M. Sana

The theoretical study of the heats of formation of organic compounds containing the substituents CH3, CF3, NH2, NF2, NO2, OH and F

In this work, the heats of formation of some small molecules containing one or more of the following groups. CH3, CF3, NH2, NHF, NF2, NO2, OH, F are studied. The calculations have been carried out theoretically, using the isodesmic procedure of Pople et al. In most cases, the thermal corrections were introduced using experimental (when available) or theoretical (at the SCF 6–31G level) vibrational frequencies. It was found that this correction is relatively negligible (compared to the accuracy of the heats of formation determined); nevertheless, the vibrational frequencies remain useful for determining the heat capacity and the entropy. We observe that the heats of formation, in families such as XCH3-nFn, XNH2-nFn, and CH4-nXn, are related by quasilinear relations (ΔHf ≈ a + bn). Finally, the NF2 and NO2 substituents have large and comparable destabilization effects on the heat of formation. All the other substituents are stabilizing; the importance of the stabilization effect depends on the number of fluorine atoms.

Theoretical Thermochemistry - the Enthalpies of Formation of Some Xhn and Xyhn Compounds

Ab initio molecular orbital theory (Moller-Plesset to fourth order with 6-31 + G(2df,p) and 6-311+ +G(3df,2p) basis sets) was used to compute the total energies of a few XH(n) and XYH(n) compounds. In each case, the fully optimized structures at the full second order of the Moller-Plesset perturbation theory were employed and the thermal corrections were included using analytical harmonic frequencies at the Hartree-Fock level. We discuss the choice of the reaction to evaluate the enthalpies of formation as accurately as possible. For XH(n) compounds, the isogyric reaction of Pople leads to valuable results (+/- 1 kcal mol-1). For XYH(n) systems, we propose the use of bond hydrogenation reactions. In any case, the isodesmic reaction remains a fairly good choice when feasible (for instance with radicals). It appears that some corrections proposed in the recent G1 procedure of Pople are not crucial for saturated molecules as long as very extended basis sets are used (without combination of separately treated corrections) or with hydrogenation reactions. Nevertheless, Poples DELTA-E(QCI) term becomes important for unsaturated radicals. This term permits the incorporation of correlation effects beyond the MP4 level.

Comment On the Accurate Theoretical Determination of Heats of Formation

Different theoretical approaches in determining heats of formation (DELTA-H(f)) at the chemical accuracy level (+/-kcal/mol) are commented on. We show that calculations at the MP4SDTQ/6-311 + + G(3df, 2p) level of molecular orbital theory reproduce experimental results best. Accurate determination of DELTA-H(f) also depends on the choice of working reactions. It is shown that, in a large number of cases, the additivity assumption of basis set extensions does not hold and that the quadratic configuration interaction QCISD(T) method does not lead to any significant improvement with respect To DELTA-H(f) obtained from the perturbative MP4SDTQ treatment.

Theoretical-study of Hydrogen Abstraction Reactions Ch4+r-]ch3+hr, Geometrical, Energetical and Kinetical Aspects

Five hydrogen abstraction reactions, CH4 + R → CH3 + HR have been studied usingab initio SCF and CI methods. R was successively chosen as H, CH3, NH2, OH and F. Geometries were fully optimized at SCF level and energies were computed at CI level for products, reactants and transition states. Quadratic hypersurfaces were fitted in the neighborhood of the most important points of the potential energy hypersurfaces and vibrational analysis were performed thereupon. Wigners and Christovs approximations were used to obtain an idea of the importance of tunneling of H atoms through the reaction barrier, and this effect was shown to be non-negligible. Finally, rate constant calculation were carried out at different temperatures.

Theoretical study of the Diels-Alder reaction

The cycloaddition of ethylene to butadiene has been studied by theab initio LCAO-SCF-MO method of Roothaan using STO-3G and 7s-3p basis sets.The potential energy hypersurface of the supersystem formed by the reactants has been calculated in order to determine the reaction path. It was found that, during the approach of the partners, the planes of the molecules form an angle around 70 °.The activated complex has a geometry which prefigures the half-chair conformation of cyclohexene and exhibits no biradical character. Our theoretical results are in reasonable agreement with the corresponding experimental ones.

