Synthesis , Spectroscopic , Intramolecular Energy Transfer and Electronic Structure Nonlinear Optical Properties of Novel

New oxygen bridged tricyclic pyrimidinone molecule is synthesized by Biginelli condensation reaction using CeCl3.7H2O as an efficient catalyst. The new molecular structural arrangement of unusual product of Biginelli reaction is analysed using experimental and theoretical techniques. The extent of intermolecular charge transfer and delocalization are estimated and discussed in terms of natural bond orbitals. The optimized geometry reveals the intramolecular hydrogen bonding exists in the studied compound and it is confirmed using NBO analysis. The shifts in vibrational wavenumber due to the hydrogen bondings are curiously analysed with potential energy distributions of vibrations. To scrutinize the nonlinear optical properties of the title molecule first order hyperpolarizability components are calculated and its shows the title molecule is promising candidate for NLO studies. In addition, the active charge sites and energy gap are also identified and discussed.


INTRODUCTION
Dihydropyrimidinones (DHPM) and their derivatives have gained greater attention in synthetic and material chemistry, because of their wide range of pharmacological and optical activities 1,2 .
Recently, strong photo physical and electrochemical active materials were reported with a series of pyrimidine-carbazole conjugates 3 . Pyridine-5carboxylic acid was used as an anchoring group in solar cells with an efficiency of 5.5% 4. Interestingly the organic molecules with donor and acceptor groups connected with π-conjugation have shown wide range of field effect transistor, organic light emitting diodes (OLED), dye-sensitized solar cells (DSSCs) and nonlinear optics [5][6][7][8] . In recent years, the direct conversion of solar energy has gained more attention in terms of low cost and higher efficiency in DSSC 9 .
Biginelli compounds contained multiple functional groups where a significant conformational flexibility was found between the aryl and the ester moiety. Dihydropyrimidinones and its corresponding derivatives are acted as vital hetero cyclic entities that possess diverse pharmacological and therapeutic properties, such as anti-inflammatory, anti-viral, anti-microbial and anti-tumour activities 10 . Moreover these compounds had emerged as α-adreno receptor antagonists, calcium channel blockers and anti-hypertensive agents 11 . Due to our interest in the Biginelli reaction, in the current manuscript, we intend to evaluate the molecular structural arrangement, shifting in wavenumbers, energy transfer occurring intramolecular level and nonlinear optical investigation for the newly synthesized oxygen bridged Biginelli compound with theoretical predictions. We intend this manuscript to be very useful for the optical investigations of unexpected product of the Biginelli condensation. The new structural arrangement of ethyl-2-(chloromethyl)-9-methoxy-4-oxo-3,4,5,6-tetrahydro-2H-2,6methanobenzo[g] 1,3,5 oxadiazocine-11-carboxylate (THPC) has been studied experimentally as well as with theoretical calculations. In addition, the vertical excitations in electronic states and identification of active sites in novel THPC also carried out.

ExPERIMENTAL S y n t h e s i s o f n o v e l o x y g e n b r i d g e d tetrahydropyrimidinone (THPC)
Ethanolic solution of urea (0.9 g) and 4-methoxysalicylaldehyde (0.76 g) is mixed to ethyl-4-chloroacetoacetate (0.82 ml). To this homogenous mixture 0.465 g of Cecl 3 .7H 2 O is added gradually and stirred continuously. The reaction mixture is further refluxed in a round bottomed flask at 90 °C for the time period of 4 hours. After the completion of reaction, the crude product is poured onto the beaker containing crushed ice and stirred for 10-15 min., to get the solid product. The crude sample is washed well with cold water and recrystallized using absolute ethanol. The reaction scheme of novel THPC is shown in the Scheme-1.

