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Dynamic properties of α,ω-dibromoalkane guest molecules constrained within urea inclusion compounds: a neutron scattering study
Physica B 180 & 181 (1992) North-Holland
PHYSICA El
687-690
Dynamic properties of a,o-dibromoalkane guest molecules constrained within urea inclusion compounds: a neutron scattering study S.P. Smarta, F. Guillaumeb, K.D.M. Harris”, C. Sourisseaub and A.J. Dianoux” “Department of Chemistry, University of St. Andrews, St. Andrews, Fife KY16 9ST, Scotland, UK bLaboratoire de Spectroscopic Moltculaire et Cristalline, Vniversite de Bordeaux I, CNRS VRA 124, 33405 Talence Cedex, France ‘Institut Laue-Langevin, 156X, 38042 Grenoble Cedex, France
Incoherent quasielastic neutron scattering from semi-oriented polycrystalline samples of o,o-dibromoalkane/urea-d, inclusion compounds has been studied as a function of sample orientation, length of guest (a,w-dibromoalkane) molecule, and temperature. In the high-temperature phase of these materials, the guest molecules, constrained within the urea tunnel structure, undergo translational (and oscillatory) motions along the tunnel axis and uniaxial rotational diffusion about the tunnel axis. Quantitative dynamic information relating to these motional models is presented.
1. Introduction
We are currently interested in the structural, dynamic and chemical properties of urea inclusion compounds containing a diverse range of organic “guest” molecules. In these inclusion compounds, the urea molecules form an extensively hydrogen-bonded “host” structure containing parallel one-dimensional tunnels that are densely packed with guest molecules [l]. Previous neutron scattering investigations [2,3] of n-alkanelurea-d, inclusion compounds have thrown considerable light upon the rotational and translational motions of the guest (n-alkane) molecules over a wide range of temperatures (encompassing both the low temperature and high temperature phases of these materials [4,5]). We have shown recently [6,7], by X-ray diffraction and other techniques, that urea inclusion compounds containing functionalized n-alkane guests can exhibit a new range of structural properties, particularly concerning the mode of intertunnel packing of the guest molecules. In this paper, we report incoherent quasielastic neutron scattering results for semi-oriented polycrystalline samples of a,odibromoalkane/urea-d, inclusion compounds; deuteration of the host molecules minimizes the incoherent scattering from the host, allowing our results to be interpreted in terms of the dynamic properties of the guest molecules alone. In these investigations, the neutron scattering was measured as a function of: (a) sample orientation (momentum transfer vector either parallel or perpendicular to the urea tunnel axis); (b) length of the guest molecule (Br(CH,),Br; n = 8-10); (c) temperature (with particular regard to differences in motional properties above and below the phase transition temperatures (indicated by differential scanning calorimetry) for these materials). Phenomeno0921-4526/92/$05.00
0
1992 - Elsevier
Science
Publishers
logical models have been developed to describe the reorientational and translational dynamic properties of the a,w-dibromoalkanes constrained within the urea tunnel structure, and preliminary results from fitting these models to the experimental data are presented in this paper.
2. Experimental Inclusion compounds of a,w-dibromoalkanes (Br(CH,),Br; n = 8-10) in urea-d, were prepared by slow cooling of warm solutions of the a,w-dibromoalkane and urea-d, in CH,OD. Powder X-ray diffraction analysis confirmed that the crystals had the conventional structure of urea inclusion compounds. Single crystals (typical dimensions 0.5 x 0.5 x 5 mm’) of these inclusion compounds were placed in aluminium containers such that the urea tunnel axes (crystallographic c axis; prism axis of the hexagonal prismatic morphology [l]) of all crystals were parallel to each other. The crystallographic Q and b axes (perpendicular to the tunnel axis) were randomly oriented with respect to rotation about the tunnel axis. Neutron scattering experiments were carried out on these semi-oriented polycrystalline samples using the time-of-tlight spectrometer IN5 (hint, = 6 A; spectral resolution (FWHM) = 67 ueV) at the Institut LaueLangevin in Grenoble. Two experimental geometries were considered: (a) with the momentum transfer vector (Q) parallel to the urea tunnel axis (denoted Q,,; fig. l(a)); and (b) with Q perpendicular to the urea tunnel axis (denoted Q, ; fig. l(b)). With these two sample orientations, motions of the guest molecules along the tunnel (studied using Q,, geometry) can be investigated separately from reorientation of the guest
