A theoretical study of the heats of formation of some small molecules using non-empirical wavefunctions

Total energies, obtained from non-empirical LCAO-MO-SCF calculations on a series of reactions involving only closed-shell molecules and ions, have been used to calculate the heats of formation H ₂₉₈ ⁰ of a large number of small molecules. The Double- basis set calculations, after empirical corrections for inadequacies in the basis set and systematic errors found in all calculations involving oxygen and carbon atoms, usually predict the heats of formation within 10 kcal/mole of the experimental value. A series of similar calculations predicts the heats of formation of some negative ions for which experimental values are either not available or are unreliable.     

Energy dependence of transverse quark flow in heavy ion collisions

Energy dependence of quark transverse flow carries information about dynamical properties (equation of state, initial conditions) of deconfined matter produced in heavy ion collisions. We assume quark-antiquark matter formation in Pb+Pb collisions at CERN SPS and Au+Au collisions at RHIC energies and determine quark transverse flow at the critical temperature of the quark-hadron phase transition. Coalescence of massive quarks is calculated in the MICOR hadronization model and hadronic final state effects are considered using the GROMIT cascade program. Comparing theoretical results to data, transverse flow values are determined and energy dependence is discussed.     

How weak an acid can be? Variations of H-bond and/or <Emphasis Type="Italic">van der Waals</Emphasis> Interaction of Weak Acids

Complex formation ability and stability of both weak and super-weak acids was studied by mean of in silico determined thermodynamic data of the complexes. While weak acids act like Brønsted acids forming hydrogen bond type Brønsted complexes, super-weak acids form Lewis complexes via van der Waals interaction. Unlike in the former type, upon complexation, C-H distances changes insignificantly, yet the complex formation is energy driven in the terms of zero-point corrected Energies, ΔE _zp  < 0 kcal mol^?1, which supports the Lewis complex formation, with the exception of CH₄, an extremely “weak acid”.

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Graphical abstract Selected NBOs of the complexes formed between NC-CH₂-H (Lewis acid) with dioxolane as well as NC-O-H (Brønsted acid) and dioxolane.

     

Ab initio calculations on the singlet and triplet energy surfaces for CsiH2

Singlet and triplet calculations, including configuration interaction, are reported for H₂CSi, HCSiH and CSiH₂, and for the transition state on both surfaces.     

Modeling rate-controlling solvent effects. The pericyclic meisenheimer rearrangement of N-propargylmorpholine N-oxide

The activation parameters of the pericyclic Meisenheimer rearrangement and a competitive rearrangement of N-propargylmorpholine N-oxide were determined by experimental and computational methods. A number of aprotic and protic solvents of different polarities and hydrogen bond-forming abilities and the roles of electron-pair acceptor additives were investigated. The reaction kinetics were followed by means of NMR. In protic solvents, isotope-labeling experiments revealed a novel inverse secondary kinetic isotope effect (k(H)/k(D) about 0.8) for the rate-determining cyclization step, probably occurring because of a C(sp) --> C(sp(2)) change in hybridization at the reaction center. In molecular computations at the B3LYP/6-31++G(d,p) level of theory, implicit, explicit, and joint explicit-implicit solvent models were used. The explicit-implicit model and molecular dynamic simulations gave the most accurate results. The components of the rate-controlling solvent effect are discussed, and general equations are proposed for accurate prediction of the solvent-dependent activation parameters.     

Non-empirical LCAO-MO-SCF-CI calculations on organic molecules with gaussian type functions

This part of the series presents the results obtained by open shell SCF computations on the G, N, O, and F Atoms.

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Zusammenfassung Dieser Teil der Reihe gibt die Ergebnisse von SCF-Rechnungen für offene Schalen am C, N, O und F an.

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Résumé Cette partie de la suite d'articles présente les résultats obtenus par des calculs SCF à couches ouvertes sur les atomet C, N, O et F.

     

Quantum chemical study of the geometries and stabilities of the two valence-tautomers of C2H2F+

Non-empirical SCF-MO molecular wavefunctions were computed for the two limiting structures of C₂H₂F⁺ with full geometry optimization using double-zeta quality atomic orbital basis sets. The bridged structure (fluorenium ion) was found to be an energy maximum (transition state) about 31 kcal/mole higher than the open structure (fluoro-vinyl cation). The latter, contrary to the unsubstituted vinyl cation, is slightly (4.5 °) bent away from fluorine at the electron deficient centre.     

Quantifying the intrinsic effects of two point mutation models of proline-proline diamino acid diamide: a first-principle computational study

Two sites of a Pro-Pro diamide were subjected to individual Pro --> Thr point mutations. The parent diamide Pro-Pro as well as selected conformers of the Pro-Thr and Thr-Pro mutant models were subjected to molecular computations at the B3LYP/6-31G(d) level of theory. At the optimized geometries, thermodynamic functions (S, H, and G) were computed. In order to assess relative stabilities of the mutant models, isodesmic reactions were constructed to calculate DeltaS, DeltaH, and DeltaG, relative to the initial Pro-Pro state. The importance of intramolecular hydrogen bonds, involving the -OH group of the Thr side chain, which emerged after the point mutations were also examined. Our findings suggest a novel approach to analyzing the stability of point mutants in peptide models through the analysis of thermodynamic functions.