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 conformational analysis of 1,5‐dithiacyclooctane, 1,5‐diselenacyclooctane,and 1,5‐ditelluracyclooctane

Energy‐minimum structures of 1,5‐dithiacyclooctane (1,5‐DTCO), 1,5‐diselenacyclooctane (1,5‐DSeCO), and 1,5‐ditelluracyclooctane (1,5‐DTeCO) were calculated by the ab initio molecular orbital method. Nine energy‐minimum structures were obtained for each compound. A twist‐boat–chair (TBC) structure is the most stable for 1,5‐DTCO and 1,5‐DSeCO, whereas a boat–boat (BB) structure is the most stable in 1,5‐DTeCO. The TBC conformer of 1,5‐DTCO has received little attention so far. The energy gap between HOMO and NHOMO in the TBC conformer of 1,5‐DTCO is in good agreement with the experimental data (photoelectron spectrum). For 1,5‐DTCO and 1,5‐DSeCO, the boat–chair (BC) conformer in which two chalcogen atoms face each other has the highest HOMO energy among the nine conformers, and the energy barriers between the TBC and BC conformers were calculated to be relatively low for these compounds. Therefore, a conformational change from the TBC to the BC is predicted to occur before these compounds are oxidized in solution. © 1999 John Wiley & Sons, Inc. Heteroatom Chem 10: 159–166, 1999     

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 study of the hydrogen-bonded pyrrole ⃛acetonitrile complex

The total dipole moments, molecular energies, and π-electron densities for the linear and orthogonal pyrrole ?acetonitrile hydrogen-bonded complexes were studied in the ab initio valence bond framework using the minimal STO -3G basis set. That the orthogonal conformation, although slightly less stable than the other, is predominant as observed in carbon tetrachloride, can be explained by its relatively high symmetry number.     

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 molecular orbital theory study of GaAs clusters: The geometry

We have studied the structure and geometry of neutral and charged atomic clusters consisting of Ga and As atoms via ab initio Hartree–Fock (HF) and second‐order Møller–Plesset methods. The Ga_mAs_n cluster with m≠n composition prefers a nontetrahedral geometry in the charge neutral (q=0) state. These clusters tend to be stable in tetrahedral geometry when appropriately charged. The Ga_mAs_n cluster with m=n composition (1:1 ratio of Ga to As atoms) tends to be stable in a tetrahedral geometry in the charge neutral (q=0) state. With increasing size of the cluster, the geometry of Ga_nAs_n cluster approaches the zinc‐belende‐type crystalline structure. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 563–573, 2000     

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 study of ground and low-lying electronic states of CoCN

The ground and low-lying excited states of CoCN have been studied by ab initio multireference single and double excitation configuration interaction (MR-SDCI) calculations with Davidson's correction Q and Cowan-Griffin's relativistic corrections. The electronic ground state of CoCN is (3)Phi(i) and the equilibrium geometry is linear with bond lengths of r(e)(Co-C)=1.8540 A and r(e)(C-N)=1.1677 A, substantially different from the experimentally derived values of r(0)(Co-C)=1.8827(7) A and r(0)(C-N)=1.1313(10) A. The first excited state is (3)Delta(i), separated from the ground state by 727 cm(-1). Larger dynamical electron correlation energy for the low-spin (3)Phi state than for the high-spin (5)Phi state makes the (3)Phi state to be the ground state, which is discussed in terms of the differences in natural orbitals. A new spin-orbit interaction scheme between the X (3)Phi(i) and 1 (3)Delta(i) states is proposed.     

Ab initio molecular orbital study of the tautomerism of 4-hydroxy-2-pyridinone

Ab initio calculations were performed on 2,4-pyridinediol, 4-hydroxy-2-pyridinone, and 2hydroxy-4-pyridinone at the HF /3-21G level with full geometry optimization. Two conformations of the hydroxyl group were considered for each tautomer. Corrections for polarization functions, electron correlation, and zero point energy were made by comparison with previous calculations on 2- and 4-pyridinone tautomerism which included these contributions. The most stable structure is 4-hydroxy-2-pyridinone, in agreement with experiment. Relative to 4-hydroxy-2-pyridinone, the energies of the other tautomers are estimated to be 1.9 kcal/mol for 2,4-pyridinediol and 8.9 kcal/mol for 2-hydroxy-4-pyridinone. These are in accord with the experimental values 0.3 ± 1.9 and 10.6 ± 1.9 kcal/mol, respectively, deduced from equilibration studies of the tautomeric methyl derivatives.     

Ab initio molecular orbital studies of nonidentity allyl transfer reactions

Ab initio molecular orbital (MO) calculations are carried out on the nonidentity allyl transfer processes, X^? + CH₂CHCH₂Y ? CH₂CHCH₂ X + Y^?, with X^? = H, F, and Cl and Y = H, NH₂, OH, F, PH₂, SH, and Cl. The Marcus equation applies well to the allyl transfer reactions. The transition state (TS) position along the reaction coordinate and the TS structure are strongly influenced by the thermodynamic driving force, whereas the TS looseness is originated from the intrinsic barrier. The intrinsic barrier, ΔE, looseness, %L?, and absolute asymmetry, %AS?, are well correlated with the percentage bond elongation, %CY? = [(d ? d)/d] × 100 and/or %CX?. The %CY? and the bond orders indicate that a stronger nucleophile and/or a stronger nucleofuge (or a better leaving group) leads to an earlier TS on the reaction coordinate with a lesser degree of bond making as well as bond breaking. These are consistent with the Bell-Evans-Polanyi principle and the Leffler-Hammond postulate. © 1995 by John Wiley & Sons, Inc.     

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 study of the insertion of H2 into POSS compounds

The insertion of one and two H₂ molecules into polyhedral oligomeric silsesquioxanes (POSS) was investigated as a function of the size of the cage, using both Hartree-Fock (HF) and second order perturbation theory (MP2) methods. Also investigated was the same reaction into the heavier groups 4 and 14 metal-substituted POSS (metallasilsesquioxanes) such as Ge-POSS, Si/Ge-mixed POSS, and Ti- and Zr-POSS. The properties of these species in comparison with those of POSS are discussed.     

An Ab initio molecular orbital study of the systematics in the photoelectron spectra of halomethanes

Ab initio molecular orbital calculations on fluoro- and chloro-methanes, CH_4—nX_n, predict the correct trends in the photoelectron spectra except in the case of the C(2s) bands of fluoromethanes. Thus, the calculated energies corresponding to the lone pair, σ (CX) and C(1s) bands increase with increasing n as found experimentally; the C(2s) energy in chloromethanes decreases with increasing n, again, in agreement with experiment.     

Ab initio molecular orbital analysis of acetaldehyde dimers.: Thermodynamic properties

The surface of the acetaldehyde dimer was studied by MP2/6-31+G^* ab initio calculations and some of the minima thus located were used in subsequent optimizations and single-point calculations at the B3-LYP/6-311+G(2d,p), MP2/6-311+G(2d,p), MP2/aug-cc-pDVZ and MP2/aug-cc-pTVZ levels. An overall of six minima in two distinct groups were detected, one group consisting of planar configurations and the other of more stable, non-planar (spatial) configurations of the two monomers. Some of the dimer thermodynamic properties were calculated from its harmonic frequencies and a comparison between the experimental values for the thermodynamic properties and the calculated values is discussed.