Phase transitions in non-centrosymmetric pyridinium trifluoromethanesulfonate crystal: Vibrational studies

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 203–214

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Phase transitions in non-centrosymmetric pyridinium trifluoromethanesulfonate crystal: Vibrational studies Dominik Jesariew, Maria M. Ilczyszyn ⇑ Faculty of Chemistry, Wrocław University, Joliot-Curie 14, 50-383 Wrocław, Poland

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

mNH band arises from Fermi resonance.  Internal modes of PyHOTf fulfill selection rules of the factor group in IR spectra. +   PyH cation and OTf anion are involved in the order–disorder phase transitions. +   PyH cation and OTf anion exhibit rotational mobility at high temperature phases.  Multiple structure of

a r t i c l e

i n f o

Article history: Received 30 September 2014 Received in revised form 19 March 2015 Accepted 27 March 2015 Available online 3 April 2015 Keywords: Pyridinium trifluoromethanesulfonate Vibrational spectroscopy Temperature-dependent infrared spectra Order–disorder phase transition Ion mobility

a b s t r a c t Infrared spectroscopy (4000–400 cm1) in the wide temperature range, from 11 to 473 K, has been used to investigate the non-centrosymmetric pyridinium trifluoromethanesulfonate crystal, exhibiting several phase transitions. The assignments of the bands observed in the studied spectra have been proposed. The temperature dependence of the wavenumbers and the full width at half maximum (FWHM) of the bands arising from some internal vibrations of the pyridinium cation and the triflate anion have been analyzed in order to achieve a knowledge of whether these both ions are involved in the phase transitions and what is the role of these both ions in these phase transitions. The infrared measurements showed that the both ions, pyridinium cation and triflate anion are involved in the high temperature phase transitions of the order–disorder type, previously reported at 305.1 and 396.7 K. They also revealed that these transitions are governed by a rotational mobility (changes in dynamical states) of both the pyridinium and triflate ions. Our results show that the multiple structures of the mNH and mND bands observed in the studied infrared spectra is due to the Fermi resonance interaction between the stretching vibration of the N–H  O hydrogen bond and the overtones and combinations of the internal vibrations of the pyridinium cation. Ó 2015 Elsevier B.V. All rights reserved.

Introduction Pyridinium trifluoromethanesulfonate (PyHOTf), universally used as a source of the trifluoromethanesulfonate anion in the diverse chemical reactions, turn out to be very attractive species ⇑ Corresponding author. Tel.: +48 71 375 7305. E-mail address: [email protected] (M.M. Ilczyszyn). http://dx.doi.org/10.1016/j.saa.2015.03.114 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.

for its interesting physical properties. Recently reported Differential Scanning Calorimetry (DSC) has revealed several fully reversible phase transitions in this crystal [1]. According to these data three reversible and well defined solid–solid phase transitions of a first order character at 305.1, 358.7 and 396.7 K and a very weak anomaly at low temperature (at about 205.3 K) of a second order nature in studied crystal were observed. The DSC results were fully supported by X-ray studies [1]. The PyHOTf crystal

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D. Jesariew, M.M. Ilczyszyn / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 203–214

belongs to the non-centrosymmetric P43212 space group of tetragonal system [1,2] in two low temperature phases (phase V and IV). The phase transition at 305.1 K (IV ? III) is accompanied by change of the space group from P43212 (in phase IV) to P2 (in the phase III), and simultaneously, by change of the crystallographic system from tetragonal (phase V and IV) to monoclinic one (phase III). Both intermediate phases (III and II) belong to a monoclinic system. In the high-temperature phase I (prototype phase) the crystal is pseudo-cubic with a slight tetragonal deformation. Additionally, the TGA measurement has shown the thermal stability of the crystal in question up to about 620 K [1]. As follows from DSC data the studied crystal is characterized by a low entropy of melting (about 14 J mol1 K1) and a relatively high melting temperature (about 490 K) [1]. According to Timmermans’ criterion for a plastic crystal [3] all these features imply that PyHOTf may appear as a promising high temperature proton conductor. In this contribution vibrational spectra of the PyHOTf crystal at room temperature and temperature dependent infrared spectra of the PyHOTf crystal in wide temperature range are presented. The temperature dependent infrared spectra can provide information on anomalous dynamical properties of crystals near structural phase transition or on structural changes near structural phase transition. Those data can be easily determined from the evolution of the infrared band shapes as well as from infrared band positions versus temperature. The utility of infrared spectroscopy in the structural phase transition investigations has been written in many publications (see for example ref [4]). Recently reported XRD data show that the low temperature phase transition is accompanied by weakness of one N–H  O hydrogen bond but the high temperature phase transitions are triggered off by the high disordered of pyridinium cations and triflate anions [1]. Our analysis of the temperature dependent infrared spectra was preceded by detailed discussion of the infrared spectra measured at room temperature based on the PyHOTf crystal structure and vibrational spectra of the pyridinium cations, triflate anions and their salts.

Experimental details PyHOTf was obtained from a saturated aqueous solution containing pyridine and triflic acid in the 1:1 molecular ration at room temperature. The N-deuterated species was prepared by threefold re-crystallization from D2O solution. The degree of the deuterium substitution at the nitrogen atom has been estimated at ca. 60 per cent on comparing the infrared spectra of the N-deuterated and non-deuterated species. The infrared spectra for polycrystalline sample of studied crystal were measured for the mulls in Nujol and fluorolube in wide temperature range (473–11 K) in the 4000–400 cm1 wavenumbers region with a Bruker IFS-66 and Bruker IFS-88 spectrometer. The resolution was 2 cm1. The low temperature spectra (11–300 K) were measured with a spectrometer equipped with closed helium cryostat (ARS Displex Model CS202-X1.Al closed cycle cryostat) to which a temperature controller of Scientific

