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Noncovalent intermolecular forces in polycrystalline and amorphous saccharides in the far infrared
Chemical Physics 288 (2003) 261–268 www.elsevier.com/locate/chemphys
Noncovalent intermolecular forces in polycrystalline and amorphous saccharides in the far infrared Markus Walther, Bernd M. Fischer, P. Uhd Jepsen * Department of Molecular and Optical Physics, Physikalisches Institut, Albert-Ludwigs Universit€at Freiburg, Stefan-Meier-Str. 19, Freiburg D-79104, Germany Received 21 October 2002
Abstract The far-infrared dielectric properties of sugars in the condensed state are dominated by vibrational modes of their intermolecular hydrogen bonded network. We have used terahertz time-domain spectroscopy to measure the full dielectric function of the monosaccharides glucose and fructose and the disaccharide sucrose in the frequency range 0.5– 4.0 THz (16–130 cm1 ). In polycrystalline sugars we observe a series of distinct absorption lines originating from the lowest intermolecular vibrational modes, whereas in amorphous sugars a broad, featureless absorption spectrum is observed. Our measurement of an anomalous temperature dependence of the absorption line positions in both protonated and deuterated sucrose shows that the effective potential of the weakest intermolecular vibrational modes in sucrose is determined by a balance between the hydrogen bond strength and van der Waals forces. Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction The hydrogen bond and other low-energy interactions such as dipole–dipole and dispersion interactions between molecules are generally accepted to be of utmost importance in biochemistry and biology. In contrast to covalent bonds, with bond energies in the range 300–400 kJ/mol, hydrogen bonds typically have energies a few times higher than the thermal energy (kT ¼ 2:4 kJ/mol) at room temperature, and dispersion interactions have energies in the range of a few kJ/mol. The
*
Corresponding author. Fax: +49-761-203-5955. E-mail address: [email protected] (P. Uhd Jepsen).
interplay between hydrogen bonding and dispersion forces in the form of a large number of noncovalent bonds therefore allows biomolecules to be stable, but still highly flexible, in their natural environments. Molecules such as DNA and RNA are prime examples of the interplay between different types of noncovalent forces. Hydrogen bonding is responsible for the highly specific binding between complementary base pairs, whereas a combination of dispersion forces and hydrophobic effects is known to stabilize the helix shape of the DNA double strand [1,2]. Vibrational spectroscopy in the mid-infrared spectral region is a frequently used technique for investigating hydrogen bonded systems. The
0301-0104/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0301-0104(03)00031-4
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observation of spectral shifts of the X–H stretching vibration is commonly used to deduce the strength of a hydrogen bond to another atom group X–H Y [3]. Here X and Y are electronegative atoms, for example oxygen. Whereas mid-IR spectroscopy only gives indirect information on the hydrogen bond itself, long-wavelength investigations (IR and Raman spectroscopy or neutron scattering) allow direct observation of the vibrational characteristics of the hydrogen bond linkage [4]. Since the bonding forces are weak and the moving masses are considerably large, vibrational modes of hydrogen bonds have resonance frequencies in the still relatively unexplored terahertz region. To some extent sugars represent prototype systems for the investigation of H-bonded networks in crystalline, amorphous and dissolved forms. In the solid phase saccharides are linked by H-bonds, which generate a highly ordered crystal structure or are randomly organized in the amorphous phase. Neutron scattering data of different crystals of monosaccharides like glucose and fructose, and disaccharides like sucrose show intermolecular hydrogen bonding between neighboring hydroxyl groups [5–7]. In the case of several disacharrides, including sucrose, additional intramolecular H-bonds stabilize the structure. However, there are only few investigations on crystalline and amorphous saccharides in the farIR below 200 cm1 [8–10].
In this work we present temperature dependent far-IR spectra of polycrystalline glucose, fructose and sucrose. In addition, direct comparison with the absorption spectrum of amorphous sucrose allows us to conclude on the origin of the observed spectral features. In order to acquire further understanding of the observed low-frequency modes we perform a detailed study of the temperature dependence of the line positions of the lowest modes in protonated and deuterated sucrose. We make the highly unusual observation that below a mode-dependent critical temperature the resonance frequencies are subject to an anomalous blueshift with increasing temperature.
2. Sample preparation and experimental technique The sugars D (+)-glucose and D ())-fructose were purchased from Merck and sucrose was purchased from Sigma–Aldrich in the form of coarse-grained powders. The chemical structures of the relevant configurations of the sugars are shown in Fig. 1. In the polycrystalline form glucose was investigated in its a-D -(+) configuration, and fructose in its naturally occuring b-D -()) fructopyranose configuration. The conformations were confirmed by measurements of the optical rotation immediately after solvation of the sugars in water. Due to mutarotation amorphous glucose exists as a mixture of a-D -glucose and b-D -glucose.
Fig. 1. Chemical structure of glucose, fructose, and sucrose in the conformations relevant to the present investigation.
