Ab initio molecular orbital study of energies and conformers of 3,4-dihydro-1,2-dithiin, 3,6-dihydro-1,2-dithiin, 4H-1,3-dithiin,and 2,3-dihydro-1,4-dithiin

Optimized geometries and energies for 3,4-dihydro-1,2-dithiin ( 1 ), 3,6-dihydro-1,2-dithiin ( 2 ), 4H-1,3-dithiin ( 3 ), and 2,3-dihydro-1,4-dithiin ( 4 ) were calculated using ab initio 6-31G* and MP2/6-31G*//6-31G* methods. At the MP2/6-31G*//6-31G* level, the half-chair conformer of 4 is more stable than those of 1 , 2 , and 3 by 2.5, 3.5, and 3.6 kcal/mol, respectively. The half-chair conformers of 1 , 2 , 3 , and 4 are 2.9, 7.1, 2.0, and 5.6 kcal/mol, respectively, more stable than their boat conformers. The calculated half-chair structures of 1 – 4 are compared with the calculated chair conformer of cyclohexane and the half-chair structures for cyclohexene, 3,4-dihydro-1,2-dioxin ( 5 ), 3,6-dihydro-1,2-dioxin ( 6 ), 4H-1,3-dioxin ( 7 ), and 2,3-dihydro-1,4-dioxin ( 8 ). © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1064–1071, 1998     

Ab initio molecular orbital calculations of 3,4-dihydro-1,2-dioxin, 3,6-dihydro-1,2-dioxin, 4H-1,3-dioxin (1,3-diox-4-ene), and 2,3-dihydro-1,4-dioxin (1,4-dioxene)

Optimized geometries and total energies for 3,4-dihydro-1,2-dioxin ( 1 ), 3,6-dihydro-1,2-dioxin ( 2 ), 4H-1,3-dioxin (1,3-diox-4-ene, 3 ), and 2,3-dihydro-1,4-dioxin (1,4-dioxene, 4 ) were calculated using ab initio 3-21G, 6-31G*, and MP2/6-31G*//6-31G* methods. The half-chair conformers of 1 (C₁), 2 (C₂), 3 (C₁), and 4 (C₂) are more stable than their respective planar structures [ 1 (C_s), 2 (C_2v), 3 (C_s), and 4 (C_2v)]. Among the four isomers 1 – 4 , the half-chair conformer of 3 is the most stable. It is 53.1, 54.6, and 3.4 kcal mol⁻¹ more stable than 1 , 2 , and 4 , respectively. The largest energy difference (19.0 kcal mol⁻¹) is observed between the half-chair and planar conformers of 2 . The boat conformers of 2 and 4 are less stable than their respective half-chair conformers, but are more stable than their planar structures. Hyperconjugative orbital interactions (anomeric effects) contribute to the greater stability of 3 (n_O(3) →σ*_C(2)—O(1), n_O(3)→σ*,n _O(3)→σ*) and of 4 (n_O(1)→ σ*). The ab initio calculated structural features of the half-chair conformations of the dihydrodioxins 1 – 4 are compared with the half-chair conformations of cyclohexene and the chair conformations of cyclohexane, oxacyclohexane (tetrahydropyran), 1,2-dioxacyclohexane (1,2-dioxane), 1,3-dioxacyclohexane (1,3-dioxane), and 1,4-dioxacyclohexane (1,4-dioxane) © 1997 by John Wiley & Sons, Inc. J Comput Chem 18 : 1392–1406, 1997     

Ab initio molecular orbital calculations

A series of non-empirical calculations on furan, pyrrole and 1,2,5-oxadiazole are reported in which the effect of polarisation functions added to the minimal 7s 3p basis on each atom is studied. The effect on these planar molecules is largely through the rather than the-system. A comparison with the results of work with scaled functions is reported. Both series are shown to lead to much improved agreement with the electron spectroscopy energy levels. The effect on the dipole moments of these changes in basis is more variable but, with the exception of furan, the agreement with experiment is improved in the present method.

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Zusammenfassung Für die Moleküle Furan, Pyrrol und 1,2,5-Oxadiazol wurde eine Reihe von nichtempirischen Rechnungen durchgeführt, in denen der Einfluß von zusätzlichen Polarisationsfunktionen zur minimalen 7s 3p-Basis an jedem Atom untersucht wird. Die Ergebnisse werden mehr durch die Art der Beschreibung des Systems der-Elektronen als durch diejenige der-Elektronen beeinflußt. Ein Vergleich mit den Ergebnissen bei Verwendung skalierter Funktionen wird durchgeführt. Beide Reihen von Ergebnissen zeigen eine verbesserte Übereinstimmung zu den Energiemeßwerten der Elektronenspektroskopie. Die Änderungen des berechneten Dipolmoments bei derartigen Basisvariationen sind größer als bei früheren Methoden. Die Übereinstimmung mit dem Experiment wird, mit Ausnahme von Furan, jedoch verbessert.

     

Ab initio molecular orbital calculations for butadiene and its ions

Ab initio calculations are reported for cis and trans butadiene and some of their ions. The calculations are compared with semi-empirical results, and used to predict coupling constants.     

