NMR investigation of substituent effects on strength of π-π stacking and hydrogen bonding interactions to supports the formation of [2 + 2] photodimerization in (para-X-ba):::(bpe)||(bpe):::(para-X-ba) complexes

1 Introduction

Boronic acids have been widely used as reagents for the targeted synthesis of compounds having applications in the areas of pharmaceuticals, agrochemicals etc. It should be noted that there are potential drawbacks to using boronic acids as supramolecular building blocks, especially in the presence of heterocyclic compounds (Zheng et al., 2010, 2012; Das et al., 2017; Cuenca et al., 2016; Hall, 2006; Li et al., 2017; Diemoz and Franz, 2019). Boronic acids can be regarded as “green” compounds due to their low toxicity and their ultimate degradation into the environmentally friendly boric acid (Hall, 2005). The reversible hydrogen bonding interactions that boronic acids can take part in has seen a significant increase in the applications of boronic acid based systems in self-assembly sensing, and separation science (James, 2016; Nishiyabu et al., 2011; Kubo et al., 2015; Fujita et al., 2008; Corbett et al., 2006). While there has been considerable work on the construction of supramolecular frameworks and architectures based on organoboronic acids; (Saleem et al., 2019; Chen et al., 2020) there has been significantly less work on the ability of boronic acids to form hydrogen-bonded complexes.

The evidence for the formation of hydrogen bonds and π-π stacking in boronic acids is substantial and its relevance at the solid state has been essentially limited to the self-associated architectures and data regarding the thermodynamics of the interactions with complementary recognition motifs in solution are essentially unknown (Shang et al., 2015; Zhao et al., 2016; Iribarren et al., 2019; Campos-Gaxiola et al., 2010; Rettig and Trotter, 1977; Cyrański et al., 2008). These interactions are among the most prevalent non-covalent interactions present in both natural systems and synthetic (Meyer et al., 2003; Burley and Petsko, 1985). These interactions play an important role in determining the conformation of organic molecules, (Hughes and Waters, 2006) the structure and stability of proteins, and directing the stereoselectivity of organic transformations (Pecsi et al., 2010).

To the best of our knowledge, we have investigated the ability of para-X-ba where: X = NO₂, CN, F, Cl, Br, C(O)CH₃, OCH₃, OH, NH₂ and H to enable reactivity of trans-1,2-bis(4-pyridyl)ethylene (bpe) to direct intermolecular [2 + 2] photodimerization via computational chemistry (Scheme 1). Para-X-ba would support the formation of discrete four-component hydrogen-bonded molecular assemblies wherein π-π stacking of a pyridyl-functionalized alkene would conform to undergo [2 + 2] photodimerization.

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Scheme 1

Molecular models used in the present study. (Red spherical = NO₂, CN, C(O)OCH₃, F, Cl, Br, H, OCH₃ OH and NH₂ substituents).

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Our efforts to utilize the complexes based on para-X-ba to enable photoreactivity of bpe are inspired by work of MacGillivray et Al. (Alvarado et al., 2018) who experimentally demonstrated a propensity of related para-X-ba and bpe to assemble by a combination of π-π stacking contacts and hydrogen bond interactions. The work of MacGillivray et al. (Alvarado et al., 2018) prompted us to explore potential to employ para-X-ba to direct an intermolecular [2 + 2] photodimerization of bpe by computational chemistry.

2 Computational chemistry

To shed further light on the electronic and geometries properties ruling the formation and the stability of the (para-X-ba):::(bpe)||(bpe):::(para-X-ba) and (para-X-ba):::tpcb:::(para-X-ba) complexes (X = NO₂, CN, F, Cl, Br, C(O)CH₃ , OCH₃, OH, NH₂ and H where || and ∙∙∙ denote π-π stacking and hydrogen bonds), DFT geometry optimization and frequency calculations were carried out at the M05-2X/6–311++G** level (Zhao and Truhlar, 2008) using the GAMESS program (Schmidt et al., 1993). The geometries have been optimized using the the gradient CP-corrected methods (Boys and Bernardi, 1970). Calculations were followed by harmonic vibrational frequencies calculations at the same level of theory and it confirms that the optimized structures have energetic minima on their potential energy surfaces.

