Normal halogen effect on light-atom NMR shielding explained

12.05.2010

Nuclear magnetic resonance (NMR) shielding contains detailed information about the electronic structure of molecules. Thus it provides a sensitive tool to probe the surroundings of nuclei. Carbon and hydrogen shielding tensors of methyl halides, CH3X (X = F, Cl, Br, and I), were studied both experimentally and theoretically by the NMR Research group at the University of Oulu. This series of molecules is prototypic for demonstrating the effect of the nearby halogen on the light atom NMR shielding, s.c. normal halogen dependence (NHD). The effects of relativity, solvent, as well as rotational and vibrational thermal motions of the molecules were systematically studied with state-of-the-art experimental and computational methods.

The experimental shielding parameters were obtained from gas-phase and liquid crystal NMR experiments providing the currently best experimental estimates for the full shielding tensors. Quantum chemical (QC) calculations at the leading-order Breit-Pauli perturbation level confirmed that the NHD arises mainly from the relativistic spin–orbit effects while the scalar relativity is important for quantitative results. Rovibrational effects were also found significant both in the theoretical shielding calculations and in the analysis of the experimental data. Overall, the obtained experimental and theoretical results for the NMR shielding parameters are in remarkable agreement setting a standard for further investigations on the normal halogen dependence of the shielding as well as for benchmarking relativistic quantum-chemical methods in the NMR of light nuclei.

Figure: Example of carbon NMR spectrum as well as comparison of experimental (EXP) and computational [non-relativistic (NR), relativistic (BPPT), and total (TOT)] carbon nuclear shielding constants (sC) and shielding anisotropies (DsC). Reproduced by permission of the PCCP Owner Societies.

The rovibrational averaging of the NMR parameters required executing non-relativistic (academic ACES II code) and relativistic (private development version of the Dalton 2.0 code) QC calculations for 100 molecular geometries for each molecule. This extensive computational task was performed using both serial and parallel resources of the CSC supercomputers as well as the local Linux cluster in University of Oulu.  All the experimental spectra were analyzed with PERCH NMR software (CSC license).

The work was carried out at the Department of Physics of the University of Oulu (Centre of Excellence in Research within the University of Oulu). PL and JV belong to the Finnish Center of Excellence in Computational Molecular Science (CMS) funded by the Academy of Finland.

More information:

Anu M. Kantola, Perttu Lantto, Juha Vaara, and Jukka Jokisaari: Carbon and proton shielding tensors in methyl halides, Physical Chemistry Chemical Physics, 12, 2679–2692 (2010) [DOI: 10.1039/b923506j]

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