Synthesis, spectroscopic investigation and computational studies of 2-formyl-4-(phenyldiazenyl)phenyl methyl carbonate and 4-((4-chlorophenyl)diazenyl)-2-formylphenyl methyl carbonate

. Two new compounds namely 2-formyl-4-(phenyldiazenyl)phenyl methyl carbonate (FPMC) and 4-((4-chlorophenyl) diazenyl)-2-formylphenyl methyl carbonate (CFPMC) have been synthesized and have characterized using FT-IR, FT-Raman, 1 H and 13 C NMR techniques. Computational optimization studies have been carried out using Hatree–Fock (HF) and Density Functional Theory (DFT–B3LYP) methods with 6–31+G(d, p) basis set of Gaussian 09W software. The stable configuration of the title compounds were achieved theoretically by potential energy surface scan analysis. The complete vibrational assignments were performed on the basis of total energy distribution (TED) and natural bonding orbital (NBO) have been studied. Various parameters such as E HOMO , E LUMO , total energy, dipole moment, polarizability, first order hyperpolarizability, zero–point vibrational energy as well as thermal properties were analyzed and reported for the title compounds.


INTRODUCTION
Azobenzenes and organic carbonate derivatives play a vital role in our modern life along with rapidly increasing applications in e.g. plastic materials, fuel additives and scientific laboratories [1−7] (e.g., solvents and synthetic blocks). Besides, organic carbonates are versatile compounds used as solvents or reagents in the chemical industry and as electrolytes in lithium batteries and fuel additives [8]. They are also frequently encountered as endower molecules in pharmaceuticals and agrochemicals. They are important precursors in biological/medicinal fields [9] and are useful synthetic intermediates [10].
Synthesis of linear carbonates mainly makes use phosgene and its derivatives [11][12][13][14] as the starting material, while cyclic carbonates are synthesized from propylene oxide. As an alternative approach, substituted chloroformates [15][16][17] are the most frequently used reagents for the preparation of organic carbonates. This reagent has gained much attention recently, because of easy handling, non-toxic nature as well as good reactivity which have led to a successful synthesis of azo organic carbonates. Literature survey reveals that DFT calculations and experimental studies on FPMC and CFPMC compounds have not been reported. Therefore, the present work deals with FT-IR, FT-Raman and NMR spectroscopic investigations of FPMC and CFPMC, utilizing HF and B3LYP methods with 6-31+G(d, p) as basis set. NBO analysis by B3LYP method revealed clear evidences of stabilization which originate from the hyper conjugation of various intramolecular interactions. The HOMO and LUMO analyses have been used to elucidate the information regarding charge transfer within the molecule.

Synthesis of FPMC and CFPMC
In the first step, 2-hydroxy-5-(phenyldiazenyl)benzaldehyde (HPDB) and 5-((4chlorophenyl)diazenyl)-2-hydroxybenzaldehyde (CPDB) have been synthesized according to the procedure as mentioned in literature [18] and the second step reaction, the reaction between HPDB and methyl chloroformate were carried out in different solvents in order to study about the feasibility of the reaction. Optimization of reaction conditions is shown in Table 1 and DCM/K 2 CO 3 yielded the product quantitatively (Entry 4). A typical experimental procedure for the synthesis of the title compounds is as follows: the stirring solution containing HPDB (0.59 g) and dichloromethane (5 mL), potassium carbonate (0.51 g, 3.75 mmol), after 15 min stirring methyl chloroformate (0.3 mL, 3.75 mmol) was added drop wise to the reaction mixture for a period of 15 min. It was stirred at an ambient temperature for 6 hours and the progress of the reaction was monitored by thin layer chromatography. Upon completion of reaction, the reaction mixture was diluted with water (20 mL) and extracted with dichloromethane (2×20 mL). The combined organic layer was washed with water (2×20 mL) and brine solution (20 mL) and dried over anhydrous sodium sulphate (2.5 g), filtered and concentrated under reduced pressure. The crude product was then recrystallised from ethanol to get the pure FPMC [19]. A similar procedure was adopted for the synthesis of CFPMC. The synthetic routes of azo carbonates are outlined in Scheme 1.

