Synthesis and study on aroylhydrazones having cyanovinylpyrrole

2.1 Preparation of ethyl 3,5-dinitrobenzoate (Vogel, 1956)

3,5-dinitrobenzoic acid (5.320 g) was dissolved in 15 ml ethanol and 1.0 ml of conc. H₂SO₄ was added as catalyst and solution was refluxed for 20 h. The solution was cooled down to room temperature and refrigerated for 8 h to precipitation. The precipitate was recrystallized in ethanol to give the product ethyl 3,5-dinitrobenzoate. m.p. – 90 °C.

2.2 Preparation of 3,5-dinitrobenzohydrazide (Wild, 1947)

Ethyl 3,5-dinitrobenzoate (1.60 g, 6.6633 mmol) and hydrazine hydrate (0.48 ml, 0.01 mol) of 100% in 10 ml of methanol were refluxed for 10 h. The solution was cooled to room temperature to obtain yellow colored precipitate. The precipitate was filtered off and recrystallized from methanol to give pure product 3,5-dinitrobenzohydrazide.

Color: dark yellow, yield: (0.9422 g, 62.54%), m.p.: 145 °C.

2.3 Preparation of 2-hydroxybenzohydrazide (salicylhydrazide) (Gup and Kırkan, 2005)

A solution of hydrazine hydrate (0.1645 g, 0.16 ml, 3.286 mmol) in 10 ml methanol was added dropwise to the stirring solution of methyl 2-hydroxybenzoate (0.500 g, 0.42 ml, 3.286 mmol) in 25 ml methanol. Reaction mixture was refluxed for 6 h. After refluxing, cream color precipitate was obtained. The precipitate was filtered off, washed with methanol and dried in air.

Color: creamy white, yield: 0.3020 g, 72.88%, m.p.: 140 °C.

2.4 Ethyl 2-cyano-3-[5-{(2-hydroxy-benzoyl)-hydrazonomethyl}-1H-pyrrol-2-yl]-acrylate (3a)

To the equimolar solution of ethyl 2-cyano-3-(5-formyl-1H-pyrrol-2-yl)-acrylate (1) (0.200 g, 0.9171 mmol) and 2-hydroxy-benzoic acid hydrazide (2a) (0.1394 g, 0.9171 mmol) in 20 ml methanol, 0.01 ml of conc. HCl acid was added as catalyst. The reaction mixture was stirred at room temperature for overnight to obtain yellow color precipitate. The precipitate was filtered off, washed with MeOH and dried in air, afforded (3a), as dark yellow solid. Yield: 0.1582 g, 48.96%. M.p.: 278–280 °C. Anal. calcd. for C₁₈H₁₆N₄O₄: C 61.34%, H 4.57%, N 15.90%; obs.: C 61.38%, H 4.54%, N 15.92%. MS (m/z): calcd. 352.1171, obs. 353 [M⁺+1]. ¹H NMR (300 MHz, DMSO-d₆) δexp/calcd 12.680/12.545 (s, 1H, —OH), δ 12.050/10.313 (s, 1H, pyrrole-NH), 12.039/9.582 (s, 1H, ArCONH), 8.367/8.085 (s, 1H, vinyl-CH⚌C), 7.850–7.876/7.872–7.169 (d, J = 7.8 Hz, 2H, Ar-CH), 6.953–6.980/7.872–7.169 (t, J = 9.75 Hz, 2H, Ar-CH), 7.407/7.97 (s, 1H, —CH⚌N), 7.444–7.457, 6.887–6.875/6.966–7.253 (m, 2H, J = 3.9, 3.6 Hz, pyrrole-CH), 4.231–4.301/4.347 (q, J = 7.00 Hz, 2H, ester-CH₂), 1.263–1.310/1.473–1.236 (t, J = 7.05 Hz, 3H, ester-CH₃). ¹³C NMR (300 MHz, DMSO-d₆) δexp/calcd C2-123.22/128.08, C3-122.26/126.82, C4-113.98/115.7, C5-132.39/137.05, C6-141.95/141.44, C7-108.27/91.89, C8-114.34/116.08, C10-163.68/161.23, C13-50.28/65.16, C14-15.23/16.72, C15-145.38/132.42, C18-168.54/162.88, C20-111.29/112.53, C21-120.42/124.33, C22-120.42/116.25, C23-129.53/133.29, C24-129.53/115.99, C25-158.48/160.96.

