Molecular Modes and Dynamics of HCl and DCl Guests of Gas

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The Journal of Physical Chemistry

Molecular Modes and Dynamics of HCl and DCl Guests of Gas Clathrate Hydrates Nevin Uras-Aytemiz,a* F. Mine Balcı,b Zafer Maşlakcı,c Hasan Özsoy,c and J. Paul Devlind* a

Department of Polymer Engineering, Karabuk University, 78050 Karabuk, Turkey

b

Department of Chemistry, Suleyman Demirel University, 32260 Isparta, Turkey

c

Department of Chemistry, Karabuk University, 78050 Karabuk, Turkey

d

Department of Chemistry, Oklahoma State University, Stillwater, OK 74078, USA

*

Corresponding Authors: [email protected] and [email protected]

Abstract Recent years have yielded advances in the placement of unusual molecules as guests within clathrate hydrates (CHs) without severe distortion of the classic lattice structures. Reports describing systems for which observable but limited distortion does occur are available for methanol, ammonia, acetone and small ether molecules. In these particular examples, the large-cage molecules often participate as nonclassical guests H-bonded to the cage walls. Here, we expand the list of such components to include HCl/DCl and HBr as small-cage guests. Based on FTIR spectra of nanocrystalline CHs from two distinct preparative methods combined with critical insights derived from on-the-fly molecular dynamics and ab initio computational data a coherent argument emerges that these strong acids serve as a source of molecular small-cage guests, ions and orientational defects. Depending on the HCl/DCl content the ions, defects and molecular guests determine the CH structures some of which form in sub-seconds within an all-vapor preparative method.

Keywords: Gas hydrate, HCl guests, all-vapor formation, nonclassical structures, density function-based molecular dynamics, L orientational defects.

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Introduction It has been established that the minimum energy structures for HCl within freestanding water clusters of 4 or more water molecules include the dissociated acid (see, for example, ref. (1-4) and citations therein). Though subject to some debate, 5 there are also published conclusions that highly diluted HCl adsorbed on an ice surface at 10 K/50K can quickly achieve the necessary coordinations with water for HCl ionization; 6-8 this despite evidence that the surface of ice nanocrystals lacks mobility to reorganize below ~60 K. 9 It is also reported that strong acids within clathrate hydrates prepared from aqueous solutions assume structures for which anions are guests within the hydrate cages while protons roam the hydrate lattice.10-12 It follows that any attempt to incorporate strong acids such as HCl in a molecular form within cages of gas hydrates could be viewed as a fool’s errand. Nevertheless, our intent is to present evidence of such molecular incorporation based on ab initio molecular dynamics simulations and DFT-level electronic structure calculations, including energy minimization and normal-mode analysis, combined with FTIR spectra obtained using two quite different methods of formation of CH nanocrystals. Both the computational and experimental data suggest that the small strong-acid molecules HCl and DCl serve as both classical (C) and nonclassical (N-C) molecular small-cage (s-c) guests distributed throughout CH nanocrystalline particles. As with other guest molecules that have been recognized as forming significant Hbonds with water of the cage walls, there is often a disruption of the clathrate hydrate structures but the resultant “nonclassical” structure retains a basic clathrate form recognizable in the infrared spectra 13, 14 as well as NMR and diffraction data.15 As a result, symbols used to denote the classic structures as structure I (s-I) and structure II (s-II) remain generally useful along with the concepts of unit-cell size and guest cage populations. The terms N-C and C are used to indicate H-bonding, or lack thereof, of particular guest molecules to water molecules of the CH host lattice within an overall N-C structure. In previous studies of N-C structures the focus has been primarily on the large-cage N-C


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molecules while the small strong-acid molecules of the present study are smallcage guests.