Spectral measurements
The melting points were measured in open capillary and are uncorrected. FT-IR spectra were recorded on a Thermo Nicolet iS5 FT-IR spectrometer in the range of 500 to 4000 cm -1 by ATR at Annamalai University, Annamalai Nagar, India. FT-IR spectra of these compounds were recorded at room temperature with number of scans equals 16. FT-Raman spectra were recorded on a Bruker: RFS 27 spectrometer. The spectral measurements were carried out at SAIF, Indian Institute of Technology (IIT), Chennai (Tamil Nadu, India). The spectral features are reported in wave number (cm -1 ). 1 H NMR spectra were recorded on a Bruker 400 MHz spectrometer and 13 C NMR spectra were recorded on a Bruker 100 MHz spectrometer at Annamalai University, From Fig. 1, the FPMC has attained minimum energy (-989.23 a.u) at 0.46 º and 360.46 º and the CFPMC has attained minimum energy (-1448.82 a.u) at -0.61 º and 359.61 º in the rotation. From the result of potential energy surface scan analysis, the CFPMC showed minimum energy which has more stability than that of FPMC besides the optimized geometrical structures of FPMC and CFPMC are shown in Figs. 2 and 9.

Molecular geometry
The molecular structural parameters like bond lengths, bond angles and dihedral angles for compounds FPMC and CFPMC have been calculated using HF and B3LYP methods with /6-

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ILCPA Volume 66 31+G(d, p). The calculated structural parameters have been compared with the crystal structure of azo compounds available in the literature [18,24]. The calculated structural parameters of FPMC and CFPMC are depicted in Tables 8 and 9, respectively. It is observed from the Figs. 2 and 9 that the basic azo dye unit in FPMC and CFPMC are planar and aromatic in nature due to the continuous delocalization of electrons in the benzene ring system. However, the computed C-H and C-C bond lengths of benzene ring were found to be in good agreement within 0.01-0.02 Å compared to the corresponding literature values [24]. ) are ascribable due to substituent effect, electronegativity and lone pair of electrons present in the molecule. Besides, from the above results compared to the C-C bond lengths of benzene (1) ring was found to be significantly small within 0.01-0.02 Å for CFPMC than that of FPMC due to chlorine atom in C1 carbon of CFPMC and the C-C bond lengths of with carbonate attached benzene ring is increases in both compounds (C16-C19/C15-C18) due to carbonate group attachment. The bond length values of C4-N12/C4-N11 (1.42/1.42/1.43 Å), N13-C14/N12-C13 (1.42/1.42/1.43 Å) are found to coincide with related compounds [18].

Vibrational analysis
The compounds FPMC and CFPMC have 33 atoms which belong to C1 symmetry and possess 93 normal modes of vibrations. All the 93 modes of vibrations are distributed as 34 stretching, 29 in-planes, 25 torsion and 5 out-of-plane vibrations in the infrared spectra of FPMC and CFPMC. The experimental and calculated scaled frequencies as well as the TED of FPMC are listed in Table 10 (CFPMC -Table 11). The calculated frequencies of the investigated molecules are found to be very close to the corresponding literature values. The mean percentage deviations of the calculated frequencies from the experimental ones are about 1.0-2.0 %. The experimental FT-IR and FT-Raman spectra with the corresponding theoretically simulated ones for FPMC are shown in Figs.3 and 4 (CFPMC -Figs. 10 and 11).
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C=O vibrations of FPMC and CFPMC
The C=O stretching vibrational band can be easily identified from the FT-IR and FT-Raman spectra, because of the degree of conjugation, strength and polarizations. The carbonyl stretching vibrations are expected in the region between 1715 -1680 cm -1 [18,24,25]. In our case, two C=O stretching bands were observed in both experimental FT-IR and FT-Raman spectra. In FT-IR spectra, C=O stretching bands were observed at 1768/1767 cm -1 for carbonate and at 1689/1695 cm -1 for aldehyde. In FT-Raman spectra, C=O stretching bands were observed at 1768/1768 cm -1 for carbonate and at 1688/1693 cm -1 for aldehyde of FPMC and CFPMC, respectively. From the B3LYP/6-31+G(d, p) calculation, two C=O stretching bands were observed, one at 1789/1783 cm -1 (mode no. 81/82) for aldehyde and another at 1714/1707 cm -1 for carbonate (mode no. 80/81) of FPMC and CFPMC, respectively which are correlated with the experimental FT-IR and FT-Raman values. These wave number shifts from the expected range is due to the substitution of carbonate in the para position of azo group.

