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An experimental and theoretical investigation of free Oxazole in conjunction with the DFT analysis of Oxazole ⋯(H 2 O) n complexes

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An experimental and theoretical investigation of free Oxazole in conjunction with the DFT analysis of Oxazole ⋯(H 2 O) n complexes
  An experimental and theoretical investigation of free Oxazole inconjunction with the DFT analysis of Oxazole ⋯ (H 2 O) n  complexes Ş enay Yurdakul ⁎ , Serdar Bado ğ lu, Lüt fi ye Özkurt Department of Physics, Faculty of Science, Gazi University, Teknikokullar, 06500, Ankara, Turkey a b s t r a c ta r t i c l e i n f o  Article history: Received 13 October 2015Received in revised form 26 January 2016Accepted 14 February 2016Available online 17 February 2016 The mid-IR spectrum of Oxazole (Oxa) is recorded. This spectrum is interpreted with the help of B3LYP/6-311++G(d,p)calculationsand potential energydistribution(PED)analysis. The experimentalspectrum iscon-cordantwiththetheoreticaldata.Geometricalparametersandtheatomicchargesarealsotheoreticallyobtainedand presented. Solvent effects on the geometrical parameters, vibrational frequencies, and electronic propertiesof Oxa are analyzed theoretically in chloroform, ethanol, and water. Besides, hydrogen bonded Oxa ⋯ (H 2 O) n (n = 1, 2, … , 10) complexes are investigated within the PCM solvation model. It is found that the interactionenergies in Oxa ⋯ (H 2 O) n  complexes are in fl uenced by the number of water molecules, and by the arrangementof water molecules.© 2016 Elsevier B.V. All rights reserved. Keywords: OxazoleVibrational spectroscopyDFTSolvent effectsPCM 1. Introduction Oxazole(C 3 H 3 ON=Oxa)isa fi ve-memberedunsaturatedheterocy-clicringcontainingaNC – Ogroup[1].Itiscloselyrelatedtofuranbyre-placement of a 3CH group in the latter by a pyridine-like 3N atom. Theparent heterocycle is a liquid at room temperature, and has a boilingpoint of 69.8 °C. It is soluble in diethyl ether and alcohols, and slightlysoluble in water.Oxazoles are an important class of pharmaceutically interestingchemical entities, and hence, many studies have been carried out toaccess their general molecular features. The chemistry of oxazoles was fi rst seriously investigated when the antibiotic penicillin was believedto contain this heterocyclic moiety. Oxazole and related compoundsarewidespreadstructuralunitsimportantinnaturalproductsofvarioussources,syntheticintermediates,andpharmaceuticalseitherasapartof the structure or as intermediates in their synthesis [2,3].Chemicalbehaviorofoxazolesiswellknown[4,5].Oxazoleisaliquidwhich has yellowish color and pyridine-like odor. The oxazole ringoccurs naturally in many living systems, such as marine organisms,plants (e.g., coffee, peanuts), and mushrooms [6 – 14]. Oxazole contain-ing molecules isolated from marine organisms is common in growingnumber of natural products which have various pharmacological uses(as anti-in fl ammatory, antibacterial, antibiotic, antiviral, analgesic, andantitumor drugs) [8,9,15 – 31]. Some oxazoles display scintillator prop-erties [32,33] and are used as  fl uorescent whitening agents [34,35]and some are used in dyes and pigments [36]. In such a case, oxazoledyes in oxazole yellow (known as YO) and its homodimer (known asYOYO) are used as DNA indicators. These dyes have strong bindingaf  fi nities for DNA. They also have high  fl uorescence sensitivity andmolar absorptivity besides large  fl uorescence enhancement uponbinding to DNA [37]. The practical uses of oxazoles extend to someother industrial applications such as pesticides, the production of electrophotographic materials, additives to detergents, and hydraulic fl uids and lubricants [38].Vibrational assignments of the fundamental bands of Oxa