Introduction
P-tert-butylphenyl salicylate (TBS) is well-known commercial ultraviolet stabilizer and ultraviolet absorber (UVA). Ultraviolet absorbers (UVA) have been extensively studied due to the importance of protecting polymeric materials from photooxidation (Schmitt and Hirt, 1960; Gottfried and Dutzer, 1961; Newland and Tamblyn, 1964; Dobashi and Ohkatsu, 2008) . As a salicylic acid derivative, 4-tert-butylphenyl salicylate (4-TBPS) exhibits antiinflammatory activity. Choi et al. investigated the anti-inflammatory effects of 4-tert-butylphenyl salicylate (4-TBPS) (Choi et al., 2016) . Salicylic acid derivatives are used for medicinal and cosmetic purposes. They are used as antipyretics, vitamins, antioxidants, NSAIDs, food preservatives, bactericidal agents, antiseptics, etc. (Hutchinson, 2003; Madan and Levitt, 2014) . The molecular chemical formula and molecular weight of p-tertbutylphenyl salicylate are C 17 H 18 O 3 and 270.328 g/mol, respectively and its melting point is reported as 65-66 °C (Pubchem, 2017) . Kasumov and Köksal studied new bidentate N-(2,5-di-tertbutylphenyl)salicylaldimines that involve X=H, HO, CH 3 O, Br, NO 2 , 3,5-di-Br, 3-NO 2 -5-Br and 5,6-benzo substituents on the salicylaldehyde moiety, L x H, and their mononuclear bis [N-(2,5-ditertbutylphenyl) salicylaldiminato] copper (II) complexes, Cu(L x ) 2 using IR, 1 H NMR, ESR spectroscopy, magnetic measurements (Kasumov and Köksal, 2002) . Raja et. al., investigated the molecular vibrations of 1-Naphthol in polycrystalline sample, at room temperature, using FT-IR and FT-Raman spectroscopy (Raja et al., 2013) .
Introduction
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 ).
Computational Details
All quantum chemical computations were performed with B3LYP, B3PW91, M06-2X and CAM-B3LYP functionals in DFT method (Lee et al., 1988; Becke, 1993; Perdew et al., 1996; Yanai et al., 2004) at the 6-311++G(d,p) basis set in the ground state of the title molecule. The calculated harmonic vibrational wavenumbers were scaled with 0.9671 for B3LYP/6-311++G(d,p) level, 0.9576 for B3PW91/6-311++G(d,p) level, 0.9489 for M06-2X/6-311++G(d,p) level and 0.9665 for CAM-B3LYP/6-311++G(d,p) level (Cruz et al., 2015; Kanagathara et al., 2015; Chase et al., 2016) .
Computational Details
Using optimized molecular geometries, 1 H and 13 C NMR isotropic chemical shift were computed with aforementioned computational levels using integral equation formalism polarizable continuum model (IEFPCM), solvent model and gauge invariant atomic orbital (GIAO) approach (London, 1937; Ditchfield, 1974; Wolinski et al., 1990) . The HOMO and LUMO analyses were performed using aforementioned computational levels. According to HOMO and LUMO energy values, the quantum molecular descriptors such as ionization potential (I), electron affinity (A), chemical hardness (), chemical softness (), electronegativity (χ), chemical potential (μ) and electrophilicity index () parameters were computed. Nonlinear optical (NLO) properties such as dipole moment (µ), mean polarizability (α total ), anisotropy of the polarizability (∆α) and first hyperpolarizability (β 0 ) were computed with the mentioned computational levels. NBO analysis was performed to investigate the intramolecular hydrogen bonding interaction (O-H ... O) in the title compound molecule. Finally, thermodynamic properties (thermal energy (E), heat capacity (C v ), entropy (S), zero-point energy, etc.) were calculated at B3LYP, B3PW91, M06-2X and CAM-B3LYP/6-311++G(d,p) levels.
