Experimental ( FTIR , Raman and NMR ) and Theoretical ( B 3 LYP , B 3 PW 91 , M 06-2 X and CAM-B 3 LYP ) Analyses of P-Tert-Butylphenyl Salicylate

The spectroscopic investigations of p-tert-butylphenyl salicylate (C17H18O3) molecule were performed using C and H NMR chemical shifts, FT-IR and Raman spectroscopies. Molecular geometric optimizations, vibrational frequencies, Carbon-13 and Proton NMR chemical shifts (in vacuum and chloroform), HOMO-LUMO properties, natural bond orbital (NBO) analysis, nonlinear optical properties and thermodynamic parameters of p-tert-butylphenyl salicylate molecule was studied using B3LYP, B3PW91, M06-2X and CAM-B3LYP functionals in DFT method at the 6-311++G(d,p) basis set. NBO analysis was carried out to investigate the intramolecular hydrogen bonding (O-HO) in the title molecule. Some of the molecular properties such as ionization potential (I), electron affinity (A), chemical hardness (), chemical softness (), electronegativity (χ), chemical potential (μ) and electrophilicity index () parameters were determined via HOMO and LUMO energies of the title molecule. Also, quantum chemical computations were performed to determine the dipole moment (μtotal), mean polarizability (α), anisotropy of the polarizability (∆α) and first hyperpolarizability (β0) values. Thermochemical properties of the title molecule were investigated with the aforementioned calculation levels. The recorded experimental spectroscopic results were found to be in good agreement with the computed data.

In this study, vibrational frequencies (FT-IR and Raman), NMR chemical shifts, optimized molecular geometric parameters, HOMO-LUMO analyses, NBO analysis, NLO properties and thermodynamic parameters of p-tert-butylphenyl salicylate were investigated both experimentally and theoretically.No detailed work was found on structural, spectral, electronic, nonlinear optical properties and thermodynamic investigations of the title molecule in the literature.The theoretical computations were performed using B3LYP, B3PW91, M06-2X and CAM-B3LYP functionals in DFT method with the 6-311++G(d,p) basis set.As theoretical analyses provide a powerful support for experimental studies, quantum chemical computations have been used by many researchers to determine the structural, spectroscopic, magnetic, electronic, optical and thermodynamic properties of the molecular systems in the literature (Buyukuslu et al., 2010;Ceylan et al., 2016;Kavitha and Velraj, 2016;Öztürk and Gökce, 2017).

Computational Details
Molecular geometric optimizations, vibrational wavenumbers, Proton and Carbon NMR chemical shifts (in vacuum and chloroform), NBO analysis, NLO properties, HOMO-LUMO analyses and thermodynamic parameters of the title molecule were studied using Gaussian 09W software package (Frisch et al., 2009).The calculated results were visualized via GaussView5.0 program (Dennington et al., 2009).
We can see from computed vibrational wavenumbers given in Table 2 that imaginary frequency value is not found at B3LYP, B3PW91 and M06-2X/6-311++G(d,p) levels, whereas one imaginary frequency value (mode no:ν 1 = -3.2cm -1 ) is computed at CAM-B3LYP/6-311++G(d,p) level.This situation shows that the minimum energy state in the available conformation was not achieved for CAM-B3LYP/6-311++G (d,p) level.This molecular structure is an unstable conformational form for CAM-B3LYP/6-311++G(d,p) level and it can be a saddle point or transition state or nonminimum energy point over PES of the title compound.