M. Walther et al. / Chemical Physics 288 (2003) 261–268
The polycrystalline samples were prepared by milling the powder and pressing it to pellets of an approximate thickness of 1 mm by applying a pressure of 7500 bar with a hydraulic press. By this procedure we obtained polycrystalline samples with a density of approximately 90% of the density of a single crystal. In order to record strong absorption features which were saturated in the pure samples we also prepared mixed pellets with polyethylene (PE) powder at various dilution ratios. PE is nearly transparent and non-dispersive with a frequency-independent index of refraction of 1:53 0:02 below 170 cm1 and is therefore a suitable filling material for far-infrared spectroscopic applications. By taking into account the densities of the sample materials and the index of refraction of PE we extracted the absorption coefficient and index of refraction of the pure sugars. Amorphous glucose samples were prepared by heating the polycrystalline a-D -glucose powder just above the melting temperature (153–156 °C) and subsequently pouring the melt into an airtight sample cell equipped with PE windows. Care was taken to apply high temperatures only for as short as possible in order not to decompose the sugar. After melting the sample had a weak yellowish color. Care was also taken to avoid air inclusions in the melt and to minimize recrystallization. The quality of the amorphous sample and the degree of recrystallization was inspected with a polarized light microscope. The deuterated sucrose samples were produced by dissolving polycrystalline sucrose in excess D2 O at elevated temperature, followed by drying under vacuum at 50 °C until constant weight. After six cycles of dissolution and evaporation all OH groups were then assumed to have been exchanged with OD groups. The dielectric properties of the samples are measured with terahertz time-domain spectroscopy (THz-TDS) based on photoconductive switches for generation and detection of the farinfrared light [11,12]. The setup allows us to record the absorption coefficient and index of refraction of samples in the frequency range 0.1–4.0 THz (6–133 cm1 ) with a spectral resolution of 15 GHz (0.5 cm1 ). THz-TDS is a useful technique for the recording of the complex dielectric function of a
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wide range of substances in the far infrared [11– 16]. Time-resolved implementations of THz-TDS have recently been applied to study ultrafast processes in the far infrared [17–21]. We note that this technique has the unique potential to perform transient vibrational studies with subpicosecond time-resolution subsequent to photoexcitation, and could therefore become an alternative technique to novel ultrafast X-ray diffraction techniques [22]. In order to record the temperature dependence of the spectral features the samples are mounted in a cryostat equipped with Teflon windows. The temperature is measured near the sample by a calibrated Si-diode with an accuracy of 1 K. The cryostat can be moved so that the THz beam passes through either the sample or through an empty aperture of the identical size as the clear aperture of the sample. The field of the THz pulse transmitted through the sample is modified by the dispersion nðxÞ and absorption aðxÞ of the sample, where x ¼ 2pm is the cyclic frequency. In the frequency domain the ratio of the electrical field strengths transmitted through the empty aperture (Er ) and through the sample aperture (Es ) is given by Es =Er ¼ T ðnÞ expðad=2 þ inxd=cÞ where d is the thickness of the sample, c is the speed of light in vacuum, and T ðnÞ is a factor which accounts for reflection losses at the sample surfaces.
3. Results and discussion The measured absorption coefficient aðmÞ and the index of refraction nðmÞ of glucose, fructose, and sucrose are shown in Figs. 2–4, as recorded in the frequency range 0.5–4.0 THz at temperatures of 10 and 300 K. For a clearer representation the 300-K data have been offset vertically by the amounts indicated in the figures. The weak oscillations at low frequencies are artifacts caused by multiple reflections of the probe beam through the sample. At high frequencies the signal-to-noise ratio degrades due to low signal strength in this spectral region. Within the bandwidth of the spectrometer we observe a range of resonances. We did not observe spectral features of interest in the region below 0.5 THz.
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Fig. 2. (a) Absorption coefficient and (b) index of refraction of crystalline a-D -glucose in the frequency range 0.2–4.0 THz, at 10 and 300 K. The 300-K curves are offset vertically for clarity.
Accompanying each absorption peak is a characteristic change of the index of refraction around its average value of approximately 1.8 in all samples. The average index of refraction decreases slightly (3–5%) when the temperature is increased from 10 to 300 K. We observe that at 300 K all absorption lines are broader than at 10 K. Additionally, most lines shift to lower frequencies when the temperature is increased, with a notable exception found in the two lowest resonances in sucrose, where a nonmonotonous temperature dependence of the line positions is observed. This peculiar temperature dependence will be discussed below. In the presence of anharmonicity of the vibrational potentials it is expected that increased temperature will lead to a redshift of vibrational transition frequencies with a simultaneous broadening of the line profile. At 10 K the thermal en-
Fig. 3. (a) Absorption coefficient and (b) index of refraction of crystalline b-D -fructopyranose in the frequency range 0.2–4.0 THz, at 10 and 300 K. The 300-K curves are offset vertically for clarity.
ergy kT is equivalent to a frequency of 0.2 THz, resulting in thermal population of only the lowest energetic level in a vibrational potential with level spacings in the low THz range. At 300 K the thermal energy kT amounts to an equivalent frequency of 6.1 THz, with the result that several vibrational levels are significantly populated. This leads to a broadening of the low-frequency wing of the absorption profile. Increased thermal motion of the atoms will lead to further homogenous broadening of the line profile. A detailed assignment of the individual vibrational modes found in the low-frequency region is a challenging task. With the aid of polarized absorption spectra recorded along the crystal axes of single-crystal samples an assignment of the various modes would be facilitated [23].
