Molecular Investigation and Nonlinear Optical Response of Dihydropyrimidinone: A Comparative Spectroscopic and Quantum Computational Studies

Organic molecule ethyl-4-(4-chloro-3-nitrophenyl)-6-methyl-2-oxo-1,2,3,4tetrahydropyrimidine-5-carboxylate has been synthesized. The molecular structure has been characterized using FT-IR, FT-Raman, 1H and 13CNMR spectral studies. The structure of the title molecule was theoretically investigated by DFT method using B3LYP/6-31G(d,p) basis set. The firm assignments of vibrational bands are allowed using experimental and computations. The nonlinear optical property of the title molecule has been calculated using first hyperpolarizability components. The intra-molecular charge transfer occurring in the molecule have been analyzed by NBO analysis. The electronic and charge transfer properties have been studied using frontier molecular orbitals. 1H and 13C-NMR spectra were recorded and calculated using the gauge independent atomic orbital (GIAO) method.


Introduction
Organic materials are attractive due to their optical properties, electronics, and integrated photonics [1][2][3]. Organic molecules with electron deficient pyrimidine ring tend to act as electron acceptor and are very effectively used in Organic light emitting diodes (OLEDs) [4]. The higher light harvesting efficiency achieved by pyrimidine adopted porphyrin sensitizers show more advantage in oxidized dyes. New organic dyes with pyrimidine-2-carboxylic acid forms coordination bond with TiO 2 improves the interaction between the anchor and semiconductor [5]. Pyrimidine show considerable efforts in the development of bipolar materials to overcome the unipolar character of the organic materials [6]. Recent work from Lin et al., reported that pyrimidine used as π-conjugated spacer in organic photosensitizers in dye sensitized solar cells (DSSCs) [7]. The electron withdrawing character of pyrimidine chromophore exhibits the white photoluminescence in both liquid and solid state [8]. Moreover, pyrimidine based iridium complex exhibit external quantum efficiency up to 28.6% due to high photoluminescence quantum yield determines the excellent device performance and high efficiency [9].
In this paper, we report the synthesis, spectral and nonlinear optical investigation of ethyl-4-(4-chloro-3-nitrophenyl)-6-methyl-2-oxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate (CNPC). Here the molecular structure and electronic structural properties of the title molecule were studied using experimental and also with theoretical approach. For unambiguous vibrational spectral assignments precisely potential energy distribution have been performed and related with recorded FT-IR and FT-Raman spectra, respectively. This work also covers the molecular electrostatic potential mapped surface and energy gap analysis along with global reactivity descriptors. The charge transfer interactions occur in the CNPC molecule and stabilization arising from the donor-acceptors interactions are examined using Natural bond orbital analysis. The first hyperpolarizability (β) components examining the nonlinear response of the title molecule. The NMR chemical shifts calculated using Gauge-independent atomic orbitals and experimental chemical shifts were also analyzed in the present work.
raised from 0˚ to 360˚ rotation by a step of 10˚ interval. The potential energy surface scan curve and optimized structure of CNPC is shown in Figure 1. From the results, there are three maximum energy conformers (0˚, 170˚ and 360˚) and two minimum conformers (100˚ and 270˚) were identified. The most stable conformer was identified at 270˚ rotation with the relative energy -1542.44631 Hartree. Hence this structure is the global minimum conformer and is used for the further investigations. The various possible conformers of CNPC during PES scan were shown in Table 1.

Vibrational Assignments
The detailed description of vibrational assignments of CNPC along with calculated IR/Raman intensities and potential energy distributions of the vibrations are listed in Table 2. For visual comparison, the recorded and simulated FT-IR and FT-Raman spectra of CNPC are shown in Figures 2 and 3, respectively. The un-scaled wavenumber obtained from DFT method, overestimate the observed wavenumber. To bring the theoretical wavenumber closer to the observed wavenumber, a selective scaling procedure was employed. The title molecule have 37 atoms, hence it gives 105 normal modes of vibrations, 36 stretching, 71 in-plane bending and 34 out-of-plane bending vibrations.