C-C and C=C vibrations of FPMC and CFPMC
The six ring carbon atoms undergo coupled vibrations, known as semicircular stretching which usually occurs in the region 1400-1625 cm -1 [26]. The actual positions of these modes are determined not so much by the nature of the substituent but by the form of substitution around the ring [27]. In the experimental FT-IR spectra, C-C vibration bands are observed with variable intensities at 1607, 1595, 1578 and 1315 cm -1 for FPMC whereas for CFPMC, notable spectrum intensities were observed at 1606, 1587 and 1280 cm -1 . FT-Raman bands were observed at 1652, 1597, 1564 and 1314 cm -1 for FPMC whereas in CFPMC, the bands appeared at around 1603, 1585 and 1303 cm -1 . The theoretically computed frequencies in B3LYP/6-31+G(d, p) method are predicted at 1596, 1588, 1573 and 1318 cm -1 (mode nos. 79, 78, 77 and 65) for FPMC similarly in CFPMC at 1589, 1575 and 1296 cm -1 (mode nos. 80, 79, and 66). Most of the theoretical modes of vibrations are close to the experimental frequencies of FPMC and CFPMC.
In experimental FT-IR spectra, the strong bands occur in the region 3438-3027 and 3521-3051 cm -1 for FPMC and CFPMC, respectively which have been assigned as C-H asymmetric stretching vibrations of aromatic benzene rings. The C-H in-plane bending harmonic vibrations occurred at around 1280, 1257, 1140, 1067 and 1024 cm -1 in FPMC spectrum whereas at 1455, 1234, 1095 and 1024 cm -1 in CFPMC spectrum. In experimental FT-Raman spectra, bands are observed at 1259, 1233, 1140, 1062 and 1000 cm -1 in FPMC and at 1451, 1261, 1228 1094 cm -1 in CFPMC which are showed good agreement with medium and strong FT-IR and FT-Raman bands.

Methyl group vibrations
The asymmetric stretching vibrations of CH 3 are expected to appear at about 2980 cm -1 and symmetric stretching vibrations around 2870 cm -1 [24]. In computed FT-IR spectrum of FPMC, C-H stretching vibrations of CH 3 were observed at 3070, 3047 and 2964 cm -1 (mode nos. 85-83) and in the experimental FT-IR spectra of FPMC and CFPMC, at 3057, 3034 and 2951 cm -1 (mode nos. 86-84) and at 2962, 2924 and 2877 cm -1 respectively. Furthermore in experimental FT-Raman spectrum of FPMC, C-H stretching vibrations of CH 3 were noted at around 2965, 2880 and 2851 cm -1 and in CFPMC spectrum, the peaks appeared at around 3047, 3016 and 2959 cm -1 respectively. The C-H in-plane bending vibrations were observed at 1424, 1416 and 1416 cm -1 (mode no. 69), at 1418, 1416 and 1416 cm -1 (mode no. 71) in theoretical, experimental FT-IR and FT-Raman spectra, respectively for FPMC and CFPMC, respectively. All these assignments are found to be in good agreement with literature values.
In addition, the stretching frequency of N12-N13 are observed at 1499, 1479 and 1489 cm -1 (mode no: 75) in theoretical, experimental FT-IR and FT-Raman spectra of FPMC, respectively and at 1486, 1479 and 1486 cm -1 (mode no. 76) in theoretical, experimental FT-IR and FT-Raman spectra of CFPMC, respectively. The correlations between the experimental and theoretical wave numbers of compounds FPMC and CFPMC are good for ring vibrations.

