Key principles needed for the rational design of thermosetting monomer crystals, in order to control the melting point, have been elucidated using both theoretical and experimental investigations of cyanate esters. A determination of the thermodynamic properties associated with melting showed that the substitution of silicon for the central quaternary carbon in the di(cyanate ester), 2,2-bis(4-cyanatophenyl)propane, resulted in an increase in the entropy of melting along with a decrease in the enthalpy of melting, leading to a decrease in the melting temperature of 21.8 ± 0.2 K. In contrast, the analogous silicon substitution in the tri(cyanate ester), 1,1,1-tris(4-cyanatophenyl)ethane, resulted in no significant changes to the enthalpy and entropy of melting, accompanied by a small increase of 1.5 ± 0.3 K in the melting point. The crystal structure of 1,1,1-tris(4-cyanatophenyl)ethane was determined via single crystal X-ray diffraction, and the structures of these four di(cyanate esters) and tri(cyanate esters) were examined. Although both the empirical models of Lian and Yalkowsky, as well as Chickos and Acree, provided reasonable estimates of the entropy of melting of 2,2-bis(4-cyanatophenyl)propane, they successfully predicted only certain effects of silicon substitution, and did not capture the difference in behavior between the di(cyanate esters) and the tri(cyanate esters). Semi-empirical molecular modeling, however, helped to validate an explanation of the mechanism for the increase in the entropy of melting of the silicon-containing di(cyanate ester), while providing insight into the reason for the difference in behavior between the di(cyanate esters) and tri(cyanate esters). Taken together, the results assist in understanding how freedom of molecular motions in the liquid state may control the entropy of melting, and can be utilized to guide the development of compounds with optimal melting characteristics for high-performance applications.

The interactions between nitrate plasticizer ingredients nitroglycerine, diethylene glycol dinitrate (DEGDN), and triethylene glycol dinitrate (TEGDN) have been investigated theoretically and experimentally. Density functional theory (DFT) and second order perturbation theory (MP2) calculations show that intermolecular N−O electrostatic interactions and hydrogen bonding between alkyl hydrogens and oxygen atoms govern the dimer structures, which have predicted binding energies ranging from 20.6 to 39.3 kJ mol−1. Vibrational frequencies observed in the nitrate band fingerprint region of the infrared spectra are in good agreement with the DFT scaled harmonic frequencies. The relative volatility observed in thermal gravimetric analysis (TGA) measurements of the pure ingredients and their 1 : 1 mixtures show reasonable correlation with the DFT dimer binding energies. The similarity in the dimer binding energies of all three plasticizers suggest that their mixtures should be uniformly miscible.

The development of applications for germanium nanocrystals has been hindered by the limited availability of synthetic methods coupled with poorly understood ligand-exchange chemistry. Herein we describe the synthesis of germanium nanocrystals and ligand exchange experiments designed to establish facile routes toward ligand replacement and, consequently, layers that are amenable to charge-transfer. After assessing thiols, carboxylates, and dithiocarbamates, sulfur dissolved in 1-ocatadecene was determined to be the most amenable to ligand exchange, with over 95% of the initial alkylamine ligand replaced as determined by Fourier transform infrared spectroscopy (FTIR). These results were in good agreement with density functional theory calculations showing a strong preference for Ge–S bonding. The materials were fully characterized via powder X-ray diffraction, FTIR, transmission electron microscopy, and X-ray and UV photoelectron spectroscopy. This new ligand exchange procedure provides a possible route toward the fabrication of thin films that may be employed in such applications as photovoltaic devices.

The back cover picture shows computed molecular electrostatic potentials (top left) and selected heterogeneous dimer interactions (top right) of nitrate plasticizer ingredients nitroglycerine (NG), diethylene glycol dinitrate (DEGDN), and triethylene glycol dinitrate (TEGDN), used as a qualitative assessment of the expected degree of miscibility between these ingredients in propellant formulations. The general similarity in the binding energies of homogenous and heterogeneous dimers, highlighted by a network of hydrogen bonds between alkyl hydrogens and the ether or nitrate oxygen atoms, suggests these plasticizers should be miscible. Predicted infrared vibrational spectra of the TEGDN‐TEGDN, DEGDN‐DEGDN, and TEGDN‐DEGDN dimers are in good qualitative agreement with the corresponding experimental spectra of TEDGN, DEGDN, and their 1:1 mixture (bottom left). Further details are presented in the Short Communication by Buszek et al. on page 115 ff.

Cyanate esters are versatile, thermosetting monomers that are commercially important due to their outstanding physical properties such as fire and heat resistivity. This class of molecules also possesses relatively low melting points, allowing for improved processability and manufacturing.While studies of this class of polymers are prominent, the monomers tend to shy away from focus.In an effort to understand organic crystal engineering of these compounds, four structures of silicon-and carbon-containing di(cyanate ester)s and tri(cyanate ester)s are presented and analyzed using single-crystal X-ray diffraction. This data, in concert with extensive thermodynamic and computational studies, assist in understanding how a subtle change, such as exchanging quaternary central carbon for a silicon, affects the molecular degrees of freedom. From a crystallographic standpoint, this affects the packing efficiency and intermolecular interactions.