T he conformational analyses of Calix[4]arenes reveals four different stable structures  conformations ; Cone, Partial Cone, 1,2-Alternate and 1,3-Alternate after employing a density functional theory  DFT  computational analysis. Intramolecular Hydrogen Bonds  IHBs  existing Calixarene core cause Cone conformation, supporting to be the best stable state in 1, 2 and 3 compounds. In addition, one needs Natural Bond Orbital  NBO  analyses of current compounds in order to understand nature of these IHBs. Specifically, it has been shown using NBO that the LP *→σ interactions for O˙˙˙O¯H IHBs and the delocalization LP → π* for O¯C=O are the major contributions to energy stabilization. Of all conformers of compound 4, Partial Cone has the lowest energy, which can be attributed to devoid of intramolecular hydrogen bond due to the absence of free phenolic group

Arzu Karayel

Hittite Journal of Science & Engineering

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Hittite Journal of Science and Engineering, 2019, 6 (3) 223-228 ISSN NUMBER: 2148-4171DOI: 10.17350/HJSE19030000151Calixarenes are macrocyclic compounds, which are utilized in important fields like pharmacy, engineering and medicine. They are applied as sen-sors in various areas of human activities, in medical applications and environmental protection (1, 2). Mo-reover, they are used in catalysis as well as in mole-cular recognition. Their cage-like structures play im-portant role in host-guest mechanism in supramole-cular chemistry (1, 2). In addition, the functionalizati-on of Calixarenes by means of substituting methylene bridges with heteroatoms is quite important in order to discover new compounds (1, 2). Calixarenes attract much attention due to the rich conformational pos-sibilities which come in four varieties: Cone, Partial Cone, 1,2-Alternate and 1,3-Alternate (3-6).The four forms of Calixarenes are mainly due to the free rotation of methylene groups. The determinati-on of stable conformations could not always be achieved correctly by spectroscopic techniques such as IR and NMR. On the other hand, the current state of theory on predicting Calixerarene derivatives remains inconclusi-ve or insufficient (7) hence there is still need for extensi-ve theoretical work. Moreover, in literature, it has been highlighted that the determination of the conformati-onal stability from experimental results of Calixarenes Article History: Received: 2019/07/17Computational Insight into Conformational Rearrangement and Intramolecular-H Bond Analysis of Some Calix[4]Arenes Including Acryloyl MoietyArzu Karayel Hitit University, Department of Physics, Corum, TurkeyAccepted: 2019/08/29Online: 2019/09/30Correspondence to: Arzu Karayel, Hitit University, Department of Physics, 19030, Corum, TURKEYE-Mail: arzukarayel@hitit.edu.trPhone: +90 364 227 70 00-1676 Fax: +90 364 227 70 05derivatives could be complicated (8), thus, justifies the importance of more theoretical studies.The main goal in this study is to scrutinize the con-formational stability and the effect of intramolecular hydrogen bonds on this equilibrium of some Calix[4]arene derivatives including acryloyl moiety using first principles calculations based on Density Functional Theory (DFT). For this purpose, the most stable states of the compounds were investigated in detail by taking into account all cases including Cone (C), Partial Cone (PC), 1,2-Alternate (1,2-A) and 1,3-Alternate (1,3-A). In the next section, we will explain material and methods and computational method used in geometry optimiza-tions of the current molecules. Later, we will discuss our results and summarize our study in conclusion part.MATERIAL AND METHODSIn Fig. 1, the structures that are previously reported experimental synthesis and