The inelastic incoherent neutron scattering spectra of 1,3-cyclohexanedione (CHD) in its crystalline enol form and its cyclamer complexes with benzene and benzene-d 6 are compared with each other, with IR and Raman spectra and with the results of calculations using density functional theory (DFT). 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. The benzene complex is a closed hexameric hydrogen-bonded cycle. A DFT treatment is applied to the full hexamer of the benzene 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 the linear chain clusters exhibits a continuous change in structure with increasing chain length, converging to values near the observed structure. Emphasis is placed on the cooperative nature of hydrogen bonding in CHD as revealed by these systematic trends. The ability of DFT methods to treat hydrogen bonding in solids appears to be roughly as accurate as the crystal structure determinations.1,3-Cyclohexanedione (CHD) exists in its enol form in the solid state where it forms hydrogen-bonded chains with an antianti configuration having short (2.56 Å) O-O hydrogen bonding distances. 1 This structure is of interest for several reasons including the ordered hydrogen bonding arrangement, an orderdisorderphasetransitionthatinvolveshydrogenbondinterchange, 2-6 and indications derived from examination of crystal structures that CHD is an example of a resonance-assisted, cooperative hydrogen bonding network. [7][8][9][10][11][12][13] This cooperativity arises from the fact that when the enol of CHD forms a hydrogen bond, this polarizes the molecule such that the formation of an additional hydrogen bond with another CHD is more favorable. This may be thought of as due to an enhanced contribution of the ionic resonance form. When crystallized from benzene, this material forms a unique cyclamer structure (CHD 6 Bz) in which six CHD molecules form a syn-anti hydrogen bonded ring surrounding a central benzene molecule. 14 The related 1,3-cyclopentanedione (CPD) 15 forms an antianti linear chain structure very much like that of CHD. Methyl substitution of CPD or CHD can result in changes to the hydrogen-bonding pattern. 2-Methyl-CPD forms a linear chain with an anti-syn arrangement. 16 In this case the methyl-CHD molecules in a given linear chain all point in the same direction. This is in contrast to the anti-anti chain of CPD and CHD (Figure 1) where alternate molecules are oriented in opposite directions. The 2-methyl derivative of CHD has a structure similar to that of CHD 17 but t...