Self hydrogenated molecules

Fig. 31. Supramolecular (hydrogen-bonded) motifs of self-complementary molecules (196).

Wyler, R. de Mendoza, J. Rebek, J., Jr. Formation of a cavity by dimerization of a self-complementary molecule via hydrogen bonds. Angew. Chem. Int. Ed. Engl. 1993,32, 1699-1701. 

Fig. 3.6 Binding energy curves for the hydrogen molecule (lower panel). HF and HL are the Hartree-Fock and Heitler-London predictions, whereas LDA and LSDA are those for local density and local spin density approximations respectively. The upper panel gives the local magnetic moment within the LSDA self-consistent calculations. (After Gunnarsson and Lundquist (1976).)...

This is plotted in the right-hand panel of Fig. 3.8 as a function of I/2 h. Remembering that h(R) - 0 as R - oo, we see that it shows the same square root distance-dependence as that displayed by the numerical self-consistent solution of the local spin density functional Schrddinger equation in Fig. 3.6. Thus, as the hydrogen molecule is pulled apart, it moves from the singlet state S = 0 at equilibrium to the isolated free atoms in doublet states with S = 2-... 

He concludes that the first (associative) mechanism gives values nearest the observed heat of adsorption determined by Beeck (30), and is therefore accepted as nearest the truth (34) (Qo (calculated) = 42 kcal./ mole Qo (observed) = 58 kcal./mole). Experiments on tungsten and nickel films (Beeck (35), Trapnell (36), and more recent work in Rideal s laboratory) have shown that when ethylene is added to a clean metal surface ethane appears in the gas phase. A self hydrogenation mechanism must be operative and at least in these cases dissociation of ethylene must occur on the catalyst. It is suggested that the calculations might be complicated by the energy of bond strain in the adsorption of an ethylene molecule to the fixed lattice distances of the metal. 

During this period, accurate solutions for the electronic structure of helium (1) and the hydrogen molecule (2) were obtained in order to verify that the Schrodinger equation was useful. Most of the effort, however, was devoted to developing a simple quantum model of electronic structure. Hartree (3) and others developed the self-consistent-field model for the structure of light atoms. For heavier atoms, the Thomas-Fermi model (4) based on total charge density rather than individual orbitals was used. 

Indeed, let us consider, for example, olefin self-hydrogenation. This is generally a multistage process beginning with disproportionation of a pair of olefin molecules to the corresponding alkane and a polyunsaturated compound (diene or alkyne), which rapidly polymerizes to form high-molecular-weight products P ... 

At higher tenqperatures self-hydrogenation and fragmentation of adsorbed molecules take dace. 

The chiral RNA isomer pyranosyl-RNA (p-RNA, Scheme 14D), which has its ribose in the pyranosyl rather than in the furanosyl form, and phosphodiester linkages between the 2 and 4 positions, instead of 3 and 5, forms stronger and more selective hydrogen bonds than RNA. Moreover, it has recently been shown that p-RNA oligomerization on a p-RNA template is chiroselective for the formation of oligomers of the same handedness, which makes p-RNA a serious candidate to be considered as a possible precursor of RNA as the first self-replicating molecule. 

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