Jerzy Moc

Hydrogen Abstraction from n-Butanol by the Hydroxyl Radical: High Level Ab Initio Study of the Relative Significance of Various Abstraction Channels and the Role of Weakly Bound Intermediates

We have investigated the mechanism of the reaction of H abstraction from n-butanol by the hydroxyl radical (HO*) using high level ab initio methods in conjunction with the correlation consistent basis sets up to quadruple-zeta quality (cc-pVQZ). This reaction is of significance in the atmosphere and combustion. The focus of the study has been on the relative importance of the abstractions from the specific n-butanol sites and on the role of reaction intermediates involved. Our results show that abstractions from the C(alpha) and C(gamma) positions are kinetically most favored and nearly barrierless, with barrier height estimates of 0.10 and 0.47 kcal/mol, respectively, at the CCSD(T)/cc-pVQZ level. We have determined that the indicated barrier height order, C(alpha) < C(gamma) < C(beta) < C(delta) < OH, parallels that for the n-butanol bond dissociation energies established recently. The kinetically and thermodynamically most favored C(alpha) abstraction occurs via a mechanism including the formation of the n-butanol...HO* prereaction complex. The weakly bound postreaction complexes between the product radicals and H(2)O have been identified for all the specific site abstraction reactions, with their calculated CCSD(T) binding energies of up to about 3 kcal/mol after correcting for the basis set superposition error. G3 method has been found to yield consistent results with those obtained from the CCSD(T) calculations for the predicted orders of both the H abstraction barrier heights and their exothermicities.

Transition structures for H2 elimination from XH4 hypervalent species (X = S, Se and Te). Ab initio MO study

Abstract Ab initio MO calculations on the fragmentation of hypervalent XH4 (X = Se and Te) to XH2 and H2 show that the most favorable transition structure is highly polarized with C1 symmetry, which can be viewed as a trigonal bipyramid where one apical and one equatorial ligand are coupled. The least-motion C2v transition state previously obtained theoretically is much higher in energy.

Zeolite-Supported Palladium Tetramer and Its Reactivity toward H2 Molecules : Computational Studies

Effects of zeolite support on reactivity of Pd 4 cluster toward dihydrogen molecules were studied at the DFT level using T6 (six-ring) and T24 (sodalite cage) clusters as models of zeolite FAU. It has been found that Pd 4 cluster binds to O-centers of T6 cluster via eta (3) and eta (2) coordination modes, leading to three different T6/Pd 4 clusters. For the energetically most stable triplet state T6/Pd 4 structures, the energy of interaction between Pd 4 and the constrained T6 ring is calculated to be ca. -5 kcal/mol. Encapsulating Pd 4 in a sodalite cage (T24) with the full relaxation of cluster geometry resulted in the Pd 4-zeolite interaction energy of -7.4 kcal/mol after correcting for basis set superposition error. The H-H bond activation barrier associated with the first H 2 addition to the triplet state T6/Pd 4 clusters (Delta E 0/Delta H, kcal/mol) varies from (2.2/0.7) to (3.2/2.0) to (4.8/3.5), depending on the path. Comparison of the calculated H 2 addition barriers for the T6-supported and gas-phase Pd 4 indicates that embedding of Pd 4 on zeolite reduces this barrier slightly (by 1.8/2.1 kcal/mol). Interestingly, the characteristic gas phase Pd 4-H 2 active site structural motif has been preserved in the T6-supported transition state structures. The heat of the reaction of the addition of first H 2 to the triplet state T6/Pd 4 ranges from (-17.6/-18.9) to (-21.8/-23.5) for the paths considered. The addition of the second, third and fourth H 2 molecules to the respective first H 2 addition products leads to the dissociative addition product only for the continuation of the single first H 2 addition path.

Conformational studies of gas-phase ribose and 2-deoxyribose by density functional, second order PT and multi-level method calculations: the pyranoses, furanoses, and open-chain structures.