Mulliken population analysis
Mulliken population analysis provides the atomic charge density or an orbital density on individual atom. The charge distributions over the atoms suggest the formation of donor and acceptor pairs involving the charge transfer in the molecule [28,29].

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The net Mulliken atomic charge distribution comparison of FPMC and CFPMC are shown in Fig. 5. The Mulliken atomic charges of CFPMC for C1, C3, C5, C18, H7, H8, H9 and H10 are found to be increased while the C2, C4, C13, C14, C15 and C16 charges decreased as compared to FPMC due to chlorine atom attached in C1. In compounds FPMC and CFPMC, the magnitude of carbon atom attached at nitrogen atoms have attained highest negative charge and behaved as electron acceptors (C4), and the large accumulation of positive charge were found to be on carbon atoms (C16/C15 and C17/C16) than the other atoms. The Mulliken atomic charge values are listed in Table 2.

NBO analysis
NBO analysis was performed using the Gaussian 09W package at the B3LYP/6-31+G(d, p) level. This analysis was carried out in order to understand the various second-order interactions between the filled orbitals of one subsystem and vacant orbitals of another subsystem, which is a measure of the intermolecular delocalization or hyper conjugation. The second order Fock matrix was carried out to evaluate the donor-acceptor interactions in the NBO basis [30]. For each donor (i) and acceptor (j), the stabilization energy E (2) associated with the delocalization is estimated as: Where q i is the donor orbital occupancy,  i and  j are diagonal elements and F(i,j) is the off diagonal NBO Fock matrix element. The NBO analysis is proved to be an effective tool for the chemical interpretation of hyper conjugative interaction and electron density transfer from the filled lone pair electrons [31]. A larger E (2) value indicates a more intense interaction between electron donors and electron acceptors which leads to a greater extent of conjugation in the system. The intramolecular hyperconjugative interactions are formed by the orbital overlap between π(C-C) and π*(C-C) bond orbitals which results intramolecular charge transfer (ICT) causing stabilization of the system. These interactions are observed as an increase in electron density (ED) in C-C antibonding orbital that weakens the respective bonds.
In FPMC, the strong intra-molecular hyperconjugative interaction of π electrons from C1-C6, C2-C3 and C4-C5 bonds to the π*(C1-C6) bond of the benzene (1) ring which increases the ED (0.319 eV) leading to stabilization of 20.43 kcal/mol. Furthermore the π-electron delocalization is maximum around the C14-C15, C16-C18 and C17-C21 of the benzene (2) ring which increases the ED (0.414 eV) leading to stabilization of 24.14 kcal/mol. This enhanced π*(C17-C21) NBO further conjugates with π*(C16-C19) resulting to a stabilization of 241.09 kcal/mol. Similarly in CFPMC, the stabilization energy of benzene (1) ring is 20.79 and 24.11 kcal/mol for benzene (2) ring which increases the ED (0.2776 eV) leading to stabilization of 241.41 kcal/mol. From the above data, the stabilization energy of benzene (1) ring is slightly increases (0.36 kcal/mol) and ED values of benzene (2) ring decreases due to chloride substitution in para position of azo group in benzene (1) ring of CFPMC.
In FPMC, the interaction energy between the filled bond orbital π(C-C) and the empty antibond orbital π*(N-N) of benzene (1) and benzene (2) rings are 21.03 and 19.52 kcal/mol, respectively, as shown in Table 3. Besides these kinds of electron transaction occurs in CFPMC which are 20.77 and 20.03 kcal/mol for benzene (1) and benzene (2) rings respectively as shown in Table 12. From the above comparison conforms CFPMC having strong delocalization than FPMC. Furthermore the NBO studies of both compounds confirm the lone pair electrons of the azo group is involved in delocalization since lone pairs lie in the molecular plane which is orthogonal to the π system. The azo group being co-planar with the benzene (1) ring increases the conjugation between them which supports electron delocalization from benzene (1) ring to the azo group which is revealed by the high value of the interaction of π*→π* energy which is equal to 133.63 kcal/mol for CFPMC but in FPMC π*→π* interaction is absent. These intra-molecular charge transfer (n→π*, π→π* and π*→π*) can induce large non linearity to the molecule.
The most important interaction energy in both compounds, related to the resonance in the molecule is electron donation from lone pair oxygen (LP(1 or 2) O23 and LP(1 or 2) O32) to the antibonding acceptor π*(C-C) of the benzene (2) ring. In FPMC, electron donation from LP(1) O23 to π*(C24-O25) is equivalent to 12.4 kcal/mol but there is no electron donation from LP(1) O23 to π*(C17-C21) which confirm electron delocalization or extension is not present in between the oxygen (O23) of the carbonate group and benzene (2) ring (C21). Beside LP(2) O32 to π*(C17-C31) does not occur therefore electron delocalization or extension is not forming in between the oxygen (O32) of the aldehyde group and benzene (2) ring (C17). Similarly the CFPMC also does not showing electron delocalization or extension in between LP(1) O22 and π*(C16-C20) or LP(2) O32 to π*(C16-C30). Therefore in both compounds delocalization is present in between the two International Letters of Chemistry, Physics and Astronomy Vol. 66 benzene rings via azo group and the delocalization or extension of benzene ring to carbonate or aldehyde group have been not showed. Table 3. Second order perturbation theory analysis of Fock matrix in NBO basis for FPMC.

