Faraday Symp. Chem. Soc. 1982 17 25-30 Nuclear Magnetic Resonance Relaxation Investigation of Tetrahydrofuran and Methyl Iodide Clathrates AND MANFRED BY MARC JUNIO DIETER 0. LEICHT D. ZEIDLER Institut fur Physikalische Chemie der RWTH Aachen Templergraben 59 D-5 100 Aachen Federal Republic of Germany Received 13th August 1982 Proton and deuteron spin-lattice relaxation rates of the guest molecules have been measured as a function of temperature in methyl iodide and tetrahydrofuran clathrates. By use of the isotopic- dilution method in proton n.m.r. i.e. employing the compounds (CH31),.(CD31)l -;17D20 and (C4H80)x*(C4D80)1 -,*17D20 with varying mole fractions x intramolecular and intermolecular contributions to the proton relaxation rate may be separated.Deuteron relaxation rates have been measured in the compounds CD31*17H20 and C4D80*17H20. The maximum temperature range covered was 0 to-120 "C. The proton relaxation rates contain rather large intermolecular contributions ca. 45% for methyl iodide and 23% for tetrahydrofuran. The correlation times obtained from the intramolecular contributions lie in the range 1-12 and 0.5-1 ps for methyl iodide and tetrahydrofuran respectively which is the range typical for liquids. This confirms the well known fact that the enclathrated guest molecules perform very fast motion. The evaluation of the deuteron relaxation rates leads to correla-tion times which are comparable to those derived from the intramolecular proton rate for tetrahydro- furan but differ in the case of methyl iodide.This indicates anisotropic rotational motion of the methyl iodide molecule in the clathrate cage. A summary of n.m.r. investigations on clathrate hydrates has been given by Davids0n.l Here we mention only those papers which include relaxation time measurements methyl chloride,2 pr~pane,~ sulphur hexafluoride and tetrahydro- furan hydrate. To our knowledge no attempt has been made to separate relaxation contributions for example by using the isotopic-dilution method,6 and thus to obtain more detailed information on the molecular motion of the clathrate components. Clathrate hydrates form according to well known rules depending on the chemical nature of the guest molecule. In the present investigation tetrahydrofuran and methyl iodide are the guest molecules and they both form clathrates of the type M.17H20.The water molecules in these clathrates are arranged in pentagonal dodeca- FIG.1.-Cage structures of type M.17H20 clathrates. Only oxygen positions are drawn. N.M.R. INVESTIGATION OF CLATHRATES hedra and hexakaidecahedra (having 4 hexagons and 12 pentagons as faces) with cage diameters 4.8 and 6.9 A respectively (see fig. 1). Only the larger cages can be occupied by the tetrahydrofuran and methyl iodide molecules because of their size. Both hydrates are stable up to 4.3 "C at normal pressure. They can be further stabilized by also filling the smaller cages with suitable molecules for example nitro- gen. It is well known that the encaged molecules show large mobility even at tempera- tures far below the ice point.This motion must be largely rotational because the water cage prohibits any translational motion except within the cage. In order to obtain more details about this motion one can employ the well established n.m.r. relaxation methods often used in liquid-state studies. (a) If dipolar interactions dominate the relaxation intra- and inter-molecular contributions are separated by the isotopic-dilution technique. Intramolecular contributions are associated with rotational motion whereas intermolecular contributions are largely effected by translational motion. (6) If quadrupolar interactions dominate only rotational motion is probed. By comparison of rotational correlation times for different quadrupolar nuclei (or with the aforementioned rotational dipolar contribution) information on anisotropic rotation is obtained.In the present paper the proton and deuteron n.m.r. relaxations (showing dipolar and quadrupolar interactions