Ab initio study of spectroscopic constants and anharmonic force field of 74GeCl2

The equilibrium structure, spectroscopy constants, and anharmonic force field of germanium dichloride have been calculated at MP2, B3LYP, and CCSD(T) levels of theory employing two basis sets, cc-pVDZ and cc-pVTZ, respectively. The computed geometries, rotational constants, and vibration-rotation interaction constants, and quartic centrifugal distortion constants are compared with the available experimental data. The harmonic frequencies, anharmonic constants, and cubic and quartic force constants are predicted. The calculated results show that the MP2 results are in excellent agreement with experiment and represent a substantial improvement over the results obtained from B3LYP. The CCSD(T) method is also an advisable choice to study anharmonic force field of molecules.     

FC(O)NCS 分子振动光谱的理论研究

采用DFT(B3LYP)方法,以6-3G*为基组对FC(O)NCS的顺式和反式两种构型的几何结构,振动谐性力场和红外光谱进行了研究。B3LYP/6-31G*计算水平和大多数有机分子的一套固定标度因子进行标度。根据标度后的理论力场进行简正坐标分析得到的势能分布(PED)和红外光谱强度值对FC(O)NCS分子的顺式和反式两种构型的振动基频进行了理论归属。     

Conventional strain energies of 1,2-dihydroazete, 2,3-dihydroazete, 1,2-dihydrophosphete, and 2,3-dihydrophosphete

The conventional strain energies of 1,2-dihydroazete, 2,3-dihydroazete, 1,2-dihydrophosphete, and 2,3-dihydrophosphete are determined within the isodesmic, homodesmotic, and hyperhomodesmotic models. Optimum equilibrium geometries, harmonic vibrational frequencies, and corresponding electronic energies and zero-point vibrational energies are computed for all pertinent molecular systems using SCF theory, second-order perturbation theory, and density functional theory and employing the correlation consistent basis sets cc-pVDZ, cc-pVTZ, and cc-pVQZ. Single-point fourth-order perturbation theory, CCSD, and CCSD(T) calculations employing the cc-pVTZ and the cc-pVQZ basis sets are computed using the MP2/cc-pVTZ and MP2/cc-pVQZ optimized geometries, respectfully, to ascertain the contribution of higher order correlation. Three DFT functionals, B3LYP, wB97XD, and M06-2X, are employed to determine whether they can yield results similar to those obtained at the CCSD(T) level.     

A theoretical study of the thermodynamics and kinetics of small organosulfur compounds

The performance of a large set of ab initio procedures in predicting geometries, thermochemical and kinetic data of small sulfur compounds is assessed. Geometries and thermochemical data for H₂S, (CH₃)₂S, H₂S₂, (CH₃)₂S₂ and H₂C=S are studied using the HF method, density functional theory methods (B3LYP, BHandHLYP, MPW1PW91 and BMK), post-HF methods [MP2, MP3, MP4, CCSD, CCSD(T) and QCISD] and composite techniques (G3, G3B3, CBS-QB3 and W1U). A set of five reactions involving these small organosulfur compounds is studied and the influence of the level of theory on transition state geometries, reaction barriers and rate coefficients is assessed. Independent of the level of theory used, accurate geometries are obtained with the 6-311G(2d,d,p) and cc-pVTZ basis sets, both reproducing experimental bond lengths and bond angles within 2 pm and 0.5°. Besides composite methods, the BMK/cc-pVTZ method is the only studied method that succeeds to predict standard enthalpies of formation within 10 kJ mol^?1 of the experimental data. The best agreement with experimental rate coefficients is obtained with the BHandHLYP/cc-pVTZ method, closely followed by the composite methods and the BMK/cc-pVTZ method. All these methods succeed to reproduce the experimental rate coefficients within a factor 4. To obtain an accurate prediction of both thermochemical and kinetic data for organosulfur compounds, the commonly used composite methods G3B3 and CBS-QB3 and the BMK/cc-pVTZ method prove to be valuable tools.     

