pubs.acs.org/JPCL
Classical to Nonclassical Transition of Ether-HCN Clathrate Hydrates at Low Temperature I. Abrrey Monreal,† Lukasz Cwiklik,‡ Barbara Jagoda-Cwiklik,‡ and J. Paul Devlin*,† †
Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 and ‡Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic and Center for Biomolecules and Complex Molecular Systems, Flemingovo nam. 2, 16610 Prague 6, Czech Republic
ABSTRACT Guest molecules of typical clathrate hydrates are stabilized by weak nonspecific interactions with the cage walls of the host lattice. Despite their ability to form hydrogen bonds, this description also generally applies to encaged ether and other moderately strong proton acceptor molecules. However, on the basis of infrared spectroscopic and molecular dynamics results, an altered structure is indicated when guests such as HCN or SO2, capable of binding to oxygen lattice sites, occupy the small cages. During cooling from 140 to 60 K, structure-II clathrate hydrates, with HCN in the small cages and either tetrahydrofuran or trimethylene oxide as the large-cage guests, convert to nonclassical structures in which most “guest” molecules establish hydrogen bonds to water. The nonclassical structure is stable at even higher temperatures for the dimethyl ether-HCN double clathrate hydrate. SECTION Kinetics, Spectroscopy
transport.8 Rather, the simulations affirmed the results of a pioneering study of guest transport within the CO2 C.H. by Demurov, et al.,9 which identified host-lattice vacancy-defects as the likely facilitator of guest transport for gas hydrates. The more recent simulations also demonstrated that the ordersof-magnitude enhancement of formation rates by H-bonding guests likely results from stabilization of the vacancy defects by H-bonding with the guest molecules. The vacancy defects, with dangling-water coordination sites, are otherwise highly energetic and, therefore, few in number. This view of guest transport in ether C.H.'s may endure, but here we describe newly observed and computed structural characteristics that imply the stabilization of such an abundance of L-defects for certain ether double C.H.'s, that existing conclusions regarding the facilitation of guest transport may ultimately need revision. More definitively, the new results point to a surprising importance and, in some cases, dominance of nonclassical configurations involving proton-acceptor lc and protondonor (or electron-acceptor) sc guests. Since extensive Fourier transform infrared (FTIR) spectroscopic and kinetic data for C.H.'s with H-bonding guests are available in a recent “Perspective” article,8 the focus here is on the limited regions of the infrared spectra that best display unusual structural effects from the simultaneous presence of ether molecules in the lc's and HCN (or SO2) in the small ones. The bands of the ether C-O/C-C stretch modes near
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ecause of their importance in nature and to society, clathrate hydrates (C.H.'s) have been thoroughly described in numerous recent publications. Here we note only that the classic structures, I and II, (s-I and s-II) are nonstoichiometric inclusion compounds with “tetrahedrally” bonded water host lattices having incorporated cages of two sizes, large and small (lc and sc), that are populated by appropriately sized molecules or atoms.1 In the “classical” structures, these guest molecules engage in multiple weak interactions with the water molecules of the cage walls. For some time, two unique characteristics of C.H.'s with guest molecules having H-bonding ability in the lc's (such as ethers, ketones, and aldehydes) have been presumed to be closely interrelated: the exceptionally great dielectric relaxation rates2 and the ability of such C.H.'s to form from ice at pressures and temperatures much lower than with guests having little or no H-bonding ability.3 These two characteristics have been thought to reflect a greatly enhanced population of orientational defects induced by acceptor H-bonding guests through transient nonclassical interactions. In particular, ether lc guests have been viewed as occasionally bonding to an O-H group from the walls of the C.H. cages,3-5 thereby creating Bjerrum orientational L-defects that enable water molecule reorientation6 as well as the transport of guest molecules necessary for C.H. growth from ice.7 However, a recent computational study, while consistent with the projected greatly enhanced L-defect populations, has concluded that the presence of an L-defect (i.e., missing hydrogen along an O 3 3 3 O coordinate) does not, of itself, constitute a hole in a cage wall of sufficient size for the observed high rates/low energetics of ether-molecule guest
r 2009 American Chemical Society
Received Date: September 30, 2009 Accepted Date: November 24, 2009 Published on Web Date: December 02, 2009
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DOI: 10.1021/jz900073n |J. Phys. Chem. Lett. 2010, 1, 290–294
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Figure 1. Left panel: Evolution of the THF C-O stretch-mode band intensities upon cooling the THF-HCN double C.H. from 130 to 70 K. The 1073 cm-1 band loses over 60% of its intensity, while the 1055 cm-1 band grows to dominate the spectrum (purple curve). Right panel: simultaneous changes in the HCN C-N stretch mode band: the 2093 classical sc band weakens while a new version, stronger by a factor of ∼5, evolves at 2084 cm-1. The bottom (red) curve is the comparative 70 K spectrum for HCN in the sc of the classical s-I double hydrate with ethylene oxide. The 2078 band is of adsorbed HCN, and the sloping background is a result of host H2O absorption.
