Site Specific Rotational Mobility of Anhydrous Glucose near the Glass Transition As Studied by 2D Echo Decay 13 C NMR Dagmar van Dusschoten,* ,²,‡ Ursula Tracht, ² Andreas Heuer, ² and Hans W. Spiess ² Laboratory of Molecular Physics, Agricultural UniVersity Wageningen, Dreijenlaan 3, 6703 HA Wageningen, The Netherlands, and Max Planck Institut fu ¨ r Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany ReceiVed: May 25, 1999; In Final Form: August 2, 1999 Site specific 13 C labeling of anhydrous glucose is used to study the time scale and geometry of reorientational motion of the exocyclic CH 2 OH group in relation to the main glucose ring. By comparison of 2D echo decay NMR experiments with Monte Carlo simulations a bimodal distribution of jump angles, a 75% fraction of 1-2° jumps and a 25% of 7-8° jumps, is found to describe the geometry of the reorientational processes of the main ring. For the CH 2 OH group the average jump angle of the larger jump process is somewhat larger. The jump rates for both the CH 2 OH group and the ring are similar. The apparent activation energy determined for the rotational motion of the CH 2 OH group and the ring is 480 ( 40 kJ/mol, which is very similar to an earlier determination using viscometry. It is concluded that the glucose ring and the exocyclic CH 2 OH-group mobility are strongly correlated and that the rotational freedom of the CH 2 OH group should not be used to explain the faster -relaxation process also found for glucose. Introduction The nature of the molecular mobility of a supercooled liquid near its glass transition temperature, T g , is of great interest for understanding the macroscopic changes that are associated with the glass transition. 1,2 In many studies that attempt to determine the nature of molecular mobility near T g , one resorts to glasses with favorable characteristics, e.g. o-terphenyl, toluene, or glycerol. 3,4 However, to better understand the molecular mobility and its relation to macroscopic characteristics in general, it is necessary to make comparisons with other low molecular mass glasses. One group of interesting molecules is provided by the monomeric carbohydrates, which have been extensively studied by differential scanning calorimetry (DSC) 5 and dielectric spectroscopy/relaxation 6-8 and are available in a wide variety with small molecular variations. Most of these carbohydrates consist of a closed ring with hydroxyl groups and an exocyclic group. The nature of the dynamics and geometry of molecular motion of carbohydrates close to T g has not been characterized yet. This question, however, is particularly important since it is believed that the exocyclic CH 2 OH group, with its additional degrees of freedom, play a major role in the -relaxation process. This assumption is based on the observation that monomeric sugars without this side group show little or no -relaxation when studied by dielectric spectroscopy. 7 To investigate this hypoth- esis, it is therefore essential to know how the relatively small CH 2 OH group moves relative to the bigger, main ring. Because of the high specificity of NMR and the ready availability of site-specific 13 C labeled glucose, such a hypothesis is open to testing. Even more, not only the time scale of (sub)molecular reorientations can be detected, but using the 2D spin echo decay technique the geometry of the reorientational motion can also be assessed. 9 R-D-Glucose (Figure 1) is a good candidate for studying molecular mobility in carbohydrates. It is comparable in size with toluene, not as a van der Waals glass but as an H-bridge forming glass, in which respect it is comparable to glycerol, which is also well studied. 3 Compared to glycerol and toluene, anhydrous glucose has a much higher glass transition temper- ature of 308 K compared to 190 K for glycerol and 117 K for toluene. 10 All three glasses, according to the classifying system developed by Angell, 11 are rather fragile, with toluene having the highest fragility index, m ) 107, followed by glucose with m ) 79 and glycerol with m ) 53. 12 Because of the many hydroxyl groups and the therewith-associated high dipolar moment, glucose, like glycerol, is highly hygroscopic. The presence of water strongly reduces the glass transition temper- ature, whereby water is thought to cut or destabilize the intraglucose H-bonds. 13 In recent NMR studies, e.g., refs 9, 10, and 14-16, the reorientational mechanism of deuterated glycerol, toluene, and o-terphenyl was studied slightly above their respective calori- metric glass temperatures by using specific multidimensional 2 H NMR methods. 17 For glycerol it was concluded that the reorientational mechanism was dominated by small angular jumps of about 2-3° superimposed on a smaller fraction with a 25° average jump angle. 3 Here, we describe similar 13 C NMR studies on glucose enriched at the C-1 or C-6 position (see Figure 1) to investigate both molecular and intramolecular * To whom correspondence should be addressed. ² Current address: Max Planck Institut fu ¨r Polymerforschung, Acker- mannweg 10, D-55128 Mainz, Germany. E-mail: dvd@mpip-mainz.mpg.de. ‡ Agricultural University Wageningen. Figure 1. Molecular structure of R-D-glucose. Enantiomerization into -D-glucose occurs when the hydroxy group at position 1 flips up. 13 C enrichment was at position 1 or 6. 8359 J. Phys. Chem. A 1999, 103, 8359-8364 10.1021/jp9917244 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/01/1999