Analysis of vibrational spectra of 4-amino-2,6-dichloropyridine and 2-chloro-3,5-dinitropyridine based on density functional theory calculations

This work deals with the vibrational spectroscopy of 4-amino-2,6-dichloropyridine (ADCP) and 2-chloro-3,5-dinitropyridine (CDNP) 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(**) methods and basis set combinations, and was scaled using various scale factors which yields a good agreement between observed and calculated frequencies. The vibrational spectra were interpreted with the aid of normal coordinate analysis based on scaled density functional force field. The results of the calculations were applied to simulated infrared and Raman spectra of the title compounds, which showed excellent agreement with the observed spectra.     

Scaled quantum chemical studies on the vibrational spectra of 4-bromo benzonitrile

The vibrational spectra of 4-bromo benzonitrile have been reported. The fundamental vibrational frequencies and intensity of vibrational bands were evaluated using density functional theory (DFT) with the standard B3LYP/6-311+G basis set combination and were scaled using various scale factors which yielded a good agreement between observed and calculated frequencies. The vibrational spectra were interpreted with the aid of normal coordinate analysis. The results of the calculations were applied to simulated infrared and Raman spectra of the title compound which showed excellent agreement with the observed spectra.     

石墨烯分子振动模式因子群分析与密度泛函计算

运用因子群分析法对石墨烯的分子振动模式进行了理论分析,得到石墨烯的分子振动模式,计算出各振动模式的光谱特性.对所建立的石墨烯晶体的布拉维单胞模型采用基于密度泛函理论的第一性原理进行分子振动频率与模式的计算,所得的振动模式数目以及各振动频率的光谱特性同因子群分析方法所得结论一致.结合上述计算结果,通过系统比较石墨与石墨烯之间的红外光谱和拉曼光谱的差别,从理论上解释了具有D6h对称的石墨烯的A2u、E1u红外活性特征振动模式没有在红外光谱中出现的原因.     

DFT studies and vibrational spectra of isoquinoline and 8-hydroxyquinoline

The geometry, frequency and intensity of the vibrational bands of isoquinoline (IQ) and 8-hydroxyquinoline (8-HQ) were obtained by the density functional theory (DFT) calculations with the B3LYP functional and 6-31 G* basis set. The vibrational spectral data obtained from the solid phase mid and far FT-IR and FT-Raman spectra of IQ and 8-HQ are assigned based on the results of the normal coordinate calculations. The observed and the calculated spectra are found to be in good agreement.     

Simulation of IR and Raman spectra of p-hydroxyanisole and p-nitroanisole based on scaled DFT force fields and their vibrational assignments

This work deals with the vibrational spectroscopy of p-hydroxyanisole (PHA) and p-nitroanisole (PNA) by means of quantum chemical calculations. The mid and far FT-IR and FT-Raman spectra were recorded 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* method and basis set combination and were scaled using various scale factors which yield a good agreement between observed and calculated frequencies. The vibrational spectra were interpreted with the aid of normal coordinate analysis based on scaled density functional force field. The results of the calculations were applied to simulate infrared and Raman spectra of the title compounds, which showed excellent agreement with the observed spectra.     

Vibrational spectroscopy of resveratrol

In this article the authors deal with the experimental and theoretical interpretation of the vibrational spectra of trans-resveratrol (3,5,4'-trihydroxy-trans-stilbene) of diverse beneficial biological activity. Infrared and Raman spectra of the compound were recorded; density functional calculations were carried out resulting in the optimized geometry and several properties of the molecule. Based on the calculated force constants, a normal coordinate analysis yielded the character of the vibrational modes and the assignment of the measured spectral bands.     

First-principles DFT study of the structural, electronic and vibrational properties of azidopentazole

In this Letter we report a density functional all-electron calculation of the structural and electronic properties of the polynitrogen high-energy molecule, azidopentazole (N₈). We have also performed a vibrational analysis to determine the IR and Raman spectra. Our calculated geometrical properties and vibrational frequencies are in good agreement with previous ab initio and density functional calculations. The weaker IR modes show a different relative ordering than previously reported. We also report calculated Raman intensities for azidopentazole.     