Experimental methods section Two different experimental methods have been used in the study of the incorporation of HCl/DCl as CH guests. Experiments based on the first method were initiated by deposition of an array of ice nanocrystals on a mid-IRtransparent ZnS substrate held near 70 K in an inner chamber of a double-walled structure. After warming to 108-120 K, the ice particles were exposed to vapor mixtures of an ether with gases of potential small-cage guests such as H2S. Under such conditions, Ice nanoparticles have been shown to fully convert to 100% binary CHs in a few min at 120 K.16 The rapid rate for the low temperature was enabled by the ether guest molecules functioning as catalysts of molecular transport through the growing CH crust to an interface with the ice particle core, 17 i.e., catalysis of a shrinking-core ice-to-CH conversion18. Formation temperatures were 108 K for CHs that included HCl along with the ether and H2S guests, with FTIR spectra subsequently obtained from 60 to 200 K. Acid attack of the CH often began near 120 K but, for samples in which the HCl/H2S ratio was less than 1/3rd, CH loss with acid-hydrate formation was sometimes delayed to > 160 K. The second CH formation method was based on our unique all-vapor methodology. As originally, 19 and more recently, 20 that method consists of premixing both the water and guest vapors with a carrier gas in a liter bulb at appropriate partial pressures. FTIR spectra obtained after pulsing into a cold chamber (140 – 220 K), (or in a supersonic nozzle expansion) 21 suggest that the vapors convert to solution nanodroplets which, in the cold chamber, generally transition to ~100% CHs on a subsecond timescale. 22 The rapid formation of the CH aerosols was immune to disruption from inclusion of as much as 5 mole % HCl with 5 mole % HCN in the vapor premixtures that also included 6 to 8 Torr of water and ~0.4 Torr of THF. The premixing was completed with addition of ~100 Torr of He (g). During the present study the vapor premixtures at different times contained a) THF alone, b) THF with HCN, c) THF with HCl/DCl, (d) THF with HCN followed by HCl/DCl added in a 2nd pulse (as previously described in a study of the 3 ACS Paragon Plus Environment


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droplet compositions) 23, e) THF with HCl with HCN added in a 2nd pulse, f) all three guests in a single pulse g) THF alone followed by HCN in the 2nd pulse and h) THF alone followed by HCl. It is noteworthy that guest molecules of any 2nd pulse are not premixed with vapors of the other guests, but occupy the CH particles only through rapid transport into pre-existing CH aerosol particles available from the 1st pulse. Examples of accessible CHs in the present study are displayed in Fig.2 based on sample preparation schemes (b), (b) plus 0.1 Torr of HCl, and (d). Preparation scheme (d) with ~ 0.1 Torr of HCL in the 2nd pulse was used for Fig.3 while schemes (d) and (f) were used for Fig.4. Experimental results with discussion DCl CH from ice nanocrystals Initially, the FTIR spectra of CHs, formed at 108 K from ice nanocrystals and vapor premixtures that included a few % HCl along with H2S and the ether catalyst, showed no obvious impact from the presence of the acid. However, during subsequent warming to 120 K the CH spectra tended to develop non-CH-like distortions including Zundel continua that, for water-rich media, are usually indicative of acid ionization.24 Further warming to ~160 K revealed the abundance of acid present within the CH samples through emergence of known intense HCl ionic-hydrate bands.25,26 A more direct effect of HCl/DCl on the CH structure at 108 K was eventually confirmed when FTIR spectra indicated that, for several different premixtures, only the CHs prepared with HCl/DCl contained structural elements associated with hydrogen bonding between guest and host components.13, 14 Spectra that show this N-C impact of HCl/DCl for premixtures with large-cage acetone and small-cage acetylene are presented on the left in Fig.1; and on the right for dimethyl ether (DME). The acetylene classical small-cage frequency is decreased weakly from the gas-phase value of 3289 27 to ~3280 cm-1, but is downshifted another ~30 cm-1 upon bonding to the cage wall. 13 Nevertheless, that downshift is small compared to the ~100 cm-1 value for C2H2 dissolved in oxygenated organics, or within the corresponding heterogeneous dimers. 23 The large shift is a testimony to an acidity of the acetylene hydrogen as well as the strength of its electron-acceptor charge-transfer interactions. 28 Observation of the HCl induced N-C bonding of acetylene and of oxygenated organic large-cage 4 ACS Paragon Plus Environment

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guest molecules, such as DME in the spectra on the right of Fig.1, became a convincing basis for a more thorough study using our all-vapor method of CH preparation.

