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The temperature dependence of 35Cl NQR spectrum and study of spin—lattice relaxation times in chloral hydrate
Journal of Molecular Structure, 192 (1989) 379-382 Elsevier Science Publishers B.V., Amsterdam - Printed
379
in The Netherlands
THE TEMPERATURE DEPENDENCE OF 36C1NQR SPECTRUM AND STUDY OF SPIN-LATTICE RELAXATION TIMES IN CHLORAL HYDRATE
J. KASPRZAK,
J. PIETRZAK
and A. PIETRZAK
Solid State Spectrosocopy Laboratory, Institute of Physics, A. Mickiewicz University, Poznari (Poland) (Received 22 January
1988)
ABSTRACT The temperature investigation of nuclear quadrupole spin-lattice relaxation times in chloral hydrate above 290 K revealed the presence of an additional mechanism of relaxation. Its activation energy is 14.5 kcal mol-’ which is almost 6 kcal mol-’ greater than the energy corresponding to the hindered rotation of the Ccl, group. We propose this additional mechanism to be the breaking of hydrogen bonds and translational diffusion of water molecules.
INTRODUCTION
Chloral hydrate, CCI,CHOH,, crystallizes in the monoclinic structure (space group PB,/c) and its elementary cell contains four molecules [l-3]. In the “UC” plane OH***0 type hydrogen bonds between two oxygen atoms of different molecules were found to occur. These bonds are different in length for two crystallographically non-equivalent hydroxyl groups and their lengths are 0.1908 and 0.1933 nm. NQR investigation [ 1,4-61 has shown that the 35C1 spectrum consists of three lines of the same intensity. Significant splitting of the spectrum (d u = 1.3 MHz) is explained by the effect of intramolecular interactions. As follows from the fact that the temperature coefficients of NQR frequency as well as spinlattice relaxation times (T,) are similar for all three lines, the intermolecular interactions are weak. An exponential change of In T1 (T) function above 250 K is interpreted as arising from the hindered rotation of the Ccl, group. The activation energy of this process has been found to be 8.8 kcal mol- ’ [ 11. EXPERIMENTAL
For our investigations we used polycrystalline chloral hydrate made by Merck. The sample to be studied was put inside a glass ampoule which was then sealed.
0022-2860/89/$03.50
0 1989 Elsevier Science Publishers
B.V.
380
The measurements of NQR frequency as well as spin-lattice relaxation times were carried out in the temperature range from room temperature to the temperature at which the NQR signal could not be distinguished from noise. For measurements we used a pulse NQR spectrometer constructed at the Institute of Physics, A. Mickiewicz University, Poznan. Details of construction and experimental methods have been described elsewhere [ 71. The NQR lines frequencies were determined with an accuracy of ? 2 kHz and the relaxation times 7’i with an accuracy better than 10%. RESULTS AND DISCUSSION
At 295 K the NQR frequencies were measured to be vl = 37.516, v2 = 38.703 and v,= 38.790 MHz and the relaxation time T1 was 5 ms. For the line v1 detailed measurements of temperature changes of NQR frequencies and relaxation times were carried out and repeated many times for temperatures ranging from 295 to 315 K. In the analysis we use the averaged values for individual temperatures. Figure 1 presents the temperature dependence of v1 for temperatures higher than 290 K. This dependence was analyzed on the grounds of Bayer’s theory [ 81 and taking into account Brown’s correction [ 91 which led to the relation vQ(T)=A,+A,T+A2T2
(1)
The experimental data were found to fit this function for A, = 38.315 MHz. A ,=-1.4~10-~MHzK~‘andA,=-4.5x10~~MHzK~~.Thesevaluesare only slightly different from those obtained for the temperature range 77-300 K [ 61, which means that the torsional motions of these molecules should be treated as the vibrations of anharmonic oscillators. T IKI I--
c
3x
T [Kl Fig. 1. Temperature dependence of
V,
3.2
103/T [K-?
line above 290 K in chloral hydrate.
Fig. 2. Temperature dependence of spin-lattice
relaxation time above 290 K in chloral hydrate.
381
The temperature changes of T, are shown in Fig. 2. The dependence 7’i = f (l/T) decreases linearly and thus the relation
Tcl =Aexp(
=EJRT)
In
(2)
is satisfied. The value obtained, E,= 14.5 kcal mol-‘, is considerably greater than 8.8 kcal mol-l obtained from the measurements carried out in the temperatures up to 300 K [ 11.This suggests that besides the hindered rotation of a Ccl, group activated above 250 K, there is another mechanism of relaxation that activates above 295 K. Analysis of the known crystallographic structure of chloral hydrate leads us to the conclusion that this other mechanism is related to an order-disorder transition in which the hydrogen bonds are broken. The energies of hydrogen bonds of the type OH..*0 extrapolated on the grounds of Newton’s results [lo] to the lengths H..*O = 0.1908 nm and 0.1933 nm are equal to 7.2 kcal mol-’ and 5.8 kcal mol-I, respectively. It seems justified to assume that above 295 K the quadrupole relaxation is a result of the joint influence of hindered rotation of Ccl, and breaking of the hydrogen bonds. The latter process may be a source of the change in the second moment of the NMR proton line from 12.9 Gs2 to 7.9 Gs’ observed above 305 K [ 111. Quadrupole relaxation may be also additionally influenced by the diffusion of water molecules weakly bonded in the polycrystalline material. This may be confirmed by a decrease in T, at room temperature from 5 ms to 3 ms after the sample has been annealed in the open ampoule above 315 K (after a few hours the value of T, comes back to 5 ms).
