Research Article  Open Access
Vibrational Spectroscopic Investigation and Conformational Analysis of Methacrylamidoantipyrine: A Comparative Density Functional Study
Abstract
FTIR and Raman spectra of methacrylamidoantipyrine (MAAP) have been reported in the region of 4000–10 cm^{−1} and 4000–100 cm^{−1}, respectively. The optimized geometric parameters, conformational analysis, normal mode frequencies, and corresponding vibrational assignments of MAAP (C_{15}H_{17}N_{3}O_{2}) have been examined by means of density functional theory (DFT) method using the Becke3LeeYangParr (B3LYP) exchangecorrelation functional and the 631G^{++}(d,p) basis sets. Vibrational assignments have been made on the basis of potential energy distribution (PED) and the thermodynamics functions, and the highest occupied and the lowest unoccupied molecular orbitals (HOMO and LUMO) of MAAP have been predicted. Calculations are carried out with the possible seven (amide1–5 and imide12) conformational isomers of MAAP. Comparison between the experimental and theoretical results indicates that the B3LYP method provides satisfactory evidence for the prediction of vibrational wavenumbers, and the amide1 conformational isomer is supposed to be the most stable form of MAAP.
1. Introduction
The MAAP, which has a electron rich aromatic ring, was first synthesized using antipyrine, a hydroxy radical capture and spectroscopic reagent for phenols, and used for a method described for removal of phenolic compounds with nitrophenol imprinted polymer based on  and hydrogen bonding interactions [1]. Then, it was used in analyzing the selective binding behavior of paraoxon and parathion compounds on surface imprinted polymers prepared using both charge transfer (MAAP) and ligandexchange (MAAPGd) monomers [2]. Today, one can easily reach many papers published where it is one of the most important organic chemical product for the molecularly imprinted polymers [1–5]. For instance, it has been used in researching the biomimetic immunoassay based on molecularly imprinted polymer [3], the 8Hydroxy2′deoxyguanosine (8OHdG), which is one of the most abundant oxidative DNA lesions resulting from reactive oxygen species, and the 8OHdG level in blood serum from a breast cancer patient [4], and new 8OHdG imprinted quartz crystal microbalance sensor [5]. Furthermore, it is taken part in adsorption of phenol and its derivatives from water using synthetic resins and lowcost natural adsorbents [6].
Vibrational spectroscopy has been widely used as the standard tool for structural characterization of molecular systems together with DFT calculations [7–14]. DFT is popular as it is a costeffective procedure for studying the physical properties of molecules. Unlike the Hartree Fock theory, DFT recovers electron correlation in the selfconsistent KohnSham procedure through the functions of electron density, so it is a costeffective and reliable method. The DFT/B3LYP model exhibits good performance particularly on vibrational frequencies and geometries of organic compounds [7–14].
Though MAAP has wide applications in science, to the best of our knowledge, there is limited information available in the literature about its spectroscopic properties. A detailed, quantum chemical study will be useful in making assignments to the fundamental normal modes and in explaining the obtained experimental data of MAAP. Furthermore, theoretically and experimentally presented data may be helpful in the context of the further studies of MAAP. For the above goals, we have reported vibrational spectra of MAAP. Vibrational frequencies with PED values, HOMO and LUMO data, structural parameters, and some thermodynamics functions of MAAP are also calculated for the most stable conformational isomer of MAAP by means of B3LYP/631G^{++}(d,p) level. The results of the theoretical and spectroscopic studies are reported here.
2. Experimental
MAAP was prepared according to the published procedure [1] and used without further purification. FTMIR and FIR spectra were recorded in the region of 4000–400 cm^{−1} and 400–10 cm^{−1} with Bruker Optics IFS66v/s FTIR spectrometer at a resolution of 2 cm^{−1}. Raman spectrum was obtained using a Bruker Senterra Dispersive Raman microscope spectrometer with 785 nm excitation from a 3B diode laser having 3 cm^{−1} resolution in the spectral region of 4000–100 cm^{−1}.
