DFT Investigations (Geometry Optimization, UV/Vis, FT-IR, NMR, HOMO-LUMO, FMO, M

MASOOME Shikhi SIYAMAK Shh LIUDMILA Filippovich EVGENIJ Dikusr MEHRNOOSH Khlghin



DFT Investigations (Geometry Optimization, UV/Vis, FT-IR, NMR, HOMO-LUMO, FMO, MEP, NBO, Excited States) and the Syntheses of New Pyrimidine Dyes①

MASOOME Sheikhia②SIYAMAK Shahabb, c, dLIUDMILA Filippovichb, cEVGENIJ DikusarbMEHRNOOSH Khaleghiane

a()b(13220072)c(220141)d()e()

In the present work, the molecular structures of two new synthesized dyes:(4,6-dimethylpyrimidin-2-ylamino)(5-p-tolylisoxazol-3-yl)methanol (PS-1) and N-(4,6-dimethylpyrimi-din-2-yl)-5-phenylisoxazole-3-carboxamide (PS-2), have been investigated using density func-tional theory (DFT) in dimethylformamide (DMF) for the first time. The electronic spectra of new dyes in a DMF solvent were carried out by time dependent density functional theory (TD-DFT) method. After quantum-chemical calculations two new dyes for the optoelectronic applications were synthesized. FT-IR spectra of the title compounds are recorded and discussed. Nucleus-Independent Chemical Shifts (NICS) calculations have also been carried out for the title compounds. The computed absorption spectral data of the title compounds are in good agreement with the experimental data, thus allowing an assignment of the UV spectra. The HOMO and LUMO molecular orbitals, excitation energies and oscillator strengths for the dyes have also been calculated and presented.

pyrimidinedye, NICS, DFT calculation, UV/Vis spectrum;

1 INTRODUCTION

Pyrimidine and its derivatives play an important role in chemistry. Many derivatives of pyrimidine were synthesized for use in the field of pharmacy[1-4]. They have biological activities such as antihyperten-sive, antibiotic, antioxidant, antiviral, antifungal,anti-convulsant, anti-HIV, inflammatory, antibacterial[5,6], antifilarial agents[7], anticancer[8]and DNA modi-ficators[9,10]. Pyrimidines are candidates for new therapeutic agents. Therefore, new methods for their synthesis have been developed. Pyrimidine-based compounds are valuable because of the the optical and physical characteristics. Pyrimidine is-electron deficient with an ionization potential value about 10.41 eV[11]. The organic-conjugated compounds have semiconducting properties and are used in optoelectronic devices[12-14], light emitting diodes (LED), field-effect transistors (FET)[15-17], sensors and solar cells[18]. The organic field-effect transistors (OFETs) were used in various electronic devices due to their cheapness,low weight, easy construction method, high flexibility and environmental friendly nature[19-22]. They have been used as the inhibitor of corrosion[23,24], electroluminescent materials and biomaterials in the past[25].

In recent years, computational chemistry has become an important tool for chemists and a well-accepted partner for experimental chemistry. Theore- tical quantum chemistry method is an important area in determining the mechanisms of chemical reac-tions[26], especialcatalysis[27], structural determina-tion of organic compounds[28], prediction of spec-troscopic data such as1H NMR and13C NMR che-mical shifts[29], IR, UV/Vis, properties calculation of organic molecules[30]. TD-DFT is used for predicting the absorption spectra of the compounds[30-32]. Besides detailed experimental studies of pyrimidines, theoretical studies (DFT calculations) were used for interpreting their structures, spectroscopic assign-ments, tautomeric studies and reaction mecha-nisms[33-37]. In this research, we report the results of DFT calculations to investigate absorption spectrum, UV/Vis, FT-IR, HOMO LUMO, FMO, MEP and NBO of two new pyrimidine derivatives such as PS-1 and PS-2. In this work, FT-IR spectra data and UV/Vis computations of the new synthesized dyes in the ground state were calculated by using DFTmethod.

2 COMPUTATIONAL METHODS

In this work, the quantum chemical calculations have performed and optimized compounds PS-1 and PS-2using the DFT method with 6-31+G* basis setsby the Gaussian 09W program package[38]on a Pentium IV/4.28 GHz personal computer.The Polarized Continuum Model (PCM)[30]was used for calculations of solvent effect. We also used TD-DFT method to calculate the electronic transitions of the title compounds and theoretical absorption spectra of the structures in a solvent DMF. The nucleus-independent chemical shift (NICS)[39]was calculated for two structures PS-1 and PS-2. The theoretical FT-IR spectra of the optimized structures PS-1 and PS-2 were calculated by the B3LYP/6-31+G* level of theory. The electronic properties such as EHOMO, ELUMO, energy gap between HOMO and LUMO, dipole moment (μ), point group and natural charge[40]of the title structures were calculated. The optimized molecular structures, MEP maps, HOMO and LUMO surfaces were visualized by GaussView05 program[41]. Also the electronic struc-ture of the compound PS-1 was studied by Natural Bond Orbital (NBO) analysis[42]using the B3LYP/6-31+G* level of theory in order to understand hyper-conjugative interactions and charge delocalization.

