СТЕКТРОСКОПИЯ АТОМОВ И МОЛЕКУЛ
539.19
DFT SIMULATIONS, FT-IR, FT-RAMAN, AND FT-NMR SPECTRA OF 4-(4-CHLOROPHENYL)-1H-IMIDAZOLE MOLECULES
© 2012 г. Y. Erdogdu*, M. T. GuHuoglu*, $. Yurdakul**, and O. Dereli***
*Department of Physics, Ahi Evran University, 40040 Kirsehir, Turkey **Department of Physics, Gazi University, 06500Ankara, Turkey ***A. Kelesoglu Education Faculty, Department of Physics, Selcuk University, 42090Konya, Turkey
Received November 24, 2011
Abstract—The FT-IR, FT-Raman, and FT-NMR spectra of the compound 4-(4-Chlorophenyl)-1H-imi-dazole (4-ClPI) was recorded and analyzed. Density functional method has been used to compute optimized geometry, vibrational wavenumbers and NMR spectra of the 4-ClPI. Only one tautomeric form was found most stable by using B3LYP functional with the 6-311++G(d,p) as basis sets. The detailed interpretation of the vibrational spectra was carried out with the aid of total energy distribution (TED) following the scaled quantum mechanical force field methodology.
ОПТИКА И СПЕКТРОСКОПИЯ, 2012, том 113, № 1, с. 26-34
УДК
INTRODUCTION
Imidazole is nitrogen containing heterocyclic ring which possesses biological, pharmaceutical, and unique optical properties. Thus, imidazole compounds have been an interesting source for researchers for more than a century [1, 2]. Heterocyclic imidazole derivatives play very important role in chemistry as mediators for synthetic reactions, primarily for preparing functionalized materials [2]. In addition, they are widely used in many fields, such as P38 MAP kinase [3], antivascular disrupting, antitumour activator [4], ionic liquids [5], anion sensors [2], as well as electrical and optical materials [6—8].
Imidazole derivatives show unique chemical and physical properties because they contain imidazole heterocycle which has better thermal stability, and benzene rings can increase the degree of conjugation of the organic molecule. Imidazole derivatives has also significant analytical applications by utilizing their fluorescence and chemiluminescence properties [9]. An important property that makes imidazole derivatives more attractive as a chelator is the appreciable change in its fluorescence upon metal binding. As a result, luminescent materials of imidazole derivative have emerged as the attractive blue-emitting materials. Therefore, imidazole derivatives have been used to construct highly sensitive fluorescent chemisensors for sensing and imaging of metal ions and its chelates in particular those with Ir3+ are major components for organic light emitting diodes [9, 10] and are promising candidates for fluorescent chemisensors for metal ions. Recently the research by Huang and Zhao [11] was aimed at the production of imidazole derivatives for luminophores, but such materials are oligomers which restricted the application.
Organic luminescent materials have recently received much attention due to their potential applica-
tions in organic lightemitting diodes [12], ceramics [13], fluorescent biological labels [14], photovoltaic cells [15], and optical sensors [16]. A great number of luminescent organic materials have been synthesized and investigated, for example, recently the better blue lightemitting materials were achieved for the pyra-zoloquinoline chromophore, where the quantum efficiency was about 1.7 [17, 18]. However, organic luminescent materials still have stability problems and the correlation between structure and fluorescence efficiency remains a big challenge in this area.
FT-IR, FT-Raman, and FT-NMR spectra of 4-phenylimidazole molecule using theoretical and experimental methods has been reported earlier [19]. In the present paper we deal with the IR, Raman and NMR spectra of 4-(4-Chlorophenyl)-1H-imidazole (4-ClPI) molecule along with the theoretical prediction using DFT method.
EXPERIMENTAL
The FT-IR spectrum of this molecule recorded in the region 4000—400 cm-1 on IFS 66V spectrophotometer using KBr pellet technique is shown in Fig. 1. The FT-Raman spectrum of 4-ClPI recorded using 1064 nm line of Nd : YAG laser as excitation wavelength in the region 50-3500 cm-1 on a Thermo Electron Corporation model Nexus 670 spectrophotome-ter equipped with FT-Raman module accessory are shown in Fig. 2. The 1H and 13C NMR spectra are taken in chloroform solutions and all signals are referenced to TMS on a BRUKER DPX-400 FT-NMR Spectrometer. All NMR spectra are measured at room temperature.
Wavenumber, cm 1 Fig. 1. FT-IR spectra of 4-ClPI molecule in the range 400-4000 cm-1.
Fig. 2. FT-Raman spectra of 4-ClPI in the range 50-3500 cm 1.