In following, the ¹HNMR calculations of the chemical shielding were performed for structure of the complexes. The chemical shielding of protons were computed at M05-2X/6–311++G** levels using SPINSPIN keyword. The shielding values were used to calculate the chemical shifts of hydrogen with respect to TMS (δ_H = σ_TMS − σ_H). Also, we employed the nucleus independent chemical shift (NICS) index that is one of the most widely employed indicators of aromaticity, which was proposed by Schleyer and co-workers (Ebrahimi et al., 2009a, 2009b; Ghafari and Gholipour, 2015, 2019; Schleyer and Jiao, 1996; Schleyer et al., 1996; Biegler Konig et al., 2001; Hammett, 1935; Zhu et al., 2003; Zhikol et al., 2005; Quinonero et al., 2008). It is defined as the negative value of the absolute shielding computed at a ring center.

The wave functions at the M05-2X/6–311++G** level were used to examine the electron density within the atoms in molecules (AIM) methodology by using the AIM2000 program (Biegler Konig et al., 2001).

3 Results and discussion:

We computationally demonstrated the ability of para-X-phenylboronic acid (para-X-ba) to assemble trans-1,2-bis(4-pyridyl)ethylene (bpe) into π-π stacks via –(B)O − H···N– hydrogen bonds in (para-X-ba):::(bpe)||(bpe):::(para-X-ba) complexes to react to afford rctt-tetrakis(4-pyridyl)cyclobutane (tpcb), (para-X-ba):::tpcb:::(para-X-ba) complexes (Scheme 1). We demonstrated para-X-ba in (para-X-ba):::(bpe)||(bpe):::(para-X-ba) complexes to act as a bifunctional hydrogen-bond-donor template by assembling bpe by way of –(B)O − H···N– hydrogen bonds in a discrete four-component supramolecular assembly to undergo an intermolecular [2 + 2] photodimerization. This photocycloaddition reaction generates (para-X-ba):::tpcb:::(para-X-ba) complexes. The components of (para-X-ba):::(bpe)||(bpe):::(para-X-ba) complexes as discrete four-component molecular assemblies sustained by four –(B)O − H···N– hydrogen bonds and π-π stacks of bpe have enabled the C = C bonds to be stacked in a suitable geometry for a [2 + 2] photodimerization (Scheme 1).

The results summarized in Table 1 allow us to analyze the electronic effects of substituents in terms of the Hammett constant (Hammett, 1935). Hammett constants of substituents demonstrated that electron-donating or electron-withdrawing substituent (X) influence π-π stacking the hydrogen bond interactions in (para-X-ba):::(bpe)||(bpe):::(para-X-ba) and (para-X-ba):::tpcb:::(para-X-ba) complexes. The electron-donating or electron-withdrawing capabilities of substituents which are including were describes using Hammett constant and the trend in electron withdrawing strength was found to be: NH₂ < OH < OCH₃ < H < F < Cl < Br < C(O)CH₃ < CN < NO₂.

Table 1 The The ΔE (kcal mol⁻¹). The most important geometrical parameters (in Å) for (para-X-ba):::(bpe)||(bpe):::(para-X-ba) and (para-X-ba):::tpcb:::(para-X-ba) complexes calculated at M05-2X/6–311++G** level of theory.

The corresponding binding energies of complexes were evaluated using Eq. (1) and are also tabulated in Table 1.

