Simulation of IR and Raman spectral based on scaled DFT force fields: a case study of 2-amino 4-hydroxy 6-trifluoromethylpyrimidine, with emphasis on band assignment

This work deals with the vibrational spectroscopy of 2-amino 4-hydroxy 6-triflouromethylpyrimidine (AHFMP) by means of quantum chemical calculations. The mid and far FTIR and FT-Raman spectra were measured in the condensed state. The fundamental vibrational frequencies and intensity of vibrational bands were evaluated using density functional theory (DFT) with the standard B3LYP/6-31G* and B3LYP/6-311+G** method and basic set combinations. Normal co-ordinate calculations were performed with the DFT force field corrected by a recommended set of scaling factors yielding fairly good agreement between observed and calculated frequencies. Simulation of infrared and Raman spectra utilizing the results of these calculations led to excellent overall agreement with the observed spectral patterns. The SQM approach applying selective scaling of the DFT force field was shown to be superior to the uniform scaling method in its ability to allow for making modifications in the band assignment, resulting in more accurate simulation of IR and Raman Spectra.     

DFT and ab initio quantum chemical studies on p-cyanobenzoic acid

The Fourier transform infrared (FTIR) and FT-Raman spectra of p-cyanobenzoic acid (CBA) have been recorded in the range 4000-400 and 4000-100 cm(-1), respectively. The complete vibrational assignment and analysis of the fundamental modes of the compound were carried out using the observed FTIR and FT-Raman data. The vibrational frequencies determined experimentally were compared with theoretical wavenumbers obtained from ab initio HF and DFT-B3LYP gradient calculations employing 6-31G**, 6-311++G** and cc-pVTZ basis sets for the optimised geometry of the compound. The geometry and normal modes of vibration obtained from the HF and DFT methods are in good agreement with the experimental data. The normal coordinate analysis was also carried out with ab initio force fields utilising Wilson's FG matrix method. The interactions of cyano and carboxylic acid groups with the skeletal vibrational modes were investigated.     

Structures and vibrational frequencies of 2-hydroxy-3-methoxy-5-nitrobenzaldehyde and 2-methoxy-1-naphthaldehyde based on density functional theory calculations

FT-IR and FT-Raman spectra of 2-hydroxy-3-methoxy-5-nitrobenzaldehyde (HMN) and 2-methoxy-1-naphthaldehyde (MN) have been recorded in the regions 4000-400 cm(-1) and 3500-100 cm(-1), respectively. The molecular structure, conformational stability, geometry optimization, vibrational frequencies have been investigated. The total energy calculations of HMN and MN were tried for various possible conformers. The spectra were interpreted with the aid of normal coordinate analysis based on density functional theory (DFT) using B3LYP/6-31G* and B3LYP/6-311+G** level and basis set combinations and was scaled using various scale factors yielding good agreement between observed and calculated frequencies. The infrared and Raman spectra were also predicted from the calculated intensities. Comparison of the simulated spectra with the experimental spectra provides important information about the ability of the computational method to describe the vibrational modes.     

Density functional and experimental studies on the FT-IR and FT-Raman spectra and structure of 2,6-diamino purine and 6-methoxy purine

FT-IR and FT-Raman spectra of 2,6-diamino purine (DAP) and 6-methoxy purine (MP) have been recorded in the regions of 4000-400cm(-1) and 3500-100cm(-1), respectively. The spectra were interpreted with the aid of normal coordinate analysis following full structure optimizations and force field calculations based on density functional theory (DFT) using standard B3LYP/6-31G* and B3LYP/6-311+G** method and basis set combinations. Normal coordinate calculations were performed with the DFT force field corrected by a recommended set of scaling factors yielding fairly good agreement between observed and calculated frequencies.     

Crystal structure and density functional theory study on structural properties and energies of a isonicotinohydrazide compound

An X-ray and a theoretical study of the structure of the isoniazid derivative N'-(4-dimethylaminobenzylidene)-isonicotinohydrazide monohydrate (1) are reported. In this work, we will report a combined experimental and theoretical study on the molecular structure, vibrational spectra and energies of N'-(4-dimethylaminobenzylidene)-isonicotinohydrazide monohydrate. The calculated parameters are in good agreement with the corresponding X-ray diffraction values. The FTIR spectrum in the range of 400-4000 cm-1 of N'-(4-dimethylaminobenzylidene)-isonicotinohydrazide monohydrate has been recorded. The molecular geometry and vibrational frequencies and energies in the ground state are calculated by using the DFT (B3LYP, PBE1PBE) methods with 6-311G** basis sets. The calculated HOMO and LUMO energies also confirm that charge transfer occurs within the molecule. The geometries and normal modes of vibrations obtained from B3LYP/PBE1PBE/6-311G** calculations are in good agreement with the experimentally observed data.     