2.5 Synthesis of ethyl 2-cyano-3-[5-{(4-nitro-benzoyl)-hydrazonomethyl}-1H-pyrrol-2-yl]-acrylate (3b)

To the equimolar solution of ethyl 2-cyano-3-(5-formyl-1H-pyrrol-2-yl)-acrylate (0.250 g, 1.1464 mmol) and isoniazid (0.1572 g, 1.1464 mmol) in 25 ml methanol, 0.01 ml of conc. HCl acid was added as catalyst. The reaction mixture was stirred at room temperature for 8 h and yellow color precipitate was obtained. The precipitate was filtered off, washed with methanol and dried in air, afforded (3b), as yellow solid. Yield: 0.3464 g, 89.63%. The compound decomposes above 280 °C. Anal. calcd. for C₁₇H₁₅N₅O₃: C 60.51%, H 4.48%, N 20.76%; obs.: C 60.54%, H 4.50%, N 20.72%. MS (m/z): calcd. 337.117; obs. 338.14 [M⁺+1]. ¹H NMR (300 MHz, DMSO-d₆) δexp/calcd 12.749/10.3155 (s, 1H, pyrrole-NH), 12.707/9.5591 (s, 1H, ArCONH), 8.790/8.0801 (s, 1H, vinyl-CH⚌C), 8.369–8.343/9.0865–9.1345 (d, J = 7.8 Hz, 2H, pyridine-CH), 8.463/7.9717 (s, 1H, —CH⚌N—), 6.974–6.995/7.8626–8.1343 (d, J = 6.3 Hz, 2H, pyridine-CH), 6.779–6.769/7.2548(d, 2H, J = 3.6, 2.7 Hz, pyrrole-CH), 8.596–8.587/6.9616 (d, J = 2.7 Hz, 1H, pyrrole-CH), 4.247–4.314/4.3452–4.3542 (q, J = 6.70 Hz, 2H, ester-CH₂), 1.283–1.243/1.2348–1.4826 (t, J = 6.0 Hz, 3H, ester-CH₃). ¹³C NMR (300 MHz, DMSO-d₆) δexp/calcd C2-125.24/128.1798, C3-123.60/126.8273, C4-116.96/115.9032, C5-133.84/136.9796, C6-138.68/141.5059, C7-114.98/92.0105, C8-113.94/116.0699, C10-157.56/161.2264, C13-52.20/65.1485, C14-15.28/16.6665, C15-141.82/132.6996, C18-159.92/158.5427, C20-129.78/139.4344, C21-117.68/118.2875, C22-142.67/148.5444, C24-142.67/148.8017, C25-117.68/122.2078.