NMR chemical shift analyses
The experimental (AIST, 2017) 1 H and 13 C NMR chemical shifts of the title molecule were measured in chloroform-d solvent.The 1 H and 13 C NMR chemical shifts of the title molecule were calculated at B3LYP, B3PW91, M06-2X and CAM-B3LYP/6-311++G(d,p) levels in vacuum and chloroform solvent with IEFPCM solvent model using GIAO method.The experimental and calculated NMR chemical shift values were listed in Table 3.The linear correlation coeffections (R 2 ) between the experimental and calculated proton and carbon-13 NMR chemical shifts were also given in Table 3. Methyl and methylene carbon atoms are shielded by their own hydrogen atoms.Therefore, 13 C NMR resonance signals of these groups occur within the region of 15-35 ppm (Silverstein and Webster, 1998;Anderson et al., 2004;Stuart, 2004;Wade, 2006).The signals recorded at 31.41 ppm and 34.55 ppm in the Carbon-13 NMR isotropic chemical shift spectrum of the title molecule indicated presence of methyl groups (C26, C27, C31 and C35).The computed values corresponding to these carbon atoms were given in Table 3.
Due to intramolecular hydrogen bonding, phenolic protons shift to the approximate range of δ 10-12 ppm (Silverstein and Webster, 1998).The NMR chemical shift value for the phenolic H15 proton is observed at 10.56 ppm, while the computed values for this proton are between 10.90 ppm and 11.34 ppm in all mentioned computational levels.Aromatic hydrogens are deshielded at a higher level than those attached to double bonds due to the large anisotropic field that is generated by the circulation of the π electrons in the ring and they are easily identified in the region of 6.5-9.0 ppm (Pavia et al., 2009).The aromatic protons (H7, H8, H9 and H10) were experimentally recorded at the interval of 6.96-8.07ppm, while they were theoretically computed at the regions of 6.97-8.44 ppm for B3LYP, 6.99-8.49ppm for B3PW91, 7.72-9.16ppm for M06-2X and 7.03-8.60ppm for CAM-B3LYP.The methyl hydrogen atoms, shielded at the highest level, have chemical shift signals in the region of 0.7-1.3ppm (Pavia et al., 2009).The methyl protons in the title molecule were experimentally and theoretically obtained at 1.34 ppm and at the interval of 1.09-1.68ppm with the mentioned computational levels, respectively.

Frontier molecular orbitals analyses
The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are the main orbitals taking part in a chemical reaction (Fukui, 1982).The HOMO is the outermost orbital filled by electrons.It is represented by the ionization potential of a molecule.It can be considered as a valance band since it behaves as an electron donor.The LUMO represents the first empty innermost orbital unfilled by electrons.It is directly related to the electron affinity.Behaving as an electron acceptor, it can be thought as the conductance band of the system.The HOMO-LUMO energy band gap is an indication of molecular chemical stability.It is a very important parameter for determination of molecular electrical properties.Furthermore, the quantum molecular descriptors such as ionization potential, electron affinity, chemical reactivity, kinetic stability, polarizability, chemical hardness and softness, aromaticity and electronegativity can be found using HOMO-LUMO energy band gap (Alpaslan et al., 2015).HOMO and LUMO energy values and HOMO-LUMO band gap values, computed with the B3LYP, B3PW91, M06-2X and CAM-B3LYP/6-311++G(d,p) levels, were summarized in Table 4. Additionally, the ionization potential, chemical potential, electron affinity, electronegativity chemical hardness, softness and electrophilicity index parameters, found using the computed HOMO and LUMO energy values, were summarized in Table 4 (Alpaslan et al., 2015).HOMO and LUMO surfaces were given in Figure 4.As seen from Figure 4, both HOMO and LUMO plots simulated with all four-computational level were localized on salicylate group of p-tertbutylphenyl salicylate molecule.