Fig.1. Clathrate deuterate nanocrystal spectra prepared at 170 K with ~2 mole % DCl (blue) and without DCl (red) in the vapor premixtures of the guests. The guests other than DCl were acetone and acetylene on the left and DME with acetylene on the right. Symbols C and N-C refer to classical and nonclassical. Scales on the left are in absorbance units.

HCl/DCl CH spectra from all-vapor premixtures The second CH formation method, based on our all-vapor methodology and used to obtain the particular spectra of Fig.1, has provided the experimental data most essential to this study. As originally, 19 that method consists of premixing both the potential host and guest component vapors within a carrier gas at appropriate partial pressures. Upon pulsing into a cold chamber the vapors convert to solution nanodroplets which transition to ~100% CHs, all on a subsecond timescale. The 2 steps in the transition from vapor to CH, occurring on a microsecond scale, have been tracked in FTIR spectra of supersonic-jet expansions.21 The differences shown in each of the two examples of Fig.1 indicate extensive HCl-induced C to N-C change for the acetylene small-cage guests of the structureII (s-II) CHs. However, our focus is on studies for which tetrahydrofuran (THF) and HCN were selected as versatile s-II guests with the particular advantage that their comparative C and N-C FTIR spectra have been most thoroughly examined.13, 14 5 ACS Paragon Plus Environment


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Also significantly, the ternary CH combination of THF and HCN with HCl/DCl yield FTIR bands that we assign to the molecular s-c HCl/DCl stretch modes.

Fig.2. FTIR spectra of ~90 % deuterated binary hydrate of THF and HCN at 170 K with varying amounts of DCl included in the premixture for the all-vapor formation. From the bottom: ~0 %, 1 %, and 5 % DCl. Weak nonclassical bands in the bottom spectrum of THF at 1056 and 890 and HCN at 2085 cm-1 reflect the impact of a trace of contaminant DCl. The scales on the left are in absorbance units, and symbols C and NC refer to classical and nonclassical guests. See text to relate frequencies to non-CH structures. From bottom to top, the samples were prepared using preparation schemes (b), (b) including 1% HCl and (d) as described in the Experimental section.

The aerosol CH particles of Fig.2, composed of D2O, THF, HCN and DCl, were highly deuterated to avoid FTIR-band interference common to the corresponding protiated samples. The spectra include regions, on the left, that display the thoroughly studied THF C-O symmetric and asymmetric stretch modes 29 and, on the right, HC-N/DC-N cyanide stretch modes plus the stretch mode of s-c DCl. Bands of the cyanide vibration can appear for 4 different versions of both HCN and DCN: namely, cyanide nanoparticles (2098/1912), surface adsorbed cyanide (2078/1890), classical s-c (2093/1904) and nonclassical s-c (~2085/ ~1897cm-1). The relative intensity of the bands at 1850 and 1882 cm-1, absorptions assigned directly to s-c DCl, depends on the stage and conditions of the CH formation. The unstable 1850 band is observed only during early stages of relaxation of CHs prepared below 180 K with a low DCl content relative to that of HCN in the allvapor premixture. These are conditions for which small-cage hydrogen cyanide is abundant but mostly classical. By contrast, the 1882 DCl band, once established, is stable over the total aerosol sampling time of 10-15 min, even at 190 K. The frequency of either DCl band, when compared with the ~2500 cm-1 value


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computed and eventually observed for s-c HCl, is consistent with the anharmonic isotopic-shift magnitude expected for DCl, i.e., an H/D frequency ratio of ~1.33. The frequencies and relative intensities of C and N-C bands of guest THF and HCN are well-known from earlier study. 13,14 Nonclassical HCN donor H-bonding, induced here by adding DCl and resulting in the band at 2085 cm1 having gained an order-of-magnitude in band intensity, is reaffirmed in the top spectrum of Fig.2. With respect to THF, the C-O asymmetric-stretch C (1073 +/- 2) and N-C (1053 +/- 2) pair of bands appear much as for the temperature-controlled version published originally for a much lower temperature.13 The classical (~914) and nonclassical (~890 cm-1) infrared bands of the THF symmetric mode, made visible through the isotopic shifting of the intense water librational band, are displayed for the first time (though the classical band is commonly invoked in Raman studies) 29. A second series of DCl-related spectra is presented in Fig.3 to demonstrate the evolution of the N-C structure as the quantity of molecular DCl and of DCl-derived ions slowly increase within the CH. This series was accessed via a 2-pulse experiment in which the 1st premixture contained deuterated water, THF, HCN, and He with < 0.1 Torr of HCl added in a 2nd helium pulse. The evolution of the NC structure was slow as the dilute HCl/DCl must be adsorbed and then transported to the core of the CH nanocrystals. Further, the ion population of the CH, that is dependent on the acid content, was low.

