Structure and bonding of H3CCuH

The reaction CH₄ + Cu(²P) → H₃CCuH → H₃C-CU + H and H₃C + CuH has been subjected to ab initio SCF calculations within the RHF formalism using a split valence shell gaussian basis set. The three electronic states (²A₁m ²E and ²E) of H₃CCuH studied had C_3V symmetry. The molecular structure and computed properties of the H₃CCuH ground state (²A₁) suggested that this molecule can be considered as a methyl radical attached to a closed-shell CuH. The results also indicate that the formation of H₃CCuH may occur through either an insertion or via an abstraction mechanism, nevertheless, the insertion mechanism appears to be the most favoured.     

Reaction competition in the fragmentation of protonated dipeptides

A major low-energy fragmentation reaction of many protonated dipeptides involves cleavage of the amide bond resulting in formation of either the y(1)" ion or the a(1) ion. For a series of protonated dipeptides H-Val-Xxx-OH it is observed that log(y(1)"/a(1)) is a linear function of the proton affinity of the variable C-terminal amino acid. For the series of protonated dipeptides H-Xxx-Phe-OH log(a(1)/y(1)") gives a poor correlation with the proton affinity or gas-phase basicity of H-Xxx-OH. However, a good limited correlation of log(a(1)/y(1)") with the Taft-Topsom sigma(alpha) for the alkyl group is observed when Xxx is an aliphatic amino acid. It is proposed that fragmentation occurs by initial formation of a proton-bound complex of an aziridinone and an amino acid which may fragment to form either a protonated amino acid (y(1)") or an N-protonated aziridinone with the corresponding neutrals being an aziridinone and an amino acid. Ab initio calculations show that the N-protonated aziridinone is unstable and fragments by loss of CO to form the a(1) immonium ion. However, the proton-bound complex of an aziridinone and an amine base is a stable species which exists in a potential well. Copyright 2000 John Wiley & Sons, Ltd.     

AB Initio molecular orbital calculations on the si2h4molecule

Ab initio molecular orbital calculations using an extended Gaussian basis set have been performed on C₂H₄, CH₂SiH₂ and Si₂H₄. The species CH₂ and SiH₂ have also been examined. Geometries were partially optimized and the energy difference between the planar singlet and orthogonal or twist triplet geometries of Si₂H₄ was studied in order to provide a measure of the strength of the Si-Si bond in this molecule. Mulliken population analyses were carried out on CH₂CH₂ and SiH₂SiH₂, to further study the nature of the Si-Si double bond in comparison with the C—C double bond.     

D and J/ψ production from deconfined matter in relativistic heavy ion collisions

We study the production of the D and J/ψ mesons from deconfined quark matter at CERN SPS energy. Using the MICOR microscopical coalescence model we determine the transverse momentum spectra of these charm mesons. We predict the slopes of their transverse momentum spectra in Pb+Pb collision at 158 GeV/nucleon beam energy.     

A theoretical study on the thermodynamic properties of the formation and decomposition of methyloxirane via triplet mechanisms

Molecular unrestricted Hartree-Fock calculations with geometry optimization have been carried out on eight triplet isomers of C₃H₆O considered to be possible intermediates in the addition of O(³P) atoms to propylene and in the Hg(³P) sensitization of methyloxirane. The computed thermodynamic stabilities reveal that four triplet states are available in the former and eight triplet states are available in the latter reaction. The isomer CH₃?HCH₂? is more stable than ?CHCH₃?H₂. This latter feature gives a satisfactory explanation for the experimental observation that in the O(³P) + C₃H₆ reaction propionaldehyde is the major carbonyl product.     

An attempted application of the extended hückel molecular orbital approach to reactions involving charged species

The semi-empirical extended Hückel molecular orbital (EHMO) and non-empirical self consistent field molecular orbital (SCF-MO) methods have been used to study protonation reactions of acids, amides and esters. The work has been extended to the charged intermediates and subsequent products in their hydrolysis reactions.

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Zusammenfassung Ein semiempirisches, erweitertes Hückel-Verfahren sowie ab initio SCF-MO-Rechnungen wurden zur Untersuchung der Protonierung von Säuren, Amiden und Estern, deren ladungstragender Ab-kömmlinge und ihrer Folgeprodukte herangezogen.

