ОПТИКА И СПЕКТРОСКОПИЯ, 2013, том 114, № 4, с. 573-584
СПЕКТРОСКОПИЯ ^^^^^^^^
КОНДЕНСИРОВАННОГО СОСТОЯНИЯ
УДК 539.19
FT-IR, FT-RAMAN, NMR SPECTRA AND DFT SIMULATIONS OF 4-(4-FLUORO-PHENYL)-1H-IMIDAZOLE
© 2013 г. Y. Erdogdu**, D. Manimaran**, M. T. GUlluoglu*, M. Amalanathan**,
I. Hubert Joe**, and S. Yurdakul***
*Department of Physics, Ahi Evran University, 40040 Kirsehir, Turkey **Department of Physics, Centre for Molecular and Biophysics Research, Mar Ivanios College, Thiruvananthapuram-695 015, Kerala, India ***Department of Physics, Gazi University, 06500Ankara, Turkey E-mail: yusuferdogdu@gmail.com Received July 19, 2012
The FT-IR, FT-Raman and FT-NMR spectra of the compound 4-(4-Fluoro-phenyl)-1H-imidazole (4-FPI) were recorded and analyzed. Density functional method (B3LYP level with the 6-311G(d, p) and 6-311++G(d, p) and cc-pVQZ as basis sets) has been used to compute optimized geometry, vibrational wave-numbers of the 4-FPI. Only one tautomeric form was found most stable by using DFT/B3LYP. The detailed interpretation of the vibrational spectra was carried out with the aid of total energy distribution following the scaled quantum mechanical force field methodology. Potential Energy Surface scan studies has also been carried out by ab initio calculations with the same basis sets.
DOI: 10.7868/S0030403413040077
INTRODUCTION
Imidazole is a heterocyclic compound of five-membered diunsaturated ring structure composed of three carbon atoms and two nitrogen atoms at nonad-jacent positions. Imidazole ring is found in histidine (an essential amino acid) and in histamine, the decar-boxylated compound from histamine. Some imidazole compounds inhibit the biosynthesis of ergo sterol, required in cell membrane in fungal. They have antibacterial, antifungal, antiprotozoal, and anthelmintic activity. Imidazole and its derivatives are widely used as intermediates in synthesis of organic target compounds including pharmaceuticals, agrochemicals, dyes, photographic chemicals, corrosion inhibitors, epoxy curing agents, adhesives and plastic modifiers [1, 2]. The phenylimidazole molecular fragment plays a primary role in the functional architecture of biologically active molecules such as novel histamine H2 receptor antagonists [3], cardio tropic agents [4] and several types of artificial enzymes [5—8].
FT-IR, FT-Raman and FT-NMR spectra of 4-phenylimidazole molecule using theoretical and experimental methods have been reported earlier [9]. The present paper deals with the Infrared, Raman and NMR spectra of 4-(4-Fluoro-phenyl)-1H-imidazole (4-FPI) molecule along with the theoretical prediction using DFT method. The change in electron density (Ed) in the a* antibonding orbitals and E(2) energies have been calculated by natural bond orbital analysis.
EXPERIMENTAL
The title compound 4-FPI (99% Aldrich) was purchased from Sigma-Aldrich and used without further purification. The FT-IR spectrum of this molecule is recorded in the region 4000—400 cm-1 on IFS 66V spectrophotometer using KBr pellet technique. The FT-Raman spectrum of 4-FPI has been 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 spectrophotometer equipped with FT-Raman module accessory. 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 recorded at room temperature.
COMPUTATIONAL DETAILS
The quantum chemical computations of 4-FPI were performed using the Gaussian 03 program package [10] at the DFT level using B3LYP (Becke's three parameter hybrid functional using Lee-Yang-Parr correlation functional) with 6-311G(d,p), 6-311++G(d,p) and cc-VPQZ basis sets. The vibrational modes were also assigned on the basis of TED analysis using SQM program [11]. The current version of the SQM package uses a modified procedure involving the scaling of individual valence coordinates (not the linear combinations present in natural internal coordinates). This has immediate advantages in terms of ease of use, as no natural internals need be generated (which may fail for
Fig. 1. Tautomeric forms of 4-FPI.
complicated molecular topologies) and it simplifies the classification and presorting of the coordinates. In addition, the extra flexibility involved in the scaling of individual primitive internals generally leads to an increase in accuracy and to more transferable scale factors [12]. The calculated vibrational wavenumbers are scaled by (0.967), (0.978) and (0.969), for B3LYP/6-311G(d,p), 6-311++G(d, p) and cc-pVQZ, respectively [13—15] to offset the systematic error caused by neglecting anharmonicity and electron density.
The GIAO (Gauge Including Atomic Orbital) method is one of the most common approaches for calculating isotropic nuclear magnetic shielding tensors [16, 17]. For the same basis set size GIAO method is often more accurate than those calculated with other approaches [18, 19]. The 1H and 13C NMR chemical shifts calculations of the T1 tautomeric form of the 4-FPI molecule were made by using B3LYP functional with 6-31G(d) basis set.
