International Conference on Gas Hydrates (ICGH) (6th : 2008)

INVESTIGATIONS ON THE INFLUENCE OF GUEST MOLECULE CHARACTERISTICS AND THE PRESENCE OF MULTICOMPONENT… Luzi, Manja; Schicks, Judith M.; Naumann, Rudolf; Erzinger, Jörg; Udachin, Konstantin A.; Moudrakovski, Igor L.; Ripmeester, John A.; Ludwig, Ralf 2008-07

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INVESTIGATIONS ON THE INFLUENCE OF GUEST MOLECULE CHARACTERISTICS AND THE PRESENCE OF MULTICOMPO- NENT GAS MIXTURES ON GAS HYDRATE PROPERTIES  Manja Luzi, Judith M. Schicks*, Rudolf Naumann, Jörg Erzinger GeoForschungsZentrum Potsdam (GFZ) Telegrafenberg, 14473 Potsdam, GERMANY  Konstantin Udachin, Igor Moudrakowski, John A. Ripmeester The Steacie Institute for Molecular Sciences National Research Council of Canada Ottawa, Ontario, CANADA  Ralf Ludwig Physikalische Chemie / Theoretische Chemie Universität Rostock 18059 Rostock, GERMANY  ABSTRACT In this study, we investigated the molecular characteristics of hydrates which were synthesized from gas mixtures containing the two isomers of butane, or the pentane isomers neopentane and isopentane, in excess methane. Thereto various techniques, including Raman spectroscopy, pow- der and single crystal X-ray diffraction and 13C NMR spectroscopy were employed. It turned out that shape and conformation of the guest molecule and hydrate structure both influence each other. In case of the mixed butane hydrate it could be confirmed that n-butane is enclathrated in its gauche conformation. This was verified by Raman spectroscopy, single crystal X-ray diffrac- tion and calculated data. While isopentane is known as a structure H former, our results from powder X-ray diffraction, 13C NMR and ab initio calculations show that it can be also incorpo- rated into structure II when the hydrate is formed from a neopentane/isopentane/methane gas mixture.   Keywords: gas hydrates, guest molecular properties, constitutional isomer, rotational isomer  INTRODUCTION Although most naturally occurring gas hydrates form structure I hydrates with methane as the main component, structure II hydrates that contain light hydrocarbon guest molecules like propane, isobu- tane or n-butane could be identified as well [1, 2]. Recently Lu et al. succeeded in verifying the exis- tence of structure H hydrates in nature. The E (51268) cavity of structure H is the largest cavity which is known for naturally occurring gas hy- drates. It incorporates large guest molecules such as isopentane, n-hexane and even methylcyclohex- ane [3]. It has been well established that the guest molecule size influences the structure obtained when a guest material reacts with water to form a hydrate.   But as it was shown before the relationship be- tween structure and molecular size is not straight- forward [4]. There are still open questions regard- ing the influence of other molecular properties such as the guest molecule shape (conformation) as well as the presence of further guest molecules on the properties of the formed gas hydrate and vice versa. Therefore hydrates were synthesized from gas mixtures containing the two isomers of butane, or the pentane isomers neopentane and isopentane, in excess methane. The composition of these gas mixtures was kept close to natural conditions [5, 6]. Recently it was shown that depending on the for- mation conditions either the shape of a guest mole- cule or its water solubility has a great impact on  Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.   *  Corresponding author: Phone: +49 331 2881487 Fax: +49 331 2881474 E-mail: the gas hydrate composition [7]. For a non-stirred water system the smaller isomer is preferentially incorporated into the hydrate phase. The hydrate composition is dominated by the different diffusion rates of the isomers, which are related to their shape. When the water phase is stirred diffusion becomes unimportant, and the isomer with the higher water solubility is enriched in the hydrate phase. In order to study these multicomponent hydrates on a molecular level several analytical tools are employed to verify the crystal structure and require the hydrate composition. Thereby, the emphasis of this work was kept on the role of the guest molecu- lar shape during the hydrate formation process and the interaction between guest molecule and hydrate lattice.  