14N nuclear quadrupole resonance and proton spin–lattice relaxation study of phase transition in pyridazine perchlorate

Solid State Communications 149 (2009) 546–549

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14

N nuclear quadrupole resonance and proton spin–lattice relaxation study of phase transition in pyridazine perchlorate J. Seliger a,b,∗ , V. Žagar a , T. Asaji c a

‘‘Jozef Stefan’’ Institute, Jamova 39, 1000 Ljubljana, Slovenia

b

Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia

c

Department of Chemistry, College of Humanities and Sciences, Nihon University, Sakurajosui 3-25-40, Setagaya-ku, Tokyo 156-8550, Japan

article

info

Article history: Received 1 July 2008 Received in revised form 19 November 2008 Accepted 7 January 2009 by T.T.M. Palstra Available online 14 January 2009 PACS: 64.70.Kt 76.60.Gv 76.70.-r

a b s t r a c t The temperature dependence of the 14 N nuclear quadrupole resonance frequencies in pyridazine perchlorate has been measured by double resonance. The results show that in the low temperature phase the pyridazinium ions are static, while in the high temperature phase the ions reorient around the normal to the plane of the ring between six equivalent orientations in agreement with the X-ray data. The 14 N NQR data have been obtained for the protonated nitrogen position in the pyridazine ring for the first time. Temperature and frequency dependence of the proton spin–lattice relaxation time T1 has been measured in both crystallographic phases. The results show the ionic mobility dominates T1 in the low temperature phase. The activation energy for the ionic mobility is 240 meV. In the high temperature phase the pyridazine ring reorientation dominates the proton spin–lattice relaxation. © 2009 Elsevier Ltd. All rights reserved.

Keywords: A. Organic crystals D. Phase transitions E. Nuclear resonances

1. Introduction Pyridinium salts of the general formula (C5 H5 NH)+ X− exhibit numerous interesting features. The appearance of ferroelectricity in pyridinium perchlorate, PyHClO4 , [1–3] and the antiferroelectric phase transition in pyridinium tetrachloroiodate (III), PyHICl4 , [4,5] is reported to be closely related to the freezing of the reorientational motion of the pyridinium cation below the ferroelectric and antiferroelectric phase transition, respectively. Recently pyridazine perchlorate (C4 H4 N2 )HClO4 , in which the pyridinium cation is replaced by the pyridazine cation, was synthesized by Kosturek et al. [6]. The authors characterized the sample by x-ray diffraction, dielectric measurements and optical studies. The crystal undergoes first-order phase transitions at 343 K and 339 K on heating and cooling, respectively. The room temperature monoclinic structure of pyridazine perchlorate is stabilized by a system of hydrogen bonds linking the distorted ClO− 4 anionic

∗ Corresponding author at: Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia. Tel.: +386 1 4766576; fax: +386 1 2517281. E-mail address: [email protected] (J. Seliger). 0038-1098/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2009.01.008

groups and flat rings of the protonated pyridazine. In the high temperature phase the interionic hydrogen bonds are broken, leading in consequence to a fully disordered structure modeled in the hexagonal space group R3m. The C4 H4 N2 H+ ion reorients among six equivalent orientations. The change in the molecular dynamics associated by the phase transition has been studied by Hatori et al. [7] using differential scanning calorimetry and 1 H nuclear magnetic resonance. The authors observed a drastic change of the proton second moment at the phase transition, consistent with the disorder of the pyridazinium ions. A decrease of the proton spin–lattice relaxation time has also been observed at the phase transition. The mechanism of the phase transition in pyridazine perchlorate is not yet completely understood. We therefore performed the 14 N nuclear quadrupole resonance (NQR) study of the phase transition. The 14 N NQR frequencies are namely an extremely sensitive indicator of the local structure and molecular dynamics in solids. In addition we probed the molecular and ionic dynamics by measuring the temperature and frequency dependence of the proton spin–lattice relaxation time. 2. Experimental The polycrystals of pyridazine perchlorate were obtained from stoichiometric aqueous solution of pyridazine and perchloric acid

J. Seliger et al. / Solid State Communications 149 (2009) 546–549

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by slow evaporation of water in a vacuum desiccator using P2 O5 as desiccant. A 14 N nucleus has in zero magnetic field three generally non-degenerated nuclear quadrupole energy levels. The NQR frequencies, labeled as ν+ ≥ ν− ≥ ν0 , are expressed as

ν+ = ν− =

e2 qQ 4h e2 qQ 4h

(3 + η) (3 − η)

ν0 = ν+ − ν− =

e2 qQ 2h

(1)

η.

