Is hydrogen electronegativity higher than Pauling’s value? New clues from the ¹³C and ²⁹Si NMR chemical shifts of [CHF₃] and [SiHF₃] molecules

¹³C and ²⁹Si δ NMR chemical shifts as a function of the Σ(r_L ) values, in tetrahedral [AL₄] (A=C, Si) systems with Σ(χLPau)≥12.4: the case of [AH_mX_n] compounds (X_n=combination of n halides; m+n=4)

The δ(¹³C) [32], [33], [44], [45], [46], [47] and δ(²⁹Si) [42], [48], [49], [50], [51] NMR chemical shifts for [AX₄] (A=C, Si; X₄=combination of four Cl and/or F halides, Tables S1, S2) and [AH_mX_n] (X_n=combination of n Cl and/or F halides; m+n=4; Σ(χLPau)≥12.4; Tables S3, S4) compounds are reported in graph vs. the overall sum of ionic radii of A bonded atomic L ligands, Σ(r_L), see Fig. 2a,b. The following ionic radii: r_F−=133 [52]; r_Cl−=181 [52]; r_Br−=196 [52]; r_I−=220 [52]; r_H−=208 pm [53], [54], [55] were used. All the considered compounds are characterized by high overall Pauling electronegativity sum, Σ(χLPau), for the A bonded atoms. Interestingly, these [AH_mX_n] hydrogenated compounds, with Σ(χLPau)≥12.4, show a specific behavior similar to that exhibited by the fully halogenated [AX₄] congeners, Fig. 2. The compounds characterized by high Pauling electronegativity overall sum of the C or Si bonded L atoms are generally located in the low Σ(r_L) region and exhibit lower than expected NMR chemical shifts, Fig. 2a,b [32], [42]. Indeed deviations from the linear correlation of δ vs. Σ(r_L), Equations 1 in the Experimental, are observed for both [AX₄] and [AH_mX_n] species. These deviations, quantified as ΔΣ(r_L) differences by Equations 2 and reported in Fig. 3a,b, have been already analyzed and discussed in a previous work, for the sole [AX₄] derivatives (Tables S1, S4) [34]. The ΔΣ(r_L) resulted linear dependent on the Pauling electronegativity overall sum for the single atomic substituents, Σ(χLPau), as indicated by Equations 3 and were found to be trigged by the specific minimum onset Σ(χLPau) value, equal to about 12.4, see Fig. 3a,b [34]. As discussed above, this effect can be expressed in term of electronegativity dependent apparent increase of the NMR effective atomic radii overall sum, ΔΣ(r_L), as shown by Equations 4. The Fig. 3a graph shows a trend of ΔΣ(r_L) vs. Σ(χLPau) values already observed for the fully halogenated [CX₄] and [SiX₄] species. In fact, these exhibit for ΔΣ(r_L) values a region where it is about zero or strongly reduced (below or in proximity of the onset Σ(χLPau) value, respectively) and a region where the ΔΣ(r_L) and Σ(χLPau) values are linearly related [34]. The mono-hydrogenated compounds in Fig. 3a graph appear to line up following the expected trend already observed for the fully halogenated species. Interestingly these H containing compounds, characterized by Σ(χLPau) higher than the onset value, appear linearly correlated with the ΔΣ(r_L) but shifted towards lower Σ(χLPau) on a line nearly parallel to that previously described for the fully halogenated species [34]. The shift from this latter line appears to be nearly double for [CH₂F₂] with respect to [CHF₃], [CHF₂Cl] and [CHFCl₂], Fig. 3a. Although limited to the restricted available data set, similar shifts are also observed for the silicon [SiHF₃] and [SiH₂F₂] derivatives, Fig. 3b.

Fig. 2:

(a, b) δ(¹³C) and δ(²⁹Si) NMR chemical shifts vs. ionic radii overall sum of carbon and silicon bonded atomic ligands, Σ(r_L), in tetrahedral [AX₄] (A=C, Si; X₄=combination of four halides) compounds. The partially hydrogenated [AH_mX_n] (X_n=combination of n Cl and/or F halides; m+n=4) compounds, with Pauling’s electronegativities overall sum of the atomic substituents, Σ(χLPau), ≥12.4 are indicated in yellow. The ΔΣ(r_L) differences are shown in the graphs by horizontal blue double arrows. The shown red lines, interpolating the data points of the sole [ABr_mI_n] (m+n=4) compounds with Σ(χLPau)≪12.4, correspond to: ΔΣ(r_L)=0.

