Molecular structure and vibrational spectra of indole and 5-aminoindole by density functional theory and ab initio Hartree–Fock calculations

The molecular geometry and vibrational frequencies of indole and 5-aminoindole in the ground state have been calculated by using the Hartree–Fock and density functional method with 6-311++G(d,p) basis set. The optimized geometrical parameters obtained by DFT calculations are in good agreement with the experimental values. Comparison of the observed fundamental vibrational frequencies of indole and 5-aminoindole with the calculated results by density functional and Hartree–Fock methods indicates that B3LYP is superior to the scaled Hartree–Fock approach for molecular vibrational problems. The theoretical spectrograms for FT-IR spectrum of 5-aminoindole have been constructed.     

Theoretical studies of molecular structure and vibrational spectra of melaminium citrate

The molecular geometry and vibrational frequencies of melaminium citrate in the ground state have been calculated using the Hartree-Fock (HF) and density functional method (B3LYP) with 6-31G(d) basis set. The optimized geometric bond lengths and bond angles obtained by using HF and density functional theory (DFT, B3LYP) show the best agreement with the experimental data. Comparison of the observed fundamental vibrational frequencies of melaminium citrate and calculated results by density functional B3LYP and Hartree-Fock methods indicate that B3LYP is superior to the scaled Hartree-Fock approach for molecular vibrational problems.     

Molecular structure and vibrational spectra of lepidine and 2-chlorolepidine by density functional theory and ab initio Hartree–Fock Calculations

The molecular geometry and vibrational frequencies of lepidine and 2-chlorolepidine in the ground state have been calculated by using the Hartree–Fock and density functional methods (B3LYP) with 6-31G (d) as the basis set. The optimized geometric bond lengths obtained by using B3LYP and bond angles obtained by HF that correspond to the experimental values of 2-cl-lepidine molecule were given. Comparison of the observed fundamental vibrational frequencies of lepidine and 2-chlorolepidine, and calculated results by density functional B3LYP and Hartree–Fock methods indicates that B3LYP is superior to the scaled Hartree–Fock approach for molecular vibrational problems.     

Molecular structure and vibrational spectra of N-mesylhydroxylamin and N-mesyl-O-methylhydroxylamin by density functional theory and ab initio Hartree–Fock calculations

The molecular geometry and vibrational frequencies of N-mesylhydroxylamin (N-MHN) and N-mesyl-O-methylhydroxylamin (N-MMHN) in the ground state have been calculated using the Hartree–Fock and density functional method (B3LYP) with 6-31G(d) basis set. The optimized geometric band lengths and bond angles obtained by using HF and DFT (B3LYP) show the best agreement with the experimental data. Comparison of the observed fundamental vibrational frequencies of N-MHN and N-MMHN and calculated results by density functional B3LYP and Hartree–Fock methods indicate that B3LYP is superior to the scaled Hartree–Fock approach for molecular vibrational problems.     

Vibrational spectra and molecular structure of chiral and racemic 4-phenyl-1,3-oxazolidin-2-one by density functional theory and ab initio Hartree-Fock calculations

The vibrational frequencies and molecular geometry of (R)- and (rac)-4-phenly-1,3-oxazolidin-2-one (4-POO) in the ground state have been calculated using the Hartree-Fock and density functional method (B3LYP) with 6-31G(d) basis set. The optimized geometric bond lengths are described better by HF while bond angles are reproduced more accurately by DFT (B3LYP). Comparison of the observed fundamental vibrational frequencies of (R)-POO and (rac)-4-POO and calculated results by density functional B3LYP and Hartree-Fock methods indicate that B3LYP is superior to the scaled Hartree-Fock approach for molecular vibrational problems.     

Vibrational spectroscopy [FTIR and FTRaman] investigation, computed vibrational frequency analysis and IR intensity and Raman activity peak resemblance analysis on 4-chloro 2-methylaniline using HF and DFT [LSDA, B3LYP and B3PW91] calculations

In the present study, the FT-IR and FT-Raman spectra of 4-chloro-2-methylaniline (4CH2MA) have been recorded in the range of 4000-100 cm(-1). The fundamental modes of vibrational frequencies of 4CH2MA are assigned. All the geometrical parameters have been calculated by HF and DFT (LSDA, B3LYP and B3PW91) methods with 6-31G (d, p) and 6-311G (d, p) basis sets. Optimized geometries of the molecule have been interpreted and compared with the reported experimental values for aniline and some substituted aniline. The harmonic and anharmonic vibrational wavenumbers, IR intensities and Raman activities are calculated at the same theory levels used in geometry optimization. The calculated frequencies are scaled and compared with experimental values. The scaled vibrational frequencies at LSDA/B3LYP/6-311G (d, p) seem to coincide with the experimentally observed values with acceptable deviations. The impact of substitutions on the benzene structure is investigated. The molecular interactions between the substitutions (Cl, CH(3) and NH(2)) are also analyzed.     