Predictions of Raman Intensities
The calculated Raman activities (Si) was converted to relative Raman intensities (Ii) using the following relationship derived from the intensity theory of Raman scattering [20, 21]
7 = f (vo -V-)4 Si s (1)
vi [1 - exp(-hcvi/kT)]
where v0 is the exciting wavenumber, Vj the vibrational wavenumber of the ith normal mode, h, c and k are fundamental constants, and f is a suitably chosen common normalization factor for all peak intensities. For simulation, the calculated FT-Raman spectra
were plotted using pure Lorentizian band shape with a bandwidth ofFull Width and HalfMaximum (FW-HM) of 10 cm-1.
RESULTS AND DISCUSSION
Tautomeric Analysis
All the possible tautomeric forms of 4-FPI 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. 1 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-FPI are presented in Table 1. It is clear that the T1 tautomer has the lowest energy and is the most stable form. The energy difference between T1 and T2 tautomer is 6.409 kJ/mol (at B3LYP/6-311G(d, p) level of theory). This confirms that the 4-FPI is the only stable tautomer in gas phase. The fluorine atom is connected to para position in 4-phenylimidazole and 4-FPI have similar tautomeric forms. The tautomeric equilibrium structures of 4-phenylimidazole molecule were investigated by several authors and is determined that T1 is more stable than T2 by 3 kJ/mol for AM1 and PM3 calculations by Ogretir et al. [22]. Maye and Venanzi calculated rotational barrier and energies of both T1 and T2 tautomeric forms of 4-PI [23]. They reported that the difference in energy is 7.5 kJ/mol by T1 compared with T2. In the earlier work it shows that T1 is more stable than T2 by 5.27 kJ/mol by B3LYP/6-311G (d, p) level of theory. Due to the most stable state of T1 tautomeric compared to T2. So, finally T1 tautomeric form was used in
Table 1. Total (a.u.) and relative energies (kcal/mol) and dipole moment (Debye) of different tautomers of 4-FPI calculated at the B3LYP levels of theory
Tau- tomer Methods Optimized ZPE corrected energy Relative energy ^HOMO, eV ^LUMO, eV ^^HOMO-LUMO> eV Dipole moment
T1 B3LYP/6-311G(d, p) -556.51303127 0.000 -5.807 -0.622 5.185 5.1913
B3LYP/6-311++G(d, p) -556.52477635 0.000 -5.968 -0.892 5.076 5.361
T2 B3LYP/6-311G(d, p) -556.51006100 7.798 -5.979 -0.969 5.010 2.7817
B3LYP/6-311++G(d, p) -556.52211306 6.992 -6.142 -1.205 4.937 2.7804
Relative energy, kJ/mol 1
-r-/A\
20
10 -
Id i'
¿1 \k 3
60 120 180 240 300 360 Dihedral angles, degrees
Fig. 2. Potential energy surface of tautomeric forms for dihedral angle N13-C11-C3-C2: T1 (1, 2), T2 (3, 4); B3LYP/6-311G(d,p) (1, 3), B2LYP/6-311 ++G(d,p) (2, 4).
the future calculations such as vibrational and NMR spectra and NBO analysis of the 4-FPI molecule.
Conformational Analysis
In order to reveal all possible conformations of 4-FPI, a detailed potential energy surface (PES) scan in N16—C2—C6—C7 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 360° rotation around the bond. The shape of the potential energy as a function of the dihedral angle is illustrated in Fig. 2. It shows that T1 tautomeric form was planar (N—C—C—C dihedral angle for 0°) and T2 tautomeric form was twisted (N-C-C-C dihedral angle for 35.3°) [23]. The T1 tautomeric form predict a near planar equilibrium structure and the dihedral angle between phenyl and imidazole ring is 9° for HF/6-31G (d) level of theory [24]. In the 4-phenylimidazole molecule, predicted at 26.45° T2 tautomeric form and at 0° T1 tautomeric form by B3LYP/6-311G(d, p) level of theory [9]. In this study the T1 tautomeric shows the planar conformation (0°) and the T2 tautomeric was twisted around (28.47°) at the B3LYP/6-311 G(d, p) level of theory. The 4-phenylimidazole [9] and 4-FPI molecules exhibit similar trends for conformational analysis.
Optimized Geometry
The optimized molecular structure of the isolated 4-FPI molecule calculated using DFT at B3LYP/6-311++G(d, p) basis set is shown in Fig. 3. The computed optimized geometrical parameters along with the experimental values [25, 26] for comparison are given in Table 2. The predicted bond lengths of
Cjj — HTS, C8—Hl2, Cl—H
l15
12>
C3-H5,
and C9-H14 are elongated from the experimental values. The calculated dihedral angles C8-C6-C2-N16 and C8-C6-C2-C1
Fig. 3. Molecular structure of 4-FPI.
are different at ~36.13° and ~39.92° respectively from the experimental values. Nielsen et al. [25] reported that T1 co
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