EXPERIMENTAL METHODS Sample preparation To synthesize hydrate 3 g freshly ground ice was loaded into a high pressure cell (internal volume: approximately 130 mL) which was precooled in a freezer at 253 K. Afterwards the pressure cell was connected to a pressure transducer and placed in a cooling bath at 271 K. Before pressurization the cell was tempered 30 min in the cooling bath. To prevent the ice powder from melting, the incoming gas was cooled and the reactor was pressurized very slowly. The pressure cell was stored in the cooling bath until no further pressure drop was observed.  In case of the isopentane hydrate sample the liquid hydrocarbon was cooled in the freezer as well. 1.9 mL isopentane were dispersed with a syringe on the ice powder. After filling a 50 mL pressure cell with 5 g ice and the liquid hydrocarbon, the cell was tempered in the cooling bath at 268 K and afterwards pressurized with methane. By the time a further pressure drop could not be observed the temperature was increased to 274 K to enhance the conversion of ice into hydrate. A list of the gas mixtures used in this study is pre- sented in Tables 1 and 2.  Gas mixture CH4 (mol %) n-C4H10 (mol %) iso-C4H10 (mol %) Ia 96.8 3.2 - Ib 94.4 - 5.6 Ic 96.0 2.0 2.0  Table 1. Butane gas mixtures. Gas mixture CH4 (mol %) neo-C5H12 (mol %) iso-C5H12 (mol %) IIIa 97.2 2.8 - IIIb 79.8 - 20.2 IIIc 98.0 1.0 1.0  Table 2. Pentane gas mixtures.  Sample analysis The hydrate crystal structure and lattice constants were obtained with a Bruker D8 Advance Powder X-ray diffractometer (Cu Kα radiation, λ = 1.5406 Å) equipped with an Anton Paar low tem- perature controller. The powder X-ray diffraction patterns were acquired at 153 K in a continuous scan mode. The patterns obtained were calibrated with reference to several ice Ih reflections.  The cage occupancies of the mixed hydrate sam- ples were determined by 13C magic angle spinning (MAS) NMR measurements which were carried out on a Bruker DSX-400 NMR spectrometer. For this, hydrate samples were finely ground and packed in a 7 mm zirconia rotor that was loaded into a variable temperature probe. The 13C NMR spectra were recorded at 173 K with ~ 2 kHz spin- ning rate. In order to distinguish the solid phase signals from that of the liquid (adsorbed higher hydrocarbons) the spectra were obtained by high power proton decoupling (HPDEC) as well as cross polarization (CP) programs. The spectra were referenced to adamantane as external chemical shift reference with assigned chemical shifts of δ = 38.56 ppm and δ = 29.50 ppm at 298 K.  The compositional analysis of the hydrate samples was supported by Raman spectroscopy. The Ra- man spectra were recorded in the C-C stretching vibration regions at 77 K on an Acton SpectroPro 2500i spectrometer equipped with a Witec confo- cal microscope. An argon ion laser operating at 514.53 nm was used as excitation source. Naphtha- lene at 1382 cm-1 was used as reference.  In case of the n-butane – isobutane – methane hy- drate it was possible to collect a single crystal from the bulk sample which turned out to be a multiple twin. This hydrate crystal was analyzed by single crystal X-ray diffraction. The data were collected with Mo Kα radiation (λ = 0.71073 Å, 2Θ = 50.0, ω scan mode) on a Bruker SMART CCD diffracto- meter at 100 K. The structure was solved by direct methods using the SHELXTL suite of programs. All atoms were refined anisotropically. Hydrogen atoms on guest molecules were placed in calculated positions and allowed to ride on the parent atoms.  In addition to the sample analysis, ab initio quan- tum mechanical calculations were performed to investigate the interaction of the guest molecule and the hydrate cage and the role of the different guest molecular conformations during the hydrate formation process in more detail. The ab initio calculations were performed with Hartree Fock (HF) and density functional (DFT) methods that are implemented in the GAUSSIAN 98 software package. The following basis set was used: 6-31 + G*. The geometries of the hydrate cages and the entrapped guest molecules were fully opti- mized using HF and B3LYP levels of theory. A more detailed description for this method is pro- vided in the literature [8]. To support the experi- mental results from 13C NMR measurements the chemical shifts were calculated for the synthesized hydrates as well.  RESULTS AND DISCUSSION All hydrate samples were analyzed with powder X- ray diffraction, Raman spectroscopy and 13C NMR spectroscopy to determine the crystal structure and the composition of the hydrate. The measurements were performed in regard to the shape and confor- mation of the guest molecule and the gas hydrate structure. In the following, three examples for the interaction of guest molecular properties and mo- lecular properties of the formed gas hydrate are presented.  