Conformational distinguishability of medium cycloalkanes in crystals via inelastic neutron scattering

This paper begins an exploration of the use of the combination of DFT computations with experimental inelastic neutron scattering (INS) spectra as a method for establishing what conformation is present in a molecular crystal at low temperature. Presented here are INS spectra of a series of medium-sized cycloalkanes: C ₆H ₁₂, C ₇H ₁₄, C ₈R ₂₀, C ₁₀H ₂₀, C ₁₂H ₂₄, and C ₁₄H ₂₈. Optimized geometries and normal mode calculations were performed at B3LYP/6-311G(d,p) on the lowest energy conformations (i.e., those thermally accessible at the experimental temperature of 30 K). The calculated and observed spectra were analyzed for the best fit from each set of conformers, allowing a prediction of the dominant conformation in a conformationally rich system. For each cycloalkane, the calculated spectrum for the lowest energy conformer shows good agreement with experiment while the higher energy conformations have a much poorer fit. With little ambiguity, the lowest energy conformer is therefore predicted to be the dominant conformation, consistent with the diffraction data available for C ₆H ₁₂, C ₁₀H ₂₀, C ₁₂H ₂₄, and C ₁₂H ₂₄. These results indicate that INS spectroscopy may be a useful tool in determining the dominant conformation in a crystal lattice in cases such as this in which the intermolecular interactions are weak and the different conformers are calculated to have distinguishable spectra. Such an analysis is applied to the cases of C ₇H ₁₆ and C ₈H ₁₆ for which no low-temperature X-ray analysis is available. Clear structure predictions result, and the conformer observed is that computed to be of lowest energy for the molecule in isolation.