Developing from certain catalytic processes required for ancient life forms, the H2 processing enzymes [NiFe]- and [FeFe]-hydrogenase (H2ase) have active sites that are organometallic in composition, possessing carbon monoxide and cyanide as ligands. Simple synthetic analogues of the 2Fe portion of the active site of [FeFe]-H2ase have been shown to dock into the empty carrier (maturation) protein, apo-Hyd-F, via the bridging ability of a terminal cyanide ligand from a low valent FeIFeI unit to the iron of a 4Fe4S cluster of Hyd-F, with spectral evidence indicating CN isomerization during the coupling process (Berggren, et al., Nature, 2013, 499, 66-70). To probe the requirements for such cyanide couplings, we have prepared and characterized four cyanide-bridged analogues of 3-Fe systems with features related to the organoiron moiety within the loaded HydF protein. As in classical organometallic chemistry, the orientation of the CN bridge in the biomimetics is determined by the precursor reagents; no cyanide flipping or linkage isomerization was observed. Density functional theory computations evaluated the energetics of cyanide isomerization in such [FeFe]-CN-Fe ⇌ [FeFe]-NC-Fe units, and found excessively high barriers account for the failure to observe the alternative isomers. These results highlight roles for cyanide as an unusual ligand in biology that may stabilize low spin iron in [FeFe]-hydrogenase, and can act as a bridge connecting multi-iron units during bioassembly of the active site.

Diamagnetic iron chloro compounds [(P(Ph)2N(Ph)2)FeCp*Cl] [1Cl] and [(P(Cy)2N(Ph)2)FeCp*Cl] [2Cl] and the corresponding hydrido complexes [(P(Ph)2N(Ph)2)FeCp*H] [1H] and [(P(Cy)2N(Ph)2)FeCp*H] [2H] have been synthesized and characterized by NMR spectroscopy, electrochemical studies, electronic absorption, and (57)Fe Mössbauer spectroscopy (P(Ph)2N(Ph)2 = 1,3,5,7-tetraphenyl-1,5-diphospha-3,7-diazacyclooctane, P(Cy)2N(Ph)2 = 1,5-dicyclohexyl-3,7-diphenyl-1,5-diphospha-3,7-diazacyclooctane, Cp* = pentamethylcyclopentadienyl). Molecular structures of [2Cl], [1H], and [2H], derived from single-crystal X-ray diffraction, revealed that these compounds have a typical piano-stool geometry. The results show that the electronic properties of the hydrido complexes are strongly influenced by the substituents at the phosphorus donor atoms of the P(R)2N(Ph)2 ligand, whereas those of the chloro complexes are less affected. These results illustrate that the hydride is a strong-field ligand, as compared to chloride, and thus leads to a significant degree of covalent character of the iron hydride bonds. This is important in the context of possible catalytic intermediates of iron hydrido species, as proposed for the catalytic cycle of [FeFe] hydrogenases and other synthetic catalysts. Both hydrido compounds [1H] and [2H] show enhanced catalytic currents in cyclic voltammetry upon addition of the strong acid trifluoromethanesulfonimide [NHTf2] (pKa(MeCN) = 1.0). In contrast to the related complex [(P(tBu)N(Bn))2FeCp(C6F5)H], which was reported by Liu et al. (Nat. Chem. 2013, 5, 228-233) to be an electrocatalyst for hydrogen splitting, the here presented hydride complexes [1H] and [2H] show the tendency for electrocatalytic hydrogen production. Hence, the catalytic direction of this class of monoiron compounds can be reversed by specific ligand modifications.