werepublished before by Borello et al. from infrared spectra in vapor andliquid phases [4], and Raman shifts from the SERS (Surface EnhancedRaman Spectroscopy) spectrum were reported by Muniz-Miranda asliquid sample and aqueous solution [39]. In 1995, El-Azhary et al.published ab initio calculations of vibrational frequencies and scalefactors for Oxa [40,41]. Hegelund et al. studied a high resolution gasphase IR spectrum in the region 600 – 1400 cm − 1 [1]. Palmer reportedB3LYP and MP2 calculations of the anharmonic vibrational frequenciesof Oxa and compared them with the earlier studies [42]. Kraka et al.studied the rotational spectrum of the Oxa – Argon complex, and foundthat Ar prefers a position above (or below) the ring [43].Hydrogen bonding is of great interest in physical and chemicalsciences.Thenatureofhydrogenbondinganditsrelationshiptomolec-ular association are main topics of current researches. It may stronglyaffect the shapes of bio-molecules, and it lies at the heart of manyphysicochemical phenomena. Pagliai et al. studied the O – H ⋯ N hydro-gen bonded water/Oxa complex and compared their results with theexperimental SERS data of aqueous solution [44]. Kaur and Khannahave reported B3LYP and MP2 calculations on adducts of isoxazole,Oxazole, and furan with a single water molecule [45]. Recently, Tanzi Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 162 (2016) 48 – 60 ⁎  Corresponding author. E-mail address: ( Ş . Yurdakul).© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and BiomolecularSpectroscopy  journal homepage:  etal.investigatedthevibrationalpropertiesofOxausingmicrosolvationmodels and mixed quantum/classical ab initio molecular dynamicssimulations [46].Solvents are amongthemostimportantcomponents ofthereactionsystems. The presence of the solvent may speed up or slow down thereaction, and the change from one solvent to another can also createstrong effects on reaction and on dissolved molecular structures. Insuch cases, dissolved particles tend to exist as a cluster of solventmolecules, and those clusters determine the stability and chemicalbehavior of the system. To sum up, studying the effects of solvation onbio-molecules assists in understanding the possible interactionsbetween any speci fi ed molecule and the solvent [47].Despite its importanceas a dye, scintillator and indicator; studies of solvent effects on Oxa are limited with its selected 1:n Oxa ⋯ (H 2 O) 2 complexes (n ≤ 2) up to now. On the other hand, Oxa has several sitesfor hydrogen bonding and this enables one to consider Oxa-wateradducts with water molecules positioned to create a continuous layeraround Oxa, which is known as the  fi rst solvation shell. Particularemphasis on the  fi rst solvation shell of isolated molecules was laid bymanyresearchers,becauseexplicitsolventmoleculesintheir fi rstsolva-tionshellhaveaclearin fl uenceontheelectronicstructureofthesolute.Nonetheless, to the best of our knowledge, there are no such studiesconducted on Oxa.In this paper, we present a combined experimental and theoreticalstudy on the free Oxa molecule. Its geometrical parameters, vibrationalfrequencies, and electronic properties are computationally obtained.TheexperimentalliquidphaseIRspectrumisrecordedandassignmentsare done with the help of potential energy distribution (PED) analysis.The density functional theory (DFT) in connection with the polarizedcontinuum model (PCM) is used to calculate those properties in threedifferent solvents, and to compare the obtained data with the sameproperties in the gas phase. The solvents chosen were; chloroform(non-polar,  ε  = 4.7113), ethanol (polar,  ε  = 24.852), and water(polar,  ε  = 78.3553). Our selection is to see the variations in thegeometry and vibrational frequencies due to a change in dielectricityof the medium. Besides, hydrogen bonded Oxa ⋯ (H 2 O) n  (n = 1, 2, … ,10) complexes are investigated within the PCM model by positioningexplicit water molecules to create a continuous layer as a solvationshell. Total energies, interaction energies, and H-bond lengths arereported. 