Molecular rotations studied by nuclear resonant scattering

Journal of Physics and Chemistry of Solids 66 (2005) 2271–2276 www.elsevier.com/locate/jpcs

Molecular rotations studied by nuclear resonant scattering T. Asthalter a,*, I. Sergueev b, U. van Bu¨rck c a

Institut fu¨r Physikalische Chemie, Universita¨t Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany b European Synchrotron Radiation Facility, B.P. 220, F-38043 Grenoble, France c Physik Department, Technische Universita¨t Mu¨nchen, James-Franck-Str. 1, D-85747 Garching, Germany

Abstract Nuclear resonant scattering techniques can be used to study both fast and slow dynamics of Mo¨ssbauer nuclei. The influence of rotational dynamics in molecular systems is studied applying three types of scattering techniques: (1) Synchrotron radiation perturbed angular correlation (SRPAC) yields direct and quantitative evidence for rotational dynamics in the ms–ns regime. (2) Nuclear inelastic scattering (NIS) monitors the relative influence of intra- and intermolecular forces via the vibrational density of states, which can be influenced by the onset of molecular rotation. (3) In nuclear forward scattering (NFS), information both on rotational and on translational dynamics can be extracted. Results using SRPAC and NIS on a plastic crystal and NFS on ferrocene confined in a molecular sieve are presented. q 2005 Elsevier Ltd. All rights reserved. Keywords: A. Organometallic compounds; A. Microporous materials; D. Phonons; D. Phase transitions PACS: 61.10.Eq; 82.80.Ej; 64.60.Cn; 82.75.Jn

1. Introduction For several years, nuclear resonant scattering has continued to yield highly valuable information on materials containing Mo¨ssbauer nuclei, both via partial phonon densities of states [1] and via magnetic structure and electronic relaxation [2]. Moreover, its usefulness for the study of glass physics [3–7] and diffusion in metallic phases [8,9] has been demonstrated in a number of studies. In the present paper, we focus on a topic of prominent interest in chemical physics: molecular rotational dynamics. The different ways how molecular rotation influences nuclear resonant scattering spectra will be briefly described in Section 2. Translational and rotational motions are basic processes that dominate the dynamics in molecular systems. The separate investigation of both types of motion, in particular in partially disordered materials, such as plastic and liquid crystals, and in guest–host systems, is highly interesting both from a fundamental (translation–rotation coupling) and an applicational (catalytic processes) point of view. We present results for two different systems:

* Corresponding author. E-mail address: [email protected] (T. Asthalter).

0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2005.09.076

1. a plastic crystal, octamethyl-ethinyl-ferrocene (OMFA), and 2. a molecule confined in a zeolite-type host lattice, ferrocene in AlPO4K5. Plastic crystals [10] form a mesophase with translational symmetry but rotational disorder between the ordered crystalline and the liquid phase. Prominent examples are fullerenes [11], higher alkanes [12,13] and Langmuir monolayers of surfactants [14]. Supercooling of such mesophases, also called rotator phases, under freezing of rotational disorder may yield an orientational glass [15], whose dynamics provides partial insight into the physics of the glass transition [16]. In Section 3.1, new results about an organometallic rotator phase, OMFA, are presented and discussed in the light of our previous study [17]. The structure and dynamics of molecules in nanoporous hosts is a topic of high interest [18] thanks to the outstanding importance of adsorption in nanoporous materials in catalytic processes, detergents, etc. The interplay between reorientational and translational dynamics may influence chemical reactivities in pores. In this context, it is particularly interesting to study the reorientational dynamics of molecules whose reactivity is enhanced by the bonding in a host lattice, since the steric effects may prevail over the electronic ones, as shown for ferrocene in zeolite-Y [19]. In Section 3.2, recent [20] and new results on ferrocene confined in the unidimensional channels of

