College Park, Maryland      June 6 - 10 , 2004

WP23: The crystalline enol of 1,3-cyclohexanedione and its complex with benzene: vibrational spectra, simulation of structure and dynamics and evidence for cooperative hydrogen bonding

B. S. Hudson, D. G. Allis, Y. Lan (Department of Chemistry, Syracuse University, Syracuse, NY), D. A. Braden (Schrödinger Inc., Portland, OR), C. T. Middleton, T. Jenkins (Department of Chemistry, Syracuse University, Syracuse, NY), R. Withnall (School of Chemical and Life Sciences, University of Greenwich, Chatham Maritime Campus, Chatham, Kent ME4 4TB, UK.), S. Baronov (Syracuse University/NSF REU Participant, Summer 2000. Current address: Department of Chemistry, Moscow State University, Moscow, Russia.), C. M. Brown (Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742-2115 and NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899-8562)

The inelastic incoherent neutron scattering spectra of 1,3-cyclohexanedione (CHD) in its crystalline enol form and its cyclamer complexes with benzene and benzene-d6 are compared with each other, with IR and Raman spectra and with the results of calculations using density functional theory (DFT). We have also determined the INS spectra of a variety of methyl and deuterium substituted derivatives of CHD. Deuterium isotopic substitution can be used to confirm spectra assignments. The crystal packing of the CHD enol is a linear hydrogen-bonded chain with conjugated donor and acceptor groups analogous to that found in peptide hydrogen bonding. Methyl substitution can have very large effects on the crystal packing of CHD. The benzene complex is a closed hexameric hydrogen-bonded cycle. A DFT treatment is applied to the full hexamer of the benzene:CHD complex. The CHD chain is treated as a series of finite linear clusters by DFT, while the infinite one-dimensional chain and the three-dimensional crystal are treated by periodic DFT. Comparison is made with both the observed crystal structures and the vibrational spectra. The very good to excellent agreement of the computed vibrational spectra with experiment demonstrates that the models and computational treatments used are reliable. The theoretical treatment of linear chain clusters exhibits a continuous change with increasing chain length, converging to values near the observed crystal structure. Emphasis is placed on the cooperative nature of hydrogen bonding in CHD as revealed by these systematic trends. The calculations show that the energy of a linear chain of hydrogen bonded CHD molecules becomes increasingly stable and increasingly more dipolar as that chain length increases. This is consistent with the hypothesis that the cooperative hydrogen bonding of CHD is due to polarization of the structure. The significance of this effect with regard to the analogous case of peptide hydrogen bonding and its relevance to the stability of the globular form of proteins will be discussed. The ability of DFT methods to treat hydrogen bonding in solids appears to be roughly as accurate as the crystal structure determinations.

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