Fig. 3:

(a, b) ΔΣ(r_L) differences vs. Σ(χLPau) for [AX₄] (A=C, Si; X₄=combination of four Cl and/or F halides) and partially hydrogenated [AH_mX_n] (X_n=combination of n Cl and/or F halides; m+n=4) compounds with Σ(χLPau)≥12.4. In blue are indicated the straight lines interpolating the data points of the [AF₄], [AClF₃], [ACl₂F₂] and [ACl₃F] compounds. As indicated, the previously calculated onset Σ(χLPau) value corresponds to the intersection between last interpolating lines and the zero line [34]. The necessary corrections to be operated to the hydrogen electronegativity, calculated for the reference [CHF₃] and [SiHF₃] compounds (i.e. ΔχH−[CHF3]NMR and ΔχH−[SiHF3]NMR), are graphically shown by violet double arrows. By using, for the hydrogen atoms the corrected NMR effective electronegativity value (χHNMR=2.75), we can calculate the Σ(χLNMR). The ΔΣ(r_L) vs. Σ(χLNMR) values are reported in (c, d). It can be observed that in this case the behavior of the shown partially hydrogenated [AH_mX_n] compounds is identical to that of the sole halogenated [AX₄] derivatives.

The observed positions of the data points for the considered carbon and silicon halo-hydrido derivatives reported in Fig. 3a,b, are characterized by the higher Σ(χLPau) values (Fig. 3a,b), suggesting that differently from halides, the NMR data derived electronegativity of the C and Si bonded hydrogens should be higher than that reported in the L. Pauling’s electronegativity scale [53], [54], [56].

Calculation of the NMR effective electronegativity value for the hydrogen atom, on the basis of the [CHF₃] and [SiHF₃] reference compounds

From the ¹³C and ²⁹Si NMR chemical shifts of [CHF₃], δ(¹³C)=+118.8 ppm [32], and [SiHF₃], δ(²⁹Si)=−77.8 ppm [42], reference compounds, reported in the graphs of Fig. 2a,b vs. Σ(r_L), it is possible to calculate the corresponding ΔΣ(r_L) values, see Experimental. As expected, these calculated values are the highest among those obtained for the [CH_mX_n] and [SiH_mX_n] hydrogenated compounds, characterized by Σ(χLPau)≥12.4, considered in this work (Tables S3, S4). This because the ΔΣ(r_L) are directly related to the Σ(χLPau). The ΔΣ(r_L) values vs. Σ(χLPau) for all considered [CH_mX_n] and [SiH_mX_n] compounds, including the fully halogenated are reported in the graphs of Fig. 3a,b. The data points of both [CHF₃] and [SiHF₃] reference compounds do not fall on the same straight lines interpolating the data points of the [CX₄] and [SiX₄] series of compounds (with Σ(χLPau)>12.4), respectively [34]. As described in the Experimental (Equations 5–7) we can easily calculate the corrections required for the Pauling’s hydrogen electronegativity, ΔχH−[AHF3]NMR, required to shift the data points of the [CHF₃] and [SiHF₃] reference compounds on the respective straight line interpolating the [CX₄] and [SiX₄] (X₄=combination of four F and/or Cl) derivatives with Σ(χLPau)>12.4,Fig. 3a,b. These corrections (ΔχH−[CHF3]NMR=0.52 and ΔχH−[SiHF3]NMR=0.59) for [CHF₃] and [SiHF₃] (Tables S3, S4), allow to calculate the following H electronegativity values: χH−[CHF3]NMR=2.72 and χH−[SiHF3]NMR=2.79 (Equations 6) required to align the data points of [CHF₃] and [SiHF₃] on the line interpolating the corresponding considered [CX₄] and [SiX₄] derivatives. Interestingly, it can be observed that the new obtained electronegativity values are very similar, producing an averaged χ¯H−[AHF3]NMR=2.75 value. It is noteworthy that the adoption of the χ¯H−[AHF3]NMR value in the calculation of the ligands’ electronegativity overall sum results in a realignment of the data reported in Fig. 3a,b for both considered series of [CH_mX_n] and [SiH_mX_n] compounds, see Fig. 3c,d. Indeed, this occurs also in the case of the dihydrogenated [CH₂X₂] and [SiH₂X₂] compounds. Therefore, taking into account the required H electronegativity correction, the behavior of hydrogenated compounds becomes similar to that observed for fully halogenated congeners. This suggests that, the hydrogen’s NMR effective electronegativity value could be simply indicated as: χHNMR=2.75, without any reference to particular compounds.