Instruments (Model SI-9700-1) was joint. The high temperature spectra (300–473 K) were recorded with the Specac high temperature cell. The infrared powder spectrum for N-deuterated sample was recorded at room temperature only. The temperature dependent IR spectra were recorded in the following way: (i) The low temperature spectra. The first spectrum was measured at 11 K. Then the sample was heated step-by-step, up to the room temperature. The maximum temperature variation in one step was 25 K and the waiting time for the temperature stabilization was 20 min. After every step and temperature stabilization the spectrum was measured. (ii) The high temperature spectra. These spectra were measured from room temperature to 473 K and an analogous stepby-step procedure was applied. The FT Raman spectra of polycrystalline spectra of both crystals, deuterated and un-deuterated were recorded with a Nicolate IFS860 instrument with a Raman attachment in the 3500–200 cm1 region. The 1064 nm line of a Nd:YAG diode pump laser (power = 400 mW) was used for excitation. The spectral resolution was 2 cm1. The programs: GRAMS/386 Galactic Industries and Origin 5.0 were used for the numerical fitting of the experimental data.

Results and discussion The crystal structure and selection rules for the PyHOTf crystal at room temperature The crystal of PyHOTf at room temperature is tetragonal and belongs to the P43212 space group with eight molecules, occupying the general position, per unit cell [1]. The pyridinium cations (PyH+) and trifluoromethanesulfonate (OTf) anions are arranged in infinite and helical chains in which they are alternately bonded by bifurcated medium strong N–H  O hydrogen bonds of 3.214 and 3.159 Å lengths [1]. The same molecular arrangement is observed in the low temperature phase (below 205 K), however, the N  O distances are shorter (2.929 and 2.925 Å, respectively). The results of the factor group analysis of studied crystal are summarized in Table 1a. The correlation diagram relating the factor group to the site group is shown in Table 1b. There it is seen that the presence of eight molecules in the unit cell can result in splitting of each internal vibrations into six components. Three of them (A2 + 2E) are predicted in infrared spectrum and five of them (A1 + B1 + B2 + 2E) should be Raman active.

Vibrational spectra at room temperature The vibrational spectra of PyHOTf and its partly N-deuterated species measured at room temperature are shown in Fig. 1. The wavenumbers of the bands observed in these spectra are reported in Table 2.

Table 1a Formal analysis of the fundamental modes (k = 0) for the PyHOTf crystal at 165 K and at room temperature (in the V and IV phase). D4

A1 A2 B1 B2 E a

Lattice vibrations

Internal vibrations

Selection rules

Na

A

T

L

C5H5NH+

CF3SO3

60 60 60 60 120

0 1 0 0 1

6 5 6 6 11

6 6 6 6 12

30 30 30 30 60

18 18 18 18 36

Used abbreviation: N – total number of modes; A – acoustical modes; T – translational modes; L – librational modes.

IR

Raman x2 + y2, z2

z

x, y

x2  y2 xy xz, yz

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D. Jesariew, M.M. Ilczyszyn / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 203–214 Table 1b The correlation diagram relating the factor group (unit cell symmetry group) to the site group.

Site group C1

factor group D4 A1 (R) A2 (IR)

A

B1 (R) B2 (R) 2 E (IR, R)

The studied compound is an interionic complex (PyH+  A; A = CF3SO 3 ). Therefore the bands observed in the 4000– 200 cm1 frequency region are due to internal vibrations of the  OTf anion and PyH+ cation. The internal vibration of latter one might be divided into the internal vibrations of pyridine ring and vibrations of the proton transferred to the pyridine what is tantamount to the N–H  O hydrogen bond vibrations. The tentative assignments of the bands apparent in the vibrational spectra of the studied crystal proposed in Table 2 has been based on the theoretical predictions of the internal vibrations of the pyridine and d5-pyridine [5–7] and OTf anion [8,9] and on the comparison of the assignments proposed for the pyridine and d5-pyridine [10–14], pyridinium, d5-pyridinium and N-deuterated of h5- and d5-pyridinium cation [15–18] and simple salts of trifluoromethanesulfonic acid [8,9,19]. The known crystal structure [1,2] is also handy at the band assignments.

Hydrogen bonds vibration There are two medium strong N–H  O hydrogen bonds, with the N  O distances of 3.214 and 3.159 Å length in the studied crystal at room temperature [1]. When the temperature is reduced the N  O distances become shorter [1,2]. At room temperature they manifest themselves as a strong absorption situated in the spectral region from 3400 to 2600 cm1 (Figs. 1 and 2) of the infrared spectra. This absorption is arising from the mNH stretching vibrations and as one can see in Fig. 2a it has a complex structure with several sharp maxima (at 3254, 3239, 3178, 3143, 3117, 3076, 2978 and 2903 cm1) and some minima (transmission windows at 3206 and 3156 cm1) (Table 2). The structure of the mNH infrared absorption in the studied crystal is very similar to that one reported by Glazunov and Odinokov [17] for the simple pyridinium salts in solution. As follows from their consideration ‘‘the multiple structure of the mNH and mND band contours in the range 3300– 1400 cm1 is due to Fermi resonance interaction of the mNH (mND) stretching mode with overtones and combination tones of the pyridinium ion internal modes’’ [13]. This is also the case of the studied crystal. All sub-maxima, except two maxima located at 3117 and 3076 cm1, and all transmission windows apparent on the mNH band, are derived from the resonance interaction (Fermi resonance) between mNH stretching vibration of the N–H  O hydrogen bond and overtones and combination modes of the internal vibrations of the PyH+ ion. The corresponding assignments are presented in Table 2. Note that the shape of the mND broad band observed at about 2326 cm1 in the infrared spectra of the N-deuterated species of the studied crystal differs from that of the mNH band and is less complex than that one observed for the mNH band. The significant difference between the shape of the bands due to mNH and mND vibrations results from a number of modes (overtones or combination modes) which perturb stretching vibrations of NH and ND bonds (mNH or mND). In the

Table 2 Wavenumbers (cm1) and intensity of the bands observed in the infrared and Raman spectra of polycrystalline sample of PyHOTf and its N-deuterated analogue measured at room temperature. Polycrystalline sample C5H6N+CF3SO 3 Infrared

