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

Abstract

This study reports experimentally and theoretically (ab initio) determined indirect CC spin−spin coupling tensors n J CC in benzene. The CC spin−spin coupling constants n J CC 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, Δ n J CC, 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.

Solvent Effects on the Indirect Spin–Spin Coupling Constants of Benzene: The DFT-PCM Approach

“…[23] obtained with the HuzIII-su3 basis set are observed. Our results corroborate the observation [23,42] that the MCSCF results significantly overestimate most of the coupling constants. In a recent paper, an attempt was made to extract a vibrationally averaged 1 J(H 13 C) coupling constant for benzene in the gas phase by extrapolation of accurately measured coupling constants in four relatively non-polar solvents to a dielectric medium with a dielectric constant of 1 (which corresponds to the dielectric constant of vacuum) [43].…”

Spin-Spin Coupling Tensors in Fluoromethanes

“…The RAS-I calculation employs an active space containing 69.7 % of the virtual MP2 particles, while in RAS-II we utilise an active space that recovers 90.9 % of the MP2 particles. Similar monitoring of the basis set convergence as in previous cases was not possible for CHF 3 , since the active space in RAS-II increased the computational effort considerably. This forced us to apply the HIII basis set only for the dominating contributions such as the FC and SD/FC terms.…”

“…[2] For the carbon ± carbon couplings, J aniso CC has been shown to be negligible regardless of the hybridisation of the carbon atoms. [3,4] For the H couplings the same has been known to hold for already a long time. [1] Recent LC NMR experiments and ab initio calculations indicate non-negligible contributions to 5 D exp FF and several types of D exp CF couplings in para-difluorobenzene.…”

“…aniso ij (3) where D eq ij corresponds to the equilibrium geometry of the molecule, and D h ij and D ah ij are the contributions from the harmonic [12] and anharmonic [13] aniso /D exp ovat negatiivisia ja suuruudeltaan alle 1.7 %, kun käytetään hyväksi mitattuja orientaatioparametreja. Siten yhden sidoksen CF-ja kahden sidoksen FF-kytkentöjen tapauksissa epäsuoran kontribuution huomiotta jättäminen on varsin pieni virhelähde molekyylin orientaation ja/tai rakenteen määrityksessä.…”

The conformational distribution in diphenylmethane determined by nuclear magnetic resonance spectroscopy of a sample dissolved in a nematic liquid crystalline solvent

“…Here we present a study of diphenylmethane by the method which relies on being able to obtain partially-averaged dipolar couplings, D i j , between pairs of nuclei by analysing the nuclear magnetic resonance ͑NMR͒ spectrum of a sample dissolved in a liquid crystalline phase. This so-called LXNMR method has a wide range of applicability, and has been used for small, rigid molecules such as benzene 1 in order to make a comparison at high precision between structures obtained in solid, liquid, and gaseous phases, and for molecules as large as ubiquitin to obtain coarse grained structures of this and similar proteins in their folded states. 2,3 A study of the conformation of diphenylmethane is important because it is the simplest case of linkage of two aromatic rings through a methylene group.…”