B.V. All rights
reserved
S. P. Smart et al.
688
I Dynamics of a,o-dibromoalkanelurea
9
inclusion compounds
’
z 0.8
0.6
1
0.4
I
jumps
27x16
0
0.4
0.8
1.2
1.6
2
Q/A-’ Fig. 1. Schematic representation of experimental geometries, with the incident and scattered wave vectors lying in the plane of the page (the scattering plane): (a) Q vector parallel to urea tunnel axis-with the tunnel axes lying in the scattering plane and forming an angle 135” with the incoming neutron beam, this condition is satisfied when the scattering angle 20 is 90”; (b) Q vector perpendicular to urea tunnel axis- with the tunnel axes perpendicular to the scattering plane, this condition is satisfied for any value of 20. molecules
about
the
axis (studied
tunnel
using Q,
geometry). 3.
Summary
Fig. 3. Graphs showing the experimental EISF, recorded in the Q, geometry, as a function of Q for Br(CH,),Br/urea-d, at different temperatures. The line represents the theoretical EISF versus Q curve for a 2rr/6 uniaxial rotational jump model (assuming radius of gyration of 1.39 A).
material in the LT phase (fig. 2(a)); these side-peaks broaden considerably at higher temperatures. Quasielastic broadening is negligible in the LT phase, whereas large quasielastic broadening is evident in the HT phase.
of main results
the results for the We consider in detail Br(CH,),Br/urea-d, inclusion compound, although qualitatively similar behaviour is observed for all the compounds studied. The phase transition temperature for Br(CH,),Br/urea-d, is in the range 145-155 K. We subsequently use LT and HT to refer to temperatures below and above the phase transition temperature, respectively. 3.1. Experiments
3.2. Experiments
in Q,
In the LT spectra there are no side-peaks and quasielastic broadening is negligible (fig. 2(b)). In the HT phase, on the other hand, there is appreciable quasielastic broadening, and the experimental elastic incoherent structure factor (EISF) exhibits a continuous decrease with increasing temperature (fig. 3). There is no evidence for side-peaks at any temperature.
in Q,, geometry
“Side-peaks”, displaying a soft mode behaviour, are observed (at ~0.7 meV at 120 K) in the spectra of this
4. Analysis and discussion of results
The experiments
in the Q,, geometry are consistent
W
J -1
-1.5
1
geometry
-0.5
Energy
0
0.5
1
1.5
+
QL
120K
+ ,t + *+
++
j&+h+&yM I
-1
5
- 1
I meV
Fig. 2. Experimental spectra for Br(CH,),Br/urea-d, at 120K in the Q,, and Q, scattering angles in the range 60” < 20 < 110”). The scale expansion is X 100.