Density functional theory study on the structure and vibrational spectra for cyanuric chloride

The molecular structure and vibrational spectra of cyanuric chloride have been investigated by density functional theory (DFT) using standard B3LYP/6-31G* and B3LYP/6-311+G** method and basis set combinations. The DFT force field transformed to natural internal coordinates was corrected by a well-established set of scale factors that were found to be transferable to the title compound. Both the calculated structural parameters and vibrational frequencies are in good agreement with the available experimental data.     

Experimental and density functional theory and ab initio Hartree-Fock study on the vibrational spectra of 2-(4-fluorobenzylideneamino)-3-mercaptopropanoic acid

2-(4-Fluorobenzylideneamino)-3-mercaptopropanoic acid (4-FC) was synthesized through the reaction of 4-fluorobenzaldehyde and l-cysteine in refluxing EtOH. Its structure was verified by (1)H NMR, FT-IR and Raman. The ground-state geometries were optimized at B3LYP/6-31G**, B3LYP/6-31G*, HF/6-31G** and HF/6-31G* levels without symmetry constrains, respectively. The vibrational wavenumbers of 4-FC were calculated at same level. The scaled theoretical spectra using B3LYP methods, which are in a good agreement with the experimental ones, are superior to those using HF methods.     

Synthesis, characterization, crystal structure and DFT studies on 3-(1H-benzo[d][1,2,3]triazol-1-yl)-1-oxo-1-m-tolylpropan-2-yl-nicotinate

3-(1H-Benzo[d][1,2,3]triazol-1-yl)-1-oxo-1-m-tolylpropan-2-yl-nicotinate (BOTN) has been synthesized and characterized by elemental analysis, IR, UV-vis and fluorescence spectroscopy. Its crystal structure has also been determined by X-ray single crystal diffraction. For BOTN, density functional theory (DFT) calculations of the structure and vibrational frequencies have been performed at B3LYP/6-311G** level. The comparisons between the experimental vibrational frequencies and the predicted data show that B3LYP/6-311G** method can simulate the IR of BOTN on the whole. Based on the vibration analysis, thermodynamic properties of BOTN have been calculated. The correlative equations between the thermodynamic properties and the temperatures have also been listed. The experimental UV-vis spectra present two peaks and theoretical UV-vis spectra obtained by TD-DFT method exhibit three peaks. The comparison between them suggests that the B3LYP/6-311G** method can only approximately simulate the UV-vis spectra of BOTN. The fluorescence determination reveals two emission bands at 423 and 489 nm, respectively.     

Density functional theory study of vibrational spectra and assignment of fundamental vibrational modes of 1-methyl-4-piperidone

The FT-IR and FT-Raman spectra of 1-methyl-4-piperidone was recorded and the observed bands were interpreted with the aid of normal coordinate analysis following a full structure optimization and force field calculation based on the density functional theory (DFT) using the standard B3LYP/6-311G** method and basis set combinations. A very good agreement obtained between the simulated and experimental spectra was established and unambiguous vibrational assignments of various modes were proposed based on the results of potential energy distribution (PED) calculations.     

Structure and vibrational assignment of the enol form of 1,1,1-trifluoro-2,4-pentanedione

Molecular structure of 1,1,1-trifluoro-pentane-2,4-dione, known as trifluoro-acetylacetone (TFAA), has been investigated by means of Density Functional Theory (DFT) calculations and the results were compared with those of acetylacetone (AA) and hexafluoro-acetylacetone (HFAA). The harmonic vibrational frequencies of both stable cis-enol forms were calculated at B3LYP level of theory using 6-31G** and 6-311++G** basis sets. We also calculated the anharmonic frequencies at B3LYP/6-31G** level of theory for both stable cis-enol isomers. The calculated frequencies, Raman and IR intensities, and depolarization ratios were compared with the experimental results. The energy difference between the two stable cis-enol forms, calculated at B3LYP/6-311++G**, is only 5.89 kJ/mol. The observed vibrational frequencies and Raman and IR intensities are in excellent agreement with the corresponding values calculated for the most stable conformation, 2TFAA. According to the theoretical calculations, the hydrogen bond strength for the most stable conformer is 57 kJ/mol, about 9.5kJ/mol less than that of AA and about 14.5 kJ/mol more than that of HFAA. These hydrogen bond strengths are consistent with the frequency shifts for OH/OD stretching and OH/OD out-of-plane bending modes upon substitution of CH(3) groups with CF(3) groups. By comparing the vibrational spectra of both theoretical and experimental data, it was concluded that 2TFAA is the dominant isomer.     

