Structural , Spectroscopic ( FTIR , Raman and NMR ) , Non-linear Optical ( NLO ) , HOMO-LUMO and Theoretical ( DFT / CAM-B 3 LYP ) Analyses of N-Benzyloxycarbonyloxy-5-Norbornene-2 , 3-Dicarboximide Molecule

The experimental spectroscopic investigation of N-benzyloxycarbonyloxy-5-norbornene-2,3-dicarboximide (C 17 H 15 NO 5 ) molecule has been done using 1 H and 13 C NMR chemical shifts, FT-IR and Raman spectroscopies. Conformational forms have been determined depending on orientation of N-benzyloxycarbonyloxy and 5-norbornene-2,3-dicarboximide (NDI) groups of the title compound. The structural geometric optimizations, vibrational wavenumbers, NMR chemical shifts (in vacuum and chloroform) and HOMO-LUMO analyses for all conformers of the title molecule have been done with DFT/CAM-B3LYP method at the 6-311++G(d,p) basis set. Additionally, based on the calculated HOMO and LUMO energy values, some molecular properties such as ionization potential (I), electron affinity (A), electronegativity (χ), chemical hardness ( h ), chemical softness ( z ), chemical potential (μ) and electrophilicity index ( w ) parameters are determined for all conformers. The non-linear optical (NLO) properties have been studied for the title molecule. We can say that the experimental spectral data are in accordance with calculated values.


Introduction
Recently, norbornene-dicarboximides based groups have attracted much research interest.Especially, polynorbornene structures have perfect dielectric features, mechanical strength and exhibit membrane properties and can be used to generate composite materials [1][2][3][4][5][6].Therefore, norbornenes are very attractive momomers for macromolecular design of poymers with desired properties [7].Besides, some norbornene-dicarboximides show binding affinity and selectivity towards serotonin receptors [8,9].The synthesis and some chemical properties of Nb e n z y l o x y c a r b o n y l o x y -5 -n o r b o r n e n e -2,3dicarboximide can be found in literature [10].
In this work, we aimed to exhibit the molecular properties such as vibrational frequencies (FT-IR and Raman), 1 H and 13  Detailed structural, spectroscopic, electronic and non-linear optical analyses for the title molecule are not available in the literature.It is well-known that the quantum chemical computations provide a powerful support for experimental studies.Many researchers apply quantum mechanical computational methods to investigate structural, spectroscopic, electronic, magnetic, thermodynamic and optical properties of the molecular systems [11][12][13][14][15][16].Recently, these methods have been widely used in order to determine the structural parameters such as vibrational frequencies, magnetic properties (NMR chemical shifts and NMR spin-spin couplings), electronic parameters (UV-Vis.spectral parameters, HOMO, LUMO, NBO, MEP, …), thermodynamic properties (enthalpy, entropy, heat capacity, thermal energy, zero-point energy, Gibbs free energy, Helmholtz free energy, …) and non-linear optical properties of the molecular systems theoretically.Our study differs from the literature in terms of computational methods.The quantum chemical investigations of the compound have been performed by DFT/CAM-B3LYP method with the 6-311++G(d,p) basis set, for the first time.