Here e2 qQ /h is the quadrupole coupling constant and η is the asymmetry parameter of the electric field gradient (EFG) tensor. The EFG tensor is a traceless second rank tensor composed of the second derivatives of the electrostatic potential with respect to the coordinates. It has three principal values labeled as VXX , VYY and VZZ , |VXX | 6 |VYY | 6 |VZZ |. The quadrupole coupling constant is the largest principal value |VZZ | = eq of the EFG tensor multiplied by the electric quadrupole moment eQ of the nitrogen nucleus and divided by Planck’s constant h¯ . The asymmetry parameter η is defined as η = (VXX − VYY )/VZZ . It ranges between 0 and 1. The 14 N NQR frequencies are usually below 5 MHz. In addition the magnetic moment of a 14 N nucleus is low. The 14 N NQR frequencies are therefore often measured by various highly sensitive 1 H–14 N nuclear quadrupole double resonance techniques based on magnetic field cycling. In the present case we used the technique based on the frequency dispersion of proton T1 [8] and the solid effect technique [9]. The magnetic field cycling was done by magnetic field cycling between two magnets. The proton spin system was first polarized in a magnet with the constant magnetic field B0 ≈ 0.75 T (νL = 32 MHz). Then the sample was pneumatically transferred into another magnet, where the low magnetic field B can be varied. The sample spent a time τ in the low magnetic field. Then the sample was pneumatically transferred back into the first magnet and the proton NMR signal S was measured. Variation of the time τ at a fixed value of B was used to measure the proton spin–lattice relaxation time T1 (B) in the low magnetic field B. Variation of B at a constant value of τ was used to observe quadrupolar dips in the Larmor frequency dependence of the proton NMR signal S. A quadrupolar dip occurs when the proton Larmor frequency matches a NQR frequency. Application of a strong radiofrequency magnetic field with the frequency ν in the low static magnetic field B was used to observe the solid effect dips in the ν -dependence of the proton NMR signal. A solid effect dip is observed when the frequency ν is equal ν = νQ ± νL . Here νQ is a 14 N NQR frequency and νL = γH B/2π is the proton Larmor frequency. 3. Results and discussion The temperature dependence of the 14 N NQR frequencies in pyridazine perchlorate is shown in Fig. 1. At room temperature we observe two sets of three 14 N NQR frequencies corresponding to two nonequivalent nitrogen positions: the nonprotonated nitrogen (N) with the quadrupole coupling constant e2 qQ /h = 5120 kHz and the asymmetry parameter η = 0.172 and the protonated nitrogen (NH) with the quadrupole coupling constant e2 qQ /h = 1615 kHz and the asymmetry parameter η = 0.402. The quadrupole coupling constant and the asymmetry parameter η for the nonprotonated nitrogen are close to the values obtained in pyridazine [10]. The NQR data for the protonated nitrogen are obtained for the first time. The quadrupole coupling constant of the protonated nitrogen is – similarly as in pyridine [11] – significantly lower than the quadrupole coupling constant of the nonprotonated nitrogen.

Fig. 1. Temperature dependence of 14 N NQR frequencies at the NH (triangles) and N(open squares) nitrogen positions in (C4 H4 N2 )HClO4 .

On increasing temperatures below Tc we observe only a weak temperature dependence of the NQR frequencies that may be due to molecular librations. No pretransitional changes of the NQR frequencies are observed. At the phase transition the NQR frequencies abruptly change. Above Tc we observe only two NQR frequencies. At 71 ◦ C the NQR frequencies are equal 1610 kHz and 420 kHz. The two independent NQR frequencies mean that the asymmetry parameter η is at both nitrogen positions equal zero. The zero value of the asymmetry parameter η is obtained if the pyridazinium ion reorients either between three, four or six equivalent orientations, or if it performs a free uniaxial rotation. In all these cases we obtain in time average a uniaxial second rank EFG tensor. The NQR results agree with the crystallographic [6] and proton NMR [7] data if a pyrazinium ion reorients around the normal to its plane between six equivalent orientations. The quadrupole coupling constant e2 qQ /h at the nonprotonated nitrogen position is equal 2150 kHz, while it is at the protonated nitrogen position equal 560 kHz. In the high temperature phase the 14 N NQR frequencies weakly decrease on increasing temperature. The two nitrogen positions are in both crystallographic phases discriminated by the solid effect technique [9], which produces stronger double resonance lines when the proton-nitrogen dipolar interaction is stronger. The solid effect lines have been in pyridazine perchlorate observed only around the NQR frequencies of the protonated nitrogen. The abrupt change of the 14 N NQR frequencies at the phase transition is characteristic for the first order phase transition. No disorder of the pyridazinium ion is observed below the phase transition temperature in contrast to the pyridinium ion in pyridinium tetrachloroiodate which already below the phase transition temperature starts to reorient around the normal to its plane for ±60◦ [5]. Above the phase transition temperature the pyridinium ion in pyridinium tetrachloroiodate reorients between six orientations which are not equally probable, while the present NQR data for pyridazine perchlorate show that the six orientations of a pyridazinium ion are equally probable within about 1%. If this would not be the case, the asymmetry parameter η would differ from zero in the high temperature phase. At both nitrogen positions in a static pyridazinium ion one of the principal axes of the EFG tensor points nearly perpendicular to the plane of the ion. If the ion reorients around the normal to the plane between six equivalent orientations, the principal value of the time averaged EFG tensor corresponding to this principal direction remains unchanged. It becomes the largest principal value of the axially symmetric time-averaged EFG tensor. To see, which principal axis of the EFG tensor points perpendicular to the