Characterization techniques
The infrared spectrum of THPC in the range of 4000-400 cm -1 was recorded with the device of Shimadzu FT-IR spectrophotometer (resolution of 4 cm -1 ). The FT-Raman spectrum in the spectral range of 3500-50 cm -1 was recorded using Bruker RFS27 spectrometer operating at 100mW laser. The NMR ( 1 H & 13 C) spectra of THPC were recorded using Bruker 300 MHz spectrometer.

Quantum calculation details
The ground state geometry of THPC has been simulated by using B3LYP hybrid functional level with 6-311++G(d,p) basis set. The entire quantum chemical calculations were performed with Gaussian 03 software 12 . The initial structural parameters of THPC were minimized in potential surface scan. The harmonic wavenumbers were calculated for the energy minimum structure and the wavenumbers were properly scaled by 0.96. The vibration bands were precisely assigned based on the percentage contributions of potential energy distributions 13. The calculated Raman activities were transformed into intensity using Raint Program 14, 15 .

Structural Analysis
The preliminary search for the stable conformer of THPC is identified from the scanned points of potential energy surface. The dihedral angles D1(C 5 -C 4 -O 34 -C 35 ), D2(C 3 -C 4 -O 34 -C 35 ), D3(C 14 -C 11 -C 20 -O 28 ) and D4(C 11 -C 20 -O 28 -C 21 ) are the internal redundant coordinates used for the free rotation of the molecule. While processing the PES scan, the structural parameters are relaxed such as D1, D2, D3 and D4 torsional angles are raised from 0 o to 360 o rotations with 10 o intervals at each step. The potential energy surface scan curves of four dihedrals of THPC are presented in Fig 1. The various conformers of THPC and its energies are presented in Table 1. The dihedral angles D1and D2 are scanned using the torsional barrier of the methoxy side chain around the bond C 4 -O 34 . Dihedral angle D3 and D4 are scanned using the torsional barrier of the exocyclic ester ring around the bonds, C 11 -C 20 and C 20 -O 28 , by varying the torsional perturbation.  21 . Finally, the minimum energy conformer is identified in the internal rotation of D3(C 14 -C 11 -C 20 -O 28 ) with relative energy of -1527.70953 Hartree, yielded more stable conformer and is used for further investigation. The optimized structure of THPC with intramolecular hydrogen bonding is shown in Figure 2.

Identification of intramolecular hydrogen bond
The existence of hydrogen bondings in a molecule have shown pronounced effects on molecular structural properties 16 . In this investigation, the intramolecular hydrogen bonding is found between the carbonyl oxygen of the ester group with the chlorine attached methylene group. The shorter distance of 2.28 Å, is calculated for the C=O•••H−C    In addition the strength of the intramolecular hydrogen bond is identified by donor-acceptor interactions. From NBO results, the charge transfer interaction from (LP)O 29 →σ*(C 17 −H 39 ) leads the moderate stabilization of 1.56 kcal/mol. The weak hydrogen bonding energy of charge transfer between the elements lies in the range between 0-4 kcal/mol, but can sometimes be rise up to 40 kcal/mol. The ED value of the C 17 −H 39 is found as 0.0184e, which is comparably less than other C−H bonds of the THPC molecular system. These results strongly assure the formation of hydrogen bonding in THPC at intramolecular level with the distance of 2.28 Å.

Vibrational Spectral Assignments
The THPC has 40 atoms and fits to C1 point group symmetry, which gives rise 114 normal modes of vibration. The scaled wavenumbers have been studied from Potential Energy Distributions (PED). The Fourier Transformed experimental and theoretical (IR & RAMAN) vibrational spectra of THPC are shown in Fig. 3. The NMR ( 1 H & 13 C) spectra of THPC are shown in Fig. 4. The experimental and calculated wavenumbers with detailed PED assignments are presented in Table S1. To overcome the discrepancies between experimental and theoretical wavenumber, the proper scale factor has used to scale down the theoretical wavenumbers suggested by Computational Chemistry Comparison in Benchmark Database 18 .