respectively) of the guest molecules are studied and the isotopic-dilution method (using deuteron dilution) is employed in the proton n.m.r. This means that clathrates of the type (resonant nucleus underlined) (C,H,O) (C4D80)1-x-17D20,(CH,I);(CD,l) -;17D20 C4]2S0.17H20 and CD31017H20 with varying composition x were measured. The maximum temperature range covered was 0 to -120 "C. RESULTS The clathrate components water (D20 was 99.9 atom %) and methyl iodide (99.5 atom % D) or tetrahydrofuran (99.0 atom % D) were separately freed from oxygen by several freeze-pump-thaw cycles and then distilled into an n.m.r.tube. A nitrogen I I I I I 3.7 L.0 4.5 5.0 5.5 6.0 I -10 I -30 I -50 1 -70 I -90 I -100 I -110 inner scale lo3 K/T outer scale T/"C FIG. 2.-Proton relaxation rates for the clathrate (CH31~;(CD31) -;17D20 top curve x = 1 ; middle curve x = 0.5; lower curve x = 0.25. M. JUNIO D. 0. LEICHT AND M. D. ZEIDLER 5.01 1.51 4.5 5.0 5.5 6.O 6.5 r -50 I -70 I -90 I -100 I -110 inner scale lo3 K/T outer scale T/"C FIG.3.-Proton relaxation rates for the clathrate (C4H80);(C4D80)l-,*17 D20 top curve x = 1; middle curve x = 0.6; lower curve x = 0.1. pressure of 0.4-0.5 bar was applied prior to sealing the tubes. Finally the clathrates were formed by keeping the samples between 0 and 4 "Cfor several hours occasionally shaking them to initiate crystallisation.To avoid inclusion of the liquids several freeze-thaw cycles were performed and it was ascertained that the relaxation-rate measurements were always reproducible after preparation of the clathrate. TI relaxation measurements were performed with a pulse spectrometer operating at 12 MHz using the 9O0-r-9O0 method. The probe temperature was controlled to A1 "C by a thermostatted nitrogen-gas stream flowing around the sample. 200-d ml-lvJ -_ 80-60-g LO-U 1 20 1 1 I I I I 3.7 4.0 L.5 5.0 5.5 6.0 I I I I 1 I I -10 -30 -50 -70 -90 -100 -110 inner scale lo3 K/T outer scale T/"C FIG.4.-Deuteron relaxation rates of the clathrates upper curve CD31*17H20; lower curve C4D80'17H20.N.M.R. INVESTIGATION OF CLATHRATES Fig. 2 and 3 show the proton relaxation rates of methyl iodide and tetrahydrofuran clathrates respectively as a function of temperature; the corresponding deuteron relaxation rates are summarized in fig. 4. DISCUSSION It is obvious from fig. 2 and 3 that dilution of the protonated guest molecule with deuterated species leads to a decrease of the proton relaxation rate. This behaviour is expected if an intermolecular relaxation contribution is present. Since the magnetic moment of the deuteron is considerably smaller than that of the proton the dipolar magnetic interaction is correspondingly reduced. In quantitative terms where l/Tl is the measured proton relaxation rate at the mole fraction x of protonated species.As required by eqn (l) plots of I/Tl against x proved to be linear and could TABLE 1.-PROTONRELAXATION CONTRIBUTIONSOF METHYL IODIDEAND TETRAHYDROFURAN CLATHRATES methyl iodide tetrahydrofur an lo3 KIT 3.8 6.44 4.96 4.0 7.36 5.74 4.2 8.37 6.73 4.4 9.76 8.03 - - 4.5 - - 1.75 0.46 4.6 11.6 9.4 - - 4.7 - - 1.82 0.56 4.8 14.4 11.4 - - 4.9 - - 1.91 0.64 5.O 17.7 15.1 - - 5.1 - - 2.04 0.70 5.2 21.6 20.4 - - 5.3 - - 2.19 0.74 5.4 29.1 25.4 - - 5.5 - - 2.37 0.77 5.6 40.6 32.6 - - 5.7 - - 2.57 0.79 5.8 54.0 47.5 - - 5.9 - - 2.77 0.82 6.1 2.98 0.87 6.3 3.21 0.93 6.5 3.53 0.94 be evaluated for the intra- and inter-molecular contributions.The results are sum- marized in table 1. The large intermolecular rates ca. 45% for methyl iodide and 23% for tetra- hydrofuran are surprising. We have not been able to account for this large rate in quantitative terms by any current theory. M. JUNIO D. 0. LEICHT AND M. D. ZEIDLER 29 The intramolecular contribution is evaluated using the extreme-narrowing formula i# i which relates the rate to a rotational correlation time zD the term " extreme narrow- ing " expressing the condition COT, < 1 where cr) = 27cv stands for the n.m.r. fre-quency. This condition was experimentally