Theoretical prediction of the Infrared and Raman spectra of O-ethyl S-2-diisopropylaminoethylmethylphosphonothiolate

We have extended our computations of the structure and of the infrared and Raman spectra of methylphosphonates and related compounds to the O-ethyl S-2-diisopropylaminoethylmethylphosphonothiolate molecule (we abbreviate the name to ESD). We have computed the optimized geometry and the vibrational infrared and Raman frequencies of ESD by means of the Guassian 92 Program Package using 6–31G * basis sets. We assign the vibrational frequencies and we correct each frequency by multiplying it with a previously derived 6–31G * correction factor. The result is a computer-generated prediction of the IR and Raman spectra of ESD . The agreement between our theoretical predictions and the experimental IR spectrum of ESD is surprisingly good. © 1994 John Wiley & Sons, Inc.     

Basis set effects and the choice of reference geometry in ab initio calculations of vibrational spectra

Tatewaki and Huzinaga's [J. Comput. Chem. 1 , 205 (1980)] basis sets, constructed to minimize superposition error, were used to calculate infrared (IR) frequencies and intensities. They were found inferior to Pople bases such as 3–21G and 6–31G*. The question of whether a theoretical vibrational spectrum should be computed at experimental or theoretical bond lengths was also investigated. If the magnitude of the correlation energy increases with bond length, Hartree-Fock bond lengths are expected to be shorter than experimental, and frequencies computed there will be higher than those computed at experimental lengths. Conversely, if this magnitude decreases with R, computed lengths should be longer than experimental and should give lower computed frequencies.     

Protonation of thymine,cytosine, adenine,and guanine DNA nucleic acid bases: Theoretical investigation into the framework of density functional theory

Gradient-corrected density functional computations with triple-zeta-type basis sets were performed to determine the preferred protonation site and the absolute gas-phase proton affinities of the most stable tautomer of the DNA bases thymine (T), cytosine (C), adenine (A), and guanine (G). Charge distribution, bond orders, and molecular electrostatic potentials were considered to rationalize the obtained results. The vibrational frequencies and the contribution of the zero-point energies were also computed. Significant geometrical changes in bond lengths and angles near the protonation sites were found. At 298 K, proton affinities values of 208.8 (T), 229.1 (C), 225.8 (A), and 230.3 (G) kcal/mol were obtained in agreement with experimental results. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 989–1000, 1998     

Molecular structure,vibrational spectra and nonlinear optical properties of orthoarsenic acid–tris-(hydroxymethyl)-aminomethane DFT study

In this work, we report a theoretical study on molecular structure, vibrational spectra and nonlinear optical properties of orthoarsenic acid–tris-(hydroxymethyl)-aminomethane (OATA). The theoretical geometrical parameters in the ground state have been investigated by density functional method (B3LYP and BLYP) with 6-311G(d,p) basis set. The influence of intermolecular interactions effects on molecular properties has been considered by calculation performed on (OATA) dimer. The optimized geometric bond lengths and bond angles are in well agreement with the experimental data. As compared to theoretical frequencies of the monomer, the calculated values obtained for (OATA) dimer are in much better agreement with the experiment. All experimental vibrational bands have been discussed and assigned to normal modes on the basis of our theoretical calculations. B3LYP method has shown better fit to experimental ones than BLYP in calculation vibrational frequencies. To investigate nonlinear optical behaviour, the electric dipole moment μ, the polarizability α and the hyperpolarizability β were computed using DFT//B3LYP/6-311G(d,p) method.     