3 RESULTS AND DISCUSSION

3.1 Optimized structuresof compounds PS-1 and PS-2

We carried out DFT calculations for the new compounds PS-1 and PS-2 by B3LYP/6-31+G* level of theory in a solvent DMF, and the optimized molecular structuresare shown in Fig. 1. The quan-tum chemical calculations were performed using the Gaussian 09 software package and GaussView 05 visualization programs on a Pentium IV/4.02 GHz personal computer. The IEFPCM (Integral Equation Formalism PCM) coupled to UAKS radii is a method that was used to calculate the solvent (DMF) effect. The Integral Equation Formalism PCMby Cances, Mennucci and Tomasiis the most popular PCM version. It employs a molecule shaped cavity com-posed of spheres centered on the nuclei, while the reaction field is modeled by placing charges on the cavity surface. The selected experimental and calcu-lated geometrical parameters, such as bond lengths (Å) and bond angles (°),ofstructures PS-1 and PS-2 have been obtained by the B3LYP/6-31+G* level of theory and listed in Tables 1 and 2, from which wecansee that the calculated parameters show good approximation and can be used as a foundation to calculate the other parameters for the title com-pounds.

Table 1. Selected Optimized Geometrical Parameters (Bond Lengths (Å) and Bond Angles (°)) ofCompound PS-1 Calculated by the B3LYP/6-31+G* Method

Table 2. Selected Optimized Geometrical Parameters (Bond Lengths (Å) and Bond Angles (°)) of Compound PS-2 Calculated by the B3LYP/6-31+G* Method

Fig. 1. Optimized molecular structures ofcompounds PS-1 and PS-2

3.2 Nucleus-independent chemical shifts of compounds PS-1 and PS-1

An aromatic ring current is an effect found in aromaticmolecules. If a magnetic field is directed perpendicular to the plane of the aromatic system, a ring current is induced in the delocalizedelectrons of the aromatic ring[43]. This is a direct consequence of Ampère's law; since the electrons involved are free to circulate, rather than being localized in bonds as they would be in most non-aromatic molecules, they respond much more strongly to the magnetic field. Aromatic ring currents are relevant to NMR spectroscopy, as they dramatically influence the chemical shifts of 1Hnuclei in aromatic molecules[44]. The effect helps distinguish these nuclear environ- ments and is therefore of great use in molecular structure determination. The nucleus-independent chemical shift (NICS) is a computational method that calculates the absolute magnetic shielding at the center of a ring. The values are reported with a reversed sign to make them compatible with the chemical shift conventions of NMR spectroscopy[45].

NICS calculations have been done forcompounds PS-1 and PS-2. NICS values are given in Table 3. In this method, the negative NICS values indicate aromaticity and positive values antiaromaticity. According to the results in Table 3, negative NICS values indicate aromaticity in the rings of com-pounds PS-1 and PS-2 (Rings a, b and c inFig. 1). Incompound PS-1, the phenyl (Ring a) and pyrimidine (Ring c) rings have the highest aromaticity at the 1Å below the ring surface, whereas the isoxazole (Ring b) ring has the highest aromaticity along the five atoms of the ring. Incompound PS-2, the phenyl (Ring a) ring has the highest aromaticity at 1Å above the ring surface, whereas the pyrimidine (Ring c) ring has the highest aromaticity at 1Å below the ring surface and the isoxazole (Ring b) ring has the highest aromaticity along the five atoms of the ring.