FT-IR, FT-Raman and UV spectral investigation; computed frequency estimation analysis and electronic structure calculations on 1-nitronaphthalene

In this work, the vibrational spectral analysis was carried out by using FT-Raman and FT-IR spectroscopy in the range 100-4000cm(-1) and 400-4000cm(-1) respectively, for 1-nitronaphthalene (C(10)H(7)NO(2)) molecule. The molecular structure, fundamental vibrational frequencies and intensity of the vibrational bands are interpreted with the aid of structure optimizations and normal coordinate force field calculations based density functional theory (DFT) and ab initio HF methods and different basis sets combination. The complete vibrational assignments of wavenumbers were made on the basis of total energy distribution (TED). The results of the calculations were applied to simulated spectra of the title compound, which show excellent agreement with observed spectra. The scaled B3LYP/6-311++G(d,p) results show the best agreement with the experimental values over the other methods. The energy and oscillator strength calculated by time-dependent density functional theory (TD-DFT) complements with the experimental findings. Thermodynamic properties of the title compound at different temperatures have been calculated. Besides, frontier molecular orbitals (FMO), molecular electrostatic potential (MEP) were performed.     

Molecular structure, vibrational spectra and NBO analysis of phenylisothiocyanate by density functional method

The molecular geometry, vibrational frequencies and NBO analysis of phenylisothiocyanate (PITC) in the ground state have been calculated by using density functional theory calculation (B3LYP) with 6-311++G(d,p) basis set. The optimized geometrical parameters obtained by DFT calculations are in good agreement with experimental values. Comparison of the observed fundamental vibrational frequencies of the PITC and calculated result by density functional theory (B3LYP) indicates B3LYP is superior for molecular vibrational problems. The entropy of the title compound was also performed at HF/B3LYP/6-311++G(d,p) levels of theory. Natural bond orbital (NBO) analysis of title molecule is also carried out. A detailed interpretation of the IR and Raman spectra of PITC is reported on the basis of the calculated potential energy distribution (PED). The theoretical spectrogram for IR spectrum of the title molecule has been constructed.     

Density functional theory studies on the structure,spectra (FT-IR,FT-Raman,and UV) and first order molecular hyperpolarizability of 2-hydroxy-3-methoxy-N-(2-chloro-benzyl)-benzaldehyde-imine: Comparison to experimental data

The infrared and Raman spectra of 2-hydroxy-3-methoxy-N-(2-chloro-benzyl)-benzaldehyde-imine (HMCBI) have been recorded and analyzed. Density functional calculations at B3LYP/6-311++G(d,p) level were carried out to study the equilibrium geometries and vibrational spectra of HMCBI. The calculations revealed that the optimized geometry closely resembled the experimental XRD data. The calculated vibrational spectra were analyzed on the basis of the potential energy distribution (PED) of each vibrational mode, which allowed us to obtain a quantitative as well as qualitative interpretation of IR and Raman spectra. The ¹H nuclear magnetic resonance (NMR) chemical shifts of the molecule in the ground state were calculated by Gauge independent atomic orbital (GIAO) method. Information about size, shape, charge density distribution and site of chemical reactivity of the molecules has been obtained by mapping electron density isosurface with electrostatic potential surface. Based on optimized ground state geometries, the NBO analysis has been done to study donor–acceptor (bond–antibond) interactions. The TD-DFT method has been used to calculate energies, oscillator strengths of electronic singlet–singlet transitions and the absorption wavelengths. Solvent effects were considered using the polarizable continuum model (PCM). Good consistency is found between the calculated results and experimental data for the electronic absorption. The calculated first hyperpolarizability may be attractive for further studies on non-linear optical properties of materials.     

Scaled quantum chemical studies of the structural and vibrational spectra of acetyl coumarin

The solid phase FT-IR and FR-Raman spectra of acetyl coumarin have been recorded in the regions 4000–50 cm⁻¹. The spectra were interpreted with the aid of normal coordinate analysis following full structure optimization and force field calculations based on density functional theory (DFT) and Hartree-Fock (HF) at 6–31G* and 6–311++G** basis sets. The resulting force fields were transformed to internal coordinates, the calculated vibrational frequencies and normal modes were utilized in the assignment of the observed vibrational fundamentals. The measured spectral data were used to refine the vibrational force constants by means of a small number of scaling factors.     

Density functional theory study of Fourier transform infrared and Raman spectra of 2-amino-5-chloro benzonitrile

The vibrational spectra of 2-amino-5-chloro benzonitrile (ACB) have been obtained by density functional theory (DFT) calculations. Normal coordinate analysis has been carried out to support the vibrational analysis. The results were compared with the experimental values. With the help of scaling procedures, the observed FTIR and FT Raman vibrational frequencies were analysed and compared with the theoretically predicted vibrational spectra. The assignments of bands to various normal modes of the molecules were also carried out.     

