Growth of Clathrate\nHydrates in Nanoscale Ice Films\nObserved Using Electron Diffraction and Infrared Spectroscopy

Clathrate hydrates (CHs) are believed to exist in cold\nregions\nof space, such as comets and icy moons. While spectroscopic studies\nhave explored their formation under similar laboratory conditions,\ndirect structural characterization using diffraction techniques has\nremained elusive. We present the first electron diffraction study\nof tetrahydrofuran (THF) and 1,3-dioxolane (DIOX) CHs in the form\nof nanometer-thin ice films under an ultrahigh vacuum at cryogenic\ntemperatures. Using reflection high-energy electron diffraction, we\nshow that THF CH grows readily on various substrates during thermal\nannealing of an amorphous ice mixture of THF and water, and the formation\nis independent of the nature of the substrate. The growth of DIOX\nCHs on a Au(111) substrate is similar. A comparison of electron diffraction\npatterns with calculated X-ray diffraction patterns indicates that\nTHF and DIOX form structure II CH (5<sup>12</sup>6<sup>4</sup>) with\na lattice constant of ∼17.2 Å (cubic <i>Fd</i>3̅<i>m</i>). Both CHs were also grown on Ru(0001)\nand were examined by reflection absorption infrared spectroscopy.\nA direct comparison of diffraction data to infrared spectra as a function\nof the temperature further demonstrates the strength of multiple probes\nin examining complex systems possessing diverse molecular interactions.