Table 3. Nucleus-independent Chemical Shifts ofCompounds PS-1 and PS-2

3.3 Electronic structures and excited states ofcompounds PS-1 and PS-2

We used the TD-DFT method for predicting the absorption spectra of the new compounds PS-1 and PS-2. The theoretical absorption spectra of the optimized compounds were calculated in a solvent DMF by TDB3LYP method with the 6-31+G* basis set. 20 excited states considered for the calculation equations that were performed using the IEFPCM (Integral Equation Formalism PCM) coupled to UAKS radii. The exact amount of the maximum absorption wavelength (max) to the title compounds is obtained using the TD-DFT method. 20 excited states and wavelengths of electronic absorption spectrum of the compound PS-1 are reported in Table 4. As can be seen from Table 4, the strong absorption atmax= 281 nm and the oscillator strength= 0.67 are due to charge transfer of electron into the excited state0→2with wave function including two configurations ((H-1→L), (H→L)). The transition from HOMO-1 to LUMO (H-1→L) is main responsible for the formation of maximum wavelength at 281 nm (Table 4). Fig. 2 shows shape molecular orbitals participant atmax= 281 nm. According to Fig. 2, the electron density of HOMO-1 is mainly focused on -C=N- groups, -C=C- groups in phenyl, isoxazole and pyrimidine rings and nitrogen atoms in N-H group, whereas the LUMO is mainly centralized on -C=N- in isoxazole ring and -C=C- in phenyl and isoxazole rings. Therefore, the electronic transition from HOMO-1 to LUMO is due to the contribution of pi () bonds. The other important excited state is0→1at 287 nm (= 0.20) with two configurations for electronic excitations ((H-1→L), (H→L)). The other excited states of compound PS-1 (max≤0.05 nm) have very small intensity that is nearly forbidden by orbital symmetry considerations (Table 4).

The calculated electronic absorption spectrum (UV/Vis) of the compound PS-1 in a solvent (DMF) and experimental spectrum at concentration 0.2´10-4M/Land in the solvent DMF is observed in Figs. 3a and 3b. The calculated and experimental magnitudes of maximum wavelength (max) are 281 nm (= 0.67, Fig. 3a) and 282 nm (= 1.3)(Fig. 3b), respectively. As can be seen, the experimental value of wave-length is in excellent agreement with the theoretical one.

According to theoretical results in Table 5, the strong absorption peak in electronic absorption spectrum of the compound PS-2 atmax= 261 nm is observed at oscillator strength= 0.34. The wavelength maximum is due to the charge transfer of electron into the excited state0→7that it is a wave function consisting of four configurations for electron excitations ((H-4→L), (H-3→L), (H-1→L), (H→L+1)). The most important transition for forma-tionmax= 261 nm is from HOMO to LUMO+1 (Table 5). The formedmolecular orbitals partici-pating at the excited state0→7are shown in Fig. 4. According to Fig. 4, the electron density ofHOMO is mainly focused on -C=C- of phenyl and isoxazole rings and-C=N- in isoxazole ring and nitrogen atom in N–H group, whereas the electron density ofLUMO+1 is mainly focused on -C=C- and -C=N-of the pyrimidine ring. Therefore, the electronic transitions from HOMO to LUMO+1 are due to the contribution of pi () bonds. The other important excited state is0→3at 273 nm (= 0.23) with five configurations for electronic excitations ((H-4→L), (H-2→L), (H-2→L+1), (H-1→L), (H→L)). The other excited states (max=271, 270, 264, 248, 247, 240, 224, 219, 218, 213, 212, 210, 209 nm) of the title compound have very small intensity that is nearly forbidden by orbital symmetry considerations (Table 5).

Fig.2. Form of MO participating in forming theabsorption spectrum of compound PS-1 atmax= 281 nm calculated by the B3LYP/6-31+G* method

Fig.3. UV/Vis spectrum ofcompound PS-1 in the solvent DMF: (a) calculated by the TDB3LYP/6-31+G* method;(b) experimental at the concentration of dye 0.2∙10-4M/L

Fig.4. Form of MO participating in the formation of the absorption spectrum of compound PS-2 atmax= 261 nm calculated by the B3LYP/6-31+G* method

Table 4. Electronic Absorption Spectrum ofCompound PS-1 Calculated by the TDB3LYP/6-31+G* Method

*H-HOMO, L-LUMO

Table 5. Electronic Absorption Spectrum ofCompound PS-2 Calculated by the TDB3LYP/6-31+G* Method

*H-HOMO, L-LUMO

The calculated (UV/Vis) of the compound PS-2 in the solvent DMF and experimental spectra at con- centration 0.24´10-4M/Land in the solvent DMF are shown in Figs. 5a and 5b. The calculated and experimental values of maximum wavelength are 261 nm (= 0.34, Fig. 5a) and 287 nm (= 1.3, Fig. 5b), respectively. As can be seen, the calculated wave- length is in excellent agreement with the experimen-tal value.