Raman

3254vs 3239vs 3206⁄ 3188sh 3178vs 3156⁄ 3143vs 3117vs 3106⁄ 3084vs 3076vs 3054sh 2998⁄ 3036sh 2978vs

3108 (0.938)

3054 (0.29) 2988(0.117) 2970(0.117)

⁄

2954 2931vs 2903vs

2731vw 2692vw 2674vw 2596vw

C5H5DN+CF3SO 3 Infrared 3255sh w 3236m 3208⁄ 3188sh m 3177m 3153⁄ 3142sh m 3116m

3084m 3072m 3058m 2995⁄ 2979m 2968m 2951⁄ 2932m 2911m 2869sh m 2838sh w 2789w

Tentative assignments

Raman 2 x m8a (3274)a 2 x m8b (3220)

3167 (0.15)

m8a + m19b(3178) m8b + m19b(3151) m8a + m19a(3127)

3105 (1.86)

m2

3074 (0.24) 3046 (0.35) 2989 (0.094)

m8b + m19a (3100) m20b m13 m7 m19a + m19b (3031) m20a 2 x m19a (2980)

2965 (0.19) 2 x m19a (2980) m19a + 1394 (2935) 2 x m19b (2906)

2687w 2653vw 2606w 2565vw 2544vw

2508vw 2469vw 2477vw 2433m 2419⁄

2441vw

2378vw 2326s b 2309vw 2256vw 2209vw 2087vw 2055vw 2033vw 1987vw

2247m 2122w 2090w 2059w 2032w 1984w 1975w 1934sh 1915w 1878w 1779w 1781w

1915vw 1881vw 1806vw 1745vw 1695vw 1667vw 1637s

1636(0.82)

1610vs

1611(0.583)

1541vs 1490vs 1453vw 1436vw 1429vw 1394w 1381w 1339m

2364 (0.12)

1635vs 1627s 1609vs 1584m 1539vs 1489vs 1485sh s 1457sh w 1425vw 1394w 1381w 1337sh 1303vs b

m3 + m15 (2506) 2 x m9a (2402) mN–D 2 x masCF3 2 x m18b (2178) 2 x m18a (2116) m18a + m1 (2063) m1 + m12 (2032) 2 x m5 (1992)

1633 (0.6) 1627 (0.75) 1585 (0.87)

1304 (0.49)

m8a (mCC) m8a (mCC) m8b (mCC) m8b (mCC) m19b (mCC) m19a (mCC) m19a (mCC) m19b (mCC)

m3 (dCH) masSO3 (continued on next page)

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Table 2 (continued) Polycrystalline sample

Tentative assignments

C5H6N+CF3SO 3

C5H5DN+CF3SO 3

Infrared

Raman

Infrared

Raman

1287vs 1260vs

1280(0.292)

masSO3 dNH (1269?)

1224vs

1227 (0.997) 1201 (1.089) 1169(0.327)

1222vs

1281 (0.283) 1261b (0.316) 1226 (0.94)

1200vs

1201 (1.04)

m9a (dCH)

1166sh 1150vs 1128sh 1089 1078 1058

1169 (0.49)

m15 (dCH) masCF3

1201vs 1166sh 1155vs 1090w 1058m

1059 (0.464)

1027vs

1031 (5.703) 1009 (8.415)

1005s 996sh s

997sh 941vs

934sh w 902m 855vw 807vw 759vs 685vs

1034sh 1024vs

1126 (0.49)

m18b (dCH) 1057 (0.55)

m18a (dCH)

1031sh (5.11) 1028 (5.18)

m12 (dCC) msSO3, m12(dCC)

1009 (0.95)

m1 (mCC)

996 (6.2) 943 (0.26)

m1 (mCC) dND

cNH

903s 857sh

763 (2.745)

645vs 636vs 611m

637 (1.142) 611 (0.731)

577vs

577 (1.266)

519vs

519(0.389)

396w 351 (2.017) 317 (2.244) 215 (0.281)

799vs 756vs 684vs 663vs 643vs 628vs 611s 605s 575vs 565sh 520sh 517vs 400m 395w 383w

msCF3

801 (0.25) 762 (2.9)

634 (0.92) 604 (0.78) 576 (1.45)

m10b (dCH) dsCF3, m10b(dCH) m11 (dCH) cND m6b (dCC) dsSO3

m6a (dCC) m6a (dCC) dasCF3

520 (0.56)

dasSO3

383 349 316 212

m16b(dCC) m16b(dCC) qSO3 mC–S qCF3

(0.39) (2.41) (2.40) (0.27)

a The wavenumbers of the unperturbed overtones and combination modes are given in the brackets. ⁄ transmission window.

case of the mND vibration, resonance interactions have been observed only for the combination m3 + m15 and for the overtone of m18b. The transmission window at 2419 and pseudo-maximum at 2433 cm1 result from the former interaction but the sub-maximum at 2247 cm1 arises from the latter one. If the center of gravity of the mNH absorption is estimated at 3087 cm1 the isotopic ratio for the mNH vibration is equal to 1.33. The in-plane dNH and out-of-plane cNH deformation mode in the infrared spectra of simple pyridinium salt have been observed near 1300 cm1 and in the 1080–903 cm1 region, respectively [16]. Moreover, as it was shown in this report the strong coupling of both deformation modes, dNH and cNH, with internal vibrations of pyridinium occur. The former mode is strongly coupled to the B1 symmetry modes in the region 1650–1200 cm1, the latter one to those of B2 symmetry in the region 1100–600 cm1 [16]. In the studied crystal, the cNH vibration appears at 902 cm1 as a medium intense band. In the N-deuterated analogue, the corresponding band, cND, is localized at 663 cm1. The isotopic ratio for this mode is equal to 1.36. The dND band was found at 941 cm1. If