Butane hydrates  Isobutane – Methane SII Hydrate  This hydrate was formed at 2.1 MPa for several days while the main pressure drop was observed during the first hours after pressurization. After verifying the crystal structure by powder X-ray diffraction the hydrate was analyzed by 13C MAS NMR methods (Figure 1). The measured chemical shifts coincide with literature data of mixed isobu- tane – methane hydrates (Table 3) [3]. Please note, that the carbon signals for isobutane in the hydrate phase are reversed relative to their order in the pure compound isobutane [9].  -15-10-505101520253035 Chemical Shift [ppm] In te n si ty   Figure 1. CP/MAS 13C NMR spectra of isobutane – methane hydrate; measuring temperature 173 K.  System Chemical shift [ppm] Peak assignment -4.46 CH4 in small cage -8.16 CH4 in large cage 26.46 CH3-signal of iso-C4H10 isobutane – methane hydrate 23.69 CH-signal of iso-C4H10 25.2 CH-signal isobutane (liq.) [9] 24.3 CH3-signal  Table 3. Chemical shifts and peak assignments for isobutane and methane in the hydrate phase and isobutane in the liquid phase.  It is important to mention that the peak assign- ments for the CH- and CH3-signal of pure isobu- tane are not consistent in the literature [10, 11]. For this reason we decided to compare our data with calculated 13C NMR data. In this case our peak assignments for 13C NMR spectra were also confirmed by the calculated data (Figure 2). In comparison to the pure compound (dashed lines) the CH-signal and CH3-signal of isobutane are reversed in the hydrate phase (solid lines). The peak positions of the calculated spectra (calculation temperature 0 K) differ from those of the experimental spectrum as they were referenced to the methane peak that was also calculated for this structure. CH3-signal of iso-C4H10 CH-signal of iso-C4H10  Figure 2. Calculated 13C NMR spectra of isobutane in the hydrate phase (solid lines) and in the gas phase (dashed lines); calculation temperature 0 K.  Apart from that it should be noted that peak posi- tions can be shifted due to changes in temperature [12-14]. So, one might assume that the change of the peak order is a consequence of the different measuring and calculation temperatures, respec- tively. But to the authors’ knowledge it was not reported so far that a change of the temperature can cause a change in the peak order. Therefore we suppose that the change of the peak order results from the enclathration of isobutane in the hydrate lattice. This assumption is supported by single crystal analyses (Figure 3). As mentioned before, it was possible to collect a single crystal from a hydrate sample that was synthesized from a gas mixture containing 2% isobutane 2% n-butane and 98% methane (gas mixture Ic). This combination of guest molecules results in a structure II hydrate. The unit cell parameter was determined with a = 17.213 Å (space group Fd-3m). The measurement has shown that the small cavities are completely filled with methane. 65% of the large cavities are filled with isobutane and 35% are filled with n- butane. As presented in Figure 3 it was found for isobutane that the molecule has moved out of the cavity centre closer to the edge of the hydrate cage. Therefore an interaction of the guest molecule and the hydrate cage appears to be possible which might arise in a change of the peak order in the 13C NMR spectrum.    Figure 3. Guest molecule isobutane in the H (51264) cavity of structure II.  n-Butane – Methane SII Hydrate  A conformational analysis of n-butane encaged in the hydrate lattice is presented as the second ex- ample for an interaction between guest molecule and hydrate cage. As a linear alkane, the CH3CH2 fragments are able to rotate around the central C-C bond which results in different rotational isomers. The energetically most stable conformer of n-butane is the trans (also called anti) form that has a C2h symmetry. A rota- tion of 120° about the central C-C bond results in the gauche conformer (symmetry C2) [15]. Several studies dealing with the rotational isomerism of n- butane demonstrate that Raman spectroscopy is an appropriate tool to distinguish the conformers [15- 17]. In the presence of a help gas like methane, n- butane forms a structure II hydrate. Two studies deal with the conformation of n-butane in the hy- drate cage. Davidson et al. have performed