Knowledge Bank

Several years ago we reported the low frequency intramolecular band assignments in the neutron spectra of the solid normal alkanes $[CH3(CH2)N-2CH3$ with $5\le N\le 8]$ on the basis of a normal coordinate calculation using a simplified valence force field. Recently we have extended our investigations to include the corresponding n-alkyl bromides $[CH3(CH2)N-2CH2Br]$ and the n-alkyl dibromides $[CH2Br(CH2)N-2CH2Br]$. By adding only four additional force constants which determine the relative motion of the bromine atom, we are able to assign the skeletal bending and torsion modes of all of these molecules in their solid phase. These assignments are consistent in that most of the bands in all three series of compounds show systematic band progressions which are closely correlated with the dispersion curves of polymethylene. One exception is n-butyl dibromide where our analysis indicates the presence of more than one conformer.