Conformer Pair Contributions to Optical Rotations in a Series of Chiral Linear Aliphatic Alcohols

Abstract

The chain length effect of four chiral aliphatic alcohols, (S)-2-butanol, (S)-2-pentanol, (S)-2-hexanol and (S)-2-heptanol, on their specific optical rotations(OR) was studied experimentally and theoretically via quantum theory. Many conformations of each chiral alcohol exist as conformer pairs in solution. The OR sum from these pairs of conformers has much smaller contributions to OR values than that contributed by the most stable conformation. These four alcohols’ OR values were also investigated using the matrix model, which employs each substituent’s comprehensive mass, radii, electronegativity and symmetry number as the elements in the matrix. These are all particle properties. This matrix determinant is proportional to its OR values within a closely related structural series of chiral compounds. The experimental OR values and the matrix determinants of these four alcohols were compared with the predicted OR values obtained from quantum theory wave functions. The ORs predicted by the matrix method, which is based on particle function statistics, agreed with the results from quantum theory. The agreement between OR predictions by the matrix method and DFT calculations illustrates the wave-particle duality of polarized light that is operating in these predictions.