Application of Hammett equation to intramolecular hydrogen bond strength in para-substituted phenyl ring of trifluorobenzoylacetone and 1-aryl-1,3-diketone malonates European Journal of Chemistry 9 (3) (2018) 213-221 European Journal of Chemistry View Journal Online View Article Online Application of Hammett equation to intramolecular hydrogen bond strength in para-substituted phenyl ring of trifluorobenzoylacetone and 1-aryl-1,3- diketone malonates Vahidreza Darugar 1,*, Mohammad Vakili 1,*, Sayyed Faramarz Tayyari 1, Fadhil Suleiman Kamounah 2 and Raheleh Afzali 1 1 Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, Mashhad 91775-1436, Iran vahidrezadarugar@mail.um.ac.ir (V.D.), vakili-m@um.ac.ir (M.V.), sftayyari@yahoo.com (S.F.T.), afzalimona@yahoo.com (R.A.) 2 Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100, Copenhagen, Denmark fadil@chem.ku.dk (F.S.K.) * Corresponding author at: Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, Mashhad 91775-1436, Iran. Tel: +98.051.38805551 Fax: +98.051.38796416 e-mail: vakili-m@um.ac.ir (M. Vakili), vahidrezadarugar@mail.um.ac.ir (V. Darugar). 10.5155/eurjchem.9.3.213-221.1713 Received: 13 April 2018 Received in revised form: 29 May 2018 Accepted: 02 June 2018 Published online: 30 September 2018 Printed: 30 September 2018 The stability of two stable cis-enol forms in two categories of β-diketones, including para- substituted of trifluorobenzoylacetone (X-TFBA) and 1-aryl-1,3-diketone malonates (X-ADM, X: H, NO2, OCH3, CH3, OH, CF3, F, Cl, and NH2) has been obtained by different theoretical methods. According to our results, the energy difference between the mentioned stable chelated enol forms for the titled compounds is negligible. The theoretical equilibrium constants between the two stable cis-enol of the mentioned molecules are in excellent agreement with the reported experimental equilibrium constant. In addition, the effect of different substitutions on the intramolecular hydrogen bond strength has been evaluated. The correlation between Hammett para-substituent constants, σp. with the theoretical and experimental parameters related to the strength of hydrogen bond in p-X-TFBA and p-X- ADM molecules also investigated by means of density functional theory calculations. The electronic effects of para-substitutions on the intramolecular hydrogen bond strength were determined by NMR and IR data related to intramolecular hydrogen bond strength, geometry, natural bond orbital results, and topological parameters. These parameters were correlated with the Hammett para-substituent constants, σp. Good linear correlations between σp and the several parameters related to the hydrogen bond strength, in this study were obtained. AIM DFT NBO Hammett LFER Substituent effect Intramolecular hydrogen bond Cite this: Eur. J. Chem. 2018, 9(3), 213-221 Journal website: www.eurjchem.com 1. Introduction A hydrogen bond is an associative interaction between molecules containing a polar H-A bond and an electron donor B. A and B are atoms with greater electronegativity than hydrogen and if A and B belong to the same molecule intramolecular hydrogen bonding the intramolecular hydrogen bond (IHB) occurs if the spatial configuration is favorable. In 1919, the concept of hydrogen bond had been proposed by Huggins [1]. After that, the properties of intramolecular and intermolecular hydrogen bonded systems have been studied theoretically and experimentally by several workers [2-6]. The cis-enol forms of β-diketones are engaged in an intramolecular hydrogen bond, IHB, system [7,8], which, as resulted by Gilli et al. [9-12], the π-electron delocalization between the donor and acceptor atoms is responsible to increase the intramolecular HB strength in malonaldehyde, β- diketones