Alkanes bond length

The expense is justified, however, when tackling polymer chains, where reconstruction of an entire chain is expressed as a succession of atomic moves of this kind [121]. The first atom is placed at random the second selected nearby (one bond length away), the third placed near the second, and so on. Each placement of an atom is given a greater chance of success by selecting from multiple locations, as just described. Biasing factors are calculated for the whole multi-atom move, forward and reverse, and used as before in the Metropolis prescription. For fiirther details see [122, 123. 124. 125]. A nice example of this teclmique is the study [126. 127] of the distribution of linear and branched chain alkanes in zeolites. 

As the number of carbon atoms in the alkane increases, so does the number of possible stractural isomers. Thousands of different alkanes exist, because there are no limits on the length of the carbon chain. Regardless of the number of the chain length, alkanes have tetrahedral geometry around all of their carbon atoms. The structure of decane, Cio H22, shown in Figure 9-15. illustrates this feature. Notice that the carbon backbone of decane has a zigzag pattern because of the 109.5° bond angles that characterize the tetrahedron. 

If the diamond lattice itself is used for the mapping of the PE chains, each internal bead represents a methylene unit, the step length is the C-C bond length, and CxH2x 2 is represented by x beads. Typical bulk densities for n-alkane melts, which are in the range 0.7-0.8 g/cm3, are achieved with occupancy of 16-19 % of the sites on this lattice. 

Bartell and coworkers investigated the structures of a series of noncyclic alkanes by means of gas electron diffraction (14, 44, 45) and invoked for the interpretation of their results a simple force field which contained to a high extent vibrational spectroscopic constants of Snyder and Schachtschneider. This force field reproduces bond lengths and bond angles of acyclic hydrocarbons well, energies of isomerisation satisfactorily. As an example, Fig. 8 shows geometry parameters of tri-t-butylmethane as observed by electron diffraction and calculated with this force field (14). 

Table 3.27. Symmetry, bond lengths Raa and Rah, bond angle 6 hah and theoretical strain energy (TSE) for small cyclic alkanes (A = C) and silanes (A = Si), with corresponding geometrical variablesT of acyclic (C2V) propane and...

Aromatic compounds have special characteristics of aromaticity, which include a low hydro-gen carbon atomic ratio, C-C bonds that are quite strong and of intermediate length between such bonds in alkanes and those in alkenes, tendency to undergo substitution reactions rather than the addition reactions characteristic of alkenes, and delocalization of n electrons over several carbon atoms. The last phenomenon adds substantial stability to aromatic compounds and is known as resonance stabilization. 

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