CONCLUSIONS
The results of NQR investigations in chloral hydrate prove that besides the hindered rotation of the Ccl:, group observed earlier above 250 K, another mechanism of quadrupole relaxation exists. The activation energy E a= 14.5 kcal mol-’ may be a sum of the contributions due to rotation, breaking of the hydrogen bonds and probably also water diffusion. Our results suggest the possibility of distinguishing three dynamically different temperature ranges in chloral hydrate: (a) T-c250 K where libration vibrations dominate, (b ) T 250-290K where Ccl, group rotation is activated, (c) T> 290K where the molecules have more freedom after the OH.-0 bonds are broken and translational diffusion of water molecules becomes possible.
382
REFERENCES
H. Chihara and N. Nakamura, Bull. Chem. Sot. Jpn., 45 (1972) 3530. S. Kondo and I. Nib, X-Rays, 6 (1950) 53. K. Ogawa, Bull. Chem. Sot. Jpn., 36 (1963) 610. D. Biedenkapp and A. Weiss, Z. Naturforsch. Teil A, 22 (1976) 1124. H.C. Allen, J. Am. Chem. Sot., 74 (1952) 6074. J. Pietrzak, Physics of Dielectrics and Radiospectroscopy, Vol. 5 PTPN, 1969, p. 143. J. Pietrzak, J. Kasprzak. P. Kamasa, and G. Kienitz. Port. Conf. Radio- and Microwave Spectroscopy, Poznan, 1971. 8 H. Bayer, Z. Phys., 130 (1951) 227. 9 J.C. Brown. J. Chem. Phys., 32 (1960) 116. 10 M.D. Newton, Acta Crystallogr., Sect. B 39 (1983) 104. 11 K. Holderna, M. Ostafin, Z. Pajak and J. Pietrzak. Rep. No. 802/PL, Institute of Nuclear Physics, Cracow, 1972, p. 13.
379
in The Netherlands
THE TEMPERATURE DEPENDENCE OF 36C1NQR SPECTRUM AND STUDY OF SPIN-LATTICE RELAXATION TIMES IN CHLORAL HYDRATE
J. KASPRZAK,
J. PIETRZAK
and A. PIETRZAK
Solid State Spectrosocopy Laboratory, Institute of Physics, A. Mickiewicz University, Poznari (Poland) (Received 22 January
1988)
ABSTRACT The temperature investigation of nuclear quadrupole spin-lattice relaxation times in chloral hydrate above 290 K revealed the presence of an additional mechanism of relaxation. Its activation energy is 14.5 kcal mol-’ which is almost 6 kcal mol-’ greater than the energy corresponding to the hindered rotation of the Ccl, group. We propose this additional mechanism to be the breaking of hydrogen bonds and translational diffusion of water molecules.
INTRODUCTION
Chloral hydrate, CCI,CHOH,, crystallizes in the monoclinic structure (space group PB,/c) and its elementary cell contains four molecules [l-3]. In the “UC” plane OH***0 type hydrogen bonds between two oxygen atoms of different molecules were found to occur. These bonds are different in length for two crystallographically non-equivalent hydroxyl groups and their lengths are 0.1908 and 0.1933 nm. NQR investigation [ 1,4-61 has shown that the 35C1 spectrum consists of three lines of the same intensity. Significant splitting of the spectrum (d u = 1.3 MHz) is explained by the effect of intramolecular interactions. As follows from the fact that the temperature coefficients of NQR frequency as well as spinlattice relaxation times (T,) are similar for all three lines, the intermolecular interactions are weak. An exponential change of In T1 (T) function above 250 K is interpreted as arising from the hindered rotation of the Ccl, group. The activation energy of this process has been found to be 8.8 kcal mol- ’ [ 11. EXPERIMENTAL
For our investigations we used polycrystalline chloral hydrate made by Merck. The sample to be studied was put inside a glass ampoule which was then sealed.
0022-2860/89/$03.50
0 1989 Elsevier Science Publishers
B.V.
