In press, available from Chemical Physics Letters. This is a first foray into the very-low frequency terahertz region (3 to 30 cm-1) for the study of hydrogen-bonded molecular solids in collaboration with J. Bjarnason and E. Brown in the Electrical and Computer Engineering Department of the University of California, Santa Barbara. The analysis of these systems is generally more complicated because the extensive hydrogen bonding networks strongly connect individual molecules, which leads to a blurring of the line between internal (intramolecular) and external (crystal cell, intermolecular) vibrational modes. It is clear from these studies that the crystal cell strongly influences the predicted positions of both the molecular vibrations and the molecular conformations. In the case of lactose, the molecule exists in a configuration that breaks an important intramolecular hydrogen bonding interaction (blue arrow in the figure) in the crystal cell of its monohydrate form. For biotin, the conformation of the molecule in its crystal is NOT the conformation of the solution-phase molecule, with the many neighboring hydrogen bonding interactions forcing the single molecule to conform to its elongated (extended) hydrocarbon tail structure. In both cases, isolated-molecule computational studies would lead to incorrect assignments for the simple reason that the molecules are conformationally different in their solid-state forms.
D.G. Allis, A.M. Fedor, T.M. Korter, J.E. Bjarnason,* and E.R. Brown*
Abstract: The narrow terahertz (THz) features in crystalline biotin and lactose monohydrate observed in recent experimental studies are considered by solid-state density functional theory (DFT) calculations. The lowest-frequency THz features in both solid-state biotin and lactose monohydrate are assigned to external hindered rotational modes and not to the lowest-frequency internal modes predicted from isolated molecule calculations. The motions of the molecules associated with these narrow THz features and the interactions between molecules in the hydrogen-bonded networks of these molecular crystals are discussed, and comparisons are made to similar studies on molecular crystals not exhibiting strong intermolecular interactions.
Keywords: Biotin, lactose monohydrate, terahertz spectroscopy, solid-state density functional theory