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The dependence of disubstituted methanes X-CH2-X and X-CH2-Y methylene protons (CH2) and trisubstituted methanes CHX3 or X2CH-Y methyne protons (CH) chemical shifts on the nature of substituents X and Y in the 1H NMR spectra. An anomeric effect action?
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1  Ukrainian Academy of Printing


In our accompanying communication (see ECSOC-19) we have hypothesized that in the 1H NMR spectra of 1,2-disubstituted propanes with general formula C1H21(X)-C2H2(Y)-C3H33 may exist an anomeric effect. It was interesting to test our hypothesis on the PMR spectra of disubstituted methanes: Х-C1H2-Х (II) and Х-C1H2-Y (IV), the substances in which the classic gem-anomeric effect of other physical and chemical properties are most pronounced. For investigation were selected 13 «heteroatomcontaing» substituents Х (Y) [Х = Y = -Het(R)m], for which we find a reliable spectral data. In these substituents, there is a simple chemical bond С1-Het between substituent -Het(R)m and methylene carbon atoms (С1). On a similar principle selected number of trisubstituted methine compounds: CHX3 (III) и X2CH-Y (V).

The forming of di- (II) and trisubstituted (III) methanes considered as the result of three successive stages of hydrogen atoms substitution in a molecule of methane: СН4СН3Х (I) → СН2Х2 (II) → СНХ3 (III). We analyzed the differencies (indicated as Δi = ΔδН,Хi) of hydrogen atoms chemical shifts (δH) for each stage of substitution (called «transition») depending on the nature of the substituent Х: Δ1, Δ2 и Δ3. For example, the first transition (Δ1) - is a differential parameter equal to the difference between the parameter δH: Δ1 = δHСН3Х - δHСН4. But the most informative was the second differential parameter (denoted as ΔΔi). It is equal to the difference between the two respective first differential parameters Δi. For example, for the second transition [СН3Х (I) → СН2Х2 (II)] the parameter ΔΔ2 is equal to Δ2 - Δ1, and for the third transition [СН2Х2 (II) → СНХ3 (III)] parameter ΔΔ3 = Δ3 - Δ1. We compared the relative values of all three second differential parameters (ΔΔ1, ΔΔ2 and ΔΔ3) for each substituent Х, wherein the first parameter ΔΔ1 taken as 100%.

All substituents Х (Y) divided into 2 groups. The first group consisted of 7 substituents, wherein heteroatom -Het is an element of the second period - nitrogen, oxygen or fluorine; second group consist of 6 substituents where -Het is an element of the third (fourth) periods, phosphorus, sulfur, chlorine, bromine, iodine. The first group includes next substituents: -N(CH3)2 (а), -NO2 (b), -OH (c), -OCH3 (d), -OC2H5 (D), -OC(O)CH3 (e), F (f). The second group: -P(CH3)2 (g), -OP(O)(O-Alk)2 (G), -S-Alk (h), Cl (j), Br (k), I(L). Compounds with similar substituents (II and III) considered separately from the compounds with different substituents (IV and V).

For compounds (II and III) with the same substituents X of first group (Het = N, O, F) was found fundamental differences from the compounds with the second group substituents. The values of the second transition parameters ΔΔ2 for substituents X of the first group (Het = N, O, F) are less than 50%. A third transition parameters ΔΔ3 values even fewer (35%). Conversely, all values ​​of the second transition parameters ΔΔ2 for substituents X of the second group (Het = P, S, Cl, Br, I), much higher 50% (71% - 92%), as well as third transition parameters ΔΔ3 (53% ÷ 76 %). Therefore, it was concluded that heteroatoms of second period N, O, F in di- СН2Х2 (II) and trisubstituted СНХ3 (III) methane derivatives can interact with each other, which resulted in reduced of their total electronegativity. This effect resembles (or is) an anomeric effect in its broadt interpretation.

In the second part of communication we compared the experimental (δHe) and calculated using the above data ΔΔ2 and ΔΔ2 (δHс) the parameters δH for compounds with different substituents Х и Y (IV and V). For compounds with a first group substituents (Het = N, O, F) good agreement between the calculated (δHс) and experimental (δHе) parameters is found, and for the compounds of the second group of substituents (Het = P, S, Cl, Br, I) in most cases, we found a significant excess of the experimental (δHе) parameters on the calculated ones (δHc). This result, in our view, confirms our assumption of the existence of the anomeric effect for the first group substituents X, where (Het = N, O, F).


Keywords: di- and trisubstituted methanes, base and differential chemical shift of methylene (methine) protons, substituents Х and Y, two groups of substituents.