NBO analysis
NBO analysis is a powerful method to determine intra-molecular and inter-molecular bonding interactions, bond species, bond structures and hyperconjugation interactions in molecular systems.The stabilization energy, E(2), depends on the interaction between Lewis type (bonding or lone pair) filled orbitals and non-Lewis type (antibonding or Rydberg) vacancy orbitals.For each donor NBO (i) and acceptor NBO (j), the stabilization energy E(2) associated with the electron delocalization between stabilization energy E(2) associated with the electron delocalization between the donor and the acceptor is estimated as (Wienhold and Landis, 2005); Where q i is the donor orbital occupancy,  i and  j are diagonal elements (orbital energies), and F ij is the off-diagonal NBO Fock matrix element.Table 5 shows the results of second-order perturbation theory analysis of the Fock Matrix computed with B3LYP, B3PW91, M06-2X and CAM-B3LYP/6-311++G(d,p) levels for the intra-molecular O-H … O interaction in the title molecule.According to the computed NBO results, the n(O13)→σ*(O12-H15) charge transfer for the intramolecular hydrogen bonding in the title molecule belongs to the stabilization energy of 17.24 kcal mol -1 for B3LYP, 19.69 kcal mol -1 for B3PW91, 14.85 kcal mol -1 for M06-2X and 19.92 kcal mol -1 for CAM-B3LYP.2) is the energy of hyperconjugative interactions.b Energy difference between donor (i) and acceptor (j) NBO.c F(i,j) is the Fock matrix element between i and j NBO.

NLO analysis
Organic, inorganic and organometallic nonlinear optical (NLO) materials have drawn much interest in the fields of physics, chemistry and engineering, due to their potential for future applications in the fields of optoelectronics and microelectronics, 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 (Nalwa and Miyata, 1997).Mean polarizability (α total ), anisotropy of polarizability (∆α) and first hyperpolarizability (β 0 ) values are important factors to determine NLO properties of molecular systems.Therefore, recent syntheses and investigations of novel nonlinear optic materials with high performance have created an interesting study area.

Thermodynamic properties
The thermodynamic parameters were calculated using B3LYP, B3PW91 and CAM-B3LYP methods with the 6-311++G(d,p) basis set at the room temperature of 298.15 K, under 1 atm pressure and in vacuum for the title molecule.The computed thermodynamic parameters were given in Table 7.The partition function, indicating the statical properties of a system in thermodynamic equilibrium is very important for thermodynamic parameters.There are four types of partition functions, namely, translational, electronic, vibrational and rotational partition functions.Partition functions can be used to compute the thermodynamic variables (such as heat capacity, entropy, equilibrium constants, total energy, free energy, pressure, thermal energy and rate constants, etc.) of a system.As known, the total energy of any molecular system is the sum of electronic, vibrational, rotational and translation energies (or E = E e + E v + E r + E t ).The computed total energy values for the title molecule were obtained as -884.59282006Hartrees for B3LYP, -884.24252309Hartrees for B3PW91, -884.22227194Hartrees for M06-2X and -884.13686115Hartrees for CAM-B3LYP.The computed zero-point vibrational energy (ZPVE) values for all four levels were found as 195.37107, 195.84427, 197.44399 and 197.70185 kcal/mol, respectively.As seen from Table 7, the computed thermal energy (E), heat capacity (C v ) and entropy (S) values for B3LYP, B3PW9, M06-2X and CAM-B3LYP/6-311++G(d,p) levels are 206.790, 207.257, 208.763 and 207.407 cal/mol×K, 71.656, 71.566, 71.003 and 68.641 cal/mol×K and 141.872, 141.859, 140.605 and 133.286 cal/mol×K, respectively.The major contributions to these values were from vibrational energies, whereas the minor contributions resulted from electronic, transitional and rotational energies.The computed rotational temperatures (Kelvin), rotational constants (GHz) and other thermodynamic parameters for the title molecule were listed in Table 7. thermodynamic parameters were computed to understand the thermochemical properties (at the room temperature of 298.15 K, under 1 atm pressure and in vacuum) of the title compound.

Table 1 .
The optimized molecular geometric parameters of p-tert-butylphenyl salicylate.

Table 3 .
The experimental (AIST, 2017) and calculated 13 C and 1 H NMR isotropic chemical shifts (with respect to TMS, all values in ppm) of p-tert-butylphenyl salicylate.

Table 5 .
The calculated results with second order perturbation theory of Fock matrix in NBO of p-tertbutylphenyl salicylate.
a E(

Table 6 .
The computed dipole moments, polarizability and first hyperpolarizability values of p-tertbutylphenyl salicylate.