-0.5
Energy geometries
4
I
0
0.5
1
1I
5
/ meV (data
summed
over detectors
at
S. P. Smart et al.
I Dynamics
of a,w-dibromoalkanelurea
with the occurrence of two types of motion of the guest molecules parallel to the tunnel: an oscillatory motion (giving rise to the side-peaks) and a translatory motion (giving rise to the large quasielastic broadening in the HT phase). Assuming that these two motions are not correlated, a scattering law can be derived (based upon that developed in a previous publication [3]) in terms of the following parameters. Oscillatory motion: ( r*) , mean-square amplitude of oscillation of the hydrogens; 4, oscillation frequency; K,, damping factor. Translatory motion: L, translational length; D,, translational diffusion coefficient. Fitting this scattering law to the spectra recorded at scattering angle 20 = 90” (since it is only at this angle that the Q vector is genuinely parallel to the tunnel axis) gives the results summarized in table 1. Although there are large uncertainties in the values relating to the oscillatory motion (due, in part, to the very low intensity of the side-peaks), the results do confirm the assumed time-decoupling of the oscillatory and translatory motions - thus, the order of magnitude of the quasielastic broadening is a few tens of PeV, whereas 0, is in the range 0.8-0.9 meV. For the three compounds studied, the translational length (L) at 240 K ranges from 1.6 to 2.1 A; these values are close to that (L = 1.63 A) reported previously [3] for n-nonadecanelurea-d, (at 200 K). The variation of D, with temperature in the HT phase for Br(CH,),Br/urea-d, (see table 1) corresponds to an activation energy for translations
inclusion
689
compounds
of 6? 1 kJ mol-’ (derived on the assumption of Arrhenius behaviour for 0,). The reorientational motion of the guest molecules (evidenced from the quasielastic broadening in the HT phase for Q, geometry) cannot involve discrete jumps (such as 21r/6 uniaxial rotational jumps), since models based upon such a motion fail to predict correctly the variation of the experimental EISF with temperature (see fig. 3). Furthermore, no low-frequency side-peaks are observed in the Q, geometry, suggesting that the motion is diffusive (rather than oscillatory) in nature. A theoretical model based upon uniaxial rotational diffusion in a onefold cosine potential [8] has been used to analyse the results for the Q, spectra in the HT phase. As outlined in detail elsewhere [3], the scattering law corresponding to this motional model is expressed in terms of the following parameters: V, (=2k,Ty), an effective, potential barrier to the rotation; D,, rotational diffusion constant; 4, mean angle of fluctuations about the tunnel axis (related to y). The results obtained from fitting this scattering law to the experimental data are given in table 2. The effective potential barriers (V,) for guest molecule reorientation in Br(CH,),Br/urea-d, and n-nonadecane/urea-d, are comparable (~8-16 kJ mol-’ and Further detailed analy-5-11 kJ mol-‘, respectively). sis is required to decide on the significance of the small differences in the fitted parameters between these two systems.
Table 1 Best fit parameters for Br(CH,),Br/urea-d, samples at 240 K and for Br(CH,),Br/urea-d, at several temperatures, for spectra recorded in the Q,, geometry. The experimental data were collected on the detector at 20 = 90”. n
8 9 9 9 9 9 10
TW
Q’(r’)
4 bW
& (mev)
D, ( 10mbcm’ s-l)
L(W)
VGW)
240 120 160 200 240 280 240
2.1 0.8 1.2 1.2 1.4 1.5 1.4
0.9 0.8 0.9 0.8 0.8 0.8 0.8
0.7 0.5 0.9 1.0 1.0 0.8 1.2
5.8 _ 0.7 1.3 2.4 7.0 2.7
2.1
1.0 0.6 0.7 0.7 0.8 0.8 0.8
0.9 1.3 1.6 2.3 1.8
Table 2 Best fit parameters for Br(CH,),Br/urea-d, samples at 240K and for Br(CH,),Br/urea-d, at several temperatures, for spectra recorded in the Q, geometry. In each case, the parameters were derived by fitting the theoretical model individually to each spectrum recorded in the range 30”~ 28 < 120”. n
8 9 9 9 9 10
T(K)
Y
D, (pss’)
i (“)
V, (kJ mol-‘)
240 160 200 240 280 240
1.9 6.0 3.2 2.4 1.9 2.4
0.23 0.16 0.19 0.20 0.21 0.19
47 21 33 41 54 41
7.6 15.9 10.6 9.5 8.8 9.6
690
S.P. Smart et al. I Dynamics of a,o-dibromoalkaneiurea
References [l]
K.D.M. Harris and J.M. Thomas, J. Chem. Sot. Trans. 86 (1990) 2985. [2] F. Guillaume, C. Sourisseau and A.J. Dianoux, Phys. 93 (1990) 3536. [3] F. Guillaume, C. Sourisseau and A.J. Dianoux, Phys. 88 (1991) 1721. [4] N.G. Parsonage and R.C. Pemberton, Trans. Sot. 63 (1967) 311, and ref. [l] cited therein.
Faraday J. Chem. J. Chem. Faraday
inclusion compounds
(51 Y. Chatani, H. Anraku and Y. Taki, Mol. Cryst. Liq. Cryst. 48 (1978) 219. [6] K.D.M. Harris and M.D. Hollingsworth, Proc. Roy. Sot. A 431 (1990) 245. [7] K.D.M. Harris, S.P. Smart and M.D. Hollingsworth, J. Chem. Sot. Faraday Trans. 87 (1991) 3423. [S] A.J. Dianoux and F. Volino, Mol. Phys. 34 (1973) 1263.