The increase in ΔE arises in (para-X-ba):::(bpe)||(bpe):::(para-X-ba) and (para-X-ba):::tpcb:::(para-X-ba) complexes with the electron withdrawing substituents (see Table 1). Whereas ΔE is in the range from −38.84 to −34.09 kcal/mol for (para-X-ba):::(bpe)||(bpe):::(para-X-ba) and from −26.01 to –22.92 kcal/mol for (para-X-ba):::tpcb:::(para-X-ba) complexes. The binding energy from NH₂ to NO₂ is in the following order: NH₂ < OCH₃ < OH < H < F < Cl < Br < C(O)CH₃ < CN < NO₂. The binding energy is more as manifested in the largest ΔE values in –NO₂ substituent compared to other substituents.

The lowest values ΔE are found for–NH₂ substituent that is equal to −34.10 and –22.92 kcalmol⁻¹ to (para-X-ba):::(bpe)||(bpe):::(para-X-ba) and (para-X-ba):::tpcb:::(para-X-ba) complexes, respectively.

The constants σ_para or σ_meta are not effective parameters to describe intermolecular interactions in these complexes. Therefore, both para and meta positions, that are, σ_para and σ_meta, certainly have an effect on the binding energies so new parameter σ_total (σ_total = (σ_para + σ_meta)) was applied to describe these interactions (see Table 1) (Ghafari and Gholipour, 2015, 2019; Zhu et al., 2003; Ebrahimi et al., 2009). Increasing binding energies with electron withdrawing character of substituents show a meaningful correlation between the Hammett constant of substituents and the interaction energies of (para-X-ba):::(bpe)||(bpe):::(para-X-ba) and (para-X-ba):::tpcb:::(para-X-ba) complexes. We demonstrated the correlation between the Hammett’s parameter and the value of ΔE (Fig. 1).

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Fig. 1

Correlation between ΔE and Hammett constant σ_total for (para-X-ba):::(bpe)||(bpe):::(para-X-ba) (left) and (para-X-ba):::tpcb:::(para-X-ba) (right) complexes.

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4 Geometrical parameters

The geometrical data of all the optimized complexes included in this study were gathered in Table 1 at the M05-2X/6–311++G** level of theory which is contained R_N···H, R_cen···cen and R_C=C geometrical parameters (see Scheme 1). Selected geometrical parameters for the related complexes are represented in Scheme 1. The R_N···H, R_cen···cen and R_C=C geometrical parameters are selected to derive the general trend of the relative strength of these interaction. R_N…H is the -(B)O − H···N- intermolecular hydrogen bond distance between para-X-ba and bpe molecule. R_cen···cen is the distance between the center of pyridine rings in two bpe molecules;. R_C=C is the distance between the C = C bonds stacked in two bpe molecules.

As it can be seen from Scheme 1, para-X-ba are connected to two bpe molecules through –(B)O − H···N– hydrogen bonds. Substitutions of the studied complexes result in essentially change in the R_N…H hydrogen bond lengths (see Table 1). The four R_N…H hydrogen bond distances of each substitute have almost the same length. The R_N…H distances found range from 1.928 to 1.983 Å (mean value) for complexes (para-X-ba):::(bpe)||(bpe):::(para-X-ba) and from 2.093 to 2.169 Å (mean value) for (para-X-ba):::tpcb:::(para-X-ba) complexes. As can be seen in Table 1, the R_N···H for these complexes increases with increasing electron-donating character of substituents.

An electron-withdrawing substituent tends to decrease the R_N…H in complexes in comparison with other substituents and this quantity increases through the incorporation of an electron- donating substituent. As depicted in Table 1, this observation signifies that R_N…H for (X = NH₂) complex is greater than any other complexes and this quantity in (X = NO₂) complex is shorter than those of the corresponding complexes. The R_N…H falls in the order NH₂ > OCH₃ > OH > H > F > Cl > Br > C(O)CH₃ > CN > NO₂. We demonstrated the correlation between the R_N…H and the value of ΔE (Fig. 2).