spectral characterization are depicted. These reported experimental results have been used as the starting point for the following computational study (9, 10). An X-Ray crystallog-raphy of the compound 2 is also available (10) and is used as initial configuration in following DFT calcu-A B S T R A C TThe conformational analyses of Calix[4]arenes reveals four different stable structures (conformations); Cone, Partial Cone, 1,2-Alternate and 1,3-Alternate after employ-ing a density functional theory (DFT) computational analysis. Intramolecular Hydrogen Bonds (IHBs) existing Calixarene core cause Cone conformation, supporting to be the best stable state in 1, 2 and 3 compounds. In addition, one needs Natural Bond Orbital (NBO) analyses of current compounds in order to understand nature of these IHBs. Specifically, it has been shown using NBO that the LP *→σ interactions for O˙˙˙O¯H IHBs and the delocalization LP → π* for O¯C=O are the major contributions to energy stabilization. Of all conformers of compound 4, Partial Cone has the lowest energy, which can be attributed to devoid of intramolecular hydrogen bond due to the absence of free phenolic groups.INTRODUCTION Keywords: Calix[4]arene; Acryloyl moiety; Conformational equilibrium; DFT; NBO.A. Karayel/ Hittite J Sci Eng, 2019, 6 (3) 223–228224were relaxed at optimization stage. While all geometries were drawn using Gauss View 5.0 program (11), X-Ray starting geometry of compound 2 was created by taking crystal information file (cif) containing X-Ray coordina-tes from literature (10).  Babel program (12) was used in order to transfer these coordinates into the Z-matrix for-mat. Gaussian 09 program (13) was used in order to relax of all molecules. The intramolecular hydrogen bonding energies were calculated using NBO 3.1 program (14) as implemented in Gaussian 09 program. The molecular electrostatic potentials (MEPs) were drawn in order to see intramolecular H-bond sites. The HOMOs and the LUMOs were illustrated and the global reactivity para-meters were computed.RESULT AND DISCUSSIONConformational Analysis of the CompoundsIn molecules having hydroxyl group, Cone conformation was found to be the most stable state (molecule 1, 2 and 3 contain three, two and one hydroxyl group, respectively, as shown Fig. 2.) Hydroxyl groups led to conformational rigid structures, i.e. fixed conformations such as Cone, by means of strong O¯H···O IHBs in the molecules. DFT analysis indicated the molecule 3 exists in Cone. Howe-ver, Cone has only 0.11 kcal/mol lower energy than Par-tial Cone as shown in Table 1. As a result, it is apparent that this molecule adopts a Cone conformation since it contains at least one free phenolic group. A study on Ca-lix[4]arene molecule which has one free hydroxyl group revealed that this molecule could exist in both confor-lations. These molecules are some Calix[4]arenes deriva-tives containing acryloyl moiety, in which they consist of p-substituted phenolic units linked by methylene bridgesin the ortho position relative to the OH groups (1, 9). The-se relevant compounds have two usual positions, lowerrim (hydroxyl groups) and upper rim (para position as tothe hydroxyl groups), as shown in Fig 1.Computational MethodThe conformational equilibrium of Calix[4]arenes de-rivatives containing acryloyl moiety was examined by using DFT method with B3LYP functional at 6-31G(d,p) level. All possible conformers, i.e. C, PC, 1,2-A and 1,3-A Figure 1. The structures of Calix[4]arene derivatives including acryloyl moiety.Figure 2. All possible conformers of Calix[4]arene derivatives including acryloyl moiety.225A. Karayel/ Hittite J Sci Eng, 2019, 6 (3) 223–228mations, where one of these conformations is  Partial Cone in according to NMR and second one is Cone in according