and derivatives. Formation of IHB causes an obvious affinity for equalization of the valence bonds in the resulting chelated ring. Thus, any parameter that affects the electron density of the chelating ring will change the IHB strength, EHB. Two stable cis-enol forms of 4,4,4-trifluoro-1-phenyl-1,3- butanedione, known as trifluorobenzoylacetone (TFBA), and 1-aryl-1,3-diketone malonates (ADM) as asymmetric β- diketones, were characterized by the position of the phenyl group, which can be attached at C2 or at C4 (i.e. adjacent to C=O and C-O bonds), respectively (Figure 1). These tautomers are labeled as X-TFBA-2, X-TFBA-4, X-ADM-2, and X-ADM-4, respectively. Replacing the hydrogen atom in the para position of phenyl ring with an electron-withdrawing group (EWG) or electron donating group (EDG) causes a charge redistribution in the π-electrons of the chelated ring. Therefore, the IHB of these molecules is affected by the substitution on the para position of the phenyl group [13-16]. Hammett quantified the effects of substituents by considering an empirical electronic substituent parameter (σ), which obtained from the acid dissociation constants, Ka’s of substituted benzoic acids [17,18]. ABSTRACT RESEARCH ARTICLE KEYWORDS European Journal of Chemistry ISSN 2153-2249 (Print) / ISSN 2153-2257 (Online) – Copyright © 2018 The Authors – Atlanta Publishing House LLC – Printed in the USA. This work is published and licensed by Atlanta Publishing House LLC – CC BY NC – Some Rights Reserved. http://dx.doi.org/10.5155/eurjchem.9.3.213-221.1713 http://dx.doi.org/10.5155/eurjchem.9.3.213-221.1713 https://crossmark.crossref.org/dialog/?doi=10.5155/eurjchem.9.3.213-221.1713&domain=pdf&date_stamp=2018-09-30 http://www.eurjchem.com/ http://dx.doi.org/10.5155/eurjchem.9.3.213-221.1713 mailto:vahidrezadarugar@mail.um.ac.ir mailto:vakili-m@um.ac.ir mailto:sftayyari@yahoo.com mailto:afzalimona@yahoo.com mailto:fadil@chem.ku.dk mailto:vakili-m@um.ac.ir mailto:vahidrezadarugar@mail.um.ac.ir http://www.eurjchem.com/ https://crossmark.crossref.org/dialog/?doi=10.5155/eurjchem.9.3.213-221.1713&domain=pdf&date_stamp=2018-09-30� 214 Darugar et al. / European Journal of Chemistry 9 (3) (2018) 213-221 Table 1. Calculated relative energies of X-TFBA-2 with respect to X-TFBA-4, as the most stable form, in gas phase and in solution (in kcal/mol), and theoretical and experimental equilibrium constants between X-TFBA-4 ⇌ X-TFBA-2 a. Calculation level TFBA-2 F-TFBA-2 CH3-TFBA-2 OCH3-TFBA-2 NH2-TFBA-2 NO2-TFBA-2 CF3-TFBA-2 OH-TFBA-2 A/6-311++G** 0.99 (0.89) 0.79 (0.72) 1.01 (0.86) 0.80 (0.93) 1.03 (1.08) 0.79 (0.71) 0.78 (0.69) 0.72 (0.74) A /6-311G** 0.63 (0.60) 0.45 (0.44) 0.67 (0.62) 0.74 (0.80) 0.76 (0.70) 0.46 (0.46) 0.48 (0.48) 0.47 (0.45) A /6-31G** 0.55 (0.50) 0.39 (0.38) 0.60 (0.54) 0.66 (0.60) 0.66 (0.59) 0.39 (0.39) 0.43 (0.40) 0.38 (0.36) B /6-31G** 0.08 0.09 0.22 0.98 0.35 0.04 0.16 0.75 C /6-311++G** 1.07 0.88 1.11 1.18 1.19 0.89 0.89 0.89 CCl4 b 1.23 (1.15) 1.06 (0.96) 1.21 (1.18) 1.15 (1.19) 1.39 (1.31) 1.01 (0.94) 1.01 (0.93) 1.07 (1.02) CH3CN b 1.49 (1.35) 1.39 (1.18) 1.58 (1.41) 1.58 (1.43) 2.27 (1.72) 1.28 (1.18) 1.31 (1.17) 1.50 (1.42) C2H5OH b 1.48 (1.62) 1.37 (1.18) 1.50 (1.40) 1.56 (1.43) 1.82 (1.71) 1.27 (1.17) 1.30 (1.17) 1.48 (1.41) Keq (exp.) C 1.03 (1.02) 1.04 (-) 1.03 (-) 1.04 (-) 1.03 (-) 1.01 (0.99) 1.01 (-) 1.06 (-) a A, B, and C are the calculated relative energies in gas phase at B3LYP, MP2, and TPSSh levels, respectively, the values of ZPE are in parentheses. b Calculated relative energies in various solvents at B3LYP/6-311++G** level of theory. c Calculated equilibrium constants are in gas phase at B3LYP/6-311++G** level of theory and experimental equilibrium constants are in parentheses from Ref. [40]. The