Accurate scaling of the vibrational spectra of aniline and several derivatives (original) (raw)
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The structure of aniline was studied by semiempirical, ab initio, and density functional methods. Complete geometry optimization of the minimum energy structure and of the transition states for internal rotation and inversion of the amino group was carried out using several levels. The performance of the different methods in calculating and describing the vibrational frequencies of aniline was determined. The normal modes were characterized by the magnitudes and direction of the displacement vectors. Three procedures were used to obtain the scaled frequencies, two of them new, using specific scale factors and scaling equations from the benzene molecule. The errors obtained were compared with those calculated through other standard procedures. A reassignment of several bands was made. A comparison of the cost-effective method and procedure of scaling was carried out.
The structure of aniline by ab initio studies
Journal of Molecular Structure: THEOCHEM, 1993
The structure of aniline has been studied by ab initio calculations. Complete geometry optimization of (1) the energy minimum structure and the transition states for (2) internal rotation and (3) inversion of the amino group were carried out at the SCF level using several different basis sets. For these three stationary geometries vibrational frequencies were calculated at the SCFf6-31G** level. The effect of electron correlation was estimated by single point MP4(SDQ) ~culations using the 6-311G ** basis set. To ~tisfacto~ly describe the confo~ation and orientation of the amino group a fully polarized (631G**) basis set is required. It is predicted that the aniline molecule has a pyramidal amino group with an angle between the C-N bond and the NH, plane of 42.3'. The angle between the C-N bond and the plane of the benzene ring is 2.0'. The barriers to inversion and internal rotation of the amino group are estimated to be 1.7 and 3.7 kcal mol-' respectively.
Infrared, Raman spectra and DFT calculations of chlorine substituted anilines
Vibrational Spectroscopy, 2006
The infrared and Raman spectra of chlorine substituted anilines (2,5-dichloroaniline, 2,4-dichloroaniline and 3,5-dichloroaniline) have been measured in the region of 4000-600 cm À1 . FAR infrared spectrum of 2,5-dichloroaniline has been also presented. Vibrational frequencies, rotational constants and energies of these molecules have also been calculated by means of quantum chemical calculations using density functional theory and Restricted Hartree-Fock method with 6-31+G* and 6-311++G** basis sets. An assignment of normal modes of vibration to the observed frequencies has been based on these calculations. Non-co-planarity of amino group with respect to the phenyl ring has been investigated. To test the reliability of the computational methods, a series of calculations has been done for aniline to substituted anilines using different basis sets. #
2016
The optimized molecular structure, vibrational frequencies, corresponding vibrational assignments and thermodynamic properties of N-(4-methoxybenzylidine) aniline (N4MBA) have been investigated by using ab initio HF/6-311++G(d,p) and DFT/B3LYP method at 6-311G(d,p) and 6-311++G(d,p) basis sets. The energy and oscillator strength calculated by TD-DFT are in line with experimental findings. Moreover, we have not only simulated HOMO and LUMO, but also determined the energy band gap. The stability of the molecule arising from hyperconjugative interaction and charge delocalization has been analyzed using natural NBO analysis. Besides, Mulliken charges were also calculated. IR and Raman intensities were calculated and TED also has been reported. © 2016 Elixir All rights reserved. Elixir Vib. Spec. 91 (2016) 38039-38046 Vibrational Spectroscopy Available online at www.elixirpublishers.com (Elixir International Journal) F.Liakath Ali Khan et al./ Elixir Vib. Spec. 91 (2016) 38039-38046 3804...