Fig.5. UV/Vis spectrum ofcompound PS-2 in the solvent DMF: a) calculated by the TDB3LYP/6-31+G* method and b) experimental at the concentration of dye 0.24∙10-4M/L

3.4 Frontier molecular orbital analysis ofstructures PS-1 and PS-2

The frontier molecular orbitals (FMO) analysis plays a significant role in the electronic and optical properties, as well as in UV/Vis spectrum and che-mical reactions[30,42]. FMO analysis was done for compounds PS-1 and PS-2 by the B3LYP/6-31+G* level of theory. The FMO results of the title com-pounds are summarized in Table 6.

Table 6. Electronic Properties ofCompounds PS-1 and PS-2

HOMO and LUMO orbitals act as electron donor andacceptor, respectively. As can be seen from Table 6, the energy values of HOMO of compounds PS-1 and PS-2 are –6.5 and –6.8 eV and their energy values of LUMO are–1.76and –2.16 eV, respec-tively. Total electronic density of states (DOSs)[46]of the title compounds were computed (Fig. 6). The DOS analysis indicates that the energy gaps between LUMO and HOMO of compounds PS-1 and PS-2 are about 4.74and 4.64 eV, respectively. The compound PS-1 has a greater energy gap value (4.74 eV), so it is less reactive with the harder electronic transfers compared with the compound PS-2.

Fig.6. Calculated DOS plots ofcompounds PS-1 and PS-2

Details of quantum molecular descriptors of the title compounds such as ionization potential (–HOMO), electron affinity (= –LUMO), global hardness (=_/2), electronegativity (=+/2), electronic chemical potential (= –()/2), electrophilicity (=2/2)[40]and chemical softness (= 1/)[42], are calculated andreported in Table 6. The energy of HOMO is directly related to the ionization potential (), while that of LUMO is related to the electron affinity (). The global hardness () corresponds to the energy gap between LUMO and HOMO. A molecule with a small energy gap has high chemical reactivity, low kinetic stability and is a soft molecule, while a hard molecule has a large energy gap[42]. The global hardness () values ofcompounds PS-1 and PS-2 are about 2.37and 2.32 eV, respectively. Therefore, compound PS-1 hashigher global hardness and it is a hard molecule compared with the PS-2. Electronegativity () is a measure of the power of an atom or a group of atoms to attract electrons[40], and the chemical softness () describes the capacity of an atom or a group of atoms to receive electrons[42].

The dipole moment parameter is used for the investigation of intermolecular interactions and study of asymmetric nature of compounds[46]. The high amounts of dipole moment lead to stronger intermo-lecular interaction[46]. As can be seen from Table 6, dipole moment values ofcompounds PS-1 and PS-2 are about 4.5880 and 9.6852 Debye. The value of the dipole moment is related to the composition and dimensionality of the 3D compounds. The point group of the title compounds is C1, which refers to their high asymmetry. When the atoms in compound are irregularly arranged, the dipole moment is increased.

3. 5 Molecular electrostatic potentials (MEP) of compounds PS-1 and PS-2

Molecular electrostatic potential (MEP) maps show the electronic density in the molecules. The MEP maps are also used to identify sites of negative and positive electrostatic potentials for electrophilic attack and nucleophilic reactions[47,48]. The dif-ference of electrostatic potential at the surfaces is represented by different colors. The negative regions of MEP with red, orange and yellow colors havehigh electron density, while it is low for the positive regions with blue color. Also, the green colorshows the neutral region. The MEPs ofcompounds PS-1 and PS-2 were obtained by theoretical calculations (Fig. 7).

Fig. 7. Molecular electrostatic potential (MEP) maps ofcompounds PS-1 and PS-2

As shown in Fig. 7, the negative regions (red color) of the compound PS-1 are mainly focused on the O(13) atom and phenyl ring, whereas the negative region of compound PS-2is mainly focused on the O(13) atom. Therefore, these regions are suitable for electrophilic attack. The parts of the title compounds with pale red or yellow color are sites with weak interaction that includes N(16) atom incompound PS-1. According to MEP map of the title compounds, the hydrogen atoms are the positive potential sites (blue color). The regions with green color of the two compounds indicate areas with zero potential (neutral sites).

3.6 Natural charge analysis of compounds PS-1 and PS-2

We calculated the charge distributions for equili-brium geometry ofcompounds PS-1 and PS-2 by the NBO (natural charge) charges[40]using the B3LYP/6-31+G* level of theory. The calculated natural charges are listed in Table 7 (Atoms labeling is according to Fig. 1).