Comparison between optimized geometries and vibrational frequencies calculated by the DFT methods for the Anderson-type heteropolyanion: hexamolybdoaluminate(III), [AlIII(OH)6Mo6O18]3-

Optimized geometries and vibrational frequencies were calculated for the hexamolybdoaluminate(III), [AlIII(OH)6Mo6O18]3-, Anderson-type heteropolyanion with the HF, B3LYP, B3PW91, B3P86 and B1LYP methods of theory using the LanL2DZ, SDD and combination of LanL2DZ with 6-31G (d, p) basis sets. The agreement between the optimized and experimental geometries was in the decreasing order: HF, B3P86, B3PW91, B1LYP and B3LYP. The calculated frequencies by the B3LYP have the smallest mean root mean square (RMS) error. The effect of the basis set on the calculated bond lengths and frequencies by the density functional calculations (DFT) methods was minor. The agreement between the previously reported IR and Raman spectra and the calculated values is, in general, good.     

Theoretical study of oxidative additions of H2 and MeCN to a nickel(0) complex: significantly large correlation effects and characteristic features of the reaction

Oxidative addition of H2 to Ni(PH3)2 was theoretically studied as a prototype of nickel-catalyzed sigma-bond activation reaction, where CASSCF, CASPT2, CCSD(T), broken symmetry (Bs) MP2 to MP4(SDTQ), and DFT methods were employed. The CASPT2 method yields a reliable potential energy curve (PEC) when the active space consists of 10 electrons and 10 orbitals including five outer 3d' orbitals. The CCSD(T) method presents almost the same PEC as the CASPT2-calculated one, when either the ANO or the cc-pVTZ basis set is used for Ni. Bs-MP4(SDTQ)-calculated PEC is similar to those calculated by the CASPT2/ANO method, while the PEC is not smooth around the transition state. In the DFT calculation, ANO, cc-pVTZ, and triple-zeta quality basis sets (SDB) with Stuttgart-Dresden-Bonn effective core potentials (ECPs) must be used for Ni. The DFT-calculated reaction energy is somewhat smaller than the CASPT2- and CCSD(T)-calculated values, while B3PW91 and mPW1PW91 present moderately better energy changes than BLYP, B1LYP, and B3LYP. Oxidative addition of MeCN to Ni(PH3)2 was investigated by the DFT(B3PW91) and CCSD(T) methods. Almost the same activation barrier was calculated by these methods, when cc-pVTZ was employed for Ni. However, the DFT method moderately underestimates the binding energy of the reactant complex and the reaction energy compared to the CCSD(T) method. This oxidative addition exhibits interesting characteristic features, as follows: The barrier height relative to infinite separation is lower, and the product is more stable than those of the oxidative addition of C2H6. These differences are discussed in detail in terms of Ni-Me and Ni-CN bond energies and the participation of the CN pi* orbital to stabilization interaction in the transition state.     

Computational study of noncovalent complexes between formamide and formic acid

The geometries and binding energies of 1:1, 1:2, and 1:4 formic acid-formamide complexes (FA-FMA) are calculated by quantum chemical procedures. Vibrational spectra and intermolecular distances of the most stable FA-FMA dimers as well as the influence of the basis set superposition error (BSSE) on the geometries and energies of the dimers are also discussed. All FA-FMA dimers are optimized at the B3LYP/cc-pVTZ, the MP2/cc-pVDZ, aug-cc-pVDZ, cc-pVTZ, and aug-cc-pVTZ levels of theory to study the influence of the level of theory on the calculated geometries and energies. CCSD(T)/cc-pVTZ single-point calculations at the MP2/aug-cc-pVTZ-optimized geometries were performed as reference for estimating the quality of lower level calculations. These calculations allow us to qualitatively describe the competition between different types of hydrogen-bonding interactions in FA-FMA complexes. FA-FMA dimers are compared to other formamide complexes and to the FA-FMA crystal structure.     

Theoretical studies on structures and reactivities of organocuprate(I) and organocopper(III) species

Systematic evaluation of method and basis set on the structure and energetics of organocuprate(I) and organocopper(III) species has been carried out. Various structures of organocuprate(I) and organocuprate(III) complexes were optimized with the HF, MP2, and B3LYP methods, and compared with the structures determined by X-ray crystallography (i.e., Me(2)Cu(I)(-), (CF(3))(4)Cu(III)(-)). Both the MP2 and B3LYP methods reasonably reproduce the X-ray structures while the HF method does not. Using larger basis set and incorporating the relativistic effects for Cu afford the best results. In the studies on the energetics of a Libond;Cu cluster model (Me(2)CuLi. LiCl) and Me(3)Cu model with the MP2, MP3, MP4DQ, MP4SDQ, CCSD(T), and B3LYP methods, the B3LYP method gives energetics similar to those obtained with the CCSD(T) method with much less cost, and hence, is judged to be the best practical method. The studies have shown that B3LYP method with the basis set incorporating the relativistic effective core potentials for Cu and the 6-31G* basis set for the rest is the theoretical method that is the most cost-effective for the studies of the structure and energetics of organocuprate(I) and organocopper(III) species.     