M. Walther et al. / Chemical Physics 288 (2003) 261–268
Fig. 4. (a) Absorption coefficient and (b) index of refraction of crystalline sucrose in the frequency range 0.2–4.0 THz, at 10 and 300 K. The 300-K curves are offset vertically for clarity.
A simple experiment can, however, be used to classify the origin of the low-frequency resonances observed in the studied sugars. In Fig. 5 we show the absorption coefficient and index of refraction of amorphous glucose, recorded at 10 and 300 K. In contrast to the spectrum of polycrystalline glucose shown in Fig. 2 we observe a featureless absorption profile with monotonously increasing absorption at high frequencies. The index of refraction is significantly higher than in the case of polycrystalline glucose. The frequency-integrated absorption in the range 0.5–2.5 THz is similar for amorphous and polycrystalline glucose. We observed very similar behavior of the absorption coefficient in samples of amorphous fructose and sucrose. The differences between the dielectric properties of polycrystalline and amorphous glucose
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Fig. 5. (a) Absorption coefficient and (b) index of refraction of amorphous glucose in the frequency range 0.2–2.5 THz, at 10 and 300 K (solid and dashed lines, respectively). Oscillations below 0.7 THz are due to interference effects.
lead us to conclude that the sharp spectral features observed in the polycrystalline sugars arise from concerted intermolecular vibrational modes of long-range order, controlled by noncovalent bonds between the sugar molecules. Due to the lack of long-range symmetry all sharp spectral features disappear in amorphous glucose, but absorption is still considerable due to the now random orientation of intermolecular bonds. If intramolecular vibrational modes were present in the frequency range considered here such modes should be visible in the spectra of the amorphous sugars. Similar differences between absorption spectra of random and ordered arrangement of water molecules in vitrous ice and ice II has been observed by Bertie et al. [24,25].
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The glass transition temperature of glucose is 304 K, slightly above the temperature range considered in this work. We therefore rule out that the absorption and index of refraction of amorphous glucose is a consequence of dielectric relaxation described by the Debye model, as observed in recent spectroscopic investigations of liquid water [26,27]. Instead, spectral broadening of the individual vibrational modes due to the amorphous environment leads to an overall decrease of the index of refraction with increasing frequency. A similar effect was observed in the dielectric function of different retinal isomers [12]. In order to shed further light on the dynamics of the observed vibrational modes we measured the temperature dependence of the absorption coefficient of protonated and deuterated sucrose in the polycrystalline phase. In Fig. 6 the absorption coefficient of the two sugars is shown in the frequency range 0.5–2.5 THz, at temperatures between 10 and 300 K. In the following we will concentrate on the temperature dependence of the two low-frequency peaks (1.38 and 1.69 THz in protonated sucrose at 10 K, 1.28 and 1.63 THz in deuterated sucrose at 10 K). The relative frequency shift upon deuteration is large (Dm=m ¼ 0:07 for peak 1 and )0.035 for peak 2). The relatively small increase in effective mass (Dm=m ¼ 0:023) would in the harmonic approximation lead to a relative frequency shift of )0.012. The large observed redshifts indicate that deuteration leads to significant softening of the vibrational potentials. This softening, on the other hand, is not reflected in the melting points of deuterated and protonated sucrose. We determined the melting point of deuterated sucrose to be 192– 194 °C, significantly higher than the 185–188 °C melting point of protonated sucrose, both measured by oil bath immersion. The increased melting point of deuterated sucrose indicates a higher dissociation energy than in protonated sucrose. In Fig. 7 we show the relative frequency shift mðT Þ=m10K 1 of peaks 1 and 2 in protonated and deuterated sucrose. The line positions were determined by a multipeak-fit to the absorption data in Fig. 6 using Lorentzian profiles with amplitudes and widths as adjustable parameters. The standard deviations of the fitted center frequencies are smaller than the size of the symbols in Fig. 7.
(a)
(b)
Fig. 6. Absorption spectra of: (a) protonated and (b) deuterated polycrystalline sucrose in the frequency range 0.5–2.5 THz, at temperatures from 10 to 300 K. The different traces are vertically offset for a clearer representation.
Fig. 7. Temperature dependence of the center frequency of the two lowest resonances in protonated and deuterated sucrose (labeled H- and D-sucrose) in the range 10–375 K.