Computational Methods
The molecular geometric optimizations, molecular conformational analysis, vibrational frequencies, 1 H and 13 C NMR chemical shifts (in vacuum and chloroform), HOMO and LUMO analyses (in vacuum and ethanol) and NLO properties of the title compound have been performed using the Gaussian 09W program package [17].The calculated results have been visualized using GaussView5 program [18].
The measured [30] and calculated vibrational wavenumbers, IR intensities, Raman scattering activities and vibrational band assignments for the most stable molecular conformation of the studied molecule can be found in Table 2.The experimental [30] and simulated (for conformer 1) IR spectra of the title compound are given in Figure 2. Similarly, the experimental [30] and simulated (for conformer 1) Raman spectra of the title molecule are given in Figure 3.The linear correlation coefficients (R 2 ) between the experimental and calculated wavenumbers have been found as 0.99975 (νcal.= 0.99955νexp.+ 2.21849) (for IR wavenumbers) and 0.99961 (νcal.= 0.99745νexp.+ 2.05360) (for Raman wavenumbers).
The CH stretching vibrations in aromatic compounds and CH bonds with sp 2 hybrid occur absorption bands in the region 3000-3100 cm -1 , whereas the CH bonds with sp 3 hybrid (such as methyl, methylene,…) cause to stretching modes at the interval 2800-3000 cm -1 [31][32][33].The observed band at 3132 (IR)-3138 (R) cm -1 is corresponded to =CH stretching mode in NDI group of the compound.The calculated wavenumbers for this band have been found at 3107 and 3084 cm -1 .Similarly, the CH stretching bands of phenyl ring are emerged at 3061 (IR), 3068 (R) and 3089 (IR) cm -1 , while the computed wavenumber values for the aromatic CH stretching vibrations are found at 3057, 3060, 3068, 3078 and 3087 cm -1 .The stretching band for the CH groups in the NDI group is recorded at 3030 cm -1 and they are computed at 3029, 3026, 3002 and 2995 cm -1 .Similarly, the symmetric stretching vibrations of methylene groups in the title molecule are found at 2965 (IR)-2969 (R) (exp.)/2965(cal.)cm -1 and 2982 (IR)-2985 (R) (exp.)/2971(cal.)cm -1 , while the asymmetric stretching modes are found at 3023 (R) (exp.)/3020(cal.)cm -1 and 3023 (cal.)cm -1 .Additionally, the C-H in-plane and out-of-plane bending vibrations cause to absorption bands in the region 1000-1600 cm -1 and 650-1000 cm -1 , respectively [31][32][33].The experimental and computed CH in-plane and out-ofplane bending modes in the phenyl and NDI groups of the title molecule are summarized in Table 2.
The O=C stretching vibration, which is highly characteristic, gives rise to strong absorption bands in the region 1540-1870 cm -1 .Also, position of this band depends on the physical state, electronic and mass effects of neighboring substituents, intra-and inter-molecular hydrogen bond interactions, conjugations and size of ring [31][32][33].In our study, the position of stretching modes of the O=C groups in five member ring are shifted towards the high frequency region due to decrease in size of ring.The O=C stretching vibrations in the carbonate ester (R1-O(C=O)O-R2) and NDI groups of the title molecule are observed at 1738 (IR), 1780 (IR)-1783 (R) and 1807 (IR/R) and 1738 (IR) cm -1 and the computed wavenumbers corresponding to them are obtained at 1769, 1791 and 1827 cm -1 , respectively.Generally, the O-C stretching mode gives rise to strong absorption bands in the region 1000-1300 cm -1 in acids, alcohols, ether and esters groups [31][32][33][34].In this connection, the O-C stretching vibrations is found at 1265 (IR)-1268 (R) (exp.)/1274(cal.), 1118 (cal.), 1019 (cal.) and 987 (cal.)cm -1 as mixed with other vibrational bands.The NO stretching mode between O17 atom in carbonate ester group and N16 atom in NDI one is observed at 1376 (IR)-1377 (R) cm -1 , while this band is computed at 1391 cm -1 .The out-ofplane bending (γO17O19O20C18 and γO17C14C15N16) vibrations in the carbonate ester group and N-Figure 2. The measured (top) [30] and computed (bottom) (for conformer 1) IR spectra of N-benzyloxycarbonyloxy-5norbornene-2,3-dicarboximide.hydroxysuccinimide one of the title compound are observed at 786 (IR)-789 (R) cm -1 and 368 (R) cm -1 and the computed values for these modes are found at 776 cm -1 and 386 cm -1 , respectively.

NMR chemical shift analyses
Nuclear magnetic resonance (NMR) spectroscopy is a powerful technique that can be used to determine the isotropic chemical shifts of atom nucleus in different Figure 3.The measured (top) [30] and computed (bottom) (for conformer 1) Raman spectra of N-benzyloxycarbonyloxy-5norbornene-2,3-dicarboximide.chemical environment, the content and purity of a compound, stable conformational form of molecule and functional groups in molecules [37][38][39][40].The experimental 1 H and 13 C NMR isotropic chemical shift values recorded in chloroform-d of the title compound are listed in Table 3 [30].Additionally, the computed 1 H and 13 C NMR isotropic chemical shifts for the title molecule have been obtained by GIAO method with the CAM-B3LYP/6-311++G(d,p) level in vacuum and chloroform solvent using IEFPCM solvent model.The calculated NMR isotropic chemical shifts are listed in Table 3.    [30] and calculated (vacuum/chloroform) 13 C and 1 H NMR isotropic chemical shifts (with recpect to TMS, all values in ppm) for conformer 1 form of N-benzyloxycarbonyloxy-5-norbornene-  3.

