Saturday, January 4, 2014

Electron correlation, zero-point vibrational and temperature effects in Nuclear Magnetic Resonance spectroscopy

A.M. Teale, O.B. Lutnæs, T. Helgaker, D.J. Tozer, J. Gauss, Journal of Chemical Physics 2013138, 024111 (Pay-wall) and
J. Kaminský, M. Buděšínský, S. Taubert, P. Bouřa, M. Straka, Physical Chemistry Chemical Physics 201315, 9223-9230 (Open Access)
Contributed by Marcel Swart

Last year has seen two important contributions in the field of determination of NMR chemical shifts by theoretical chemistry. More and more it is recognized that the computational prediction of 1H and 13C chemical shifts is a useful tool for natural product, mechanistic, and synthetic organic chemistry.[1] There are however doubts about how accurate these results are, and if any chemically relevant conclusions can be drawn from them.

The first paper[2] compares the chemical shifts as obtained by both density functional theory and wavefunction theory (RHF, CCSD, CCSD(T), extrapolated) for a total of 28 molecules (for which previously already rotational g-tensors and magnetizabilities were computed[3]). The authors also included zero-point vibrational effects on the computed chemical shieldings, and used extrapolation techniques to estimate uncertainties related to basis-set incompleteness[4]. First, the authors established an accurate benchmark set of data, for which the accuracy was established by comparison with experimental data (including zero-point vibrational corrections). They found good agreement between CCSD(T)/aug-cc-pCVQZ and experiment (empirical equilibrium values), with a mean-absolute-error of 2.9 ppm for the chemical shieldings. Afterwards, these reference data were used to compare how well a variety of density functionals was able to reproduce them, with a sobering conclusion: "None of the existing approximate functionals provide an accuracy competitive with that provided by CCSD or CCSD(T) theory".[2] The best performing functional (as shown before) was KT2 with a mean-absolute-error compared to the CCSD(T) data (both with the same aug-cc-pCVQZ basis set) of 10.2 ppm.

The second paper[5] has a completely different approach, and deals with the characterization of fullerenes. For this purpose, computational chemistry might be used, but one again should be sure about the methods used. The authors used quantum vibrational averaging, a dielectric continuum model for the solvent (CPCM, 1,1,2-trichloroethane), classical (MM3) and first-principle (BP86/def-SVP) molecular dynamics simulations, and did experiments. These authors used a different set of density functionals, and found the best results for wB97X-D/IGLO-III (a root-mean-square deviation compared to experiment of only 0.4 ppm). However, surprisingly, in the analysis of the dynamical part they used either BP86 or BHandHLYP for the NMR chemical shifts, even though these showed larger RMSD values of respectively 1.7 and 0.8 ppm. Moreover, big differences were found in the chemical shifts from the snapshots of the 1 ns classical MD, and those of the 1.2 ps first-principles MD. For the five different atom types these differences were found in the range 1.8-7.9 ppm.[6]

References and notes
[1] M.W. Lodewyk, M.R. Siebert, D.J. Tantillo, Chem. Rev. 2012, 112, 1839-1862 [DOI 10.1021/cr200106v]
[2] A.M. Teale, O.B. Lutnæs, T. Helgaker, D.J. Tozer, J. Gauss, J. Chem. Phys. 2013, 138, 024111 [DOI 10.1063/1.4773016]
[3] O. B. Lutnæs, A. M. Teale, T. Helgaker, D. J. Tozer, K. Ruud, J. Gauss, J. Chem. Phys. 2009, 131, 144104 [DOI 10.1063/1.3242081]
[4] These formulas have been developed for energies; hence, their use for the direct extrapolation of molecular properties is less well founded, and indeed can not be used with a two-point extrapolation for prediction of the basis set limiting value.
[5] J. Kaminský, M. Buděšínský, S. Taubert, P. Bouřa, M. Straka, Phys. Chem. Chem. Phys. 2013, 15, 9223-9230 [DOI 10.1039/C3CP50657F]
[6] Strangely enough, while the CPCM solvent model only had a modest effect on the chemical shifts of 0.2-0.3 ppm, the authors showed that first-principles MD (FPMD) simulations including solvent effects (COSMO) led to drastically different results for the chemical shifts of snapshots (0.1-4.1 ppm) from those from FPMD without them.

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