A series of four [S2Ni(μ-S)2FeCp*Cl] compounds with different tetradentate thiolate/thioether ligands bound to the Ni(II) ion is reported (Cp* = C5Me5). The {S2Ni(μ-S)2Fe} core of these compounds resembles structural features of the active site of [NiFe] hydrogenases. Detailed analyses of the electronic structures of these compounds by Mössbauer and electron paramagnetic resonance spectroscopy, magnetic measurements, and density functional theory calculations reveal the oxidation states Ni(II) low spin and Fe(II) high spin for the metal ions. The same electronic configurations have been suggested for the Cred1 state of the C-cluster [NiFeu] subsite in carbon monoxide dehydrogenases (CODH). The Ni-Fe distance of ∼3 Å excludes a metal-metal bond between nickel and iron, which is in agreement with the computational results. Electrochemical experiments show that iron is the redox active site in these complexes, performing a reversible one-electron oxidation. The four complexes are discussed with regard to their similarities and differences both to the [NiFe] hydrogenases and the C-cluster of Ni-containing CODH.

Base metal, molecular catalysts for the fundamental process of conversion of protons and electrons to dihydrogen, remain a substantial synthetic goal related to a sustainable energy future. Here we report a diiron complex with bridging thiolates in the butterfly shape of the 2Fe2S core of the [FeFe]-hydrogenase active site but with nitrosyl rather than carbonyl or cyanide ligands. This binuclear [(NO)Fe(N2S2)Fe(NO)(2)](+) complex maintains structural integrity in two redox levels; it consists of a (N2S2)Fe(NO) complex (N2S2 = N,N'-bis(2-mercaptoethyl)-1,4-diazacycloheptane) that serves as redox active metallodithiolato bidentate ligand to a redox active dinitrosyl iron unit, Fe(NO)(2). Experimental and theoretical methods demonstrate the accommodation of redox levels in both components of the complex, each involving electronically versatile nitrosyl ligands. An interplay of orbital mixing between the Fe(NO) and Fe(NO)(2) sites and within the iron nitrosyl bonds in each moiety is revealed, accounting for the interactions that facilitate electron uptake, storage and proton reduction.

Nitrophorins are proteins occurring in the saliva of the blood-sucking insect Rhodnius prolixus to carry NO as a vasodilator and blood-coagulation inhibitor into the victim's tissue. It was suggested that the rate of NO release can be enhanced by the blood-plasma component L-cysteine [J.M.C.Ribeiro, Insect Biochem. Mol. Biol. 26 (1996) 899-905]. However, the mechanism of the reaction is not clear. In the attempt to exploit the reaction in detail, complexes of nitrophorin 4 (NP4) with the thiols 2-mercaptoethanol, L-cysteine, and L-homocysteine and with HS(-) were formed and characterized under anaerobic conditions using absorption spectroscopy, X-ray crystallography, and EPR spectroscopy. In contrast to met-myoglobin, which is reduced by L-cysteine, all four compounds form low-spin Fe(III) complexes with NP4. The weak equilibration constants (167-5200 M(-1)) neither support significant complexation nor the simple displacement of NO in vivo. Both amino acid based thiols form additional H-bonds with side chains of the heme pocket entry. Glutathione and L-methionine did not form a complex, indicating the specificity of the complexes with L-cysteine and L-homocysteine. Continuous wave EPR spectroscopy reveals the simultaneous existence of three low-spin systems in each case that are attributed to various protonation and/or conformational stages in the heme pocket. Electron nuclear double resonance (ENDOR) spectroscopy demonstrates that the thiol sulfurs are, at least in part, protonated. Overall, the results not only demonstrate the good accessibility of the NP4 heme center by biologically relevant thiols, but also represent the first structural characterization of a ferriheme protein in complex with L-cysteine L-homocysteine.