C_3O_2分子结构和光谱的密度泛函理论研究

使用密度泛函理论,在B3LYP/6-31G(d)和B3LYP/6-311G(2d)水平上,研究了C_3O_2分子的可能几何构型,并在6-31G(d)水平上计算了其中2种总能量最小的构型的振动频率,同时与实验观察值进行了比较, 计算结果当C_3O_2分子具有C2v对称性的W型弯曲结构（键角C－C－C和C－C－O分别为162.3°和178.8°）时,振动频率的计算值和实验观察值非常吻合。     

硝胺及其甲基衍生物的从头计算研究 3: 甲硝胺及其同位素衍生物的谐性力场和振动光谱

用TEXAS从头计算程序,取STO-4-21G基组,计算了甲硝胺的谐性力场和振动光谱.直接理论计算的谐性力场经由其他分子转移来的经验校正因子校正后,提供了甲硝胺振动基频的预测.预测值和甲硝胺分子在气相中的振动光谱实验值之间的平均偏差为31cm^-1.为了获得更合适的气相甲硝胺振动力场和预测它的同位素衍生物的振动光谱,我们优化了一组新的校正因子,使理论值和实验值的平均偏差减为8.9cm^-1.用这组校正因子得到的力场预测了三个同位素衍生物的振动光谱,其同位素位移的理论预测值和实验值符合良好.     

Vibrational assignment of the spectral data and thermodynamic properties of 2-chloro-4-fluorobenzophenone using DFT quantum chemical calculations

The FT-Raman (3500-100 cm⁻¹) and FT-IR (4000-450 cm⁻¹) spectra of 2-chloro-4-fluorobenzophenone were recorded in the solid phase. Density functional theory calculations with B3LYP/6-31G (d, p) basis set was used to determine the ground state molecular geometries (bond lengths and bond angles), harmonic vibrational frequencies, infrared intensities and Raman activities of this compound. Potential energy distributions (PEDs) and normal modes, for the spectral data computed at B3LYP/6-31G (d, p) level, have also been obtained from force-field calculations. The wavenumbers found after scaling of the force field showed very good agreement with the experimentally determined values. A comparison of the theoretical spectra and experimental FT-IR and FT-Raman spectra of the title molecule has been made and full vibrational assignments of the observed spectra have been proposed. On the basis of vibrational analyses, the thermodynamic properties of title compound at different temperatures have been calculated.     

Vibrational absorption intensities in chemical analysis—VIII. Dimethyl sulphide

The structure and electric dipole moment of dimethyl sulphide have been calculated with a variety of basis sets. It is shown that the force fields computed with the 3-21G, 3-21G and 6-311G** bases are very similar after scaling. The vibrational assignments are discussed. The force field is compared with that of trimethyl phosphine. There is an excellent accord for the CH stretching modes, but the ab initio calculations indicate that the X-C/X-C (X = S,P) interaction terms have opposite signs. This appears to be incompatible with the experimental data. Effective atomic charges and atomic polar tensors (APT) are computed with the above bases and also with bond centred sp functions added to the 3-21G* basis. It is shown that the APT are much more sensitive to the basis set than are the effective atomic charges. The effects of the lone pair electrons on the bond electrical properties are much less than with oxy-ethers, though the overall perturbation of the sulphur atom on the intensities is greater. A number of inadequacies of the SCF calculations are identified.     

FT-IR, FT-Raman vibrational spectra and molecular structure investigation of 2-chloro-4-methylaniline: A combined experimental and theoretical study

In this work, the experimental and theoretical vibrational spectra of 2-chloro-4-methylaniline (2Cl4MA, C₇H₈NCl) were studied. FT-IR and FT-Raman spectra of 2Cl4MA in the liquid phase have been recorded in the region 4000–400 cm⁻¹ and 3500–50 cm⁻¹, respectively. The structural and spectroscopic data of the molecule in the ground state have been calculated by using Hartree-Fock (HF) and density functional method (B3LYP) with the 6-31G(d), 6-31G(d,p), 6-31+G(d,p), 6-31++G(d,p) and 6-311G(d), 6-311G(d,p), 6-311+G(d,p), 6-311++G(d,p) basis sets. The vibrational frequencies have been calculated and scaled values have been compared with experimental FT-IR and FT-Raman spectra. The observed and calculated frequencies are found to be in good agreement. 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 DFT-B3LYP/6-311++G(d,p) calculations have been found more reliable than the ab initio HF/6-311++G(d,p) calculations for the vibrational study of 2Cl4MA. The optimized geometric parameters (bond lengths and bond angles) were compared with experimental values of aniline and p-methylaniline molecules.     