we assume that the dNH/dND isotopic ratio is the same as for the out-of-plane mode, the dNH absorption should be expected at about 1260 cm1. Unfortunately, the intense bands due to the anion internal vibrations (masSO3 and msCF3) are present near this wavenumber. Internal vibrations of PyH+ cation The wavenumbers of the bands due to internal vibrations of the pyridinium cation in the studied crystal are summarized in Table 2 and they are very similar to those reported for h5-pyridinium and for N-deuterated h5-pyridinium cations [16]. The most intense band observed in the Raman spectra of both undeuterated and deuterated crystal, as expected, was assigned to the m1(mCC) vibration. For the former crystal corresponding band is apparent at 1009 cm1, for the latter one it is shifted to 996 cm1. Our results confirm also the strong coupling between internal vibrations of the pyridine ring and the deformation modes, dNH and cNH, of the N–H  O hydrogen bonds postulated by Glazunov and Odinokov [16]. Some bands assigned to the pyridine internal vibrations shift to lower or higher wavenumbers when the proton transferred to the pyridine is substituted by deuterium. One can observe such frequency changes for the m8a, m8b, m10b, m6a and m16b vibrations (see Table 2). Internal vibrations of OTf anion Many reports on the OTf anion internal vibrations are available in the scientific literature [8,9,19–23]. Unfortunately, the assignments of the corresponding bands given there are ambiguous and very often disagree with each other. Those are probably due to significant mixing of the SO3, CF3 and CS motions in some normal modes [8,9,20]. On attribution of the bands due to internal vibrations of OTf in the studied crystal the OTf ion bands assignment proposed by Huang et al. [9] we considered. An isolated CF3SO 3 anion exhibits C3v symmetry. In this case eighteen internal vibrations belonging to the 5A1 + A2 + 6E irreducible representations are expected. All modes, except the mode of the A2 symmetry, are infrared and Raman active. In studied crystal, the OTf occupies position (site) of C1 symmetry. The effect of site symmetry results in lowering of the effective molecular symmetry of OTf anion from C3v to C1. As a result the doubly degenerate species (E) split into two components and eighteen internal vibrations of OTf anion of A symmetry are infrared and Raman active. The presence of eight anions per primitive unit cell results in correlation splitting of each A-species internal vibration of OTf group into six components (A1 + A2 + B1 + B2 + 2E). Three of them (A2 + 2E) are infrared, five of them (A1 + B1 + B2 + 2E) are Raman active. Correlation diagram of the OTf internal vibrations and their wavenumbers in the studied crystal are given in Table 3. Two bands at 1287 cm1 and 1027 cm1 in the infrared spectrum are associated with the maSO3 and msSO3 stretching, respectively. Their Raman counterparts are observed at 1280 cm1 and 1031 cm1, respectively. The proposed assignments are based on the following determinations: (1) for the wavenumbers of the stretching vibrations of SO3 group, the relation maSO3 > msSO3 holds both in infrared and Raman spectra; (2) the FWHM value for the maSO3 band in the infrared spectrum is much larger than that of the msSO3 band; (3) the relative intensities of the msSO3 and maSO3 vibrations are reversed in the infrared and Raman spectra of PyHOTf; the ImaSO3 > ImsSO3 holds for the infrared bands, whereas the relation ImaSO3 < ImsSO3 is valid for the Raman bands. The bands arising from the CF3 stretching vibrations have been localized at 1224 and 1155 cm1. The former band has been assigned to the msCF3 vibration, the latter one to the maCF3 vibration. Note that the frequencies of the bands arising from the stretching vibrations of CF3 group in PyHOTf are in reverse order with respect to the general expectation; i.e. usually a band due

759

350 317

637 611 577 520

1060

1227 1201

1636 1611

3109 3053

762

1031

685 645

519

577

902 1027 1010

1224

1155

1636 1610

1540 1489

2911

3238 3176

3083

(b)

2000

1500

518

350 317 637 604 577 520

1057

1227 1201

1305

1627 1585

2370

3046

3000

1127

762

(d)

684 629 576

756

801

943 903 1028 1025 996

1489

1152

1540

1635 1610

2433 2371

2912

3237 3176

(c)

3105

Raman intensity transmittance Raman intensity

207

(a)

3083

transmittance

D. Jesariew, M.M. Ilczyszyn / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 203–214

1000

500

-1

Wavenumber [cm ] Fig. 1. Vibrational (IR and Raman) spectra of the PyHOTf crystal (a and b) and its N-deuterated species (c and d) measured at room temperature on the polycrystalline sample. Infrared spectra were measured as Nujol and polychlorotrifluoroethylene emulsion.

0,6

0,4

0,4

3600

3400

3200

3000

2800

2600

2800

2600

2400

2ν18b

0,6

ν3+ν15

0,8

2ν19b

0,8

2ν19a

1,0

ν13

1,0

PyDOTf

2ν14

1,2

PyHOTf

2ν18a 2ν18b ν8a+ν19b ν8b+ν19b

Transmittance

1,2

2200

2000

-1

Wavenumber [cm ] Fig. 2. Infrared spectra of PyHOTf (left) and its partly N-deuterated species (right) in the N–H  O (left) and N–D  O (right) hydrogen bond stretching region (mNH/mND). Spectra were recorded at room temperature.

to the asymmetric stretching vibration of this group is expected to have a higher frequency than that due to its symmetric vibration [24]. The reversible order of the stretching frequencies of the CF3

group in triflatic anion in solid state was explained by Huang et al. [9] by ‘‘indirect interaction of the sulfur atom with the fluorine or oxygen atoms’’. In our case, the assignments proposed for

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Table 3 Correlation diagram for the internal vibrations of the OTf anion in the PyHOTf crystal.

a b c

Isolated aniona C3v

Site C1

m1 (msCF3), A1 m2 (msSO3), A1 m3 (dsCF3), A1 m4 (dsSO3), A1 m5 (mCS), A1 m6 (maSO3), E m7 (masCF3), E m8 (dasCF3), E m9 (dasSO3), E m10 (qSO3), E m11 (qCF3), E m12 (sCS), A2

? ? ? ? ? ? ? ? ? ? ? ?