2.6 Synthesis of ethyl 2-cyano-3-[5-{(3,5-dinitro-benzoyl)-hydrazonomethyl}-1H-pyrrol-2-yl]-acrylate (3c)

To the equimolar solution of ethyl 2-cyano-3-(5-formyl-1H-pyrrol-2-yl)-acrylate (1) (0.250 g, 1.1464 mmol) and 3,5-dinitro-benzohydrazide (2c) (0.2592 g, 1.1464 mmol) in 25 ml methanol, 0.01 ml of conc. HCl acid was added as catalyst. The reaction mixture was stirred at room temperature. After stirring for 4 h, dark yellow color precipitate was obtained. The precipitate was filtered off, washed with methanol and dried in air, afforded (3c), as yellow solid. Yield: 0.3844 g, 78.68%. Anal. calcd. for C₁₈H₁₄N₆O₇: C 50.69%, H 3.31%, N 19.71%; obs.: C 50.72%, H 3.28%, N 19.74%. MS (m/z): calcd. 426.0924; obs. 427.18 [M⁺+1]. ¹H NMR (300 MHz, DMSO-d₆) δ exp/calcd 12.675/10.4148, (s, 1H, pyrrole-NH), 12.110/9.0539, (s, 1H, ArCONH), 8.366/9.4455, (s, 1H, Ar-CH), 7.867/9.3966, (s, 1H, Ar-CH), 7.566/9.0367, (s, 1H, Ar-CH), 7.496/7.9468, (s, 1H, vinyl-CH⚌C), 7.447/7.8184, (s, 1H, —CH⚌N), 6.877/7.0459, (s, 1H, pyrrole-CH), 6.127/6.8171, (s, 1H, pyrrole-CH), 4.230–4.299/4.3266–4.3243, (q, J = 6.90 Hz, 2H, ester-CH₂), 1.263–1.310/1.2642–1.4851, (t, J = 7.05 Hz, 3H, ester-CH₃). ¹³C NMR (300 MHz, DMSO-d₆) δexp/calcd C2-126.04/128.6533, C3-122.58/126.153, C4-117.98/116.3081, C5-133.04/135.7931, C6-138.96/141.43, C7-116.64/93.2815, C8-113.89/115.4622, C10-158.74/160.6803, C13-53.03/65.6575, C14-15.94/16.7965, C15-144.05/133.6347, C18-157.66/155.1949, C31-128.88/136.2025, C32-127.15/130.9513, C33-139.64/148.7423, C34-119.57/122.1914, C35-139.64/147.9564, C36-127.15/125.8432.

4.1 Molecular geometry and stability of conformers

The route for the formation of products (3a–c) involved in chemical reactions is shown in Scheme 1. The optimized geometry for the ground state lower energy conformer of (3a–c) is shown in Fig. 1. Selected optimized geometrical parameters of (3a–c), calculated at B3LYP/6-31G(d, p) are listed in Supplementary material (S Table 1). The ground state lower energy conformer of all the molecules (3a–c) possesses C₁ symmetry. The asymmetry of the N1—C2 and N1—C5 bonds i.e. difference between their bond lengths can be explained due to the presence of the two different groups as ‘ethyl 2-cyano-acrylate’ and ‘aroyl hydrazonomethyl’ at C2, C5 of pyrrole ring, respectively. The E–configuration about the vinyl C6⚌C7 bond with respect to the higher priority group ethoxycarbonyl and pyrrole gives lower energy conformer. The molecule also exists in E-configuration with respect to the pyrrole and —NH—CO—Ar group located on the opposite side of the Schiff base C15⚌N16 bond. The E-configuration about Schiff base C15⚌N16 bond is not only observed in our theoretical study but also reported in crystal structure of various aroylhydrazone derivatives (Wei, 2012; Zong, 2012). In (3a–c), due to the presence of intramolecular hydrogen bonding (N1—H27/26/20⋯N9) the pyrrole N—H bond is elongated than free N—H bond (Rawat and Singh, 2014a, 2014b, 2014c, 2015a, 2015b; Singh et al., 2013a, 2013b, 2013c, 2013e, 2013e, 2014a, 2014b).

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Scheme 1

Optimized geometry of the reactants (1, 2a–c) and products (3a–c).

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Figure 1

Optimized geometry for the ground state lower energy conformer of (3a–c).

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4.2 ¹H and ¹³C NMR spectroscopy

NMR calculations are now attainable and accurate enough to be useful exploring the relationship between chemical shift and molecular structure. Density functional theory (DFT) has emerged in recent years as a promising alternative to conventional ab initio methods in quantum chemistry. It therefore seems reasonable to investigate in detail how well DFT predicts magnetic response properties, in particular shielding tensors (Kim et al., 2000). A number of methods have been developed for the calculation of molecular second-order magnetic response properties. It is generally accepted that accurate prediction of these properties within the finite basis approximation, requires gauge-invariant procedures (Wolinski et al., 1990). The geometry of (3a–c) compounds, together with that of tetramethylsilane (TMS) is fully optimized. ¹H and ¹³C NMR chemical shifts are calculated with GIAO approach at B3LYP/6-31G(d, p) method. The experimental ¹H NMR spectra of products (3a–c) in DMSO-d₆ are given in Supplementary Fig. S1a–c. The ¹H NMR chemical shifts (δ/ppm) of (3a–c) are assigned in Sections 2.4-2.6. In the ¹H NMR spectra of (3a–c), a quartet and a triplet chemical shift designate the presence of ester —CH₂, —CH₃ group in all the molecules. In (3a–c), the observed chemical shifts as a singlet at 7.404, 8.463, 7.447 ppm indicate presence of the hydrazone linkage (CH⚌N—NH—) in these molecules. The big difference in the certain experimental and calculated chemical shift of the compound is due to the formation of intermolecular hydrogen bonding with solvent. The pyrrolic NH and hydrazide NH appear at downfield in experimental spectra due to the formation of intermolecular hydrogen bonding with DMSO solvent. The higher chemical shift for ArCONH is observed due to intermolecular hydrogen bonding with solvent than calculated in gaseous state. In order to compare the chemical shifts, correlation graph between the experimental and calculated ¹H NMR chemical shifts is shown in Supplementary Fig. S2. The correlation coefficients (R² = 0.937, 0.893, 0.927) for (3a–c), show that experimental ¹H NMR data are consistent with the calculated data from optimized structure of probed molecules.