to both IR and DFT. It is also reported that there is a swift exchange between Cone and Partial Cone conformations (15). Moreover, our conformational analy-sis of compound 4 revealed that the Partial Cone is the most stable state. In the literature, it is emphasized that Calix[4]arenes prefer the Partial Cone or 1,3-Alternate conformers since they are lacking of free phenolic groups (16). In order to find exact conformation of compound 4, we employed a DFT method which predicts the exact conformation as Partial Cone.When the energy of the model geometry of compo-und 2 is compared with that of experimental geometry, it is seen a difference of 0.71 kcal/mol, as shown in Table 1. The reason of this difference can be attributed to hook-sha-ped direction of carbonyl groups of the acryloyl moiety in X-ray geometry (Fig. 3). This is due to steric effect of solvent toluene, as understood from experimental study (10). The experimental realization also confirms that compound 2 prefers Cone conformation due to the intramolecular Table 1. Optimization energies of conformers of compounds 1, 2, 3 and 4.Comp. Conformer E (hartree) ∆E kcal/mol1Cone -1573.1375732 0.0Partial cone -1573.1275896 6.261,2-Alternate -1573.1216164 10.01 1,3-Alternate -1573.1214976 10.092Cone -1763.8625073 0.0Experimentala -1763.8613754 0.71Partial cone -1763.8601678 1.471,3-Alternate -1763.8572642 3.291,2-Alternate -1763.8536385 5.573Cone -1954.5940627 0.0Partial cone -1954.593894 0.111,3-Alternate -1954.5922281 1.151,2-Alternate -1954.5911824 1.81Partial cone -2145.3287422 0.01,3-Alternate -2145.3283142 0.2741,2-Alternate -2145.3263679 1.49Cone -2145.3154581 8.34a Experimental starting geometry was taken from X-ray coordinates (10).Figure 3. The comparison of optimized model geometry (blue) with optimized experimental geometry (pink) of compound 2, where O¯H···O distances are 1.893 Å and 1.878 Å, respectively.Table 2. Hydrogen-bond geometries (Å, °) of compound 1, 2 and 3.Molecule D—H˙˙ Ȧ D—H H˙˙ Ȧ D˙˙ Ȧ D—H˙˙ Ȧ1 Model optimizationO1—H1˙˙˙O2 0.980 1.741 2.687 161.2O2—H2˙˙˙O3 0.984 1.729 2.686 163.2O3—H3˙˙˙O4 0.977 1.800 2.773 173.32ExperimentalaO1—H1˙˙˙O2 0.91 (11) 2.00 (11) 2.897 (3) 170 (9)O3—H3˙˙˙O4   0.79 (8)  2.11 (8) 2.855 (4) 157 (8)Exp_optimizationO1—H1˙˙˙O2 0.970 1.878 2.843 173.5O3—H3˙˙˙O4 0.970 1.877 2.843 173.5Model_optimizationO1—H1˙˙˙O2 0.970 1.893 2.852 169.4O3—H3˙˙˙O4 0.970 1.893 2.852 169.43 Model _ptimization O1 — H1˙˙˙O2 0.968 2.024 2.975 166.64 There is no intramolecular hydrogen bondinga These results were taken from literature (10).A. Karayel/ Hittite J Sci Eng, 2019, 6 (3) 223–228226hydrogen bond between hydroxyl and acryloyl groups (9). This result is in accordance with our theoretical results in terms of both conformational research and intramolecular hydrogen bond analysis. The optimization results regarding to intramolecular hydrogen bonding of X-Ray geometry of compound 2 were listed in Table 2 as Exp optimization line.The intramolecular hydrogen bond of 1 is stronger than that of both 2 and 3 (Table 2). These results are in line with NBO analysis results, as shown in Table 3. The short O ···O distance means the presence of robust intramolecularH-bonds. Also, the molecular electrostatic potential graphs represent the intramolecular H-bond sites as the electron-rich region in acceptor oxygens of both acryloyl moiety and the hydroxyl groups, while presenting the phenyl rings as the neutral region.As shown in Fig. 4, HOMOs are partially distributed on Calix[4]arene core with a hydroxyl group, while LUMOs are located partially on acryloyl groups of all molecules. Accor-ding to Column ∆E from top to bottom in Table 4, HOMO-LUMO energy gaps are gradually increasing, which indicate the molecules become more rigid as it can also