A detailed characterization of a close synthetic model of the [2 Fe]H subcluster in the [FeFe] hydrogenase active site is presented. It contains the full primary coordination sphere of the CO-inhibited oxidized state of the enzyme including the CN(-) ligands and the azadithiolate (adt) bridge, [((μ-SCH2 )2 NR)Fe2 (CO)4 (CN)2 ](2-) , R=CH2 CH2 SCH3 . The electronic structure of the model complex in its Fe(I) Fe(II) state was investigated by means of density functional theory (DFT) calculations and Fourier transform infrared (FTIR) spectroscopy. By using a combination of continuous-wave (CW) electron paramagnetic resonance (EPR) and hyperfine sublevel correlation (HYSCORE) experiments as well as DFT calculations, it is shown that, for this complex, the spin density is delocalized over both iron atoms. Interestingly, we found that the nitrogen hyperfine coupling, which represents the interaction between the unpaired electron and the nitrogen at the dithiolate bridge, is slightly larger than that in the analogous complex in which the CN(-) ligands are replaced with PMe3 ligands. This reveals, first, that the CN(-) /PMe3 ligands coordinated to the iron core are electronically coupled to the amine in the adt bridge. Second, the CN(-) ligands in this complex are somewhat stronger σ-donor ligands than the PMe3 ligand, and thereby enable more spin density to be transferred from the Fe core to the adt unit, which might in turn affect the reactivity of the bridging amine.

A series of [FeFe]-hydrogenase active site analogues, with the general formula [Fe(2)(dt)(CO)(4)(BC)] 1-3 (dt = dithiolate, pdt = propyl-1,3-dt (1), bdt = benzene-1,2-dt (2), edt = ethyl-1,2-dt (3); BC = 1,2-bisdiphenylphosphine-1,2-o-carborane), has been prepared and structurally characterized. While the electrochemical reductions of 1-3 are largely invariant to the different nature of their dt bridges, the oxidations differ by more than 120 mV in between the series. Remarkably, all three compounds are reversibly oxidized, with complex 1 that contains the most electron-donating pdt ligand at the mildest potential of -0.09 V vs. Fc/Fc(+). The one-electron oxidized state 1(ox) is stable for several minutes and was spectroscopically characterized by FTIR and EPR. EPR spectroscopy provided evidence that in the mixed-valence [Fe(I)Fe(II)] state most of the spin density is located on the iron with the BC-ligand. This is monitored through the strong (31)P hyperfine coupling of the phenyl groups of the BC ligand, while further delocalization into the o-carborane unit is negligible.

The compounds of this study have yielded to complementary structural, spectroscopic (Mössbauer, EPR/ENDOR, IR), and computational probes that illustrate the fine control of electronic and steric features that are involved in the two structural forms of (μ-SRS)[Fe(CO)2PMe3]2(0,+) complexes. The installation of bridgehead bulk in the -SCH2CR2CH2S- dithiolate (R = Me, Et) model complexes produces 6-membered FeS2C3 cyclohexane-type rings that produce substantial distortions in Fe(I)Fe(I) precursors. Both the innocent (Fc(+)) and the noninnocent or incipient (NO(+)/CO exchange) oxidations result in complexes with inequivalent iron centers in contrast to the Fe(I)Fe(I) derivatives. In the Fe(II)Fe(I) complexes of S = 1/2, there is complete inversion of one square pyramid relative to the other with strong super hyperfine coupling to one PMe3 and weak SHFC to the other. Remarkably, diamagnetic complexes deriving from isoelectronic replacement of CO by NO(+), {(μ-SRS)[Fe(CO)2PMe3] [Fe(CO)(NO)PMe3](+)}, are also rotated and exist in only one isomeric form with the -SCH2CR2CH2S- dithiolates, in contrast to R = H ( Olsen , M. T. ; Bruschi , M. ; De Gioia , L. ; Rauchfuss , T. B. ; Wilson , S. R. J. Am. Chem. Soc. 2008 , 130 , 12021 -12030 ). The results and redox levels determined from the extensive spectroscopic analyses have been corroborated by gas-phase DFT calculations, with the primary spin density either localized on the rotated iron in the case of the S = 1/2 compound, or delocalized over the {Fe(NO)} unit in the S = 0 complex. In the latter case, the nitrosyl has effectively shifted electron density from the Fe(I)Fe(I) bond, repositioning it onto the spin coupled Fe-N-O unit such that steric repulsion is sufficient to induce the rotated structure in the Fe(II)-{Fe(I)((•)NO)}(8) derivatives.

