Alkene potential energy

It has been known for more than a century that hydrocarbons containing double bonds are more reactive than their counterparts that do not contain double bonds. Alkenes are, in general, more reactive than alkanes. We call electrons in double bonds 71 electrons and those in the much less reactive C—C or CH bonds Huckel theory, we assume that the chemistry of unsaturated hydrocarbons is so dominated by the chemistry of their double bonds that we may separate the Schroedinger equation yet again, into an equation for potential energy. We now have an equation of the same fomi as Eq. (6-8), but one in which the Hamiltonian for all elections is replaced by the Hamiltonian for Ji electrons only... 

The purpose of this article is to review some of the current endeavors in this developing field. To maintain brevity, the focus is on recent studies carried out in our own laboratory and in conjunction with Professor M.T. Bowers at the University of California at Santa Barbara, with emphasis on the use of kinetic energy release distributions and infrared laser multiphoton excitation to probe potential energy surfaces for the reactions of atomic metal ions with alkenes and alkanes. 

The potential energy surface for the hydroformylation of ethylene has been mapped out for several catalytic model systems at various levels of theory. In 1997, Morokuma and co-workers [17], considering HRh(CO)2(PH3) as the unsaturated catalytic species that coordinates alkene, reported free energies for the full catalytic cycle at the ab initio MP2//RHF level. Recently, in 2001, Decker and Cundari [18] published CCSD(T)//B3LYP results for the HRh(CO)(PH3)2 catalytic complex, which would persist under high phosphine concentrations. Potential energy surfaces for both Rh-catalyzed model systems were qualitatively very similar. The catalytic cycle has no large barriers or deep thermodynamic wells to trap the... 

Alkenes strained by twist or r-bond torsion, such as E-cyclooctene, exhibit much lower barriers due to relief of strain in the TS for the oxygen transfer step. While the epoxidation of symmetrically substituted alkenes normally involve a symmetrical approach to the TT-bond, the TSs for epoxidation of E-cyclooctene and E-l-methylcyclooctene exhibit highly asymmetric transition structures. The AAE = 3.3 kcalmol" for E- versus Z-cyclooctene is clearly a reflection of the relative SE of these two medium ring alkenes (16.4 vs 4.2 kcalmol ) ". The classical activation barrier (AE ) for the highly strained bicyclo[3.3.1]non-l-ene is also quite low (Table 10, Figure 26). In these twist-strain alkenes, the approach of the peracid deviates markedly from the idealized spiro approach suggesting fliat this part of the potential energy surface is quite soft. 

Theoretical calculations on the cycloaddition reactions of a range of 1,3-dipoles to ethene in the gas phase have been carried out (85) with optimization of the structures of these precursor complexes and the transition states for the reactions at the B3LYP/6-31G level. Calculated vibration frequencies for the orientation complexes revealed that they are true minima on the potential energy surface. The dipole-alkene bond lengths in the complexes were found to be about twice that in the final products and binding was relatively weak with energies <2 kcal mol . Calculations on the cycloaddition reactions of nitrilium and diazonium betaines to ethene indicate that the former have smaller activation energies and are more exothermic. 

FIGURE 2. Potential energy surfaces of Si, T and So states of alkenes and their dependence on the torsional angle. Reprinted with permission of Academic Press, Inc. from Ref. 3... 

FIGURE 3. Potential energy surfaces proposed for one-way (a,c) and two-way (b) isomerization of alkenes. Reprinted with permission from Ref. 9c. Copyright (1993) American Chemical Society... 

Hopkinson was hired by York to teach theoretical organic chemistry (the Woodward-Hoffmann rules were then a hot topic) and to carry out experimental chemistry. Despite the limited computing capacity at York at the time, he managed to complete some work on the electrophilic addition to alkenes. He is probably best known, however, for his work on proton affinities, destabilized carbocations,234 organosilicon compounds, silyl anions and cations, and more recently, on the calculation of potential energy surfaces and thermodynamic properties. He has had a particularly fruitful collaboration with Diethard Bohme.235... 

The energetics of the low-, intermediate-, and high-spin states of the key species on the potential energy surface for oxidation of alkenes by Mn(salen) (and the related chromium species [50]) complexes have received considerable attention, following the suggestion [51] by Linde et al that spin-state changes play an important role in the diastereoselectivity of this... 

Generally, as the potential energy level of SOMO increases (becomes a more reactive radical), free radicals have nucleophilic character, while as the potential energy level of SOMO decreases (becomes a stable radical), free radicals have electrophilic character. Thus, when effective radical reactions are required, small energy difference in SOMO-HOMO or SOMO-LUMO interactions is necessary. For example, the relative reactivities of radical addition reactions of a nucleophilic cyclohexyl radical to alkenes,... 

Fig. Graph of potential energy versus reaction coordinate for an uncatalysed and a catalysed hydrogenation reaction of an alkene.

Heats of hydrogenation are usually I exothermic. A larger amount of heat I given off implies a less stable alkene, because the less stable alkene starts from a higher potential energy. 

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