Vibrational spectra, NBO, HOMO–LUMO and conformational stability studies of 4-hydroxythiobenzamide

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Highlights

  • Synthesis and spectroscopic analysis of 4-hydroxythiobenzamide have been reported.

  • Conformational analysis on 4-hydroxythiobenzamide reveals two stable conformers.

  • The crystals are monoclinic, space group P21/n, with a = 13.508 Å, b = 6.780 Å, c = 15.878 Å and Z = 8.

  • Stability of the molecule arising from hyper conjugative interactions, charge delocalization has been analyzed.

  • The MEP and HOMO–LUMO energy gap were theoretically predicted.

Abstract

In this work, the experimental and theoretical study on molecular structure, vibrational spectral analysis of 4-hydroxythiobenzamide (HTB) have been reported. The solid phase FTIR (4000–400 cm−1) and FT-Raman spectra (3500–50 cm−1) were recorded. The molecular geometry, harmonic vibrational frequencies and bonding features of HTB in the ground-state have been calculated by the density functional method (B3LYP) with 6-311+G(d,p) and 6-311++G(d,p) as basis sets. Utilizing the observed FTIR and FT-Raman data, a complete vibrational assignment and analysis of the fundamental modes of the compound were carried out. Stability of the molecule arising from hyperconjugative interactions, charge delocalization has been analyzed using natural bond orbital (NBO) analysis. The results show that the value of electron density (ED) in the σ antibonding orbitals and E(2) energies confirms the occurrence of ICT (intra-molecular charge transfer) within the molecule. The UV spectrum was measured in ethanol solution. The energy and oscillator strength calculated by time-dependent density functional theory (TD-DFT) correlates with the experimental findings. The calculated molecular electrostatic potential (MESP), HOMO and LUMO energies show that charge transfer occurs within the molecule. Besides, the simulated infrared and Raman spectra of the title compound which show good agreement with observed spectra.

Introduction

Salicylates, including salicylic acid (4-hydroxythiobenzamide) are natural products of plant metabolism that possess therapeutic properties. Methyl salicylate is used in food flavorings and preservatives. The acetylsalicylic acid (aspirin) is the most widely used common antiseptic and antipyretic agent. Phenyl salicylate (salol) is also used as an antiseptic and antipyretic agent. In addition to its analgesic and antipyretic properties, 4-hydroxythiobenzamide (HTB) possesses keratinolytic properties and fungicidal properties. Its derivatives are used in the treatment of hyperkeratotic, dandruff, ichthyosis and psoriasis as well as in the treatment of fungal skin infections such as tinea [1], [2], [3], [4], [5], [6], [7], [8]. 4-Hydroxythiobenzamide is the most popular model systems for studying intramolecular hydrogen bonds [9], [10]. Thus, owing to the vast biological significance of 4-hydroxythiobenzamide in an enormous field, an extensive structural, spectroscopic and quantum chemical studies of the title compound have been undertaken. The present study has been aimed, to investigate the vibrational spectra of 4-hydroxythiobenzamide. Furthermore, the HOMO–LUMO energy gap, natural bond orbital (NBO) analysis, chemical hardness, chemical potential and delocalization activity of the electron clouds in the optimized molecular structure.

All these investigations have been done on the basis of the optimized geometry by using the density functional theory method (DFT/B3LYP) with 6-311+G(d,p) and 6-311++G(d,p) basis sets. Theoretical studies on bioactive compounds are of interest in order to gain a deeper insight on their action and thus helping in the design of new compounds with therapeutic effects. The knowledge of physico-chemical properties and sites of reaction of investigated compound will provide a deeper insight of its probable action. Particularly, molecular electrostatic potential (MESP) is related to the electronic density and is a very useful descriptor in understanding sites for electrophilic attack and nucleophilic reactions as well as hydrogen bonding interactions. For the title compound, the molecular electrostatic potential is calculated at B3LYP/6-311++G(d,p) level.