Hammett equation have correlated some parameters, such as the equilibrium constants, rate constants, and different physical properties with Hammett constant to show the effect of electron donating/withdrawing ability of substituents on the mentioned properties. A few studies have been reported in connection with the Hammett equation [19,20]. p-X-ADM molecules were studied by Jimenez-Cruz et al. [21,22]. They reported the effects of para substitutions on the aromatic systems, by a correlation between 13C NMR chemical shifts and Hammett substituent constant (σp). Darugar et al. reported the correlation between theoretical and experimental parameters related to IHB strength with σp in para substituted benzoylacetones [23]. The aim of the present work is to predict the molecular structure, tautomeric stabilities, and IHB strength of the titled molecules by means of density functional theory (DFT), Atoms-In-Molecules (AIM) [24], and Natural Bond Orbital (NBO) analyses. Afterwards, the results related to IHB strength have been compared with the experimental enolic proton chemical shifts, δOH which shows the effect of different substitutions in para-positions of phenyl ring on the IHB strength of the title molecules. The parameters related to IHB, such as EHB, ѵOH, γOH, δOH, geometrical and topological parameters would be correlated with the Hammett’s para function, σp [25]. So, the electron donating/withdrawing substituent effects are discussed quantitatively by applying the Hammett equation. 2. Method of calculations All calculations were performed using Gaussian 09 software package [26]. The cis-enol structure of all molecules has been optimized at the B3LYP [27-28], using 6-31G**, 6- 311G**, and 6-311++G** basis sets, the second-order Møller- Plesset (MP2) [29,30], using 6-31G** basis set, and the TPSSh [31] levels, using 6-311++G** basis set. All of these levels and basis sets have been applied to confirm the relative stability of the cis-enol forms of the titled molecules. The zero-point vibrational energy, ZPE, corrections were obtained at the B3LYP level, without applying any scaling. The vibrational frequencies of the cis-enol forms were calculated at the B3LYP level of theory. The SCRF-PCM method [32] at the B3LYP/6-311++G** level, was selected for calculations in solutions. Different polar and non-polar solvent such as, acetonitrile, carbon tetrachloride, and ethanol, were used to investigate the solvent dependence of tautomeric equilibrium. The electronic charge density, ρ(r), its corresponding Laplacian, ∇2ρ(r), at the critical point of hydrogen bond, O…H, and EHB were carried out by using the AIM2000 program [33,34]. The NBO 5.0 program [35] used to calculate the second-order interaction energies E (2), and natural charge of the bridged atom in the chelated ring (H). To obtain the chemical shift of the enolic proton, δOH, NMR calculations were done by using gauge independent atomic orbital (GIAO) method [36, 37] at the B3LYP/6-311++G** level of theory in chloroform as solvent, by SCRF-PCM method. The predicted 1H chemical shifts are derived from δ = σo − σ. In this equation, δ and σ are the chemical shift and the absolute shielding of bridged hydrogen, respectively. The σo is the absolute shielding of hydrogen nuclei in TMS (Tetramethylsilane) as reference. To end, some theoretical and experimental parameters related to IHB strength were correlated with σp Hammett equation. Graphs were drawn and regression analyses were performed using Microsoft Office Excel, 2016 software. 