HOMO-LUMO analysis
The HOMO and LUMO are important quantum chemical parameters to determine the molecular interaction with other species and to characterize the chemical reactivity, global hardness, softness and kinetic stability of the molecule etc. [32]. The energy gap between HOMO and LUMO orbitals of compounds FPMC and CFPMC are in decreasing order: CFPMC > FPMC and are given in Table 4.

NMR spectral analysis
The 1 H and 13 C chemical shift calculations [34] of FPMC have been carried out by HF and B3LYPs method with 6-31+G(d, p) basis set and compared with the experimental values and are presented in Fig. 7 and Table 5 (CFPMC - Fig. 13 and Table 13). In 1 H NMR, one signal observed in up field region at 3.99 ppm (s) with three protons integral corresponds to H28, H29 and H30 atoms, respectively. In the downfield region the signals at 10

Molecular electrostatic potential
To predict the reactive sites for electrophilic and nucleophilic attack for the compounds FPMC and CFPMC, the MEP have been calculated at B3LYP/6-31+G (d, p) method. Each molecule is encompassed by a characteristic three dimensional surface corresponding to contour of 0.002 a.u. isodensity surface. As seen in the Fig. 8.and Fig. 14., the potential region of the map is in between the range of -1.152 to 1.152 and -1.403 to 1.403 for FPMC and CFPMC, respectively. Furthermore, the reactive sites of compounds FPMC and CFPMC are in favor of electrophilic attack. The negative region is mainly localized on the oxygen atom of the carbonate (O25/O24).