A A A A A 2A 2A 2A 2A 2A 2A A

Factor groupa D4

? ? ? ? ? ? ? ? ? ? ? ?

A1 + A2 + B1 + B2 + 2E A1 + A2 + B1 + B2 + 2E A1 + A2 + B1 + B2 + 2E A1 + A2 + B1 + B2 + 2E A1 + A2 + B1 + B2 + 2E 2(A1 + A2 + B1 + B2 + 2E) 2(A1 + A2 + B1 + B2 + 2E) 2(A1 + A2 + B1 + B2 + 2E) 2(A1 + A2 + B1 + B2 + 2E) 2(A1 + A2 + B1 + B2 + 2E) 2(A1 + A2 + B1 + B2 + 2E) A1 + A2 + B1 + B2 + 2E

PyHOTfb

PyDOTfb

PyHOTfc

Infrared T = 295 K

Raman T = 295 K

Infrared T = 11 K

Infrared T = 295 K

Raman T = 295 K

Infrared

Raman

1224vs 1027vs 759vs 636vs

1227(0.997) 1031(5.703) 763(2.745) 637(1.142) 319(2.244) 1280(0.292)

1255vs, 1238vs, 1227vs (very broad) 1039vs, 1033vs, 1028vs 764vs 759vs 755vs (broad) 637vs 631vs 626vs (broad)

1222vs 1024vs 756vs 628vs

1223 1032 752 638

1282 (remains still broad) 1154vs 1140vs 1134vs 580vs 578sh, 576vs 525vs 523sh, 520vs

1303vsb 1150vs 575vs 520sh

1226(0.94) 1028(5.18) 762(2.9) 635(0.92) 316(2.4) 1304(0.49)

1225 1027 750 637 311 1271 1151 572 517 348 210

1287vs 1155vs 577vs 519vs

577(1.266) 519(0.389) 351(2.017) 215(0.281)

576(1.45) 520(0.56) 349(2.41) 212(0.27)

1270 1148 572 517

Selection rules for C3v: A1 (IR, R), A2 (i.a.), E (IR, R); selection rules for D4: A1 (R), A2 (IR), B1 (R), B2 (R), E (IR, R). This work. Wavenumbers taken from [9].

the symmetric and asymmetric stretching vibrations of CF3 group have been supported by the relative intensities of the bands due to these group vibrations; while the former (msCF3) yields a medium- intensity band, the later (maCF3) yields no observable band in the Raman spectrum. The assignments of the other bands arising from the internal vibrations of OTf anion are presented in Table 3. It should be emphasized that no excess of the bands arose from the internal vibrations of the OTf anion are observed in the infrared and Raman spectra at room temperature. The number of bands due to the internal vibrations of this anion observed in the studied vibrational spectra agrees well with that expected for isolated OTf anion (Table 3). This may show that the symmetry of the OTf anion in the studied crystal at room temperature is higher than follows from XRD data. According to this the OTf anion occupies position of a C1 symmetry. Contrary to the above observation, the number of bands associated with the internal vibrations of OTf anion significantly increase in the infrared spectrum recorded at 11 K. All bands arose from the internal vibrations of the OTf anion, expect the band due to maSO3, split into three components. From Table 4 it is seen that these experimental results are in agreement with the factor-group selection rules. This indicates that the vibrational couplings between eight OTf anions in the unit cell are present. Temperature dependent vibrational spectra Low temperature infrared studies Selected temperature dependent infrared spectra measured below room temperature, in the 11–296 K range of temperature, are shown in Fig. 3, the wavenumbers of the bands observed there are collected in Table S1. Detailed analysis of these spectra allowed one to make the following observations: (i) On cooling down the most bands observed in the spectra split into two or three components. These observations concern the bands arising from the internal vibrations of the both ions, PyH+ cation and OTf anion. Moreover the bands arising from the internal vibrations of the OTf anion, expect band arising from maSO3, become narrower on cooling sample to 11 K. It is worthwhile to note that the splitting of the individual bands is observed at various temperatures, sometimes at temperatures significantly different from the low temperature phase transition (about 205.3 K).

(ii) Some bands arising mainly from the internal vibrations of the PyH+ cation (m8a(mCC), m19a (mCC), m19b(mCC), m18b(dCH), m18a(dCH), m11(dCH)) shift to higher wavenumbers when the temperature is reduced to 11 K. It is necessary to stress that all observed changes are small and have monotonous character. (iii) The band due to the cNH vibration observed at about 900 cm1 at room temperature is split into four components: three intense bands at 892, 881 and 873 cm1 and shoulder at 886 cm1 in low temperature spectrum (11 K, Fig. 4). This splitting starts at various temperatures (at 196, 161 and 36 K) considerably different from the low phase transition temperature (205.3 K). (iv) The band (absorption) arising from the mNH vibration becomes distinctly narrower at 11 K than at room temperature (Fig. 5). Its approximate half width at room temperature is equal about 520 cm1 but the corresponding value at 11 K is equal about 190 cm1. The transmission windows become deeper than these observed in the spectrum measured at room temperature. Simultaneously, the sub-maxima become sharper and better defined. Moreover the maximum of the mNH absorption shifts from about 3110 cm1 (at 300 K) to about 3200 cm1 (at 11 K). Unfortunately, the last values as well as the half width of the mNH absorption quoted above are only approximate because of the complex structure of the mNH absorption in both temperatures, at 300 and 11 K (Fig. 5). In spite of this one can state that the changes above mentioned are monotonous and do not show any abrupt changes at 205.3 K. The findings presented above show that the changes observed in the spectra recorded below room temperature rather do not result from the V ? IV transition. This conclusion seems to be supported by X-ray diffraction data [1]. As follows from them the studied crystal crystallizes in the same space group above and below 205.3 K. Although the molecular arrangements are the same for both phases (above and below) 205.3 K, one can observe small differences between the corresponding bond lengths. On the other hands, the monotonous/smooth character of the observed changes shows that they were rather produced by decreasing the oscillation amplitudes of the atoms, accompanied by the reduction in temperature. The splitting of the bands observed in the low temperature spectra might be explained by the vibrational coupling between