Angewandte ChemieVolume 123, Issue 6 p. 1475-1479 Zuschrift Ein Modell des aktiven Zentrums der [FeFe]-Hydrogenasen mit biologisch relevanter Azadithiolat-Brücke: eine spektroskopische und theoretische Untersuchung† Dr. Özlen F. Erdem, Corresponding Author Dr. Özlen F. Erdem erdem@mpi-muelheim.mpg.de Max-Planck-Institut für Bioanorganische Chemie, Stiftstraße 34–36, 45470 Mülheim an der Ruhr (Deutschland), Fax: (+49) 208-306-3955 Özlen F. Erdem, Max-Planck-Institut für Bioanorganische Chemie, Stiftstraße 34–36, 45470 Mülheim an der Ruhr (Deutschland), Fax: (+49) 208-306-3955 Sascha Ott, Department of Photochemistry and Molecular Science, Uppsala University, Box 523, 75120, Uppsala (Schweden) Wolfgang Lubitz, Max-Planck-Institut für Bioanorganische Chemie, Stiftstraße 34–36, 45470 Mülheim an der Ruhr (Deutschland), Fax: (+49) 208-306-3955Search for more papers by this authorDr. Lennart Schwartz, Dr. Lennart Schwartz Department of Photochemistry and Molecular Science, Uppsala University, Box 523, 75120, Uppsala (Schweden)Search for more papers by this authorDr. Matthias Stein, Dr. Matthias Stein Max-Planck-Institut für Dynamik komplexer technischer Systeme, Sandtorstraße 1, 39106 Magdeburg (Deutschland)Search for more papers by this authorDr. Alexey Silakov, Dr. Alexey Silakov Max-Planck-Institut für Bioanorganische Chemie, Stiftstraße 34–36, 45470 Mülheim an der Ruhr (Deutschland), Fax: (+49) 208-306-3955Search for more papers by this authorDr. Sandeep Kaur-Ghumaan, Dr. Sandeep Kaur-Ghumaan Department of Photochemistry and Molecular Science, Uppsala University, Box 523, 75120, Uppsala (Schweden)Search for more papers by this authorDr. Ping Huang, Dr. Ping Huang Department of Photochemistry and Molecular Science, Uppsala University, Box 523, 75120, Uppsala (Schweden)Search for more papers by this authorDr. Sascha Ott, Corresponding Author Dr. Sascha Ott sascha.ott@fotomol.uu.se Department of Photochemistry and Molecular Science, Uppsala University, Box 523, 75120, Uppsala (Schweden) Özlen F. Erdem, Max-Planck-Institut für Bioanorganische Chemie, Stiftstraße 34–36, 45470 Mülheim an der Ruhr (Deutschland), Fax: (+49) 208-306-3955 Sascha Ott, Department of Photochemistry and Molecular Science, Uppsala University, Box 523, 75120, Uppsala (Schweden) Wolfgang Lubitz, Max-Planck-Institut für Bioanorganische Chemie, Stiftstraße 34–36, 45470 Mülheim an der Ruhr (Deutschland), Fax: (+49) 208-306-3955Search for more papers by this authorDr. Edward J. Reijerse, Dr. Edward J. Reijerse Max-Planck-Institut für Bioanorganische Chemie, Stiftstraße 34–36, 45470 Mülheim an der Ruhr (Deutschland), Fax: (+49) 208-306-3955Search for more papers by this authorProf. Dr. Wolfgang Lubitz, Corresponding Author Prof. Dr. Wolfgang Lubitz lubitz@mpi-muelheim.mpg.de Max-Planck-Institut für Bioanorganische Chemie, Stiftstraße 34–36, 45470 Mülheim an der Ruhr (Deutschland), Fax: (+49) 208-306-3955 Özlen F. Erdem, Max-Planck-Institut für Bioanorganische Chemie, Stiftstraße 34–36, 45470 Mülheim an der Ruhr (Deutschland), Fax: (+49) 208-306-3955 Sascha Ott, Department of Photochemistry and Molecular Science, Uppsala University, Box 523, 75120, Uppsala (Schweden) Wolfgang Lubitz, Max-Planck-Institut für Bioanorganische Chemie, Stiftstraße 34–36, 45470 Mülheim an der Ruhr (Deutschland), Fax: (+49) 208-306-3955Search for more papers by this author Dr. Özlen F. Erdem, Corresponding Author Dr. Özlen F. Erdem erdem@mpi-muelheim.mpg.de Max-Planck-Institut für Bioanorganische Chemie, Stiftstraße 34–36, 45470 Mülheim an der Ruhr (Deutschland), Fax: (+49) 208-306-3955 Özlen F. Erdem, Max-Planck-Institut für Bioanorganische Chemie, Stiftstraße 34–36, 45470 Mülheim an der Ruhr (Deutschland), Fax: (+49) 208-306-3955 Sascha Ott, Department of Photochemistry and Molecular Science, Uppsala University, Box 523, 75120, Uppsala (Schweden) Wolfgang Lubitz, Max-Planck-Institut für Bioanorganische Chemie, Stiftstraße 34–36, 45470 Mülheim an der Ruhr (Deutschland), Fax: (+49) 208-306-3955Search for more papers by this authorDr. Lennart Schwartz, Dr. Lennart Schwartz Department of Photochemistry and Molecular Science, Uppsala