Drora Cohen

An Effective Fragment Method for Modeling Solvent Effects in Quantum Mechanical Calculations

An effective fragment model is developed to treat solvent effects on chemical properties and reactions. The solvent, which might consist of discrete water molecules, protein, or other material, is treated explicitly using a model potential that incorporates electrostatics, polarization, and exchange repulsion effects. The solute, which one can most generally envision as including some number of solvent molecules as well, is treated in a fully ab initio manner, using an appropriate level of electronic structure theory. In addition to the fragment model itself, formulae are presented that permit the determination of analytic energy gradients and, therefore, numerically determined energy second derivatives (hessians) for the complete system. Initial tests of the model for the water dimer and water‐formamide are in good agreement with fully ab initio calculations.

Reactions of coordinated cyclic polyolefins. The mechanism of the pericyclic [4,4]-sigmahaptotropic rearrangement; a kinetic study. Crystal structure of tricarbonyl[(2,3,4,5-η)bicyclo[4.2.1]nona-2,4-diene-7,7,8,8-tetracarbonitrile]iron

Abstract The novel [4,4]-sigmahaptotropic rearrangement II → IV, in which a σ-bonded group and a Fe(CO) 3 unit exchange bonding sites antarafacially across a four carbon skeleton, has been shown by kinetic data to involve a concerted, non-synchronous, one step process. First order rate constants for the rearrangement at 23°C are k 1.25 × 10 −6 s −1 (acetone) and k 2.2 × 10 −5 s −1 (methanol), with activation parameters Δ H # 21 kcal mol −1 and Δ S # −15 e.u. (acetone). The moderate value of the ratio of k in methanol to that in acetone, viz. 18, indicates that although a minor charge separation develops upon activation, no intermediate is formed. The least motion pathway mechanism is shown to involve a Berry pseudorotation about the metal, which maintains the bonding interaction between the metal and the organic fragment orbitals during the rearrangement. Topologically this rearrangement corresponds to a [σ2 a + (σ2 s + π2 a )] thermally allowed pericyclic reaction. The structure of the rearranged complex IV was determined by single-crystal X-ray diffraction.

Reactions of coordinated cyclic polyolefins. A kinetic study of the pericyclic [3,3]-sigmahaptotropic rearrangement

Abstract The reaction of tricarbonyl(tropone)iron with tetracyanoethylene (TCNE) was reinvestigated. Two primary cycloadducts, the 3 + 2 and 4 + 2 isomers are formed in a 96/4 ratio. The second order rate constant for the reaction is k 4.4 × 10 −2 M −1 s −1 (at 24°C, in acetone) with a free activation energy of Δ G # 19.2 kcal mol −1 The 3 + 2 cycloadduct undergoes a facile sigmahaptotropic rearrangement to the formal 5 + 2 adduct with rate constants of k 3.8 × 10 −10 s −1 (24°C, acetone) and k 5.1 × 10 −4 s −1 (24°C, methanol), and with activation parameters Δ H # 20 kcal mol −1 and Δ S # −7 e.u. (acetone). The kinetic results suggest that both the cycloaddition and the rearrangment are concerted, nonsynchronous, one-step reactions, involving a slightly polar transition state. Frontier molecular orbital considerations imply that the 3 + 2 and 4 + 2 cycloadditions are symmetry allowed reactions. The structural reorganization which takes place in the [3,3]-sigmahaptotropic rearrangement suggests that the metal migration proceeds by way of a Berry pseudorotation, and the simultaneous 1,3-sigmatropic shift occurs with configuration retention at the migrating carbon. A detailed molecular orbital analysis of the rearrangement is given.

Molecular structure of the hydroperoxyl anion (HO−2 )

The equilibrium geometry of ground state HO−2 (1A′) has been determined by an ab initio multiconfiguration self‐consistent field gradient optimization method using 14 active space molecular orbitals in a double zeta+polarization+(two different) diffuse Gaussian function basis sets. The calculated geometry is R(O–O)=1.498 A, R(O–H)=0.962 A, and ∠HOO=99.8°. For comparison purposes, parallel results were also obtained for the ground states of O, O−, O2, O−2 , and HO2. For a given basis set the calculated adiabatic electron affinities of O, O2, and HO2 are all found to differ by a constant value ±0.05 eV relative to their respective experimental values, suggesting that the errors in the molecular cases are essentially atomic in nature.