N-H and C-N Vibrations
Aromatic N-H stretching wavenumbers normally occur in the 3500-3300 cm -1 spectral region 19

C=O and C-O vibrations
Normally the C=O bonds have an intense peaks in the region 1800-1600 cm -1 21 . The νC=O vibrations are mainly based on the bond strength, which depends on the conjugative, steric effect and lone pair electron present in it 22

Aromatic ring vibrations
The νC-H vibrations in aromatic ring exhibit bands usually in the spectral range of 3100-3000 cm -1 24 . In THPC, there are three C−H bonds namely C 3 −H 7 , C 5 −H 8 and C 6 −H 9  Similarly, the out-of-plane bending C−H vibrations occur in the region 1000-750 cm −1 as coupled vibrations 26 . The weak band at 821 cm -1 in Raman spectrum is the corresponding band of out-of-plane C−H bending of THPC. The calculated wavenumber at 915, 820 and 794 cm −1 are also assigned relatively with 80% of PED contribution. The νC=C vibrations of phenyl ring usually observed in the spectral region of 1625-1430 cm −1 27 . In this investigation, sharp bands at 1619, 1501 cm -1 in IR and 1600, 1491 cm −1 in Raman bands represents νC=C vibrations of phenyl ring. These vibration modes are calculated at 1609, 1569 and 1490 cm −1 from the DFT method. In 13 C-NMR spectrum the signals observed at 101. 37-129.40 ppm are the responsible carbon signals of phenyl ring.

C−H Vibrations
The νC−H (as) vibrations of methyl group is appeared in the spectral region 3010-2940 cm −1 and νC−H (s) stretching vibrations in the spectral range of 2970-2840 cm −1 28 . In THPC, there are two methyl groups one at methoxy and another one at ethyl group of the acetate chain. The wavenumber observed at 2930 cm -1 in Raman spectrum and at 2924 cm -1 in IR spectrum are allocated to symmetric stretching C-H bands of the methyl groups present in methoxy and ethyl side chains. The asymmetric vibrations of methyl moities are identified at 3014 and 2966 cm −1 . The vibrations of symmetric νC−H stretching are identified at 2942 and 2905 cm -1 with 90% of PED. In 13 C-NMR spectrum, the carbon signals appeared at 55. 21 and 13.90 ppm are corresponding to C 13 and C 22 methyl groups. Whereas, the singlet and triplet observed at 3.714 and 1.215- 1.262 ppm confirms the protons of methoxy and methyl group in ester chain of the THPC.
The stretching modes of methylene groups are expected to occur at 2935 and 2865 cm −1 , respectively 29 . The acetate moiety methylene vibrations are calculated at 3000 and 2959 cm -1 . The quar tet signal observed at 4.143-4.253 ppm is corresponds to the methylene protons. The asymmetric stretching vibrations of chlorine attached CH 2 group absorb nearly 3100 cm −1 , and symmetric CH 2 stretching nearly 2986 cm −1 30 . The asymmetric and symmetric wavenumbers of chlorine attached methylene group is calculated at 3116 and 3012 cm −1 , respectively. The blue-shifting in the wavenumber indicating the improper intramolecular hydrogen bonding occur in the THPC. The aliphatic C 10 -H 9 and C 11 -H 33 vibrations are calculated at 3004 and 2958 cm -1 . The sharp band observed at 3002 cm −1 is attributed for the tetrahydropyrimidine aliphatic C-H vibration of the ring.

Vibrations of C−Cl
T he νC-X (x=Cl) vibration bands are usually observed in the region of 760-505 cm −1 ; here the combinations of band are possible due to heavy atom and lowering the molecular symmetry 31 . The νC−Cl vibration is appeared at 784 cm -1 in FT-IR spectrum and 773 cm -1 in FT-Raman with 83% of PED contribution. The theoretical band at 783 cm −1 well matched with the experimental finding. The chlorine substituted methylene group shown doublet signal at 3.937-3.976 ppm, instead of singlet the splitting is may be due to the improper hydrogen bonding existing in the THPC molecule. The chlorine bonded carbon signal is observed at 47.09 ppm in carbon NMR spectrum.

