Ab initio calculation of the inversion barrier in the germyl radical

Equilibrium geometry, vibrational frequencies and inversion barrier of the GeH₃ radical have been calculated using ab initio molecular orbital methods at both the SCF and correlated levels. At the SCF level, the effect of several small and medium size basis sets of Ge on these properties are studied systematically. Electron correlation corrections as incorporatedvia fourth-order unrestricted Møller-Plesset perturbation theory in conjunction with large Gaussian basis sets were found to reduce the GeH₃ inversion barrier appreciably relative to the UHF values. The final barrier height of 4.5 kcal/mol can be compared with a recent estimate of 4.4 kcal/mol inferred from the REMPI study.     

Ab initio study of the barriers to methyl torsion and torsional frequencies of acetyl molecules

A wide range of ab initio and hybrid density functional methods and basis sets have been employed to calculate the barriers to methyl internal rotation in a range of molecules with the acetyl moiety. Comparison is made of the computed torsional frequency with the experimental torsional frequency, nu(obs), for each molecule. With the MP2/6-311+G(3df,2p) combination of method and basis set, the agreement is better than 4 cm-1 for most of the molecules, where nu(obs) or the V3 barrier is well-determined experimentally.     

Ab initio relativistic calculation of the RbCs molecule

We apply the relativistic configuration-interaction valence-bond method to calculate various characteristics of the alkali-metal RbCs dimer. These include the electronic potentials and transition dipole moments between the ground and first excited states and permanent dipole moments of the X 1sigma+ and a 3sigma+ states of the ground configuration. In addition, we estimate the lifetime of the rovibrational levels of the X state due to blackbody radiation. These data can help experimentalists to optimize photoassociative formation of ultracold RbCs molecules and their longevity in a trap or in an optical lattice. Extended basis sets, constructed from Dirac-Fock and Sturm's orbitals, have been used to ensure convergence of our calculations. We compare our data with other theoretical and experimental results when they were available.     

Ab initio calculations of the inversion transitions of cyanamide

Ab initio calculations of the inversion transitions of the cyanamide molecule are carried out to calculate inversion frequencies. Using a polarized double-zeta basis set and a double-minimum potential function of the superposed gaussian and harmonic oscillator type, these a priori calculations led to correct analysis and interpretation of the infrared spectroscopic data.     

Ab initio calculation of molecular chiroptical properties

This review describes the first-principles calculation of chiroptical properties such as optical rotation, electronic and vibrational circular dichroism, and Raman optical activity. Recent years have witnessed a flurry of activity in this area, especially in the advancement of density-functional and coupled cluster methods, with two ultimate goals: the elucidation of the fundamental relationship between chiroptical properties and detailed molecular structure, and the development of a suite of computational tools for the assignment of the absolute configurations of chiral molecules. The underlying theory and the basic principles of such calculations are given for each property, and a number of representative applications are discussed.     

Ab initio study of the inversion barrier in NF3+

Ab initio Hartree–Fock, Møller–Plesset perturbation theory (MP 2), and quadratic configuration interaction, using single and double substitutions (QCISD ), calculations were carried out for the NF₃⁺ ion. Optimized structures were examined at the various levels of theory. Calculation of the inversion barrier height shows the importance of optimizing the geometry at the post-Hartree–Fock level and the inclusion of polarization functions. The best calculated inversion barrier was 13.3 kcal/mol, compared to an experimental value of 17.3 kcal/mol. The dissociation transition state was computed to determine the well depth of the NF₃⁺ ion and its stability toward dissociation. The computed well depth was 28 and 48 kcal/mol at the SCF and MP 2 levels, respectively. © 1994 John Wiley & Sons, Inc.     

Anab initio investigation of rotation and inversion barriers in formylphosphine

Rotation about N–CO bonds in amides has been extensively investigated, but a corresponding barrier to rotation about the P–CO bond in an acylphosphine has yet to be observed. In the present 4-31Gab initio study of formylphosphine, rotation barriers of 9.6 and 13.5 kJ mol^–1 and a phosphorus pyramidal inversion barrier of 108.0 kJ mol^–1 are predicted. A comparison of STO-3G and STO-3G^* barriers suggests that polarization functions are not needed to describe rotation in this system.

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Eine ab initio Untersuchung von Rotations- und Inversions-Barrieren in Formylphosphin

Zusammenfassung Die Rotation um N–CO-Bindungen in Amiden wurde bisher intensiv untersucht, eine entsprechende Rotationsbarriere für Drehungen um die P–CO-Bindung in Acylphosphinen wurde jedoch nicht beobachtet. Eine 4-31Gab initio-Untersuchung an Formylphosphin ergibt Rotationsbarrieren von 9,6 und 13,5 kJ mol^–1 und eine pyramidale Inversionsbarriere von 108,0 kJ mol^–1 als Voraussage. Ein Vergleich der STO-3G und STO-3G^* Barrieren legt nahe, daß Polarisationsfunktionen für die Beschreibung der Rotation in diesen Systemen nicht nötig sind.

     

Ab initio calculation on the H3 activated complex

A full configuration interaction treatment has been carried out for linear, symmetrical H₃ activated complex, with a limited basis set of Slater orbitals. A similar calculation is performed on H₂ in order to obtain an estimate of the activation energy of the reaction H+H₂=H₂+H. The variation of nonlinear parameters in the basis set is studied and the different behaviour of the H₃ and H₂ resulting wave functions examined. — A larger basis set is needed.