380
The measurements of NQR frequency as well as spin-lattice relaxation times were carried out in the temperature range from room temperature to the temperature at which the NQR signal could not be distinguished from noise. For measurements we used a pulse NQR spectrometer constructed at the Institute of Physics, A. Mickiewicz University, Poznan. Details of construction and experimental methods have been described elsewhere [ 71. The NQR lines frequencies were determined with an accuracy of ? 2 kHz and the relaxation times 7’i with an accuracy better than 10%. RESULTS AND DISCUSSION
At 295 K the NQR frequencies were measured to be vl = 37.516, v2 = 38.703 and v,= 38.790 MHz and the relaxation time T1 was 5 ms. For the line v1 detailed measurements of temperature changes of NQR frequencies and relaxation times were carried out and repeated many times for temperatures ranging from 295 to 315 K. In the analysis we use the averaged values for individual temperatures. Figure 1 presents the temperature dependence of v1 for temperatures higher than 290 K. This dependence was analyzed on the grounds of Bayer’s theory [ 81 and taking into account Brown’s correction [ 91 which led to the relation vQ(T)=A,+A,T+A2T2
(1)
The experimental data were found to fit this function for A, = 38.315 MHz. A ,=-1.4~10-~MHzK~‘andA,=-4.5x10~~MHzK~~.Thesevaluesare only slightly different from those obtained for the temperature range 77-300 K [ 61, which means that the torsional motions of these molecules should be treated as the vibrations of anharmonic oscillators. T IKI I--
c
3x
T [Kl Fig. 1. Temperature dependence of
V,
3.2
103/T [K-?
line above 290 K in chloral hydrate.
Fig. 2. Temperature dependence of spin-lattice
relaxation time above 290 K in chloral hydrate.
381
The temperature changes of T, are shown in Fig. 2. The dependence 7’i = f (l/T) decreases linearly and thus the relation
Tcl =Aexp(
=EJRT)
In
(2)
is satisfied. The value obtained, E,= 14.5 kcal mol-‘, is considerably greater than 8.8 kcal mol-l obtained from the measurements carried out in the temperatures up to 300 K [ 11.This suggests that besides the hindered rotation of a Ccl, group activated above 250 K, there is another mechanism of relaxation that activates above 295 K. Analysis of the known crystallographic structure of chloral hydrate leads us to the conclusion that this other mechanism is related to an order-disorder transition in which the hydrogen bonds are broken. The energies of hydrogen bonds of the type OH..*0 extrapolated on the grounds of Newton’s results [lo] to the lengths H..*O = 0.1908 nm and 0.1933 nm are equal to 7.2 kcal mol-’ and 5.8 kcal mol-I, respectively. It seems justified to assume that above 295 K the quadrupole relaxation is a result of the joint influence of hindered rotation of Ccl, and breaking of the hydrogen bonds. The latter process may be a source of the change in the second moment of the NMR proton line from 12.9 Gs2 to 7.9 Gs’ observed above 305 K [ 111. Quadrupole relaxation may be also additionally influenced by the diffusion of water molecules weakly bonded in the polycrystalline material. This may be confirmed by a decrease in T, at room temperature from 5 ms to 3 ms after the sample has been annealed in the open ampoule above 315 K (after a few hours the value of T, comes back to 5 ms).
CONCLUSIONS
The results of NQR investigations in chloral hydrate prove that besides the hindered rotation of the Ccl:, group observed earlier above 250 K, another mechanism of quadrupole relaxation exists. The activation energy E a= 14.5 kcal mol-’ may be a sum of the contributions due to rotation, breaking of the hydrogen bonds and probably also water diffusion. Our results suggest the possibility of distinguishing three dynamically different temperature ranges in chloral hydrate: (a) T-c250 K where libration vibrations dominate, (b ) T 250-290K where Ccl, group rotation is activated, (c) T> 290K where the molecules have more freedom after the OH.-0 bonds are broken and translational diffusion of water molecules becomes possible.
382
REFERENCES
H. Chihara and N. Nakamura, Bull. Chem. Sot. Jpn., 45 (1972) 3530. S. Kondo and I. Nib, X-Rays, 6 (1950) 53. K. Ogawa, Bull. Chem. Sot. Jpn., 36 (1963) 610. D. Biedenkapp and A. Weiss, Z. Naturforsch. Teil A, 22 (1976) 1124. H.C. Allen, J. Am. Chem. Sot., 74 (1952) 6074. J. Pietrzak, Physics of Dielectrics and Radiospectroscopy, Vol. 5 PTPN, 1969, p. 143. J. Pietrzak, J. Kasprzak. P. Kamasa, and G. Kienitz. Port. Conf. Radio- and Microwave Spectroscopy, Poznan, 1971. 8 H. Bayer, Z. Phys., 130 (1951) 227. 9 J.C. Brown. J. Chem. Phys., 32 (1960) 116. 10 M.D. Newton, Acta Crystallogr., Sect. B 39 (1983) 104. 11 K. Holderna, M. Ostafin, Z. Pajak and J. Pietrzak. Rep. No. 802/PL, Institute of Nuclear Physics, Cracow, 1972, p. 13.