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Fig. 2

Correlation between ΔE and R_N…H for (para-X-ba):::(bpe)||(bpe):::(para-X-ba) (left) and (para-X-ba):::tpcb:::(para-X-ba) (right) complexes.

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The information on the geometry (i.e. R_N…H, R_cen···cen and R_C=C) were important in the complexes, thus, R_cen···cen and R_C=C of bpe molecules are suitable geometry parameters for strength of π-π stacking interaction. The π-π stacking of the bpe molecule would place the C = C bonds in positions to undergo intermolecular [2 + 2] photodimerizations in (para-X-ba):::(bpe)||(bpe):::(para-X-ba) complexes.

The R_cen···cen distances found range from 3.670 to 3.609 Å (mean value) (para-X-ba):::(bpe)||(bpe):::(para-X-ba) complexes. Inspection of the data in Table 1 shows that the distant between center of pyridine rings in (para-X-ba):::(bpe)||(bpe):::(para-X-ba) complexes (R_cen···cen) is affected by the substituents. Noteworthy, R_cen···cen of the π-π stacking interaction can be reduced by the electron donating substituents, the lowest R_cen-cen is observed for (para-X-ba):::(bpe)||(bpe):::(para-X-ba) complexes (X = NH₂) (see Table 1), also in agreement with the energy analysis, the trend in the R_cen···cen is: NH₂ < C(O)CH₃ < H < Br < F < Cl < OCH₃ < OH < CN < NO₂. Table 1 shows that (para-X-ba):::(bpe)||(bpe):::(para-X-ba) complexes have shorter R_cen-cen which increases with increasing electron- withdrawing character of the substituents, and a reverse behavior is shown for electron- donating substituents. It is largest in (para-NO₂-ba):::(bpe)||(bpe):::(para-NO₂-ba) complex and smallest in (para-NH₂-ba):::(bpe)||(bpe):::(para-NH₂-ba) complex.

The R_C=C distances found range from 3.896 to 3.882 Å (mean value) (para-X-ba):::(bpe)||(bpe):::(para-X-ba) complexes. Inspection of the data in Table 1 shows that the distant between center of pyridine rings in (para-X-ba):::(bpe)||(bpe):::(para-X-ba) complexes (R_C=C) is affected by the substituents. The lowest R_cen-cen is observed for (para-X-ba):::(bpe)||(bpe):::(para-X-ba) complexes (X = NH₂) (see Table 1), also in agreement with the energy analysis, the trend in the R_C=C is: NH₂ < C(O)CH₃ < H < Br < F < Cl < OCH₃ < OH < CN < NO₂. Table 1 shows that (para-X-ba):::(bpe)||(bpe):::(para-X-ba) complexes have shorter R_C=C which increases with increasing electron- withdrawing character of the substituents, and a reverse behavior is shown for electron- donating substituents. It is largest in (para-NO₂-ba):::(bpe)||(bpe):::(para-NO₂-ba) complex and smallest in (para-NH₂-ba):::(bpe)||(bpe):::(para-NH₂-ba) complex.

5 NMR spectroscopy

The NMR analysis will be carried out to understand chemical shielding of δ_N-H, δ_Ha, δ_Hb δ_Hc, δ_Ha′, δ_Hb′ and δ_Hf proton that gathered in Table 2 for (para-X-ba):::(bpe)||(bpe):::(para-X-ba) and (para-X-ba):::tpcb:::(para-X-ba) complexes with electron withdrawing and donating substituents at M05-2X/6–311++G** level of theory. The strength of the hydrogen bonding interactions was explained when setting the electron withdrawing and donating substituents by NMR data. For direct comparison of the ¹H shielding values, we calculated the same for H atoms of tetramethyl silane (TMS) at the same level of theory (δ_H = σ_TMS - σ_H). For TMS protons, NMR shielding appears at 31.62 ppm.

