Reaction pathway rectangular

Here we make a number of sweeping approximations in order to obtain an estimate of the tunneling matrix element fi. The multidimensional tunneling problem is first projected onto one dimension corresponding to the reaction pathway, in line with a number of other related approaches.The path is then modeled by two equivalent rectangular potential wells of width a and depth V i, separated by a distance b. The textbook solution to the single well problem is ... 

The biochemistry, however, is a little less direct. The pathway to make urea involves a sequence of reactions called the urea cycle. This, like the TCA cycle, was discovered by Hans Krebs. In fact he found the urea cycle first, and it seems likely that one provided the inspiration for the other. In the urea cycle, the molecule of urea is built up on a sort of molecular handle, provided hy ornithine. Ornithine is an amino acid, but not one of the 20 that make up proteins. It is not important for you to know its stmcture, and so in Fig. 24.1 it is just shown as a little rectangular box. 

Cycloaddition of species with triple bonds, which should logically be addressed at this point, will be postponed to later chapters. The reluctance of acetylene to dimerize to cyclobutadiene (CBD) on the ground-state surface follows directly from Fig. 6.2. It is sufficient to note that when two acetylene molecules approach one another in the plane-rectangular (D2/1) orientation, the two additional tt orbitals in acetylene are retained as such in CBD, so they cannot alleviate the forbiddenness of the [ 2g + pathway [5, Fig. 4]. Discussion of the reaction between dioxygen and acetylene to form 1,2-dioxetene and the cycloreversion of tetraalkyl-l,2-dioxetanes to two ketonic fragments has to be postponed until the relation between space and spin symmetry has been introduced in Chapter 9. 

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