Controlling Nonclassical Content of Clathrate Hydrates Through the Choice of Molecular Guests and Temperature

Abstract

Low-temperature, low-pressure studies of clathrate hydrates (CHs) have revealed that small ether and other proton-acceptor guests greatly enhance rates of clathrate hydrate nucleation and growth; rapid formation and transformations are enabled at temperatures as low as 110 K, and cool moist vapors containing small ether molecules convert to mixed-gas CHs on a subsecond time scale. More recently, FTIR spectroscopic studies of the tetrahydrofuran (THF)-HCN double clathrate hydrate revealed a sizable frequency shift accompanied by a four-fold intensification of the C-N stretch-mode absorption of the small cage HCN, behavior that is enhanced by cooling and which correlates precisely with similar significant changes of the ether C-O/C-C stretch modes. These temperature-dependent correlated changes in the infrared spectra have been attributed to equilibrated extensive hydrogen bonding of neighboring large- and small-cage guest molecules with water molecules of the intervening wall. An ether guest functions as a proton acceptor, particularly so when complemented by the action of a proton-donor (HCN)/electron-acceptor (SO(2)) small-cage guest. Because guest molecules of the classic clathrate hydrates do not participate in hydrogen bonds with the host water, this H-bonding of guests has been labeled "nonclassical". The present study has been enriched by comparing observed FTIR spectra with high-level molecular orbital computational results for guests and hydrogen-bonded guest-water dimers. Vibrational frequency shifts, from heterodimerization of ethers and water, correlate well with the corresponding observed classical to nonclassical shifts. The new spectroscopic data reveal that the nonclassical structures can contribute at observable levels to CH infrared spectra for a remarkable range of temperatures and choice of guest molecules. By the choice of guest molecules, it is now possible to select the abundance levels of nonclassical configurations, ranging from ∼0 to 100%, for a given temperature. This ability is expected to hasten understanding of the role of guest-induced nonclassical structures in the acceleration or inhibition of the rates of CH formation and transformation.

Catalytic activity of methanol in all-vapor subsecond clathrate-hydrate formation

“…The assignments follow from (1) loss of the 1033 band (blue spectrum) during replacement of MeOH by DME as the largecage guest, and (2) a fading of the 1017 cm −1 band as CO 2 partially displaces MeOH from the small cages during equilibration over time with 80 Torr of CO 2 gas at 170 K. The extra width of these MeOH CO stretch bands (∼18 cm −1 ) is consistent with previous reports for MeOH and other guest molecules H-bonded with cage walls. 3,24,25 Bands of the hydrate CO 2 asymmetric stretch mode are hidden by gas-phase saturation of absorption in the 2340 cm −1 region of the spectrum so, as previously, 20 the loading of CO 2 that depleted the MeOH small-cage population in the inset of Fig. 2 was followed using the weak in-tensity of the 3703 cm −1 band of the combination mode (not shown).…”

“…25 The joint action with MeOH, as with DME, parallels that described for THF with HCN wherein the small-cage HCN enhances the non-classical character by stabilizing the cage-wall H 2 O as O-H bonding is switched to the oxygen of the THF. 12,24 That is, like for HCN, an acid end of C 2 H 2 H-bonds with oxygen of the cage walls producing a sizable shift of the C 2 H 2 asymmetric C-H stretch mode.…”

“…10 of Ref. 25). The greater shift suggests a stronger H-bonding of C 2 H 2 with the small-cage wall that is favored by a greater stability of a non-classical MeOH structure that dominates even in the absence of a small-cage guest.…”

“…24 This is somewhat as observed for even stronger C 2 H 2 H-bonds with the oxygen of solvent MeOH, TMO, or THF molecules in liquid solution The growth of the non-classical small-cage band of C 2 H 2 at 3245 cm −1 from bottom to top, that parallels the weakening of the 3289 cm −1 band of ice, signals the increasing % CH of the aerosols while also identifying the nature of the CH as largely non-classical. 25 The C 2 H 2 chamber partial pressure increases from 4 to 5 to 7 Torr for red to blue to green.…”

Ammonia clathrate hydrates as new solid phases for Titan, Enceladus, and other planetary systems

“…Pure ammonia clathrate hydrate synthesis cannot be done by simply cooling aqueous ammonia solutions, as a variety of stoichiometric hydrates of ammonia are known to form preferentially (2,11,18,19). Other ways of forming clathrate hydrates include vapor deposition of water at low temperatures to yield amorphous ice, followed by exposure of the ice to a pressure of guest gas and annealing (29), or vapor codeposition of water and the potential guest material at low temperatures, again, followed by annealing (23,30,31). It has been shown that below approximately 140 K ice surfaces are relatively inert unless strong hydrogen-bond donors or acceptors are present.…”