ArH＋势能曲线的从头计算

ArnH^ 是非常重要而又较简单的簇体系,对于它们的深入研究,有助于人们认识质子在溶液中的溶剂化行为,该簇中最为简单而又重要的存在形式是ArH^ 和ArnH^ 。ArH^ 可以在气相中稳定存在,在所有的ArnH^ 族中被研究得最早,实验数据也最为齐全,人们对ArH^ 体系进行了许多理论计算,但只局限于平衡键长、简正振动频率、解离能等分子参数,计算结果与已知的振动－转动光谱数据基本符合。     

Molecular Conformation of 2,2'-Dinitrodiphenylamine According to Dielectric Studies and Ab Initio Quantum Chemical Calculations

According to the results of ab initio calculations employing the HF/6-31G* approach, the isolated 2,2'-dinitrodiphenylamine molecule exists as an sc, sc conformer stabilized by a symmetric intramolecular bifurcated (three-center) hydrogen bond. In protophilic solvents (1,4-dioxane), the conformational equilibrium is also shifted in the direction of this rotational isomer.     

Ab initio program for treatment of related molecules. I. Integral part

Anab initio integral program is described. It utilizes the local symmetries to avoid the redundant computation of integrals over spatially equivalent subsets of the basis. The integrals are grouped in a particular way to facilitate their transfer. The program is very suitable for the treatment of related systems with model geometries. The computing times of different programs are compared and the efficiency of the presented one is demonstrated.     

Ab initio calculations on large molecules using molecular fragments. Characterization of the zwitterion of glycine

The ab initio molecular fragment approach is applied to a characterization study of the ground state of the zwitterion of glycine. Included among the properties studied are the — conformational energy surface, the electronic structure, and the magnitude and direction of the dipole moment. The results of the present study are compared to the results of other theoretical and experimental studies.This work was supported in part by the National Science Foundation, the University of Kansas, and the Upjohn Company, Kalamazoo, Michigan 49001.NSF Trainee (1969-1972).Alfred P. Sloan Research Fellow (1971–1973).     

Hyperpolarizabilities of hydrogen-bonded complexes of phenol derivatives with ammonia: PM3 and ab initio studies

Polarizability and first hyperpolarizability values of the hydrogen-bonded complexes formed by nitrosubstituted phenols with ammonia have been calculated using PM3 and ab initio (STO-3G) methods. It has been shown that enhancement of the polarizability (Δα) as well as the first hyperpolarizability (Δβ) of the complex arises from the hydrogen bond interaction between the phenol derivative and ammonia.     

Ab initio MO studies of nuclear spin-spin coupling constants in CH4, SiH4, AlH 4 − and GeH4 systems

Summary Ab initio molecular orbital calculations of electron coupled nuclear spin-spin coupling constants are performed for CH₄, SiH₄, AlH ₄ ^– and GeH₄ systems using the SCF perturbation theory. Basis set dependence of the major contributing terms such as orbital diamagnetic, orbital paramagnetic, spin dipolar and Fermi contact terms are studied. The study also illustrates the relative importance of bond centred functions and nuclear centred polarization functions in predicting the directly bonded and geminal couplings in the systems selected. Basis sets having uncontracted cores functions and augmented with bond functions seem to predict most of these couplings fairly satisfactorily when compared to the experimental values.     

An Ab Initio Study of the Relative Stabilities and Molecular Structures of 3-Substituted 2,5-Dihydrofurans and 4-Substituted 2,3-Dihydrofurans

The geometry optimized structures and total energies of 3-substituted (R) 2,5-dihydrofurans (a) and their isomers, 4-substituted 2,3-dihydrofurans (b), have been determined by ab initio calculations at the MP2/6-31G^*//HF/6-31G^* level. The nature of the moiety R has a marked effect on the relative total energies of the isomeric forms: at the calculation level cited, the reaction enthalpies for the a b isomerization range from +4.7 kJ mol^–1 for R = MeO to –30.5 kJ mol^–1 for both R = COOMe and R = NO₂. The reaction enthalpies appear to be controlled by the electronic effect of R on the strength of p- conjugation in b. The a isomer has a planar ring, independent of R (excluding NH₂), whereas the planarity of b depends on the electronic nature of R: the 2,3-dihydrofuran ring is planar for both R = COOMe and R = NO₂, but nonplanar for less conjugation-enhancing substituents.     