Quasilinear molecule par excellence, SrCl2: structure from high-temperature gas-phase electron diffraction and quantum-chemical calculations--computed structures of SrCl2.argon complexes

The molecular geometry of strontium dichloride has been determined by high-temperature electron diffraction (ED) and computational techniques. The computation at the MP2 level of theory yields a shallow bending potential with a barrier of about 0.1 kcal mol(-1) at the linear configuration. The experimentally determined thermal average Sr--Cl bond length, r(g), is 2.625+/-0.010 A and the bond angle, angle-spherical(a), is 142.4+/-4.0 degrees . There is excellent agreement between the equilibrium bond lengths estimated from the experimental data, 2.607+/-0.013 A, and computed at different levels of theory and basis sets, 2.605+/-0.006 A. Based on anharmonic analyses of the symmetric and asymmetric stretching as well as the bending motions of the molecule, we estimated the thermal average structure from the computation for the temperature of the ED experiment. In order to emulate the effect of the matrix environment on the measured vibrational frequencies, a series of complexes with argon atoms, SrCl(2)Ar(n) (n=1-7), with different geometrical arrangements were calculated. The complexes with six or seven argon atoms approximate the interaction best and the computed frequencies of these molecules are closer to the experimental ones than those computed for the free SrCl(2) molecule.     

Spectral analysis of acetylcholine halides by density functional theory calculations

The optimized molecular structures, vibrational frequencies and ¹H and ¹³C NMR chemical shifts of acetylcholine halides (F, Cl, and Br) have been investigated using density functional theory (B3LYP) method with 6-311G(d) basis set. The comparison of their experimental and calculated IR, R and NMR spectra of the compounds has indicated that the spectra of three optimized minimum energy conformers can simultaneously exist in one experimental spectrum. Thus, it was concluded that the compounds simultaneously exist in three conformations in the ground state. The calculated optimized geometric parameters (bond lengths and bond angles), vibrational frequencies and NMR chemical shifts for the minimum energy conformers were seen to be in a good agreement with the corresponding experimental data. All the assignments of the theoretical frequencies were performed by potential energy distributions using VEDA 4 program.     

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.     

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.     

Theoretical study of [Li(H2O)n]+ and [K(H2O)n]+ (n = 1−4) complexes

The geometries, successive binding energies, vibrational frequencies, and infrared intensities are calculated for the [Li(H₂O)_n]⁺ and [K(H₂O)_n]⁺ (n = 1?4) complexes. The basis sets used are 6-31G* and LANL 1DZ (Los Alamos ECP +DZ ) at the SCF and MP 2 levels. There is an agreement for calculated structures and frequencies between the MP 2/6-31G* and MP 2/LANL 1DZ basis sets, which indicates that the latter can be used for calculations of water complexes with heavier ions. Our results are in a reasonable agreement with available experimental data and facilitate experimental study of these complexes. © 1995 John Wiley & Sons, Inc.     

Spectroscopic investigation, computed IR intensity, Raman activity and vibrational frequency analysis on 3-bromoanisole using HF and DFT (LSDA/MPW1PW91) calculations

In this work, the experimental and theoretical spectra of 3-bromoanisole (3-BA) are studied. FT-IR and FT-Raman spectra of title molecule in the liquid phase have been recorded in the region 4000-100 cm(-1). The structural and spectroscopic data of the molecule in the ground state have been calculated by using Hartree-Fock and density functional method (LSDA and MPW1PW91) with the 6-31G (d, p) and 6-311G (d, p) basis sets. The vibrational frequencies are calculated and scaled values have been compared with the experimental FT-IR and FT-Raman spectra. The observed and calculated frequencies are found in good agreement. The DFT-LSDA/6-311G (d, p) calculations have been found are more reliable than the ab initio HF/6-31G (d, p) calculations for the vibrational study of 3-BA. The optimized geometric parameters (bond lengths and bond angles) are compared with experimental values of the molecule. The alteration of vibrational bands due to the substitutions in the base molecule is also investigated from their characteristic region of linked spectrum.     