PHYSICA El
687-690
Dynamic properties of a,o-dibromoalkane guest molecules constrained within urea inclusion compounds: a neutron scattering study S.P. Smarta, F. Guillaumeb, K.D.M. Harris”, C. Sourisseaub and A.J. Dianoux” “Department of Chemistry, University of St. Andrews, St. Andrews, Fife KY16 9ST, Scotland, UK bLaboratoire de Spectroscopic Moltculaire et Cristalline, Vniversite de Bordeaux I, CNRS VRA 124, 33405 Talence Cedex, France ‘Institut Laue-Langevin, 156X, 38042 Grenoble Cedex, France
Incoherent quasielastic neutron scattering from semi-oriented polycrystalline samples of o,o-dibromoalkane/urea-d, inclusion compounds has been studied as a function of sample orientation, length of guest (a,w-dibromoalkane) molecule, and temperature. In the high-temperature phase of these materials, the guest molecules, constrained within the urea tunnel structure, undergo translational (and oscillatory) motions along the tunnel axis and uniaxial rotational diffusion about the tunnel axis. Quantitative dynamic information relating to these motional models is presented.
1. Introduction
We are currently interested in the structural, dynamic and chemical properties of urea inclusion compounds containing a diverse range of organic “guest” molecules. In these inclusion compounds, the urea molecules form an extensively hydrogen-bonded “host” structure containing parallel one-dimensional tunnels that are densely packed with guest molecules [l]. Previous neutron scattering investigations [2,3] of n-alkanelurea-d, inclusion compounds have thrown considerable light upon the rotational and translational motions of the guest (n-alkane) molecules over a wide range of temperatures (encompassing both the low temperature and high temperature phases of these materials [4,5]). We have shown recently [6,7], by X-ray diffraction and other techniques, that urea inclusion compounds containing functionalized n-alkane guests can exhibit a new range of structural properties, particularly concerning the mode of intertunnel packing of the guest molecules. In this paper, we report incoherent quasielastic neutron scattering results for semi-oriented polycrystalline samples of a,odibromoalkane/urea-d, inclusion compounds; deuteration of the host molecules minimizes the incoherent scattering from the host, allowing our results to be interpreted in terms of the dynamic properties of the guest molecules alone. In these investigations, the neutron scattering was measured as a function of: (a) sample orientation (momentum transfer vector either parallel or perpendicular to the urea tunnel axis); (b) length of the guest molecule (Br(CH,),Br; n = 8-10); (c) temperature (with particular regard to differences in motional properties above and below the phase transition temperatures (indicated by differential scanning calorimetry) for these materials). Phenomeno0921-4526/92/$05.00
0
1992 - Elsevier
Science
Publishers
logical models have been developed to describe the reorientational and translational dynamic properties of the a,w-dibromoalkanes constrained within the urea tunnel structure, and preliminary results from fitting these models to the experimental data are presented in this paper.
2. Experimental Inclusion compounds of a,w-dibromoalkanes (Br(CH,),Br; n = 8-10) in urea-d, were prepared by slow cooling of warm solutions of the a,w-dibromoalkane and urea-d, in CH,OD. Powder X-ray diffraction analysis confirmed that the crystals had the conventional structure of urea inclusion compounds. Single crystals (typical dimensions 0.5 x 0.5 x 5 mm’) of these inclusion compounds were placed in aluminium containers such that the urea tunnel axes (crystallographic c axis; prism axis of the hexagonal prismatic morphology [l]) of all crystals were parallel to each other. The crystallographic Q and b axes (perpendicular to the tunnel axis) were randomly oriented with respect to rotation about the tunnel axis. Neutron scattering experiments were carried out on these semi-oriented polycrystalline samples using the time-of-tlight spectrometer IN5 (hint, = 6 A; spectral resolution (FWHM) = 67 ueV) at the Institut LaueLangevin in Grenoble. Two experimental geometries were considered: (a) with the momentum transfer vector (Q) parallel to the urea tunnel axis (denoted Q,,; fig. l(a)); and (b) with Q perpendicular to the urea tunnel axis (denoted Q, ; fig. l(b)). With these two sample orientations, motions of the guest molecules along the tunnel (studied using Q,, geometry) can be investigated separately from reorientation of the guest