D. Jesariew, M.M. Ilczyszyn / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 203–214

209

T=473 K liquid

cubic T=413K phase I

Transmittance [a.u]

T=373K phase II

T=345K phase III

P43212 T=296K phase IV

P43212 T=186K phase V

P43212 T=11K phase V

4000

3000

2000

1500

1000

500

-1

Wavenumber [cm ] Fig. 3. The temperature dependent infrared spectra recorded for the polycrystalline sample of the PyHOTf crystal in the 11–473 K temperature range.

molecules apparent in the unit cell [25]. The asymmetric unit of studied crystal consists of one PyH+ cation and OTf anion. There are eight asymmetric units in the primitive cell and they occupy a set of site of the C1 symmetry. The twenty atoms of the isolated asymmetric unit provides forty-eight degrees of freedom (3  12–6 = 30 and 3  8–6 = 18) to the internal vibrations, and these vibrations of species A would be expected to show activity both in infrared and Raman, if the effect of the site symmetry (C1) was considered only. If an interaction between eight molecules in the unit cell was taken into consideration each of the vibrations of species A in the site group would split into six components of the species A1, A2, B1, B2 and 2E in the factor group. The results of the factor group analysis are summarized in Table 1a. From this Table it is seen that only components of species A2 and E, are infrared active. As a result, each infrared band observed in the spectrum should be split into a triplet (A2 + 2E). This seems to be our case. The triplet splitting of the most bands arising from the internal vibrations of the PyH+ cation and OTf anion probably results from the vibrational coupling between the molecules (eight) in the unit cell (correlation splitting or Davydov splitting). Detailed inspection of Fig. 4 shows that the cNH band is split into four components, three intense bands and shoulder, at 11 K instead of three as it is expected under factor group selection rules (Table 1a). In our opinion, the cNH band splitting one should explain considering two effects simultaneously: (1) vibrational coupling between the molecules in the unit cell (correlation splitting); and (2) the presence of two distinct N–H. . .O hydrogen bonds in the crystal. Although both N–H  O hydrogen bonds were found to have the same N  O distances and very close < NHO angles at 165 K [1], their probably vary noticeably in bond strengths at very

low temperature. Note that the fourth component of cNH bond appears in the spectrum measured at 36 K. As it is easily seen from Fig. 4 the band derived from the cNH shifts to lower wavenumbers with decrease in temperature. Simultaneously, the maximum of the mNH absorption shifts to higher wavenumber (Fig. 5). These both findings mean that the N–H  O hydrogen bonds become weaker at low temperature. Unfortunately this conclusion is not consistent with the XRD data [1]. According to those the both N–H  O bonds are stronger in the phase V than in the phase IV. Summarizing the above considerations one can find that the changes observed in the low temperature infrared spectra of the studied crystal do not result from the phase transition reported at about 205.3 K (V ? IV). The observed changes are the consequence of the vibrational couplings between molecules apparent in the primitive cell (correlation splitting or Davydov splitting). A detailed examination of the PyHOTf crystal structure reveals small changes in the position of both ions (PyH+ and OTf) at the V ? IV transition. The position of the PyH+ cation in the phase IV is shifted slightly (about 0.08 Å) along an a axis, in comparison to its position in the phase V. The triflate anion leans out from the position at the phase V at about 10°. Such changes, however, will not be detected in the infrared spectra unless they are accompanied with changes in the bond strengths. The low temperature spectra reported here confirm that this is the case for the studied crystal. High temperature infrared studies When the high temperature infrared spectra reported here (recorded above room temperature) are looked over, one can easily

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γNH 909

473K

473K 413K 373K

413K Transmittance [a.u.]

345K

373K Transmittance [a.u.]

296K

186K

11K

345K 296K

186K

1100

1050

1000

950

900

873

892 886 881

11K

850

-1

wavenumbers [cm ] 3400

Fig. 4. Temperature evolution of the infrared spectra of the PyHOTf crystal in the 1100–800 cm1 wavenumber region.

distinguish that the changes occurred there differ from those observed in the low temperature spectra, considerably. First of all no splitting of the bands are observed when the temperature is increasing from 300 to 473 K. At the same time, a step increase or a step decrease in band wavenumbers and their FWHM are observed at the IV ? III and II ? I transitions. These changes are noted for the internal vibrations of the PyH+ cation as well as for the OTf anion. An anomalous behavior of the band arising from the cNH mode is also observed. However one has to admit that above changes although noticeable are rather small. Temperature evolution of the band positions In Figs. 6 and 7, an evolution of the wavenumber versus temperature is given for some selected infrared bands, all of which arise from the internal vibrations of the PyH+ cation and OTF anion. Almost all bands presented in Figs 6 and 7 show a step increase or decrease in their wavenumbers at phase transition IV ? III. Over phases III and II they shift slightly towards to higher or lower wavenumbers and for some of them next step changes in positions are observed at the II ? I transition. It is worthwhile to note that character of the observed changes agree well with the DSC data [1]. The largest changes in the band positions are observed at the IV ? III and II ? I transitions which are characterized by high thermal effect [1]. These indicate a key role of the both ions, PyH+ and OTf, in the phase transitions. This conclusion is consistent with the data available in the scientific literature that show different participation of the pyridine and triflatic acid in various phase transitions [26–32]. The temperature evolution of the band at 902 cm1 (at room temperature) corresponding to the cNH vibration is presented in Fig. 8. The abrupt increase in its wavenumber, from 902 to about 908 cm1, is observed at the IV ? III phase transition. Then over

3200

3000

2800

2600

-1

wavenumbers [cm ] Fig. 5. Temperature evolution of the infrared spectra of PyHOTf in the 3400– 2500 cm1 wavenumbers range (the range of an absorption associated with the mNH vibration).