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We make the interesting and important observation that below a certain critical temperature the resonance frequencies are subject to a strong, anomalous blueshift with increasing temperature. Above this mode-dependent critical temperature, a normal redshift of the linepositions is observed. This temperature dependence is difficult to explain in terms of an anharmonic vibrational potential of the hydrogen bonds. The resulting softening of the intermolecular hydrogen bonds due to change of molecular distance with temperature [28] would lead to a monotonous redshift of the line position with increasing temperature. It is also interesting to note that the anomalous blueshift is larger in the deuterated sample than in the protonated sample, whereas the normal redshift at above the critical temperature is of approximately equal amplitude in both samples. The temperature dependence of the vibrational frequencies is consistent with a scenario where weak noncovalent forces such as van der Waals forces weaken the hydrogen bond modes, analogous to the well-known softening and associated redshift of the covalent X–H Y stretch mode in the presence of a low-energy hydrogen bond [29]. Depending on the nature of the donor atom Y the frequency of the X–H stretch may be subject to an improper blueshift [30]. Owing to their low interaction energies van der Waals forces will have the largest influence on the higher-energy hydrogen bonds at low temperatures. This influence will be reduced at higher temperatures. We propose that this could be the origin of the observed initial blueshift when the temperature is increased. At higher temperatures a normal redshift due to anharmonicity of the hydrogen-bond vibrational potential begins to dominate the temperature dependence of the vibrational frequency. The crossover temperature Tx (240 K for peak 1, 120 K for peak 2, as indicated in Fig. 7) from blue- to redshift is determined by the balance of the strength of the bond itself and that of the competing van der Waals forces. Therefore it can be expected that Tx is highest for the weakest hydrogen bond, as observed in Fig. 7. This may also explain why the effect is not observed for higher-frequency modes. Note that the
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crossover temperature Tx does not indicate a phase transition of the crystal structure.
4. Conclusions The dielectric function of glucose, fructose, and sucrose in the frequency range 0.5–4 THz show multiple narrow resonances. A comparison between polycrystalline and amorphous samples of the sugars indicates that the resonances are due to concerted intermolecular vibrational modes of the hydrogen-bonded network of the crystalline structure. Below a certain crossover temperature the low-frequency resonances in sucrose shows an anomalous blueshift with increasing temperature. This blueshift can be qualitatively explained by softening at low temperatures of the intermolecular hydrogen bonds by weak van der Waals forces. A quantitative understanding of weak, noncovalent interactions in the condensed phase is a formidable task in terms of refinement of the theoretical foundations, and at present also in terms of computational costs. We believe that the spectroscopic data presented here will be useful for critical benchmark evaluation of future theoretical models of low-energy interactions in molecular crystals.
Acknowledgements We acknowledge Prof. Hanspeter Helm for careful reading of the manuscript, fruitful discussions, and ongoing support. This work was supported by the DFG under SFB 276, TP C14 and BMBF Biophotonics Programme.
References poner, Chem. Rev. 99 (1999) 3247. [1] P. Hobza, J. S [2] M. Elstner, P. Hobza, T. Frauenheim, S. Suhai, E. Kaxiras, J. Chem. Phys. 114 (2001) 5149. [3] P. Schuster, G. Zundel, C. Sandorfy, The Hydrogen Bond, North-Holland, Amsterdam, 1976. [4] K.D. M€ oller, W.G. Rothshild, Far-Infrared Spectroscopy, Wiley–Interscience, New York, 1971. [5] G.M. Brown, H.A. Levy, Acta Cryst. B 35 (1979) 656.
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[6] J.A. Kanters, G. Roelofsen, B.P. Albas, I. Meinders, Acta Cryst. B 33 (1977) 665. [7] G.M. Brown, H.A. Levy, Acta Cryst. B 29 (1973) 790. [8] S. S€ oderholm, Y.H. Roos, N. Meinander, M. Hotokka, J. Raman Spectrosc. 30 (1999) 1009. [9] S. S€ oderholm, Y.H. Roos, N. Meinander, K. Steinby, J. Raman Spectrosc. 31 (2000) 995. [10] L. Yang, S. Weng, J.R. Ferraro, J. Wu, Vib. Spectrosc. 25 (2001) 57. [11] M. Schall, P. Uhd Jepsen, Phys. Rev. B 64 (2001) 094301. [12] M. Walther, B. Fischer, H. Helm, P. Uhd Jepsen, Chem. Phys. Lett. 332 (2000) 389. [13] D.R. Grischkowsky, Ch. Fattinger, M. van Exter, S.R. Keiding, J. Opt. Soc. Am. B 7 (1990) 2006. [14] H. Harde, N. Katzenellenbogen, D. Grischkowsky, Phys. Rev. Lett. 74 (1995) 1307. [15] A.G. Markelz, A. Roitberg, E.J. Heilweil, Chem. Phys. Lett. 320 (2000) 42. [16] M. Walther, P. Plochocka, B. Fischer, H. Helm, P. Uhd Jepsen, Biopolymers (Biospectroscopy) 67 (2002) 310. [17] R. McElroy, K. Wynne, Phys. Rev. Lett. 79 (1997) 3078. [18] M.C. Beard, G.M. Turner, C.A. Schmuttenmaer, Phys. Rev. B. 62 (2000) 15764.