COMPUTATIONAL DETAILS
Gaussian 09 quantum chemical software was used in all calculations [20]. The optimized structural parameters and vibrational wavenumbers for the 4-ClPI molecule were calculated by using B3LYP functional with 6-311G++(af,^) as basis set. The vibrational modes were assigned on the basis of TED analysis using SQM program [21]. Normal coordinate analysis of the title molecules has been carried out to obtain a more complete description of the molecular motions involved in the fundamentals. The calculated harmon-
ic vibrational wavenumbers were scaled down uniformly by a factor of 0.967 (for wave numbers under 1800 cm-1) and 0.955 (for those over 1800 cm-1) for B3LYP/6-311++G(a^) level of theory, which accounts for systematic errors caused by basis set incompleteness, neglect of electron correlation and vibrational anharmonicity [22-24].
The 1H and 13C NMR chemical shifts calculations of the T2 tautomeric form of the 4-ClPI molecule were made by using B3LYP functional with 6-311G++(af,^) basis set. The GIAO (gauge including atomic orbital) method is one of the most common
T optimized energy = - 917.01990756 a.u.
Relative optimized energy = 6.70 kJ/mol
Fig. 3. Tautomeric forms and atomic numbers of 4-ClPI.
T2 optimized energy = - 917.02245977 a.u. Relative optimized energy = 0 kJ/mol
approaches for calculating isotropic nuclear magnetic shielding tensors [25, 26]. For the same basis set size GIAO method is often more accurate than those calculated with other approaches [27, 28]. The NMR spectra calculations were performed by Gaussian 09 program package. The calculations reported were performed in chloroform solution using IEF-PCM model as well as gas phase in agreement with experimental chemical shifts obtained in chloroform solution.
Predictions of Raman Intensities
It should be noted that Gaussian 09 package able to calculate the Raman activity. The Raman activities were transformed into Raman intensities using Raint program [29] by the expression
I = 10-12(vo - v;-)4RAJv„
where Ii is the Raman intensity, RAi is the Raman scattering activities, v, is the wavenumber of the normal modes and v0 denotes the wavenumber of the excitation laser [30].
RESULTS AND DISCUSSION
Tautomeric Analysis
All the possible tautomeric forms of 4-ClPI were calculated which are optimized by B3LYP/6-311++G(d,p) level of theory. The possible two stable tautomeric forms are given in Fig. 3 which shows the possibility of proton transfer between the nitrogen atoms of imidazole ring. The total energies and the relative energies of the different tautomeric forms of 4-ClPI are presented in Fig. 3. Many attempts to investigate the tautomeric equilibrium structure of this molecule have been made using different calculation methods. It is determined that T2 is more stable than T1 by 3 kJ/mol for AM1 and PM3 calculations by Ogretir and Yarligan [31]. Maye and Venanzi have calculated rotational barrier and energies of both T2 and T1 tautomeric forms of 4-PI [32]. They reported that the difference in energy is 7.5 kJ/mol
by T2 compared with T1. In the present work it is clear that the T2 tautomer has the lowest energy and is the most stable form. The energy difference between T1 and T2 tautomer is 6.70 kJ/mol (at B3LYP/6-311G++(d,p) level of theory). This confirms that the 4-ClPI is the only stable tautomer in gas phase. The chloride atom is connected to para position in 4-phe-nylimidazole and 4-ClPI have similar tautomeric forms. The tautomeric equilibrium structures of 4-phenylimidazole molecule were investigated in our previous paper [19] and is determined that T2 is more stable than T1 by 5.27 kJ/mol for B3LYP/6-311G(d,p) level of theory. So finally we taken T2 tautomeric form was used in the future calculations such as vibrational and NMR spectra of the 4-ClPI molecule.
Conformational Analysis
In order to reveal all possible conformations of 4-ClPI a detailed potential energy surface (PES) scan in N1з—C11—Cз—C2 dihedral angles was performed. The scan was carried out by minimizing the potential energy in all geometrical parameters by changing the torsion angle for every 10° for a 180° rotation around the bond. The shape of the potential energy as a function of the dihedral angle is illustrated in Fig. 4. It shows that T2 tautomeric form was planar (N-C-C-C dihedral angle for 0°) and T1 tautomeric form was twisted (N-C-C-C dihedral angle for 30°).
Optimized Structure
The optimized bond lengths and bond angles of the T2 tautomeric form of the 4-ClPI molecule at B3LYP/6-311++G(d,p) level are collected in Table 1. These optimized geometric parameters of 4-PI are compared with those of X-ray data [33].
Vibrational Analysis
The 4-ClPI molecule consists of19 atoms. So there are 51 vibrational modes. The 51 vibrational modes of 4-ClPI have been assigned according to the detailed motion of the individual atoms. This molecule belongs to C1 symmetry group. The experimental FT-IR and FT-Raman along with the calculated wavenumbers are given in Table 2. As seen in table, IR and Raman intensities of 4-ClPI are in consistency with the TED results. The theoretically predicted IR and Raman spectra at B3LYP/6-311++G(d,p) level of calculations along with experimental FT-IR and FT-Raman spectra are shown in Figs. 1, 2.
The C-H stretching vibrations give rise to bands in the region 3100-3000 cm-1 in all t
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