University, Box 523, 75120, Uppsala (Schweden)Search for more papers by this authorDr. Matthias Stein, Dr. Matthias Stein Max-Planck-Institut für Dynamik komplexer technischer Systeme, Sandtorstraße 1, 39106 Magdeburg (Deutschland)Search for more papers by this authorDr. Alexey Silakov, Dr. Alexey Silakov Max-Planck-Institut für Bioanorganische Chemie, Stiftstraße 34–36, 45470 Mülheim an der Ruhr (Deutschland), Fax: (+49) 208-306-3955Search for more papers by this authorDr. Sandeep Kaur-Ghumaan, Dr. Sandeep Kaur-Ghumaan Department of Photochemistry and Molecular Science, Uppsala University, Box 523, 75120, Uppsala (Schweden)Search for more papers by this authorDr. Ping Huang, Dr. Ping Huang Department of Photochemistry and Molecular Science, Uppsala University, Box 523, 75120, Uppsala (Schweden)Search for more papers by this authorDr. Sascha Ott, Corresponding Author Dr. Sascha Ott sascha.ott@fotomol.uu.se Department of Photochemistry and Molecular Science, Uppsala University, Box 523, 75120, Uppsala (Schweden) Özlen F. Erdem, Max-Planck-Institut für Bioanorganische Chemie, Stiftstraße 34–36, 45470 Mülheim an der Ruhr (Deutschland), Fax: (+49) 208-306-3955 Sascha Ott, Department of Photochemistry and Molecular Science, Uppsala University, Box 523, 75120, Uppsala (Schweden) Wolfgang Lubitz, Max-Planck-Institut für Bioanorganische Chemie, Stiftstraße 34–36, 45470 Mülheim an der Ruhr (Deutschland), Fax: (+49) 208-306-3955Search for more papers by this authorDr. Edward J. Reijerse, Dr. Edward J. Reijerse Max-Planck-Institut für Bioanorganische Chemie, Stiftstraße 34–36, 45470 Mülheim an der Ruhr (Deutschland), Fax: (+49) 208-306-3955Search for more papers by this authorProf. Dr. Wolfgang Lubitz, Corresponding Author Prof. Dr. Wolfgang Lubitz lubitz@mpi-muelheim.mpg.de Max-Planck-Institut für Bioanorganische Chemie, Stiftstraße 34–36, 45470 Mülheim an der Ruhr (Deutschland), Fax: (+49) 208-306-3955 Özlen F. Erdem, Max-Planck-Institut für Bioanorganische Chemie, Stiftstraße 34–36, 45470 Mülheim an der Ruhr (Deutschland), Fax: (+49) 208-306-3955 Sascha Ott, Department of Photochemistry and Molecular Science, Uppsala University, Box 523, 75120, Uppsala (Schweden) Wolfgang Lubitz, Max-Planck-Institut für Bioanorganische Chemie, Stiftstraße 34–36, 45470 Mülheim an der Ruhr (Deutschland), Fax: (+49) 208-306-3955Search for more papers by this author First published: 05 January 2011 https://doi.org/10.1002/ange.201006244Citations: 11 † Gudrun Klihm wird gedankt für technische Unterstützung bei den EPR-Experimenten. Wir danken der Max-Planck-Gesellschaft, dem Swedish Research Council (S.O.), der Wenner Gren Foundation (S.K.-G.), der Schwedischen Energiebehörde, der Knut und Alice Wallenberg-Stiftung, der Klaus Tschira Stiftung (M.S.) und der EU (FP7 Energy 212508 "SOLAR-H2") für finanzielle Unterstützung. Read the full textAboutPDF ToolsRequest permissionAdd to favorites ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract Überzeugende Beweise für das Vorhandensein eines Stickstoffatoms in der Dithiolatbrücke des aktiven Zentrums von natürlichen [FeFe]-Hydrogenasen B liefert die spektroskopische, elektrochemische und theoretische Untersuchung eines gut charakterisierten Strukturmodells der [FeFe]-Hydrogenasen (siehe 14N-Matched-HYSCORE-Spektrum der Modellverbindung A). Dies ist von großer Bedeutung für das Verständnis des Mechanismus der Wasserstoffkonversion und -produktion in diesem hochaktiven Enzym. Citing Literature Supporting Information Detailed facts of importance to specialist readers are published as "Supporting Information". Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Filename Description ange_201006244_sm_miscellaneous_information.pdf578 KB miscellaneous_information Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article. Volume123, Issue6February 7, 2011Pages 1475-1479 This is the German version of Angewandte Chemie. Note for articles published since 1962: Do not cite this version alone. Take me to the International Edition version with citable page numbers, DOI, and citation export. We apologize for the inconvenience. RelatedInformation