[19] M. Schall, P. Uhd Jepsen, Opt. Lett. 25 (2000) 13. [20] P. Uhd Jepsen, W. Schairer, I.H. Libon, U. Lemmer, N. Hecker, M. Birkholz, K. Lips, M. Schall, Appl. Phys. Lett. 79 (2001) 1291. [21] M. Schall, P. Uhd Jepsen, Appl. Phys. Lett. 80 (2002) 4771. [22] C. Rischel, A. Rousse, I. Uschmann, P.A. Albouy, J.-P. Geindre, P. Audebert, J.-C. Gauthier, E. F€ orster, J.-L. Martin, A. Antonetti, Nature 390 (1997) 490. [23] V. Volovsek, D. Kirin, L. Bistricic, G. Baranovic, J. Raman Spectrosc. 33 (2002) 761. [24] J.E. Bertie, E. Whalley, J. Chem. Phys. 46 (1967) 1271. [25] J.E. Bertie, H.J. Labbe, E. Whalley, J. Chem. Phys. 49 (1968) 775. [26] L. Thrane, R.H. Jacobsen, P. Uhd Jepsen, S.R. Keiding, Chem. Phys. Lett. 240 (1995) 330. strand, A. Wallquist, K.V. [27] C. Rønne, L. Thrane, P.-O. A Mikkelsen, S.R. Keiding, J. Chem. Phys. 107 (1997) 5319. [28] H.O. Desseyn, K. Clou, R. Keuleers, R. Miao, V.E. Van Doren, N. Blaton, Spectrochim. Acta A 57 (2001) 231. [29] E.T.G. Lutz, J.H. van der Maas, J. Mol. Struct. 324 (1994) 123. [30] P. Hobza, Z. Havlas, Chem. Rev. 100 (2000) 4253.
Noncovalent intermolecular forces in polycrystalline and amorphous saccharides in the far infrared Markus Walther, Bernd M. Fischer, P. Uhd Jepsen * Department of Molecular and Optical Physics, Physikalisches Institut, Albert-Ludwigs Universit€at Freiburg, Stefan-Meier-Str. 19, Freiburg D-79104, Germany Received 21 October 2002
Abstract The far-infrared dielectric properties of sugars in the condensed state are dominated by vibrational modes of their intermolecular hydrogen bonded network. We have used terahertz time-domain spectroscopy to measure the full dielectric function of the monosaccharides glucose and fructose and the disaccharide sucrose in the frequency range 0.5– 4.0 THz (16–130 cm1 ). In polycrystalline sugars we observe a series of distinct absorption lines originating from the lowest intermolecular vibrational modes, whereas in amorphous sugars a broad, featureless absorption spectrum is observed. Our measurement of an anomalous temperature dependence of the absorption line positions in both protonated and deuterated sucrose shows that the effective potential of the weakest intermolecular vibrational modes in sucrose is determined by a balance between the hydrogen bond strength and van der Waals forces. Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction The hydrogen bond and other low-energy interactions such as dipole–dipole and dispersion interactions between molecules are generally accepted to be of utmost importance in biochemistry and biology. In contrast to covalent bonds, with bond energies in the range 300–400 kJ/mol, hydrogen bonds typically have energies a few times higher than the thermal energy (kT ¼ 2:4 kJ/mol) at room temperature, and dispersion interactions have energies in the range of a few kJ/mol. The
*
Corresponding author. Fax: +49-761-203-5955. E-mail address: [email protected] (P. Uhd Jepsen).
interplay between hydrogen bonding and dispersion forces in the form of a large number of noncovalent bonds therefore allows biomolecules to be stable, but still highly flexible, in their natural environments. Molecules such as DNA and RNA are prime examples of the interplay between different types of noncovalent forces. Hydrogen bonding is responsible for the highly specific binding between complementary base pairs, whereas a combination of dispersion forces and hydrophobic effects is known to stabilize the helix shape of the DNA double strand [1,2]. Vibrational spectroscopy in the mid-infrared spectral region is a frequently used technique for investigating hydrogen bonded systems. The
0301-0104/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0301-0104(03)00031-4
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M. Walther et al. / Chemical Physics 288 (2003) 261–268
observation of spectral shifts of the X–H stretching vibration is commonly used to deduce the strength of a hydrogen bond to another atom group X–H Y [3]. Here X and Y are electronegative atoms, for example oxygen. Whereas mid-IR spectroscopy only gives indirect information on the hydrogen bond itself, long-wavelength investigations (IR and Raman spectroscopy or neutron scattering) allow direct observation of the vibrational characteristics of the hydrogen bond linkage [4]. Since the bonding forces are weak and the moving masses are considerably large, vibrational modes of hydrogen bonds have resonance frequencies in the still relatively unexplored terahertz region. To some extent sugars represent prototype systems for the investigation of H-bonded networks in crystalline, amorphous and dissolved forms. In the solid phase saccharides are linked by H-bonds, which generate a highly ordered crystal structure or are randomly organized in the amorphous phase. Neutron scattering data of different crystals of monosaccharides like glucose and fructose, and disaccharides like sucrose show intermolecular hydrogen bonding between neighboring hydroxyl groups [5–7]. In the case of several disacharrides, including sucrose, additional intramolecular H-bonds stabilize the structure. However, there are only few investigations on crystalline and amorphous saccharides in the farIR below 200 cm1 [8–10].
