Transition states theoretical calculations

Theoretical calculations by Yamabe <2001PCA7281> supported Olsen s conclusion of an unsymmetrical yet concerted biradical as transition state. Theoretical calculations for activation and entropy change for tetramethyl diazetines 66b are in good agreement with the experimentally observed value (AH cxi = 35.3 kcal mol-1, AH exp = 31.7 kcal mol-1 = 0.94 cal mol-1 AY xp = 0.3 0.8calmol-1). Tetramethyl diazetine 66b was... 

In a study of the methane complex [(diimine)Pt(CH3)(CH4)]+ (diimine = HN=C(H)-C(H)=NH), relevant to the diimine system experimentally investigated by Tilset et al. (28), theoretical calculations indicate preference for the oxidative addition pathway (30). When one water molecule was included in these calculations, the preference for oxidative addition increased due to the stabilization of Pt(IV) by coordinated water (30). The same preference for oxidative addition was previously calculated for the ethylenediamine (en) system [(en)Pt(CH3)(CH4)]+ (151). This model is relevant for the experimentally investigated tmeda system [(tmeda)Pt(CH3)(solv)]+ discussed above (Scheme 7, B) (27,152). For the bis-formate complex Pt(02CH)2, a a-bond metathesis was assumed and the energies of intermediates and transition states were calculated... 

Molecular modeling, like all other technical disciplines, has its own jargon. Much of this is described in Appendix B (Common Terms and Acronyms), and only one aspect will be addressed here. This concerns specification of theoretical model used for property calculation together with theoretical model used for equilibrium (or transition-state) geometry calculation. 

A theoretical study of the thermal isomerization and decomposition of oxalic acid has attempted to account for the predominant formation of C02 and HCOOH from the vapour at 400-430 K.41 Transition-state theory calculations indicate that a bimolecular hydrogen migration from oxygen to carbon of intermediate dihydroxycarbene (formed along with C02) achieved through a hydrogen exchange with a second oxalic acid... 

As far as relative reactivity is concerned, much effort continues to be directed towards the evaluation of reactivity ratios. While some of this is experimentally based, there has also been a number of essentially theoretical papers.Some transition-state theory calculations on the propagation reaction in cationic polymerization have also been reported. 

Poor agreement is observed between the experimental and theoretical values [calculated by a combined method, that is, quantum-chemical calculation of the activation energy (Table 37.7 and Table 37.8) and the frequencies of the vibrations of the bonds in the prereaction complex and in the transition state and calculation of the rate constants on the basis of RRKM theory] for certain processes where the effects of electron correlation and the contributions of the excited electronic configurations are not predominant [68-73]. 

The results from theoretical calculations by DFT methods DFT/BPW91 and B3LYP with the standard BASIS 6-31G (d) showed that the addition reactions between O3 and double bonds of terpinolene (75), /1-phellandrene (76), and limonene (77) were possible and thermodynamically favoured the transition states were calculated and stereoselectivity of the reactions was discussed from thermodynamic and orbital points of view. ... 

Fig. 23 The theoretical rate coefficient calculated using several methods. The uppermost curve is the full calculation with the vibationally adiabatic treatment of the torsion. The TS2 and TS2 curves are the results using the normal mode treatment of the two saddlepoints. The TS2 + TS2 curve is the result of summing the flux through both transition states. All calculations include tunneling.

The above discussion represents a necessarily brief simnnary of the aspects of chemical reaction dynamics. The theoretical focus of tliis field is concerned with the development of accurate potential energy surfaces and the calculation of scattering dynamics on these surfaces. Experimentally, much effort has been devoted to developing complementary asymptotic techniques for product characterization and frequency- and time-resolved teclmiques to study transition-state spectroscopy and dynamics. It is instructive to see what can be accomplished with all of these capabilities. Of all the benclunark reactions mentioned in section A3.7.2. the reaction F + H2 —> HE + H represents the best example of how theory and experiment can converge to yield a fairly complete picture of the dynamics of a chemical reaction. Thus, the remainder of this chapter focuses on this reaction as a case study in reaction dynamics. 

Theoretical work by the groups directed by Sustmann and, very recently, Mattay attributes the preference for the formation of endo cycloadduct in solution to the polarity of the solvent Their calculations indicate that in the gas phase the exo transition state has a lower energy than the endo counterpart and it is only upon introduction of the solvent that this situation reverses, due to the difference in polarity of both transition states (Figure 1.2). Mattay" stresses the importance of the dienophile transoid-dsoid conformational equilibrium in determining the endo-exo selectivity. The transoid conformation is favoured in solution and is shown to lead to endo product, whereas the cisoid conformation, that is favoured in the gas phase, produces the exo adduct This view is in conflict with ab initio calculations by Houk, indicating an enhanced secondary orbital interaction in the cisoid endo transition state . 

To account for the course of this reaction theoretical calculations of the coordination of ketomalonate 37 to copper(II) and zinc(II) have revealed that the six-membered ring system is slightly more stable than the five-membered ring system (Scheme 4.30). The coordination of 37 to catalyst (l )-39 shows that the six-membered intermediate is C2-symmetric with no obvious face-shielding of the carbonyl functionality (top), while for the five-membered intermediate (bottom) the carbonyl is shielded by the phenyl substituent. Calculations of the transition-state energy for the reaction of the two intermediates with 1,3-cyclohexadiene leads to the lowest energy for the five-membered intermediate this approach is in agreement with the experimental results [45]. 

One cannot discuss Lewis acid-catalyzed cycloaddition reactions in the present context without trying to understand the reaction course mechanistically, e.g. using a frontier molecular orbital (FMO) point of reasoning, or theoretical calculations of transition state structures. 

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