Spectroscopic , Structural and Density Functional Theory ( DFT ) Studies of Two Oxazol-5-one Derivatives

In this study, two oxazol-5-one derivatives, C20H20N2O2 (1) and C21H22N2O2 (2), were synthesized by getting condensed p-N,N-diethylaminobenzaldehyde with two presented hippuric acid derivatives and in further studies they were analysed spectrochemically. Molecular and crystal structures of the compounds were determined by single-crystal X-ray diffraction and the results revealed that the molecular packing of the crystal structures were stabilized by weak intraand intermolecular interactions also with C–O∙∙∙π, C–H∙∙∙π and π∙∙∙π stacking interactions. Computational studies were also performed using DFT method at B3LYP/6-311G(d,p) level of theory. Vibrational modes and chemical shifts were calculated and compared with the experimental data. In addition, frontier molecular orbitals and molecular electrostatic potential surfaces were simulated. The calculated results show that the optimized geometries can well reproduce the crystal structure. Purpose of this study was to survey the effects of the reactants, which were condensed with each other to produce oxazol-5-one, upon the characteristic properties and crystal forms of the final oxazol-5-one.


Introduction
[9][10] Oxazol-5-ones are also used in dye industry owing to the fact that oxazol-5-ones are easily obtainable in crystalline states and they possess promising photochemical/photophysical properties due to their chromophore group. 10,11erein, we report on the synthesis, spectral characterization and theoretical studies of two oxazol-5-one derivatives.The experimental FT-IR, 1 H NMR, 13 C NMR studies were performed.Structures of the compounds were confirmed by single-crystal X-ray diffraction studies.Theoretical cal-culations were also carried out in order to corroborate the experimental results.

Materials and Methods
Some reagents (toluene, ethanol, ethyl acetate, sodium acetate, p-N,N-diethylaminobenzaldehyde) were obtained from commercial sources and used without further purification, acetic anhydride was purified by distillation, hippuric acid derivatives were synthesised and used after purification.

1. Analytical Instruments and Spectroscopy Techniques
Melting points were determined by Barnstead Electrothermal 9,100 instrument.FT-IR spectra were recorded by Perkin-Elmer Spectrum BX FTIR spectrometer using KBr pellets.NMR data were measured by Varian 3.2 400 MHz spectrometer in CDCl 3 solutions and chemical shifts were expressed in ppm downfield from tetramethylsilane.

1. Synthesis of 4-(p-N,N-Diethylaminophenylmethylene) -2-phenyloxazol-5-one (1)
2.82 mmol p-N,N-diethylaminobenzaldehyde, 2.82 mmol N-benzoylglycine (hippuric acid) and 2.82 mmol sodium acetate was added to 3 mL of redistilled acetic anhydride.Reaction mixture was stirred under dry conditions for 4-5 hours at 100 °C, thereafter stirred at room temperature overnight.3 mL of ethanol was added to cooled oily-solid like mixture and left in refrigerator for 2-3 hours.Precipitation occurred so that mixture was filtered, solid obtained washed with ethanol and recrystallized from hexane-ethyl acetate solution.Determined melting point of 1 is 134.5 °C.

4. Computational Details
The synthesized compounds 1 and 2 have been optimized at DFT/B3LYP method, using 6-311G(d,p) basis set.Also, the harmonic vibrational frequencies and NMR spectra were calculated at the same levels of theory for the optimized structures.The calculated frequencies were scaled down by using single scaling factor 0.9669 for DFT/ B3LYP/6-311G(d,p) level, in order to improve the agreement with the experimental values. 16The 1 H and 13 C iso-Nazlı et al.: Spectroscopic, Structural and Density Functional ... tropic shielding tensors referenced to the TMS calculations were performed by using gauge invariant atomic orbital (GIAO) method in chloroform solvent. 17Highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) and MEP have been calculated from optimized geometry of the molecules.All calculations were carried out with the Gaussian 09W and Gauss View molecular visualization program. 18,19

1. Crystal Structure
The atomic numbering scheme of the crystal structures and the optimized geometries which has the most favourable conformation of the compound 1 and 2 are shown in Figures 1a and b.Molecules crystallize in triclinic system with P-1 space group.Selected bond distances, bond angles and torsion angles together with corresponding values obtained by means of X-ray crystallographic analysis and DFT calculations are compared and listed in Table 2.