Vibrational frequencies and assignments for some isotopomers of urcail using a scaled ab initio force field

Vibrational frequencies and IR band intensities for 18 isotopomers of uracil, including deuterated ¹⁵N and ¹⁸O species, have been calculated using the scaled ab initio force field of Ref. 1. The results obtained are compared with available experimental data, and a number of refinements in former assignments are proposed. The good agreement between the calculated and experimental frequencies confirms the reliability of the scaled quantum mechanical-force field.     

Effects of hydration on scale factors for ab initio force constants

Experimentally measured vibrational frequencies from the polar groups of peptides in aqueous solutions do not agree with frequencies calculated from scaled quantum mechanical force fields (SQMFF) using differential scale factors developed for molecules in the vapor phase. Measured stretching frequencies for carbonyl groups are more than 50 wavenumbers lower than the calculated values. On the other hand, frequencies for non-polar groups calculated using these scale factors are relatively accurate. Our goal is to develop a SQMFF that yields accurate calculated frequencies for peptides in aqueous solutions. To this end, we have calculated scale factors for ab initio force constants for formic acid, acetic acid, and acetone using a least squares fit of calculated and experimental frequencies. We compare these scale factors with changes observed in the ab initio force constants calculated for these molecules at various states of hydration. These force constants are calculated using fully optimized geometries for these hydrated molecules using the 4-31G basis. We present a comparison of the experimental and calculated frequencies, along with their potential energy distributions, for both vapor and aqueous phases. The results indicate that scale factors can simulate the effects of solvation on molecular force constants to yield accurate scaled ab initio force fields.     

丁二酰亚胺的结构、振动频率和热力学性质计算

用密度泛函理论（ＤＦＴ）和从头计算（ａｂ ｉｎｉｔｉｏ）方法,在Ｂ３ＬＹＰ／６－３１Ｇ、Ｂ３ＬＹＰ／６－３１１Ｇ和ＭＰ２／６－３１１Ｇ水平上,全优化计算了了二酰亚胺的分子几何构型和电子结构．进行了简正振动频率分析并用校正后的频率计算了２００－６００Ｋ温度范围的标准热力学函数,对计算结果进行了比较和讨论．     

Theoretical study on gas-phase conformations and vibrational assignment of methylphenidate

The molecular geometry, the normal mode frequencies and corresponding vibrational assignments of methylphenidate in the ground state were performed by DFT/B3LYP level of theory using the 6-311++G(d, p) basis set. Harmonic vibrational frequencies were calculated. The complete assignments were performed on the basis of the total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics (SQM) method. The conformational stabilities and optimized geometrical parameters have been carried out with density functional theory with 6-311++G(d, p) basic set by the DFT/B3LYP method. The frequency calculations have been performed with DFT to study the vibrational properties and their dependence on the molecular conformation.     

Theoretical studies of molecular structure and vibrational spectra of 2-amino-5-phenyl-1,3,4-thiadiazole

The molecular geometry and vibrational frequencies of 2-amino-5-phenyl-1,3,4-thiadiazole (C8H7N3S) in the ground state has been calculated using the Hartree-Fock and density functional method (B3LYP) with 6-31G(d) basis set. The optimized geometric bond lengths and bond angles obtained by using HF and DFT (B3LYP) show the best agreement with the experimental data. Comparison of the observed fundamental vibrational frequencies of 2-amino-5-phenyl-1,3,4-thiadiazole (C8H7N3S) and calculated results by density functional B3LYP and Hartree-Fock methods indicate that B3LYP is superior to the scaled Hartree-Fock approach for molecular vibrational problems.     

Theoretical studies of molecular structure and vibrational spectra of O-ethyl benzoylthiocarbamate

O-Ethyl benzoylthiocarbamate has been synthesized and characterized by elemental analysis and FT-IR. The crystal structure was determined by X-ray diffraction analysis. Title compound crystallizes in the orthorhombic space group Pna2(1), with Z=4. Unit cell parameters a=9.941(3)A, b=9.352(3)A, c=10.962(3)A and V=1019.1(5)A(3). The molecular geometry and vibrational frequencies of O-ethyl benzoylthiocarbamate in the ground state has been calculated using the Hartree-Fock and density functional using Becke's three-parameter hybrid method with the Lee, Yang, and Parr correlation functional (B3LYP) methods with 3-21G and 6-31G(d) basis sets. The computational frequencies are in good agreement with the observed results. Comparison of the observed fundamental vibrational frequencies of O-ethyl benzoylthiocarbamate and calculated results by density functional B3LYP and Hartree-Fock methods indicate that B3LYP is superior to the scaled Hartree-Fock approach for molecular vibrational problems.     