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a zeolite-like molecular sieve, AlPO4-5, are presented and discussed. 2. Nuclear resonant scattering methods In nuclear resonant scattering, a nuclear level is excited by a synchrotron pulse that is short in comparison to the lifetime of the excited state. The re-emitted photons are delayed with respect to the electronic scattering and can be observed using avalanche photodiode (APD) detectors in combination with fast timing electronics. In order to increase the ratio of incoming resonant to non-resonant X-ray quanta and thus to reduce the load on the APD detectors, the primary beam is monochromatized in two steps [21]. The first, fixed-exit monochromator takes up the heat load of the undulator beam, whereas the second, high-resolution monochromator has either a nested design [22] or a three-bounce design [1]. All nuclear resonant scattering data presented in this paper were collected at the Nuclear Resonance beamline ID18 of the European Synchrotron Radiation Facility [23]. 2.1. Synchrotron radiation perturbed angular correlation (SRPAC) SRPAC is basically a scattering variant of time-differential perturbed angular correlation (TDPAC) [24]. In contrast to TDPAC, the intermediate nuclear level is not excited ‘from above’ via a cascade originating from the decay of a radioactive parent, but ‘from below’, i.e. from the ground state, by spatially incoherent, single-nucleus resonant scattering of synchrotron radiation [25–27]. The time evolution of the spatially incoherent delayed re-emission of the resonant quanta is measured in 908 scattering geometry. For stationary nuclei, the hyperfine splitting of the excited state introduces a quantum beat (QB) pattern in the SRPAC time spectrum, which is due to intra-nuclear coherent scattering. If the molecular axis and hence the electric field gradient rotate during the decay of the intermediate state, an enhanced fading/overdamping of the QB oscillations will occur, revealing slow/fast rotational relaxation, respectively. For the 57Fe nucleus in ferrocene/dibutylphthalate, SRPAC was found to cover a dynamic range of rotational relaxation times of about five orders of magnitude, from z10 ps up to 1 ms [26]. SRPAC can be viewed as a method complementary to NMR [28], which has not been applied to the 57Fe nucleus owing to its low gyromagnetic ratio.

2.2. Nuclear inelastic scattering (NIS) Principles and technical realization of nuclear inelastic scattering [29,30,1] are described in detail in [31]. In molecular crystals, intermolecular interactions are mirrored directly in the low-energy part of the vibrational density of states (DOS). Slow processes, which give rise to a quasielastic broadening in the range of neV–meV, cannot be resolved using NIS. Faster processes, however, e.g. cage rattling processes in glasses and plastic crystals, as well as the much-debated Boson peak, appear in the low-energy part of the DOS, usually at energies well below the intramolecular modes. If the centre of mass of the molecular probe coincides with the Mo¨ssbauer nucleus, then the low-energy part of the spectrum monitors exclusively translational modes of the probe molecule, and we have a selective probe for fast translational processes on the lengthscale of several molecular diametres and larger. This fact was used to establish universal scaling laws in the energy range above the Boson peak in structural glasses [7]. If, however, the centre of mass does not coincide with the Mo¨ssbauer nucleus, then hindered rotations, i.e. librations, will contribute to the low-energy DOS. If contributions from translational motions can be neglected, as it is the case for both high- and low-temperature phases of plastic crystals, then we have a selective probe for librational dynamics on a very local scale. Finally, also the high-energy molecular modes may be influenced by changes in the intermolecular potential owing to onset of molecular rotation. 2.3. Nuclear forward scattering (NFS) In NFS [32,33], the coherence of the resonant signal emitted by the nuclear ensemble in forward direction creates a QB pattern arising from the hyperfine splitting of both ground and excited state [34]. The coherence of the signal is destroyed by random diffusive motion of the resonant nuclei [35]. Translational motion leads to an accelerated decrease of the signal intensity. Molecular rotations that involve a reorientation of the electric field gradient (EFG) of the molecule change the QB pattern completely. Isotropic rotation reduces the effective quadrupole splitting, which finally collapses into an overdamped function that corresponds to a broadened single line in the energy domain. Anisotropic rotation yields a more complex picture, as was already shown for the case of in-plane rotation in classical Mo¨ssbauer spectroscopy [36]. 3. Results and discussion

c b

3.1. OMFA—an organometallic plastic crystal a

Fig. 1. The OMFA molecule and the unit cell of its high-temperature phase.

