Defect Modes as Structural Probes in Polymethylene Chains

13 Defect Modes as Structural Probes in Polymethylene Chains 

The strongest effort by our group and by other authors was aimed at revealing the structure and molecular dynamical evolution of the very many classes of technologically and scientifically relevant molecules consisting of n-alkanes or n-alkyl long chains. We report below some of the results obtained and will show some applications. 

Let us introduce a CD2 group as a defect in n-alkanes (CH3-(CH )nCH3) or n-alkyl chain (CH3(CH2)xX or in any polymethylene chain. This is a typical case of mass defect which leaves the force constant matrix (Eq. 3-3)) unchanged because of Born-Oppenheimer approximation. The site where the CD2 group is located can be changed along the chain. Chemically, this can be easily achieved and selectively deuterated n-alkane or alkyl derivatives are either commercially available or can be (and have been) easily synthesized. 

The main characteristic defect modes calculated (and observed) for trans planar segments are the following  

GG defect. The wagging vibration of a CH2 group located between two adjacent gauche conformations CH2(G)-CH2(G)-CH2- wagging. The calculated (and 

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