Table 7. Natural Charges Distribution (NBO Charges, e) of Compounds PS-1 and PS-2

The total charge of the investigated compounds is equal to zero. Also Fig. 8 shows results of the natural charges in graphical form. The results of natural charge (NBO) analysis of compounds PS-1 and PS-2 show that carbon atoms have both positive and negative chargevalues. According to the results, positive carbons are observed for the carbonatoms attached to the electron-withdrawing nitrogen and oxygen atoms. The C(7), C(10), C(12), C(15), C(17) and C(19) atoms in PS-1 and PS-2 havepositive charges. The other carbon atoms have negative charges. The oxygen and nitrogen atoms have negative charges. The highest positive charge of the compound PS-1 is observed for the C(15) atom (0.608e) due to the attachment to the electron-withdrawing nitrogen atoms (N(14), N(16), N(12)), whereas the highest positive charge ofcompound PS-2 is observed for the C(12) atom (0.688e) due to the attachment to electron-withdrawing nitrogen atom (N(14)) and2hybridization of the C(12) atom. Also, the highest negative charge is observed for the carbon atoms of methyl groups due to hyperconju-gation effect including the C(21) (–0.717e), C(22) (–0.718 e) and C(23) (–0.705e) atoms in PS–1 and C(21) (–0.718e), C(22) (–0.719e) atoms in PS-2. According to Natural charge’s plot (Fig. 8), all hydrogen atoms have positive charges. Incompound PS-1, the H(30) atom ofO–H group has the highest positive charge (0.521e) compared with other hydrogen atoms due to the electron-withdrawing nature ofO(13), and in the compound PS-2the H(29) atom ofN-H group has the highest positive charge (0.459e) compared with other hydrogen atoms due to the electron-withdrawing nature of the N(14), so they are acidic hydrogen atoms. Also, all oxygen and nitrogen atoms of the title compounds havenegative charges.

Fig. 8. Natural chargedistribution ofcompounds PS-1 and PS-2

3.7 NBO analysis ofcompound PS-1

Natural bond orbital (NBO) analysis is an impor-tant method for studying intra- and intermolecular bonding interaction between bonds in the molecular systems[46]. Electron donor orbitals, acceptor orbitals and the interacting stabilization energy ((2)) resul-ting from the second-order micro disturbance theory for the compound PS-1 are reported in Table 8.

Table 8. Significant Donor-acceptor Interactions and the Second Order Perturbation Energies ofCompound PS-1Calculated Using the B3LYP/6-31+G* Level of Theory

a(2)Energy of hyperconjucative interactions.bEnergy difference between donor and acceptor i and j NBO orbitals,cF(,) is the Fock matrix element between i and j NBO orbitals.

The electron delocalization from filled NBOs (donor orbitals) to the empty NBOs (acceptor orbitals) describes a conjugative electron transfer process between them. For each donor () and accep-tor (), the stabilization energy(2)associated with the delocalization→is estimated[49]:

where qiis the donor orbital occupancy, εjand εiare diagonal elements and F(i,j) is the off diagonal NBO Fock matrix element. The resonance energy ((2)) detected the quantity of participation of electrons in the resonance between atoms of molecule[46]. The larger(2)value, the more intensive the interaction is between the electron donorand acceptor, i.e. the more donation tendency from electron donors to electron acceptors and the greater extent of conjuga-tion of the whole system[26]. Delocalization of electron density between occupied Lewis-type (bond or lone pair) NBO orbitals and formally unoccupied (antibond or Rydberg) non Lewis NBO orbitals correspond to a stabilization donor-acceptor interac-tion. NBO analysis has been performed for the com-pound PS-1 byB3LYP/6-31+G* method in order to elucidate the intramolecular, rehybridization and delocalization of electron density within the title compound (PS-1). We reported the highest strong intramolecular hyperconjugative interactions of the title compound, such as→*,→*,→*,→* and→* transitions in Table 8. The(C(1)–C(6)) orbital in phenyl ring participates as donor and the anti-bonding*(C(4)–C(5)) and*(C(7)–C(11)) orbitals as acceptor with resonance energies ((2)) 16.51 and 20.68 kcal/mol, respectively. These values indicate(C(1)–C(6))→*(C(7)–C(11)) transition has the highest resonance energy (20.68 kcal/mol) compared with the(C(1)–C(6))→*(C(4)–C(5)) transition. The(C(4)–C(5))→*(C(1)–C(6)) transi-tion has the highest resonance energy (68.14 kcal/mol) rather than other→* transitions of the compound PS-1. The other important intramolecular hyperconjugative interactions of→* in the pyrimidine ring that lead to a strong delocalization are such as C(15)–N(20)→C(18)–C(19), N(16)–C(17)→C(15)–N(20) andC(18)–C(19)→N(16)–C(17)with the resonance energies ((2)) 32.18, 37.32 and 37.45 kcal/mol, respectively. As can be seen from Table 9, the intramolecular hyperconjugative interactions of→* transitions havehigher resonance energy ((2)) rather than the→* transi-tions. The(C(23)–H(39))→*(C(2)–C(3)) transi-tion has the highest resonance energy (4006.94 kcal/mol) rather than other→* transitions of the compound PS-1.The(C(21)-H(35)) orbital in pyrimidine ring participates as donor and the anti-bonding*(C(1)–C(2)),*(C(2)–H(24)),*(C(3)–H(25)),*(C(23)–H(40)) orbitals as acceptor with resonance energies ((2)) of 189.02, 112.09, 120.38 and 723.22 kcal/mol, respectively. These values indicate(C(21)–H(35))→*(C(23)–C(40)) transi-tion has the highest resonance energy (723.22 kcal/mol) compared with(C(21)–H(35))→*(C(1)–C(2)),(C(21)–H(35))→*(C(2)–H(24)) and(C(21)–H(35))→*(C(3)–H(25)) transitions. The highest resonance energy of compound PS-1 is observed for(C(23)–H(41))→*(C(23)–H(40)) transition in methyl group with resonance energy ((2)) 9823.96 kcal/mol that leads to most stability of the title compound. The most important transitions→* are observed for2(O(8))→*(C(7)–C(11)) and1(N(14))→*(C(15)–N(20)) with resonance energies ((2)) 32.32and 53.81 kcal/mol, respectively. Also, the1(N(20))→*(C(23)–H(40)) transition has the highest resonance energy (308.09 kcal/mol) rather than other→* transitions of the compound PS-1.