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Résumé La méthode de Hückel étendue semi-empirique (EHMO) et la méthode du champ selfconsistant non empirique (SCF MO) ont été utilisées pour l'étude des réactions de protonation des acides, des amides et des esters. Ce travail a été étendu aux intermédiaires chargés et aux produits résultants lors de leur «hydrolyse».

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Contribution from the Department of Chemistry, University of Toronto, Toronto 5, Ontario, Canada.     

Allylic hydrogen abstraction II. H-abstraction from 1,4 type polyalkenes as a model for free radical trapping by polyunsaturated fatty acids (PUFAs)

Unsaturated radicals, containing different number of delocalized electrons, are formed via H-atom abstractions with CH(3), iso-C(3)H(7), OOH and OH radicals from (Z,Z) and (E,E)-hepta-2,5-dienes. These reactions and the relative stability of the different allyl-type radicals formed, were studied within the BH&HLYP method, using a 6-311+G(3df,2p) basis set, as well as within the G3MP2 level of theory on BH&HLYP/6-31G(d) geometries. The biallyl type radicals (involving 5 electrons delocalized on 5 carbon atoms) are more stable, by about 47.6 +/- 0.4 kJ mol(-1), than monoallyl type radicals (which involve 3 electrons delocalized on 3 carbon atoms). Three types of the H-atom abstractions were distinguished: direct H-abstraction with CH(3), indirect abstraction with a higher barrier height with iso-C(3)H(7), OOH and a non-direct quasi-barrierless H-abstraction with OH radicals. These observations were also confirmed by the activation entropy versus activation enthalpy as well as the Evans-Polányi's plots. The OOH-hepta-2,5-diene complexes are found to be extremely stable (from -19.6 to 22.3 kJ mol(-1)). The room temperature rate constants were calculated with transition state theory. Formations of monoallyl and biallyl radicals through H-abstraction with OH are fast; the calculated rate constants range from 5.84 x 10(-11) to 1.92 x 10(-9) cm(3) molecule(-1) s(-1) at room temperature. These reactions may play a key role in the "very low temperature combustion" like biological oxidations.     

The s-monotone index selection rules for pivot algorithms of linear programming

In this paper we introduce the concept of s-monotone index selection rule for linear programming problems. We show that several known anti-cycling pivot rules like the minimal index, Last-In–First-Out and the most-often-selected-variable pivot rules are s-monotone index selection rules. Furthermore, we show a possible way to define new s-monotone pivot rules. We prove that several known algorithms like the primal (dual) simplex, MBU-simplex algorithms and criss-cross algorithm with s-monotone pivot rules are finite methods.     

Peptide models XXIII. Conformational model for polar side‐chain containing amino acid residues: A comprehensive analysis of RHF,DFT, and MP2 properties of HCO‐L‐SER‐NH2

Geometric and energetic properties of a diamide of serine, HCO‐NH‐L ‐CH(CH₂OH)CO‐NH₂, are investigated by standard methods of computational quantum chemistry. Similarly to other amino acid residues, conformational properties of HCO‐L ‐Ser‐NH₂ can be derived from the analysis of its E=E(ϕ,ψ;χ₁,χ₂) hypersurface. Reoptimization of 44 RHF/3‐21G conformers at the RHF/6‐311++G** level resulted in 36 minima. For all conformers, geometrical properties, including variation of H‐bond parameters and structural shifts in the torsional space, are thoroughly investigated. Results from further single‐point energy calculations at the RHF, DFT, and MP2 levels, performed on the entire conformational data set, form a database of 224 energy values, perhaps the largest set calculated so far for any single amino acid diamide. A comprehensive analysis of this database reveals significant correlation among energies obtained at six levels of ab initio theory. Regression parameters provide an opportunity for extrapolation in order to predict the energy of a conformer at a high level by doing explicit ab initio computations only for a few selected conformers. The computed conformational and relative energy data are compared with structural and occurrence results derived from a nonhomologous protein database incorporating 1135 proteins. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 626–655, 2000