The experimental ¹³C NMR spectra of (3a–c) in DMSO-d₆ are given in Supplementary Fig. S3a–c. Additional support for molecular structures of (3a–c) is provided by their ¹³C NMR spectra, in which chemical shifts of the C15 carbon atom at 145.38, 143.39, 144.05 ppm for —CH⚌N—NH and C18 carbon atom at 168.54, 160.46, 157.66 ppm for —NH—CO—Ar confirm the aroylhydrazone character of these molecules. In order to compare the chemical shifts, correlation graph between the experimental and calculated ¹³C NMR chemical shifts is shown in Supplementary Fig. S4. The correlation coefficients (R² = 0.937, 0.952, 0.967) show that there is a good agreement between experimental and calculated ¹³C NMR chemical shifts.

The mass spectra of (3a–c) showed [M + 1] peaks with agreement of their molecular formula. The molecular ion M + 1 peaks observed at m/z: 353, 338 and 427 corresponding to their relative molecular mass peaks in 3a, 3b and 3c, mass spectra respectively are shown in Supplementary material Fig. S5a–c.

4.3 Electronic absorption (UV–Visible) and natural bond orbitals (NBO) analysis

The nature of excitations in the observed UV–Visible spectra of (3a–c) compounds has been studied by the time dependent density functional theory (TD-DFT). The calculated and experimental electronic excitations of high oscillatory strength are listed in Table 1. The experimental UV–Visible spectra of (3a–c) are shown in Fig. 2 and molecular orbital plots are shown in Supplementary material Fig. S6. The vicinal orbitals of HOMO and LUMO play the same role of electron donor and electron acceptor, respectively. The HOMO–LUMO energy gap is an important reactivity index. The HOMO–LUMO energy gap of 3.25, 3.26, 2.74 eV for (3a–c), reflects the chemical reactivity of these molecules.

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Figure 2

The experimental UV–Visible spectra of (3a-c).

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A combined experimental and theoretical UV–Visible spectrum analysis of (3a) indicates that the first observed electronic transitions at λ = 244 nm correspond to the electronic excitations calculated at λ = 295.39 nm, f = 0.1662. The second observed λ_max at 412 nm is in good agreement with the calculated λ_max = 426.38 nm, f = 0.9548. For (3b), the observed λ at 233, 389 nm agrees with the calculated electronic excitations at λ = 252.27 nm, f = 0.4711, λ = 416.01 nm, f = 0.9518, respectively. The UV–Visible spectrum analysis of (3c) indicates that the observed λ at 242, 402 nm matches well with the electronic excitations calculated at λ = 252.47 nm, f = 0.5936; λ = 413.00 nm, f = 0.8984, respectively. Therefore, the observed λ is slightly blue shifted compared with the calculated λ. On the basis of molecular orbital coefficients and molecular orbital plots the nature of electronic excitations for (3a) is assigned as π → π∗, whereas for (3b), (3c) as n → π^∗.