Peculiar basis set dependence of the energetics of C2S2H2 isomers. In search of adequate and affordable basis set for routine calculations

Quantum mechanical calculations using the Hartree-Fock (HF), post-Hartree-Fock {MP2 and CCSD(T)}and gradient-corrected hybrid density functional variant, B3LYP methodology in conjugation with basis sets like 6-31G*, 6-311G** and cc-pVTZ, were employed to critically probe the right quality of basis set and the appropriate level of theory that can be applied in modeling the organo-sulfur compounds. The relative stability ordering of 1,2-dithiete and dithioglyoxal reveal the delicate basis set dependence especially on ‘S’ atom. Single point calculations at the B3LYP, MP2 and CCSD(T) levels using a series of basis set ranging from double ζ quality all the way up to the quintuple ζ quality were done to illustrate the effect of electron correlation and the basis set dependency for these compounds. Basis set requirements are much higher than affordable for medium sized molecules as very slow convergence is seen even when the calculations are carried out with basis set as high as cc-pV5Z at the B3LYP and MP2 level of theories.     

An ab initio study of the Cl(P)+C2H6→C2H5+HCl abstraction reaction

Electronic energies, geometries, and harmonic vibration frequencies for the reactants, products, and transition state for the Cl(³P)+C₂H₆→C₂H₅+HCl abstraction reaction were evaluated at the HF and MP2 levels using several correlation consistent polarized-valence basis sets. Single-point calculations at PMP2, MP4, QCISD(T), and CCSD(T) levels were also carried out. The values of the forward activation energies obtained at the MP4/cc-pVTZ, QCISD(T)/cc-pVTZ, and CCSD(T)/cc-pVTZ levels using the MP2/cc-pVTZ structures are equal to −0.1, −0.4, and −0.3 kcal/mol, respectively. The experimental value is equal to 0.3±0.2 kcal/mol. We found that the MP2/aug-cc-pVTZ adiabatic vibration energy for the reaction (−2.4 kcal/mol) agrees well with the experimental value −(2.2–2.6) kcal/mol. Rate constants calculated with the zeroth-order interpolated variational transition state (IVTST-0) method are in good agreement with experiment. In general, the theoretical rate constants differ from experiment by, at most, a factor of 2.6.     

An ab initio and density functional theory study of keto-enol equilibria of hydroxycyclopropenone in gas and aqueous solution phase

Keto-enol tautomerism in hydroxycyclopropenone (2-hydroxy-2-cyclopropen-1-one) has been studied using ab initio methods, the B3LYP functional of density functional theory, as well as complete basis set (CBS-QB3 and CBS-APNO) and G3 methods. Absolute and relative energies were calculated with each of the methods, whereas computations of geometries and harmonic frequencies for hydroxycyclopropenone and 1,2-cyclopropanedione were computed in the gas phase but were limited to HF, MP2 and CCSD levels of theory, and the B3LYP functional, in combination with the 6-31++G** basis set. Using the MP2/6-31++G** gas phase optimized structure, each species was then optimized fully in aqueous solution by employing the polarizable continuum model (PCM) self-consistent reaction field approach, in which HF, MP2 and B3LYP levels of theory were utilized, with the same 6-31++G** basis set. In both gas and aqueous solution phases, the keto form is higher in energy for all of the model chemistries considered. The presence of the solvent, however, is found to have very little effect on the bond lengths, angles and harmonic frequencies. From the B3LYP/6-31++G** Gibbs free energy, the keto-enol tautomeric equilibrium constant for 2-hydroxy-2-cyclopropen-1-one <==> 1,2-cyclopropanedione is computed to be K(T)(gas) = 2.35 x 10(-6), K(T)(aq) = 5.61 x 10(-14). It is concluded that the enol form is overwhelmingly predominant in both environments, with the effect of the solvent shifting the direction of equilibrium even more strongly in the favor of hydroxycyclopropenone. The almost exclusive nature of this species is attributed to stabilization resulting from aromaticity. Confirmation is provided by comparison of the simulated vibrational spectra of hydroxycyclopropenone with the measured infrared spectrum in an argon matrix.     