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Zusammenfassung Eine vollstÄndige CI-Behandlung wurde für einen linearen, symmetrischen, aktivierten H₃-Komplex mit einem begrenzten Basissatz von Slaterorbitalen durchgeführt. Eine Ähnliche Rechnung wird für H₂ ausgeführt, um eine AbschÄtzung der Aktivierungsenergie der Reaktion H+H₂=H₂+H zu erhalten. Die Variation von nichtlinearen Parametern im Basissatz wird untersucht und das verschiedenartige Verhalten der resultierenden H₃ und H₂ Wellenfunktion überprüft. — Ein grö\erer Basissatz wird benötigt!

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Résumé Interaction de configurations totale pour le complexe activé H₃ linéaire et symétrique, dans une base limitée d'orbitales de Slater. Un calcul analogue a été effectué sur H₂ afin d'évaluer l'énergie d'activation de la réaction H+H₂=H₂+H. La variation des paramétres non linéaires de la base est étudiée, et l'on examine le comportement différent des fonctions d'onde de H₃ et de H₂. Une base plus étendue serait nécessaire.

     

Ab initio derivative calculation of vibrational circular dichroism

The recent formalism of Stephens for the calculation of rotational strength in vibrational circular dichroism has been implemented, exploiting the analytical derivative technique for ab initio Hartree-Fock wavefunctions.     

Ab initio calculation of the dipole polarizabilities of unsaturated hydrocarbons

Finite-field SCF MO calculations of polarizability are reported for ethene, benzene and naphthalene using three good gaussian basis sets Double-zeta wavefunctions yield values of α_LL and α_MM in agreement with experiment but the calculated α_NN is half the experimental value. Polarization functions when present preferentially improve α_NN. (L,M) and N refer to the molecular long, medium and normal axes, respectively.) STO-4G results are given for benzene, naphthalene, azulene, anthracene and phenanthrene, and the agreement with the experiment is reasonable after scaling the calculated α_LL and α_MM by a constant factor. The calculated α_MM are an order of magnitude too low.     

Ab initio investigation of the hydration of deprotonated amino acids

The complexation of five deprotonated anionic amino acids (glycine, L-alanine, L-valine, L-Aspartic acid, and L-glutamine) with one water molecule, has been investigated using a MP2/63-11++G(d,p) approach fully accounting for the basis set superposition errors. For each amino acid, several energetic minima have been identified, and we provide spectroscopic information allowing to discriminate them. Our results strongly suggest that two complexes should coexist under the experimental conditions for [Ala − H]⁻, [Val − H]⁻, and [Asp − H]⁻. Comparisons with the experimental enthalpies, entropies, and Gibbs free energies recently obtained by Wincel [J. Am. Soc. Mass Spectrom. 2008, 19, 1091–1097] show that our simulation reproduces the most significant structure/energy experimental trends, though the entropic changes induced by hydration are slightly overestimated.     

Ab initio calculation of core-electron binding energies in small molecules

Vertical ionization potentials for core orbitals of HF, H₂O, CO, HCN, and H₂CO are calculated with an ab initio transition operator method, followed by Rayleigh-Schrödinger perturbation theory. The results are improved by using a transition atomic basis set. Relativistic corrections are estimated. The average absolute deviation of our final results from experiment is 0.4 eV for the eight cases studied.     

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.     

Ab initio calculation of the electronic structure of TCNQ and its ions

Ab initio SCF-LCAO-MO calculations have been performed for TCNQ and its positive and negative ions in various electronic states. A basis set consisting of 412 primitive Gaussian type orbitals contracted to 180 was used in the investigation. The electron density distribution in TCNQ and the negative ions, and the redistribution upon ionization has been illustrated by plotting difference density contour maps. The quinone structure of the neutral TCNQ system undergoes a transformation to a benzenoid structure when electrons are added. Electronic transitions, ionization potentials and electron affinities have also been calculated.     

Ab initio calculation of the band structure of some boron polymers

Ab initio calculations using a STO-3G basis set have been performed on the polymer systems (HBX) _n where X = Be, BH, CH₂, NH, and O. Energy band diagrams and accompanying density-of-states plots have been obtained. The highest filled orbital of (HBNH) _n and (HBO) _n occurs at the X-point and possesses character while the - framework orbital at the X- point is the highest filled level for (HBBe) _n , (HBBH) _n , and (HBCH₂) _n . The conduction band for all five species has symmetry and the band gap of the (HBX) _n species increases in the order X = Be < BH < NH < O < CH₂. An estimate of the energy of polymerisation of the (HBX) _n systems suggests that HBNH is particularly stabilised by polymerisation. The electron distribution in (HBBe) _n shows a - electron drift towards boron, while in the other four systems the net electron transfer is directed away from boron. There is significant electron back-donation to boron in (HBO) _n and (HBNH) _n .     

Ab initio calculation of the dipole moment function of hydrogen iodide

SCEP/CEPA and MC SCF potential energy and dipole moment functions for hydrogen iodide have been calculated. Spectroscopic constants and vibrational dipole matrix elements obtained from the CEPA functions are in good agreement with experimental data. In contrast to previous results for hydrogen fluoride, the MC SCF dipole moment function is less accurate than the CEPA function.     

Ab initio CI calculation of the molecular quadrupole moment of benzene

The molecular quadrupole moment of benzene has been calculated from ab initio CI wavefunctions of different levels of accuracy The calculated value from the wavefunction near the Hartree-Fock limit is reduced ≈10% in magnitude by the CI treatment.