Next-nearest-neighbour direction

Computer simulations of monolayers also showed the presence of different phases at different temperatures206 207. A phase transition at 40 K in the monolayer of tridecanethiol refers to the change in the orientation of alkane chains from the nearest-neighbour direction to the next nearest neighbour. The melting of alkane chains was observed at ca 400 K208. 

An example of tacticity effects in poly(epoxides) is shown in the backbone spectrum of anionically polymerised poly(butylene oxide) in Figure 1.27 [35]. The CH resonance is narrowly split into eight peaks of essentially equal intensity, consistent with chemical shift sensitivity at the tetrad level. Evidently the CH carbon senses the tacticity of its nearest neighbours in both directions in the chain, but senses the tacticity of its next-nearest neighbour in only one direction. The CH2 carbon is split into four equal peaks, consistent with sensitivity to tacticity at the triad level. The fact that the triad and tetrad peaks are essentially equal in intensity indicates a perfectly random tacticity distribution, i.e., = 0.5. 

Compare the result of the calculation with the interatomic distance calculated for a NaCl crystal with a density of = 2.165g/cm l The molar mass of Na is 23g/mol, that of chlorine is 35.4g/mol. Avogadro s constant (the number of molecules in a mol) is Aa = 6.022 x lO mol b Estimate Young s modulus of NaCl in the (100) direction Neglect the bonds between next-nearest neighbours and those even further away Note leV= 1.602 X 10 J. 

In other systems we may encounter chain tilts in the next-nearest-neighbour (NNN) direction or in general (non-symmetry) directions. 

Both models are the tutorial model of Fig. 7,2. In model 1 the molecules tilt towards nearest neighbours, in model 2 towards next nearest neighbours. All other parameters are essentially determined from the reflectivity data. The dashed lines represent a perturbation of the models The molecules were tilted in a direction 8 from the symmetry directions. 

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