Ab initio Study of Anharmonic Force Field and Spectroscopic Constants for Germanium Dichloride

Ab initio study of the equilibrium structure, spectroscopy constants, and anharmonic force field for several isotopomers of germanium dichloride (⁷⁰GeCl₂, ⁷²GeCl₂, and ⁷⁶GeCl₂) have been carried out at the MP2 and CCSD(T) levels of theory using cc-pVTZ basis set. The calculated geometries, rotational constants, vibration-rotation interaction constants, harmonic frequencies, anharmonic constants, quartic and sextic centrifugal distortion constants, cubic and quartic force constants are compared with experimental data. For small mass differences of the Ge isotopes, the isotopic effects for germanium dichloride are much weaker. The agreements are satisfactory for these two methods, but the deviations of CCSD(T) results are slightly larger than that of MP2, because of CCSD(T)'s inadequate treatment of electron correlation in hypervalent Cl atom.     

Benchmark database of accurate (MP2 and CCSD(T) complete basis set limit) interaction energies of small model complexes, DNA base pairs, and amino acid pairs

MP2 and CCSD(T) complete basis set (CBS) limit interaction energies and geometries for more than 100 DNA base pairs, amino acid pairs and model complexes are for the first time presented together. Extrapolation to the CBS limit is done by using two-point extrapolation methods and different basis sets (aug-cc-pVDZ - aug-cc-pVTZ, aug-cc-pVTZ - aug-cc-pVQZ, cc-pVTZ - cc-pVQZ) are utilized. The CCSD(T) correction term, determined as a difference between CCSD(T) and MP2 interaction energies, is evaluated with smaller basis sets (6-31G** and cc-pVDZ). Two sets of complex geometries were used, optimized or experimental ones. The JSCH-2005 benchmark set, which is now available to the chemical community, can be used for testing lower-level computational methods. For the first screening the smaller training set (S22) containing 22 model complexes can be recommended. In this case larger basis sets were used for extrapolation to the CBS limit and also CCSD(T) and counterpoise-corrected MP2 optimized geometries were sometimes adopted.     

Computational studies of the structure and vibrational spectra of hexafluorodisilane

Ab initio calculations predict that D_3d symmetry of Si₂F₆ is more stable than D_3h symmetry. The calculated potential barrier to internal rotation was 0.77, 0.73 and 0.78 kcal/mol using HF/6-31G*, B3LYP/6-31G* and MP2/6-31G* methods respectively, which was in good agreement with the experimental value between 0.51±0.10 and 0.73±0.14 kcal/mol. The optimized geometries, harmonic force fields, infrared intensities, Raman activities, and vibrational frequencies are reported for D_3d symmetry of Si₂F₆ from HF/6-31G* and B3LYP/6-31G*. A normal coordinate analysis was carried out. The average error between the scaled DFT frequencies obtained from the B3LYP/6-31G* calculation and observed frequencies was 4.2 cm⁻¹ and the average error between the scaled HF and observed frequencies was 2.2 cm⁻¹.     

DFT studies, vibrational spectra and conformational stability of 4-hydroxy-3-methylacetophenone and 4-hydroxy-3-methoxyacetophenone

The FT-IR and FT-Raman spectra of 4-hydroxy-3-methylacetophenone (HMA) and 4-hydroxy-3-methoxyacetophenone (HMOA) have been recorded. The total energy calculations of HMA and HMOA were tried for various possible conformers. The spectra were interpreted with the aid of normal coordinate analysis based on density functional theory (DFT) using standard B3LYP/6-31G* and B3LYP/6-311+G** methods and basis sets combinations for the most optimized geometries. 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. On the basis of the comparison between calculated and experimental results, assignments of fundamental modes were examined.     

