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Deuteron quadrupole coupling in squaric acid
Volume 104, number 2,3
CHEMICAL PHYSICS LETTERS
3 February 1984
DEUTERON Q'UADRUPOLE COUPLING IN SQUARIC ACID J. SELIGER, V. ZAGAR, R. BLINC J. Stefan Institute, E. Kardel] University of L]ubljana, P.O. Box 53, 61111 Ljubl]ana, Yugoslavia and A. NOVAK CNRS, Groupe des Laboratoires de Vitry-Thiais, Thiais, France Received 31 October 1983; in final form 29 November 1983
The deuteron quadrupole coupling constant eQVzz/h in squaric acid amounts at room temperature to 142 -+ 2 kHz and the asymmetry parameter r~ is 0.21. The data show that the deuteron is located in an off-centre position below Tc and that all O-D---O bonds are equivalent within the fimits of experimental error. The weak temperature dependence of eQVzz/h suggests an order-disorder rather than a displacive nature for the phase transition.
Squaric acid (C4H402) at room temperature consists of ordered layers of C404 groups [l,2]. Each C40 4 group is linked by four O - H - - - O bonds to neighbouring molecules within the same layer, thus forming a pseudo-two-dimensional structure. The layers are held together by van der Waals forces. They are ferroelectrically ordered but antiferroelectrically stacked, so that the dipole moments o f adjacent layers are antiparallel. At TC, H = 95 °C, squaric acid undergoes a phase transition [3] from the ordered low-temperature monoclinic (P21/m, z = 2) phase to the disordered high-temperature tetragonal phase (I4/m, z = 1). The transition seems to be triggered by the dynamic disordering of the hydrogens between the two equilibrium sites in the O - H - - - O bonds, in analogy to the transition in KH2PO 4 [4]. This is supported by the large isotope effect on T c (Tc, D = 243°C) which clearly shows the role o f the hydrogens in the phase-transition mechanism. The motion of the protons between the two equilibrium sites in the O - H - - - O bonds was recently detected directly by 170 quadrupole resonance and 170-proton magnetic dipolar coupling measurements [5]. The length o f the hydrogen bond (Ro... O = 2.55 A) as determined by diffraction studies [1,2] is signifi0 009-2614/84/$ 03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
cantly larger than in KH2PO 4 (Ro. ..O = 2.49 A) [6]. This is somewhat surprising in view o f the infrared absorption data [7] which show that the OH-stretching frequency (vOH = 1300 cm -1) is lower and not higher than in KH2PO 4 (uOH -~ 2200 cm-1). Other open problems are the transition entropy - which is rather small for an order-disorder transition - and the ferroelasticity of the transition, which contributes a displacive component to the transition mechanism. In order to throw some additional light on the nature of the hydrogen bond in this system, we decided to measure the deuteron quadrupole coupling constant, which is a rather sensitive indicator of the length of the O - D - - - O bond. It strongly decreases with increasing length of O - D and decreasing length of the O - D - - - O bond [8]. The deuteron (I = 1) quadrupole coupling tensor was determined by p r o t o n - d e u t e r o n double-resonance spectroscopy in an 80% deuterated powdered sample of squaric acid via the solid effect [9]. The two high-frequency deuteron quadrupole transitions 1
u+ = 3(eQVzz/h)(1 + ~rl),
(1)
277
Volume 104, number 2,3
CHEMICAL PHYSICS LETTERS
BO% DEUTERAIED SQUARIC ACID IH-2H DOUBLERESONANCE VLB =
I
t
37kHz
i
llO
i
13O
t
i
150
I
v [kHz ]
Fig. 1. Proton deuteron double-resonance spectrum of 80% deuterated squaric acid at room temperature. The u_ and u+ lines represent direct transitions between the quadrupole energy levels of the H-bonded deuteron whereas the v + ULH and u+ + ULH lines represent combined deuteron-proton spin flips due to the solid effect [9].