Fig.3. Spectra that show the parallel growth at 170 K of three critical N-C bands of the ternary CH of THF (1055), HCN (2086) and DCl (1882 cm-1) prepared with a limited amount of DCl. Less than 0.1 Torr (or ~ 1% of all guest molecules) of HCl/DCl was delivered in a 2nd vapor pulse after formation of the (mostly) classical binary THF-DCN CH of the blue spectrum. Overall time delays for the subsequent scans were: 7 ACS Paragon Plus Environment


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purple-1 min, green-3 min and red-6 min. Weak N-C bands of the three guests that appear in the bottom spectrum indicate presence of a trace amount of contaminant DCl in the 1st pulse. The extensive isotopic exchange is further evidence of contaminant strong acid content of the 1st pulse. The strong 1912 band is of DCN nanocrystals from an excessive quantity (~1.0 Torr) of HCN included in the vapor premixture.

The weak initial DCl band of Fig. 3 peaks near 1872, between the 1882 and 1850 bands of Fig.2, and maximizes at 1882 cm-1 after ~6 min. Most informative is the parallel nature of the growth of the N-C bands of each guest versus time. This signals a common primary source of N-C growth; namely a cooperative molecular H-bonding of the three guests with the lattice water. However, we will see from computations that the 1882 N-C band of DCl is dependent on orientational point defects, generated by DCl upon incorporation into the host lattice. So the shifting of the band to 1882 cm-1 is attributed to a switch from molecular to a slightly weaker defect promoted N-C bonding. Observation of the s-c HCl band was hindered by interfering absorptions: of the asymmetric stretch of CO2 (g) at 2349 cm-1 and the O-D stretch of HDO within the CH (~2440 cm-1). The CO2 interference was a result of time-variable incomplete flushing of the spectrometer. A DFT-computed shift of the CH N-C HCl stretch placed it under a wing of the broad HDO band (see computational results); that interference was managed using FTIR subtraction methods and a reduction of the mole % of HDO which is included routinely as a reliable monitor of the quality of CH samples 22. From the frequency, width, and intensity of the 1882 DCl band, the HCl band, placed both computationally and by the isotopic frequency ratio near 2500 cm-1, was expected in that region with a half-width of ~60 cm-1 and peak intensity of ~ 0.003 absorbance units. The bottom 3 spectra of Fig.4 each indicate an isolated HCl-induced band at ~2482 cm-1 that we assign to N-C HCl in the s-c of the s-II ternary clathrate of HCl, THF and HCN.


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Fig.4. FTIR spectra of four different s-II CH aerosols prepared using the single-pulse all-vapor method with primarily an H2O host but including 10-15 % HDO. The vapor partial pressures for the bottom 3 aerosols (A-C) were, in Torr, H2O – 8, THF – 0.4, HCN - ~1.4, HCl – 0.45 and He – 120. The vapor premixture for (D) was the same except 0.6 Torr HBr replaced the 0.45 Torr of HCL. The HCl and DCl bands in (D) indicate contamination from HBr displacement of HCl/DCl from the extensive surfaces of the experimental system. Chamber temperatures were 190 K (A and B) and 180 K (C and D). The absorbance scale is an approximation except for (A).

