untitled European Journal of Chemistry 6 (3) (2015) 248‐253 European Journal of Chemistry ISSN 2153‐2249 (Print) / ISSN 2153‐2257 (Online)  2015 Atlanta Publishing House LLC ‐ All rights reserved ‐ Printed in the USA http://dx.doi.org/10.5155/eurjchem.6.3.248‐253.1060 European Journal of Chemistry Journal webpage: www.eurjchem.com Molecular structure, vibrational spectroscopic and HOMO/LUMO studies of some organotellurium compounds by quantum chemical investigations Rafid Hmedan Al‐Asadi, Bahjat Ali Saeed * and Tarik Ali Fahad Department of Chemistry, College of Education for Pure Sciences, University of Basrah, 61004, Iraq * Corresponding author at: Department of Chemistry, College of Education for Pure Sciences, University of Basrah, 61004, Iraq. Tel.: +964.60. 0107802410050. Fax: +964.61.0013203062957. E‐mail address: bahjat.saeed@yahoo.com (B.A. Saeed). ARTICLE INFORMATION ABSTRACT DOI: 10.5155/eurjchem.6.3.248‐253.1060 Received: 25 March 2015 Received in revised form: 31 May 2015 Accepted: 10 June 2015 Published online: 30 September 2015 Printed: 30 September 2015 Quantum mechanical calculations of geometries, energies and vibrational frequencies of organic mercury and tellurium compounds containing azomethine group, molecules a1‐a5 and containing azo group, molecules a6‐a10 have been undertaken using density functional theory. The optimized geometrical parameters such as bond lengths, bond angles and dihedral angles showed that only organomercuric compounds have planer structures. The calculation of the total energy and HOMO‐LUMO energy gap were showed that organotellurium compounds have higher reactivity than the corresponding organomercuric compounds. As well it showed the HOMO orbitals are localized mainly on tellurium, nitrogen and bromine atoms moieties, while the LUMO of π nature are mostly located on the phenyl ring. The calculated vibrational frequencies of molecules a1 and a7 are in good agreement with experimental frequencies with correlation coefficient r2 value is 0.9875 and 0.9987, respectively. KEYWORDS DFT DNP Organotellurium Vibrational spectra Computational study HOMO‐LUMO energy gap Cite this: Eur. J. Chem. 2015, 6(3), 248‐253 1. Introduction Tellurium chemistry has been the subject of intensive research in the last three decades due to the interest of several research groups in organometallic and supramolecular chemistry of organotellurium compounds [1,2]. There is increasing interest in the synthesis of aromatic organo‐ tellurium compounds containing electron donor nitrogen atom at position ortho to the tellurium atom [3,4], such as azo group [5] and azomethine group [6,7]. These compounds have high stability due to intra‐molecular interaction between tellurium and nitrogen atoms [8,9]. Calculations performed with the use of Density Functional Theory (DFT) have been successfully employed in a number of previous theoretical studies of organotellurium compounds [10‐12]. In this study, we report molecular geometry, HOMO‐LUMO energy gap and the assignments of IR spectra of some organomercuric and organotellurium compounds containing azo or azomethine groups. 2. Experimental 2.1. Instrumentation and materials The compounds a1‐a10 were prepared according to previously published procedure [13,14]. Infrared spectra were recorded as KBr discs in the range of 4000‐400 cm‐1 using a Shimadzu FT‐IR spectrophotometer at Department of Chemistry, College of Education for Pure Sciences, University of Basrah, Iraq. 2.2. Computational details All calculations of studied molecules (Figure 1) were performed with Material studio/DMol3 Version 5.5 program [15‐17] and using the DFT method [15‐17], at the PBE level of theory [18,19] along with standard DNP basis set [17]. 