Density-Functional and Coupled-Cluster Singles-and-Doubles Calculations of the Nuclear Shielding and Indirect Nuclear Spin-Spin Coupling Constants of o-Benzyne Trygve Helgaker* ,† Department of Chemistry, UniVersity of Durham, South Road, Durham, DH1 3LE, United Kingdom Ola B. Lutnæs Department of Chemistry, UniVersity of Oslo, P.O.B. 1033 Blindern, N-0315 Oslo, Norway Michał Jaszun ´ski Institute of Organic Chemistry, Polish Academy of Sciences, 01-224 Warszawa, Kasprzaka 44, Poland Received July 17, 2006 Abstract: Density-functional theory (DFT) and coupled-cluster singles-and-doubles (CCSD) theory are applied to compute the nuclear magnetic resonance (NMR) shielding and indirect nuclear spin-spin coupling constants of o-benzyne, whose biradical nature makes it difficult to study both experimentally and theoretically. Because of near-equilibrium triplet instabilities that follow from its biradical character, the calculated DFT NMR properties of o-benzyne are unusually sensitive to details of the exchange-correlation functional. However, this sensitivity is greatly reduced if these properties are calculated at the equilibrium of the chosen functional. A strong correlation is demonstrated between the quality of the calculated indirect spin-spin coupling constants and the quality of the calculated lowest triplet excitation energy in o-benzyne. Orbital- unrelaxed coupled-cluster theory should be less affected by such instabilities, and the CCSD NMR properties were only calculated at the experimental equilibrium geometry. For the shielding constants, the results in best agreement with experimental results are obtained with CCSD theory and with the Keal-Tozer KT1 and KT2 functionals. For the triply bonded carbon atoms, these models yield an isotropic shielding of 1.3, -3.3, and -1.2 ppm, respectively, compared with the experimentally observed shielding of 3.7 ppm for incarcerated o-benzyne. For the indirect spin-spin coupling constants, the CCSD model and the Perdew-Burke-Ernzerhof functional both yield reliable results; for the most interesting spin-spin coupling constant, 1 J (CtC), we obtain 210 and 209 Hz with these two models, respectively, somewhat above the recently reported experimental value of 177.9 ( 0.7 Hz for o-benzyne inside a molecular container, suggesting large incarceration effects. 1. Introduction The NMR shielding tensor of the triply bonded carbon atom of the 1,2- 13 C-substituted o-benzyne was first measured by Orendt et al. 1 in 1996, who also presented quantum-chemical calculations using Hartree-Fock (HF) theory, second-order Møller-Plesset theory, and density-functional theory (DFT) with the Becke-Lee-Yang-Parr (BLYP) functional. These calculations indicated that there are large electron-correlation contributions to the shielding of the triply bonded carbon atomsin particular, for the individual tensor components. Moreover, only the DFT results were found to agree well * Corresponding author phone: +47 22855428; fax: +47 22855441; email: trygve.helgaker@kjemi.uio.no. † Permanent address: Department of Chemistry, University of Oslo, P.O.B. 1033 Blindern, N-0315 Oslo, Norway. 86 J. Chem. Theory Comput. 2007, 3, 86-94 10.1021/ct600234n CCC: $37.00 © 2007 American Chemical Society Published on Web 11/22/2006