The δ_N-H values are presented in Table 2 that have found range from 7.68 to 8.77 ppm (mean value) for complexes (para-X-ba):::(bpe)||(bpe):::(para-X-ba) and 5.29–7.07 ppm (mean value) for (para-X-ba):::tpcb:::(para-X-ba) complexes, respectively. The variation of δ_N-H can be characterized as the effect of the substituents by influencing –(B)O − H···N– bond that results in stabilization of hydrogen bonding (Table 2).

In order to study the influence of the substituents on the H shielding, the value of H shielding in substituted complexes has been calculated. This hydrogen bonding in (para-X-ba):::(bpe)||(bpe):::(para-X-ba) and (para-X-ba):::tpcb:::(para-X-ba) complexes manifests itself by a shielded N-H proton signal. The influence of the electron withdrawing and the electron-donating substituents on the δ_N-H is observed and presented in Table 2. The (para-X-ba):::(bpe)||(bpe):::(para-X-ba) and (para-X-ba):::tpcb:::(para-X-ba) complexes with electron withdrawing substituents have a larger δ_N-H. In particular, the δ_N-H decreases for these complexes which bear an electron-donation substituent (Table 2). The largest magnitude of δ_N-H correspond to –NO₂ electron donating substituent that is equal to 8.77 and 7.07 ppm in (para-X-ba):::(bpe)||(bpe):::(para-X-ba) and (para-X-ba):::tpcb:::(para-X-ba) complexes, respectively. We see from Table 2 that in fact δ_N-H is lowest for NH₂ substituent with the values of 7.68 and 5.29 ppm for (para-X-ba):::(bpe)||(bpe):::(para-X-ba) and (para-X-ba):::tpcb:::(para-X-ba) complexes, respectively. The reduction of –(B)O − H···N– hydrogen bonds distances is also accompanied by increasing H shielding in complextion. The minimum value of H shielding is accompanied by the highest R_N…H hydrogen bond lengths.

The δ_Ha, δ_Hb δ_Hc, δ_Ha′, δ_Hb′ and δ_Hf of (para-X-ba):::(bpe)||(bpe):::(para-X-ba) and (para-X-ba):::tpcb:::(para-X-ba) complexes were calculated by ab initio method at the M05-2X/6–311++G** level of theory (see Scheme 2), which shows features in reasonable agreement with experimental data (Alvarado et al., 2018). The δ_Ha;δ _Hb δ_Hc, δ_Ha′, δ_Hb′ and δ_Hf are also changed due to influence of the electron withdrawing and donating substituents that summarized in Table 2.

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Scheme 2

The δ_N-H, δ_Ha, δ_Hb δ_Hc, δ_Ha′, δ_Hb′ and δ_Hf of (para-X-ba):::(bpe)||(bpe):::(para-X-ba) and (para-X-ba):::tpcb:::(para-X-ba) complexes.

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The nucleus independent chemical shift (NICS) index is one of the most widely employed indicators of aromaticity, which was proposed by Schleyer and co-workers (Chen et al.; Schleyer and Jiao, 1996; Schleyer et al., 1996). It is defined as the negative value of the absolute shielding computed at a ring center.

We have also studied the aromatic character of para-X-ba by means of NICS calculations. the aromaticity of the para-X-ba acid in (para-X-ba):::(bpe)||(bpe):::(para-X-ba) and (para-X-ba):::tpcb:::(para-X-ba) complexes changes when para-X-ba participate in hydrogen bonding interactions. There are observed changes in aromaticity of these complexes by predictions of NICS due to electron withdrawing and donating substituents. NICS(1) (1 Å above the plane of the ring) essentially reflects π effects. The electron withdrawing and donating effect of the substituent manifests itself by increasing and decreasing the aromaticity of para-X-ba ring in the (para-X-ba):::(bpe)||(bpe):::(para-X-ba) and (para-X-ba):::tpcb:::(para-X-ba) complexes, it is apparent that X  = NO₂ in these complexes has the largest NICS(1). If we compare the NICS(1) of (para-X-ba):::(bpe)||(bpe):::(para-X-ba) and (para-X-ba):::tpcb:::(para-X-ba) complexes with different substituents, we can see that the –NH₂ substituent have lowest one.