Results and Discussion

Molecular structure analysis
The optimized molecular structure of p-tertbutylphenyl salicylate was given in Figure 1 with numbering of the atoms. The optimized molecular geometric parameters including bond lengths and angles, as given in Table 1 , were calculated using B3LYP, B3PW91, M06-2X and CAM-B3LYP/6-311++G(d,p) theoretical methods. The calculated molecular geometric parameters were compared with the experimental and theoretical geometric parameters of similar structures in the literature (Bilgram et al., 1982; Hammond et al., 2002; Hanuza et al., 2004 (Hammond et al., 2002) reported the molecular structure and molecular geometric parameters of salol molecule in meta-stable phase using X-Ray powder diffraction analysis. C3-C11 and C16-O14 bond lengths were respectively found as 1.469 Å, 1.465 Å, 1.473 Å, 1.467 Å and 1.405 Å, 1.398 Å, 1.395 Å, 1.399 Å with the aforementioned computational methods. Experimental values of these bond lengths were reported as 1.464 Å and 1.411 Å, respectively (Bilgram et al., 1982) . The C-C bond lengths in ring 1 and ring 2 of the title compound were calculated at the interval of 1. (Nyquist et al., 1996; Kalampounias et al., 2003; Baran and Davydova, 2010) . The linear correlation coeffections (R 2 ) between experimental and computed vibrational wavenumbers were also given in Table 2 . The OH vibrations are very sensitive due to effect of hydrogen bonding and the OH stretching modes observe in the region of 2800-3250 cm -1 (Colthup et al., 1964; Bellamy, 1975 The aromatic CH stretching vibrational modes occur in the region of 3000-3100 cm -1 (Colthup et al., 1964; Bellamy, 1975; Silverstein and Webster, 1998; Stuart, 2004) . The bands recorded at 3023, 3068 and 3073 cm -1 in IR and Raman spectra of the title compound can be attributed to the aromatic CH stretching modes. These modes were computed at the interval of (585)586-9853 cm -1 for B3LYP, (585)586-9853 cm -1 for B3PW91, (585)586-9853 cm -1 for M06-2X and (585)586-9853 cm -1 for CAM-B3LYP. Symmetric and asymmetric stretching modes for methyl and methylene groups were observed between 2800 cm -1 and 3000 cm -1 (Colthup et al., 1964; Bellamy, 1975; Silverstein and Webster, 1998; Stuart, 2004) . As seen in Table  2 , CH 3 symmetric stretching bands were appeared at 2869 and 2900 cm -1 in IR spectrum and at 2872, 2905 and 2932 cm -1 in Raman spectrum and the corresponding calculated scaled frequencies were computed in the regions of 2921. 
NMR chemical shift analyses
The experimental (AIST, 2017) 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 . (Anderson et al., 2004; Wade, 2006; Pavia et al., 2009 (Silverstein and Webster, 1998; Anderson et al., 2004; Stuart, 2004; Wade, 2006) . The signals boonenathan@example.com 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 .
NMR chemical shift analyses
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 boonenathan@example.com 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 6.96-8.07 ppm, while they were theoretically computed at the regions of 6.97-8.44 ppm for B3LYP, 6.99-8.49 ppm for B3PW91, 7.72-9.16 ppm for M06-2X and 7.03-8.60 ppm for CAM-B3LYP. The methyl hydrogen atoms, shielded at the highest level, have chemical shift signals in the region of 0.7-1.3 ppm (Pavia et al., 2009 ). The methyl protons in the title molecule were experimentally and theoretically boonenathan@example.com ppm and at the interval of 1.09-1.68 ppm 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 . 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 . 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. Figure 4 . The HOMO and LUMO plots of p-tertbutylphenyl salicylate (1= B3LYP, 2= B3PW91, 3= M06-2X and 4= CAM-B3LYP).
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) ;
NBO analysis
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. 
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.
NLO analysis
Dipole moments, polarizabilities and the first hyperpolarizabilities of the title molecule were computed with B3LYP, B3PW91, M06-2X and CAM-B3LYP functionals at the 6-311++G(d,p) basis set by using the finite-field approach and they were summarized in Table 6 . Total dipole moment (µ total ), mean polarizability (α total ), anisotropy of the polarizability (∆α) and the first hyperpolarizability (β 0 ) magnitudes were computed using the following equations (Temel et al., 2015) ; 
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 -884.59282006 Hartrees for B3LYP, -884.24252309 Hartrees for B3PW91, -884.22227194 Hartrees for M06-2X and -884.13686115 Hartrees 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 . 
Conclusions
The molecular geometric, spectroscopic (FT-IR, Raman and NMR), HOMO-LUMO, NBO, NLO and thermodynamic analyses for p-tert-butylphenyl salicylate molecule were investigated using experimental and theoretical methods. In order to support the experimental spectroscopic data, theoretical computations were performed using B3LYP, B3PW91, M06-2X and CAM-B3LYP methods with the 6-311++G(d,p) basis set. The computed total molecular energy was obtained in the following order: B3LYP<B3PW91<M06-2X<CAM-B3LYP. As seen from the tables and figures, the computed structural, spectroscopic, electronic, nonlinear optical and thermodynamic properties varied depending on the calculation levels. In the present study, due to O-H … O intramolecular hydrogen bonding interaction in title compound, O-H vibrational frequencies, 1 H NMR isotropic chemical shifts for mentioned proton, and NBO analysis of n(O)→σ*(O-H) interaction were investigated using experimental and theoretical methods. The calculated α total , ∆α and β 0 values show that p-tert-butylphenyl salicylate molecule may be a good nonlinear optical material. The 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.

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