Ab Initio Studies on the Molecular Conformation of Lignin Model Compounds I. Conformational Preferences of the Phenolic Hydroxyl and Methoxy Groups in Guaiacol

Summary. The conformational preferences of the lignin guaiacyl structural unit were studied at the MP2/6-311G(d,p) level of theory using guaiacol (2-methoxyphenol) as model compound. The potential energy surface of guaiacol was investigated by the ab initio method with full geometry optimization by varying the torsion angles of the guaiacol functional groups (hydroxyl and methoxy). An overall of nine stationary points were located, four of which were found to be minima and the other five transition structures between them. The energy minima of guaiacol can adopt one cisoid and three transoid conformations for the hydroxyl and methoxy groups. The transoid structures differ by the orientation of the methoxy group inside and outside of the aromatic plane. The most stable cisoid conformer has an intramolecular hydrogen bond between phenolic hydrogen and methoxy oxygen with a binding energy of 18.09–18.51 kJ/mol as calculated with the second-order (MP2) and fourth-order (MP4SDQ) M?ller-Plesset methods and with larger polarized basis sets including diffuse functions. When comparing the geometrical parameters of the global energy structure with relevant experimental data from crystallographic structures good agreement between the data was found. The saddle points, the effect of calculation level on the energy relative stability, the rotational barrier heights, and the relative concentrations of the conformers are also discussed.     

Ab initio study of the complexes of first-row transition-metal ions with CH, CH2, and CH3

The geometries and bonding characteristics of the complexes of the first-row transition-metal ions with CH, CH₂ and CH₃ were investigated byab initio molecular orbital theory. MCH⁺ and MCH₂ ⁺ are linear and coplanar, respectively. Both of them are with obvious treble or double bond characteristics, but these multiple bonds are mostly “imperfect”. The calculated bond dissociation energies of , and are mostly close to the experimental values, and appear in similar periodic trends from Sc to Zn. Project supported by the National Natural Science Foundation of China (Grant No. 29170070).     

Infrared and Raman Spectra,Conformational Stability,Ab Initio Calculations,and Vibrational Assignments for Cyclopropyldifluorosilane

The infrared (3200–30 cm^–1) spectra of gaseous and solid Cyclopropyldifluorosilane, c-C₃H₅SiF₂H, and the Raman spectra (3200–20 cm^–1) of the liquid with quantitative depolarization values and the solid have been recorded. Both the syn (cis) and skew (gauche) conformers have been identified in the fluid phases, but only the syn conformer remains in the solid. Variable temperature (–55 to –100°C) studies of the infrared spectra of the sample dissolved in liquid xenon have been carried out. From these data, the enthalpy difference has been determined to be 73 ± 10 cm^–1 (209 ± 29 cal mol^–1), with the syn conformer being the more stable rotamer, which is at variance with the predictions from ab initio calculations. A complete vibrational assignment is proposed for both conformers based on infared band contours, relative intensities, depolarization values, and group frequencies. The vibrational assignments are supported by normal coordinate calculations utilizing the force constants from ab initio MP2/6-31G* calculations. Utilizing the frequencies of the silicon–hydrogen sketch, the rm Si—H bond distances of 1.474 and 1.472 Å have been obtained for the syn and skew conformers, respectively. Complete equilibrium geometries have been determined for both rotamers by ab initio calculations employing the 6-31G* and 6-311 +G** basis sets at levels of restricted Hartree-Fock (RHF) and/or Moller–Plesset (MP) to second order. The potential energy terms for the conformer interconversion have been obtained from the MP2/6-31G* calculation. The results are discussed and compared to those obtained for some similar molecules.     

Crystal Structure and Quantum Chemical Ab Initio Calculations of Ibotenic Acid,an Excitatory Amino Acid Receptor Agonist

Ibotenic acid, a constituent of Amanita muscaria, is a potent NMDA receptor agonist. The structure of the ibotenic acid zwitterion monohydrate in the crystalline state has been determined by X-ray crystallography. The crystal structure has two ibotenic acid monohydrate formula units in the asymmetric unit and features an extensive hydrogen bond network. The ibotenic acid zwitter-ions in the crystalline state are compared to the in vacuo structure calculated by quantum chemical ab initio calculations at the HF/6-31+G* level and the effects of the hydrogen bond network on the structures in the solid state are discussed. The calculated potential energy curve with respect to side-chain orientation displays a single energy minimum. The conformations corresponding to the solid-state conformations are calculated to be ca. 2 kcal/mol higher in energy than the minimum-energy conformation in vacuo.     