3. Calculations
All the calculations were performed using Gaussian 09.A1 program [15] on HP DL380G7 server system, and GaussView 5.0.8 [16] was used for visualization of the structure and simulated vibrational spectra. Many possible conformational isomers could be proposed for MAAP; however, the five amide and two imide conformational isomers of MAAP (Figure 1) were chosen using HF/631G(d,p) level by Spartan 10 program [17]. For these calculations, the possible seven conformational isomers of MAAP were first optimized in the gas phase at B3LYP level of theory using 631G^{++}(d,p) basis set, and no geometric restrictions were applied. Its amide1 conformational isomer was found more stable than the others (Figure 1). According to the theoretical calculations, the vibrational frequencies and assignments of the two conformational isomers amide1 and amide2 are about the same. So, it has been given the data for one of them.
Therefore, after the optimization, harmonic vibrational frequencies and corresponding vibrational intensities for the amide1 conformational isomer of MAAP were calculated by using the same method and basis set and then scaled by 0.955 (above 1800 cm^{−1}) and 0.977 (under 1800 cm^{−1}) [10, 13]. In order to show the relative contributions of the redundant internal coordinates to each normal vibrational mode of the molecule, PED calculations were carried out by the vibrational energy distribution analysis 4 (VEDA) [18].
4. Results and Discussion
4.1. 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 differences in frequencies. Gibbs free energy and relative stability of the optimized geometries in gas phase for seven conformational isomers of MAAP with B3LYP/631G^{++}(d,p) method are given in Figure 1. Regarding the calculated free energies, all conformational isomers relative to the amide1 and amide2 forms of MAAP could be neglected for the calculation of equilibrium constant since their energy differences are larger than 2 kcal/mol [10–14]. The amide1 conformational isomer of MAAP is more stable than amide2 by 0.03 kcal/mol. Consequently, MAAP in the gas phase prefers the amide1 and amide2 conformational isomers with preference of 51% and 49%, respectively.
Some of the optimized geometric parameters such as bond lengths and bond angles calculated by B3LYP/631G^{++}(d,p) are listed in Table 1 for the most stable conformational isomer (amide1) of MAAP. To the best of our knowledge, experimental data on the geometric structure of MAAP is not available in the literature. However, some geometric parameters for 4([(1E)(2hydroxynaphthyl)methylidene]amino)1,5dimethyl2phenyl2,3dihyrdo1Hpyrazol3one and pmethacryloylaminophenylarsonic acid monomer were identified in previously reported studies [19, 20]. Henceforth, the theoretical results have been compared with data for some parts of the related molecules as given in Table 1.

Generally, it is expected that the bond distances calculated by electron correlated methods are longer than the experimental distance. This situation can be seen in Table 1 as expected. Overall, the calculated bond lengths are in good agreement with experimental results. The mean absolute deviation (MAD) and root mean square deviation (RMSD) of bond lengths are 0.022 and 0.032 Å, respectively. The biggest difference between the experimental and calculated bond distances is 0.061 Å. As can be seen in Table 1, big differences are observed in the calculated C6–N5–C7, C2–C1–N20 and C7–C1–N20 bond angles compared to experimental values. The calculated C2–C1–N20 and C6–N5–C7 bond angles are about 11° smaller than the experimental result whereas the calculated C7–C1–N20 angle is about 11° larger than the experimental value. All the other bond angles are reasonably close to the experimental data.
The MAD and RMSD of bond angles are 3.15 and 4.96°, respectively. The observed differences in bond distances and angles are not due to the theoretical shortcomings, as experimental results are also subject to variations owing to the insufficient data to calculate equilibrium structure which are sometimes averaged over zero point vibrational motion. Furthermore, it can be noted that theoretical results have been compared with available experimental data.
For the optimized geometric parameters, magnitude of dihedral angles, D (10; 11; 12; 13) = 0.70°, D (1; 7; 4; 5) = 177.49°, D (8; 7; 5; 4) = 178.46°, D (20; 1; 2; 4) = 178.68°, D (1; 20; 15; 16) = 165.90°, D (17; 16; 15; 18) = 175.60°, D (14; 9; 5; 4) = 21.60°, D (1; 2; 3; 7) = 173.40°, D (15; 16; 18; 20) = 17.85°, D (1; 20; 15; 19) = 15.35°, and D (15; 16; 17; 19) = 13.24°, indicate that a large part of molecule is nearly in the same plane. In this compound, two intramolecular hydrogen bonds can be observed as N20H36⋯O19 and N20H36⋯O3. Theoretical results are N20H36: 3.17 Å/N20H36O19: 25.1° and N20H36: 2.52 Å/N20H36O19: 100.6°. The complete set of optimized geometric parameters for the amide1 conformational isomer of MAAP is given in Table 3. Several thermodynamic parameters (zero point energy, entropy, etc.) calculated by B3LYP/631G^{++}(d,p) method are also presented in Table S1 (see Supplementary Material available online at http://dx.doi.org/10.1155/2013/386247). The total energy and change in total entropy for the amide1 conformational isomer of MAAP are at room temperature.
