Benson barrier

Isomerizations are important unimolecular reactions that result in the intramolecular rearrangement of atoms, and their rate parameters are of the same order of magnitude as other unimolecular reactions. Consequently, they can have significant impact on product distributions in high-temperature processes. A large number of different types of isomerization reactions seem to be possible, in which stable as well as radical species serve as reactants (Benson, 1976). Unfortunately, with the exception of cis-trans isomerizations, accurate kinetic information is scarce for many of these reactions. This is, in part, caused by experimental difficulties associated with the detection of isomers and with the presence of parallel reactions. However, with computational quantum mechanics theoretical estimations of barrier heights in isomerizations are now possible. 

Heats of formation assume resonance stabilizations 10.8 kcal mole-1 in ( CHaCN) 12.6 kcal mole-1 in (CH3CHCN) and in [(CH3)2CCN]. " Na = doubly bonded nitrogen in azo compounds. h This correction assumes that the barrier to rotation in the radical R is two-thirds the barrier in the corresponding alkane RH. See O Neal and Benson for further discussion of this point. AH° and to +2 cal mole-1 °K 1 for 5°. The following example shows how the table is used to calculate thermodynamic properties for the 2-butyl radical (12).41 H3C—ch—ch2—ch3 12 ... 

Eykholt GR, Elder CR, Benson CH. Effects of aquifer heterogeneity and reaction mechanism uncertainty on a reactive barrier. J Hazard Mater 1999 68 73-96. 

The results of Benson and McLaughlin104 indicate that no barrier to reaction exists, nor do they show a minimum, which is in contrast to the stable HeH4 complex bound by 0.44 eV reported by Poshusta, Haugen, and Zetik255 using a minimal valence-bond Cl technique. 

The data necessary for thermodynamic estimates are available from experimental as well as computational methods. In many systems AGh can be approximated by experimentally accessible AGJ. The approximation is valid (to within 0.05-0.15 eV) if the radical coupling has no barrier (is diffusion limited) and the thermolysis is carried out under conditions selected to minimize the cage recombination [79]. The homolytic bond strengths can also be obtained in many cases from the Benson group-additivity tables [80] or semiempirical quantum or molecular mechanics calculations [81]. With appropriate entropy corrections [75f], relatively accurate AGh values can be obtained in that way. 

The second mechanism, suggested originally by Rabinovitch and coworkers but later elaborated by (and usually attributed to) Benson involves trimethylene, the biradical generated by homolysis of one C-C bond in cyclopropane, as an intermediate in the reaction. The thermochemical calculations of Benson suggested that this intermediate might sit in a potential energy well some 9 kcal mol" deep. Since the barrier to rotation about the C-CH2 bonds would be expected to be much smaller than this, it followed that the intermediate should lose stereochemical information by essentially random rotation at both methylenes. 

II), RRKM calculations were performed to determine the rate constants. The program Fall-Off", available from the Quantum Chemistry Exchange Program (2J) and modified by Shandross and Howard (24) was used lor the RRKM calculations. Transition states were located at the centrifugal barrier on the minimum energy path from reactants to products. Bond vibration and rotation frequencies were estimated using the techniques of Benson (25). 

Our current estimates for a-allyl are currently based on calculations for CH3 with corrections to allyl based on Benson-type estimates (18). In addition, we have not yet estimated the stability of the TT-allyl species. Most important, we have not calculated the transition states and energy barriers for any of the steps. 

AA Benson, RE Summons. Arsenic accumulation in Great Barrier Reef invertebrates. Science 211 482-483, 1981. 

O Neal and Benson have carried out a detailed analysis of the entropies and heat capacities of alkyl free radicals, and we refer the reader to their work. It is worth noting that entropies and heat capacities for free radicals are not measured directly, but either calculated by one of the techniques outlined earlier or inferred from experimental kinetic measurements in conjunction with some calculations. Principal sources of uncertainty in such calculations have been, and continue to be, questions of structural symmetry and barriers to internal rotations in the radicals. The calculation of butyl radical illustrates both of these uncertainties. O Neal and Benson and later Benson assumed that the radical site in planar, and thus has a symmetry number <7 = 162 (6 for... 

Benson, A. A., and Summons, R. E., 1981, Arsenic accumulation In Great Barrier Reef Invertebrates, Science, 211 482. 

Khire, M., Benson, C., Bosscher, P., 2000. Capillary barriers in semiarid and arid climates design variables and the water balance. Journal of Geotechnical and Geoenvironmental Engineering, ASCE 126 (8), 695—708. 

In place of using statistical mechanics formulas to evaluate one can estimate it by thermochemical arguments using procedures developed by Benson (1976) and co-workers. The value of this method is that with some intuition and practice one can make reasonable guesses of the transition structure, compute its entropy of formation, and thus find preexponential factors. As with TST generally, no prediction is made for the barrier height and so the activation energy must be taken from experiments. We present here an outline of the thermochemical data manipulations required in this formulation and illustrate them with an application. 

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