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J. Seliger et al. / Solid State Communications 149 (2009) 546–549

Table 1 Principal values of the EFG tensor at the nitrogen positions below and above Tc . Position

T /◦ C

eQV XX /h or eQV ZZ /h / kHz

eQV YY /h / kHz

eQV ZZ /h or eQV XX /h / kHz

N N NH NH

65 71 65 71

2130 2150 ±440 ±560

2980 −1075 ±1155 ±280

−5110 −1075 ±1585 ±280

plane of the ring, we calculate the principal values below and above Tc . The results are tabulated in Table 1. The sign of the quadrupole coupling constant of the nonprotonated nitrogen is — similarly as in case of pyridine [11], taken to be negative. The sign of the quadrupole coupling constant of the protonated nitrogen cannot be determined from the NQR measurement. In both cases the principal value, labeled as VXX in the low temperature phase, remains nearly unchanged at the phase transition. In case of the non-protonated nitrogen this principal value changes negligibly, while in case of the protonated nitrogen it changes more. This larger change of the principal value may be associated with the break of the N–H. . . O hydrogen bonds above the phase transition temperature. The NQR data thus show that at both nitrogen positions within a pyridazinium ion the principal axis X of the EFG tensor corresponding to the lowest-magnitude principal value of the EFG tensor points perpendicular to the plane of the ring. The dynamics of solid pyridazine perchlorate has already been studied by the measurements of proton spin–lattice relaxation time T1 at the Larmor frequency νL = 10 MHz [7]. In order to get some more data on the molecular and ionic dynamics in this compound we measured the temperature dependence of proton T2 and proton T1 at νL = 32 MHz and the frequency dependence of proton T1 in both crystallographic phases. The proton spin–spin relaxation time T2 is in the low temperature phase equal T2 ≈ 13 µs. In the high temperature phase it increases to about T2 ≈ 25 µs. For a pair of protons the dipolar Hamiltonian reads:

E)(EI2 nE)). HD = C D (EI1EI2 − 3(EI1 n

(2)

E is Here CD = µ0 h¯ γ /4π r , r is the interproton distance and n E||x, the dipolar Hamiltonian the unit vector in the H–H direction. If n reads 2

2 H

3

HD = C D (EI1EI2 − 3I1x I2x ).

(3)

When the pair of protons isotropically reorients around an axis z which is perpendicular to the H–H direction, the dipolar Hamiltonian averages. In case of fast reorientation we observe the time-averaged dipolar Hamiltonian 1

Fig. 2. Proton spin–lattice relaxation time T1 at νL = 32 MHz as a function of temperature.

hHD i = − CD (EI1EI2 − 3I1z I2z ).

(4) 2 The same result is obtained when the pair of protons reorients around the axis z between three equivalent orientations, four equivalent orientations, or six equivalent orientations. Assume, we are dealing with identical pairs of protons in a polycrystalline sample in a high static magnetic field. The directions x are randomly distributed with respect to the direction of the static magnetic field. The directions of the reorientation axes z are randomly distributed with respect to the direction of the static magnetic field as well. Factor 1/2 in expression (4) tells that in case of an isotropic uniaxial reorientation the proton NMR linewidth reduces by factor 2 and the spin–spin relaxation time T2 increases by factor 2. This is not exactly true in the present case where each pyridazinium ion isotropically reorients around the normal to its plane. There are namely five protons in a ring. Therefore there are various H–H directions x (expression (3)), while the reorientation axis z (expression (4)) is unique. Nevertheless a similar reduction

Fig. 3. Larmor frequency dependence of the proton spin–lattice relaxation time at T = 295 K() and 353 K ().

