Methylene bending motion

The simplest vibrational motions in molecules giving rise to the absorption of infrared radiation are stretching and bending motions. Illustrated in Figure 11.3 are the fundamental stretching and bending vibrations for a methylene group. 

The simulation by Bareman et al. involved 90 chains, each of which was made up of 20 identical pseudoatoms meant to represent methylene groups and the terminal methyl. Changes in chain conformations, including bond-bending and chain torsional motions, were allowed, with potentials known to provide good representations of short-chain alkanes. Interchain interactions were computed from pairwise additive Lennard-Jones 12-6 potentials. A 9-3 potential, which is appropriate for the interaction of a L-J particle above the surface of a bulk medium, was taken for the interaction of each methylene with the surface, with a well depth 5 times deeper than that for the CHj—CH2 interaction. The simulations were carried out with periodic boundary conditions for three fixed areas, 21,26, and 35 A per chain. 

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