C-Cl vibrations
The stretching and bending vibrations of C-Cl normally occur in the low wavenumber region 760-505 cm −1 [14]. Vibrational couplings are possible due to lowering of the molecular symmetry and the presence of heavy atom. The C-Cl stretching vibration of CNPC is observed as strong band at 688 cm −1 in FT-IR spectrum along with the vibration of β CCC bending vibration. The calculated wavenumber corresponds to the C-Cl stretching mode is 693 cm −1 with 68% of PED contribution. The chloro substituted aromatic compounds have a band of strong to medium intensity in the region 385-265 cm −1 due to C-Cl in-plane deformation [15]. The in-plane β CCCl is observed at 338 cm −1 in FT-Raman spectrum with medium intensity. The calculated in-plane and out-of-plane Experimental Synthesis of ethyl 4-(4-chloro-3-nitrophenyl)-6methyl-2-oxo-1,2,3,4-tetrahydro pyrimidine-5carboxylate 4-Chloro-3-nitrobenzaldehyde (1.84 ml, 0.01 mmol) and urea (1.8 g, 0.03 mmol) was added to an ethanolic solution of ethyl acetoacetate (1.34 ml, 0.01 mmol). To the mixture CeCl 3 .7H 2 O (0.465 g, 25%) was added and stirred well. Then, the reaction mixture was refluxed at 90°C for 2-3 hours and the completion of the reaction was monitored by thin layer chromatography. After completion, the reaction mixture was poured onto crushed ice and stirred up to 5-10 minutes. The solid product was separated, filtered under suction, washed with ice-cold water and then recrystallized from absolute ethanol. The synthesis of CNPC molecule is shown in Scheme 1. Melting point=185˚C; Yield=87%.

Scheme 1
The reaction scheme of synthesis of CNPC molecule.

Computational Details
The quantum chemical calculations of CNPC was performed using the B3LYP level of theory supplemented with 6-31G(d,p) basis set, using Gaussian 03 program package invoking geometry optimization [10]. Initial geometry generated from geometrical parameters was minimized without any constraint in the potential energy surface at DFT level. The optimized minimum structure parameters were used in the vibrational wavenumber calculations at the DFT level to characterize all stationary points as minima. The harmonic vibrational wavenumber calculations resulting in IR and Raman intensities and Raman depolarization ratios. The vibrational modes were assigned based on potential energy distribution analysis (PED) using VEDA4 program [11]. The Raman activities were transformed into Raman intensities using Raint program [12] by the expression: Where I i is the Raman intensity, A i is the Raman scattering activities, ν i is the wavenumber of the normal modes and ν 0 denotes the wavenumber of the excitation laser [13].

Molecular conformational analysis
In order to investigate the stable conformer, potential energy surface scan was performed to CNPC molecule. In this PES scan process, the internal redundant coordinate of the dihedral angle D(C11-C13-C16-C20) chosen for the conformational flexibility within the molecule. During this scan the geometrical parameters was relaxed, while the D(C11-C13-C16-C20) torsional angle

NO 2 vibrations
The nitro stretching vibrations are the most characteristic bands in the spectra of nitro compounds, not only because of their spectral positions but also for their strong intensities. The nitro substituted aromatic compounds show asymmetric stretching mode in the region of 1600-1500 cm −1 and symmetric stretching mode in the region of 1385-1325 cm −1 [16]. In the present case, the asymmetric stretching vibrations of NO 2 are observed at 1500 cm −1 /FT-IR with medium intensity (27.45) and at 1502 cm −1 / FT-Raman spectrum. The calculated wavenumber at 1538 cm −1 is assigned for the asymmetric NO 2 vibrations. The symmetric stretching vibration of NO 2 is observed at 1323 cm −1 in FT-IR and calculated wavenumber computed at 1336 cm −1 with medium intensity. Aromatic nitro compounds show C−N stretching vibrations nearly ~870 cm −1 [16]. The theoretical band at 913 cm −1 is assigned for νC 7 −N 34 vibration of the nitro group. The inplane deformation of NO 2 is observed at 563 cm −1 in FT-IR and well agreed with the computed wavenumber at 562 cm −1 .