Octamethyl-ethinyl-ferrocene (OMFA), like some other highly substituted ferrocenes, exhibits a sharp decrease of the Lamb-Mo¨ssbauer factor (also called recoilless or elastic fraction) far below the melting point TmZ436 K [37–39]. This phenomenon was shown to be due to a first-order solid– solid phase transition to a nearly cubic rotator phase at

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Tpcz240 K (as obtained from calibrated DSC) having a wide thermal hysteresis [17]. X-ray powder diffraction revealed that the rotation is most probably nearly free, however, direct experimental evidence has been lacking so far. The molecular structure (geometry optimization was carried out using densityfunctional calculations [40]) and the high-temperature phase are depicted in Fig. 1. Dielectric spectroscopy has been used extensively to study the dynamics of plastic crystals over a wide frequency range [41,42]. In the case of molecules with a small molecular dipole moment, however, X-ray or neutron scattering techniques are more promising. We therefore studied OMFA by SRPAC and NIS experiments in order to learn more about the phase transition and the mechanism of the rotation. For the SRPAC experiment, a nested high-resolution monochromator (HRM) yielding an overall energy resolution of 6 meV was used. For the NIS experiment, a three-bounce HRM providing an energy resolution of about 0.5 meV was used. The sample, a pressurized pellet of about 0.35 mm thickness, was mounted between thin Be sheets into a copper holder sealed with Kapton windows in a closed-cycle cryostat. Comparison of NFS spectra during the first and subsequent heating cycles showed that the pressurizing process did not induce any noticeable texture that would affect the depth of the QB minima. All

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temperatures in the following are set temperatures of the cryostat. The true sample temperatures are expected to be somewhat higher than the set temperatures owing to heat transfer from the Kapton window close to the sample, in particular for the NIS experiment. Using SRPAC, time spectra were obtained as shown in Fig. 2. The increasing degree of the damping of the QB above Tpc is clearly visible. We obtained the rotational relaxation rate l of OMFA versus temperature through a full hysteresis cycle, as shown in Fig. 3. For data fitting, the strong-collision model, which assumes arbitrarily large random angular jumps [43,27], gave a satisfactory description of the time spectra over the entire temperature range. We can therefore conclude that SRPAC has provided us with the first direct evidence of quasifree rotation in the rotator phase of OMFA. SRPAC is not directly sensitive to mutual ’gear-wheel’-type rotations of the two substituted cyclopentadienyl rings located at the same molecule, which were postulated in [39]. Instead, SRPAC provides us with a proof of reorientations of the EFG (which is oriented parallel to the ring-Fe-ring axis) and hence to reorientations of the entire OMFA molecule. SRPAC is therefore complementary, e.g. to quasielastic neutron scattering, which ’sees’ a superposition of both types of motion. As for the temperature dependence of l, it is interesting to note that a hysteresis is observable, which, however, does not simply follow the hysteresis observed by nuclear forward scattering, X-ray diffraction and differential calorimetry [17]. There are sharp jumps in each of the quantities as observed by those three methods at T Z Tpc on heating and at TZ210 K on cooling. In contrast, the rotational motion as observed by SRPAC sets in smoothly between 220 and 230 K, well below Tpc, i.e. we observe a pronounced precursor effect. Upon further heating, l increases smoothly across the transition temperature T Z Tpc and follows Arrhenius behaviour (dotted line) above Tpc, with an activation energy of 29.7 kJ molK1 and a frequency factor of 8.3!1012 sK1. These values are roughly comparable with the ones found on formyl ferrocene [44].

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t/ns Fig. 2. SRPAC time spectra of OMFA for three different temperatures.

