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Title: Focal Point Analysis of the Singlet-Triplet Energy Gap of Octacene and Larger Acenes
Authors: Hajgato, Balazs
Huzak, Matija
Deleuze, Michael S.
Issue Date: 2011
Citation: JOURNAL OF PHYSICAL CHEMISTRY A, 115, p. 9282-9293
Abstract: A benchmark theoretical study of the electronic ground state and of the vertical and adiabatic singlet-triplet (ST) excitation energies of n-acenes (C4n+2H2n+4) ranging from octacene (n = 8) to undecacene (n = 11) is presented. The T1 diagnostics of coupled cluster theory and further energy-based criteria demonstrate that all investigated systems exhibit predominantly a 1Ag singlet closed-shell electronic ground state. Singlet-triplet (S0-T1) energy gaps can therefore be very accurately determined by applying the principle of a focal point analysis (FPA) onto the results of a series of single-point and symmetry-restricted calculations employing correlation consistent cc-pVXZ basis sets (X = D, T, Q, 5) and single-reference methods [HF, MP2, MP3, MP4SDQ, CCSD, and CCSD(T)] of improving quality. According to our best estimates, which amount to a dual extrapolation of energy differences to the level of coupled cluster theory including single, double, and perturbative estimates of connected triple excitations [CCSD(T)] in the limit of an asymptotically complete basis set (cc-pV∞Z), the S0-T1 vertical (adiabatic) excitation energies of these compounds amount to 13.40 (8.21), 10.72 (6.05), 8.05 (3.67), and 7.10 (2.58) kcal/mol, respectively. In line with the absence of Peierls distortions (bond length alternations), extrapolations of results obtained at this level for benzene (n = 1) and all studied n-acenes so far (n = 2-11) indicate a vanishing S0-T1 energy gap, in the limit of an infinitely large polyacene, within an uncertainty of 1.5 kcal/mol (0.06 eV). Lacking experimental values for the S0-T1 energy gaps of n-acenes larger than hexacene, comparison is made with recent optical and electrochemical determinations of the HOMO-LUMO band gap. Further issues such as scalar relativistic, core correlation, and diagonal Born-Oppenheimer corrections (DBOCs) are tentatively examined.
Notes: Reprint Address: Deleuze, MS (reprint author), Hasselt Univ, Res Grp Theoret Chem & Mol Modelling, Agoralaan Gebouw D, B-3590 Diepenbeek, Belgium - Addresses: 1. Hasselt Univ, Res Grp Theoret Chem & Mol Modelling, B-3590 Diepenbeek, Belgium - E-mail Address: michael.deleuze@uhasselt.be
URI: http://hdl.handle.net/1942/13101
DOI: 10.1021/jp2043043
ISI #: 000294076500023
ISSN: 1089-5639
Category: A1
Type: Journal Contribution
Validation: ecoom, 2012
Appears in Collections: Research publications

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