A useful aspect of the NBOs is that it provides an accurate method for studying intramolecular bonding and interaction among bonds and also gives an efficient basis for investigating charge transfer or conjugative interaction in various molecular systems. Second-order perturbation theory analysis of the Fock matrix in NBO basis for (3a–c) is presented in Supplementary material (S Table 2a–c). In (3a–c), the interactions π(C2—C3) → π^∗(C4—C5), π(C4—C5) → π^∗(C2—C3) are responsible for the conjugation of respective π-bonds in pyrrole ring due to the high electron density at conjugated π bonds (1.666–1.678) and low electron density at π^∗ bonds (0.420–0.433) and stabilized the molecules with energy 83.01–95.60 kJ/mol. The interaction n₁(N1) → π^∗(C2—C3)/π^∗(C4—C5) shows that loan pair of pyrrole N atom takes part in electron delocalization within ring. In the same manner, the π → π^∗ interactions of benzene or pyridine ring designate the conjugation of respective π-bonds within ring and stabilized the molecules in a broad region 57.43–110.52 kJ/mol, due to presence of the different substituent at benzene ring as 2-OH, 3,5-NO₂, for (3a), (3c) and pyridine ring in (3b), respectively. The interactions π(C2—C3) → π^∗(C6—C7), π(C6—C7) → π^∗(C2—C3) are responsible for the conjugation of bonds C2—C3, C6—C7 with C2—C6 and stabilized the molecule with energy 50.70–104.20 kJ/mol. In the same manner, the interactions π(C4—C5) → π^∗(C15—N16) and π(C15—N16) → π^∗(C4—C5) are responsible for the conjugation of these bonds with C5—C15. It is to be noticed that the charge transfer interactions are formed by the orbital overlap between bonding (π) and antibonding (π^∗) orbitals, which results in intramolecular charge transfer (ICT) causing stabilization of the system. The movement of π-electron cloud from donor to acceptor i.e. intramolecular charge transfer (ICT) can make the molecules more polarized, which must be responsible for the NLO properties of molecules. Therefore, the titled compounds may be used for non–linear optical materials. The primary hyperconjugative interaction π₁(C8—N9) → σ^∗(N1—H27)/σ^∗(N1—H26)/σ^∗(N1—H20) designates the intramolecular hydrogen bonding as N1—H27⋯N9, N1—H26⋯N9, N1—H20⋯N9 in molecules (3a–c), respectively. In (3a), the interaction n₂(O19) → σ^∗(O26—H42) shows existence of intramolecular hydrogen bonding as O26—H42⋯O19. The presence of nitro group in (3c) stabilized the molecule to a greater extent of 679.30, 693.10 kJ/mol due to the interactions n₃(O41) → π^∗(N40—O42), n₃(O38) → π^∗(N37—O39), respectively. In (3a–c), the secondary hyperconjugative interactions σ → σ^∗ associated with pyrrole and benzene ring stabilized the molecules with energy 9.57–19.48 kJ/mol. Selected Lewis orbitals (occupied bond or lone pair) of (3a–c) with their NBO hybrid orbitals are listed in Supplementary material (S Table 3a–c). The NBO hybrid orbitals analysis shows that all the N—H/C—N bond orbitals are polarized toward the nitrogen atom (ED = 57.39 − 74.75% at N), whereas C—O/N—O bond orbitals toward oxygen atom (ED = 51.34 − 79.02% at O). The electron density distribution (occupancy) around the imino group (N—H) also influences the polarity of the compound. Therefore, they consist with the maximum electron density on the oxygen and nitrogen atoms, which is responsible for the polarity of the molecule.

4.4 Emission (photoluminescence) spectroscopy

The optical properties of (3a–c) have been explored using electronic absorption and photoluminescence spectra. Quantum mechanically, Photoluminescence (PL) can be described as an excitation to a higher energy state and then a return to a lower energy state accompanied by the emission of a photon. The emitted radiation is generally of a longer wavelength than the wavelengths imposed on the photoluminescent material and difference between wavelength absorption maxima and wavelength emission maxima is known as the Stoke’s shift.