Fig. 3. Rotational relaxation rate of OMFA from SRPAC experiments.

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On re-cooling, the Arrhenius plot of l reveals a significant deviation from simple activated behaviour below Tpc; l lies above the Arrhenius line and only drops to the low-temperature curve below the lower hysteresis point around 210 K. Deviations from Arrhenius behaviour and a Vogel–Fulcher– Tammann scaling of the relaxation rate have been described for some orientational glasses [43,28], where a slowing-down on cooling with respect to Arrhenius scaling was observed, whereas in our case, the opposite is true. We speculate that coupling of the ring-Fe-ring axis motion to other degrees of freedom, such as the ’gear-wheel’ rotation mentioned above, might affect the effective activation energy on cooling, since different ’gear-wheel’ configurations alter the molecular shape and hence also the intermolecular potential energy. However, any direct evidence for the true reason of the observed difference between rotational and structural hysteresis is lacking so far. Using NIS, five spectra were measured, four for OMFA cooled down slowly from room temperature to 20 K and subsequently re-heated to the set temperature and one for OMFA quenched by immersion into liquid nitrogen. The extraction of the one-phonon contribution and the evaluation of the vibrational density of states (VDOS) was carried out using the DOS V2.1 software [45]. Fig. 4 shows the resulting reduced VDOS gðEÞ=E2 . The experimental error bars are about the line thickness for E>0.5 meV and about five times higher for E%0.5 meV, where additional uncertainties arise from the subtraction of the elastic line from the raw spectrum. At the lowest set temperature (60 K), we only observe a slight deviation from Debye behaviour, i.e. a weakly pronounced Boson peak, which is believed to be a signature of disorder [46]. Fast quenching to 60 K does not enhance the Boson peak intensity but rather cuts off of the reduced VDOS for lowest energies, much like the VDOS of glassy ferrocene/dibutylphthalate in nanopores [6]. Whether this can be attributed to phonon confinement owing to the formation of nanostructured domains in the undercooled crystal is currently unclear. Between 60 and 230 K, the lowenergy contributions slowly increase. We attribute this to an increasing amplitude of librational molecular motion, which

enhances both the mean-square amplitude of the central Fe atom and the anharmonicity of the lattice. Strong anharmonicities are known to play an important role in rotator phases of n-alkanes [47]. The spectrum taken at 240 K set temperature, directly above the phase transition, shows a very strongly increased low-energy VDOS, which can most probably be traced back to the conversion of librational into unhindered rotational motion. Whereas at low energies mainly intermolecular vibrations are seen, higher energies mirror the intramolecular vibrational spectrum. Fig. 5 shows this spectral range for 60, 230, and 240 K, in comparison to a room-temperature Raman spectrum taken with a commercial FT-Raman spectrometer (RFS 100/S, Bruker) having an overall spectral resolution of 4 cmK1, which is equal to the resolution in the NIS experiment. Error bars for the NIS spectra are about 3–4 times the symbol size. Some changes in the molecular modes can be observed clearly in the 240 K spectrum but start already 10 K below: the 40 and 51 meV bands disappear, and the band for 60.5 meV splits. The latter phenomenon is also observed in the roomtemperature Raman spectrum, where all atomic displacements, not only the ones from the Fe atom, are seen. A tentative assignment of the phonon bands to different molecular modes [40] reveals that the energy and intensity of those modes that are modified most strongly at Tpc tend to disagree with the theoretical predictions. Since the calculations assume an isolated molecule, it appears that such disagreements can mostly be traced back to intermolecular interactions. The onset of molecular rotation in the high-temperature phase will modify these interactions, which in turn may influence intramolecular vibrations via rotation–vibration coupling. 3.2. Ferrocene in AlPO4K5 The dynamics of non-immobilized metallocenes in some zeolitic hosts was studied by optical spectroscopy, EPR, and IR absorption [48]. Ferrocene in NaX zeolite was investigated by classical Mo¨ssbauer spectroscopy, NMR, and quasielastic neutron scattering [49,50,19]. 0.14

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Fig. 4. Reduced VDOS of OMFA for different temperatures.