Spectra and Structures of Silicon-Containing Compounds. XXIV.* Raman and Infrared Spectra,r 0 Structural Parameters,Vibrational Assignment,Barriers to Internal Rotation,and Ab Initio Calculations of Ethylsilane

The infrared (3200 to 400 cm^–1) and Raman (3200 to 20 cm^–1) spectra of gaseous and solid ethylsilane, CH₃CH₂SiH₃, have been recorded. Additionally, the Raman spectrum of the liquid has been obtained with quantitative depolarization values. The SiH₃ torsional mode has been observed as sum and difference bands with the silicon-hydrogen stretching vibration. Utilizing the torsional fundamental frequency of 132 cm^–1 the threefold periodic barrier of 590 cm^–1 (7.06 kJ/mol) has been obtained. Utilizing the frequencies of the silicon-hydrogen stretches, Si-H bond distances of 1.485 and 1.484 Å have been obtained for the bonds gauche and trans to the methyl group, respectively. Using previously reported rotational constants from seven different isotopomers, the r ₀ parameters have been calculated and are compared to the corresponding r _s parameters. A complete vibrational assignment is proposed that is consistent with the predicted frequencies utilizing the force constants from ab initio MP2/6-31G(d) calculations. Both the infrared intensities as well as the Raman activities and depolarization values have been obtained from the ab initio calculations. Complete equilibrium geometries have been determined by ab initio calculations employing the 6-31G(d), 6-311 + G(d,p), and 6-311+G(2d,2p) basis sets at levels of restricted Hartree–Fock (RHF) and/or Moller–Plesset (MP) to second order. The results are discussed and the theoretical values are compared to the experimental values when appropriate.     

Molecular Structures and Thermodynamic Stabilities of Cycloalkadienes and Their Methoxy Derivatives: An Ab Initio and DFT Study

The geometry-optimized molecular structures and total energies of 4- to 6-membered cycloalkadienes, and of a number of their monoand dimethoxy derivatives, have been calculated by ab initio (HF/6-31G^*, MP2/6-31G^*//HF/6-31G^*) and DFT (B3LYP/6-31G^*) methods. By comparison with available experimental data, the reliability of these computational methods for an estimation of the relative stabilities (enthalpies) of the isomeric forms of the title compounds was tested. The experimental enthalpies of isomerization proved to agree best with the respective theoretical data based on the mean of the HF/6-31G^* and B3LYP/6-31G^* energies. The theoretical calculations were then extended to several isomeric methoxy-substituted cycloalkadienes, for which no previous thermodynamic data exist. Some structural features of the title dienes were also discussed.     

Cyclohexanone Dimethyl Acetals: A 13C NMR, Thermodynamic, and Ab Initio Study

The spatial structures of a number of mono- and disubstituted 1,1-dimethoxycyclohexanes (cyclohexanone dimethyl acetals) were studied by ¹³C NMR spectroscopy. In the monosubstituted acetals, substituents (Me, Et, i-Pr, and MeO) on C-2 are axially oriented, contrary to their normal, equatorial orientation on C-3 and C-4. Besides the spectroscopic study, the relative thermodynamic stabilities of the cis-trans isomers of a few 2,X-dialkyl (X = 3, 4, 5, or 6) derivatives of the parent cyclohexanone dimethyl acetal were determined by acid-catalyzed chemical equilibration in MeOH solution. In the most stable isomeric form, the 2-substituent is axial and the other equatorial. In the less stable isomer, both substituents are equatorial, excluding the cis-2,6-dimethyl derivative, where the ¹³C NMR shift data point to a predominance of the diaxial form. In general, the enthalpy difference between the isomeric forms is ca. 9 kJ mol^–1, while the entropy term favors the less stable isomer by 4 to 16 J K^–1 mol^–1. In the 2,6-dimethyl derivatives, however, the trans form is favored by only 0.8 kJ mol^–1 in G _m at 298.15 K. The main findings of the experimental work are in good agreement with ab initio calculations.     