HOMO and LUMO analyses
The main orbitals taking part in chemical reaction is frontier molecule orbitals (FMOs) called as the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) [41].
The HOMO is directly correlated with the ionization potential of the compound.Behaving as an electron donor, it can be considered as valance band of the system.The LUMO is directly correlated with the electron affinity.It can be thought as conductivity band of the system due to behaving as an electron acceptor.The HOMO-LUMO energy band gap that is an important parameter for investigation of molecular electrical transport properties can use to determine molecular chemical stability.The molecules can be defined as "soft molecules" or "hard molecules" based on the HOMO-LUMO energy band gap that is the small or large, respectively.Therefore, the molecules that are belonged to the low HOMO-LUMO energy gap are more reactive [42].
Additionally, the molecular quantum mechanical parameters such as ionization potential, electron affinity, chemical reactivity, kinetic stability, polarizability, chemical hardness and softness, chemical potential, aromaticity, electronegativity and electrophilicity index can be found using HOMO-LUMO energy band gap [43].The computed HOMO and LUMO energies and computed molecular quantum mechanical descriptors with the CAM-B3LYP/6-311++G(d,p) level are listed in Table 4 for the title molecule.The HOMO is mainly localized over whole molecule, whereas the LUMO is mostly placed on Nbenzyloxycarbonyloxy group.Likewise, the HOMO-1 and LUMO+1 are centered on phenyl and 5norbornene-2,3-dicarboximide, respectively.

NLO properties
The organic, inorganic and organometallic non-linear optical (NLO) materials have been effectively used due to their future potential applications in the fields of physics, chemistry and engineering.Especially, the effective NLO materials with high performance have been very interesting in the optoelectronic and microelectronics fields such as optical telecommunications, signal processing, optical interconnections, optical computing, optical information processing, sensor protection, optical switching, dynamic image processing and various other photonic technologies [44].The mean polarizability (αtotal), the anisotropy of polarizability (∆α) and first hyperpolarizability (β0) values are important key factors to determine NLO properties of molecular systems.The polarizabilities, static first hyperpolarizabilities and dipole moments of the title molecule are computed with the CAM-B3LYP/6-311++G(d,p) level using the finite-field approach.

Conclusions
The molecular structure, vibrational wavenumbers (FT-IR and Raman), proton and carbon-13 NMR isotropic chemical shifts, HOMO-LUMO analyses and NLO properties of N-benzyloxycarbonyloxy-5norbornene-2,3-dicarboximide have been studied using experimental and computational (DFT/CAM-B3LYP/6-311++G(d,p) level) methods, for the first time.According to conformational analysis results, the most stable structural form is conformer 1 with 118.8°, 86.2° and 180.0° values of τ0 (C3-C2-C1-C14), τ1 (C14-N16-O17-C18) and τ2 (N16-O17-C18-O20) torsional angles and with energy values of -1087.45869209Hartrees, respectively.The HOMO and LUMO energy analyses of the title molecule have been theoretically studied.The quantum molecular descriptors have been determined depending on HOMO and LUMO energy values.We can declare that the molecule may be a good non-linear optical material with regard to the NLO results of the title compound.
C NMR chemical shifts, molecular geometric structural parameters, HOMO-LUMO analyses and quantum molecular descriptors that are depended on HOMO and LUMO energy values of Nb e n z y l o x y c a r b o n y l o x y -5 -n o r b o r n e n e -2,3dicarboximide molecule.
The carbon-13 NMR chemical shifts are observed at values varying from 169.51 ppm to 44.70 ppm, while they are computed in the region 42.67-174.18ppm in vacuum and 42.67-177.01ppm in chloroform for the title molecule.The electronegativity, conjugation and inductive effects can cause an increase in carbon-13 NMR chemical shifts due to polarization of the electron distribution of the molecule.In this connection, the C14 and C15 carbon atoms that are bonded electronegative N and O atoms are given rise to resonance signal at 169.51 ppm, while they computed at 174.18/177.01 and 174.78/177.01ppm (in vacuum/chloroform), respectively.Similarly, the C18 atom bonded electronegative oxygen atom is recorded at 151.36 ppm and computed at 156.92 ppm in vacuum and 156.99 ppm in chloroform.The obtained chemical shifts for these carbon atoms are higher than those of other carbon in the title molecule.It is well-known that the carbons with sp 2 hybrid and aromatic carbon atoms occur signals in the region 100-150 ppm[37][38][39][40].As expected, the C5, C7, C24, C25, C26, C27, C29 and C31 carbon atoms are given signals in the region 128.46-134.67 ppm, the computed values for these carbon atoms are between 132.34 -144.51 ppm in vacuum and 132.62 -144.70

4 )
The calculated dipole moment values for the title molecule have been found as -0.5912Debye for μx, 0.5116 Debye for μy, 1.3407 Debye for μz and 1.5520 Debye for μtotal.

Table 1 .
The

Table 4 .
The