Comparative HBr CH spectra As with studies of HCl/DCl bonded to the surface of ice nanocrystals, 25 one can expect spectra of HBr encaged within a CH to add confidence to evaluation of the spectra of similarly encaged HCl and DCl. However, the FTIR vibrational band of encaged HBr, labeled at ~2350 cm-1 in the top spectrum of Fig.4, is apparently exceptionally broad and difficult to observe. Accordingly, its identity and characteristics are considered tentative, in part because of interference from the 2482 cm-1 band of contaminant HCl. Nevertheless, it is interesting that the misshapen form of the 2350 band resembles that of the asymmetric C-O stretch mode of the DME simple CH (Fig.11, ref.14). In that instance the width and form are attributed to fluctional bonding bridging C and N-C large-cage DME states, i.e., a parallel to similar fluctuations between Eigen and Zundel ion states of the diand tri-hydrates of HCl 30. By analogy, the N-C HBr frequency can be estimated as ~2300 and the classical as >2400, but significantly below the HBr (g) value of 2560 cm-1. 9 ACS Paragon Plus Environment


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Going forward, similar considerations will play a significant role as more quantitative cage-population values are sought for CHs of these strong acids. Fluctional and N-C bonding, for modes that are sensitive to H-bonding, greatly increase the difficulty of determining cage populations from FTIR spectra. A description similar to that of the HBr spectrum may also apply to spectra of certain DCl states. Intermediate states of DCl, with frequencies that range towards the gas-phase value of 2090 cm-1, are the apparent end point of a collapse of the unstable DCl 1850 band of Fig.2, a collapse that usually occurs during the first minute following single-pulse CH formation with dilute HCl and abundant HCN. For reasons revealed computationally and described later a stronger second pulse of DCl, that increases ion/defect involvement, appears to rapidly convert these weakly observed but abundant classical states to intensely absorbing N-C ones that contribute to the strong 1882 cm-1 band of the top spectrum of Fig.2. A more rapid growth observed for that band than in Fig.3 is consistent with classical HCl, along with HCN, being already present in the small cages as the ions/defects become abundant. Aspects of the HCl-HCN interdependency With such evidence along with a computational prediction of classically encaged molecular acids, the nature and impact of the steps of the sampling procedure assume added interest. For example, only classical CHs form from all-vapor premixtures for which HCN is present but HCl/DCl is lacking, or for which HCl/DCl is present without HCN, (i.e., for all-vapor sample preparation schemes labeled (b) and (c) in the Experimental section). This is shown for sample preparation scheme (b) by the very weak nature of the nonclassical 1056, 890 and 2086 cm-1 bands of the bottom spectrum of Fig.2, and of Fig.3. This result for scheme (b) is expected since it is known that the binary HCN-THF CH is classical above ~140 K. 13 Of course, similar information does not exist for HCl/DCl with THF. Molecular HCl/DCl guest bands are not readily observed for such aerosols, not necessarily because the small cages are empty, but because of an order-ofmagnitude reduced vibrational oscillator strength of C vs N-C HCl as projected from our Gaussian 09 computations. In the absence of H-bonded nonclassical structures that intensify the HCl/DCl bands, the molecular strong-acid presence is difficult to recognize by FTIR. This is true even for the relatively narrow 2093 band 10 ACS Paragon Plus Environment

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of classical HCN in Fig.2. However, HCl/DCl with no HCN does sometimes give rise to strong THF bands at 1048 and 870 cm-1 that we attribute not to the CH, but to an H-bonded THF-HCl solid complex that appears to form when excess HCl/DCl reaches the surface of the CH particles. Then, HCN from a 2nd pulse is observed to reclaim THF from the acid complex with formation of the N-C CH. There are other examples where the proton donor strength of HCl/DCl is activated. For two noteworthy 170 K aerosol samples, the simple CH of THF with a classical infrared spectrum was first prepared. Then, HCl/DCl added in a 2nd pulse (i.e., as in preparation scheme (h)), extracted most of the classical large-cage THF within a few min. This converted the CH aerosol particles to ice-like nanocrystals while also generating a quantity of the solid-state HCl/DCl-THF complex. This transition does not occur if the THF CH is first stabilized by small-cage HCN included with THF in the initial pulse. However, generally for CH samples formed with more than 2 mole % DCl, some acid ionization is indicated by a broad Zundellike deuterium-based continuum ranging from ~1200 to ~2300 cm-1.24 Nevertheless, if HCN is also present in abundance, a large fraction of the DCl appears as the molecular nonclassical guest, at 1882 cm-1. Computational methods Most of the key aspects of the systems under study came into focus only through extensive DFT MD computations that examined actions of the individual components, both molecular and ionic. In this manner a complexity has been recognized that was not envisioned from the FTIR data. To get enhanced molecular-level insight to the experimental data, we have used two simulation packages: Quickstep31 and Gaussian 09 (G09)32. DFT-level calculations have been conducted in both packages. However, the DFT method is known to overestimate the tendency of H-Cl to stretch and ionize in systems such as HCl-water clusters as compared with high-level calculations like MP2/aug-cc-pvdz.25 Therefore, validation of different DFT functionals in the two simulation packages against the published MP2 data25 was conducted initially. More details are given in Supporting Information. Ab initio molecular dynamic simulations were conducted for each MD step by using atomic pseudo potential of the Goedecker, Teter and Hutter (GTH) type 33 with a Gaussian valence basis set of quadruple- ζ quality augmented by three sets 11 ACS Paragon Plus Environment