Ab initio molecular orbital calculations of the infrared spectra of hydrogen-bonded complexes of water,ammonia and hydroxylamine

Summary The geometries of three hydrogen-bonded dimers of hydroxylamine have been optimized, at the MP2 level of theory, using the 6-31G** basis set. These calculations yielded three separate local minima on the dimer potential energy surface. The interaction energies of these three species have been calculated, and corrected for basis set superposition error. The infrared band wavenumbers and intensities have been computed, and the monomer-dimer wavenumber shifts and intensity enhancements rationalized in terms of the types and strengths of hydrogen bonds present. The predicted wavenumbers have been correlated with those measured in a recent matrix isolation spectroscopic study, and an argument for the structure of the preferred dimer has been presented.     

The Equilibrium C–Cl,C–Br,and C–I Bond Lengths from Ab Initio Calculations,Microwave and Infrared Spectroscopies,and Empirical Correlations

The equilibrium structures of a few small molecules containing halogens: methyl halides, cyanogen halides, haloethynes, and halocyanoethynes (from chloride to iodide) are reviewed and redetermined if necessary using high-level ab initio calculations and experimental data from microwave and infrared spectroscopies. The correlation of the r(C–X) (X=Cl, Br, I) bond length with other properties (electronegativity or another bond length) is analyzed. It is shown that simple empirical correlations may be used to make a reliable prediction of the r(C–X) bond length.     

Ab Initio Calculation of the Structure of 5-Chloro-1,2,4-triazole

Three tautomeric forms of 5-chloro-1,2,4-triazole were calculated using the Hartree-Fock (ab initio) and Meller-Plesset methods in the 6-31G(d) basis. The ³⁵Cl NQR parameters were calculated using the occupancies of the 3p-components of the valence p-orbitals of the chlorine atom. The structure of this triazole was derived from the data obtained.     

Molecular Structure of ortho-Fluoronitrobenzene Studied by Gas Electron Diffraction and Ab Initio MO Calculations

The molecular structure of ortho-fluoronitrobenzene (o-FNB) has been investigated by gas-phase electron diffraction and ab initio MO calculations. The geometrical parameters and force fields of o-FNB were calculated by ab initio and DFT methods. The obtained force fields were used to calculate vibrational amplitudes required as input parameters in an electron diffraction analysis. Within the experimental error limits, the geometrical parameters obtained from the gas-phase electron diffraction analysis are mostly in agreement with the results obtained from the ab initio calculations. The main results are: the molecular geometry of o-FNB is nonplanar with a dihedral angle about C–N of 38(3)°. The r _g (C–F) bond is shortened to 1.307(13) Å in comparison with r _g (C–F) = 1.356(4) Å in C₆H₅F.     

Ab initio calculation of maximum bond order hybrid orbitals

Summary The maximum bond order hybrid orbital (MBOHO) procedure is tested onab initio level by use of the density matrix in Löwdin orthogonalized atomic orbital basis. The direct MBOHO calculation based on the whole density matrix includes also the hybridization of the inner atomic orbitals, and the MBOHO calculation based on the valence orbital part of the density matrix considers only the hybridization of the valence atomic orbitals. The concrete MBOHO calculations based on theab initio calculation with STO-3G basis show that the components of the s atomic orbitals in MBOHOs and the maximum bond orders obtained from the two kinds of MBOHO calculations are very close to each other, and that the two kinds of MBOHOs all have the excellent correlativity with the nuclear spin-spin coupling constants.The project supported by National Natural Science Foundation of China and the Excellent Young University Teacher's Foundation of State Education Commission of China.     

Theoretical studies of molecular conformation. Derivation of an additive procedure for the computation of intramolecular interaction energies. Comparison withab initio SCF computations

An additive procedure (SIBFA) is developed for the rapid computation of conformational energy variations in very large molecules. The macromolecule is built out of constitutive molecular fragments and the intramolecular energy is computed as a sum of interaction energies between the fragments. The electrostatic and the polarization components are calculated using multicenter multipole expansions of theab initio SCF electron density of the fragments. The repulsion component is obtained as a sum of bond and lone pair interactions.Tests of the procedure on a series of model compounds containing ether oxygens and pyridine-like nitrogens are reported and compared with the results of correspondingab initio SCF calculations. The resulting methodology is compatible with the simultaneous computation of intermolecular interactions.