Vibrational assignment and structure of dibenzoylmethane. A density functional theoretical study

Molecular structure and vibrational frequencies of 1,3-diphenyl-1,3-propanedione, known as dibenzoylmethane (DBM), have been investigated by means of density functional theory (DFT) calculations. The results were compared with those of benzoylacetone (BA) and acetylacetone (AA), the parent molecule. IR and Raman spectra of DBM and its deuterated analogue were clearly assigned.The calculated hydrogen bond energy of DBM is 16.15 kcal/mol, calculated at B3LYP/6-311++G** level of theory, which is 0.28 kcal/mol more than that of AA. This result is in agreement with the vibrational and NMR spectroscopy results. The molecular stability and the hydrogen bond strength were investigated by applying the Natural Bond Orbital analysis (NBO) and geometry calculations. The theoretical calculations indicate that the hydrogen bond in DBM is relatively stronger than that in BA and AA.     

Molecular structure, spectroscopic studies and first-order molecular hyperpolarizabilities of ferulic acid by density functional study

Quantum chemical calculations of energies, geometrical structure and vibrational wavenumbers of ferulic acid (FA) (4-hydroxy-3-methoxycinnamic acid) were carried out by using density functional (DFT/B3LYP/BLYP) method with 6-31G(d,p) as basis set. The optimized geometrical parameters obtained by DFT calculations are in good agreement with single crystal XRD data. The vibrational spectral data obtained from solid phase FT-IR and FT-Raman spectra are assigned based on the results of the theoretical calculations. The observed spectra are found to be in good agreement with calculated values. The electric dipole moment (μ) and the first hyperpolarizability (β) values of the investigated molecule have been computed using ab initio quantum mechanical calculations. The calculation results also show that the FA molecule might have microscopic nonlinear optical (NLO) behavior with non-zero values. A detailed interpretation of the infrared and Raman spectra of FA was also reported. The energy and oscillator strength calculated by time-dependent density functional theory (TD-DFT) results complements with the experimental findings. The calculated HOMO and LUMO energies shows that charge transfer occur within the molecule. The theoretical FT-IR and FT-Raman spectra for the title molecule have been constructed.     

Vibrational spectra, NMR and theoretical studies of the enantiomers and rotamers of alpha-cypermethrin

NMR, infrared and Raman vibrational spectra of alpha-cypermethrin have been measured at room temperature. Infrared spectra were also recorded to low temperature. The spectra were analyzed by means of ab initio calculations. The conformational space of both enantiomers and some rotamers A, B and C of alpha-cypermethrin has been scanned using molecular dynamics and complemented with functional density calculations that optimize the geometry of the lowest-energy conformers of each species as obtained in the simulations. The vibrational frequencies and the 1H and 13C NMR chemical shifts were assigned using functional density calculations. The molecular electrostatic potential maps were obtained and analyzed.     

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.     

DFT calculation of the chromyl nitrate, CrO2(NO3)2 The molecular force field

We have carried out a structural and vibrational theoretical study for chromyl nitrate. The density functional theory has been used to study its structure and vibrational properties. The geometries were fully optimised at the B3LYP/Lanl2DZ, B3LYP/6-31G* and B3LYP/6-311++G levels of theory and the harmonic vibrational frequencies were evaluated at the same levels. The calculated harmonic vibrational frequencies for chromyl nitrate are consistent with the experimental IR and Raman spectra in the solid and liquid phases. These calculations gave us a precise knowledge of the normal modes of vibration taking into account the type of coordination adopted by nitrate groups of this compound as monodentate and bidentate. We have also made the assignment of all the observed bands in the vibrational spectra for chromyl nitrate. The nature of the Cr-O and Cr<--O bonds in the compound were quantitatively investigated by means of Natural Bond Order (NBO) analysis. The topological properties of electronic charge density are analysed employing Bader's Atoms in Molecules theory (AIM).     

Theoretical and experimental study of montmorillonite intercalated with tetramethylammonium cation

Structural and vibrational features of Na-montmorillonite and montmorillonite intercalated with tetramethylammonium cation (TMA⁺) were characterized theoretically and experimentally. Theoretical study was performed using density functional theory with inclusion of dispersion corrections. The analysis of the hydrogen bonds in the calculated models has shown that the Na⁺ cations coordinated by six water molecules (Na-M model) are bound to montmorillonite layers by moderate hydrogen bonds between water molecules and basal oxygen atoms of the tetrahedral sheets. Hydrated Na⁺ cations are stabilized by relatively strong hydrogen bonds among water molecules. In the intercalate model, the TMA⁺ cation is fixed in the interlayer space by weak hydrogen bonds between the methyl groups and basal oxygen atoms of montmorillonite layers. The calculated vibrational spectra are in a good agreement with the measured infrared spectra. The detailed analysis of the simulated vibrational spectra allowed unambiguous identification of corresponding bands in the measured spectra and their assignment to the particular vibrational modes. For example, calculations clearly distinguished between AlMgOH and AlAlOH stretching vibrations and also between the coupled vibrations of the methyl groups of the TMA⁺ cations.