In this work we present temperature dependent far-IR spectra of polycrystalline glucose, fructose and sucrose. In addition, direct comparison with the absorption spectrum of amorphous sucrose allows us to conclude on the origin of the observed spectral features. In order to acquire further understanding of the observed low-frequency modes we perform a detailed study of the temperature dependence of the line positions of the lowest modes in protonated and deuterated sucrose. We make the highly unusual observation that below a mode-dependent critical temperature the resonance frequencies are subject to an anomalous blueshift with increasing temperature.
2. Sample preparation and experimental technique The sugars D (+)-glucose and D ())-fructose were purchased from Merck and sucrose was purchased from Sigma–Aldrich in the form of coarse-grained powders. The chemical structures of the relevant configurations of the sugars are shown in Fig. 1. In the polycrystalline form glucose was investigated in its a-D -(+) configuration, and fructose in its naturally occuring b-D -()) fructopyranose configuration. The conformations were confirmed by measurements of the optical rotation immediately after solvation of the sugars in water. Due to mutarotation amorphous glucose exists as a mixture of a-D -glucose and b-D -glucose.
Fig. 1. Chemical structure of glucose, fructose, and sucrose in the conformations relevant to the present investigation.
M. Walther et al. / Chemical Physics 288 (2003) 261–268
The polycrystalline samples were prepared by milling the powder and pressing it to pellets of an approximate thickness of 1 mm by applying a pressure of 7500 bar with a hydraulic press. By this procedure we obtained polycrystalline samples with a density of approximately 90% of the density of a single crystal. In order to record strong absorption features which were saturated in the pure samples we also prepared mixed pellets with polyethylene (PE) powder at various dilution ratios. PE is nearly transparent and non-dispersive with a frequency-independent index of refraction of 1:53 0:02 below 170 cm1 and is therefore a suitable filling material for far-infrared spectroscopic applications. By taking into account the densities of the sample materials and the index of refraction of PE we extracted the absorption coefficient and index of refraction of the pure sugars. Amorphous glucose samples were prepared by heating the polycrystalline a-D -glucose powder just above the melting temperature (153–156 °C) and subsequently pouring the melt into an airtight sample cell equipped with PE windows. Care was taken to apply high temperatures only for as short as possible in order not to decompose the sugar. After melting the sample had a weak yellowish color. Care was also taken to avoid air inclusions in the melt and to minimize recrystallization. The quality of the amorphous sample and the degree of recrystallization was inspected with a polarized light microscope. The deuterated sucrose samples were produced by dissolving polycrystalline sucrose in excess D2 O at elevated temperature, followed by drying under vacuum at 50 °C until constant weight. After six cycles of dissolution and evaporation all OH groups were then assumed to have been exchanged with OD groups. The dielectric properties of the samples are measured with terahertz time-domain spectroscopy (THz-TDS) based on photoconductive switches for generation and detection of the farinfrared light [11,12]. The setup allows us to record the absorption coefficient and index of refraction of samples in the frequency range 0.1–4.0 THz (6–133 cm1 ) with a spectral resolution of 15 GHz (0.5 cm1 ). THz-TDS is a useful technique for the recording of the complex dielectric function of a
263
wide range of substances in the far infrared [11– 16]. Time-resolved implementations of THz-TDS have recently been applied to study ultrafast processes in the far infrared [17–21]. We note that this technique has the unique potential to perform transient vibrational studies with subpicosecond time-resolution subsequent to photoexcitation, and could therefore become an alternative technique to novel ultrafast X-ray diffraction techniques [22]. In order to record the temperature dependence of the spectral features the samples are mounted in a cryostat equipped with Teflon windows. The temperature is measured near the sample by a calibrated Si-diode with an accuracy of 1 K. The cryostat can be moved so that the THz beam passes through either the sample or through an empty aperture of the identical size as the clear aperture of the sample. The field of the THz pulse transmitted through the sample is modified by the dispersion nðxÞ and absorption aðxÞ of the sample, where x ¼ 2pm is the cyclic frequency. In the frequency domain the ratio of the electrical field strengths transmitted through the empty aperture (Er ) and through the sample aperture (Es ) is given by Es =Er ¼ T ðnÞ expðad=2 þ inxd=cÞ where d is the thickness of the sample, c is the speed of light in vacuum, and T ðnÞ is a factor which accounts for reflection losses at the sample surfaces.