2. Computational and experimental methods The DFT calculations of the ground state geometries of Oxa wereperformed at B3LYP/6-311++G(d,p) level with the default conver-gence criteria without any constraint on the geometry. The stationarystructure is found by ascertaining that all the calculated frequenciesare real. All the calculations were carried out in a Gaussian 09Wprogram package [48]. The fundamental vibrational modes were char-acterized by their PED (potential energy distribution) obtained byusing the VEDA 4 program [49]. In all computations for investigatingthe solvent effects, the solvent environment was evaluated by usingthepolarizedcontinuummodel(PCM).HydrogenbondedOxa ⋯ (H 2 O) n complexes are pre-optimized at semi-empirical AM1 level and theoutputs are used as input structures for the DFT level.While calculating Oxa ⋯ (H 2 O) n  complexes in the solvent environ-ment,thebasissetsuperpositionerrors(BSSE)werecorrectedaccordingto the procedure published before [50].The interaction energies of Oxa ⋯ (H 2 O) n  complexes are computedaccording to the formula Δ E inter  ¼  E complex −  E Oxa  þ nE H 2 O    ð 1 Þ where E complex  is the energy of the optimized Oxa ⋯ (H 2 O) n  complex,E Oxa  and nE H 2 O  are the dimer-centered basis set (DCBS) energies of Oxa and water molecules.A commercialsample of Oxazole (Oxa)was purchased from Aldrichandusedwithoutfurtherpuri fi cation.AninfraredspectrumofOxa wasrecorded between 4000 and 550 cm − 1 on a Bruker Vertex 80 FT-IR spectrometer, and the sample of the free ligand was examined by aPike MIRacle ATR apparatus. 3. Results and discussion  3.1. Molecular parameters Experimentally obtained geometrical parameters of Oxa are avail-able in literature [43]. The systems with loosely bound electrons (ionic structures, systems containing lone pairs, etc.) should be handled byusing diffuse basis sets, hence we have employed 6-311++G(d,p)basis set. Our B3LYP/6-311++G(d,p) level calculation results arecollected in Table 1 together with the experimental results publishedin the work of Kraka et al. The geometry and atoms numbering of theoptimizedstructureareshowninFig.1.AsseenfromTable1,calculated bond lengths and angles are in very good accordance with the experi-mentaldata.Themaximumdeviationswithrespecttotheexperimentaldata are 0.004 Å and 0.4°. Our results are quiet similar with thepublished data in the abovementioned work. Hence, adding diffusefunctions caused no signi fi cant improvement on the geometricalparameters.  Table 1 Geometry of Oxazole a .CalculatedCoordinates Exp. b Vacuum CHCl 3  EtOH Water3C – 7O 1.370 1.370 1.373 1.373 1.3731C – 7O 1.357 1.357 1.355 1.355 1.3552C – 8N 1.396 1.392 1.395 1.396 1.3971C – 8N 1.291 1.290 1.293 1.293 1.2942C – 3C 1.352 1.353 1.352 1.352 1.3523C – 7O – 1C 103.9 104.3 104.6 104.8 104.92C – 8N – 1C 103.9 104.3 104.6 104.6 104.67O – 3C – 2C 108.1 107.8 107.7 107.6 107.68N – 2C – 3C 109.0 109.1 108.9 108.9 108.97O – 1C – 8N 114.9 114.5 114.1 114.0 114.0 a Bond lengths are in Å, bond angles are in degrees. b Taken from Ref. 43. Fig. 1.  Structure and atoms numbering of Oxazole.49 Ş  . Yurdakul et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 162 (2016) 48 – 60  Furthermore, we have looked for the solventeffects on thegeomet-rical parameters of Oxa theoretically. The data presented in Table 1showthatthebondlengthsandbondanglesofOxaareonlymarginallychangedwhenitsolvated.Themaximumdeviationsfromthecomputedgas phase data are 0.005 Å and 0.6°. Fig. 2 shows how bond lengths inOxa affected by the solvents. In the  fi gure every bond is representedby an index number (1: 3C – 7O, 2: 1C – 7O, 3: 2C – 8N, 4: 1C8N, and 5:2C3C). 