Reactions of coordinated cyclic polyolefins. A novel perispecific Diels-Alder reaction of tricarbonyl(η4-cycloheptatriene)iron and (carbomethoxy)maleic anhydride controlled by secondary orbital overlap☆

Abstract The cycloaddition reactions of (carbomethoxy)maleic anhydride (CMA) with (η 4 -cycloheptatriene)Fe(CO) 3 and with uncoordinated cycloheptatriene were investigated. Both reactions are highly stereospecific and give single Diels-Alder adducts. The structure of the metal-bonded adduct X was determined by single-crystal X-ray diffraction. A full analysis of the 1 H and 13 C NMR spectra of X is described. The corresponding uncomplexed adduct XI was obtained by Diels-Alder reaction of CMA with norcaradiene. The unusual Diels-Alder periselectivity and the stereochemical consequences of the 4 + 2 cycloaddition to X are best rationalized in terms of a concerted reaction controlled by secondary orbital interactions in the transition state.

Reactions of coordinated cyclic polyolefins. Regio- and stereo-specific cycloadditions of ketenes and tricarbonyl(η4-polyene)iron complexes. Structural evidence for a concerted 2 + 2 mechanism

Abstract The cycloaddition of ketenes R 1 R 2 CCO (R 1 = Me, R 2 = Ph; R 1 = R 2 = Ph; R 1 = Ph, R 2 = p -tolyl) (III) and tricarbonyl(η 4 -cycloheptatriene)iron (II) at room temperature gave regiospecifically the cyclobutanones IV, in which the carbonyl is bonded to the uncoordinated terminal of the conjugated triene. Tricarbonyl(η 4 -cyclooctatetraene)iron, VI, similarly reacts with dipenylketene to give the regioisomeric cyclobutanone VIII. Single crystal X-ray analysis of tricarbonyl[(η-2,3,4,5)-9-methyl-9-phenylbicyclo[5.2.0]nonan-2,4-dien-8-one]iron (IVa) confirmed the presence of the phenyl group at the endo position, providing a stereochemical evidence for a concerted 2 s + 2 a cycloaddition mechanism. The adducts IV undergo cyclobutanone ring cleavage catalyzed by acid to give the isomeric ketones VII. The diaryl adducts (IVb and IVc) rearrange thermally to the σ,π-allylic complexes IX, which may also be obtained directly upon heating cycloheptatriene complex II and the diarylketenes.

The electronic states of TeO2

Abstract The electronic properties and geometry of TeO 2 have been calculated using an ab initio complete active space, multi-configuration self-consistent field method with a double-zeta plus polarization Gaussian basis set. Compact effective potentials, derived from relativistic atomic orbitals for the tellurium atom, were used to replace the atomic core electrons. Like ozone, the 1 A 1 ground state is found to have a double minimum consisting of a lower-energy open form and a higher-energy ring geometry with an OO bond. The calculated ground-state equilibrium bond length ( R =1.83 A) and angle (θ=111.9°) agree well with experiment. The ring structure ( R =2.02 A and θ=63.6°) is calculated to lie 2.02 eV above the ground-state equilibrium geometry with a 3.13 eV barrier for open → ring. The first excited state, 3 B 1 , is predicted to be bound relative to the ground-state dissociation energy.

Ethylene and acetylene complexes of the silver atom

Calculs SCF ab initio, avec potentiel de cœur effectif relativiste pour Ag, sur les complexes Ag(C 2 H 4 ) 1,2 et Ag(C 2 H 2 ) 1,2 : conformations, energies totales, densites de spin, distributions de charge electronique

The ellipsoidal gaussian basis in molecular orbital theory: I. Integral formulas and test calculations

New analytic integral formulas are presented for the potential energy integrals over ellipsoidal Gaussian basis functions [∼ exp (-αx2 - βy2 - γz2)] that enter into solving the conventional expansion self-consistent field equations. Near minimal atomic orbital bases combined from large nuclear-centered primitive Gaussian sets are used in test calculations on the HF and CO molecules. The ellipsoidal exponential parameters for the valence atomic orbitals are fully optimized using a single scale factor for each atomic orbital and nuclear coordinate. The results are compared with those obtained using an unoptimized nuclear centered double-zeta spherical Gaussian basis.