3. Results and discussion 3.1. Tautomeric and IHB strength Cis-enol forms of β-dicarbonyl compounds stabilized by an intramolecular hydrogen bond. In asymmetric β-diketones two different cis-enol forms are noticeable, such as titled molecules (Figure 1). According to this figure, in the X-ADM-4 and X- TFBA-4, the phenyl group and hydroxyl group are adjacent, were the phenyl group, C=C, and C=O creating a longer conjugate system is expected, while in X-TFBA-2 and X-ADM-2, which ph and C=O are neighbored, a conjugation between Ph and C=O can occur, as reported by Afzali et al. [38] and Tayyari et al. [39]. The relative stabilities of the mentioned stable forms of the titled molecules, along with the calculated and reported experimental equilibrium constants (Keq) [21, 40], calculated at different levels of theory in the gas phase and solutions, are listed in Tables 1 and 2. According to these values, the cis-enol- 4 and cis-enol-2 forms in the X-TFBA and X-ADM molecules are the most stable forms, respectively. The values show, the energy differences between the stable cis-enol forms of X- TFBA are in the range: 0.04-1.19, 1.01-1.39, 1.28-2.27, and 1.27-1.82, and for X-ADM are 0.16-0.64, 0.26-0.67, 0.10-0.50, and 0.10-0.42 kcal/mol both in the gas phase and in CCl4, CH3CN, C2H5OH solutions, respectively. Upon Zero-point energy (ZPE) corrections, these energy differences reduce to 0.36-1.08, 0.93-1.31, 1.17-1.72, and 1.17-1.71 for X-TFBA, and 0.02-0.48, 0.06-0.43, 0.03-0.21, and 0.03-0.18 kcal/mol for X- ADM, respectively. Therefore, coexisting of the these two- stable forms of X-TFBA and X-ADM in the samples are expected, which is in agreement with the reported experimental equilibrium constants. So, according to these values there is no significant difference between reported experimental and theoretical equilibrium constants, Keq. We obtained the calculated equilibrium constants by Equation (1), ∆G° = -RTln(Keq) (1) 2018 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.9.3.213-221.1713 Darugar et al. / European Journal of Chemistry 9 (3) (2018) 213-221 215 Table 2. Calculated relative energies of X-ADM-4 with respect to X-ADM-2, as the most stable form, in gas phase and in solution (in kcal/mol), and theoretical and experimental equilibrium constants between X-ADM-2 ⇌ X-ADM-4 a. Calculation level ADM-4 Cl-ADM-4 F-ADM-4 CH3-ADM-4 OCH3-ADM-4 NH2-ADM-4 NO2-ADM-4 CF3-ADM-4 A/6-311++G** 0.32(0.04) 0.47(0.08) 0.55(0.16) 0.35(0.20) 0.64(0.48) 0.38(0.17) 0.54(0.27) 0.51(0.34) A /6-311G** 0.23(0.12) 0.39(0.02) 0.51(0.08) 0.25(0.21) 0.56(0.36) 0.33(0.15) 0.41(0.22) 0.46(0.23) A /6-31G** 0.18(0.15) 0.33(0.02) 0.50(0.07) 0.21(0.11) 0.32(0.23) 0.25(0.11) 0.24(0.18) 0.27(0.14) B /6-31G** 0.17 0.36 0.49 0.21 0.43 0.45 0.35 0.33 C /6-311++G** 0.18 0.36 0.45 0.17 0.20 0.23 0.16 0.19 CCl4 0.26(0.06) 0.40(0.30) 0.28(0.09) 0.67(0.43) 0.43(0.07) 0.29(0.21) 0.60(0.31) 0.30(0.29) CH3CN 0.10(0.04) 0.23(0.16) 0.11(0.10) 0.50(0.21) 0.32(0.03) 0.26(0.15) 0.43(0.20) 0.26(0.21) C2H5OH 0.10(0.04) 0.17(0.10) 0.10(0.08) 0.38(0.14) 0.32(0.03) 0.18(0.13) 0.42(0.18) 0.17(0.12) Keq (exp.) b 2.90 (2.77) 3.10 (3.08) 2.25 (2.20) 2.76 (2.71) 2.49 (2.31) 2.45 (-) 3.01(-) 2.99 (2.95) a A, B, and C are the calculated relative energies in gas phase at B3LYP, MP2, and TPSSh levels, respectively, the values of ZPE are in parentheses. b Calculated equilibrium constants are in gas phase at B3LYP/6-311++G** level of theory and experimental equilibrium constants are in parentheses at 20 °C from Ref. [21]. Table 3. Some theoretical and experimental parameters related to the hydrogen bond strength for the TFBA-2/-4 and ADM-2/-4 molecules and the averaged values. a Parameters TFBA ADM -2 -4 Avg. -2 -4 Avg. X-ray e δ OH b 15.32 15.04 15.18 (15.20) 15.67 15.20 15.44(16.09) ν OH b 3036 3083 3060 3000 3036 3018 γ OH b 930 937 934 995 960 978 R O…O c 2.523 2.542 2.533 2.519 2.531 2.525 2.488 R O-H c 1.006 1.001 1.004 1.007 1.004 1.006 0.963 R O…H c 1.619 1.632 1.626 1.602 1.613 1.608 1.654