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of polarization functions (QZV3P). These calculations were performed using the BP86 functional 34-36 with the D3 correction of Grimme 37 and the Becke 88 34, 38 for the exchange correlation functional. The geometric parameters for the crystal structure of s-I were taken from ref. 39 and a single unit cell with 46 water molecules was used for simulations. A cubic box of length 12.03 A was extended in three dimensions by applying periodic boundary conditions. Within the s-I unit cell, the large cages are occupied by ethylene oxide (EO) and small cages by HCl molecules (a simulation named s-I HCl + EO without ions). In addition to this, we have simulated a similar unit cell but with one H2O molecule replaced by one HCl molecule within the water framework (named s-I HCl + EO with ions). HCl is known as a strong adsorbate that, above 60 K, breaks normal Hbonds within an ice network, forming ionic acid hydrates.40 Since the simulation time frame is too short to observe HCl insertion that is presumed to occur during our experimental sampling, HCl was placed manually in the water network. It should be noted here that we have used s-I HCl + EO for modelling whereas our experimental results are for s-II HCl (+HCN) + THF. However, we do not expect to obtain accurate quantitative results for the considered systems. Rather, we would like to understand the observed H-bonding behaviour of small and large cage molecules within the clathrate hydrate cages in the presence of HCl. For MD simulations, the average temperature was ~170 K. Each system has been subjected to a minimization procedure by using Quickstep before the constantenergy MD simulations. In order to obtain the harmonic frequencies, Gaussian electronic structure code was used for a model containing HCl in an isolated dodecahedral water cage. Ab initio calculations were performed by using the WB97 functional with the aug-ccpvdz basis set. Ab initio calculations were performed with several different DFT functionals with the dispersion addition that is coded in the G09 package program. WB97 gave more reasonable results compared with the other DFT functionals. Computational results with discussion Snapshots from an ab initio trajectory of s-I HCl + EO without ions are shown in Fig.5A, B, C. During the ion-free trajectory, the HCl molecules in the small cages mostly rotated freely (Fig.5A) while remaining molecular but occasionally making 12 ACS Paragon Plus Environment

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transient H-bonds with the cage water molecules as a proton donor (Fig.5B). The oxygen atom of the ether also forms weak transient H-bonds by accepting a proton from the cage water (Fig.5C). However, these H-bond formations are very rare events (see below).

(A)

(D)

(B)

(E)

(C)

(F)

Fig.5. Snapshots from s-I HCl + EO trajectory without ions (A, B, C) and from the s-I HCl + EO simulation with ions (D, E, F). Red circles show oxgyen of water, green is chlorine, ethylene oxide’s oxygen is dark blue and the CH2 groups of EO are represented as dark gray. Oxygen of the H3O+ ion is yellow.

The most interesting and informative events were observed after the HCl in the water network ionizes. Snapshots from the s-I HCl + EO clathrate hydrate trajectory with ions are shown in Fig.5D, E, F. The water of the hydrate lattice is initially 4 coordinated, while the inserted HCl establishes only 3 coordinations; 2 13 ACS Paragon Plus Environment