Vibrational spectra and potential energy distributions for 4,5-dichloro-3-hydroxypyridazine by density functional theory and normal coordinate calculations

The solid phase FT-IR and FT-Raman spectra of 4,5-dichloro-3-hydroxypyridazine have been recorded in the regions 4000-400 cm(-1) and 3500-100 cm(-1), respectively. The spectra were interpreted with the aid of normal coordinate analysis following a full structure optimization and force field calculations based on the density functional theory (DFT) using the standard B3LYP/6-31G* and B3LYP/6-311+G** method and basis set combinations. The DFT force field transformed to natural internal coordinates was corrected by a well-established set of scale factors that were found to be transferable to the title compound. The IR and Raman spectra were predicted theoretically and compared with the experimental spectra.     

Vibrational spectra and structure of 5,6-diamino uracil and 5,6-dihydro-5-methyl uracil by density functional theory calculations

The molecular vibrations of 5,6-diamino uracil and 5,6-dihydro-5-methyl uracil were investigated in polycrystalline sample, at room temperature, by FT-IR and FT-Raman spectroscopies. The spectra were interpreted with the aid of normal coordinate analysis following a full structure optimization and force field calculations based on the density functional theory (DFT) using standard B3LYP/6-31G* and B3LYP/6-311+G** methods and basis set combinations. The DFT force field transformed to natural internal coordinates was corrected by a well-established set of scale factors that were found to be transferable to the title compounds. The infrared and Raman spectra were also predicted from the calculated intensities. Comparison of the simulated spectra with the experimental spectra provides important information about the ability of the computational method to describe the vibrational modes.     

FTIR and FTRaman spectra,assignments, ab initio HF and DFT analysis of 4-nitrotoluene

In this work, the experimental and theoretical study on molecular structure and vibrational spectra of 4-nitrotoluene are studied. The FTIR and FTRaman experimental spectra of the molecule have been recorded in the range of 4000–100 cm^?1. Making use of the recorded data, the complete vibrational assignments are made and analysis of the observed fundamental bands of molecule is carried out. The experimental determinations of vibrational frequencies are compared with those obtained theoretically from ab initio HF and DFT quantum mechanical calculations using HF/6-31G (d, p), B3LYP/6-31++G* (d, p) and B3LYP/6-311++G* (d, p) methods. The differences between the observed and scaled wave number values of most of the fundamentals are very small in B3LYP than HF. The geometries and normal modes of vibrations obtained from ab initio HF and B3LYP calculations are in good agreement with the experimentally observed data. Comparison of the simulated spectra provides important information about the ability of the computational method (B3LYP) to describe the vibrational modes. The vibrations of NO₂ and CH₃ groups coupled with skeletal vibrations are also investigated.     

Scaled quantum chemical calculations and FT-IR, FT-Raman spectral analysis of 2-methyl piperazine

The vibrational spectra of 2-methyl piperazine (2MPZ) have been computed using B3LYP methodology and 6-31G* and 6-31G** basis sets. The solid phase FT-IR and FT-Raman spectra were recorded in the region 4000-400 and 3500-100 cm(-1), respectively. A close agreement was achieved between the observed and calculated frequencies by employing normal coordinate calculations. The observed and simulated spectra were found to be well comparable.     

Scaled quantum chemical calculations and FTIR, FT-Raman spectral analysis of 3,4-diamino benzophenone

The vibrational spectra of 3,4-diamino benzophenone (DABP) have been computed using B3LYP methodology and 6-31G* and 6-31G** basis sets. The solid phase FTIR and FT-Raman spectra were recorded in the region 4000-400 cm-1 and 3500-100 cm-1, respectively. A close agreement was achieved between the observed and calculated frequencies by employing normal coordinate calculations. The observed and simulated spectra were found to be well comparable.     