The Journal of Chemical Physics, 2003
Comprehensive studies of the molecular and electronic structures, vibrational frequencies, and infrared and Raman intensities of the aniline radical cation, C 6 H 5 NH 2 ϩ have been performed by using the unrestricted density functional ͑UB3LYP͒ and second-order Møller-Plesset ͑UMP2͒ methods with the extended 6-311ϩϩG͑df,pd͒ basis set. For comparison, analogous calculations were carried out for the closed-shell neutral aniline. The studies provided detailed insight into the bonding changes that take place in aniline upon ionization. The natural bond orbital ͑NBO͒ analysis has revealed that the p-radical conjugative interactions are of prime importance in stabilizing the planar, quinoid-type structure of the aniline radical cation. It is shown that the natural charges calculated for aniline are consistent with the chemical properties of this molecule ͑an ortho-and para-directing power of the NH 2 group in electrophilic substitutions͒, whereas Mulliken charges are not reliable. The theoretical vibrational frequencies of aniline, calculated by the B3LYP method, show excellent agreement with the available experimental data. In contrast, the MP2 method is deficient in predicting the frequencies of several modes in aniline, despite the use of the extended basis set in calculations. The frequencies of aniline radical cation, calculated at the UB3LYP/ 6-311ϩϩG͑df,pd͒ level, are in very good agreement with the recently reported experimental data from zero kinetic energy photoelectron and infrared depletion spectroscopic studies. The clear-cut assignment of the IR and Raman spectra of the investigated molecules has been made on the basis of the calculated potential energy distributions. Several bands in the spectra have been reassigned. It is shown that ionization of aniline can be easily identified by the appearance of the very strong band at about 1490 cm Ϫ1 , in the Raman spectrum. The redshift of the N-H stretching frequencies and the blueshift of the C-H stretching frequencies are observed in aniline, upon ionization. As revealed by NBO analysis, the frequency shifts can be correlated with the increase of electron density ͑ED͒ on the antibonding orbitals ( NH * ) and decrease of ED on CH * , respectively. These effects are associated with a weakening of N-H bonds and strengthening of C-H bonds in the aniline radical cation. The simulated theoretical Raman and infrared spectra of aniline and its radical cation, reported in this work, can be used in further spectroscopic studies of their van der Waals clusters and hydrogen bonded complexes.
In this study, 4-Methoxy-N-(3-phenylallylidene) aniline has been synthesized and characterized by FTIR and NMR spectroscopic techniques. The optimized geometrical structure, vibrational frequencies and NMR shifts of title molecule were obtained by using ab initio HF and density functional method (B3LYP) with 6-31G* basis set. The experimental and calculated geometrical parameters were compared with each other. The calculated infrared (IR) and NMR data were compared with experimental values using HF and B3LYP/6-31G* level of theory. It was found to be a good correlation between experimental and calculated data. In addition, HOMO and LUMO analysis of title molecule were calculated using corresponding methods with 6-31G* basis set. The calculated HOMO-LUMO energies were used to calculate some properties of title molecule.
Structure and vibrational spectra of benzidine
Journal of Molecular Structure, 2003
The geometry and vibrational spectrum of benzidine have been computed by ab initio calculations using the DFT/B3LYP method with 6-31 þ G(d,p) basis set. In the most stable geometry, the dihedral angle between the two phenyl rings was found to be around 388. Calculated wavenumbers were scaled by a single factor 0.965 to approximately correct for vibrational anharmonicity as well as for overestimation of the force constants. Normal coordinate analysis of benzidine and some of its deuterated derivatives have also been performed in valance force field approximation in order to demonstrate the transferability of the force field of aniline. Good agreements between the two different calculation results (ab initio and force field refinement methods) and between the calculated and observed values are found. q
International Journal of Quantum Chemistry, 2006
Geometry, vibrational wavenumbers, and several thermodynamic parameters have been calculated using ab initio quantum chemical methods for the 3‐aminobenzonitrile molecule for the first time. The results were compared with experimental values. With the help of specific scaling procedures, the observed vibrational wavenumbers were analyzed and assigned to different normal modes of the molecule. In general, the error obtained was very low. Using potential energy distribution (PED), the contributions of the different modes to each wavenumber were determined. Other general conclusions were also deduced. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006