Isobutane, molecular model

Isobutane, C4H10, has the same molecular formula as butane, the straight-chain hydrocarbon. However, isobutane and butane have different structural formulas and, therefore, different molecular structures. Butane and isobutane are constitutional (or structural) isomers, compounds with the same molecular formula but different structural formulas. Figure 24.4 depicts molecular models of isobutane and butane. Because these isomers have different structures, they have different properties. For example, isobutane boils at - 12°C whereas butane boils at 0°C. Here the difference in boiling point can be attributed to the fact that isobutane has a more compact molecular structure than butane, which results in weaker intermolecular interactions between isobutane molecules. 

Molecular mechanics (MM), molecular dynamics (MD), and Monte-Carlo (MC) methods were employed to simulate the adsorption of methane, ethane, propane and isobutane on silicalite and HZSM-5. The silicalite was simulated using the same cluster-model adopted in the diffusion calculations. The H-ZMS-5 structure was constructed according to the procedure suggested by Vetrivel et al. [32], which consists in replacing one atom at the channel intersection by and protonating the oxygen atom bridging the Ta and Tg sites in order to preserve the lattice neutrality. 

The model was also applied to the substitution of alkyl groups by tritium to give the alkanes of lower molecular weight which are observed as products. That these products are not formed by some deep-seated rearrangement is evidenced by the fact that the tritiated alkanes always relate to the parent in a simple way. For example, n-pentane gives only tritiated methane, ethane, propane and butane. Isopentane also gives isobutane ... 

Figure 4.18 Adsorption isotherms of isobutane in Silicalite. Molecular simulations (open symbols) using the models of Vlugt et al. [36] (a = 3.60A, circles), Smit et al. [142] (cr = 3.64A, squares) and June et al. [141] (a = 3.364A, triangles), see table 4.1. Experimental data (closed symbols) from Zhu etal. [135] (circles) and Sun etal. [136] (diamonds).

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