4.2. Vibrational Studies of MAAP
To the best of our knowledge, the vibrational wavenumbers and assignments of MAAP in the middle and far infrared regions of the spectrum have not been reported in the literature. But, a few selected bands of MAAP were reported by Ersöz et al. [1]. The measured and calculated vibrational frequencies for MAAP along with corresponding vibrational assignments and intensities are given in Table 2.
 
: unscaled wavenumbers, : scaled with 0.955 above 1800 cm^{−1} and 0.977 under 1800 cm^{−1}. IR and R: calculated infrared and Raman intensities. PED data are taken from VEDA4. v: very, s: strong, m: medium, w: weak, b: broad, and sh: shoulder. , , , and denote stretching, bending, torsion, and out modes, respectively. 

The theoretical and experimental vibrational spectra of MAAP are shown in Figure 2. All calculated frequency values presented in this paper are obtained within the harmonic approximation. This allows us to identify vibrational motion in terms of independent vibrational modes, each of them is governed by a simple onedimensional harmonic potential. It is difficult to determine the MAAP’s vibrational assignments in the observed spectrum due to its low symmetry. Therefore, the assignments of vibrational modes for the amide1 conformational isomer of MAAP have been provided by VEDA 4 [18] in Table 2. The MAAP molecule consists of 37 atoms having 105 normal vibrational modes, and its most stable form belongs to the point group C_{1} with only identity (E) symmetry element or operation. According to the calculations, 18 normal vibrational modes of MAAP are below 400 cm^{−1} while 87 modes are between 4000 cm^{−1} and 400 cm^{−1}.
(a)
(b)
(c)
(d)
The high wavenumber region contains characteristic wavenumbers of NH stretching that are observed at 3249 cm^{−1} (IR) and at 3254 cm^{−1} (R) as strong broad band. The NH infrared band is consistent with previously reported data for MAAP [1] where this band was found as 3260 cm^{−1}. The corresponding scaled theoretical value for this mode is 3426 cm^{−1}. The amide band is the most intense and is predominantly C=O stretch supports also the amide form of MAAP. Hydrogen bonding plays an important role in the broadening of the spectral bands [21]. The broadening of the NH bands is attributed to the intramolecularly bonded hydrogen to the C=O group. The CH stretching region encompasses four and six CH modes in IR and Raman spectrum, respectively. The vibrational bands at 3094, 3059 cm^{−1} in IR spectrum and 3080, 3044, and 3005 cm^{−1} in Raman spectrum are attributed to the CH stretching vibrations while the bands at 2969, 2919 cm^{−1} in IR spectrum and 2968, 2934, and 2863 cm^{−1} in Raman spectrum are assigned to the CH_{2} and CH_{3} stretchings. Corresponding scaled calculated values for these bands are found as 3090, 3073, 3062, 3040, 3012, 2971, 2914, and 2892 cm^{−1}. In the high wavenumber region of the spectra, the anharmonicity can explain substantial differences between the experimental and calculated values. Alternatively, these differences may be due to intra/intermolecular interactions or to the laser used for Raman.
Carbonyl stretchings are observed at 1673 cm^{−1}—IR (1683 cm^{−1}—R) and at 1648 cm^{−1}—IR (1631 cm^{−1}—R) as intense bands in the vibrational spectra. The former band is clearly assigned to the carbonyl band at cyclic ketone position, whereas the latter is attributed to the amide carbonyl band. The corresponding scaled theoretical values of these modes are 1703 cm^{−1} and 1694 cm^{−1}. C=C stretchings are observed at 1620 cm^{−1}—IR and at 1592 cm^{−1}—IR (1605 and 1570 cm^{−1}—R) as a very strong band in the vibrational spectra. The former band is assigned to the methacrylate double band, whereas the latter is attributed to the C=C band of rings. The corresponding theoretical values of these modes are 1652 cm^{−1} and 1609 cm^{−1} (1595 cm^{−1}).