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Fig. 5. High-energy vibrational modes of OMFA from NIS and Raman scattering.

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Fig. 7. NFS time spectra of ferrocene in randomly oriented AlPO4K5 crystals.

jump mechanism might involve complex anisotropic re-orientations. In this case, the orientation and polarization of the synchrotron beam would have to be taken into account [54] assuming discrete reorientations of the EFG that are compatible with the symmetry of the host lattice. 4. Conclusion We have shown that SRPAC and NFS are powerful tools that yield information both about rotation rates and rotation mechanisms. In contrast, NIS mirrors the appearance of quasielastic librational modes as well as the coupling between inter- and intramolecular interactions caused by the onset of molecular rotations. Acknowledgements The authors are indebted to E. Reichel and Prof. H. Schottenberger for the synthesis of the enriched OMFA sample, to Dr J. Kornatowski for the AlPO4-5 crystals, to V. Krishnan and Prof. H. Bertagnolli for support with the Raman measurements and to Dr A.I. Chumakov for efficient support at the ESRF beamline. Support from the BMBF projects 05 SK8WOB/5 and 05 KS1WOC/2 is gratefully acknowledged. References

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Ferrocene loaded into powdered AlPO4-5, a molecular sieve that possesses parallel, unbranched channels arranged in a hexagonal pattern, was studied by Mo¨ssbauer spectroscopy at room temperature, where fast rotation of the included molecules was postulated [51]. However, only a limited degree of loading with ferrocene could be achieved, and thermal decomposition was reported above 2008 C as found by EXAFS [52]. A loaded zeolite powder, especially in the case of a broad grain size distribution, may exhibit not only dynamics of the molecules in the pores; other dynamic processes, such as grainboundary diffusion, can interfere with the intra-channel diffusion that we wish to study [18]. For this reason, we investigated the dynamics of ferrocene in large single crystals of AlPO4K5 [53] using NFS. Two subsequent studies were carried out, one with the hexagonal axis of the single crystals oriented parallel to the synchrotron beam [20] and one with the single crystals oriented randomly. If the EFG of the ferrocene molecules can isotropically reorient in all directions in space, the effective quadrupolar frequency is decreased and finally collapses because the Fe nucleus ’sees’ an effective EFG reduced by rotation. If, however, the rotation becomes anisotropic thanks to the symmetry of the host cage and/or the guest molecule, then additional frequencies appear in the NFS time spectrum. Gibb [36] calculated a classical Mo¨ssbauer spectrum for the case of an EFG rotating in a plane, yielding a spectrum that contains two quadrupolar frequencies. However, agreement with our experimental data is limited, since the ratio of the two quadrupolar frequencies is not reproduced correctly [20]. The temperature dependance of the NFS spectra of both samples is depicted in Figs. 6 and 7. It is immediately evident that although the shapes of the NFS spectra of both samples are almost identical at 150 K, their further evolution with temperature is different. Fourier analysis of the time spectra of the oriented sample gives a major QB frequency component with the admixture of a smaller one up to 225 K, whereas in the randomly oriented sample the smaller QB frequency appears to dominate above 210 K. We therefore conclude that the true

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Fig. 6. NFS time spectra of ferrocene in oriented AlPO4-5 single crystals.