3. Results and discussion The measured absorption coefficient aðmÞ and the index of refraction nðmÞ of glucose, fructose, and sucrose are shown in Figs. 2–4, as recorded in the frequency range 0.5–4.0 THz at temperatures of 10 and 300 K. For a clearer representation the 300-K data have been offset vertically by the amounts indicated in the figures. The weak oscillations at low frequencies are artifacts caused by multiple reflections of the probe beam through the sample. At high frequencies the signal-to-noise ratio degrades due to low signal strength in this spectral region. Within the bandwidth of the spectrometer we observe a range of resonances. We did not observe spectral features of interest in the region below 0.5 THz.
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Fig. 2. (a) Absorption coefficient and (b) index of refraction of crystalline a-D -glucose in the frequency range 0.2–4.0 THz, at 10 and 300 K. The 300-K curves are offset vertically for clarity.
Accompanying each absorption peak is a characteristic change of the index of refraction around its average value of approximately 1.8 in all samples. The average index of refraction decreases slightly (3–5%) when the temperature is increased from 10 to 300 K. We observe that at 300 K all absorption lines are broader than at 10 K. Additionally, most lines shift to lower frequencies when the temperature is increased, with a notable exception found in the two lowest resonances in sucrose, where a nonmonotonous temperature dependence of the line positions is observed. This peculiar temperature dependence will be discussed below. In the presence of anharmonicity of the vibrational potentials it is expected that increased temperature will lead to a redshift of vibrational transition frequencies with a simultaneous broadening of the line profile. At 10 K the thermal en-
Fig. 3. (a) Absorption coefficient and (b) index of refraction of crystalline b-D -fructopyranose in the frequency range 0.2–4.0 THz, at 10 and 300 K. The 300-K curves are offset vertically for clarity.
ergy kT is equivalent to a frequency of 0.2 THz, resulting in thermal population of only the lowest energetic level in a vibrational potential with level spacings in the low THz range. At 300 K the thermal energy kT amounts to an equivalent frequency of 6.1 THz, with the result that several vibrational levels are significantly populated. This leads to a broadening of the low-frequency wing of the absorption profile. Increased thermal motion of the atoms will lead to further homogenous broadening of the line profile. A detailed assignment of the individual vibrational modes found in the low-frequency region is a challenging task. With the aid of polarized absorption spectra recorded along the crystal axes of single-crystal samples an assignment of the various modes would be facilitated [23].
M. Walther et al. / Chemical Physics 288 (2003) 261–268
Fig. 4. (a) Absorption coefficient and (b) index of refraction of crystalline sucrose in the frequency range 0.2–4.0 THz, at 10 and 300 K. The 300-K curves are offset vertically for clarity.
A simple experiment can, however, be used to classify the origin of the low-frequency resonances observed in the studied sugars. In Fig. 5 we show the absorption coefficient and index of refraction of amorphous glucose, recorded at 10 and 300 K. In contrast to the spectrum of polycrystalline glucose shown in Fig. 2 we observe a featureless absorption profile with monotonously increasing absorption at high frequencies. The index of refraction is significantly higher than in the case of polycrystalline glucose. The frequency-integrated absorption in the range 0.5–2.5 THz is similar for amorphous and polycrystalline glucose. We observed very similar behavior of the absorption coefficient in samples of amorphous fructose and sucrose. The differences between the dielectric properties of polycrystalline and amorphous glucose
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Fig. 5. (a) Absorption coefficient and (b) index of refraction of amorphous glucose in the frequency range 0.2–2.5 THz, at 10 and 300 K (solid and dashed lines, respectively). Oscillations below 0.7 THz are due to interference effects.
lead us to conclude that the sharp spectral features observed in the polycrystalline sugars arise from concerted intermolecular vibrational modes of long-range order, controlled by noncovalent bonds between the sugar molecules. Due to the lack of long-range symmetry all sharp spectral features disappear in amorphous glucose, but absorption is still considerable due to the now random orientation of intermolecular bonds. If intramolecular vibrational modes were present in the frequency range considered here such modes should be visible in the spectra of the amorphous sugars. Similar differences between absorption spectra of random and ordered arrangement of water molecules in vitrous ice and ice II has been observed by Bertie et al. [24,25].