NLO Properties
The first order hyperpolarizabilities (β 0 ) and related properties (µ, α total, α 0 ) of the compounds FPMC and CFPMC have been calculated using B3LYP method with 6-31+G(d, p) basis set. Molecules having higher values of dipole moment, molecular polarizability and first hyperpolarizability show more active NLO properties [36][37][38]. The total dipole moment, mean polarizability, anisotropy of polarizability and mean first order hyperpolarizability, using the x, y, z components are defined as The first hyperpolarizability (β) and the components of hyperpolarizability β x , β y and β z of FPMC and CFPMC along with related properties (μ D , α total and α 0 ) are listed in Table 6.
Besides, efficient second order NLO properties are usually related to compounds that present intramolecular charge transfer (ICT). These ICT processes are usually of one dimensional character, being associated to appreciable dipole moment changes, between the ground and the first excited state, currently identified as the "charge transfer state" [39,40] related to the electron transfer between these states. Typical compounds (FPMC and CFPMC) showing these types of properties are p conjugated molecules with a D-p-A structure, where D and A are respectively electron donor and acceptor groups and p is a para conjugated system. This push-pull characteristic generally lead to high <beta> value, which can be further, maximized through an adequate substitution of D and A groups. The parameters δ D and δ A are used to characterize the effectiveness of donor and acceptor groups. Here, δ D is the stands for the energy 50 ILCPA Volume 66

difference between the calculated HOMO (highest occupied molecular orbital) of the donorsubstituted HPDB (2-hydroxy-5-(phenyldiazenyl)benzaldehyde) as a model compound and that of HPDB and δ A is the stands for the energy difference between the calculated LUMO of the acceptorsubstituted HPDB as a model compound and that of HPDB ie δ A =E(LUMO) A -E(LUMO) P where E(LUMO) A the represents lowest unoccupied orbital energy of the A acceptor substituted HPDB.
The strength of donor-acceptor pairs is characterized by δ DA = δ D -δ A . From the literature, around the interval of δ DA is 1.2 to1.5 eV almost in the quinoid form having weak first-order (β) and second-order (γ) NLO susceptibilities [41]. From our calculated results, we find that the δ DA value of FPMC is 1.46 eV and CFPMC is 0.077 eV. The δ DA value of CFPMC is decreased than FPMC due to the presence of chlorine atom in the para position of CFPMC.

Thermodynamic function analysis
The total energy of a molecule is the sum of translational, rotational, vibrational and electronic energies. The statistical thermochemical analysis of FPMC and CFPMC were carried out at room temperature 298.15 K at 1 atm. pressure. The temperature dependence of thermodynamic properties of heat capacity at constant pressure (Cp), entropy (S) and zero-point vibrational energy for FPMC and CFPMC were also determined by HF and B3LYP levels with 6-31+G(d, p) basis set and the values of compounds FPMC and CFPMC are listed in Table 7. The FPMC and CFPMC having minimum total energy -989.23 and -1448.82 a.u, respectively by B3LYP compared with HF method. The scale factors [33] have been used for accurate determination of thermodynamic properties.

CONCLUSION
Novel compounds 2-formyl-4-(phenyldiazenyl)phenyl methyl carbonate (FPMC) and 4-((4chlorophenyl)diazenyl)-2-formylphenyl methyl carbonates (CFPMC) have been synthesized and characterized by spectroscopic methods. The harmonic vibrational frequencies and thermodynamic properties of FPMC and CFPMC were determined and analyzed by both HF and B3LYP methods with standard basis set 6-31+G(d, p). The complete vibrational band assignments have been made for FPMC and CFPMC using FT-IR and FT-Raman spectra. The optimized geometrical parameters and the vibrational frequencies of the fundamental modes of compounds FPMC and CFPMC were performed and the vibrational modes are assigned on the basis of TED. The scaled vibrational frequencies were found in good agreement with the experimental values. The NBO analysis and thermodynamic parameters confirmed the ability of the methodology applied for the interpretation of the vibrational spectra of the title compounds in the solid phase. From the dipole moment, polarizability and hyperpolarizability data indicate that the compounds FPMC and CFPMC possess less NLO behavior due to pull-push character of molecules. The difference in HOMO and LUMO energies of compounds FPMC and CFPMC support the charge transfer takes place within the molecule.

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Supplementary Information Figure 9. Optimized structure of CFPMC.