Vibrational spectra and conformations of 1,4-cyclohexadiene and its oxygen analogues: ab initio and density functional calculations

The vibrational spectra and ring-puckering potential energy functions of 1,4-cyclohexadiene, 4H-pyran and 1,4-dioxin have been examined using a density functional theory (DFT) method as well as the Hartree–Fock (HF) and second-order Møller–Plesset (MP2) methods. The calculated vibrational frequencies and potential energy functions of those molecules have been compared with previously reported experimental data and MM3 results. For all three molecules, the DFT method using Becke's three-parameter functional (B3LYP) has led to the prediction of more accurate vibrational frequencies than the HF and MP2 methods. The enlargement of the basis set at the B3LYP levels has improved the accuracy of calculated vibrational frequencies. In particular, the C–O–C=C torsional force field parameters obtained from the B3LYP method have correctly predicted the ring-puckering potential energy functions of the oxygen-containing analogues, 4H-pyran and 1,4-dioxin, which could not be done by the MM3 method.     

N-萘基氨基甲酸甲酯振动光谱的理论研究

采用HF和DFT(B3LYP)方法及6-31G基组对N-萘基氨基甲酸甲酯的几何构型、振动谱性力场和红外光谱进行了研究,使用Pulay标度法对HF/6-21G和B3LYP/6-31G的理论力场进行标度。根据标度后的理论力场进行了简正坐标分析,得到势能分布和红外振动频率,与红外频率实验相比较,HF方法和DFT(B3LYP)方法的误差分别为37.8cm^-^1和8.68cm^-^1,此外,还根据B3LYP方法得到的势能分布和红外光谱强度对N-萘基氨基甲酸甲酯的振动基频进行了理论归属,并对前人的频率指认进行了修正和补充。     

Quantum chemical studies on structure, conformation and isomerization of nitroso, nitro substituted benzene and 1,3-cyclopentadiene

The molecular structure, conformational stability and isomerization of nitroso, nitro substituted benzene and 1,3-cyclopentadiene in gas phase have been investigated using ab initio and density functional theory methods. The molecular geometries and energetics of possible conformers were obtained by employing MP2, B3LYP and B3PW91 levels of theory implementing 6-31G* basis set. The relative stabilities of the conformations were evaluated from the energy differences of the structure. Chemical hardness (η) and chemical potential (μ) were calculated at HF/6-31G* level of theory for all the positional and geometrical isomers to study the maximum hardness principle. Each optimized structure has been tested against the imaginary frequencies at MP2/6-31G* level of theory in order to be sure they are located at energy minimum.     

Si~2Cl~6分子的内旋转能垒和振动基频的理论研究

用从头算方法HF/6-31G^*^*和密度函方法B3LYP/6-31G^*^*,对Si~2Cl~6分子的平衡几何构型进行优化,优化的结果与实验结果吻合得较好.并用上述两种不同的方法计算Si~2Cl~6分子的内旋转能垒,结果分别为8.786和6.694kJ/mol,其中DFT方法的计算结果与实验结果4.18kJ/mol吻合得较好.对Si~2Cl~6分子的振动基频进行计算.用HF/6-31G^*^*SQM力场所计算的频率理论值与实验值的平均误差为7.3cm^-^1,用B3LYP/6-31G^*^*未标度的力场所计算的频率理论值与实验值的平均误差为6.0cm^-^1.该密度泛函方法(B3LYP/~6-31G^*^*)的理论计算值比用HF/6-31G^*^*标度后的SQM力场计算的频率与实验值(除Si--Si键扭转振动基频之外的11条振动基频)吻合得更好.并给出了Si--Si键扭转振动基频的预测值。     

Theoretical study of the vibrational frequencies of carbon disulfide

A benchmark comparison for different computational methods and basis sets has been presented. In this study, five computational methods (Hartree–Fock (HF), MP2, B3LYP, MPW1MP91, and PBE1PBE) along with 18 basis sets have been applied to optimize the geometry of carbon disulfide (CS₂), and further calculate the vibrational frequencies of the optimized geometries. The differences between the calculated frequencies and corresponding experimental data are used to evaluate the efficiency of each combination of computational method and basis set. The comparison of frequency difference indicates that B3LYP generally gives the best prediction of frequencies for CS₂, whereas the other two density functional theory (DFT) methods, i.e., MPW1PW91 and PBE1PBE, often give parallel results. Although MP2 predicts the frequencies with accuracy almost as good as those from DFT methods, in a particular case, HF calculation outperforms MP2 as well as MPW1PW91 and PBE1PBE for prediction of the frequency of asymmetrical stretching for CS₂. © 2013 Wiley Periodicals, Inc.