[1] A.I. Chumakov, W. Sturhahn, Experimental aspects of inelastic nuclear resonance scattering, Hyperfine Interact 123–124 (1999) 781–808. [2] O. Leupold, H. Winkler, Relaxation experiments with synchrotron radiation, Hyperfine Interact 123–124 (1999) 571–593. [3] A. Meyer, H. Franz, J. Wuttke, W. Petry, N. Wiele, R. Ru¨ffer, C. Hu¨bsch, Nuclear resonant scattering of synchrotron radiation for the study of dynamics around the glass transition, Z. Phys. B 103 (1997) 479–484. [4] T. Asthalter, I. Sergueev, H. Franz, R. Ru¨ffer, W. Petry, K. Messel, P. Ha¨rter, A. Huwe, Quasielastic nuclear forward scattering as a background-free probe of slow glass dynamics in confined geometries, Eur. Phys. J. B 22 (2001) 301–306. [5] I. Sergueev, H. Franz, T. Asthalter, W. Petry, U. Van Bu¨rck, G.V. Smirnov, Structural relaxation in a viscous liquid studied by quasielastic nuclear forward scattering, Phys. Rev. B 66 (2002) 184210.

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T. Asthalter et al. / Journal of Physics and Chemistry of Solids 66 (2005) 2271–2276

[6] T. Asthalter, M. Bauer, U. van Bu¨rck, I. Sergueev, H. Franz, A.I. Chumakov, Confined phonons in glasses: a study by nuclear inelastic absorption and Raman scattering, Eur. Phys. J. E 12 (2003) S9–S12. [7] A.I. Chumakov, I. Sergueev, U. van Bu¨rck, W. Schirmacher, T. Asthalter, R. Ru¨ffer, O. Leupold, W. Petry, Collective nature of the boson peak and universal trans-boson dynamics of glasses, Phys. Rev. Lett. 92 (2004) 245508. [8] B. Sepiol, A. Meyer, G. Vogl, H. Franz, R. Ru¨ffer, Diffusion in a crystal lattice with nuclear resonant scattering of synchrotron radiation, Phys. Rev. B 57 (1998) 10433–10439. [9] H. Thiess, A. Baron, D. Ishikawa, T. Ishikawa, D. Miwa, B. Sepiol, S. Tsutsui, Self-diffusion of Sn atoms in Cu3Sn probed by quasielastic nuclear resonant scattering of synchrotron radiation, Phys. Rev. B 67 (2003) 184302. [10] J.N. Sherwood (Ed.), The Plastically Crystalline State (Orientationally Disordered Crystals), Wiley, Chichester, 1979. [11] P.A. Heiney, J.E. Fischer, A.R. McGhie, W.J. Romanow, A.M. Denenstein, J.P. McCauley Jr., A.B. Smith III, D.E. Cox, Orientational ordering transition in solid C60, Phys. Rev. Lett. 66 (1991) 2911–2914. [12] A. Mu¨ller, An X-ray investigation of normal paraffins near their melting points, Proc. R. Soc. A 138 (1932) 514–530. [13] H.E. King Jr., E.B. Sirota, H. Shao, D.M. Singer, A synchrotron X-ray scattering study of the rotator phases of the normal alkanes, J. Phys. D: Appl. Phys. 26 (1993) B133–B136. [14] E.B. Sirota, Remarks concerning the relation between rotator phases of bulk n-alkanes and those of