Theoretical investigation on H1 and C13 NMR chemical shifts of small alkanes and chloroalkanes

Abstract

Using density functional theory (DFT) with the B3LYP, PBE, and PBE0 exchange-correlation functionals as well as the Moller-Plesset second-order perturbation theory (MP2) combined with a series of rather extended basis sets, 1H and 13C chemical shifts of small alkanes and chloroalkanes (with different numbers of chlorine atoms on specific positions) have been simulated and compared to experimental data. For the 1H chemical shifts, theory tends to reproduce experiment within the limits of the experimental errors. In the case of 13C chemical shift, the differences between theory and experiment increase monotonically with the number of chlorine atoms and exhibit a deviation from additivity. This behavior is related to the saturation of the experimental 13C chemical shifts with the number of chlorine atoms, whereas the evolution is mostly linear at both DFT and MP2 levels of approximation. This difference has been traced back to the relativistic spin-orbit coupling effects, which are exalted as a result of the enhancement of the s character of the C atom when increasing the number of linked Cl atoms. Thus, it was demonstrated that not only electron correlation but also relativistic effects have to be considered for estimating the 13C chemical shifts when several Cl atoms are directly attached to the C atom. Linear (theory/experiment) regressions have then been performed for the different types of C atoms, i.e., bearing one, two, and three Cl atoms, with excellent correlation coefficients. The linear correlation relationships so obtained can then serve to predict and facilitate the interpretation of the nuclear magnetic resonance spectra of more complex compounds. Furthermore, by investigating the basis set effects, the correlation between the chemical shifts calculated using the 6-311 + G(2d,p) basis set and the more extended 6-311 + G(2df,p) and aug-cc-pvtz basis sets is excellent, demonstrating that the choice of the 6-311 + G(2d,p) basis set for calculating the 1H and 13C chemical shifts is relevant.

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

“…Even for the second row elements the omission of the relativistic effects introduces errors to the NMR parameter values comparable to the errors of other approximations (10,18,24,(55)(56)(57). For the molecule containing a heavy atom these effects become substantial for the NMR parameters concerning not only the heavy nucleus itself but also the nuclei of atoms directly bonded to the heavy atom (18,(54)(55)(56)(57)(58)(59)(60)(61)(62)(63).…”

“…When the calculations have been performed for testing the efficiency of a theoretical or numerical method, and especially when they have been done for a single and simple molecule, the most reasonable procedure seems to be using directly the definition of the chemical shifts (Eq. [1] 13 C chemical shifts these deviations can be of the order of several ppm even for quite advanced methods (57,(64)(65)(66). Such deviations could sometimes mask all the interesting molecular-structure-dependent effects.…”

“…C NMR chemical shifts for a series of monosubstituted benzenes (C 6 H 5 X, X ¼ H, CH 3 , F, Cl, Br, I, CCH, HgCCH) by theoretical calculations of magnetic shielding constants. The deviations from the regression line are given for two theoretical methods: (A) GIAO DFT PBE1PBE functional and LANL2DZ quasirelativistic effective core potential for Cl, Br, I and Hg and 6-311þþG(2d,p) basis set for the remaining atoms, solvent effects included by PCM; (B) GIAO DFT B3LYP functional, TZP basis set, scalar and spin-orbit relativistic effects included into calculations, solvent effects modelled by COSMO (57). In variant A the chemical shifts for carbon atoms directly bonded to heavy atoms (the points distinguished in the diagrams) were omitted in correlation, while in variant B all the data were used.…”

Sensing mechanism for a fluoride chemosensor: invalidity of excited-state proton transfer mechanism

“…3. The B3LYP function with 6-311+G(2d,p) basis set, which has been proven appropriate for calculating 1 H chemical shifts, 40,41 was used to calculate the 1 H NMR spectra. For DBM, the signals at d = 8.38, 7.75, and 7.45 ppm were assigned to protons H1, H2, and H3, respectively.…”

13C shielding tensors of crystalline amino acids and peptides: Theoretical predictions based on periodic structure models

“…Amino acids are essential structural units of peptides and proteins and are primary substrates for various enzymatic reactions. In despite of the availability of experimental shielding tensor values for a variety of amino acids, [7][8][9][10][11][12][13][14][15][16][17][18][19][20] reliable theoretical predictions of their NMR parameters are invaluable in providing supports for the assignment of spectra, testing of hypotheses, as well as design of experiments. [21][22][23][24][25] In particular, theoretical results obtained from quantum chemical calculations are also sustainable for the detailed correlations between experimental NMR parameters and molecular structures (bond lengths and bond angles).…”