2. Frontier Molecular Orbitals
The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are the basic orbitals that play an important role in chemical stability.The HOMO shows the ability to donate an electron, whereas the LUMO as an electron acceptor shows the ability to obtain an electron.This also predicts the nature of electrophiles and nucleophiles at the atom where the HOMO and LUMO are stronger. 34The energy gaps of the compounds 1 and 2 were calculated using B3LYP/ 6-311G(d,p) level (Figure 4).For both compounds, highest electron density lies mainly on the oxazol-5-one ring and on (diethylamino)benzylidene moiety.The HOMO energy levels are calculated at -5.271(2) and -5.214 eV, respectively.On the other hand, the electrons are more distributed over the phenyl for the LUMO with the energy of -2.27 and -2.203 eV, respectively.The energy gaps of HOMO and LUMO could be determined to be about 3.001 eV for the compound 1 and 3.011 eV for 2, which indicate the molecules become less stable and more reactive.

Molecular Electrostatic Potential
The molecular electrostatic potential (MEP) is a reactivity map displaying probable region for the electrophilic and nucleophilic attacks and hydrogen bonding interactions of the molecules. 35In order to predict the reactive part of the electrophilic and nucleophilic attack, the MEP of the title compounds were also calculated from B3LYP/ 6-311G(d,p) optimized geometry.In the compounds 1 and 2, the negative regions (red) of the MEP which are around the O2 and O1 atoms bounded to oxazol-5-one ring, were related to electrophilic reactivity that is responsible for intermolecular hydrogen bonding for compound 1, intramolecular hydrogen bonds for compound 2 and positive regions (blue) which are around the hydrogen atoms correspond to nucleophilic reactivity (Figure 5).

4. Analysis of the Vibrational Spectra
The infrared spectra of the title compounds were recorded in the 4000-600 cm -1 region using FT-IR spectrophotometer and are presented in Figure 6.The vibrational band assignments were determined at B3LYP/ 6-311G(d,p) theory level.It is well-known that the vibrational wavenumbers obtained by DFT computations usually overestimate their experimental counterparts.These discrepancies can be corrected either by computing anharmonic corrections or by introducing a scaled field. 36The visual check for the vibrational band assignments were also performed by using GaussView molecular visualization program.There are no negative frequencies in the calculated IR spectra, which indicates a stable optimized geometry.The selected harmonic vibrational IR frequencies and the corresponding experimental values are listed in Table 4.The infrared spectra of the compounds have some Nazlı et al.: Spectroscopic, Structural and Density Functional ... characteristic bands of the stretching vibrations of the =C-H, -C-H, C=O, C=C-O etc. groups, in plane bending vibrations of C-H, C-H 2 , C-H 3 groups and out of plane bending vibrations for =C-H, C-H 3 groups.In addition to these vibrations, some wagging, scissoring, twisting and rocking vibrations are obtained by theoretical study.There are some discrepancies between the observed and calculated data.This is because the experimental data were taken as KBr pellets, whereas the theoretical calculations were performed for isolated molecule in the gaseous phase.
The most characteristic bands of aliphatic -CH 2and -CH 3 groups are those arising from C-H stretching vibrations which experimentally occur in general region of 3000-2840 cm -1 .The asymmetrical/symmetrical stretching for -CH 2 -groups is observed near 2926/2853 cm -1 and for -CH 3 groups near 2962/2872 cm -1 , respectively.These standard values can be slightly changed depending on the surrounding of the alkyl moiety.Besides the aromatic C-H stretching bands which occurred at 3100-3000 cm -1 , all obtained experimental C-H bending vibrational data are compatible with expected values as well as stretching vibrational data.
C-H stretching vibrations were calculated at 3106/3104 cm -1 for symmetric and 3081/3063 cm -1 for   Nazlı et al.: Spectroscopic, Structural and Density Functional ... asymmetric bands which are in the characteristic region for the identification of C-H stretching vibrations in the compounds 1 and 2, respectively.The symmetric stretching vibrations of C-H 2 are determined at 2886/2851 cm -1 and asymmetric stretching vibrations are determined at 2929/2917 cm -1 .Similarly, C-H 3 symmetric vibrational modes are observed at 2901 cm -1 for both compounds, whereas the asymmetric modes are identified at 2969/2965 cm -1 . 37These results are considerably compatible with the experimental data.
Other characteristic bands C=O, C=C, etc. were also detected.As usually, carbonyl stretching band at five -membered heterocyclic core shows around 1780 cm -1 , C=C stretching bands at exo positions shows around 1640 cm -1 , experimentally.These two values are also overlapping with our experimental results.In contrast to experimental value, calculated ν(C=O) vibration band was detected quite high for compound 1 (1857 cm -1 ).This can be attributed to the strong intermolecular hydrogen bond (C11-H11B•••O2).