FTIR, Raman spectra and ab initio calculations of 2-mercaptobenzothiazole

FTIR and Raman spectra of a rubber vulcanization accelerator, 2-mercaptobenzothiazole (MBT), were recorded in the solid phase. The harmonic vibrational wavenumbers, for both the toutomeric forms of MBT, as well as for its dimeric complex, have been calculated, using ab initio RHF and density functional B3LYP methods invoking different basis sets upto RHF/6-31G** and B3LYP/6-31G** and the results were compared with the experimental values. Conformational studies have been also carried out regarding its toutomeric monomer forms and its dimer form. With all the basis sets the thione form of MBT (II) is predicted to be more stable than thiol form (I) and dimeric conformation (III) is predicted to be more stable with monomeric conformations (I) and (II). Vibrational assignments have been made, and it has been found that the calculated normal mode frequencies of dimeric conformation (III) are required for the analysis of IR and Raman bands of the MBT. The predicted shift in NH- stretching vibration towards the lower wave number side with the B3LYP/6-31G** calculations for the most stable dimer form (III), is in better agreement with experimental results. The intermolecular sulfur-nitrogen distance in N-H...S hydrogen bond was found to be 3.35 angstroms from these calculations, is also in agreement to the experimental value.     

Modeling of the influence of conformation,isotopic substitution,and aqueous environment on the vibrational spectrum of L-glutamic acid

Quantum-mechanical calculations of optimized structures, harmonic force fields, and vibrational spectra were performed for 10 L-glutamic acid conformers. The vibrational spectra were interpreted using B3LYP/6-31+G** calculations for the stablest conformer. Satisfactory agreement between the experimental and theoretical data was obtained. Vibrational frequency shifts caused by isotopic substitution of various types in the L-glutamic acid molecule were analyzed taking into account the conformational structure and the influence of water medium and molecule ionization. Isotopic tags are suggested that can be used in biochemical studies taking into account their special characteristics.     

An ab initio study of intermolecular interactions of nitromethane dimer and nitromethane trimer

Different geometries of nitromethane dimer and nitromethane trimer have been fully optimized employing the density functional theory B3LYP method and the 6-31++G** basis set. Three-body interaction energy has been obtained with the ab initio supermolecular approach at the levels of MP2/6-31++G**//B3LYP/6-31++G** and MP2/aug-cc-pVDZ//B3LYP/6-31++G**. The internal rotation of methyl group induced by intermolecular interaction has been observed theoretically. For the optimized structures of nitromethane dimer, the strength of C--H...O--N H-bond ranges from -9.0 to -12.4 kJ mol(-1) at the MP2/aug-cc-pVDZ//B3LYP/6-31++G** level, and the B3LYP method underestimates the interaction strength compared with the MP2 method, while MP2/6-31++G**//B3LYP/6-31++G** calculated DeltaE(C) is within 2.5 kJ mol(-1) of the corresponding value at the MP4(SDTQ)/6-31G**//B3LYP/6-31++G** level. The analytic atom-atom intermolecular potential has been successfully regressed by using the MP2/6-31++G**//B3LYP/6-31++G** calculated interaction energies of nitromethane dimer. For the optimized structures of nitromethane trimer the three-body interaction energies occupy small percentage of corresponding total binding energies, but become important for the compressed nitromethane explosive. In addition, it has been discovered that the three-body interaction energy in the cyclic nitromethane trimer is more and more negative as intermolecular distances decrease from 2.2 to 1.7 A.     

Experimental and density functional theory and ab initio Hartree-Fock study on the vibrational spectra of 2-(4-fluorobenzylideneamino)-3-(4-hydroxyphenyl) propanoic acid

2-(4-Fluorobenzylideneamino)-3-(4-hydroxyphenyl) propanoic acid (4-FT) was synthesized through the reaction of 4-fluorobenzaldehyde and l-tyrosine in refluxing EtOH. The structure of 4-FT was verified by measuring 1H NMR, FTIR and Raman. The ground-state geometries were optimized at B3LYP/6-31G**, B3LYP/6-31G*, HF/6-31G** and HF/6-31G* levels without symmetry constrains. The vibrational wavenumbers of 4-FT were calculated at same levels. The scaled spectra using B3LYP methods, which are in a good agreement with the measured spectra, are superior to those calculated using HF methods.