be unders-tood from η value.CONCLUSIONAn exhausting search of conformal configurations of Calix[4]arene derivatives including acryloyl moiety is the main promise of this work. Exclusively, compound 4 aside from others, the exact conformation could not be determined exactly by experimental methods (NMR or IR) since there is no -OH group being capable of intra-Table 3. Stabilization energies (kcal/mol) of selected NBO donor-acceptor pairs in NBO basis at B3LYP/6-31g(d,p)//B3LYP/6-31g(d,p) level for 1, 2 and 3 compounds.Comp. Φi Φj Eij(2)a (kcal/mol) εj-εib (a.u.) Fijc (a.u.)1LP1O9 σ* O55¯H58 9.25 1.05 0.088LP2O9 σ* O55¯H58 12.92 0.79 0.092LP1O34 σ* O₉¯H59 16.90 1.02 0.118LP₂O34 σ* O₉¯H59 7.38 0.78 0.069LP₁O47 σ* O34¯H57 14.46 1.03 0.110LP2O47 σ* O34¯H57 1.26 0.80 0.029LP2O47 π* C48¯O49 39.01 0.34 0.1052LP1O₉ σ* O62¯H66 9.47 1.05 0.090LP₂O₉ σ* O62¯H66 0.73 0.82 0.023LP₁O54 σ* O41¯H64 9.48 1.05 0.090LP₂O54 σ* O41¯H64 0.73 0.82 0.023LP₂O₉ π* C10¯O11 40.44 0.34 0.106LP₂O54 π* C55¯O56 40.44 0.34 0.1063LP₁O54 σ* O41¯H71 4.51    1.05    0.062LP₂O54 σ* O41¯H71 1.90    0.81    0.037LP₂O9 π* C10¯O11 42.65 0.34 0.108LP₂O54 π* C55¯O56 38.58 0.34 0.104LP₂O62 π* C63¯O64 43.07 0.33 0.109 a E(2) : stabilization energy,  b Energy difference of i (donor) and j (acceptor) NBO orbitals,  c Fij: the Fock matrix element.Table 4. HOMO-LUMO energies and calculated global reactivity parameters of Calix[4]arenes.Comp. Ehomo(eV) Elumo(eV) ∆E(eV) χ(eV) µ(eV) η(eV) σ(eV)-1 ω(eV)1(Cone) -5.49969 -1.66969 3.8300 3.5847 -3.5847 1.9150 0.2611 3.35512(Cone) -5.60010 -1.62071 3.9794 3.6104 -3.6104 1.9897 0.2513 3.27563(Cone) -5.74541 -1.68711 4.0583 3.7163 -3.7163 2.0291 0.2464 3.40324(Partial Cone) -6.33944 -1.48057 4.8589 3.9100 -3.9100 2.4294 0.2058 3.1465∆E: ELUMO - EHOMO, χ:Electronegativity, µ:Chemical potential, η:Chemical hardness, σ:Global softness, ω:Electrophilicity index227A. Karayel/ Hittite J Sci Eng, 2019, 6 (3) 223–228molecular hydrogen bonding. In addition, it is only esti-mated that the conformational structure can be a Partial Cone or 1,3-Alternate due to the steric effect. However, the exact conformation was determined as Partial Cone by DFT method. It has been shown that molecules (1, 2 and 3) having hydroxyl group adopt Cone conformation, being most stable state, while Partial Cone is the lowest energy state for compound 4 without free phenolic gro-ups. The robust O¯H···O type bonds of 1, 2 and 3 com-pounds have given rise to be planarity of the molecules. In NBO analyses, it is understood that the LP → σ* inte-ractions for O˙˙˙O¯H IHBs and the delocalization LP → π* for O¯C=O are the major contributions to energy sta-bilization. The determination of accurate structures as a result of laborious conformational search will shed light on understanding of host-guest mechanisms of Calix[4]arene molecules.ACKNOWLEDGEMENTWe thank to Associate Prof. Dr. Sevil Özkınalı for giving permission to use the structures synthesized previously to be used in theoretical calculations. The numerical calculations reported in this paper were fully performed at TUBITAK ULAKBIM, High Performance and Grid Computing Center (TRUBA resources).R e f e r e n c e s1. Gutsche CD. Calixarenes Revisited, The Royal Society of Chemistry, Cambridge, UK, 1998.2. Furer VL, Potapova LI, Vatsouro IM, Kovalev VV, Shokova EA,Kovalenko VI. 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Computational Insight into Conformational Rearrangement and Intramolecular-H Bond Analysis of Some Calix[4]Arenes Including Acryloyl Moiety

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Computational Insight into Conformational Rearrangement and Intramolecular-H Bond Analysis of Some Calix[4]Arenes Including Acryloyl Moiety

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