dielec- tric relaxation measurements on an S II hydrate consisting of hydrogen sulphide and n-butane [18]. The condition for dielectric absorption is a perma- nent dipole moment of the guest molecule. In con- trast to trans n-butane gauche n-butane does have a permanent dipole moment. Apart from that, trans n-butane (van-der-Waals diameter: 7.9 Å) is sup- posed to be too large to fit into the large cage of structure II. As a dielectric absorption was ob- served for this hydrate Davidson et al. assumed that the smaller conformer gauche n-butane (van- der-Waals diameter: 7.1 Å) is encaged in the hy- drate lattice. Secondly, Subramanian and Sloan presented Ra- man data for an n-butane – methane hydrate syn- thesized at 0.22 MPa and 273.7 K [19]. Comparing the Raman spectra of liquid n-butane and the gas hydrate they showed that trans-gauche band pairs  10   8   6   4   2   0 -20         -10          0           10         20          30          40 Chemical Shift [ppm] 1  CH-signal of iso-C4H10 in the  hydrate phase 2  CH3-signal of iso-C4H10 in the  hydrate phase  3 CH-signal of iso-C4H10 in the gas  phase 4 CH3-signal of iso-C4H10 in the gas  phase 1    2  3 4 occurring in the liquid spectrum reduce to single bands in the hydrate spectrum. They claimed that these single bands correspond to the gauche bands of the pure liquid n-butane. As they found only bands generated by the gauche form of n-butane in the hydrate spectrum they concluded that the trans form is excluded from the hydrate lattice. For this work an n-butane – methane hydrate was synthesized at 4.3 MPa and 271 K. In addition to the hydrate sample liquid n-butane was analyzed by Raman spectroscopy as well. The liquid n- butane was measured at room temperature and the hydrate spectrum was recorded at 77 K. While the Raman spectra of Subramanian and Sloan cover a wave number region of 700 cm-1– 1150 cm-1 this study shows a region of 750 cm-1 – 1600 cm-1 (Figure 4) [19].  750 850 950 1050 1150 1250 1350 1450 1550 wave number [cm-1] in te n s ity  [a.  u . ]   Figure 4. Comparison of the Raman spectra of pure n-butane (liquid) and n-butane – methane hydrate. The bands of the liquid n-butane spectrum were labelled with “t” and “g” corresponding to the trans and gauche rotational isomers of n-butane.  The liquid and hydrate spectra obtained correspond very well to the literature data [15-17, 19-22]. A complete overview of all peak positions and vibra- tional assignments for the liquid and the hydrate phase spectra is presented in Table 4. Based on the literature data the observed bands for liquid n- butane were assigned to the corresponding trans and gauche conformer, respectively [15-17, 20-22]. Accordingly the peaks were labelled as “t” (trans) and “g” (gauche). The band at 1453 cm-1 is very broad and can be attributed to both conformers. Due to its width it interferes with several neighbouring bands which are assigned to both conformers as well. It is important to mention that the literature data are not consistent in this fre- quency area. Vibrational mode ν liquid n-butane  [cm-1] ν n-butane – methane hydrate [cm-1] trans conformer gauche conformer 792 808  C-C stretch, CH3/2 rock 832 840  C-C stretch, CH3 rock 840 – C-C stretch 959 968  C-C stretch 983 989  CH3 rock 1060 – C-C stretch 1079 1083  C-C stretch 1137* [21] 1139  CH3 rock 1150 – CH3 rock 1172 1177  CH3/2 rock 1263* [21] 1261 CH2 twist, CH3 rock 1281 1282  CH2 twist 1304 – CH2 twist 1343* [16] 1345  CH2 wag 1383* [21] 1383 CH3 sym deform CH3 sym deform 1453 (very broad) 1446** 1465** 1474** CH3 deg deform CH3 deg deform  Table 4. Observed Raman frequencies ν [cm-1] and assignments to the main vibrational modes for liquid n-butane and n-butane – methane hydrate (stretch = stretching, rock = rocking, twist = twist- ing, scis = scissors, wag = wagging, deform = de- formation, deg = degenerate, sym = symmetric) * Literature data for pure n-butane that were not observed in the liquid phase spectrum ** Due to overlapping vibrational modes of both conformers a reliable assignment is not possible.  