3. Results and discussion 3.1. Optimized structure The important structural parameters of the optimized geometries such as bond lengths, bond angles and dihedral angles of the studied molecules a1‐a10 are summarized in Table 1, the optimized structures of these compounds are shown in Figure 2. In the ArHgCl moiety a1 and a6 each mercury atom is linearly coordinated to a chloride and a carbon atom (Angle C‐ Hg‐Cl = 177.307 and 179.451 °), which is fully characterized structurally. Al‐Asadi et al. / European Journal of Chemistry 6 (3) (2015) 248‐253 249 Table 1. Bonds lengths, bond angles and dihedral angles of the studied compounds *. Compound Bonds lengths (Å) Angles bonds (o) Dihedral angles (o) C1‐Hg Hg‐Cl C1‐Hg‐Cl Hg‐C1‐C2 N=C3‐C4‐C6 C1‐C2‐N=C3 a1 2.244 2.449 177.307 105.940 178.939 177.437 C1‐Hg Hg‐Cl C1‐Hg‐Cl Hg‐C1‐C2 N1=N2‐C3‐C4 C1‐C2‐N1=N2 a6 2.190 2.475 179.451 101.706 180.000 180.000 Bonds lengths (Å) Angles bonds (o) Dihedral angles (o) Compound C1‐Te Te‐Br1 Te1‐Te2 C1‐Te‐Br1 C1‐Te‐C1 C1‐Te‐Te Te‐C1‐C2 N=C3‐C4‐C6 C1‐C2‐N=C3 C‐Te‐Te‐C a2 2.165 2.601 94.845 106.311 170.624 166.484 a3 2.164 2.166 2.770 103.000 106.063 122.107 123.758 134.502 ‐131.369 66.470 a4 2.174 2.192 2.724 90.419 93.095 107.389 108.223 127.836 160.184 129.806 a5 2.171 2.160 106.070 128.554 120.409 106.614 94.521 Compound C1‐Te Te‐Br1 Te1‐Te2 C1‐Te‐Br1 C1‐Te‐C1 C1‐Te‐Te Te‐C1‐C2 N1=N2‐C3‐C4 C1‐C2‐N1=N2 C‐Te‐Te‐C a7 2.188 2.596 106.100 121.829 ‐167.505 ‐164.770 a8 2.169 2.153 2.842 97.263 98.903 115.156 118.392 ‐167.761 ‐14.995 92.534 a9 2.172 2.232 2.756 93.175 86.790 90.360 127.410 119.667 141.818 65.906 a10 2.150 2.157 93.926 119.968 121.197 159.255 154.019 * Experimental values: C‐Te : 2.158 Å , Te‐Br : 2.65 Å , Te‐Te: 2.77 Å. a1 a2 Te a3 a4 HOOC N C H OH OH 2 Te a5 HgCl HOOC N N N OH a6 a7 a8 a9 a10 Figure 1. Molecular structure of studied molecules. The Hg‐C distances are 2.24 and 2.19 Å, respectively, are in close agreement with experiment value 2.065 Å [20] for almost linear Ar‐Hg‐Cl. Similarly, Hg‐Cl distance, which are 2.449 and 2.475 Å are close agreement with experiment value 2.326 Å [20]. As could be seen from Table 1, there is fair agreement between the calculated bonds lengths of C‐Te, Te‐Br and Te‐Te bonds with the measured bond lengths [21‐26]. Generally, there is no significant difference between the calculated bond lengths. Only a slight increase in N=N length from 1.243 to 1.278 Å on going from molecule a2 to molecule a7. This may be due to intramolecular coordination between tellurium and nitrogen atom [27]. This interaction can be attributed to the overlap of p‐orbital on the nitrogen atom with the σ* (Te‐ Ctrans) molecular orbital (partly responsible for such an interaction) feasible [2]. 250 Al‐Asadi et al. / European Journal of Chemistry 6 (3) (2015) 248‐253 a1 a2 a3 a4 a5 a6 a7 a8 a9 a10 Figure 2. Optimization geometries structures of the studied molecules. For tellurium(II) compounds a5 and a10 both lone pairs of electrons around tellurium should be stereo chemically active according to VSEPR (Valence Shell Electron Pair Repulsion) theory [28], the geometry of the tellurium atom in compound a5 is relative to the tetrahedral geometry, where C‐Te‐C angle is 106.07°. While geometry of the tellurium atom in compound a10 is a distorted