verified by a few test measurements at v = 60 MHz and the results were identical with those at v = 12 MHz. The geo- metrical factor containing the interatomic distances rijaveraged over all n protons of the molecule is calculated from the known molecular structure y is the magnetogyric ratio of the proton.The total factor in front of zDamounts to 4.49 x 1o'O and 3.81 X 1O'O s-~ for methyl iodide and tetrahydrofuran respectively the error in these figures being 10%. The correlation times therefore lie in the range 1-12 ps for methyl iodide and 0.5-1 ps for tetrahydrofuran depending on temperature (see table 2). For TABLE2.-ROTATIONAL CORRELATION TIMES OF METHYL IODIDE AND TETRAHYDROFURAN CLATHRATES FROM PROTON (ZD)AND DEUTERON (ZQ)RELAXATION RATES methyl iodide tetrahydrofuran 103~/~ TD/10-l2 s ZQ/lO-s ZD/lO-l2 s ZQ/10-l2s 3.8 1.43 0.68 4.0 1.64 0.79 4.2 1.86 0.92 4.4 2.17 1.09 - 4.5 -0.46 0.52 4.6 2.58 1.31 4.7 -0.48 0.52 4.8 3.21 1.60 - 4.9 -0.50 0.52 5.0 3.94 2.00 - 5.1 -0.54 0.54 5.2 4.81 2.54 5.3 -0.58 0.56 5.4 6.49 3.28 5.5 -0.62 -5.6 9.04 4.39 5.7 -0.67 5.8 12.03 6.06 - 5.9 -0.73 6.1 0.78 6.3 0.84 6.5 0.93 comparison the rotational correlation times in the pure liquids at 25 "Care 1.2 ps for methyl iodide and 0.6 ps for tetrahydrofuran.* The motion of the guest molecules is therefore as fast as in the liquid.From the deuteron relaxation measurements in fig. 4 another rotational correlation time zQ can be calculated through the relation N.M.R. INVESTIGATION OF CLATHRATES where the quantity in brackets is termed the quadrupole coupling constant.This quantity is taken as 153 kHz for methyl iodide determined in the solid ~tate,~ and 198 kHz for tetrahydrofuran a value derived from the liquid-state relaxation measure-ments ' assuming isotropic rotation. The correlation times calculated from eqn (3) are included in table 2. If the correlation times derived from proton relaxation Z and from deuteron relaxation zQ in table 2 are compared it is seen that about the same figures for tetra- hydrofuran but by a factor of 2 differing values for methyl iodide exist. Different correlation times can arise only if the rotational motion is anisotropic because they are associated with either proton-proton vectors (z,) or carbon-hydrogen bonds (zQ but only if the electric field gradient has cylindrical symmetry with its main axis directed along this bond) and both must have different directions within the molecule.Thus it seems that tetrahydrofuran reorients isotropically but this statement is valid only if the same is true for the pure liquid because the quadrupole coupling constant was derived under this assumption. On the other hand methyl iodide re- orients anisotropically behaviour which is also observed in the pure 1iquid;'O however in the clathrate it is even more pronounced. These conclusions can be drawn without going into detailed model calculations which seem not be justified in the clathrate case. However if one does it turns out that motion around the molecular symmetry axis is faster than perpendicular to it the latter motion being slower in comparison to the pure liquid by a factor of 2.We gratefully acknowledge continuing generous financial support by the Fonds der Chemischen Industrie. D. W. Davidson in Water-A Comprehensive Treatise ed. F. Franks (Plenum Press New York 1973) vol. 2 chap. 3. ' C. A. McDowell and P. Ragunathan J. Mol. Struct. 1968 2 359. C. A. McDowell and P. Ragunathan J. Mol. Struct. 1970,5 433. M. B. Dunn and C. A. McDowell Chern. Phys. Lett. 1972,15 508. S. K. Garg D. W. Davidson and J. A. Ripmeester J. Magn. Reson. 1974 15 295. M. D. Zeidler Ber. Bunsenges. Phjs. Chem. 1965 69 659. ' A. Abragam The Principles of Nuclear Magnetism (Oxford University Press London 1970). E. v. Goldammer and M. D. Zeidler Ber. Bunsenges. Phys. Chern. 1969 73 4. M. Rinn6 and J. Depireux Ado. Nucl. Quadrupole Reson. 1974,7 357. lo K. T. Gillen M. Schwartz and J. H. Noggle Mol. Phys. 1971 20 899. H. Versmold 2.Naturforsch. Teil A 1970,25 367.