The measured region in the Raman spectra of liq- uid n-butane contains four trans-gauche band pairs. These band pairs are 832 cm-1 (g) – 840 (t) cm-1, 792  g  832     g 840   t      959     g     983      g       1060              t     1079          g    1150    t           1172           g  1281      g   1304    t 1453 t + g  808  840       968        989   1083      1177     1139  1261     1282    1345          1383  1446+1465  +1474     liquid n-C4H10      n-C4H10 – CH4             hydrate 1060 cm-1 (t) – 1079 cm-1 (g), 1150 cm-1 (t) – 1172 cm-1 (g) and 1281 cm-1 (g) – 1304 cm-1 (t). On the basis of the Pimentel-Charles “Loose Cage – Tight Cage Model” Subramanian and Sloan de- termined that the Raman bands for n-butane trapped in the hydrate cage are slightly shifted to higher frequencies compared to the liquid phase spectrum [19]. Thus, we can assign the band occur- ring at 808 cm-1 in the hydrate spectrum as the C-C stretching vibration that was detected at 792 cm-1 in the liquid spectrum. Considering the trans- gauche band pairs of the liquid spectrum we can designate the single bands in the hydrate phase spectrum at 840 cm-1, 1083 cm-1, 1177 cm-1 and 1282 cm-1 as the remaining gauche bands. The four corresponding trans bands are nonexistent in the hydrate phase spectrum. In contrast to the liquid spectrum the hydrate spectrum includes four bands at 1139 cm-1, 1261 cm-1, 1345 cm-1 and 1383 cm-1. As shown in Table 4 these four bands were com- pared with literature data of n-butane. Keeping in mind that the vibrational frequencies of n-butane trapped in the hydrate cage are slightly shifted, the bands at 1139 cm-1, 1261 cm-1, 1345 cm-1 and 1383 cm-1 presumably correspond to bands of pure n-butane which are located at 1137 cm-1, 1263 cm-1, 1343 cm-1 and 1383 cm-1. The literature bands at 1137 cm-1, 1263 cm-1 and 1343 cm-1 are assigned to gauche n-butane. The signal at 1383 cm-1 which is caused by a symmetric CH3 deformation vibration can result from both con- formers [20]. Therefore we assume that compared to the liquid phase the gauche conformer was en- riched in the hydrate phase which results in the appearance of these four bands. Regarding the multiple peak at 1450 cm-1 it was already men- tioned that several vibrational modes of both con- formers overlap at this position. For this reason an exact assignment of the three bands appearing in the hydrate phase Raman spectra is only specula- tive. Thus, the presence of a vibrational mode re- sulting from the trans conformer cannot be ex- cluded, but it is very unlikely. To sum up, four distinctive trans bands present in the liquid spectrum of n-butane are absent in the hydrate phase spectrum. The signal at 1383 cm-1 as well as the multiple peak at 1450 cm-1 can include vibrational modes of both conformers in theory. Considering these results and due to fact that the hydrate spectrum shows three additional bands resulting from the gauche conformer it can be con- cluded that the gauche conformer of n-butane is incorporated into the hydrate phase. These results are supported by the single X-ray diffraction data which were obtained for the n- butane – isobutane – methane hydrate. As shown in Figure 6 n-butane occupies the H (51264) cavity of structure II. It turned out that n-butane is en- caged in its higher energy gauche form.    Figure 6. Guest molecule n-butane in a gauche conformation inside the large cavity of structure II.  On the basis of ab initio calculations it was possi- ble to generate a picture of n-butane in the large cavity of structure II. At the end of the optimiza- tion process it was found as well that n-butane can be incorporated in its gauche conformation as shown in Figure 7.  Figure 7. Optimized structure of n-butane – meth- ane hydrate. n-Butane is encaged in its gauche conformation.  Pentane hydrates  Isopentane – Neopentane – Methane Hydrate  As the existence of structure H hydrates in nature could be recently verified this study was extended to hydrates containing constitutional isomers with five carbon atoms [3]. Isopentane is known as structure H former [23]. Its smaller structural iso- mer is neopentane which occupies the large cavity of structure II [24]. Isopentane has two stable con- formations – trans and gauche with C1 and Cs symmetry, respectively [25]. To come close to natural conditions, a gas mixture was employed which included 1% isopentane 1% neopentane and 98% methane (gas mixture IIIc). In order to evaluate the results from this gas mixture hydrate samples were synthesized which contained only one pentane isomer and methane. According to the 13C NMR spectra and powder X-ray diffrac- tion pattern it could be verified for the samples containing only one isomer that neopentane is in- corporated into the H (51264) cavity of structure II and isopentane is encaged in the E (51268) cavity of structure H (Figure 8a and 9a and 8b and 9b, re- spectively). Figure 8 shows the results of CP/MAS 13C NMR measurements for all three mixed hydrates. The peak positions and assignments, which were con- firmed by literature data, are presented in Table 5 [9, 26].  -15-10-50510152025303540 Chemical shift [ppm] In te n si ty   Figure 8. CP/MAS 13C NMR spectra of a) neopen- tane – methane hydrate, b) isopentane – methane hydrate, c) isopentane – neopentane – methane hydrate. In order to show the isopentane signals (marked by an arrow) the CH3-signal for neopen- tane was cut.  The carbon skeleton of isopentane was numbered according to IUPAC rules. It should be noted that Shin et al. have shown before that isopentane is incorporated in its gauche conformation into 51268 cavity of structure H [26]. Although the trans con- former is the low energy form and would fit into the large cavity of structure H, Shin et al. con- firmed by HPDEC 13C NMR techniques that the smaller gauche conformer is preferred.  System Chemical shift [ppm] Peak assignment -4.62 methane in small cage -8.19 methane in large cage 33.10 CH3-signal of neo-C5H12 a) neopen- tane – methane hydrate  27.24 C-signal of neo-C5H12 -4.39 methane in small cage -4.77 methane in medium cage 22.31 CH3-signal of iso-C5H12 (C1) 30.79 CH-signal of iso-C5H12 (C2) 32.29 CH2-signal of iso-C5H12 (C3) b) isopen- tane – methane hydrate 11.62 CH3-signal of iso-C5H12 (C4) -4.58 methane in small cage -8.13 methane in large cage 33.17 CH3-signal of neo-C5H12 27.33 C-signal of neo-C5H12 29.42 CH-signal of iso-C5H12 (C2) 21.83 CH3-signal of iso-C5H12 (C1) c) isopen- tane – neo- pentane – methane hydrate (2.3 MPa) 8.61 CH3-signal of iso-C5H12 (C4)  Table 5. Chemical shifts and peak assignments for the pentane hydrates.  The 13C NMR spectra of the isopentane – neopen- tane – methane hydrate sample (Figure 8c) shows only traces of isopentane in the hydrate phase. Neopentane is the dominant pentane isomer that was trapped in the hydrate cage. In order to influ- ence the hydrate composition the samples were synthesized at four different pressures: 2.3 MPa, 3.0 MPa, 3.5 MPa and 4.0 MPa. But no distinctive change in the hydrate composition could be achieved. Therefore the sample that was synthe- sized at 2.3 MPa was chosen as representative (Figure 8c and Figure 9c). For isopentane in the mixed pentane hydrate only carbon signals for C1, C2 and C4 could be ob- served which were marked by an arrow in Figure 8c. Please note that the peak positions for isopen-  b)  a)  c) tane in the mixed pentane hydrates are slightly shifted upfield compared to isopentane in the isopentane – methane hydrate (Figure 8b). Only the isopentane – neopentane – methane sample that was prepared at 3.0 MPa shows a double peak for the CH3-group of isopentane where one peak coin- cides with the peak position of the C1-signal for isopentane in the structure H hydrate. To verify the crystal structure of the isopentane – neopentane – methane hydrates powder X-ray dif- fraction measurements were performed and com- pared with hydrate samples containing only one pentane isomer (Figure 9). As both pentane iso- mers are known to form different hydrate struc- tures a mixture of reflections from structure II and structure H was expected. In fact, for the mixed pentane samples only structure II reflections could be recorded. Once again only the isopentane – neopentane – methane sample synthesized at 3.0 MPa shows one small reflection that can be as- signed to structure H hydrate. 16 18 20 22 24 26 28 30 32 34 36 2-Theta-Scale In te n si ty   Figure 9. Powder X-ray diffraction pattern of a) neopentane – methane hydrate, b) isopentane – methane hydrate, c) isopentane – neopentane – methane hydrate.  The 13C NMR measurements have shown that the amount of isopentane is very low in the hydrate phase. So one might assume that the concentra- tion of structure H hydrate was too low to be de- tected by powder X-ray diffraction. But due to the fact that the carbon signals for isopentane of the mixed pentane hydrate are shifted upfield these results can also lead to an- other assumption. We calculated the van-der- Waals diameter of gauche isopentane at first from the molar volume with an assumed spherical struc- ture of the molecule: 6.23 Å and secondly from an Onsager cavity: 7.86 Å. As gauche isopentane has no ideal spherical structure the “real” van-der- Waals diameter lies between those two values. Regarding the ratio of the molecular diameter of gauche isopentane to the cavity diameter of the 51264 cage of structure II this leads to the conclu- sion that the gauche isopentane does also fit