Ab initio molecular orbital calculations on the pyrazine-lithium ion pair

The results of ab initio molecular orbital calculations are presented for the pyrazine^?/Li⁺ ion pair. It is shown that the lowest energy conformation is where the lithium occupies a position in the plane of the pyrazine ring, along the C ₂ axis passing through the nitrogen atoms.     

Ab Initio molecular orbital calculations

Linear combinations of Gaussian orbitals were contracted to minimal basis sets. The binding energies (B.E.) obtained for the three compounds furan, pyrrole and 1,2,5-oxadiazole are 0.75, 0.84 and 0.26 a.u. respectively. Correlation of these figures and those obtained in related calculations leads to the relationship (B.E.)_exp.=0.7347 (B.E.)_calc.+633 (Kcal/mole). The photoelectron spectra of these compounds are extended and correlated with the orbital energies; the -electron excitations are given by (I.P.)_exp.=1.00 (I.P.)_calc.–2.20 eV. Mulliken population analyses give dipole moments in reasonable agreement with experiment.

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Zusammenfassung Linearkombinationen von Gauß-Orbitalen wurden zu minimalen Basissätzen kontrahiert.Die Bindungsenergien (B.E.) für die drei Komponenten Furan, Pyrrol und 1,2,5-Oxadiazol haben die Werte: 0,75, 0,84 und 0,26 a.u. Setzt man diese Werte zu den berechneten in Beziehung, so erhält man die Gleichung: (B.E.)_exp.=0,7347 (B.E.)_calc.+633 (Kcal/Mol).Die Photoelektronenspektren dieser drei Komponenten werden erweitert und mit den Orbitalenergien korreliert; die -Elektronen-Anregungsenergien werden durch die Beziehung: (I.P.)_exp. =1.00 (I.P.)_calc.–2,20 eV gegeben. Mulliken-Populations-Analysen ergeben die Dipolmomente in bemerkenswerter Übereinstimmung mit experimentellen Werten.

     

Ab initio molecular orbital calculations on beryllium and magnesium atom reactions with water

Ab initio molecular orbital calculations are reported for beryllium and magnesium atom complexes with water, Be. OH₂ and Mg OH₂, and for species resulting from insertion of the metal atom(s) into the water molecule, HBeOH, HMgOH, HBeOBeH, and HMgOMgH     

Ab initio molecular orbital calculations on the water-carbon dioxide system: Molecular complexes

An ab initio LCAO MO SCF calculation has been carried out on three different complexes between water and carbon dioxide. It has been found that a planar complex in which the carbon atom is bound to the oxygen atom in the water is the most stable, with a calculated energy of formation of ?25.7 kJ/mol.     

Ab initio molecular orbital calculations on some selected boron(I) hydrides

Ab initio LCAO-MO-SCF Hartree-Fock-Roothaan calculations have been carried out for a series of closed-shell boron hydrides (B³⁺, B⁺ BH, BH₂⁻,BH₂⁺, BH₃ and BH₄⁻). Koopmans' theorem vertical ionization potentials for the core and highest occupied molecular orbitals are presented. Proton and hydride affinities of some of these molecules have been calculated along with the energies of reaction between various pairs of these boron hydrides.     

Ab initio molecular orbital studies for compounds of magnesium

Using a 6-311G** basis set with estimation of correlation energy at the MP2 level, structural and energetic data for 40 molecular species containing magnesium have been calculated. For about half the species studied, further energetic data were obtained using Pople's G2 method. Enthalpy changes at 298.15 K were obtained for isogyric reactions and standard enthalpies of formation were derived from these. Comparison of the standard enthalpies of formation with the sparse literature data suggests the MP2/6-311G** standard enthalpies of formation are accurate to ± 15 kJ mol^?1 and the corresponding G2 enthalpies accurate to ± 10 kJ mol^?1. The calculated ΔH [MgN, g] revealed a gross error in the currently accepted value for this function. It is intended that these results will be used to parameterize the semiempirical molecular orbital package, MOPAC, for the element magnesium. © John Wiley & Sons, Inc.     

Ab initio molecular orbital calculations on the associated complexes of lithium cyanide with ammonia

Ab initio molecular orbital (MO ) calculations with the 3-21G and 6-31G basis sets are carried out on a series of complexes of NH₃ with Li⁺, C?N^?, LiCN, and its isomer LiNC. The BSSE -corrected interaction energies, geometrical parameters, internal force constants, and harmonic vibrational frequencies are evaluated for 15 species. Complexes with trifurcated (C_3v) structures are calculated to be saddle points on the potential energy surfaces and have one imaginary frequency each. Calculated energies, geometrical parameters, internal force constants, and harmonic vibrational frequencies of the various species considered are discussed in terms of the nature of association of LiCN with ammonia. The vibrational frequencies of the relevant complexed species are compared with the experimental frequencies reported earlier for solutions of lithium cyanide in liquid ammonia. © 1995 John Wiley & Sons, Inc.     

Ab initio molecular orbital studies of polyethylene deformation

A molecular LCAO Hartree-Fock procedure was used to calculate total energies of axially stretched normal paraffins containing up to nine carbon atoms. The results are used to model the mechanical properties of polyethylene.     