For any photoluminescent species, the quantum yield (Φ_F) of its luminescence is a basic property, and its measurement is an important step in the characterization of the species. According to the definition of the Φ_F (Brouwer, 2011), only two quantities need to be known, viz. the number of photons absorbed and the number of photons emitted per unit of time. The quantum yields (Φ_F) of (3a–c) were determined in dilute solutions with an absorbance below 0.1 at the excitation wavelength. Quinine sulfate (in 0.1 M H₂SO₄ λ_ex = 347 nm, Φ = 0.57) was used as a standard (Lakowicz, 2006). Quantum yields were calculated using the following equation: $$({\mathrm{\Phi }}_F)=[F^s\times f_r{({\eta }_s)}^2/F^r\times f_s{({\eta }_r)}^2]\times {{\mathrm{\Phi }}_F}^r$$ where F^s and F^r denote the area under the fluorescence band of sample and reference, f_r and f_s denote the absorbance at the excitation wavelength of sample and reference, and η denotes the refractive index. Refractive index was calculated by the Lorentz–Lorentz equation ((4π/3) ∗ (α_λ/V_mol)). Integration of the emission bands was performed using Origin 7.1.

The experimental emission spectra of (3a–c) were recorded in DMSO, at excitation λ_ex.max, and are shown in Fig. 3. The experimental PL spectral data of (3a–c) are listed in Table 2 with quantum yield. The most striking feature is that the (3a) gives an intense emission at λ_em.max = 572 nm in yellow region upon irradiation by ultraviolet light λ_ex.max = 244 nm, whereas another intense emission at λ_em.max = 521 nm in green region upon irradiation by visible light λ_ex.max = 412 nm. The compound (3b) gives two intense emissions at λ_em.max = 480, 522 nm in blue and green region, respectively, upon irradiation by ultraviolet light λ_ex.max = 233 nm. Another ultraviolet irradiation at λ_ex.max = 389 nm gives also an intense emission at same λ_em.max = 480 nm in blue region. The compound (3c) gives an intense emission at same λ_em.max = 520 nm in green region upon irradiation by λ_ex.max = 242, 440 nm. Therefore, the emission spectra show that (3a–c) are good photoluminescent material due to intense emission at higher wavelength in yellow, green and blue region with high Stoke’s shift in the region 80–328 nm.

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Figure 3

The experimental emission spectra of (3a–c).

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The data with the model compounds have shown that the quantum yield of aromatic hydrazone of 3a is essentially affected by an ortho-hydroxyl substituent. Furthermore, the compound 3c shows only one excitation peak at 402 and 442 excitation wavelengths.

4.5 Vibrational band assignments

The aim of the vibrational analysis is to decide which of the vibrational modes in the molecule give rise to each of the observed bands at specific wave numbers in the FTIR spectra. The functional groups present in the molecule were identified and a satisfactory vibrational band assignment has been made for the fundamental modes of vibration by observing the position, shape and intensity of the bands. Vibrational frequencies of similar pyrrole hydrazones (Singh et al., 2013a, 2013b, 2013c, 2013d, 2013e, 2014a, 2014b; Rawat and Singh, 2015a, 2015b), compounds and their derivatives have been taken into consideration for the assignment of fundamental vibrations of studied compounds (3a–c). The theoretical (selected) and experimental vibrational modes of (3a–c), calculated at B3LYP/6-31G(d, p) method and their approximate assignments are given in Table 3. Comparison between experimental and theoretical IR spectra in the region 4000–400 cm⁻¹ is shown in Supplementary Fig. 7a–c. The calculated vibrational wave numbers are higher than their experimental values for the majority of the normal modes. Two factors may be responsible for the discrepancies between the experimental and computed wave numbers. The first is caused by the environment (gas and solid phase) and the second is due to the fact that the experimental values are an anharmonic wave numbers while the calculated values are harmonic ones. Therefore, calculated wave numbers are scaled down using scaling factor 0.9608 (Rawat and Singh, 2014a, 2014b, and 2019), to discard the anharmonicity present in real system.