B.V. All rights
reserved
S. P. Smart et al.
688
I Dynamics of a,o-dibromoalkanelurea
9
inclusion compounds
’
z 0.8
0.6
1
0.4
I
jumps
27x16
0
0.4
0.8
1.2
1.6
2
Q/A-’ Fig. 1. Schematic representation of experimental geometries, with the incident and scattered wave vectors lying in the plane of the page (the scattering plane): (a) Q vector parallel to urea tunnel axis-with the tunnel axes lying in the scattering plane and forming an angle 135” with the incoming neutron beam, this condition is satisfied when the scattering angle 20 is 90”; (b) Q vector perpendicular to urea tunnel axis- with the tunnel axes perpendicular to the scattering plane, this condition is satisfied for any value of 20. molecules
about
the
axis (studied
tunnel
using Q,
geometry). 3.
Summary
Fig. 3. Graphs showing the experimental EISF, recorded in the Q, geometry, as a function of Q for Br(CH,),Br/urea-d, at different temperatures. The line represents the theoretical EISF versus Q curve for a 2rr/6 uniaxial rotational jump model (assuming radius of gyration of 1.39 A).
material in the LT phase (fig. 2(a)); these side-peaks broaden considerably at higher temperatures. Quasielastic broadening is negligible in the LT phase, whereas large quasielastic broadening is evident in the HT phase.
of main results
the results for the We consider in detail Br(CH,),Br/urea-d, inclusion compound, although qualitatively similar behaviour is observed for all the compounds studied. The phase transition temperature for Br(CH,),Br/urea-d, is in the range 145-155 K. We subsequently use LT and HT to refer to temperatures below and above the phase transition temperature, respectively. 3.1. Experiments
3.2. Experiments
in Q,
In the LT spectra there are no side-peaks and quasielastic broadening is negligible (fig. 2(b)). In the HT phase, on the other hand, there is appreciable quasielastic broadening, and the experimental elastic incoherent structure factor (EISF) exhibits a continuous decrease with increasing temperature (fig. 3). There is no evidence for side-peaks at any temperature.
in Q,, geometry
“Side-peaks”, displaying a soft mode behaviour, are observed (at ~0.7 meV at 120 K) in the spectra of this
4. Analysis and discussion of results
The experiments
in the Q,, geometry are consistent
W
J -1
-1.5
1
geometry
-0.5
Energy
0
0.5
1
1.5
+
QL
120K
+ ,t + *+
++
j&+h+&yM I
-1
5
- 1
I meV
Fig. 2. Experimental spectra for Br(CH,),Br/urea-d, at 120K in the Q,, and Q, scattering angles in the range 60” < 20 < 110”). The scale expansion is X 100.
-0.5
Energy geometries
4
I
0
0.5
1
1I
5
/ meV (data
summed
over detectors
at
S. P. Smart et al.
I Dynamics
of a,w-dibromoalkanelurea
with the occurrence of two types of motion of the guest molecules parallel to the tunnel: an oscillatory motion (giving rise to the side-peaks) and a translatory motion (giving rise to the large quasielastic broadening in the HT phase). Assuming that these two motions are not correlated, a scattering law can be derived (based upon that developed in a previous publication [3]) in terms of the following parameters. Oscillatory motion: ( r*) , mean-square amplitude of oscillation of the hydrogens; 4, oscillation frequency; K,, damping factor. Translatory motion: L, translational length; D,, translational diffusion coefficient. Fitting this scattering law to the spectra recorded at scattering angle 20 = 90” (since it is only at this angle that the Q vector is genuinely parallel to the tunnel axis) gives the results summarized in table 1. Although there are large uncertainties in the values relating to the oscillatory motion (due, in part, to the very low intensity of the side-peaks), the results do confirm the assumed time-decoupling of the oscillatory and translatory motions - thus, the order of magnitude of the quasielastic broadening is a few tens of PeV, whereas 0, is in the range 0.8-0.9 meV. For the three compounds studied, the translational length (L) at 240 K ranges from 1.6 to 2.1 A; these values are close to that (L = 1.63 A) reported previously [3] for n-nonadecanelurea-d, (at 200 K). The variation of D, with temperature in the HT phase for Br(CH,),Br/urea-d, (see table 1) corresponds to an activation energy for translations