Ab initio MO calculations on 1,2-dithietes and valence isomers

The valence isomerization of the 1,2-dithiete parent compound to the open-chain dithial was studied by CASSCF multiconfiguration methods including the CASPT2 perturbational treatment. The isomerization energy remains small at the highest level of theory. In agreement with Jonas and Frenking, the cyclic structure is only then preferred over the acyclic ones if f-functions on the sulfur atoms are considered. If they are included, the 1,2-dithiete is more stable by 3.8 kcal/mol and the barrier amounts to 24.9 kcal/mol at the CASPT2(8,8)/6-31G(2df)//MP2/6-31G* level of theory. According to MP2/6-31G* geometry optimizations, substitution of H by NH₂ and CH₃ reduces the stability of the 1,2-dithiete ring structures relative to the open-chain dithiocarbonyl structures, whereas the inverse holds for acceptor substitution by CN and CF₃. A higher stability of benzodithiete relative to ortho-dithiobenzoquinone is predicted at all employed levels of theory. This is in good agreement with conclusions drawn from experimental results. Whereas the experimental microwave geometry of 1,2-dithiete is well reproduced theoretically, the experimental electron diffraction geometry of 3,4-bis-(trifluoromethyl)-1,2-dithiete differs from the calculated one. © 1996 John Wiley & Sons, Inc.     

Ab initio molecular orbital calculation on the rotational isomerism of 1,2-dichloroethane and 1-chloropropane

Ab initio MO calculations on the total energies of the rotational isomers of 1,2-dichloroethane and 1-chloropropane were performed to estimate the energy differences and the potential functions. The torsional energy levels were calculated by the harmonic rotator model. The calculation including the configuration interaction with single and double excitations on 1,2-dichloroethane improved the agreement with the experimental energy difference between the trans and gauche forms. Vibrational frequencies of 1,2-dichloroethane were also calculated by the ab initio MO method. The results are in good correlation with experimentally established values by Shimanouchi.     

Ab initio molecular orbital study of 1-fluorosilatrane

The molecular geometry of 1-fluorosilatrane was optimized fully by restricted Hartree–Fock (HF) calculations using the 3-21G, 3-21G(d) and 6-31G(d) basis sets, with the aim of locating the positions of the local minima on the energy hypersurface. The optimized geometries were compared with available experimental (X-ray and ED) and semiempirical data. The ab initio calculations using polarized basis sets are in good agreement with those of previously reported semiempirical calculations, giving a slightly longer equilibrium Si? N distance (～ 256 pm) in the case of the endo minimum. However, the exo minimum predicted by the semiempirical methods is not supported. There was no experimental evidence for the existence of this exo minimum, and the present ab initio calculations suggest that it is highly unstable. There is considerable disagreement among the experimental results in the C? N and C? C bond lengths in various silatranes, their differences being as large as 13 pm. The present calculations predict that these differences may appear because the silatrane skeleton is flexible with low-energy, large-amplitude internal motions which introduce considerable uncertainties into the position of ring carbon atoms. © 1994 by John Wiley & Sons, Inc.     

Ab initio molecular dynamics calculations to study catalysis

The modern versions of the density functional theory (DFT), especially those using the generalized gradient approximation (GGA), have reached (almost) chemical accuracy and thus can be applied to study problems of real chemical interest such as catalysis. The important equations for the DFT, the local density approximation (LDA), and GGA are given. The full-potential linearized augmented plane wave method (LAPW) is used to check the accuracy of GGA in solids. The basic concepts of the ab initio molecular dynamics (MD) method by Car and Parrinello and its recent implementation using the projector augmented Wave (PAW) method which use a similar augmentation as LAPW are described. PAW applications to ferrocene and beryllocene are summarized, which illustrate that vibrational or fluxional behavior are well described. Sodalite, an aluminosilicate, is discussed as a generic zeolite in comparison with gmelinite. A study of the dynamics of such a system allows the determination of, e.g., the proton stretch vibrations which can be related to infrared spectra. This is illustrated for the OH stretch vibration of the acid site in silicon-rich sodalite. With this methodology, we are able to study the interaction of methanol trapped inside the cage structure of silicon-rich sodalite and to gain new insight into crucial steps of catalytic reactions, namely, the hydrogen-bonding and the possible protonation in this system, or a proton-exchange reaction. The strategies for parallelizing the PAW code are outlined. © 1997 John Wiley & Sons, Inc.     

Ab initio and indo molecular orbital calculations of the geometries and electronic structures of substituted sulfines

Molecular orbital calculations are presented for the parent sulfine and some mono-and dihalogensubstituted sulfines, using ab initio and INDO methods. A partial geometry optimization was performed for nine different sulfines. Charge distributions, potential surfaces and dipole moments were calculated from the wave functions of the optimized geometries. Cis-trans interconversion barriers and electronic spectra are also given. The atomic charges of the S and O atoms are insensitive to substitutions at carbon, and substituents greatly influence the potential in the environment of the molecule. The implications of the results for the chemical behavior of sulfine derivates are briefly discussed and compared with experimental data.