Theoretical modeling of ¹³C NMR chemical shifts—How to use the calculation results

Abstract

Owing to wide accessibility of the general utility quantum chemistry programs, theoretical calculations of spectral parameters defining NMR spectra have become relatively easy for medium size molecules. This new and powerful tool allows investigators to perform much deeper interpretation of the NMR data and gain new information valuable for structural chemistry studies. It is usually realized and well understood that theoretical calculations of NMR parameters, as for any quantum mechanical calculations, have several limitations, difficult to overcome, and that the accuracy of calculation results is strongly dependent on the theoretical method applied. On the other hand, it is less commonly recognized that the effectiveness of all the combined calculational/spectroscopic approach depends meaningfully on the method of comparison of the experimental and theoretical data as well. This article is addressed primarily to experimental chemists who use in their everyday work 13 C NMR spectroscopy to solve various structural and physicochemical problems. After recalling some basic concepts concerning NMR parameters and their calculations, some suggestions concerning the rational choice of the calculational method and the experiment/theory comparison method are formulated. To support these recommendations and convince the reader of their utility, several practical examples are presented in the text. All these concepts reflect the author's beliefs which, albeit well-grounded, by no means should be treated as indispensable truths; they are rather worth considering as hints about how to solve various practical dilemmas.

An efficient DFT method of predicting the one-, two- and three-bond indirect spin–spin coupling constants involving a fluorine nucleus in fluoroalkanes

“…Taking into account some literature hints and also our earlier results we have decided to test the effectiveness of three hybrid functionals: B3LYP, 41 PBE0 (BPE1BPE in Gaussian 03) 42 and BHandH. 36,37 The two rst functionals are very popular in NMR calculations 20,24,25,43,44 and the third was shown to yield the best values of one-bond uorine-carbon spin-spin coupling constants for uoroaromatics. 11,12,18,39 In our calculations we have used three basis sets: 6-311++G(2d,p), 6-311++G(3df,3pd) and aug-pcJ-3-2006 abbreviated from here on: s, m and l, respectively.…”

“…11,12,18,39 In our calculations we have used three basis sets: 6-311++G(2d,p), 6-311++G(3df,3pd) and aug-pcJ-3-2006 abbreviated from here on: s, m and l, respectively. The rst two basis sets are of moderate size and more or less standard for NMR calculations, 20,24,25 whereas the third basis has been dedicated especially to calculations of the spin-spin coupling constants. 45 It would be the most comfortable situation if a universal and relatively small basis set appears effective for NMR calculations, although some Jdedicated basis can also be considered.…”

“…Such methods are presently standard in calculations of NMR parameters for organic compounds composed of C, H, N, O and some other atoms. [20][21][22][23][24][25][26] Even when molecule contains heavier atoms, good results can be obtained using DFT methods provided that relativistic effects are included into the theoretical model. 25,[27][28][29] It has been shown that also for uoroorganic compounds the magnetic shielding of carbon and uorine nuclei can be calculated with an acceptable accuracy by more or less standard DFT methods.…”

Crystallographic and Dynamic Aspects of Solid‐State NMR Calibration Compounds: Towards ab Initio NMR Crystallography

“…In reality,a ll molecular crystals undergo motion and lattice expansion due to the effect of temperature, and averaging over thermally occupied states is of significance to reduce the discrepancy between ss-NMRt heoretical calculations ande xperiments, even for non-plastic phases.T hese phenomena have been studied in combinationwith, for example, vibrational averaging on as eries of generated configurations at zero and finite temperatures, [15,20] classical molecular dynamics (MD) simulations [21] and ab initio MD simulations (including Born-Oppenheimer MD and path integral MD). [15,16,[21][22][23][24][25][26] On the one hand, the size of the MD simulation boxes should be as large as possible (fore xample, 30 30 30 3 )t o capturethe physics of an infinite crystal as realistically as possible. Ab initio MD simulations are inadequate for MD simulations with large supercells due to the computational cost;i n most cases, ab initio MD simulations with the constant number-volume-temperature (NVT) ensemble for 1 1 1u nit cells were used-this is not always physically realistic and can give problemsf or certains ystems.…”

“…Herein, we report ap rotocol to calculate ac omputational reference shielding by investigating five commonly used ss-NMR standard reference systems, adamantane, hexamethylbenzene, glycine( a and g forms)a nd 3-methylglutaric acid (Scheme 1), by single-crystal X-ray diffraction and in silico methods, including MD and DFT-D. The determination of 1 H, 13 Ca nd 15 Nc hemical shieldings for aw ide range of reference compounds hasb een investigated by gas-phase quantum mechanicalm ethods, also in combinationw ith solvent effects; [28][29][30] however,a binitio studies of solid-phase chemical shieldings,e specially for benchmarking compounds, are much fewer.The aim of the study was to reproduce the ss-NMRspectra of the five reference solid phases computationally,w hich provedt ob es urprisingly difficult. At room temperature, adamantane is ar otator phase, whereas hexamethylbenzene moleculesh op in their crystal structure;n oc hemically meaningful average atomic coordinates can be assigned.…”