of the proton linewidth and increase of proton T2 as in case of isolated proton pairs is expected also in this case in agreement with the experimental data. Hatori et al. [7] observed a larger decrease of the proton linewidth at the phase transition. The ratio of the proton linewidths below and above Tc respectively is about four. The reason for this discrepancy is not known. However, one possibility is that their reported values are not linewidths but second moments. Proton spin–lattice relaxation time T1 is as a function of 1000 K /T shown in Fig. 2. Below Tc proton T1 strongly increases on decreasing temperature. We are thus in the slow motion regime where T1 ∝ τ ∝ exp(Ea /kB T ). The activation energy Ea as determined from Fig. 2. is equal Ea = kB 2800 K = 240 meV = 23 kJ/mol. This value is in agreement with the previously determined value Ea = 0.2 eV [7]. Above Tc the spin–lattice relaxation time drops to T1 = 1.8 s and exhibits only a weak temperature dependence. In order to probe the nature of the slow motion below and above Tc we measured the Larmor frequency dependence of proton spin–lattice relaxation time at room temperature and at 80 ◦ C. The results are presented in Fig. 3. At room temperature the proton spin–lattice relaxation time T1 exhibits a non-BPP frequency dependence. Above approximately νL = 2 MHz it is nearly proportional to the Larmor frequency. A linear relation between νL and T1 is observed in ionic conductors [12]. Kosturek et al. [6]

J. Seliger et al. / Solid State Communications 149 (2009) 546–549

interpreted the results of the measurement of dielectric losses in terms of the ionic mobility. The measurements of proton T1 in the low temperature phase agree with this interpretation. Most probably T1 reflects the proton mobility which may be associated with the proton jump in a N–H. . . O hydrogen bond to the N. . . H–O position associated with the reorientation of the HClO4 molecule. Above Tc we observe a frequency independent T1 for frequencies larger than 2 MHz. It is thus dominated by a fast motion, presumably the reorientations of the rings. The decrease of T1 below approximately 2 MHz is most probably produced by the quadrupole nuclei (nitrogen and chlorine). 4. Conclusions In the low temperature phase of pyridazine perchlorate we observe two sets of three 14 N NQR frequencies corresponding to two nitrogen positions – NH and N - in a pyridazinium ion. All the pyridazinium ions in the unit cell are crystallographically equivalent. The NQR data for the protonated nitrogen position in a pyridazinium ion are obtained for the first time. On approaching the phase transition temperature from below we observe nearly no change of the 14 N NQR frequencies until the phase transition occurs. The orientational order parameter of a pyridazinium ion is thus equal to one in contrast to pyridinium ion in pyridinium tetrachloroiodate [5] where on approaching the phase transition from below the orientational order parameter of a pyridinium ion first decreases and then the first order phase transition occurs. In the high-temperature phase there are only two 14 N NQR frequencies. The EFG tensor is at both nitrogen positions axially symmetric. Nearly no temperature dependence of the 14 N NQR frequencies is observed in the high-temperature phase.

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The axially symmetric EFG tensor is associated with a fast reorientation of a pyridazinium ion between six equivalent orientations. The situation is again different to that in pyridinium tetrachloroiodate [5], where in the high-temperature phase a pyridinium ion reorients between two more probable and four less probable orientations while the occupation probabilities vary with temperature. The proton spin–lattice relaxation time is in the hightemperature phase below νL = 32 MHz frequency independent. It only exhibits shortening below νL ≈ 2 MHz, when the cross relaxation effects with the quadrupole nuclei take place. In the low-temperature phase proton T1 exhibits a non-BPP behavior characteristic for ionic mobility. Most probably the hydrogen ions move between the hydrogen bonds. A probable mechanism may be the proton jump N–H. . . O → N . . . H–O in a hydrogen bond associated with the reorientation of a HClO4 molecule. References [1] P. Czarnecki, W. Nawrocik, Z. Pajak, J. Wasicki, J. Phys. C 6 (1994) 4955. [2] J. Wasicki, S. Lewicki, P. Czarnecki, C. Ecolivet, Z. Pajak, Mol. Phys. 98 (2000) 643. [3] P. Czarnecki, J. Wasicki, Z. Pajak, R. Goc, H. Maluszynska, S. Habrylo, J. Mol. Struct. 404 (1997) 175. [4] T. Asaji, K. Eda, H. Fujimori, T. Adachi, T. Shibusawa, M. Oguni, J. Mol. Struct. 826 (2007) 24. [5] J. Seliger, V. Žagar, T. Asaji, A. Konnai, Magn. Reson. Chem. 46 (2008) 756. [6] B. Kosturek, A. Waskowska, S. Dacko, Z. Czapla, J. Phys.: Condens. Matter 19 (2007) 086219. [7] J. Hatori, K. Tanigawa, T. Kato, Y. Yoshida, S. Ikehata, Y. Mastuo, Z. Czapla, J. Korean Phys. Soc. 51 (2007) 832. [8] J. Seliger, R. Blinc, H. Arend, R. Kind, Z. Phys. B 25 (1976) 189. [9] J. Seliger, V. Žagar, J. Magn. Reson. 193 (2008) 54. [10] E. Schempp, P.J. Bray, J. Chem. Phys. 46 (1967) 1186. [11] G.V. Rubenacker, T.L. Brown, Inorg. Chem. 19 (1980) 392. [12] K. Funke, D. Wilmer, Europhys. Lett. 12 (1990) 363.