The results of NBO analysis such as the occupa-tion numbers with their energies for the interacting NBOs (interaction between natural bond orbital A and natural bond orbital B (A–B)) and the polariza-tion coefficient amounts of atoms in the compound PS-1 are presented using the B3LYP/6-31G* method, as summarized in Table 9 (Atoms labeling is according to Fig. 1).

The size of polarization coefficients shows the importance of the two hybrids in the formation of the bond in molecules. The differences in electronega-tivity of the atoms involved in the bond formation are reflected in the larger differences in the pola-rization coefficients of the atoms (C–O, C–N, C–H bonds)[26]. As can be seen from Table 9, the calcula-ted bonding orbital for the(C(7)–O(8)) bond is the=0.5620(3.51)+0.8271(1.91) with high occupancy 1.99198a.u. and low energy –0.94443a.u. The polarization coefficients of C(7)=0.5620 and O(8)=0.8271 shows the importance of O(8) in forming the(C(7)–O(8)) bond compared with the C(7) atom. As seen from Table 10, the calculated bonding orbital for the(N(14)–C(15)) bond is=0.7807(1.79)+0.6249(2.29)formed from1.79and2.29hybrids on the N(14) and C(15) atoms, which is the mixture of(35.77%)(64.19%) for N14 and(30.33%)(69.56%) for C(15). The natural hybrid orbital2(O(8)) with high occupancy 1.71851a.u. and high energy –0.35437a.u. has-character (99.89%). Therefore, pure-type lone pair orbital2(O(8)) participates as electron donation to*(C(7)–C(11)) in the2(O(8))→*(C(7)–C(11)) interaction with high resonance energy ((2)) 32.32 kcal/mol in the title compound (see Table 8). According to NBO analysis, the natural hybrid orbital1(N(14)) occupies a high energy orbital (–0.26865a.u), high occupation number (1.73229a.u) and high-character (98.28%). Therefore, n1(N(14)) participates as electron donation to*(C(12)–O(13)),*(C(12)–H(29)) and*(C(15)–N(20)) in the1(N(14))→*(C(12)–O(13)),1(N(14))→*(C(12)–H(29)) and1(N(14))→*(C(15)–N(20)) interac-tions with resonance energies ((2)) 14.20,3.09and 53.81 kcal/mol in the title compound (see Table 8).

Table 9. Calculated Natural Bond Orbitals (NBO) and the Polarization Coefficient for Each Hybrid in Selected Bonds of the Compound PS-1Using the B3LYP/6-31+G* Level of Theory

aA–B is the bond between atoms A and B (A: natural bond orbital and the polarization coefficient of atom; A-B: natural bond orbital and the polarization coefficient of atom B)

3.8 Vibrational frequencies of compounds PS-1 and PS-2

In order to confirm the accuracy of our findings, IR spectroscopy was used. The theoretical IR spectra of the optimized compounds PS-1 and PS-2 were calculated using B3LYP/6-31+G* level of theory. The vibrational frequency assignments were made using the GaussView 05 program. The important calculated and experimental vibrational frequencies of the title compounds are summarized in Tables 10 and 11, respectively. The experimental and calcu-lated spectra of the title compounds are found to be in good agreement with each other. The important vibrational frequencies ofcompounds PS-1 and PS-2 are reported as follows.

