Mustafa Şenyel a, Cemal Parlak b and Esma Güneş a a Department of Physics, Science Faculty, Anadolu University, Eskişehir, 26470, Turkey b Department of Physics, Dumlupinar University, Kütahya, 43100, Turkey INDEX Abstract Why thiophene-2-carbaldehyde and its halogeno derivatives ? Calculations • Conformational analysis • Geometrical structures • Vibrational studies • Electronic properties Results References 2 ABSTRACT The effects of halogen and solvent on structural, vibrational and electronic properties of thiophene-2-carbaldehyde (C5H4OS) and thiophene-2-carbonyl-halogeno [C5H3XOS; X = F, Cl or Br] were explored by DFT and TD-DFT methods. B3LYP functional was used with 6-31++G(d,p) basis set. Computations were focused on the two conformers of the compounds in the gas phase and both in a non-polar and in a polar solvent. The findings of this research work can be useful to those systems involving changes in the conformations analogous to the compounds studied. 3 Thiophene-2-carbaldehyde and its halogeno derivatives 4 Why thiophene-2-carbaldehyde and its halogeno derivatives ? They exhibit pharmacological activity. They have employed for the design of new medicines and biologically active substances. They play a role in commerce as constituents of reactive dyes, conductive polymers, ligands for catalyst systems, and in flavor and fragrances [1-3]. 5 Calculations All the computations were carried out by a cluster system with a precompiled set of Gaussian 09 program, configured for parallel computing. All structural and spectroscopic illustrations were done with the program GaussView 5.0.8 and GaussSum 2.2, while graphs were drawn with the program Origin 8 [4-5]. For the calculations, two forms of the compounds in C1 symmetry were first optimized in the gas phase, benzene and methanol at B3LYP using 6-31++G(d,p) basis set. The polarizable continuum model (PCM) model was used to evaluate the solvent effect. To determine conformational isomers, the potential energy surface (PES) analysis was also performed by the same basis set. 6 Conformational Analysis According to literature, conformers of thiophene-2-carbaldehyde (T2C) is shown in figure. It has two probable conformations with respect to rotation through the angle Φ (S5-C3-C6-O8): trans and cis [6]. 7 PES (Potential Energy Surface) Analysis To determine conformational isomers, PES analysis was also performed by the rotations of C2-C1-C6-O7 torsion angle, scanning from 180o to 360o, with 20o increments. The trans conformer was found more stable than the cis form. 8 Conformational Analysis Free - relative energies and mole fractions of T2C and its halogeno derivatives; Gas Compound Cis Free Energy (Hartree) T2C Relative Stability Mole Fractions (%) Free Energy (Hartree) T2C-F Relative Stability Mole Fractions (%) Free Energy (Hartree) T2C-Cl Relative Stability Mole Fractions (%) Free Energy (Hartree) T2C-Br Relative Stability Mole Fractions (%) 9 Benzene Trans Cis Methanol Trans Cis Trans -666.386139 -666.387956 -666.389579 -666.391571 -666.393802 -666.396084 1.14 0 1.25 0 1.43 0 13 87 11 89 8 92 -765.688736 -765.688928 -765.692103 -765.692308 -765.696068 -765.696337 0.12 0 0.13 0 0.16 0 44 55 45 55 43 57 -1126.036279 -1126.037202 -1126.039334 -1126.040298 -1126.043061 -1126.044080 0.58 0 0.6 0 0.64 0 27 73 26 74 25 75 -3239.957967 -3239.959151 -3239.961026 -3239.962203 -3239.964760 -3239.965998 0.74 0 0.74 0 0.78 0 22 78 22 78 21 79 Conformational Analysis Regarding the calculated free energies of T2C, in the gas phase, trans conformer is more stable than cis form by 1.14 kcal/mol. According to the calculations for mole fractions of the individual conformers, T2C prefers trans and cis conformers with approximate probabilities of 87% and 13%, respectively. Similarly, the calculated free energies, in benzene, show that trans conformer is more stable than other conformer and the compound prefers trans and cis conformers with approximate probabilities of 89% and 11%, respectively. The calculated free energies in methanol also indicate that trans conformer is more stable. The compound prefers these conformers with approximate probabilities of 92% and 8%, respectively. 10 Conformational Analysis Similarly, turning to T2C-F, T2C-Cl and T2C-Br, in the gas phase, benzene and methanol, the trans conformer is more stable than the cis. It is observed that all the conformers of the compounds are stabilized as the polarity of the solvents increases. Also, the most stable conformation of T2C and its halogeno derivatives is independent on the medium and halogen. 11 Geometrical Structures To clarify the vibrational frequencies, it is essential to examine the geometry of any compound as small changes in geometry can potentially cause substantial changes in frequencies. 12 Geometrical Structures Compound / Bond Length Gas Benzene Methanol Some found T2C C=O 1.2136 1.2175 1.2227 C-S 1.7448 1.7462 1.7484 C=O 1.1901 1.1918 1.1945 C-S 1.7499 1.7512 1.7528 T2C-Cl T2C-Br significant in changes the are geometric parameters when the compounds in solvated. From lower to higher dielectric, the C=O and C-S bond lenghts increase and there are C=O 1.1879 1.1894 1.1916 C-S 1.7525 1.7537 1.7552 very significant changes in C=O bond lenghts of the compounds T2C-F C=O 1.1884 1.1907 1.1938 C-S 1.7436 1.7448 1.7463 13 due to halogen atom. Vibrational Studies Theoretical vibrational frequencies for all compounds, along with corresponding vibrational assignments and intensities are performed. The simulated vibrational spectra are also depicted; All the computed frequency values are obtained within the harmonic approximation. The compounds consist of 11 atoms, with 27 normal vibrational modes, and it belongs to the point group C1. 