Comment on “Jump in the Rotational Mobility of Benzene Induced by the Clathrate Hydrate Formation”

Recently it was observed that when a saturated solution of benzene in water was cooled below 253 K in glass capillaries, there were both a shift in the 2 H resonance frequency of the benzene-d 6 and a change in the spin-lattice relaxation time. 1 These changes were interpreted in terms of enclathration of benzene molecules in a clathrate hydrate lattice that caused an increase in motional mobility of the benzene molecules with respect to benzene in aqueous solution.We wish to offer an alternative explanation of these observations, which, we think, is consistent with the observations but does not require the postulation of benzene clathrate formation.First of all, it should be pointed out that benzene, which is known to act as a guest suitable for the large cage (5 12 6 4 ) of structure II hydrate, requires a small "help gas" guest such as Xe, CH 4 , or H 2 S in the small cage (5 12 ) for stability.2,3 A second point is that hydrates become thermodynamically less stable when they are confined in small pores.4 This makes it unlikely that hydrate will form in capillaries if it does not form in the bulk.Hence, the observation of a benzene hydrate under the conditions of the experiment seems rather unlikely.Further experimental evidence can be offered to show that some of the experimental observations are, in fact, inconsistent with a hydrate hypothesis.The 2 H resonance line attributed to benzene hydrate has a width of only a few hertz; that is, it is similar in width to that of benzene dissolved in water inside the capillary tubes. 1 This is inconsistent with the presence of benzene in a solid hydrate lattice.Previous experiments have shown that even for the pseudospherical structure II large cage the 2 H NMR line width of deuterated guests is much larger.5 To illustrate this, we have prepared a sample of benzene-d 6 hydrate with xenon as the small-cage guest and measured the 2 H line width at a temperatures of 270 K.As expected, the line width is close to 10 kHz (Figure 1).This is due to the fact that the local symmetry of the structure II large cage is lower than that given by diffraction, which gives only the spaceaverage symmetry, and unless the motion of the water molecules in the hydrate lattice is fast enough to give time-averaging on an NMR time scale, the different H-bond configurations impose * To whom correspondence should be addressed.