Table 10. Experimental and Calculated Vibrational Frequencies and Their Assignment ofCompound PS-1 by Using the B3LYP/6-31+G* Method

Abbreviations:, stretching;, in plane bending;, out of plane bending;, torsion; asym, asymmetric deformation; arom, aromatic;

Table 11. Experimental and Calculated Vibrational Frequencies and Their Assignment ofCompound PS-2 by Using the B3LYP/6-31+G* Method

Abbreviations:, stretching;, in plane bending;, out of plane bending;, torsion; asym, asymmetric deformation; arom, aromatic

3. 8. 1 O–H vibrations

The O–H stretching vibrationappears near about 3600～3400 cm-1[50]. In the present work, the O–H stretchingband in the IRspectrum ofPS-1 is assigned at 3352 cm-1and its corresponding theore-tical value is calculated at 3625 cm-1. The O–H out-of-plane bending vibration ofcompound PS-1 is assigned at 498cm-1in IR spectrum with its calcula-ted valuepredicted at 532 cm-1.

3. 8. 2 C=O and C–O vibrations

The carbonyl stretching vibrations in amides are expected at 1715～1680 cm-1as medium or strong bands[51]. In this work, the very strong stretching band at 1728 cm-1in IR spectrum of compound PS-2 is related to the C=O carbonyl and the calculated value is predicted at 1746 cm-1. According to the results, the C=O stretching vibrationof compound PS-2 appears at a higher frequency than the range 1715～1680 cm-1due to the conjugated resonance of N–H group with the pyrimidine ring and decrease resonance N–H with C=O group. The C–O stretching vibration of the hydroxyl group is expected in the range 1260～1180 cm-1[52,53]. The C–O stretching vibration ofcompound PS-1 is assigned at 1211 cm-1and the corresponding calculated valueis predicted at 1281 cm-1.

3. 8. 3 N–H vibrations

The N–H stretching modes usually appearin the range 3500～3300 cm-1[54-56]. The weak band observed in the IR spectrum of compound PS-1 at 3125 cm-1is expected to theN–H amine stretching vibration and the corresponding theoretical value is predicted at 3614 cm-1(Table 10). The medium band observedin the IR spectrum ofcompound PS-2 at 3401 cm-1is assigned to the N–H amide stretching mode and the theoretical value is predicted at 3588 cm-1(Table 12). The N–H bending vibrations related to secondary amine are observed at about 1500 cm-1[57].The bands occurring in the IR spectrum of compound PS-1 at 1521 and 1513 cm-1are assigned to the N–H bending vibrations and the calculated values are predicted at 1564, 1544 and 1515 cm-1. The N–H bending vibrations related to secondary amideappearin the range 1640～1550 cm-1[57]. In the IR spectrum ofcompound PS-2, the bands located at 1570and 1524 cm-1are assigned to the N–H bending modes and the calculated values are present in the range 1638～1536 cm-1.

3. 8. 4 C–Nand C=N vibrations

According to opinion Silverstein, the C–N stre-tching modes are assigned in the range 1382～1266 cm-1[58]. The C–N stretching vibrations of aminesappearin the range 1350～1000cm-1[57]. In the current study, the band observed in the IR spectrum ofcompound PS-1 at 1076 cm-1is assigned to the C–N stretching vibration and the calculated valuesare predicted at 1143and 1088 cm-1. The C–N stretching vibration in the IR spectrum ofcompound PS-2 is observed at 1231 cm-1and the corresponding theoretical value is located at 1236 cm-1.

Pyrimidines havestrong bands in the range 1600～1500 cm-1as to the C=N and C=C stretching vibrations[57]. For compound PS-1, the C=N stre-tching vibrations are observed at 1594, 1566, 1521, 1513 and 1438 cm-1in the IR spectrum and the DFT calculations predicted the C=N stretching modes at 1636～1500 cm-1. In the IR spectrum ofcompound PS-2,the C=N stretching modes are assigned at 1570, 1524, and 1451cm-1and the calculated values are predicted at 1638～1459cm-1.