5. 1 H and 13 C NMR Analysis
Experimental 1 H and 13 C NMR of the title compounds were recorded in CDCl 3 .Theoretical calculations carried out in the chloroform solvent (with respect to TMS) at the B3LYP/6-311G(d,p) method by adopting GIAO method and compared to the experimental chemical shift values, are presented in Tables 5 and 6.For the B3LYP/6-311G(d,p) method, the chemical shift value of tetramethylsilane (TMS) σ 0 ( 13 C) = 179.7024ppm and σ 0 ( 1 H) = 31.3919ppm was obtained. 38For the compound 1, NMR spectral data show that the C2 atom has the highest chemical shift value (168.68 ppm), whereas the methyl C12 and C14 atoms have the least one (12.64ppm).Similarly in 2, the highest chemical shift is for C2 with the value of 168.78 ppm and the least for C12 and C14 atoms at the range of 12.64 ppm.In the experimental spectrum, the signal of the C18 atom of the phenyl moiety was observed at 133.44 ppm in the compound 1.But this value is higher in the 2 (142.92ppm), because of the presence of a methyl group.

Conclusions
Compounds 1 and 2 have been synthesized and characterized by FT-IR, 1 H NMR, 13 C NMR and X-ray single -crystal diffraction.In addition, density functional modelling studies of the oxazol-5-one derivatives have been reported in this study.The calculated geometric parameters by using the DFT with the 6-311G(d,p) basis set are mostly compatible with the X-ray structure.The vibrational band assignments and NMR shift values were performed at the same theory level to compare the experimental and calculated values of the compounds.These calculated and experimental results are in good agreement with the explanatory differences.

Figure 1 .
Figure 1.(a) The molecular structure of the compounds 1 and 2 with atom numbering scheme and 30% probability displacement ellipsoids and (b) optimized structures for DFT/ B3LYP/6-311G(d,p) level.

Figure 2 .Figure 3 .
Figure 2. The formation of the hydrogen bond motif through C11-H11A•••O2 i hydrogen bonds for the compound 1 [ i 1+x, 1+y, +z].For the sake of clarity, H atoms not involved in the motif have been omitted, and only the interacting atoms are labeled.

Figure 4 .
Figure 4. Frontier molecular orbital surfaces and energy levels for the HOMO and LUMO of the compounds 1 and 2 computed at B3LYP/6-311G(d,p) level.

Table 1 .
Crystal data and structure refinement parameters for the compounds 1 and 2.

Table 4 .
Comparison of the observed and calculated vibrational spectrum of the compounds.

Table 5 .
Comparison of the experimental and calculated 1 H NMR values in chloroform.

Table 6 .
13mparison of the experimental and calculated13C NMR values in chloroform.