1000 cm-1 and of the CN stretch mode of HCN near 2100 cm-1 possess this desired sensitivity. Tetrahydrofuran (THF) has been the guest molecule for many studies of ether C.H.'s (see, for example, a recent paper that describes transient H-bonding of THF with the s-II lc walls).5 Partly for that reason we emphasize the new data for the THF-HCN double s-II hydrate. This choice is further motivated by the fact that this C.H. clearly displays a temperature-induced transition from nearly fully classical to a largely “nonclassical” structure over the temperature range best probed by the FTIR sampling method. However, limited comparisons with “parallel” experimental data for the trimethylene oxide (TMO)-HCN, dimethyl ether (DME)-HCN and acetone-HCN double s-II C.H.'s, as well as computational results for the DME-HCN C.H., are included, along with brief references to results with SO2 rather than HCN as the sc guest. The THF molecule in an s-II lc has a vibrational band, generally assigned to the C-O antisymmetric-stretch mode, that normally appears at 1072 ( 2 cm-1.8,10-13 This band, as previously observed for simple as well as many mixed s-II C. H.'s of THF, responds weakly to decreasing temperature.13 However, in this study a remarkable 18 cm-1 downshift of the band position with decreasing temperature is noted when the sc's are occupied by HCN (Figure 1, left panel). Most of the THF band intensity at 1073 cm-1 moves 18 cm-1 in a single step, to ∼1055 cm-1, in a continuous and reversible manner as the temperature drops from 130 to 70 K. The switch of band intensity from 1073 to 1055 is slow at ∼130 K, accelerates through the 120-110 K range, and continues before tailing off below 80 K. Meanwhile, the O-D stretch band, for ∼5% HDO isolated in the C. H., behaved normally by retaining its full-width halfmaximum value of ∼50 cm-1 and a nearly symmetric shape while downshifting the typical few reciprocal centimeters. A 50% dilution of the sc HCN with CO2 did not reduce either the frequency or intensity shifts from the HCN effect on the THF band; this is an anticipated result since, at 50% dilution, there is still approximately one HCN molecule for every THF guest in the s-II structure.1 A temperature-driven frequency shift of precisely the same magnitude was also
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Figure 2. FTIR spectra showing the effect of sc loading with HCN on the C-O stretch band of DME in the lc of the s-II C.H. The 930 cm-1 band (red) is for the classical structure with DME lc guests only. Loading of the s-II C.H. sc with HCN shifts the DME lc band to 918 cm-1. The nonclassical 918 cm-1 band is on the same intensity scale (top: 80 K; bottom: 160 K). Deuterated water was used for the host lattice to avoid interference from the host librational-mode band.
observed for the double THF C.H. with SO2, an electronseeking sc guest.14,15 This strongly implies that the infraredband shift reflects the binding of the THF molecules with the host water rather than neighbor guest molecules. Concurrently, during cooling, the vibrational bands of the sc guests also change remarkably. For the CN stretch mode of HCN, this is observed as a single-step downshift of ∼9 cm-1 accompanied by an ∼5-fold increase in band intensity (Figure 1, right panel). With the strong THF-band downshift clearly correlated with the marked intensification and downshift of the HCN band, the evolution of the spectra strongly suggests that the low-energy C.H. structure is “non-classical”. Implied is a structure that includes coordination of HCN with the oxygen abandoned by a coordination partner as THF accepts that O-H group of the intervening cage wall (see an analogous structure for the DME-HCN case in Figure 3).
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DOI: 10.1021/jz900073n |J. Phys. Chem. Lett. 2010, 1, 290–294
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Figure 3. Typical structure for neighbor sc's and lc's as obtained in energy minimization of the DME-HCN s-II C.H. employing the empirical force-field calculations. HCN donates to two weak hydrogen bonds, while DME accepts one O-H group (common values of bond lengths are in Å). Color coding: violet - nitrogen atom, red - oxygen atoms, blue - carbon atom of HCN and CH3 groups of DME, white - hydrogen atoms, gray - water molecules not directly engaged in guest-binding.
The position of the dominant infrared band of the C-O stretch modes of TMO in the lc of the s-II C.H. has been reported as ∼990 cm-1.3,8,16,17 When HCN is the sc guest, cooling reversibly downshifts this C-O band by ∼15 cm-1 to 975 cm-1, just as noted above for the THF C.H. The response of the HCN “non-classical” CN stretch band also resembles that of the THF-HCN hydrate band (Figure 1, right panel) as it approaches an intensity-enhanced maximum at 2086 cm-1 near 90 K. However, the TMO C.H. favors the nonclassical structure more strongly than does the THF-HCN double C.H. The HCN nonclassical band retains significant intensity at 150 K, as does the 975 cm-1 TMO band, and below ∼90 K nearly the entire structure is nonclassical. The “trend” suggested in the THF-TMO sequence, toward greater preference for the nonclassical C.H. structure with decreasing size of the ether, reaches a zenith with DME. For the DME-HCN s-II double C.H., it is no longer necessary to reduce the temperature to obtain the ∼100% nonclassical structure. This is made clear in Figure 2, where the spectra show that the classical band (930 cm-1) has