The electronic structure of 4H-pyran-4-one and its sulfur analogues

The electronic structure of 4-H-pyran-4-one and its sulfur analogues were studied using ab initio wave-functions. Bond lengths and overlap populations suggest low aromaticity for this group of compounds. Examination of Jorgensen plots of the lowest π orbitals of I--IV leads to the aromaticity order 4H-thiopyran-4-thione (IV) > 4H-thiopyran-4-one (II) > 4H-pyran-4-thione (III) > 4H-pyran-4-one (I). The effects of including d orbitals were studied using the 3-21G, 3-21G* (6d), and 3-21G* (5d) basis sets. Optimized bond lengths, vibrational frequencies, ionization energies, and dipole moments were also obtained, and results for different basis sets were compared.     

Vibrational assignment of aluminum(III) tris-acetylacetone

The geometry, frequency and intensity of the vibrational bands of aluminum(III) Tris-acetylacetone Al(AA)3 and its 1,3,5-(13)C derivative were obtained by the Hartree-Fock (HF) and Density Functional Theory (DFT) with the B3LYP, B1LYP, and G96LYP functionals and using the 6-31G* basis set. The calculated frequencies are compared with the solid IR and Raman spectra. All of the measured IR and Raman bands were interpreted in terms of the calculated vibrational modes. Most computed bands are predicted to be at higher wavenumbers than the experimental bands. The calculated bond lengths and bond angles are in good agreement with the experimental results. Analysis of the vibrational spectra indicates a strong coupling between the chelated ring modes. Four bands in the 500-390 cm(-1) frequency range are assigned to the vibrations of metal-ligand bonds.     

High level ab initio study of the structure and vibrational spectra of HO(2)NO(2)

A high-level ab initio study has been performed on the conformational structure and vibrational spectra of HO(2)NO(2). Calculations carried out with coupled-cluster methods using a series of Pople and Dunning basis sets reveal that there is a significant basis set dependence on the predicted ab initio structure. Higher angular momentum basis sets are shown to be necessary in order to bring the calculated structure into agreement with experimental rotational constants. Harmonic vibrational frequencies of HO(2)NO(2) are computed at the CCSD(T)/aug-cc-pVTZ level of theory while the corresponding vibrational anharmonicities are calculated at the MP2/cc-pVTZ level. In addition, the absorption cross sections of OH stretching overtones in HO(2)NO(2) are calculated using a dipole function computed at the QCISD level of theory and found to be in good agreement with the available experimental data.     

Vibrational spectroscopic studies and ab initio calculations of sulfanilamide

FT-Raman and FT-IR spectra of sulfanilamide were recorded and analyzed. The vibrational frequencies of the compound have been computed using the Hartree-Fock/6-21G(*) basis and compared with the experimental values. The assignments of the observed bands were made on the basis of available literature.     

The computed force constants and vibrational spectra of toluene

The complete harmonic force field and dipole moment derivatives have been computed for toluene at the Hartree-Fock level using a 4-21G basis set. The six scale factors optimized for benzene were used to scale the computed harmonic force constants of toluene. The vibrational frequencies of toluene computed from this scaled quantum mechanical force field are quite good. After a correction was made to two previously proposed spectral assignments, the mean deviation from the experimental frequencies is only 7.8 cm^?1 except for the frequencies related to the methyl group. Five more scale factors for the vibrational modes of the methyl group were reoptimized. The final comparison showed an overall mean deviation of 7.5 cm^?1 between the theoretical spectrum and the experimental spectrum. Computed intensities are qualitatively in agreement with experiments. They are highly useful in the investigation of questionable assignments.