Convincing evidence for the presence of a nitrogen atom in the dithiolate bridge of the active site of native [FeFe] hydrogenases (B) is provided by a spectroscopic, electrochemical, and theoretical study of a well-characterized structural mimic of the [FeFe] hydrogenase subcluster (picture: 14N matched-HYSCORE spectrum of the model compound A). This result should help to understand the mechanism of dihydrogen conversion and production.

Nuclear magnetic resonance (NMR) and broadband dielectric spectroscopy are used to investigate the dynamics of small glass-forming molecules confined to restricted geometries. Ethylene glycol molecules are embedded in the supercages of NaX zeolites. The combined application of NMR and broadband dielectric spectroscopy advances the understanding of the slowing down of the motion near the glass transition temperature of these confined molecules. In combination with nuclear spin relaxation and nuclear magnetic resonance spectroscopy, dielectric relaxation studies on glass forming molecules allow conclusions on the character of the motion. High resolution 1H magic angle spinning (MAS) NMR measurements not only enable a characterisation of the state of the adsorbed molecules via a chemical shift analysis. By means of an analysis of MAS spinning sidebands we may also estimate a correlation time the meaning of which will be discussed in comparison to the results of longitudinal proton spin relaxation measurements. In addition to broadband dielectric spectroscopy slow molecular motions of partially deuterated ethylene glycol adsorbed in NaX are studied by means of 2H NMR line-shape analysis.

. 1 H nuclear magnetic resonance (NMR) magic-angle spinning (MAS) sideband patterns of molecules adsorbed in zeolites or related materials both depend on the MAS frequency and the thermal mobility. By using a statistical NMR line shape theory a valuable information on the thermal motion can be derived from the MAS sidebands. The conclusions are compared with the results of longitudinal proton spin relaxation studies at different temperatures and Larmor frequencies. The measurements are performed on samples of ethylene glycol molecules adsorbed in zeolites of the NaX and sodalite type which were prepared under well-defined conditions in vacuum. It is shown that the comparison of the respective results is very suitable for a deeper understanding of the dynamics of the adsorbed molecules.