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from the acceptor Cl side and one as a proton donor which ionizes. The one missing coordination creates a defect between the Cl- ion and the oxygen of the neighbour water that had originally accepted a proton from the water replaced by HCl. This creates the Bjerrum L-defect–like structure (i.e. O…Cl-). This water, in turn, becomes stabilized by accepting hydrogen from a neighbour small-cage HCl guest molecule (see Fig.5D): that HCl thus becomes N-C. This 2nd HCl stays bonded to that oxygen for 4 fs during the trajectory run. Because of Cl- ion and O atom repulsion, the structure is distorted. After the optimization procedure, the proton was initially in contact with the Clion. However, within 0.3 ps it jumped to a neighbour water molecule and then immediately moved to another. At that point, EO in the big cage rotated to point to the hoping proton (see Fig.5E). The proton then stayed on the hydronium ion most of the time, but occasionally jumped to the neighbour water. Since the oxygen of the hydronium ion does not want to accept a proton from the neighbour water, that hydrogen of the neighbour water is used in H-bonding with an EO as shown in Fig.5F. For the simulation time scale, the proton seems localized on these waters through interplay with the oxygen of the large-cage guest. The hydrogen bonding between guest molecules and the hydrate water network can be verified by plotting the radial distribution functions (RDFs) given in Fig.6A for H2O…HCl and in Fig.6B for EO…HOH. The first peaks in the RDF plots indicate H-bonding behaviour of the cage molecules that is more obvious for the simulation that contains ions (black lines in RDF plots). That is, the presence of ions enhances the H-bonding numbers of the small and large cage guests. The total number of H-bonds out of 4000 steps is 106 for EOs of s-I HCl + EO without ions whereas it is 1300 out of 4000 steps for s-I HCl + EO with ions.


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A

B

Fig.6. RDFs for H2O…HCl (A) and EO…HOH (B) for two sample runs.

The time evolution of all HCl bond lengths from the two trajectory runs are given in Fig.7. Among all HCl molecules, only HCl attached to the water missing an Hbond (shown in Fig.7 with black) undergoes large amplitude fluctuations (a movie related to this HCl is also given in Supporting Information). One HCl in the small cage (like the one shown in Fig.5B) was chosen as a model for the G09 calculation to get the normal mode of HCl within a small cage. The analysis gave a 330 cm-1 shift vs. the monomer for this N-C HCl in the s-c.

Fig.7. HCl bond lengths (Angstroms) versus time (fs) from two runs each for s-I HCl + EO without ions (blue and green) and s-I HCl + EO with ions (black and red).



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Conclusions We conducted an FTIR spectroscopic study of incorporation of molecular HCl/DCl into the small cages of the s-II THF CH using two different CH preparative methods. A molecular view of the system was then achieved using on-the-fly molecular dynamics in combination with quantum mechanical calculations. The multi-faceted spectral data gained meaning only through MD tracking of the activity of HCl molecules after insertion into the CH host lattice structure. Such HCl were observed to be a source of lattice defects and ions which lead, in turn, to H-bonding of molecular HCl (and HCN) guests, as well as of large cage THF, with the cage walls; i.e., a pervasive nonclassical characteristic that dominated throughout much of the FTIR study independent of the CH preparative method. Combining and matching the computational results with the experimental ones, we have concluded the following: The ternary CH of THF, HCl and HCN is stable with classical/fluctional/non-classical guest structures. When the network is enriched with ions through additional HCl, the CH structure is further stabilized through H-bonding by both small and large-cage guests with the network water. This generates the 1882 DCl (as in the snapshot from on-the-fly dynamics in Fig. 5D), the 2085 HCN and 1056 THF bands. Furthermore, large-amplitude fluctuations in the guest H-Cl bonding to the H2O with the missing coordination (Fig.5D and black plot in Fig.6) are consistent with the large FTIR bandwidths; e.g., ~60 cm-1 for N-C HCl and perhaps >100 cm-1 for HBr. There is also a remarkable frequency match with the even broader bands of HCl/DCl single coordinated to 50 K ice nanocrystal surfaces (2480 cm-1/1870 cm-1) 25 versus N-C HCl/DCl within the CH s-c (2482 cm-1/1882 cm-1). A similar match seems to carry over to the HBr frequencies as well. Matches for more highly coordinated acid molecules on the ice surface with the N-C small-cage CH molecules are not expected or observed. For samples with low levels of ions and similar levels of N-C HCN and HCl, there is a close correlation of the magnitudes of N-C HCl, HCN, and N-C THF guest populations, as seen in Fig.3. However, at higher ion/defect levels, the FTIR band intensities of Fig. 2 indicate a different N-C population dependence for THF than for the small-cage acids. This difference is anticipated. In the absence of ions the guest N-C populations are all interdependent, being controlled by the same factor; namely the THF capture of a proton of a lattice O-H group. However, a divergent level of control is expected between that of the L-defect and hydronium ion produced by HCl in the network. The L-defect, with a dangling-oxygen coordination, carries a negative charge 41 that can stabilize a proton-donor N-C 16 ACS Paragon Plus Environment