langmuir monolayers of alkyl-chain surfactants on water, Langmuir 13 (1997) 3849–3859. [15] K. Adachi, H. Suga, S. Seki, Phase changes in crystalline and glassycrystalline cyclohexanol, Bull. Chem. Soc. Jpn 41 (1968) 1073–1087. [16] M.A. Ramos, S. Vieira, F.J. Bermejo, J. Davidowski, H.E. Fischer, H. Schober, M.A. Gonza´lez, C.K. Loong, D.L. Price, Quantitative assessment of the effects of orientational and positional disorder on glassy dynamics, Phys. Rev. Lett. 78 (1997) 82–85. [17] T. Asthalter, H. Franz, U. van Bu¨rck, K. Messel, E. Schreier, R. Dinnebier, Structure and dynamics of octamethyl-ethinyl-ferrocene: an organometallic rotator phase, J. Phys. Chem. Solids 64 (2003) 677–684. [18] J. Ka¨rger, D.M. Ruthven, Diffusion in Zeolites and Other Microporous Solids, Wiley, New York, 1992. [19] E. Kemner, I.M. De Schepper, G.J. Kearley, How van der Waals bonds orient molecules in zeolites, Chem. Commun. (2001) 2466–2467. [20] T. Asthalter, J. Villanueva Garibay, V. Olszowka, J. Kornatowski, Dynamics of ferrocene in AlPO4-5 single crystals probed by nuclear forward scattering, J. Chem. Phys. 122 (2005) 014508. [21] T.S. Toellner, Monochromatization of synchrotron radiation for nuclear resonant scattering experiments, Hyperfine Interact 125 (2000) 3–28. [22] T. Ishikawa, Y. Yoda, K. Izumi, C.K. Suzuki, X.W. Zhang, M. Ando, S. Kikuta, Construction of a precision diffractometer for nuclear bragg scattering at the photon factory, Rev. Sci. Instrum. 63 (1992) 1015–1018. [23] R. Ru¨ffer, A.I. Chumakov, Nuclear resonance beamline at ESRF, Hyperfine Interact 97–98 (1996) 589–604. [24] T. Butz, Nuclear quadrupole interactions studied by time differential perturbed angular correlations of g-rays, Z. Naturforsch. 51a (1996) 396–410. [25] A.Q.R. Baron, A.I. Chumakov, R. Ru¨ffer, H. Gru¨nsteudel, H.F. Gru¨nsteudel, O. Leupold, Single-nucleus quantum beats excited by synchrotron radiation, Europhys. Lett. 34 (1996) 331–336. [26] I. Sergueev, U. van Bu¨rck, A.I. Chumakov, T. Asthalter, G.V. Smirnov, H. Franz, R. Ru¨ffer, W. Petry, Synchrotron radiation-based perturbed angular correlation, ESRF Highlights, 2002, pp. 61–62. [27] I. Sergueev, Nuclear Resonant Scattering for the Study of Dynamics of Viscous Liquids and Glasses, PhD Thesis, TU Mu¨nchen, 2004, pp. 37–60. [28] R. Bo¨hmer, G. Diezemann, G. Hinze, E. Ro¨ssler, Dynamics of supercooled liquids and glassy solids, Prog. Nucl. Magn. Reson. Spectrosc. 39 (2001) 191–267. [29] M. Seto, Y. Yoda, S. Kikuta, X.W. Zhang, M. Ando, Observation of nuclear resonant scattering accompanied by phonon excitation using synchrotron radiation, Phys. Rev. Lett. 74 (1995) 3828–3831.