inclusion
689
compounds
of 6? 1 kJ mol-’ (derived on the assumption of Arrhenius behaviour for 0,). The reorientational motion of the guest molecules (evidenced from the quasielastic broadening in the HT phase for Q, geometry) cannot involve discrete jumps (such as 21r/6 uniaxial rotational jumps), since models based upon such a motion fail to predict correctly the variation of the experimental EISF with temperature (see fig. 3). Furthermore, no low-frequency side-peaks are observed in the Q, geometry, suggesting that the motion is diffusive (rather than oscillatory) in nature. A theoretical model based upon uniaxial rotational diffusion in a onefold cosine potential [8] has been used to analyse the results for the Q, spectra in the HT phase. As outlined in detail elsewhere [3], the scattering law corresponding to this motional model is expressed in terms of the following parameters: V, (=2k,Ty), an effective, potential barrier to the rotation; D,, rotational diffusion constant; 4, mean angle of fluctuations about the tunnel axis (related to y). The results obtained from fitting this scattering law to the experimental data are given in table 2. The effective potential barriers (V,) for guest molecule reorientation in Br(CH,),Br/urea-d, and n-nonadecane/urea-d, are comparable (~8-16 kJ mol-’ and Further detailed analy-5-11 kJ mol-‘, respectively). sis is required to decide on the significance of the small differences in the fitted parameters between these two systems.
Table 1 Best fit parameters for Br(CH,),Br/urea-d, samples at 240 K and for Br(CH,),Br/urea-d, at several temperatures, for spectra recorded in the Q,, geometry. The experimental data were collected on the detector at 20 = 90”. n
8 9 9 9 9 9 10
TW
Q’(r’)
4 bW
& (mev)
D, ( 10mbcm’ s-l)
L(W)
VGW)
240 120 160 200 240 280 240
2.1 0.8 1.2 1.2 1.4 1.5 1.4
0.9 0.8 0.9 0.8 0.8 0.8 0.8
0.7 0.5 0.9 1.0 1.0 0.8 1.2
5.8 _ 0.7 1.3 2.4 7.0 2.7
2.1
1.0 0.6 0.7 0.7 0.8 0.8 0.8
0.9 1.3 1.6 2.3 1.8
Table 2 Best fit parameters for Br(CH,),Br/urea-d, samples at 240K and for Br(CH,),Br/urea-d, at several temperatures, for spectra recorded in the Q, geometry. In each case, the parameters were derived by fitting the theoretical model individually to each spectrum recorded in the range 30”~ 28 < 120”. n
8 9 9 9 9 10
T(K)
Y
D, (pss’)
i (“)
V, (kJ mol-‘)
240 160 200 240 280 240
1.9 6.0 3.2 2.4 1.9 2.4
0.23 0.16 0.19 0.20 0.21 0.19
47 21 33 41 54 41
7.6 15.9 10.6 9.5 8.8 9.6
690
S.P. Smart et al. I Dynamics of a,o-dibromoalkaneiurea
References [l]
K.D.M. Harris and J.M. Thomas, J. Chem. Sot. Trans. 86 (1990) 2985. [2] F. Guillaume, C. Sourisseau and A.J. Dianoux, Phys. 93 (1990) 3536. [3] F. Guillaume, C. Sourisseau and A.J. Dianoux, Phys. 88 (1991) 1721. [4] N.G. Parsonage and R.C. Pemberton, Trans. Sot. 63 (1967) 311, and ref. [l] cited therein.
Faraday J. Chem. J. Chem. Faraday
inclusion compounds
(51 Y. Chatani, H. Anraku and Y. Taki, Mol. Cryst. Liq. Cryst. 48 (1978) 219. [6] K.D.M. Harris and M.D. Hollingsworth, Proc. Roy. Sot. A 431 (1990) 245. [7] K.D.M. Harris, S.P. Smart and M.D. Hollingsworth, J. Chem. Sot. Faraday Trans. 87 (1991) 3423. [S] A.J. Dianoux and F. Volino, Mol. Phys. 34 (1973) 1263.