N−H and C− N vibrations
In heterocyclic molecules, the N−H stretching vibrations are usually appearing in the region of 3500-3300 cm −1 [17].

C=O and C−O vibrations
The carbonyl stretching vibrations are normally occurs in the region 1715-1600 cm −1 , it is moderately active in Raman and intense in IR [19]. In the title molecule, the strong band observed at 1699 cm

C−H vibrations
The C−H stretching vibrations of aromatic ring absorb in its characteristic region 3100-3000 cm −1 [21].

Ring vibrations
The ring C-C stretching vibrations occur in the region 1625-1430 cm −1 . In the phenyl ring, the six carbon atoms undergo coupled vibrations called skeletal vibration [24,25]. In present investigation, the ring C-C stretching vibrations are observed as a strong band at 1593, 1503 cm −1 in FT-Raman and medium band at 1500 cm −1 in FT-IR are assigned for the ring vibrations. The theoretical wavenumber in the range 1585-1538 cm −1 represents the ring C-C vibrations. The strong bands at 1168 and 1023 cm −1 in FT-Raman and weak band at 1165 cm −1 in FT-IR outcomes the phenyl ring breathing and trigonal bending vibration of the title molecule. These fundamental wavenumber computed at 1173 and 1026 cm −1 represents the ring breathing and trigonal bending vibrations of aromatic ring system.

CH 3 and CH 2 group vibrations
In methyl groups, the symmetric stretching vibrations are

Archives in Chemical Research ISSN 2572-4657
hyperconjugative interaction of π(C−C)→π*(C−C) bonds of the phenyl ring increases ED at the six conjugated π-bonds. From the NBO analysis, the π-electron delocalization in phenyl ring revealed by the ED at the three conjugated π-bond (≈1.63-1.67e) and π*(≈0.30-0.45e) resulting to the stabilization energies of ≈69.54-96.99 kJ/mol, respectively. The π-electron cloud movement can make the molecule highly polarized and causes internal charge transfer, which is responsible for the activity of the title molecule. The orbital overlap between n(Cl)→π*(C-C) bond orbital, which increases ED

NLO Optics
Nonlinear optical studies is at the forefront of current research intensity varying from medium to strong and expected in the range 1380 ± 25 cm −1 [26]. This band has been observed at 1377 cm −1 in the FT-IR spectrum and 1375 cm −1 in FT-Raman spectrum. The rocking vibrations of methyl group usually observed in the region 1100 ± 95 cm −1 [28]. In present case, the medium band observed at 1016 cm −1 in FT-IR denotes the rocking vibration of methyl group attached to dihydropyrimidine rind and its corresponding computed wavenumber at 1019 cm −1 . The rocking mode for the methyl group present in the carboxylate side chain shows weak band at 1157 cm −1 in FT-Raman spectrum and its calculated wavenumber at 1137 cm −1 .

NBO Analysis
The molecular interactions occur in both the occupied and unoccupied molecular orbitals are examined by the Natural bond orbital analysis. Second order perturbation theory analysis of Fock matrix in NBO basis of CNPC is tabulated in Table 3.  F(i,j) Table 3 The donor-acceptor interactions in NBO basis for CNPC. because of its prominence in providing the key functions of optical switching, optical modulation, optical logic and optical memory for the emerging technologies in areas such as signal processing, telecommunications and optical interconnections [29,30]. The molecular polarizability and hyperpolarizability are calculated about 4.71 × 10 −30 esu and 2.61 × 10 −30 esu, respectively. The β 0 value of the title compound is seven times greater than that of reference urea. Urea is one of the prototypical molecules used in the study of the NLO properties of molecular systems. Therefore, it was used frequently as a threshold value for comparative purposes [31]. The first hyperpolarizability values of similar pyrimidine derivatives are reported as, 1.19, 1.35 and 0.30 × 10 −30 esu, respectively [32][33][34]. The first hyperpolarizability components of CNPC were listed in Table 4.

