Co-crystals, Salts, and Ionic Co-crystals of Ethanol and Ammonia
journal contributionposted on 2020-08-14, 14:07 authored by A. Dominic Fortes
Compounds of ethanol and ammonia have been known for over a century but no structural characterization has been carried out. Solid crystalline specimens of molecular ethanol mono-ammoniate and ethanol hemi-ammoniate were prepared by flash freezing of stoichiometric liquids in liquid nitrogen followed by annealing between 175 and 195 K. High-resolution neutron powder diffraction measurements were used to determine the complete structures of the two compounds. Both are triclinic (P1̅, Z = 2): ethanol mono-ammoniate is characterized by linear tapes of four-sided hydrogen-bonded rings, which extend along the crystal’s b-axis; the ethanol hemi-ammoniate structure is defined by sheets (in the crystal’s a – b plane) comprised of two different types of four- and six-sided H-bonded rings. Measurements collected as a function of temperature provide a precise determination of the thermal expansion of the two ethanol ammoniates and solid deutero-ammonia. Density functional theory calculations yield insights into the response of each structure to hydrostatic stress, including a proton transfer to form either wholly ionic ammonium ethoxide crystals or ionic co-crystals of ethanol ammonium ethoxide. The switch from molecular to ionic co-crystal in ethanol hemi-ammoniate may occur on cooling at ambient pressure, with kinetic frustration due to the low temperature of the transition. Proton transfer at ambient pressure, or at modest impact shock pressures, suggests that there may be extensive ammonium salt formation amongst compounds of the simplest alcohols. This may represent an important additional mechanism for the generation of NH4+ spectroscopic signatures in remotely sensed data from interstellar, cometary, and planetary ices.