u = ~(eQVzz/h)(1
- ~1) ,
(2)
were observed (fig. 1) at room temperature at v+ = 114 + 2 kHz and u_ = 99 + 2 kHz. With increasing temperature, ~,+ and v_ are practically constant. Here I Vzz[ > I Vyyl > I Vxxl are the eigenvalues o f the electric field gradient (EFG) tensor and r~ = (Vxx Vyy)/Vzz. The experimental data thus yield a deuteron quadrupole coupling constant eQVzz/h o f 142 + 2 kHz and an asymmetry parameter r / o f 0.2 I. This can be compared [4,8] with eQVzz/h = 121.5 kHz and r / = 0.05 in CsD2AsO 4 at T = 30°C > Tc, and with eQVzz/h = 126.7 kHz in the same compound below T c = - 6 0 ° C . The deuteron quadrupole coupling constants [4,8] for other members o f the KH2PO 4 family (Ro... O = 2.49 A) are eQVzz/h = 118.5 kHz for KD2AsO4, 119.7 kHz for KD2PO4, 119.6 kHz for ND4D2PO 4 and 117.5 kHz for ND4D2AsO 4. In triglycine sulfate, where Ro... O = 2.44 A, eQVzz/h = 87 kHz whereas in the K - H - m a l e a t e ion, where the H b o n d is short and symmetric (Ro... O = 2.40 A, R o _ D = 1.20 A), the deuteron quadrupole coupling
278
3 February 1984
constant amounts [4,8] to only 56 kHz. The result for squaric acid is close to that for the carboxyl O - D - - - O bond in oxalic acid dihydrate (COOD)2. 2D20 , where [8] eQVzz/h = 139.2 kHz, r / = 0.1 and Ro... o = 2.58 A. It should be noted that the asymmetry parameter in squaric acid is twice as large as in oxalic acid dihydrate. Its magnitude is unusual for a hydrogen bond o f length 2.55 A. Our results thus show that (i) the deuteron in squaric acid is located in an offcentre position in the O - D - - - O bond below Tc; (ii) all O - D - - - O bonds are equivalent within the limits of experimental error and their nature does not vary significantly with increasing temperature; and (iii) the length of the O - D - - - O bond in squaric acid is indeed somewhat larger than in KD2PO 4. The above results thus suggest an o r d e r - d i s o r d e r rather than a displacive nature for the phase transition in deuterated squaric acid. This agrees with the 170 quadrupole resonance results [5 ] in the undeuterated sample.
References [1] D. Semmingsen, Acta Chem. Scand. 27 (1973) 3961; A29 (1975) 470; D. Semmingsen, F.J. Hollander and T.F. Koetzle, J. Chem. Phys. 66 (1977) 4405. [2] Y. Wang, G.D. Stucky and J.M. Williams, J. Chem. Soc. Perkin II (1974) 35. [3] D. Semmingsen and J. Feder, Solid State Commun. 15 (1974) 1369. [4] R. Blinc and B. Zek~, Soft modes in ferroelectric and antiferroelectrics (North-Holland, Amsterdam, 1974), and references therein. [5 ] J. Seliger, V. Zagar and R. Blinc, J. Magn. Reson., to be published. [6] G.E. Bacon and R.S. Pease, Proc. Roy. Soc. A230 (1955) 359. [7] D. Bougeard and A. Novak, Solid State Commun. 27 (1978) 453. [8] T. Chiba, J. Chem. Phys. 41 (1964) 1352; R. Blinc and D. Hadgi, Nature 212 (1966) 1307. [9] J. Seliger, R. Blinc, M. Marl, R. Osredkar and A. Prelesnik, Phys. Rev. B11 (1975) 27.
CHEMICAL PHYSICS LETTERS
3 February 1984
DEUTERON Q'UADRUPOLE COUPLING IN SQUARIC ACID J. SELIGER, V. ZAGAR, R. BLINC J. Stefan Institute, E. Kardel] University of L]ubljana, P.O. Box 53, 61111 Ljubl]ana, Yugoslavia and A. NOVAK CNRS, Groupe des Laboratoires de Vitry-Thiais, Thiais, France Received 31 October 1983; in final form 29 November 1983
The deuteron quadrupole coupling constant eQVzz/h in squaric acid amounts at room temperature to 142 -+ 2 kHz and the asymmetry parameter r~ is 0.21. The data show that the deuteron is located in an off-centre position below Tc and that all O-D---O bonds are equivalent within the fimits of experimental error. The weak temperature dependence of eQVzz/h suggests an order-disorder rather than a displacive nature for the phase transition.
Squaric acid (C4H402) at room temperature consists of ordered layers of C404 groups [l,2]. Each C40 4 group is linked by four O - H - - - O bonds to neighbouring molecules within the same layer, thus forming a pseudo-two-dimensional structure. The layers are held together by van der Waals forces. They are ferroelectrically ordered but antiferroelectrically stacked, so that the dipole moments o f adjacent layers are antiparallel. At TC, H = 95 °C, squaric acid undergoes a phase transition [3] from the ordered low-temperature monoclinic (P21/m, z = 2) phase to the disordered high-temperature tetragonal phase (I4/m, z = 1). The transition seems to be triggered by the dynamic disordering of the hydrogens between the two equilibrium sites in the O - H - - - O bonds, in analogy to the transition in KH2PO 4 [4]. This is supported by the large isotope effect on T c (Tc, D = 243°C) which clearly shows the role o f the hydrogens in the phase-transition mechanism. The motion of the protons between the two equilibrium sites in the O - H - - - O bonds was recently detected directly by 170 quadrupole resonance and 170-proton magnetic dipolar coupling measurements [5]. The length o f the hydrogen bond (Ro... O = 2.55 A) as determined by diffraction studies [1,2] is signifi0 009-2614/84/$ 03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
cantly larger than in KH2PO 4 (Ro. ..O = 2.49 A) [6]. This is somewhat surprising in view o f the infrared absorption data [7] which show that the OH-stretching frequency (vOH = 1300 cm -1) is lower and not higher than in KH2PO 4 (uOH -~ 2200 cm-1). Other open problems are the transition entropy - which is rather small for an order-disorder transition - and the ferroelasticity of the transition, which contributes a displacive component to the transition mechanism. In order to throw some additional light on the nature of the hydrogen bond in this system, we decided to measure the deuteron quadrupole coupling constant, which is a rather sensitive indicator of the length of the O - D - - - O bond. It strongly decreases with increasing length of O - D and decreasing length of the O - D - - - O bond [8]. The deuteron (I = 1) quadrupole coupling tensor was determined by p r o t o n - d e u t e r o n double-resonance spectroscopy in an 80% deuterated powdered sample of squaric acid via the solid effect [9]. The two high-frequency deuteron quadrupole transitions 1
u+ = 3(eQVzz/h)(1 + ~rl),
(1)
277
Volume 104, number 2,3
CHEMICAL PHYSICS LETTERS
BO% DEUTERAIED SQUARIC ACID IH-2H DOUBLERESONANCE VLB =
I
t
37kHz
i
llO
i
13O
t
i
150
I
v [kHz ]
Fig. 1. Proton deuteron double-resonance spectrum of 80% deuterated squaric acid at room temperature. The u_ and u+ lines represent direct transitions between the quadrupole energy levels of the H-bonded deuteron whereas the v + ULH and u+ + ULH lines represent combined deuteron-proton spin flips due to the solid effect [9].