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state of a small-cage acid molecule; by contrast, the hydronium ion interacts weakly with a neighbour water molecule that ultimately H-bonds with the ether oxygen. It follows that equal participation in non-classical H-bonding by the smallcage donor and large-cage acceptor guests is not expected. Accordingly, relative to the middle spectrum, there is little growth of the 1056 cm-1 band with added HCl in the top spectrum of Fig.2, while the 1882 and 2085 cm-1 bands increase markedly.

Forward It is expected that a successful attempt to obtain spectroscopic and computational data for molecular hydrogen halides encaged within clathrate hydrates would reveal interesting structural, kinetic and thermodynamic properties. That expectation increases when the data also indicate that, beyond some level of content, the HX/DX molecules within the water lattice ionize in part and, with the aid of a weak acid (HCN), act to further stabilize the encaged HX/DX molecules as H-bonded non-classical guests within the small cages of THF/EO CHs. The unexpected strong-acid behaviour assures that these results offer new opportunities for increased understanding of strong acids within water-rich systems in general and clathrate hydrates in particular. At this stage of study the nature of the new opportunities are not generally known. However, the following short list notes some possibilities. 1. Thermodynamics/Chemical reactions within gas hydrates. An example is reported wherein HCl from a 2nd pulse attacks the aerosol particles of the simple THF CH, removing the THF and forming ice particles (or possibly approaching the empty s-II CH) 42 plus a solid complex of THF with HCl. In related cases, a pulse of HCN has been observed to recover the THF CH, while N-C HCl and N-C HCN populate the small cages and the THF-HCl complex vanishes. This demonstrates a remarkable increase in stability of the CH from the presence of the N-C acid molecules and suggests that such stability may be useful in future gas-hydrate studies. 2. Kinetics: Controlling CH formation rates using small amounts of HX/DX. We find that, with a few % of HCl/DCl, the all-vapor CH formation rates are not disrupted sufficiently to place them in a range where our methods allow kinetic observations, i.e., CH nanocrystals continue to form on a sub-second basis. This suggests that, with HCl soluble in the aqueous droplets formed in 17 ACS Paragon Plus Environment


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the 1st step of the all-vapour method, the presence and retention of HCl, with its stabilization of N-C CH structures, may facilitate catalytic sampling at temperatures higher than normally accessible to the all-vapour method; i.e. > 230 K. It can certainly be anticipated that the presence of HX in the droplet solutions will increase/decrease the solubility of other guest molecules in the droplets and, therefore, the CH guest concentrations. 3. Structural: Throughout this study there has been evidence that bonding of the HX/DX molecules to cage walls is sometimes fluctional. With advantages that come with CH structures that are better characterized than liquid aqueous solutions, specific factors that favour fluctional bonding, or other structural patterns, are more likely to be identified. 4. Charge transport: The structural influence of strong acids, through ions/ defects, is apparently transmitted rapidly within the CHs, as observed with the double-pulse sampling. A more rigorous control of the HCl populations in future studies may reveal factors that further determine the CH chargetransport properties and their relationship to Zundel-like continua. Close observation of the extent of HCN-DCN isotopic exchange may also be useful in that respect.

Supporting Information Validation of DFT functionals that have been used in two simulation packages (i.e. Quickstep and Gaussian09) within this study were conducted by using HCl-(H2O)n, n= 1-6, clusters. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgements Support of this research by the National Science Foundation (NSF) through Grant CHE-1213732 is gratefully acknowledged. N.U.-A. is appreciative of partial support for this project by Karabuk University, Turkey through No. KBU-BAP-13/2-DR-001.


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Table of Contents Figure:


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Molecular Modes and Dynamics of HCl and DCl Guests of Gas

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