The molecular structure and vibrational spectra of 1-amino-5-benzoyl-4-phenylpyrimidin-2(1H) by Hartree-Fock and density functional methods

The molecular structure and vibrational spectra of 1-amino-5-benzoyl-4-phenylpyrimidin-2(1H) (C(17)H(13)N(3)O(2)) have been investigated by Hartree-Fock and density functional method using standard B3LYP with 6-31G(d) basis set. The calculated results of the geometric bond lengths and bond angles obtained by using HF and DFT (B3LYP) are in very good agreement with the experimental values. Comparison of the observed fundamental vibrational frequencies of 1-amino-5-benzoyl-4-phenylpyrimidin-2(1H) (C(17)H(13)N(3)O(2)) and calculated results by density functional B3LYP and Hartree-Fock methods indicate that B3LYP is superior to the scaled Hartree-Fock approach for molecular vibrational problems.     

Experimental (FT-IR, FT-Raman) and theoretical (HF and DFT) investigation and HOMO and LUMO analysis on the structure of p-fluoronitrobenzene

FT-IR and FT-Raman spectra of p-fluoronitrobenzene (FNO(2)C(6)H(4)) have been recorded in the region 4000-100 cm(-1). In this work, the experimental and theoretical spectra of p-fluoronitrobenzene (p-FNBz) are studied. The molecular geometry and vibrational frequencies are calculated in the ground state of molecule using ab initio Hartree-Fock (HF) and DFT (B3LYP and LSDA) methods with 6-31++G(d,p) and 6-311++G(d,p) basis sets. The computed values of frequencies are scaled to yield good coherence with observed values by using suitable factor. The complete assignments are performed on the basis of the total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics (SQM) method. The observed and calculated frequencies are found to be in very good agreement. The alteration of vibration bands due to the substitutions at the first and fourth position of the skeletal ring is also investigated from their characteristic region of linked spectrum. A study on the electronic properties, such as absorption wavelengths, excitation energy, dipole moment and frontier molecular orbital energies, are performed by time dependent DFT (TD-DFT) approach. The electronic structure and the assignment of the absorption bands in the electronic spectra of steady compounds are discussed. The calculated HOMO and LUMO energies show that charge transfer occurs within the molecule. The thermodynamic properties of the title compound at different temperatures have been calculated in gas phase, revealing the correlations between standard heat capacities (C) standard entropies (S), standard enthalpy changes (H) and temperatures.     

FT-IR and FT-Raman investigation, computed vibrational intensity analysis and computed vibrational frequency analysis on m-Xylol using ab-initio HF and DFT calculations

The FT-IR and FT-Raman spectra of m-Xylol molecule have been recorded using Bruker IFS 66V spectrometer in the range 4000-100cm(-1). The molecular geometry and vibrational frequencies in the ground state are evaluated using the Hartree-fock (HF) and B3LYP with 6-31+G (d, p), 6-31++G (d, p) and 6-311++G (d, p) basis sets. The computed frequencies are scaled using a suitable scale factors to yield good agreement with the observed values. The HF and DFT analysis agree well with experimental observations. Comparison of the fundamental vibrational frequencies with calculated results by HF and B3LYP methods indicate that B3LYP/6-311++G (d, p) is superior to HF/6-31+G (d, p) for molecular vibrational problems. The complete data of this title compound provide some useful information for the study of substituted benzenes. The influences of Methyl groups on the geometry of benzene and its normal modes of vibrations have also been discussed.     

Density functional theory study on the structure and vibrational frequencies of glycylglycine

Eleven possible conformers of glycylglycine have been studied by using the BLYP, B3LYP methods of density functional theory and the HF method at the basis set of 6-311++G**. BLYP (using Becke's and Lee-Yang-Parr's correlation functionals), ab initio Hartree-Fock (HF) and hybrid DFT/HF B3LYP calculations have been carried out to study the structure and vibrational spectra of glycylglycine. Glycylglycine crystal structure has been determined by X-ray diffraction analysis. The title compound has been crystallizes in the orthorhombic space group C1, with Z=4. And the unit cell parameters are: a=8.1184(12)A, b=9.5542(14)A, c=7.8192(11)A and V=577.95(15)A(3). Molecular conformation calculations have got 11 possible conformers. In these possible conformers, the most stable one has been selected. The BLYP/6-311++G** and scaled HF/6-311++G** frequencies correspond well with available experimental assignments of the normal vibrational modes. Comparison of the observed fundamental vibrational frequencies of glycylglycine and calculated results by density functional B3LYP and Hartree-Fock (HF) methods indicates that B3LYP is superior to the scaled Hartree-Fock (HF) for molecular vibrational issues.     