2C – 8N deserves a special mention. It is the most affected bondby the solvent and it is affected in various amounts dependingof dielectricity of the medium. Deviation from the gas phase valuegradually rises with increasing dielectric constant, and the minimumdeviationisspottedforthenon-polarsolventchloroform.Thisdifferentbehavior in 2C – 8N may refer to a high activity of an 8N site of Oxa.  3.2. Analysis of vibrational spectra Oxa has eight atoms and hence 18 fundamental vibrational modes.We have recorded the experimental FT-IR spectrum of Oxa in 4000 – 550 cm − 1 range which is shown in Fig. 3. Calculated vibrationalmodes and frequencies of Oxa are given in Table 2 together with theexperimental data and %PED assignments. Calculated frequencies arescaled by 0.983 for the wavenumbers lower than 3000, and by 0.958for the wavenumbers higher than 3000 cm − 1 before interpreted [51].It has been seen that the calculated and experimental data are concor-dant with each other.The ring torsion of Oxa is observed at 645 cm − 1 as a very strongband. This band is calculated at 650 cm − 1 , and only marginally shiftedin solvation phase. An out of plane bending of CH is observed at755 cm − 1 as a strong band. It is calculated at 749 cm − 1 , and virtuallynot shifted in solvation phase. Another out of plane bending of CH isobserved at 837 cm − 1 as a medium band, but the DFT calculations areunderestimated the position of this peak likewise in earlier study [40].Calculations are predicted this peak at 822 cm − 1 . On the other hand,it shifted up to 9 cm − 1 in solvation phase. The ring deformation modeis observed at 900 cm − 1 with strong intensity. It is calculated at901 cm − 1 in the gas phase and showed only marginal shifting. Thestrong band at 1044 cm − 1 is assigned as the ring breath. This mode isoverestimated and calculated at 1050 cm − 1 in the gas phase. On theother hand, when in solvation it shifted upto22 cm − 1 whichis signi fi -cant. Another signi fi cant shift is predicted for the mixed bending andstretching mode observed at 1327 cm − 1 in medium intensity.1332 cm − 1 is the wavenumber predicted for this mode in the gasphase, and it is shifted up to 12 cm − 1 in solvation. The band computedat 1497 cm − 1 (observed at 1500 cm − 1 ) is a mixed mode of CC and CNstretching vibrations with C – H rock. It is shifted to 1490, 1486, and1485 cm − 1 in chloroform, ethanol, and water, respectively. Such aband computed at 1546 cm − 1 (observed at 1540 cm − 1 ) is shifted upto 7 cm − 1 in considered dielectric media. CH stretchings are themodeswhichstronglyaffectedbythepresenceofsolventenvironment.These modes are shifted by up to 82, 86, and 86 cm − 1 in varioussolutions. Fig. 4 visually summarizes the effect of solvent media on thevibrational modes of Oxa.  3.3. Theoretical electronic properties Atomic charge and dipole moment values of Oxa in different mediaare computed and presented in Table 3. The oxygen and nitrogenatoms are found to have highly negative charge as expected. Whenimplicit PCM solvation model employed the negative charges on bothatomsincreased. Note that, thenitrogen 8N is theatom mostlyaffectedfrom the dielectric media.Experimental dipole moment of Oxa has been measured as 1.50 Dbefore, and it is available in the literature [40]. Our computations reproduced the dipole moment value very well for the gas phase Fig. 2.  Deviations from the gas phase calculated bond lengths (Å) in various solvents. Fig. 3.  Experimental FT-IR spectrum of Oxazole.50  Ş  . Yurdakul et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 162 (2016) 48 – 60  (1.57 D). Besides, our calculation predicted a signi fi cant jump in thedipole moment when passing from gas phase to dielectric media. 