14 Vibrational Studies Compound T2C v(C=O) IR (cm-1) Gas 1745 Benzene 1722 Methanol 1691 Medium IR Intensity (km/mol) 390 581 901 T2C-F Gas Benzene Methanol 1860 1720 1689 526 568 853 T2C-Cl Gas Benzene Methanol 1823 1721 1690 421 631 962 T2C-Br Gas Benzene Methanol 1832 1721 1691 415 662 1009 15 The frequencies C=O of T2C stretching and its halogeno derivatives are collected in table together with their intensities. The relations between the C=O vibrations/intensities and medium/compounds are explored. Vibrational Studies The carbonyl vibrations of all compounds decrease from the gas phase to methanol. In the gas phase, frequencies increase with halogen atom and F-compound has the highest value. However, in benzene and methanol, frequencies decrease or don’t change with halogen and F-compound has the lowest data. 16 Vibrational Studies Intensities of C=O vibrations of all compounds increase from the gas phase to methanol. In the gas phase, intensities increase with halogen atom and F-compound has the highest value. However, in benzene and methanol, intensities decrease for F- compound whereas they increase for Cl- and Br-compounds. 17 Electronic Properties The results of the absorption wavelengths (), excitation energies (E) and oscillator strengths (f) are collected in table together with the contributions of these transitions. Medium λ (nm) 338.1 Gas E (eV) f Contribution 3.66800 0.0002 H-1>L (97%), H-1>L+2 (3%) 268.7 4.613634 0.1366 H-2>L (33%), H>L (64%) 251.4 4.932537 0.1935 H-2>L (63%), H>L (33%) 332.8 3.725928 0.0002 H-1>L (97%), H-1>L+2 (3%) Benzene 276.7 4.481233 0.2045 H-2>L (25%), H>L (73%) 258.3 4.799836 0.2395 H-2>L (72%), H>L (25%) 325.9 3.803728 0.0002 H-2>L (97%), H-2>L+2 (2%) Methanol 279.3 4.438433 0.1757 H-1>L (30%), H>L (68%) 259.9 4.769235 0.2489 H-1>L (67%), H>L (30%) H and L denote HOMO and LUMO, respectively. 18 Electronic Properties UV spectra of T2C: From the UV spectra, the absorption bands are centered at 338.1, 332.8 and 325.9 nm in the gas phase, benzene and methanol for T2C, respectively. The area of these absorption bands decreases gradually from gas phase to the polar solvent. 19 Electronic Properties Energies HOMO and LUMO and values of chemical hardness, electronegativity, chemical potential and electrophilicity index (eV) of the most stable trans conformer of the compounds; Compound T2C T2C-F T2C-Cl T2C-Br HOMO–LUMO Gap Chemical Hardness (h) Electrophilicity Index (ω) HOMO LUMO Gas -7.259 -2.325 4.934 2.467 4.792 -4.792 4.655 Benzene -7.217 -2.331 4.885 2.442 4.774 -4.774 4.666 Methanol -7.196 -2.369 4.827 2.413 4.783 -4.783 4.739 Gas -7.577 -2.462 5.116 2.558 5.019 -5.019 4.925 Benzene -7.497 -2.441 5.056 2.528 4.969 -4.969 4.884 Methanol -7.427 -2.444 4.983 2.491 4.935 -4.935 4.888 Gas -7.596 -2.659 4.936 2.468 5.127 -5.127 5.326 Benzene -7.517 -2.643 4.873 2.437 5.080 -5.080 5.295 Methanol -7.445 -2.650 4.795 2.398 5.047 -5.047 5.313 Gas -7.599 -2.720 4.880 2.440 5.159 -5.159 5.455 Benzene -7.525 -2.697 4.827 2.414 5.111 -5.111 5.411 Methanol -7.457 -2.699 4.758 2.379 5.078 -5.078 5.419 20 Electronegativity(χ) Chemical Potential (µ) Medium Electronic Properties Frontier molecular orbitals of T2C in the gas phase; The energy of HOMO-LUMO gap decreases gradually from the gas phase to the polar solvent for all compounds. The F-compound has the largest HOMOLUMO energy gap of about 5.1 eV. From the gas phase to the polar solvent, the values of the chemical hardness and electronegativity of the all compounds also show same trends with the energy gaps. 21 Conclusions The results can be useful for analysis of the conformers involving these moieties. The main conclusions of this theoretical research work are as follows: (i) The minimum energies of the optimized structures decrease with the solvent polarity and the size of halogen. (ii) Conformational preference is independent on the solvent and halogen effects. (iii) The C=O vibrations of all compounds decrease from the gas phase to methanol whereas their intensities increase. (iv) There are also both halogen and solvent effetcs on the geometric parameters, UV spectra (optical band gaps), HOMO-LUMO gaps (electrical gaps), the chemical hardness and electronegativity of the all compounds. (v) It is observed that fluorine has an atypical characteristic affecting the vibrational frequencies and their intensities, electrical band gap, chemical hardness and electronegativity. 22 REFERENCES [1] Buchin, P.I. and Lipkin, A.E., Saratov. Gos. Univ., 1974. [2] Queiroz, M.J.R.P., Ferreira, I.C.F.R., Barbosa, S., Calhelha, R., and Estevinho, L., Moscow, 2004, p. 156. [3] Jonathan Swanston, Degussa Knottingley Limited, Knottingley, Yorkshire, UK. [4] Gaussian 09, Revision D.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. 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