Section snippets

Experimental methods

All the chemicals obtained from Lancaster chemical Company, UK and were used without any further purification. Phenol is mixed with (1:1) ratio of ethoxycarbonyl isothiocyanate which is taken in a dry round bottom flash with traces of AlCl3 crystal for 12 h. This is stored in refrigerator for 24 h and the resultant precipitate is filtered, washed with ether and then the filtered compound is mixed with dil HCl and neutralized by Na2CO3 till white precipitate is formed. The resultant compound is

Molecular geometry

The optimized molecular structure and numbering scheme of HTB are shown in Fig. 1. The geometry of the compound under investigation is considered by possessing C1 point group symmetry. The calculated optimized geometrical parameters of HTB are presented in Table 3. Detailed description of vibrational modes can be given by means of normal coordinate analysis. The internal coordinates describe the position of the atoms in terms of distances, angles and dihedral angles with respect to an origin

Vibrational frequencies analysis

The molecular structure of HTB belongs to C1 point group symmetry. For C1 symmetry there would not be any relevant distribution. The molecule consists of 17 atoms and expected to have 45 normal modes of vibrations of the same A species. These modes are found to be active in both IR and Raman spectra. The experimental and theoretically calculated IR, Raman frequencies, IR intensities, Raman activities and force constants are presented in Table 7. The FTIR and FT-Raman spectra of the title

Frontier molecular orbitals

This electronic absorption corresponds to the transition from the ground to the first excited state and is mainly described by one electron excitation from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) [29], [30], [31]. The HOMO energy characterizes the ability of electron giving; LUMO characterizes the ability of electron accepting. The energy gap between HOMO and LUMO determines the kinetic stability, chemical reactivity and, optical

Global and local reactivity descriptors

Based on density functional descriptors, the global chemical reactivity descriptors of compound such as hardness, chemical potential, softness, electronegativity and electrophilicity index as well as local reactivity have been defined [32], [33], [34], [35], [36]. Pauling introduced the concept of electronegativity as the power of an atom in a compound to attract electrons to it. Hardness (η), chemical potential (μ) and electronegativity (χ) and softness are defined as,η=1/2(2E/N2)V(r)=1/2(μ/

NBO analysis

The NBO analysis is carried out by examining all possible interactions between ‘filled’ (donor) Lewis-type NBOs and ‘empty’ (acceptor) non-Lewis NBOs, and estimating their energetic importance by 2nd order perturbation theory. Since these interactions lead to loss of occupancy from the localized NBOs of the idealized Lewis structure into the empty non-Lewis orbitals, they are referred to as delocalization corrections to the zeroth-order natural Lewis structure. For each donor NBO (i) and

UV-Spectral analysis

Ultraviolet spectral analysis of HTB have been investigated by the TD-DFT/B3LYP/6-311++G(d,p) method in ethanol. The calculated visible absorption maxima (λmax) are a function of the electron availability have been reported in Table 10. The calculations on molecular orbital geometry show that the visible absorption maxima of this molecule correspond to the electron transition from HOMO to LUMO orbital. In HTB, the UV–Vis spectra absorption maxima values have been found to be 382.86, 329.77,

Analysis of molecular electrostatic potential (MESP)

The molecular electrostatic potential surface (MESP) is a method of mapping electrostatic potential onto the iso-electron density surface. It simultaneously displays electrostatic potential (electron + nuclei) distribution, molecular shape, size and dipole moments of the molecule and it provides a visual method to understand the relative polarity. Electrostatic potential maps illustrate the charge distributions of molecule three dimensionally. These maps allow us to visualize variably charged

Temperature dependence of thermodynamic properties

The temperature dependence on the thermodynamic properties such as heat capacity at constant pressure (Cp), entropy (S) and enthalpy change (ΔH0→T) for HTB were also determined by B3LYP/6-311++G(d,p) method and are listed in Table 11. The correlation of entropy (S), heat capacity at constant pressure (Cp) and enthalpy change (ΔH0→T) with temperature are shown in Fig. 15, Fig. 16, Fig. 17, respectively. From Table 11, one can find that the entropies, heat capacities, and enthalpy changes are

Conclusion

Density functional theory (B3LYP) calculations on the structure and vibrational spectra of 4-hydroxythiobenzamide have been investigated. The vibrational frequency analysis by B3LYP/6-311++G(d,p) method agrees satisfactorily with the experimental results. On the basis of agreement between the calculated and experimental results, assignments of all the fundamental vibrational modes of 4-hydroxythiobenzamide are examined and proposed. Therefore, the assignment made at higher level of the theory

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