The NICS values are presented in Table 2 that have found range from −9.38 to −11.01 ppm for (para-X-ba):::(bpe)||(bpe):::(para-X-ba) complexes and from −9.63 to −10.82 ppm for (para-X-ba):::tpcb:::(para-X-ba) complexes, respectively. We demonstrated the correlation between the NICS and the value of δ_N-H. (Fig. 3).

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Fig. 3

Correlation between δ_N-H and NICS(1) for (para-X-ba):::(bpe)||(bpe):::(para-X-ba) (left) and (para-X-ba):::tpcb:::(para-X-ba) (right) complexes.

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6 AIM analysis

To rationalize the observed trends, the electron density at bond critical points (ρ_BCP) and at cage critical points (ρ_CCP) of computed (para-X-ba):::(bpe)||(bpe):::(para-X-ba) and (para-X-ba):::tpcb:::(para-X-ba) complexes were performed using the AIM analysis at M05-2X/6–311++G** level together with QTAIM that reported in Table 3. The effect of substituents on the hydrogen bonds was completely surveyed by the electron density at bond critical points ρ_BCP (Table 3). The ρ_BCP values are presented in Table 3 that have found range from 5.128 to 5.947 a.u (mean value) for complexes (para-X-ba):::(bpe)||(bpe):::(para-X-ba) and from 2.799 to 3.445 a.u (mean value) for (para-X-ba):::tpcb:::(para-X-ba) complexes, respectively.

Table 3 Electron densities-ρ (in e/a₀³), laplacians of electron densities-∇²ρ (in e/a₀⁵), energy density (H) and potential energy density V(r) at -N∙∙∙H- BCP for (para-X-ba):::(bpe)||(bpe):::(para-X-ba) and (para-X-ba):::tpcb:::(para-X-ba) complexes and Electron densities-ρ (in e/a₀³), laplacians of electron densities-∇²ρ (in e/a₀⁵), energy density (H) and potential energy density V(r) at CCP for (para-X-ba):::(bpe)||(bpe):::(para-X-ba) complexes calculated at M05-2X/6–311++G** level of theory. The ρ and ∇²ρ data multiplied by 10².

Comparison of ρ_BCP indicates that the largest magnitude of ρ_BCP occurred in the presence of –NO₂ substituent and the lowest value of ρ_BCP is related to –NH₂ complex, as given in Table 3. The ρ_BCP varies in the order: NH₂  < OCH₃ < OH < H <  F < Cl < Br < C(O)CH3 < CN < NO₂. This trend can be expected because the strength decreases from electron withdrawing to electron donating substituents.

The ρ_BCP was well correlated with the ΔE, in summary, as the electron density ρ_BCP increases, there is seen a significant enhancement in the binding energies ΔE. The ρ_BCP and interaction energy of these molecules also reveal the same trend (see Tables 1 and 3).

The ρ_BCP is at a maximum in NO₂ substituent that presents the shortest -(B)O − H···N- hydrogen bonds distances, confirming that the ρ_BCP at the bond critical points is a good indicator of the strength of the interaction. In other words, the -(B)O − H···N- hydrogen bonds weakening can be reflected from a decrease in ρ_BCP.

The electron density at cage critical points (ρ_CCP) is in good correlation with strength of π-π stacking interaction which was previously reported (Ebrahimi et al., 2009; Zhikol et al., 2005; Quinonero et al., 2008). To explain the strength of π-π stacking interaction, the ρ_CCP was applied to give reliable results about strength and stabilization of this interaction. The increase in ρ_CCP is a direct consequence of the presence of electron withdrawing substituents, which predicts that electron withdrawing substituents have reinforced effects on (para-X-ba):::(bpe)||(bpe):::(para-X-ba) complexes (Table 3). The lowest value of ρ_CCP is attributed to the NH₂ and NO₂ substituent exhibit largest values of ρ_CCP than the corresponding ρ_CCP for other substituents. The results also indicate that the NO₂ substituent apparently enhances the π-π stacking interaction.

