Assignment of the lowest-lying THz absorption signatures in biotin and lactose monohydrate by solid-state density functional theory D.G. Allis a , A.M. Fedor a , T.M. Korter a, * , J.E. Bjarnason b , E.R. Brown b a Department of Chemistry, Syracuse University, Syracuse, NY 13244, United States b Electrical and Computer Engineering Department, University of California, Santa Barbara, CA 93106, United States Received 7 January 2007; in final form 30 March 2007 Available online 18 April 2007 Abstract The narrow terahertz (THz) features in crystalline biotin and lactose monohydrate observed in recent experimental studies are con- sidered 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. Ó 2007 Elsevier B.V. All rights reserved. 1. Introduction Room temperature (RT) terahertz (THz) spectra of molecular crystals often reveal absorption features with 5–10 cm 1 peak widths. RT biotin [1] and lactose mono- hydrate [2] have been shown to exhibit narrow (1 cm 1 in lactose monohydrate) features at sub-33 cm 1 (1 THz) frequencies, a result with relevance to many research groups. One possible explanation for narrow THz line shapes is the decoupling of single vibrational modes from nearby modes, combined with predominantly harmonic motion. A second may be the intermolecular interaction strength in molecular crystals where such narrow THz fea- tures have been observed. In the weakly-interacting molec- ular solids HMX [3] and PETN [4], the RT THz features were found to be broad (5 cm 1 ), while biotin [5] and lac- tose monohydrate [6] diffraction studies reveal hydrogen bond-rich networks that cross unit cells. The key to under- stand observed THz line shapes is the assignment of the molecular motions themselves, a task for which solid-state density functional theory (DFT) has received recent atten- tion [3,4,7,8]. This work reports the THz spectrum of biotin obtained with a high resolution photomixer-based spectrometer, compares it to the recently reported spectrum of lactose monohydrate [2], and employs solid-state DFT calculations to predict the motions associated with the narrow observed THz features in both. As a continuation of previous solid- state DFT THz studies [3,4], this work also considers the differences between isolated-molecule and solid-state vibra- tions in the biotin and lactose molecules. The computa- tional study includes isolated-molecule results based on the B3LYP functional, one commonly employed for the vibrational assignments of molecules but only recently made available for solid-state vibrational studies. Previous THz studies [3,4] have demonstrated that B3LYP-based isolated-molecule calculations perform better at geometry reproduction than many of the density functionals available in current solid-state DFT codes, but only in the absence of the crystal cell interactions. The inclusion of the crystal cell remarkably improves bond length and THz spectral 0009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.04.032 * Corresponding author. Fax: +1 3154434070. E-mail address: tmkorter@syr.edu (T.M. Korter). www.elsevier.com/locate/cplett Chemical Physics Letters 440 (2007) 203–209