the phase III, II and I the cNH band shifts to lower wavenumbers on approaching the melting temperature but it does not achieve wavenumber value observed below 310 K. In the same time, the band associated with mNH becomes broader, its center of gravity shifts to lower wavenumbers (from around 3070 cm1 at 298 K to around 2966 cm1 at 473 K) and its structure becomes more blurred. These findings may indicate a strengthening for the N–H  O hydrogen bonds when the temperature is increased above room temperature. However, this hypothesis is, unfortunately, too hard to confirm by available XRD data [1]. At high temperature phases, the studied crystal is twinned, and in these phases, the refinement of its structure with a satisfying accuracy was not possible (see for further details [1]). Temperature dependence of the mode damping The IV ? III phase transition is accompanied by complete disorder of both, PyH+ and OTf ions [1]. In such case, the shapes (FWHM) of the vibrational bands due to the internal vibrations of these ions are closely related to their dynamics and they are assumed to be described by the self-diffusion model [33]. In this model the rate of ions jumping from one orientation to another is described by a correlation time sc, that is given by the Eyring equation:

sc ¼ s0 exp



 U ; kb T

where s0 ¼ h=kb T and U is an activation energy corresponding approximately to the height of the potential barrier between two equivalent ion positions. For relaxation processes the FWHM of

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D. Jesariew, M.M. Ilczyszyn / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 203–214 1638,0

-1

ν8a(νCC), 1636 cm

ν8b(νCC), 1610 cm

1614,5

1637,8

-1

1614,0

I

II

1637,6

1613,0

1637,2

I

II

III

IV

-1

ν/cm

ν/cm

-1

1637,4

1613,5

III

IV

1612,5 1612,0

1637,0

1611,5

1636,8

1611,0

1636,6 280

300

320

340

360

380

400

420

440

460

480

280

300

320

340

360

T/K ν19b(νCC), 1541 cm

1541,8

380

400

420

440

460

480

T/K

-1

1541,6 1541,4

ν/cm

-1

1541,2

III

IV

II

1541,0

I

1540,8 1540,6 1540,4 1540,2 1540,0 280

300

320

340

360

380

400

420

440

460

480

T/K

761,5

ν10b(δCH), 759 cm

761,0

685

-1

ν11(δCH), 682 cm

684

-1

760,5

683

-1

759,5 759,0

IV

ν/cm

ν/cm

-1

760,0

III

758,5

I

II

682

III

IV

681

I II

758,0

680

757,5 280

300

320

340

360

380

400

420

440

460

480

679 280

300

320

340

360

T/K

380

400

420

440

460

480

T/K

Fig. 6. Temperature evolution of the wavenumbers of the selected infrared bands assigned to the internal vibrations of the PyH+ (m8a, m8b, m19b, m10b and m11) in the 298–475 K temperature region.

Raman or infrared bands is proportional to a spectral density which is the Fourier transformation of the correlation function of the fluctuation. In such case the experimental data for the FWHM (C(x)) is assumed to be described by [34]:

reorientation mechanism of diffuse nature. Usually x2 s2c  1, therefore the last equation is transformed to the following form [35]:

sc ; CðxÞ ¼ ða þ bTÞ þ C 1 þ x2 s2c

C ¼ ða þ bTÞ þ C 0 exp 

where x is the frequency of the considered vibrational mode. The first term of last equation corresponds to vibrational relaxation contribution to the FWHM, the second one corresponds to the thermal

The experimental values of C of selected bands arising from the internal vibrations of the PyH+ and OTf ions (m8a(mCC), m11(dCH), dsCF3, dasSO3) in 300–360 K temperature region were fitted by







 U : kb T

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δsCF3, 750 cm

753

577,0

-1

576,5

752

750

-1

IV

III

ν/cm

749

-1

575,5

-1

ν/cm

δasCF3, 576 cm

576,0

751

I II

748

575,0

III

IV

574,5

I

II

747 574,0

746 280

300

320

340

360

380

400

420

440

460

573,5

480

280

300

320

340

360

380

T/K

400

420

440

460

480

T/K

520,0

δasSO3, 519 cm

-1

ν/cm

-1

519,5

519,0

IV

III

518,5

I II

518,0

280

300

320

340

360

380

400

420

440

460

480

T/K Fig. 7. Temperature evolution of the wavenumbers of the infrared bands assigned to the internal vibrations of the OTf anion (dsCF3, dasCF3 and dasSO3) in the 298–475 K temperature range.

908 907

γNH

906

901 cm IV

-1

905

ν/cm

-1

III

904

I

903

II

902 901 280

300

320

340

360

380

400

420

440

T/K Fig. 8. The temperature dependence of the cNH band wavenumber.

the last equation where a, b, C0 and U were treated as constants. The final results are shown in Fig. 9 and in Table 4. As one can see from Fig. 9, the values of FWHM of selected bands exhibit

abrupt changes at the IV ? III phase transition temperature. But over phase III, the values of FWHM of selected bands noticeably increase. As is seen from Table 4, the values estimated from the b parameters are rather low for all of the considered bands, while the C0 parameters exhibit high values. The former observation indicates that the modes: m8a(mCC), m11(dCH), dsCF3, dasSO3 are weakly anharmonic. On the other hand, the latter indicates that the dynamics of ions is dominated by the reorientation mechanism [35]. This conclusion is consistent with XRD data reported recently [1]. The value of the estimated activation energy for the pyridinium cations rotation in the PyHOTf crystal (about 36 kJ mol1) in the 306–360 K temperature range (phase III) is significantly higher than that found for the PyH+ cation in pyridinium iodide at room temperature (about 15 kJ mol1) [36] and in bis-thiourea pyridinium chloride in high temperature phase (about 1.2 kJ mol1) [37] from quasi-elastic scattering study and in (C5H5NH)6Bi4Cl18 and (C5H5NH)5Bi2Br11 in low temperature phases (about 7 kJ mol1 and about 5 kJ mol1, respectively) from infrared studies [32,38]. These determinations suggest that reorientation of the PyH+ cation in the investigated PyHOTf crystal is strongly hindered because of the interionic hydrogen bonds in which the PyH+ cation is involved. But the value of the estimated activation energy