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The glass transition temperature of glucose is 304 K, slightly above the temperature range considered in this work. We therefore rule out that the absorption and index of refraction of amorphous glucose is a consequence of dielectric relaxation described by the Debye model, as observed in recent spectroscopic investigations of liquid water [26,27]. Instead, spectral broadening of the individual vibrational modes due to the amorphous environment leads to an overall decrease of the index of refraction with increasing frequency. A similar effect was observed in the dielectric function of different retinal isomers [12]. In order to shed further light on the dynamics of the observed vibrational modes we measured the temperature dependence of the absorption coefficient of protonated and deuterated sucrose in the polycrystalline phase. In Fig. 6 the absorption coefficient of the two sugars is shown in the frequency range 0.5–2.5 THz, at temperatures between 10 and 300 K. In the following we will concentrate on the temperature dependence of the two low-frequency peaks (1.38 and 1.69 THz in protonated sucrose at 10 K, 1.28 and 1.63 THz in deuterated sucrose at 10 K). The relative frequency shift upon deuteration is large (Dm=m ¼ 0:07 for peak 1 and )0.035 for peak 2). The relatively small increase in effective mass (Dm=m ¼ 0:023) would in the harmonic approximation lead to a relative frequency shift of )0.012. The large observed redshifts indicate that deuteration leads to significant softening of the vibrational potentials. This softening, on the other hand, is not reflected in the melting points of deuterated and protonated sucrose. We determined the melting point of deuterated sucrose to be 192– 194 °C, significantly higher than the 185–188 °C melting point of protonated sucrose, both measured by oil bath immersion. The increased melting point of deuterated sucrose indicates a higher dissociation energy than in protonated sucrose. In Fig. 7 we show the relative frequency shift mðT Þ=m10K 1 of peaks 1 and 2 in protonated and deuterated sucrose. The line positions were determined by a multipeak-fit to the absorption data in Fig. 6 using Lorentzian profiles with amplitudes and widths as adjustable parameters. The standard deviations of the fitted center frequencies are smaller than the size of the symbols in Fig. 7.
(a)
(b)
Fig. 6. Absorption spectra of: (a) protonated and (b) deuterated polycrystalline sucrose in the frequency range 0.5–2.5 THz, at temperatures from 10 to 300 K. The different traces are vertically offset for a clearer representation.
Fig. 7. Temperature dependence of the center frequency of the two lowest resonances in protonated and deuterated sucrose (labeled H- and D-sucrose) in the range 10–375 K.
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We make the interesting and important observation that below a certain critical temperature the resonance frequencies are subject to a strong, anomalous blueshift with increasing temperature. Above this mode-dependent critical temperature, a normal redshift of the linepositions is observed. This temperature dependence is difficult to explain in terms of an anharmonic vibrational potential of the hydrogen bonds. The resulting softening of the intermolecular hydrogen bonds due to change of molecular distance with temperature [28] would lead to a monotonous redshift of the line position with increasing temperature. It is also interesting to note that the anomalous blueshift is larger in the deuterated sample than in the protonated sample, whereas the normal redshift at above the critical temperature is of approximately equal amplitude in both samples. The temperature dependence of the vibrational frequencies is consistent with a scenario where weak noncovalent forces such as van der Waals forces weaken the hydrogen bond modes, analogous to the well-known softening and associated redshift of the covalent X–H Y stretch mode in the presence of a low-energy hydrogen bond [29]. Depending on the nature of the donor atom Y the frequency of the X–H stretch may be subject to an improper blueshift [30]. Owing to their low interaction energies van der Waals forces will have the largest influence on the higher-energy hydrogen bonds at low temperatures. This influence will be reduced at higher temperatures. We propose that this could be the origin of the observed initial blueshift when the temperature is increased. At higher temperatures a normal redshift due to anharmonicity of the hydrogen-bond vibrational potential begins to dominate the temperature dependence of the vibrational frequency. The crossover temperature Tx (240 K for peak 1, 120 K for peak 2, as indicated in Fig. 7) from blue- to redshift is determined by the balance of the strength of the bond itself and that of the competing van der Waals forces. Therefore it can be expected that Tx is highest for the weakest hydrogen bond, as observed in Fig. 7. This may also explain why the effect is not observed for higher-frequency modes. Note that the
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crossover temperature Tx does not indicate a phase transition of the crystal structure.
4. Conclusions The dielectric function of glucose, fructose, and sucrose in the frequency range 0.5–4 THz show multiple narrow resonances. A comparison between polycrystalline and amorphous samples of the sugars indicates that the resonances are due to concerted intermolecular vibrational modes of the hydrogen-bonded network of the crystalline structure. Below a certain crossover temperature the low-frequency resonances in sucrose shows an anomalous blueshift with increasing temperature. This blueshift can be qualitatively explained by softening at low temperatures of the intermolecular hydrogen bonds by weak van der Waals forces. A quantitative understanding of weak, noncovalent interactions in the condensed phase is a formidable task in terms of refinement of the theoretical foundations, and at present also in terms of computational costs. We believe that the spectroscopic data presented here will be useful for critical benchmark evaluation of future theoretical models of low-energy interactions in molecular crystals.
Acknowledgements We acknowledge Prof. Hanspeter Helm for careful reading of the manuscript, fruitful discussions, and ongoing support. This work was supported by the DFG under SFB 276, TP C14 and BMBF Biophotonics Programme.
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