pseudo‐tetrahedral [27], C‐Te‐C angles are 93.9°. This is due to high secondary intramolecular coordination between tellurium and nitrogen atom in this compound [29]. On the other hand, the C‐Te‐C angle for the tellurium(IV) compounds a4 and a9 (covering the range 90.08‐107.38 °) are significantly lower than the putative value of 120 ° for trigonal bipyramidal geometry due to the stereo chemical activity of the lone pair on tellurium atom [30]. Al‐Asadi et al. / European Journal of Chemistry 6 (3) (2015) 248‐253 251 Table 2. Values of Total energy and LUMO‐HOMO energy gap of studied compounds in Hartree unit. Compound Total energy HOMO LUMO ∆E(LUMO‐HOMO) Energy Energy a1 ‐19767.3034778 ‐0.20750 ‐0.11965 0.08785 a2 ‐15229.8885967 ‐0.17632 ‐0.13959 0.03673 a3 ‐15016.6729209 ‐0.17360 ‐0.10248 0.07112 a4 ‐13550.5167344 ‐0.20574 ‐0.12309 0.08265 a5 ‐8402.8083298 ‐0.17584 ‐0.10006 0.07578 a6 ‐19877.6762863 ‐0.21148 ‐0.14248 0.06900 a7 ‐15334.2428367 ‐0.21059 ‐0.16402 0.04657 a8 ‐15237.4436071 ‐0.18269 ‐0.13639 0.04630 a9 ‐2044.5778558 ‐0.16680 ‐0.13649 0.03031 a10 ‐2017.828634 ‐0.17381 ‐0.13448 0.03933 In the case of aryl tellurium (IV) tribromide a2 and a7, the presence of an electron‐rich Br atom of the neighboring molecule (close to Te(IV) atom having a lone pair as per VSERP theory) in the lattice is of particular interest [30]. The overall coordination geometry around the tellurium atom is trigonal bipyramidal. Due to the presence Br group, that will weaken the Lewis acidity of the tellurium atom will further prevent intramolecular interaction, leading to the formation of molecular species with conformation consistent with the VSEPR theory [31], therefore the distance of C1‐Te in compound a7 is longer (2.188 Å) and angle Te‐C1‐C2 is largest (121.82 °) compared with other compounds. While in the compound a2 the case is reverse (C1‐Te is 2.165 Å and Te‐C1‐ C2 is 94.84 °), this may be due to steric or electronic effects for bromine atoms. In the ditelluride system a3 and a8, the Te‐Te bond is likely to influence the repulsion between loin pairs. The steric interaction between aromatic rings is due to small C‐Te‐Te‐Te dihedral angles (66.47 and 92.53 °). This is a consequence of rotation around the C‐Te bonds which take place because of the proximity of the phenyl rings of each other. Due to a substantial secondary intramolecular coordina‐ tion between Te and N, it is of interest to note that the twist dihedral angles of both the N‐phenyl and C‐phenyl ring out of the plane C‐C=N‐C in the Schiff base moiety, molecules a1 to a5 or C‐N=N‐C in the azo moiety, molecules a6 to a7 is less than 15 °. From the dihedral angles measurement, observed two molecules a1 and a6 have a planar structure (dihedral angle ≈180 °), the angle N=C2‐C4‐C6 is 178.93 ° and C1‐C2‐ N=C3 is 177.43 ° for compound a1, while angle N1=N2‐C3‐C4 is 180 ° and C1‐C2‐N1=N2 is 180.00 ° for compound a6. The rest compounds have non planar structures (dihedral angles are 49‐170 °), Table 1. 