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12,5

ν8a(νCC), 1636 cm

-1

11,0

I

II

III

IV

10,5

Δν1/2/cm

-1

11,5

liquid

18

liquid

-1

12,0

Δν1/2/cm

ν11(δCH), 682 cm

20

-1

16

14

II

III

IV

12

10,0

10

9,5 280

300

320

340

360

380

400

420

440

460

280

480

300

320

340

360

380

400

420

440

460

480

440

460

480

T/K

T/K

18

24 22

δsCF3, 750 cm

17

-1

δasSO3, 519 cm

16

20

-1

-1

liquid

14 12

II

III

IV

I

Δν1/2/cm

-1

16

liquid

15

18

Δν1/2/cm

I

14 13

IV

12

III

I

II

11

10

10

8

9 280

300

320

340

360

380

400

420

440

460

480

280

300

320

340

360

380

400

420

T/K

T/K

Fig. 9. Temperature dependence of the FWHM of the 1636 cm1 band (m8a(mCC)); 682 cm1 band (m11(dCH)); 750 cm1 band (dsCF3); 519 cm1 band (dasSO3).

Table 4 Activation energy and a, b, and C0 parameters estimated by fitting of the theoretical   bandwidth at half maximum temperature dependence, C ¼ ða þ bTÞ þ C 0 exp kUT , to b the experimental temperature relation recorded for chosen bands. Infrared band (cm1) 1636 682 750 576 519

m8a(mCC) m11(dCH) dsCF3 dasCF3 dasSO3

a (cm1)

b (cm1 K1)

C0 (cm1)

U (kJ mol1)

11.01 11.22 12.3 6.7 9.50

3  105 5.9  104 7.8  104 6.43  103 2.11  103

2.45  105 2.37  105 2.27  104 7.21  103 6.25  104

38.6 ± 0.1 33.9 ± 0.1 28.1 ± 0.1 25.8 ± 0.5 31.6 ± 1.0

for the pyridinium cations rotation in the PyHOTf crystal is the same as those one obtained for imidazolium rings in imidazolium methanesulfonate from ionic conductivity and solid state NMR data above room temperature (about 36 kJ mol1 [39] and about 38 kJ mol1 [40], respectively). It is worthwhile to note that imidazolium methanesulfonate is a protic organic ionic plastic crystal with a plastic phase ranging from 174 to 188 °C which is characterized by high ionic conductivity (about 1.0  102 S cm1) [39]. However, value of the activation energy estimated by us is two times lower than activation energy determined for trimethylammonium cations in trimethylammonium trifluoroacetate in high temperature phases [41] (about 56 kJ mol1) from solid state NMR and three times lower than that one estimated for tetrabutylammoniumiodide [42] in a plastic phase (82.02 kJ mol1) from ionic conductivity.

Considering the anion (OTf anion) dynamics two separate activation energies were estimated: one of them associated with the CF3 – reorientation, the second one with the reorientation of the SO3 group. The activation energies estimated by us for former process (CF3 reorientation) is equal to approximately 26.9 kJ mol1 but for the latter one (SO3 group reorientation) is equal to 31.6 kJ mol1. The former value is almost the same as the activation energy value found for CF3 reorientation in LiCF3SO3 (about 25.9 kJ mol1) [43,44] but the latter one is lower than obtained for SO3 group reorientation in this compound (about 45 kJ mol1) [43]. In turn, the activation energy estimated by us for the SO3 group of the OTf anion is three orders of magnitude higher than that found for the SeO2 anion in CsHSe solid elec4 trolyte in protonic superionic state (about 44.06 J mol1) [45]. In this case ‘‘almost free rotation of the SeO4 tetrahedrons’’ and H-bond breaking is postulated. In the studied crystal, reorientation of the SO3 group seems to be more hindered, and this situation may be indicating that, even at high temperature phases, the SO3 groups in the PyHOTf crystal are still involved in hydrogen bond network. Indeed, this hypothesis could not be supported by recently reported powder XRD data [1] because of the crystal twinning appeared in high temperature phases as well as the limited accuracy achieved in the refinement of the crystal structure. Nevertheless, the existence of a hydrogen bond network in the high temperature phases was proved by probing the temperature dependent variation of the mNH absorption band (Fig. 5); when the temperature is raised from room temperature to 473 K, its

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maximum shifts slightly to lower wavenumbers, indicating that the N–H  O hydrogen bonds will not get slightly weaker than before. Concluding remarks 1. The shape of the absorption band observed for the stretching mNH/mND mode in the infrared spectra of the studied PyOTF crystal was explained by the Fermi resonance interactions between the fundamental mNH/mND mode and the overtones and combinations of the internal modes of the pyridinium cation. 2. The low infrared spectra of the studied PyHOTF crystal did not indicate any weakness in the N–H  O bonds depending on the V ? IV transition, which was confirmed by the previously reported XRD data [1]. In such a phase transition occurred when the sample is heated from 11 to 298 K, the absorption infrared band due to the mNH vibration becomes broader and its maximum slightly shifts to lower wavenumbers. These changes resulting from the increase of the oscillation amplitudes of the atoms depending on the rise in temperature are monotonous and not sharp at 205.3 K. 3. Our findings confirmed the order–disorder character of the high temperature phase transitions. The step increase or decrease in wavenumbers of the infrared bands associated with the internal vibrations of the PyH+ cation and the OTf anion show that both ions are involved in high temperature phase transitions. Simultaneously, the step increase in bandwidth of chosen bands arising from PyH+ as well as OTf at the transition temperatures show that both these ions exhibit rotational mobility in high temperature phases.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2015.03.114. References [1] [2] [3] [4]

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