u = ~(eQVzz/h)(1
- ~1) ,
(2)
were observed (fig. 1) at room temperature at v+ = 114 + 2 kHz and u_ = 99 + 2 kHz. With increasing temperature, ~,+ and v_ are practically constant. Here I Vzz[ > I Vyyl > I Vxxl are the eigenvalues o f the electric field gradient (EFG) tensor and r~ = (Vxx Vyy)/Vzz. The experimental data thus yield a deuteron quadrupole coupling constant eQVzz/h o f 142 + 2 kHz and an asymmetry parameter r / o f 0.2 I. This can be compared [4,8] with eQVzz/h = 121.5 kHz and r / = 0.05 in CsD2AsO 4 at T = 30°C > Tc, and with eQVzz/h = 126.7 kHz in the same compound below T c = - 6 0 ° C . The deuteron quadrupole coupling constants [4,8] for other members o f the KH2PO 4 family (Ro... O = 2.49 A) are eQVzz/h = 118.5 kHz for KD2AsO4, 119.7 kHz for KD2PO4, 119.6 kHz for ND4D2PO 4 and 117.5 kHz for ND4D2AsO 4. In triglycine sulfate, where Ro... O = 2.44 A, eQVzz/h = 87 kHz whereas in the K - H - m a l e a t e ion, where the H b o n d is short and symmetric (Ro... O = 2.40 A, R o _ D = 1.20 A), the deuteron quadrupole coupling
278
3 February 1984
constant amounts [4,8] to only 56 kHz. The result for squaric acid is close to that for the carboxyl O - D - - - O bond in oxalic acid dihydrate (COOD)2. 2D20 , where [8] eQVzz/h = 139.2 kHz, r / = 0.1 and Ro... o = 2.58 A. It should be noted that the asymmetry parameter in squaric acid is twice as large as in oxalic acid dihydrate. Its magnitude is unusual for a hydrogen bond o f length 2.55 A. Our results thus show that (i) the deuteron in squaric acid is located in an offcentre position in the O - D - - - O bond below Tc; (ii) all O - D - - - O bonds are equivalent within the limits of experimental error and their nature does not vary significantly with increasing temperature; and (iii) the length of the O - D - - - O bond in squaric acid is indeed somewhat larger than in KD2PO 4. The above results thus suggest an o r d e r - d i s o r d e r rather than a displacive nature for the phase transition in deuterated squaric acid. This agrees with the 170 quadrupole resonance results [5 ] in the undeuterated sample.
References [1] D. Semmingsen, Acta Chem. Scand. 27 (1973) 3961; A29 (1975) 470; D. Semmingsen, F.J. Hollander and T.F. Koetzle, J. Chem. Phys. 66 (1977) 4405. [2] Y. Wang, G.D. Stucky and J.M. Williams, J. Chem. Soc. Perkin II (1974) 35. [3] D. Semmingsen and J. Feder, Solid State Commun. 15 (1974) 1369. [4] R. Blinc and B. Zek~, Soft modes in ferroelectric and antiferroelectrics (North-Holland, Amsterdam, 1974), and references therein. [5 ] J. Seliger, V. Zagar and R. Blinc, J. Magn. Reson., to be published. [6] G.E. Bacon and R.S. Pease, Proc. Roy. Soc. A230 (1955) 359. [7] D. Bougeard and A. Novak, Solid State Commun. 27 (1978) 453. [8] T. Chiba, J. Chem. Phys. 41 (1964) 1352; R. Blinc and D. Hadgi, Nature 212 (1966) 1307. [9] J. Seliger, R. Blinc, M. Marl, R. Osredkar and A. Prelesnik, Phys. Rev. B11 (1975) 27.