The results obtained for the Laplacian (∇²ρ), potential energy density, V(r) and the energy density (H) at the critical points (CPs) are evaluated by means of the AIM approach at the M05-2X/6–311++G** level for the studied complexes.

Moreover, we have found that the sign of the Laplacian of the electron density at the bond critical point (∇²ρ) and that of the energy density (H) could characterize the strength of hydrogen bonding interaction. If the classification of Rozas et al. (Rozas et al., 2000) has been corrected for individual interactions in hydrogen bonding interactions. Thus, weak hydrogen bonding show both ∇²ρ_BCP and H_BCP > 0, and medium hydrogen bonding show ∇²ρ_BCP > 0 and H_BCP < 0, while strong hydrogen bonding show both ∇²ρ_BCP and H_BCP < 0. In these interactions, with regard to ∇²ρ_BCP > 0 and H_BCP < 0 at BCPs in the complexes, they will be classified as medium hydrogen bonding interaction (see Table 3).

7 Conclusion

The computational work that was done in this study complements and extends the general research on the prediction of binding energy (Alvarado et al., 2018); chemical shielding and electron density of (para-X-ba):::(bpe)||(bpe):::(para-X-ba) and (para-X-ba):::tpcb:::(para-X-ba) complexes.

It was found that the hydrogen bonds and π-π stacking in (para-X-ba):::(bpe)||(bpe):::(para-X-ba) and (para-X-ba):::tpcb:::(para-X-ba) complexes with electron-withdrawing substituents were generally observed better activities than the one with electron-donating substituents. Complexes involving electron withdrawing substituents obtained more favorable binding energy values than those involving electron donating substituents. (para-NO₂-ba):::(bpe)||(bpe):::(para-NO₂-ba) Complex involving NO₂ obtained the largest interaction energy value of the study (-38.84 kcal/mol). On the contrary, (para-NH₂-ba):::(bpe)||(bpe):::(para-NH₂-ba) complex involving the NH₂ substituent obtained the lowest interaction energy value of the study (−34.10 kcal/mol).

The hydrogen bond interaction was further proven by ¹H NMR. A clear upfield shift (from δ_N-H = 7.68 to 8.77 ppm in (para-X-ba):::(bpe)||(bpe):::(para-X-ba) complexes) and (from δ_N-H = 5.29 to 7.07 ppm in (para-X-ba):::tpcb:::(para-X-ba) complexes) of the δ_N-H proton signal was observed after complextion with electron-withdrawing, indicating these substituents is making the hydrogen bond stronger. The system bearing the electron withdrawing substituents yields an increase of the corresponding δ_N-H. It was demonstrated that for these complexes with electron donating, the calculated shielding of the proton engaged in the hydrogen bonding δ_N-_H was reduced. The ρ_BCP increases in the complexes having electron-donating or withdrawing substituents and the calculated ρ_BCP values suggest that –NO₂ substituent has the maximum ρ_BCP for both (para-X-ba):::(bpe)||(bpe):::(para-X-ba) and (para-X-ba):::tpcb:::(para-X-ba) complexes, it is clear that this substituent is making the hydrogen bond stronger. The maximum value of H shielding is accompanied by the highest ρ_BCP hydrogen bond lengths (see Fig. 4).

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Fig. 4

Correlation between δ_N-H and ρ_BCP for (para-X-ba):::(bpe)||(bpe):::(para-X-ba) (left) and (para-X-ba):::tpcb:::(para-X-ba) (right) complexes.

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