We present an extensive computational study of a complex conformational isomerism of two gas phase pentoses of biological and potential astrobiological importance, d-ribose and 2-deoxy-d-ribose. Both cyclic (α- and β-pyranoses, α- and β-furanoses) and open-chain isomers have been probed using second order Møller-Plesset perturbation theory (MP2), M06-2X density functional, and multi-level G4 methods. This study revealed a multitude of existing minima structures. Numerous furanose conformers found are described with the Altona and Sundaralingam pseudorotation parameters. In agreement with the recent gas-phase microwave (MW) investigation of Cocinero et al., the calculated free ribose isomers of lowest energy are the two β-pyranoses with the (1)C4 and (4)C1 ring chair conformations. Both β-pyranoses lie within 0.9kJ/mol in terms of ΔG(298K) (G4), thus challenge the computational methods used to predict the ribose global minimum. The calculated most favoured ribofuranose is the α-anomer having the twist (2)T1 ring conformation, put 10.4kJ/mol higher in ΔG than the global minimum. By contrast with d-ribose, the lowest energy 2-deoxy-d-ribose is the α-pyranose, with the most stable 2-deoxy-d-furanose (the α-anomer) being only 6.2kJ/mol higher in free energy. For both pentoses, the most favoured open-chain isomers are significantly higher in energy than the low-lying cyclic forms. A good overall agreement is observed between the M06-2X and MP2 results in terms of both the existing low-energy minima structures and intramolecular H-bonding geometrical parameters. The natural orbital analysis confirms the occuring of the endo- and exo-anomeric effects and maximization of intramolecular H-bonding in the lowest-lying pyranoses and furanoses of both sugars.

The Unimolecular Decomposition and H Abstraction Reactions by HO and HO2 from n‐Butanol

By using correlated ab initio (MP2, CCSD(T)) and multi‐level (G3, CBS‐QB3) methods we have studied unimolecular and bimolecular reactions of n‐butanol in the gas phase. The specific processes investigated include H2O elimination and hydrogen abstraction by the hydroxy (HO) and hydroperoxy (HO2) radicals from this alcohol.

THEORETICAL STUDY OF THE CH2BR, CHBR2 AND CBR3 RADICALS

Abstract Equilibrium structures, vibrational spectra, inversion barriers, and adiabatic ionization energies are determined for the series of bromomethyl radicals CH 2 Br, CHBr 2 and CBr 3 using both ab initio correlated and hybrid UB3LYP density functional theory (DFT) methods in conjunction with the large TZ2P and 6-311++G(3df,3pd) basis sets. In particular, the trends in the calculated molecular properties within the series are indicated and the performance of the two theoretical approaches is assessed.

Comparative study of the structures and energies of the SiX3, GeX3 and SnX3 series of radicals (X = H, F, Cl)

Abstract A comparative all-electron ab initio study for the structures and energies of the silicon-, germanium-, and tin-centered radicals of formulas SiX 3 , GeX 3 and SnX 3 (X = H, F, Cl) has been presented. I particular, changes in both the degree of non-planarity and barriers of inversion (vertex or edge) within the title series have been discussed. A comparison of the predictions by the UHF and ROHF methods has been made.

Structures and energies of the lithiated silanes

Preliminary ab initio calculations for the entire series of lithiated silanes SiH4 –nLin are reported. Tetrahedral and planar structures of these species are fully optimized and the resulting tetrahedral–planar energy differences are estimated and compared with those of the lithiated methanes. Basis set effects on the calculated structures of lithiated silanes are discussed. In addition performance of the semiempirical MNDO method in the case of the silicon–lithium systems considered is assessed.

Theoretical study of the geometry of GeH3 and electronic structure of SiH3 and GeH3

Abstract Unrestricted Hartree—Fock (UHF) calculations on the geometry of GeH3 and SW Xα calculations on the electronic structure of SiH3 and GeH3 have been carried out. In particular, SW Xα ionization potentials and electron affinities for both the pyramidal and planar forms of SiH3 and GeH3 have been computed and are compared with available ab initio estimates and experimental data.

A theoretical study of the SiCl3 and GeCl3 radicals

Abstract Ab initio UHF and SW xα calculations have been performed on the SiCl3 and GeCl3 radicals. Geometrical structures and inversion barriers were predicted from ab initio calculations, and ionization potentials, electron affinities and electronegativities were obtained via the SW Xα method. Changes in geometrical and electronic properties along the series (XCl3; X = C, Si, Ge) are discussed.