4.6 Quantum theory of atoms in molecules (QTAIM) analysis

Topological as well as geometrical parameters are useful tool to characterize the strength and nature of hydrogen bond (Bader, 1990; Lee et al., 1994). Molecular graphs of (3a–c) using AIM program at B3LYP functional are shown in Supplementary Fig. S8. Topological as well as geometrical parameters calculated at B3LYP/6-31G(g, p) and ωB97X/6-31G(d, p) level for bonds of interacting atoms for 3a, 3b and 3c are given in Tables 4 and 5, respectively. For the intramolecular interactions N1—H27/H26/H20⋯N9 electron density (ρ_H…_A) and its Laplacian (∇²ρ_BCP) follow the Koch and Popelier (1995) criteria and the distance between interacting atoms (d_{H27/H26/H20⋯N9} = 2.44–2.45 Å) is less than the sum of van der Waals radii of these atoms. Therefore, these interactions are in the category of hydrogen bonds. The nature of N1—H27/H26/H20⋯N9 hydrogen bonds is weak due to (∇²ρ_BCP) > 0 and H_BCP > 0, whereas O26—H42⋯O19 is a medium hydrogen bond due to (∇²ρ_BCP) > 0 and H_BCP < 0. In this article, QTAIM theory is used to estimate hydrogen bond energy (E), and the energy of intramolecular hydrogen bonds N1—H27/H26/H20⋯N9, O26—H42⋯O19 is calculated as 9.23, 9.23, 9.41, 55.72 kJ/mol, respectively. In (3a), the presence of bond critical point (BCP) at H37···H38 contact designates presence of the dihydrogen bonding due to short d_H37⋯H38 of 1.97 Å. Energy of hydrogen bonding is calculated using AIM and DFT calculations. With respect to change of both the method used and the basis set the reliability and stability of values of QTAIM parameters have been studied and found that they were almost independent of basis set in case of used B3LYP functional in DFT (Jablonski and Palusiak, 2010). Energy of hydrogen bonding has been calculated using both B3LYP functional and ωB97X with 6-31G(d, p) basis set for describing hydrogen bond interactions in molecules. The calculated values tabulated in Tables 4 and 5 found to vary only slightly.

4.7 Chemical reactivity

The chemical reactivity of molecule is described in three ways as: (i) Molar refractivity (MR) and (ii) Global and local electronic reactivity descriptors.

4.8 Static dipole moment (μ₀), mean polarizability (|α₀|), anisotropy of polarizability (Δα) and first hyperpolarizability (β₀)

As hyperpolarizability is difficult task to measure directly, computational calculation is an alternate choice and provides another method to investigate extensive properties of materials. Polarizabilities and hyperpolarizabilities are described as response of a system in the presence of an applied electric field (Kleinmann, 1962). In order to investigate the relationship between molecular structure and NLO response, first hyperpolarizability (β₀) of this novel molecular system, and related properties (|α₀| and Δα) are calculated using B3LYP/6-31G(d, p), based on the finite-field approach and their calculated values are given in Table 9. Total static dipole moment (μ₀), mean polarizability (|α₀|), anisotropy of polarizability (Δα) and first hyperpolarizability (β₀), using x, y, z, components are defined as

• μ₀ = (μ²_x + μ²_y + μ²_z)^1/2

• |α₀| = 1/3(α_xx + α_yy + α_zz)

• Δα = 2^–1/2[(α_xx − α_yy)² + [(α_yy − α_zz)² + (α_zz − α_xx)² + 6α²_xx]^1/2

• β₀ = [(β_xxx + β_xyy + β_xzz)² + (β_yyy + β_xxy + β_yzz)² + (β_zzz + β_xxz + β_yyz)²]^1/2

Large value of particular component of the polarizability and hyperpolarizability indicates a substantial delocalization of charge in these directions. Since the value of the polarizability (|α₀|) and first hyperpolarizability (β₀) of Gaussian 09 output is reported in atomic unit (a.u.) these values are converted into electrostatic unit (esu) using converting factors as for α₀: 1 a.u. = 0.1482 × 10⁻²⁴ esu; for β₀: 1 a.u = 0.008639 × 10⁻³⁰ esu. The first hyperpolarizabilities (β₀) of the title molecules (3a–c) are calculated as 28.48, 14.62, 35.76 × 10⁻³⁰ esu, respectively with respect to p-Nitroaniline (p-NA) as a reference. The first hyperpolarizabilities of (3a–c) have been found to increase with electron withdrawing substituents and might be used as non-linear optical (NLO) material.