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13 C− 13 C Spin−Spin Coupling Tensors in Benzene As Determined Experimentally by Liquid Crystal NMR and Theoretically by ab Initio Calculations

1996; American Chemical Society; Volume: 118; Issue: 37 Linguagem: Inglês

10.1021/ja961263p

1943-2984

Jaakko Kaski, Juha Vaara, Jukka Jokisaari,

Electron Spin Resonance Studies

This study reports experimentally and theoretically (ab initio) determined indirect CC spin−spin coupling tensors nJCC in benzene. The CC spin−spin coupling constants nJCC between the ortho, meta, and para (n = 1, 2, and 3) positioned carbons were experimentally determined in two ways: firstly by utilizing the 2H/1H isotope effect on the carbon shieldings in neat monodeuteriobenzene and recording the 13C satellite spectrum in a 1H-decoupled 13C NMR spectrum, and secondly by recording the 13C NMR spectrum of fully 13C-enriched benzene (13C6H6) and carrying out its complete analysis. The anisotropies of the corresponding coupling tensors, ΔnJCC, were resolved experimentally by liquid crystal 1H and 13C NMR using dipolar couplings corrected for both harmonic vibrations and deformations. The results obtained in three thermotropic liquid crystal solvents are in good mutual agreement, indicating the reliability of the determinations. The anisotropy of the ortho, meta, and para CC indirect couplings are ca. +17, −4, and +9 Hz, respectively. Also, the signs of the coupling constants are unambiguously determined. The ab initio calculations were performed using multiconfiguration self-consistent field linear response theory with both single-reference and multireference wave functions. The results confirm the signs of the experimental anisotropies in all cases. The magnitude of the ortho coupling anisotropy is excellently reproduced, but the anisotropies are somewhat overestimated in the two other theoretical coupling tensors. The importance of the different physical contributions to the couplings and anisotropies is discussed.