Molecular Electrostatic Potential
Molecular electrostatic potential is related to the electronic density of the molecule and is a very useful descriptor in understanding the charge sites as well as hydrogen bonding interactions [35,36]. The electrostatic potential V(r) are also well suited for analyzing processes based on the ''recognition'' of one molecule by another, as in drug-receptor, and enzyme substrate interactions, because it is through their potentials that the two species first ''see'' each other [37,38]. Being a real physical property V(r)s can be determined experimentally by diffraction or by computational methods [39]. The MEP mapped surface and 2D contour map of CNPC were shown in Figure 5. In order to predict the possible electrophilic and nucleophilic charge sites for the investigated molecule, MEP mapped surfaces are   Table 4 The first hyperpolarizability components of CNPC

Figure 5
The molecular electrostatic potential mapped surface of CNPC.

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generated. The positive (blue) regions to nucleophilic reactivity and the negative (red and yellow) regions of MEP were related to electrophilic reactivity shown in Figure 5. The negative potentials are commonly observed in the region of electronegative atoms with lone pair electrons. In our study, the negative regions are localized in the oxygen atoms present in the nitro group and also with the oxygen atoms present in the two carbonyl groups, which are electrophilic nature. The positive potentials are localized over the hydrogen atoms bonded with the nitrogen atoms of dihydropyrimidinone ring, which are nucleophilic in nature.

Energy Gap Analysis
Frontier molecular orbitals and their energies are very useful for the physicists and chemists. The analysis of the wave function indicates that the electron absorption corresponds to the transition from the ground to the first excited state and is mainly described by one-electron excitation from the highest occupied molecular orbital (HOMO) to the lowest unoccupied orbital (LUMO). The eigen values of HOMO/LUMO and their energy gap reflect the chemical activity of the molecule. In order to gain insight into the electronic structure of CNPC, it's theoretical molecular orbital distributions were calculated with the Gaussian program at B3LYP/6-31G(d,p) level using the density functional theory. As shown in the Figure 6 the highest occupied molecular orbitals of CNPC were mostly dispersed on the 4-chloro-3-nitrophenyl moiety. In contrast, the LUMO were localized on the electron deficient pyrimidine ring together with the carboxylate side chain.

NMR Analysis
Gauge independent atomic orbital method is used as a default method to calculate the NMR chemical shifts of the CNPC molecule. The recorded 1 H-NMR and 13 C-NMR spectra of the CNPC molecule are shown in Figure 7.

Conclusion
Nonlinear organic molecule ethyl-4-(4-chloro-3-nitrophenyl)-6methyl-2-oxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate has synthesized and characterized using spectral techniques. The stable conformer was identified with the relative energy of about -1542.44631 Hatree. Using potential energy distribution the vibrational modes of the recorded wavenumbers were assigned. The NBO analysis reveals the strong hyperconjugative interactions occur in the molecule leads to the stabilization of the molecular system. The small energy gap value 4.0899 eV is responsible for the nonlinear activity of the investigated molecule. The first hyperpolarizability calculated as 2.6143 x 10-30 esu, which is   seven times greater than that of reference urea and their similar NLO molecules. Thus our title molecule is good candidate for the nonlinear optical studies. The molecular charge sites are identified by the molecular electrostatic potential mapped surface. The structural confirmation by 1 H and 13 C-NMR reports with computed chemical shifts shown very good agreement.