Donor-acceptor interaction analysis
The bonding and anti-bonding interactions can be significantly described by natural bond orbitals, which is expressed in terms of the second order perturbation energies (E (2) ) 32-35 . These interaction energies represent the estimation of NBO Fock matrix. The E (2) values of donor-acceptor bonds are given in Table 2 and it reveals, the various interactions between the donor/acceptor orbitals of THPC.

Nonlinear optical analysis
The first hyperpolarizabilities (β 0 , α 0 and Δα) of THPC is calculated using DFT level of theory. The first hyperpolarizability tensors can be described by Kleinman symmetry 36 . The calculated dipole moment (α), polarizability (μ) and first hyperpolarizability (β0) of THPC is, The first hyperpolarizability β tot of THPC calculated as 2.75×10 -30 esu. The dominated longitudinal compounds β XXX , β YYY , β XXZ and β YZZ shows large value of specific components indicates the significant charge delocalization in this direction.
The calculated polarizability 'α' has non-zero value and dominated by the diagonal components. The first hyperpolarizability components of THPC are presented in Table 3. The first hyperpolarizability of THPC has seven times higher than reference Urea. Hence THPC has good nonlinear response and it is a good candidate in optical studies.

Energy gap analysis
The energy gap of THPC explains the charge transfer interaction, which influences the biological activity of the molecule. The negative and positive phases are represented in blue and red colour, respectively. The energy from the HOMO represents the ionization potential and LOMO represents the electron affinity. The energy gap of an organic molecule is an important stability factor for chemical structures 37 . The three dimensional images of HOMO and LUMO orbitals are shown in Fig. 5. The frontier molecular orbitals of THPC are presented in Table 4.  The energies of HOMO = -5.9099 eV, LUMO = -0.4473 eV and the energy gap is calculated as 5.4617 eV. The electron density at HOMO level in localized over the tetrahydropyrimidone and methoxy phenyl ring. The electron density at LUMO level is localized over the phenyl ring and carboxyl group of the exocyclic ester chain. The electron transition from HOMO→LUMO implies the transfer of electron density from tetrahydropyrimidinone ring to aromatic ring of the title molecule.

Molecular Electrostatic Potential mapped surface
The MEP mapped surface and the contours of the THPC are shown in Fig 6. The red region in MEP refers the area that would favour lone pair interaction region predicting a site of hydrogen bonding donor. The blue region in MEP refers the area that would favour acceptor nature. In the title molecule, the red regions are observed at the oxygen atom of tetrahydropyromidinone carbonyl group and ester carbonyl group. The blue regions are observed over the two N−H of the tetrahydropyrimidinone ring. The green regions represent the zero potential of the total molecule. The contour map of title molecule, represent the two dimension image of the electron density in the title molecule. In closer curved line represent the fractional region of the molecule, which represents the electrostatic potentially active area. These regions are responsible for the biologically active molecular interactions.

CONCLUSION
Novel organic oxygen bridged tricyclic pyrimidinones molecule is synthesized using Biginelli condensation reaction. The structural features of title molecule are studied experimentally and theoretically. The relative energy of the stable conformer is identified as -1527.70 Hartree. The conformer exists an intramolecular hydrogen bonding between ester C=O•••H−C chlorine attached methylene with a bond distance of 2.28 Å. The stabilization energy of 1.56 Kcal/mol reveals the existing hydrogen bond is weak and its blue shift the C−H wavenumber to ~16 cm -1 . The predicted non-linear optical property (2.7495 x10 -30 esu) of the title compound is much higher than urea. The THPC is a good candidate for second-order NLO material.
The active charge sites give information about where the THPC can have intermolecular interactions and metallic bonding.