Several electronegativity scales, calculated by using many physical parameters, have been proposed over the years after the original Pauling’s concept definition [56]. Due to the importance of the Pauling’s electronegativity scale [54], the others are routinely normalized to the former, obtaining an overall ranking range defined by single dimensionless numbers between 0.78 and 4.00. A slight variability of the electronegativity values ascribed to the same element in the different scales is generally observed. Hydrogen should be highlighted among the atoms having the wider range of attributed electronegativities, with values from 2.0 to 2.8 (χHLang−Smith=2.00; χHAllred−Rochov≈χHPauling=2.20;χHSanderson= 2.31; χHMulliken= 2.80) [56]. Interestingly, our calculated χHNMR value is higher than the hydrogen electronegativity reported in the Pauling’s scale, but similar to the normalized value reported by Mulliken, which is the highest for H. It should be noted, that Pauling’s and Mulliken’s electronegativities of halides are both similarly NMR effective in the studied systems, but this does not occur for Hydrogen, where a correspondence is found only with the value reported by Mulliken. Noteworthy, it appears clear from the data of the present paper, that the H electronegativity should be better located above that of both C and Si (according to Mulliken [57] and the absolute electronegativity scale calculated with the Pearson–Parr approach [58], [59]) and not between them (according to Pauling). This buttresses our approximation of considering hydrogens as C and Si bonded hydrido ligands with respect to the considered central atoms. As stated by L. Pauling, the electronegativity can be defined as “the power of an atom in a molecule to attract electrons to itself” [53], [54]. This definition often loses the link to a molecule related property in the basic statement defining other scales (including Mulliken) [57]. It should be noted that the present approach allows a realignment of Hydrogen electronegativity to that of the Mulliken scale (based on atomic data) by using exclusively experimental NMR data of molecular compounds.

Influence of steric hindrance and electronegativity of atomic substituents on the ¹³C and ²⁹Si NMR chemical shifts of [AH_mX_n] (A=C, Si; X_n=combination of n Cl and/or F halides; m+n=4) compounds

The ¹³C and ²⁹Si NMR chemical-shift of [AX₄] (X₄=combination of four halides) compounds are reported in Tables S1, S2 together with the overall sum of halides ionic radii, Σ(r_L). Instead in Tables S3, S4 are reported the ¹³C and ²⁹Si NMR chemical shifts of the considered [AH_mX_n] compounds whose Σ(χLPau)≥12.4, together with the Σ(r_L) values. Last NMR data are plotted in the graphs of Fig. 2a,b vs. Σ(r_L). In these graphs the red straight lines of Fig. 2a,b are obtained by interpolation of the data points of the subgroup of [ABr_mI_n] (A=C, Si; m+n=4) compounds, characterized by a Σ(χLPau)≪12.4, as previously shown [34]. These straight lines correspond to the following Equations:

where i_C=2126.5 ppm; s_C=–2.7488 ppm/pm.

where i_Si=2031 ppm; s_Si=−2.703 ppm/pm.

We can calculate the horizontal distance, indicated as ΔΣ(r_L) in the graphs of Fig. 2a,b (blue double arrows), between a given data point and the corresponding straight line, as follows:

We previously shown that when Σ(χLPau)>12.4, for the two groups of cabon ([CF₄], [CF₃Cl], [CF₂Cl₂], [CFCl₃]) and silicon ([SiF₄], [SiF₃Cl], [SiF₂Cl₂], [SiFCl₃]) tetrahalido coordination compounds there exist linear correlations (R²≈0.999) between ΔΣ(r_L) and Σ(χLPau) as described by following equations [34]:

where j_C=−731 pm; t_C=+58.3 pm.

where j_Si=−892 pm; t_Si=+72.3 pm.

Last straight lines are represented in the graphs of Fig. 3a,b, by blue skew lines crossing the zero line in proximity of the averaged onset electronegativity value corresponding to: Σ(χLPau)≈12.4 [34].

Following Equations deriving from Equations 1 directly relate the δ(¹³C) and δ(²⁹Si) NMR chemical shifts to the Σ(r_L) and ΔΣ(r_L) values in the Σ(χLPau)>12.4 range, given that when Σ(χLPau)<12.4 it results ΔΣ(r_L)≈0 [34]:

It is noteworthy that none of the considered hydrogenated [AH_mX_n] derivatives, showing a Σ(χLPau)≥12.4, falls on the straight lines calculated by Equations 3, evidenced in blue, in Fig. 3a,b.

In the case of [CHF₃] and [SiHF₃] compounds, the following Equations give the NMR effective electronegativity sum, Σ(χH−[AHF3]NMR), of C and Si bonded atoms:

By subtracting the contribution of the Pauling’s fluorine atoms electronegativity (χFPau=3.98) from the last calculated sums we can obtain the NMR effective electronegativity of the single bonded hydrogen atom in the considered [CHF₃] and [SiHF₃] compounds:

The differences between the NMR effective and the Pauling’s electronegativities overall sums are as follows:

These are graphically shown by horizontal violet double arrows in the graphs of Fig. 3a,b, for both reference [CHF₃] and [SiHF₃] compounds. Because the hydrogen electronegativity value is expected to be constant and independent from molecular environment, we can indicate the averaged NMR effective electronegativity of hydrogen (χ¯H−[AHF3] NMR=2.75) simply as: χHNMR and use this value as NMR effective electronegativity of the bonded hydrogen atom.