[30] W. Sturhahn, T.S. Toellner, E.E. Alp, X. Zhang, M. Ando, Y. Yoda, S. Kikuta, M. Seto, C.W. Kimball, B. Dabrowski, Phonon density of states measured by inelastic nuclear resonant scattering, Phys. Rev. Lett. 74 (1995) 3832–3835. [31] W. Sturhahn, Nuclear resonant spectroscopy, J. Phys.: Condens. Mater. 16 (2004) S497–S530. [32] J.B. Hastings, D.P. Siddons, U. van Bu¨rck, R. Hollatz, U. Bergmann, Mo¨ssbauer spectroscopy using synchrotron radiation, Phys. Rev. Lett. 66 (1991) 770–773. [33] U. van Bu¨rck, D.P. Siddons, J.B. Hastings, U. Bergmann, R. Hollatz, Nuclear forward scattering of synchrotron radiation, Phys. Rev. B 46 (1992) 6207–6211. [34] U. van Bu¨rck, Coherent pulse propagation through resonant media, Hyperfine Interact 123–124 (1999) 483–509. [35] H. Franz, W. Petry, A.Q.R. Baron, Quasielastic scattering: slow dynamics of glasses, Hyperfine Interact 123–124 (1999) 865–879. [36] T.C. Gibb, Anisotropic relaxation of the electric field gradient tensor in the 57Fe Mo¨ssbauer spectra of a thiourea-ferrocene clathrate, J. Phys. C: Solid State Phys. 9 (1976) 2627–2642. [37] R.H. Herber, I. Nowik, Strong metal atom motion anomaly in ironorganometallics, Hyperfine Interact 126 (2000) 127–130. [38] R.H. Herber, The quadrupole hyperfine interaction of iron organometallics: an interpretation of its temperature dependence, Inorg. Chim. Acta 291 (1999) 74–81. [39] H. Schottenberger, K. Wurst, R.H. Herber, X-ray structural and Mo¨ssbauer spectroscopic investigation of terminal ethynylmetallocenium salts, J. Organomet. Chem. 625 (2001) 200–207. [40] T. Asthalter, G. Rauhut, unpublished results. [41] M. Jime´nez-Ruiz, M.A. Gonzalez, F.J. Bermejo, M.A. Miller, N.O. Birge, I. Cendoya, A. Alegrı´a, Relaxational dynamics in the glassy, supercooled liquid, and orientationally disordered crystal phases of a polymorphic molecular material, Phys. Rev. B 59 (1999) 9155–9166. [42] R. Brand, P. Lunkenheimer, A. Loidl, Relaxation dynamics in plastic crystals, J. Chem. Phys. 116 (2002) 10386–10401. [43] S. Dattagupta, M. Blume, Stochastic theory of line shape. I. Nonsecular effects in the strong-collision model, Phys. Rev. B 10 (1974) 4540–4550. [44] M.F. Daniel, A.J. Leadbetter, R.M. Richardson, Intramolecular and intermolecular dynamics in the two crystal phases of ferrocene carboxaldehyde, J. Chem. Soc.: Faraday Trans. 2 77 (1981) 1851–1863. [45] V.G. Kohn, A.I. Chumakov, DOS: evaluation of phonon density of states from nuclear resonant inelastic absorption, Hyperfine Interact 125 (2000) 205–221. [46] W. Schirmacher, G. Diezemann, C. Ganter, Harmonic vibrational excitations in disordered solids and the ‘boson peak’, Phys. Rev. Lett. 81 (1998) 136–139. [47] E.B. Sirota, D.M. Singer, H.E. King Jr., Structural effects of high pressure gas on the rotator phases of normal alkanes, J. Chem. Phys. 100 (1993) 1542–1551. [48] G.A. Ozin, J. Godber, Intrazeolite metallocenes: chemistry, spectroscopy, and dynamics, J. Phys. Chem. 93 (1989) 878–893. [49] A.R. Overweg, H. Koller, J.W. De Haan, L.J.M. Van de Ven, A.M. Van der Kraan, R.A. Van Santen, Ferrocene and dicarbonylcyclopentadienylcobalt in faujasite-type zeolites: a study of molecular motion, J. Phys. Chem. B 103 (1999) 4298–4308. [50] E. Kemner, I.M. de Schepper, A.J.M. Schmets, H. Grimm, A.R. Overweg, R.A. van Santen, Molecular motion of ferrocene in a faujasite-type zeolite: a quasielastic neutron scattering study, J. Phys. Chem. B 104 (2000) 1560–1562. [51] A. Lund, D.G. Nicholson, R.V. Parish, J.P. Wright, 57Fe Mo¨ssbauer spectroscopic studies of the ferrocene molecular reorientation in AlPO4-5 and AlPO4-8 frameworks, Acta Chem. Scand. 48 (1994) 738–741. [52] A. Lund, D.G. Nicholson, G. Lamble, B. Beagley, X-Ray absorption spectroscopic study of the AlPO4-5: ferrocene inclusion compound and its thermally decomposed products, J. Mater. Chem. 4 (1994) 1723–1730. [53] J. Kornatowski, G. Finger, Growth of large crystals of aluminophosphate molecular sieve AlPO4-5, Bull. Soc. Chim. Belg. 99 (1990) 857–859. [54] W. Sturhahn, CONUSS and PHOENIX: evaluation of nuclear resonant scattering data, Hyperfine Interact 125 (2000) 149–172.