into the 51264 cavity. Thereto, ab initio calculations were performed which should show if the large cavity of structure II could be occupied by the gauche conformer of isopentane as well. In fact, the optimizing process for a structure II hydrate with isopentane in its gauche conformation was successful (Figure 10) and shows that this con- former can also occupy the large cavity structure II. Thus, the observed upfield shift for isopentane in the mixed pentane hydrate might result from its enclathration in the 51264 of structure II. The driv- ing force for this effect is supposedly the presence of neopentane, another higher hydrocarbon guest molecule that forms structure II.  Figure 10. Optimized structure of isopentane – methane hydrate. Isopentane is encaged in its gauche conformation in the H (51264) cavity of structure II.  For this reason it can be concluded that although isopentane is a typical structure H former it might be also incorporated into the large cavity of struc- ture II when another higher hydrocarbon guest molecule is present. There is only one mixed pen- tane sample that shows traces of a structure H hy- drate. But the diffraction pattern of all other sam- ples shows only structure II reflections. Therefore we conclude that depending on the formation con- ditions isopentane is able to form either a structure II or a structure H hydrate or probably even coex- isting structure II and structure H hydrates. But to  a)  b)  c) verify this assumption further investigations are necessary.  CONCLUSION The aim of this study was to investigate the inter- action of guest molecular properties e.g. the con- formation and molecular properties of the gas hy- drate formed. Therefore gas hydrates were synthe- sized which contained the constitutional isomers of butane or two constitutional isomers of pentane in excess methane, respectively. By means of three examples this interaction was illustrated. For the isobutane – methane hydrate it was ob- served that the peak positions for isobutane in the 13C NMR spectrum are reversed relative to their order in the pure compound. With the help of sin- gle crystal X-ray measurements it could be shown that isobutane is moved out of the cavity centre. The resulting interaction of the guest molecule and the cavity wall is assumed to be the reason for the change in the peak positions. On the basis of Raman spectra, single crystal X-ray diffraction data and quantum mechanical calcula- tions it could be verified that n-butane occupies the large cavity of structure II in its gauche conforma- tion. The results for isopentane – neopentane – methane hydrates led to the assumption that similar to n- butane the conformation of isopentane plays an important role during the hydrate formation proc- ess although the conformation itself is not expected to change significantly. Depending on the forma- tion conditions e.g. the presence of another higher hydrocarbon like neopentane the “ambivalent” gauche conformer of isopentane does influence the crystal structure of the hydrate and vice versa. In order to examine this phenomenon in more detail single crystal X-ray measurements for these hy- drates and additional ab initio calculations are un- der way.  SUPPLEMENTARY INFORMATION Details of single crystal data collections and re- finement:  Structure II isobutane – n-butane – methane hy- drate. Crystal size 0.15 x 0.15 x 0.15, cubic, space group: Fd-3m, a=17.213(1)Å, V = 5099.9(2)Å3, T=100.0(1)ºK, ρcalc= 1.033 mg/m3, 2θMax=50.00º, 117 parameters, 33 restrains, residual electron den- sity max. 0.53, min. -0.22 eÅ-3. Final R indices (I>2σ(I)): R1= 0.064, wR2= 0.172 (28603 reflec- tions total, 275 unique, 246 (I>2 σ (I)). ACKNOWLEDGMENTS Manja Luzi gratefully acknowledges the financial support for her studies at the National Research Council Canada provided by the DAAD (German Academic Exchange Service) and thanks Dr Stephen Lang from the National Research Council Canada for his technical and scientific support.  