The molecular structure and vibrational spectra of 3-acetyl-4-[N-(2'-aminopyridinyl)-3-amino]-3-buten-2-one by Hartree-Fock and density functional theory calculations

The molecular structure and vibrational spectra of 3-acetyl-4-[N-(2'-aminopyridinyl)-3-amino]-3-buten-2-one (C(11)H(13)N(3)O(2)) in the ground state have been investigated by Hartree-Fock and density functional method (B3LYP and BLYP) with 6-31G(d) basis set. The optimized geometric bond lengths and bond angles obtained by using HF and DFT show the best agreement with the experimental data. Comparison of the observed fundamental vibrational frequencies of title compound and calculated results by HF and DFT methods indicate that B3LYP is superior to the scaled HF approach for molecular problems.     

An experimental and theoretical study of molecular structure and vibrational spectra of 2-chloronicotinic acid by density functional theory and ab initio Hartree–Fock calculations

In this work, the Fourier transform Raman and Fourier transform infrared spectra of 2-chloronicotinic acid (2-CNA) are recorded in the solid phase. The molecular geometry, vibrational frequencies, infrared intensities and Raman scattering activities of 2-CNA in ground state have been calculated by using ab initio Hartree–Fock (HF) and density functional (B3LYP and B3PW91) methods with 6-31G(d) and 6-311G(d) basis sets level. On the basis of the comparison between calculated and experimental results and the comparison with related molecule, assignments of fundamental vibrational modes are examined. The optimized geometric parameters (bond lengths and bond angles) obtained by using HF show the best agreement with the experimental values of 2-CNA. Comparison of the observed fundamental vibrational frequencies of 2-CNA and calculated results by density functional (B3LYP and B3PW91) and Hartree–Fock methods indicates that B3LYP is superior to the scaled Hartree–Fock and B3PW91 approach for molecular vibrational problems.     

An evaluation of scaling factors for multiparameter scaling procedures based on DFT force fields

An extended database of scaling factors for calculating fundamental frequencies within multiparameter scaled quantum mechanical (SQM) force field, and effective scaling frequency factor (ESFF) methods, based on various DFT force fields is reported. Twenty-six density functionals have been examined in conjunction with various Pople's and Dunning's basis sets of VDZ and VTZ quality. The calculations were based on a standard training set of 30 molecules proposed by Baker et al., for which 660 vibrational modes were assigned. Six functionals turned out to be particularly well-suited to the calculations oriented toward determination of scaled frequencies. They are B3LYP, B3PW91, B97, B97-1, B97-2, and O3LYP; they are all capable of providing reasonable scaled frequencies even for the small, 6-31G* basis set (rms <15 cm(-1)). Correlations between the quality of the scaled frequencies and the basis set quality as well as the accuracy of the predicted molecular geometry were investigated. The 6-311+G** basis set turned out to be preferable. In addition, correlation between the rms values in the scaled frequencies and the errors in the predicted geometric parameters was found. Both 11- and 9-parameter scaling frames are compared. It is shown that 9-parameter scaling is preferable in the middle range of the vibrational spectrum (500-2500 cm(-1)), provided it was based on high-quality force fields. Finally, statistical uncertainties of the calculated scaling factors are reported.     

Molecular structure, vibrational spectral studies of pyrazole and 3,5-dimethyl pyrazole based on density functional calculations

In this work, the experimental and theoretical vibrational spectra of pyrazole (PZ) and 3,5-dimethyl pyrazole (DMP) have been studied. FTIR and FT-Raman spectra of the title compounds in the solid phase are recorded in the region 4000-400 cm(-1) and 4000-50 cm(-1), respectively. The structural and spectroscopic data of the molecules in the ground state are calculated using density functional methods (B3LYP) with 6-311+G** basis set. The vibrational frequencies are calculated and scaled values are compared with experimental FTIR and FT-Raman spectra. The observed and calculated frequencies are found to be in good agreement. The complete vibrational assignments are performed on the basis of the total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanical (SM) method. 13C and 1H NMR chemical shifts results are compared with the experimental values.