3. 8. 5 C=C vibrations

The C=C aromatic ring stretching vibrations are expected in the range 1650～1200 cm-1[59]. In this investigation, the bands located at 1594, 1566, 11521, 1513, 1438, 1370, 1336, 1299 and 1211 cm-1in the IR spectrum of compound PS-1 are assigned to C=C stretching vibrations and the corresponding theore-tical values are observed in the region 1659～11276 cm-1. The C=C stretching vibrations in the IR spectrum of PS-2 are assigned at 1570, 1524, 1451, 1345, 1231 cm-1and its corresponding theoretical values are calculated at 1638～1286 cm-1.

3. 8. 6 C–H vibrations

The C–H stretching vibrations of aromatic com-pounds are expected in the range 3100～3000 cm-1[57]. The C–H stretching vibrations of the aryl ringsofcompound PS-1 are assigned at 3032, 3005 cm-1and the corresponding theoretical values are 3297, 3223, 3208, 3187, 3186 cm-1. In the IR spectrum of com-pound PS-2, the C–H stretching vibrations of the aryl rings are observed at 3130, 3062 cm-1and the calculated values are predicted at 3223, 3217, 3209, 3201, 3193 cm-1. The aromatic C–H in-plane ben- ding vibrations are expected in the range 1500～1100 cm-1[60]. The calculated values of vibrations in-plane ofcompound PS-1 in the region 1354～1051 cm-1are in good agreement with the experimental frequencies in the range of 1299～1011 cm-1. Also in the IR spectrum ofcompound PS-2, the aromatic C–H in-plane bending frequencies are assigned at about 1231, 1193, 1140 cm-1and the calculated values are predicted in the range of 1278～1077cm-1.

All the experimental vibrations ofcompounds PS-1 and PS-2 are ingood agreement with the calcula-ted frequencies by DFT method (Tables 10 and 11).

3.9 Experimental

The characterization data of the title compounds are given below:

3. 9. 1 (4,6-Dimethylpyrimidin-2-ylamino)(5-p-tolylisoxazol-3-yl)methanol (PS-1)

IR (KBr)max/cm-1: 3352, 3124, 3005, 2942, 1594, 1566, 1521, 1438, 1142, 1076, 814.1H NMR (DMSO-d6,250.13 MHz;):H(ppm) 2.89 (s, 6H, 2CH3), 2.36 (s, 3H, CH3), 3.46 (s, 3H, CH3), 6.23 (d,= 7 MHz, 1H, CH), 6.25 (s, 1H, CH), 6.40 (s, 1H, CH), 6.52 (d,= 7 MHz, 1H, NH), 7.22 (d,= 7.5 MHz, 2H, Arom.), 7.61 (d,= 7.5 MHz, 2H, Arom.).13C NMR (DMSO-6, 62.90 MHz):C(ppm) 54.71, 77.82, 97.20, 111.75, 124.68, 125.85, 129.74, 140.65, 161.23, 163.48, 167.84, 170.79.

3. 9. 2 N-(4,6-Dimethylpyrimidin-2-yl)-5-phenylisoxazole-3-carboxamide (PS-2)

IR (KBr)max/cm-1: 3401, 3146, 3130, 2920, 1728, 1604, 1524, 1451, 1231, 1193, 773.1H NMR (DMSO-d6,250.13 MHz;):H(ppm)2.38 (s, 6H, 2CH3), 7.04 (s, 1H, Arom.), 7.48 (s,1H,Arom.), 7.55 (m, 3H, Arom.), 7.94 (m, 2H, Arom.).13C NMR (DMSO-6, 62.90 MHz):C(ppm) 23.89, 100.61, 117.17, 126.35, 126.85, 129.93, 131.47, 157.07, 157.91, 160.28, 168.47, 170.97.

4 CONCLUSION

We have modeled by Density functional theory (DFT) and synthesized two newpyrimidine dyes ((4,6-dimethylpyrimidin-2-ylamino)(5-p-tolylisoxa-zol-3-yl)methanol (PS-1), N-(4,6-dimethylpyrimidin-2-yl)-5-phenylisoxazole-3-carboxamide (PS-2)) absorbed in the UV region of spectrum. A high degree of approximation between the calculated and experimental data was established. These synthesized dyes will be used to develop polarizing UV-films at the Laboratory of Polarizing films of the Institute of Physical Organic Chemistry of the National Academy of Sciences of Belarus.

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10 November 2017;

7 March 2018

①This project was supported by the National Academy of Sciences of Belarus

.E-mail:m.sheikhi2@gmail.com (M. Sheikhi)

10.14102/j.cnki.0254-5861.2011-1887