Single oxides of Ti and Zr incorporated SBA-15 were prepared and characterized by N2 adsorption, NMR, and XPS techniques. 29Si MAS NMR results suggest the formation of Si–O–X linkages (X: Ti or Zr) by an increase in the ratio of Q 3/Q 4 in the presence of Ti or Zr. XPS analysis of Ti–SBA-15 catalysts indicate the presence of Ti–O–Si bonds in addition to Ti–O–Ti and Si–O–Si bonds, supporting the NMR evidence.

In order to prepare high surface area highly acidic catalysts, different weight loadings of ZrO2 were incorporated in the SBA-15 structures which are subsequently sulfated by treating in 0.25 M H2SO4. The catalysts were characterized by means of TEM, XRD, N2 adsorption, and 1H MAS NMR. Brønsted type acidities of sulfated zirconia included SBA-15 materials were identified by a sharp 1H MAS NMR line at 10.6 ppm. The highest acidity was obtained in the 25 mol% ZrO2 included SBA-15 catalyst with a BET surface area of 246 m2/g.

Proton spin relaxation and broadband dielectric spectroscopy are applied to study the dynamics of ethylene glycol adsorbed in NaX, EG/NaX. The molecular mobility strongly depends on the pore filling factor which may be controlled by high-resolution 1H MAS NMR measurements. Although EG in bulk shows a Vogel–Fulcher–Tammann type of activation, temperature dependent relaxation rate measurements for the EG/NaX systems always follow an Arrhenius plot, independent of the loading degree. There is no hint to a glass-forming behaviour of EG/NaX which could be understood in terms of the strong influence of surface–molecule interactions playing a dominant role compared to the molecule–molecule interactions.

By means of broadband dielectric spectroscopy the molecular dynamics of glass-forming ethylene glycol (EG) adsorbed in zeolites is investigated by varying the loading degree, the type of zeolites, and the Si/Al ratio. We only concentrate to the frequency and temperature range where relaxation processes may be observed which allow conclusions about the glass-transition. Although EG molecules in the bulk liquid show a Vogel–Fulcher–Tammann (VFT) type of activation at lower temperatures, measurements for EG molecules adsorbed in NaX zeolites always follow Arrhenius-type of behaviour independent of the loading degree. This effect is explored in more detail by measuring the dielectric relaxation rate for EG adsorbed in zeolite beta, EG/beta, with different Si/Al ratios. For the case of zeolite beta with a very large Si/Al ratio, a glass-transition is observed, i.e., a VFT-type of activation is detectable. For the system EG/beta with Si/Al ratio of 56, however, a clear Arrhenius-type of activation is observed. Obviously, due to a higher number of adsorption sites, surface–molecule interactions are of greater influence and suppress the glass-transition. For EG/beta with a Si/Al ratio between 56 and infinity, there is subtle interplay between “cooperative” and “local” motions, i.e., between VFT- and Arrhenius-type of activation behaviour.

The application of 1H MAS NMR allows a detailed study of the behavior of ethylene glycol adsorbed in NaX zeolites which may be used to understand the effect of confinement. Typical changes in the chemical shift values for the CH2 and OH groups were found which are very sensitive to interactions between the molecules and the internal surfaces. This allows clear differentiation between molecules within the zeolite cages and those adsorbed at the outer surface of the zeolite grains and also allows study of the dynamics of the different species. Selective 1H T1 measurements were carried out for various pore-filling degrees where large differences were found in the thermal mobility. It is shown that for the molecules inside the supercages, a dynamic heterogeneity occurs which may be related to the competing influences of molecule-internal surface interactions and molecule-molecule interactions within a network of intermolecular hydrogen bonds.