1.97,2.14,and2.17Darethedipolemomentvaluescalculatedinchloroform,ethanol, andwater,respectively.Wehaveseenthat thedipolemomentmagnitude in chloroform is close to the values calculated for ethanoland water. Hence, we may conclude that the polarity of the solventenvironment is not much effective in the case of the solvation of theOxa ring.Frontier molecular orbitals properties of Oxa are determined in allmedia considered in this work, and the data obtained are tabulated onTable 4. We have seen that the total energy, and the highest occupiedmolecular orbital (HOMO) and lowest unoccupied molecular orbital(LUMO) energies of an Oxa molecule decrease with increasingdielectricity of the media. On the other hand, the data on Table 4 showthat the HOMO – LUMO energy gap of Oxa is affected only marginallyby dielectric media. Fig. 4.  Deviations from the gas phase calculated frequencies (cm − 1 ) in various solvents.  Table 3 Atomic charges (e) and dipole moment (Debye) of Oxazole in various media.Vacuum CHCl 3a EtOH a Water a 1C 0.354 0.356 0.356 0.3572C  − 0.098  − 0.108  − 0.112  − 0.1133C 0.080 0.078 0.078 0.0784H 0.189 0.209 0.217 0.2185H 0.211 0.226 0.232 0.2336H 0.205 0.224 0.232 0.2337O  − 0.459  − 0.465  − 0.466  − 0.4678N  − 0.483  − 0.522  − 0.536  − 0.539 μ   1.57 1.97 2.14 2.17 a ε  (chloroform) = 4.9,  ε  (ethanol) = 24.55,  ε  (water) = 78.39.  Table 4 HOMO, LUMO energies (au) and the ∆ E L  − H  energy band gap (eV) of Oxazole.Vacuum CHCl 3a EtOH a Water a E HOMO  − 0.26666  − 0.26491  − 0.26467  − 0.26463E LUMO  − 0.02268  − 0.02117  − 0.02118  − 0.02119 ∆ E L  − H  6.64 6.63 6.63 6.62 a ε  (chloroform) = 4.9,  ε  (ethanol) = 24.55,  ε  (water) = 78.39.  Table 2 The theoretical and experimental vibrational frequencies and PED (%) distribution of Oxazole molecule.Mode Vacuum CHCl 3a EtOH a Water a Exp. PED (%)1 612 608 607 607 609 s  Γ CCOC (72) +  Γ NCOC (15) +  Γ HCOC (10)2 650 647 647 647 645 vs  Γ NCOC (62) +  Γ HCNC (18) +  Γ CCOC (11)3 749 751 751 751 755 s  Γ HCOC (82)4 822 829 831 831 837 m  Γ HCNC (70) +  Γ NCOC (21)5 861 861 860 860  Γ HCNC (79)6 901 899 897 897 900 s  δ CCO (34) +  δ NOC (26) +  δ COC (25)7 913 913 913 913 907 sh  δ COC (53) +  δ NOC (21) +  ν OC (11)8 1050 1034 1029 1028 1044 s  δ OC (39) +  δ HCN (16) +  δ CCO (15)9 1072 1069 1067 1066 1081 s  ν OC (38) +  δ HCO (16) +  δ NCO (14) +  ν CC (11)10 1100 1085 1080 1079  ν OC (42) +  δ NCO (21) +  δ CCO (19)11 1139 1134 1132 1131 1142 m  ν OC (31) +  δ HCO (23) +  δ HCN (19) +  ν CC (14)12 1250 1246 1244 1244 1259 vw  δ HCN (79) +  δ HCO (15)13 1332 1324 1321 1320 1327 m  δ HCN (30) +  ν CN (23) +  δ HCO (14) +  δ NCO (13)14 1497 1490 1486 1485 1500 m  ν CC (33) +  ν CN (28) +  δ HCO (15) +  δ COC (11)15 1546 1542 1540 1539 1540 m  ν NC (41) +  ν CC (37) +  δ HCN (15)16 3121 3072 3045 3039  ν CH (91)17 3134 3074 3053 3048 3134 m  ν CH (98)18 3158 3102 3077 3072 3160 w  ν CH (89) a ε  (chloroform) = 4.9,  ε  (ethanol) = 24.55,  ε  (water) = 78.39.51 Ş  . Yurdakul et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 162 (2016) 48 – 60   3.4. Oxa ⋯ (H   2 O) n  complexes in the solvation phase Despite the simplicity of Oxa structure, its presence in manyimportant molecules, and studies published in the past, further explicitsolvationstudiesneedtobedone.HavingtheNandOheteroatomswithlone pair (lp) electrons, Oxa can form intermolecular H-bonds of thetype lpN ⋯ H – O, lpO ⋯ H – O, and C – H ⋯ O with water molecules.There are two published studies in which the Oxa ⋯ (H 2 O) n complexes investigated. In the study of Kaur et al. [45], the H-bondedcomplexes are investigated by quantum mechanics (QM) calculations Fig. 5.  Geometries of Oxa ⋯ (H 2 O) complexes.52  Ş  . Yurdakul et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 162 (2016) 48 – 60
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