of the ionized states of various acenes have been studied by MR-AQCC and MR-CI computations 954.…”