3.2. Energies calculation Molecular orbital and their properties are very useful for physicists and chemists. In particular, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) and their energy gap reflect the chemical activity of the molecule [32,33]. Higher value of HOMO of a molecule has a tendency to donate electrons to appropriate accepter molecule with low energy, empty molecular orbitals [34]. The total energy and HOMO‐LUMO energy gaps the studied molecules a1‐a10 are summarized in Table 2. The values of LUMO‐HOMO energy gap and total energy of the organomercuric compounds a1 and a6, which are ∆ELUMO‐HOMO energy gap 0.0878 and 0.0690 Hartree; Total energy ‐19767.3 and ‐19877.6 Hartree, are relatively higher compared with the corresponding organtellurium compounds. This indicates a high stability and high chemical hardness of these compounds. Molecules a2 and a9 showed the lowest gap values (0.0367 and 0.0303 Hartree), reflecting their chemical reactivity compared with other studied molecules. This may be due to presence bromine atoms. 3.3. HOMO LUMO analysis The HOMO orbitals are localized mainly on tellurium, nitrogen and bromine atoms moieties. Whereas the LUMO of π nature are mostly located on the phenyl ring. The HOMO‐ LUMO transition implies an electron density transfer to the phenyl ring from tellurium atom. The visualization of HOMO‐ LUMO gap and HOMO and LUMO orbitals for compound a2 and a8 are shown in Figure 3 and 4. Figure 3. HOMO and LUMO orbitals of compound a2. Figure 4. HOMO and LUMO orbitals of compound a8. 3.4. Vibrational frequencies Assignments for the complex systems can be proposed on the basis of frequency agreement between the computed harmonics and the observed fundamentals. The calculated frequencies for the optimized geometry and the experimental wave numbers together with the proposed assignments for compounds a1 and a7 are given in Table 3 and 4, respectively. The vibrational spectral data obtained from the solid‐phase FT‐IR spectra based on the results of the normal coordinates calculations. The observed and the calculated spectra reflect a reasonable agreement for the vibrational frequencies. Based on the comparison of the calculated and experimental results, assignments of fundamental frequencies incorporate the observed band frequencies in the infrared spectra of the studied species confirmed by establishing a one‐to‐one correlation between observed and theoretically calculated frequencies. 252 Al‐Asadi et al. / European Journal of Chemistry 6 (3) (2015) 248‐253 Table 3. Calculated vibrational frequencies (cm‐1) and the observed frequencies of compound a1. Calculated Experimental Assignment 3544 3321 υ(O‐H) 3499 3225 υ(O‐H) 2949 3095 υ(C‐H) aromatic 2844 2963 υ(C‐H) aliphatic 2818 2876 υ(C‐H) aliphatic 1739 1690 υ(C=O) 1652 1624 υ(C=N) 1522 1518 υ(C=C) 1479 1477 δ(C‐H) aliphatic 1311 1389 δ(C‐N) 1222 1225 δ(C‐O) 1097 935 δ(C‐H) aromatic 840 841 δ(C‐H) aromatic 789 770 δ (O‐H) Correlation coefficient = 0.9875 Table 4. Calculated vibrational frequencies (cm‐1) and the observed frequencies of compound a7. Calculated Experimental Assignment 3511 3550 υ(O‐H) 3465 3435 υ(O‐H) 3018 3150 υ(C‐H) aromatic 2995 3066 υ(C‐H) aromatic 1660 1685 υ(C=O) 1528 1601 υ(N=N) 1522 1531 υ(C=C) 1342 1359 δ(C=C) 1279 1282 δ(C‐N) 1241 1251 δ(C‐O) 1028 1030 δ(C‐C) 778 775 δ(C‐H) aromatic 668 688 δ (O‐H) 613 636 δ (O‐H) Correlation coefficient = 0.9987 The calculated frequencies are slightly higher than the observed values for the majority of the normal modes. Many different factors may be responsible for the discrepancies between the experimental and computed spectra of the compound. Factors such as environment, anharmonicity, intermolecular interaction and limited basis set [34]. The vibrational analyses are summarized in Table 3 and 4. A linearity between the experimental and the calculated wave numbers can be estimated by plotting the calculated vs. experimental wavenumbers, Figure 5 and 6. 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