An Internal Coordinate Invariant Reaction Pathway

In this work we show that some properties of a potential energy surface are not independent of the choice of the coordinate frame. So the reaction pathway often described as steepest descent way does not correspond to an invariant curve under coordinate transformations. We propose an internal intrinsic reaction pathway by using some quasi-dynamical considerations (like instantaneous internal acceleration). Our work precises the intrinsic-reaction coordinates of Fukui to any set of 3N-6 internal parameters. Finally, from the equations of motion we deduce the form of the normal reaction coordinates frame anywhere along the postulated reaction pathway.

The Influence of Gem-substitution On the Stability of Open-shell and Closed-shell Systems - the Captodative and Anomeric Effects Reexamined

In this paper we examine the influence of gem-substitution on the stability of carbon-centred radicals and their parent molecules, using the unifying concept of thermodynamic stabilization energy. We deduce this quantity either from available experimental data or from theoretical enthalpies of formation calculated at various theoretical levels. It is found that captodative substitution significantly stabilizes carbon-centred radicals and generally destabilizes methane. Moreover, most captodative (cd) radicals studied in this work possess a sizable extra stabilization which demonstrates a synergetic effect of the captor and donor substituents at the radical centre. Didative (dd)-substituted methyl radicals and parent molecules are most often stabilized by a non-negligible anomeric effect. Finally most cc-substituted species are destabilized. On the whole, cdCH2 compounds have systematically lower C-H bond strengths than ccCH2 and ddCH2 species due to the joint action of the captodative and the generalized anomeric effects. We have also shown that enthalpies of reactions can not be used to analyse the influence of substituents on the stability of chemical species. More particularly, the energy changes of the processes R. + CH4 --> RH + CH3 and those of group separation reactions are far from being equal to radical and anomeric stabilization energies, respectively.

A theoretical investigation of the structure and reactivity of nitrogen-centred radicals

Abstract It is shown that N-centred radicals carrying two electron-donor substituents are persistent or stable due to their large thermodynamic stabilization energy. On the other hand, aminyl radicals carrying both an electron-donor and an electron-acceptor substituent (the so-called push-pull aminyl radicals) may exhibit, even in the gas phase, an appreciable thermodynamic stabilization quantitatively measured by their extra- or merostabilization energy. They are also kinetically stabilized, essentially because of the destabilization of their dimer. Finally N-centred radicals carrying two electron-acceptor substituents are largely destabilized. They are either transient or persistent depending on how much their dimer is destabilized.

Heats of Formation of Some Small Molecules and Radicals

Abstract In this work, we determine the heat of formation of some small neutral molecules and radicals (NH 2 F, NHF, HOOH, HOO, NH 2 NH 2 , NH 2 OH, NH 2 O, NHOH, CH 3 NH 2 , CH 3 NH, CH 2 NH 2 , HNO 2 and CH 3 BH 2 ). Atomization energies of these compounds are computed at the Moller—Plesset level to full fourth order using a series of extended basis sets of 6-31G type. The results that we obtain confirm some experimental values (NH 3 , NH 2 , H 2 O 2 ,…); nevertheless, some other computed heats of formation call into question the experimental values of CH 3 NH 2 and HOO. We also predict Δ H f298 0 for CH 3 BH 2 (24.25 kcal mol −1 ), HNO 2 (−16.72), NHOH (25.25), NH 2 O (−16.73) and NH 2 F (−9.64).

Etude theorique de la cycloaddition de la nitrone sur l'ethylene

Abstract A theoretical study of the concerted 1,3-dipolar cycloaddition of nitrone on ethylene has been carried out by the LCAO-SCF-MO method. The potential energy hypersurface for the reaction has been investigated and the characteristics of the transition state have been determined.