The CN stretching modes of the amide group are observed at 1491 cm^{−1} (IRvs), 1295 cm^{−1} (IRvs), 1285 cm^{−1} (IRvs), 1245 cm^{−1} (Rw,sh), and 1060 cm^{−1} (IRm) while the present theoretical values are 1520 cm^{−1}, 1263 cm^{−1}, and 1058 cm^{−1}. The CN and NN stretch vibrations at cyclic ketone position are observed at 1368 cm^{−1} (IRs), 1368 cm^{−1} (Rvs), 1200 cm^{−1} (IRs), and 1211 cm^{−1} (Rm) while the present theoretical values are 1339 cm^{−1}, 1205 cm^{−1}, and 1191 cm^{−1}. CC or NC stretching, CCC, CCN, CNC or HCC bending, some torsion, and out modes dominate the regions of 1000–500 cm^{−1} while CCC, CCN, CNC, or CNN bending and CCCN, CCNC, CNCC, CCNN, HCCC, or CCCC torsion modes are seen in the low frequency region. Similar situations have been shown in for calculations. Vibrational modes in the low wavenumber region of the spectrum contain contributions of several internal coordinates and their assignments have reduction approximation to one of two of the internal coordinates.
To make a comparison between the experimental and theoretical frequencies, we have calculated RMSD. This is used to measure the difference between values predicted by a model and those actually observed from the thing being modeled. In this study, RMSD values have been obtained as 29 cm^{−1} (IR) and 32 cm^{−1} (R). Omitting the stretch from the analysis results substantially reduced RMSD values as 15 cm^{−1} (IR) and 17 cm^{−1} (R). Furthermore, the correlation values between the experimental and calculated vibrational frequencies are found to be as 0.99898 (IR) and 0.99889 (R). Similarly, omitting the stretch from the analysis results substantially reduced correlation values as 0.99963 (IR) and 0.99960 (R). It can be seen that the B3LYP calculation is reliable for both IR and Raman spectra.
Regarding the calculated fundamentals, in general, the computed vibrational intensities are in agreement with the experimental results. Although the calculated vibrational intensities of some modes are about zero, these bands can be seen in the vibrational spectra. The opposite situation has also been observed. Similarly, the and modes are observed as double peaks in the infrared spectrum which can be attributed to molecular interactions (Figure 2). It is also important to note that theoretical studies were conducted for an isolated molecule in the gaseous state contrary to the experimental values recorded for the presence of interactions in the solid phase.
The HOMO and LUMO orbitals are called the frontier orbitals and determine the way the molecule interacts with other species. The HOMO is the orbital that could behave as an electron donor, since it is the outermost orbital containing electrons. The LUMO is the orbital that could act as the electron acceptor, as it is the innermost orbital that has room to accept electrons. The transitions can be described from HOMO to LUMO. A single orbital, however, may be both the LUMO and the HOMO. The HOMO is dominated by nitrogen, oxygen, and carbon atoms. The LUMO is located over all atoms except from some CH_{3} and NH groups in MAAP. Frontier molecular orbital and their orbital energy are shown in Figure 3 together with the HOMOLUMO gap. The energy gap given from the ground state to first excited state is calculated at around 4.66 eV. The laser used for Raman analysis in the present study has an energy around half of this gap. Therefore, electronic excitement due to Raman laser seems unlikely.
5. Conclusion
The experimental and theoretical vibrational investigations of MAAP are performed, for the first time, by using FTIR, Raman, and quantum chemical calculations. In conclusion, the following results can be summarized.(1)Results of energy calculations for gas phase indicate that the amide1 conformational isomer is the most stable conformer of MAAP. Furthermore, relative energies of the other five conformational isomers, except for the amide2, are larger than 2.0 kcal/mol. Therefore, relative mole fractions of the five forms could be neglected, and these results suggest that the MAAP molecule prefers the amide1 and amide2 conformational isomers with preference of 51% and 49%, respectively, and the conformational energy barrier is independent of the imide form.(2)The RMSD and correlation values between the experimental and calculated vibrational frequencies indicate that B3LYP/631G^{++}(d,p) method is reliable, and this theoretical approximation makes the understanding of vibrational spectrum of MAAP easier.
Conflict of Interests
The authors declare that no conflict of interests exists.
Supplementary Materials
Table S1. “Several thermodynamic parameters of amide1 conformational isomer of MAAP.”
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Copyright © 2013 Özgür Alver et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.