LITERATURE [1] Sassen R, MacDonald IR. Evidence of structure H hydrate, Gulf of Mexico continental slope. Or- ganic Geochemistry 1994, 22 (6): 1029-1032. [2] Davidson DW, Garg SK, Gough SR, Handa YP, Ratcliffe CI, Ripmeester JA, Tse JS, Lawson WF. Laboratory analysis of a naturally occurring gas hydrate from sediment of the Gulf of Mexico. Geochimica et Cosmochimica Acta 1986, 50 (4): 619-623. [3] Lu H, Seo Y-t, Lee J-w, Moudrakovski I, Rip- meester JA, Chapman NR, Coffin RB, Gardner G, Pohlman J. Complex gas hydrate from the Cas- cadia margin. Nature 2007, 445 (7125): 303-306. [4] Schicks JM, Naumann R, Erzinger J, Hester KC, Koh CA, Sloan Jr. ED. Phase Transitions in Mixed Gas Hydrates: Experimental Observations versus Calculated Data. Journal of Physical Chem- istry B 2006, 110 (23): 11468-11474. [5] Milkov AV. Molecular and stable isotope compositions of natural gas hydrates: A revised global dataset and basic interpretations in the context of geological settings. Organic Geochemis- try 2005, 36 (5): 681-702. [6] Sassen R, Roberts HH, Carney R, Milkov AV, DeFreitas DA, Lanoil B, Zhang C. Free hydrocar- bon gas, gas hydrate, and authigenic minerals in chemosynthetic communities of the northern Gulf of Mexico continental slope: relation to microbial processes. Chemical Geology 2004, 205 (3-4): 195-217. [7] Schicks JM, Luzi M, Spangenberg E, Erzinger J. Clathrate Hydrate Formation and Growth: Ex- perimental Observations versus Predicted Behav- iour. In: 11th International Conference on the Physics and Chemistry of Ice, Bremerhaven, Ger- many, 2006. [8] Ludwig R, Appelhagen A. Calculation of Clathrate-Like Water Clusters Including H2O- Buckminsterfullerene. Angewandte Chemie Inter- national Edition 2005, 44 (5): 811-815. [9] Stothers JB. Carbon-13 NMR Spectroscopy. New York, London: Academic Press, Inc., 1972. [10] Breitmaier E, Voelter W. Carbon-13 NMR Spectroscopy: high resolution methods and appli- cations in organic chemistry and biochemistry. Weinheim, New York: VCH Verlagsgesellschaft, 1989. [11] Kalinowski H-O, Berger S, Braun S. 13C- NMR-Spektroskopie. Stuttgart, New York: Georg Thieme Verlag, 1984. [12] de Haan JW, van de Ven LJM, Wilson ARN, van der Hout-Lodder AE, Altona C, Faber DH. Temperature effects on 13C n.m.r. chemical shifts of normal alkanes and some linear and branched 1-alkenes. Organic Magnetic Resonance 1976, 8 (9): 477-482. [13] Cheng HN, Bovey FA. Temperature depend- ence of the carbon-13 chemical shifts of paraffinic hydrocarbons. Organic Magnetic Resonance 1978, 11 (9): 457-460. [14] Litchman WM, McLaughlin DR. Temperature effects on the carbon-13 NMR shifts of selected compounds. Chemical Physics Letters 1973, 22 (2): 424-426. [15] Szasz GJ, Sheppard N, Rank DH. Spectro- scopic Studies of Rotational Isomerism. I. Liquid n-Butane and the Assignment of the Normal Modes of Vibration. The Journal of Chemical Physics 1948, 16 (7): 704-711. [16] Harada I, Takeuchi H, Sakakibara M, Matsu- ura H, Shimanouchi T. Vibration Spectra and Ro- tational Isomerism of Chain Molecules. II. Butane, Pentane, Hexane, Pentane-d12, and Hexane-d14. Bulletin of the Chemical Society of Japan 1977, 50 (1): 102-110. [17] Kint S, Scherer JR, Snyder RG. Raman spec- tra of liquid n-alkanes. III. Energy difference be- tween trans and gauche n-butane. The Journal of Chemical Physics 1980, 73 (6): 2599-2602. [18] Davidson DW, Garg SK, Gough SR, Hawkins RE, Ripmeester JA. Characterization of natural gas hydrates by nuclear magnetic resonance and dielectric relaxation. Canadian Journal of Chemis- try 1977, 55: 3641-3650. [19] Subramanian S, Sloan Jr. ED. Trends in Vi- brational Frequencies of Guests Trapped in Clath- rate Hydrate Cages. Journal of Physical Chemistry B 2002, 106 (17): 4348-4355. [20] Durig JR, Wang A, Beshir W, Little TS. Bar- rier to asymmetric internal rotation, conforma- tional stability, vibrational spectra and assign- ments, and Ab Initio calculations of n-butane-d0, d5 and d10. Journal of Raman Spectroscopy 1991, 22 (11): 683-704. [21] Murphy WF, Fernandez-Sanchez JM, Ragha- vachari K. Harmonic force field and Raman scat- tering intensity parameters of n-butane. Journal of Physical Chemistry 1991, 95 (3): 1124 - 1139. [22] Shimanouchi T. Tables of Molecular Fre- quencies - Consolidated Volume I. Washington, D. C.: United States Department Of Commerce, 1972. [23] Sloan Jr. ED. Clathrate hydrates of natural gases. New York: MARCEL DEKKER, INC., 1998. [24] Ripmeester JA, Ratcliffe CI. Xenon-129 NMR studies of clathrate hydrates: new guests for struc- ture II and structure H. Journal of Physical Chem- istry 1990, 94 (25): 8773-8776. [25] Mirkin NG, Krimm S. Ab initio analysis of the vibrational spectra of conformers of some branched alkanes. Journal of Molecular Structure 2000, 550-551: 67-91. [26] Shin K, Park Y, Hong J, Lee H. Gauche con- formation of acyclic guest molecules appearing in the large cages of structure-H clathrate hydrates. Korean Journal of Chemical Engineering 2007, 24 (5): 843-846.  


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