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Vibrational spectra and the phase transition in 4-aminopyridine hemiperchlorate
Journal of Molecular Structure, 178 (1988) 23-28 Elsevier Science Publishers B.V.. Amsterdam - Printed
23 in The Netherlands
VIBRATIONAL SPECTRA AND THE PHASE TRANSITION 4-AMINOPYRIDINE HEMIPERCHLORATE
E. GRECH, Z. MALARSKI
IN
and L. SOBCZYK
Institute of Chemistry, University of Wroctaw, 50 383 Wroclaw (Poland) J. POTIER
and J. ROZIBRE
Laboratoire des Acides Minkraux USTL, 34060 Montpellier CCdex (France) (Received 9 June 1987)
ABSTRACT The IR and Raman spectra of monocrystals and polycrystalline samples of 4-aminopyridine hemiperchlorate have been studied at various temperatures. The phase transition at 290 K is revealed in the behaviour of NH, stretching vibration bands, low frequency vibrations below 130 cm-’ and broad IR absorption in the fingerprint region. The results confirm that the transition is related to a freezing of rotational motion of ClO,- anions which are responsible for a smoothing of the discrete low frequency branch of the Raman spectrum in the (Yphase. As a rule, this phase undergoes supercooling down to liquid nitrogen temperature.
INTRODUCTION
In the studies of homoconjugated cations NHN+ in the solid state, special attention was paid to crystals of 4-aminopyridine hemiperchlorate ( 1) in which asymmetric [NH.. .N] + bridges of length 2.69 A are present [ 11.These bridges are characterized in the IR spectrum by a doublet in the 2000-2400 cm-’ range, and in some cases by a broad absorption in the fingerprint region which is sensitive to temperature. Crystals of 1 grown at room temperature and quickly cooled show a temperature effect consisting of an increase in absorption below 800 cm-l on cooling. However, this effect only appears when the sample is cooled quickly. If a polycrystalline sample is cooled slowly or a crystal grown below 290 K is used, such an effect is not observed. It has been found that 1 undergoes a first order phase transition /%-+a at 290 K. On the grounds of crystallographic studies performed up to now [1,2] it seems that the main difference between these phases lies in freedom of rotational motion of Clodanions. In the high temperature Q!phase the oxygen atoms of the perchlorate anions exhibit very large isotropic temperature factors, whereas the /? phase contains well-ordered tetrahedral perchlorate groups. As far as the structure of the cation shown in Fig. 1 is concerned, it does not undergo any marked
0022-2860/88/$03.50
0 1988 Elsevier Science Publishers
B.V.
24
Fig. 1. Schematic structure of the 4-aminopyridine
homoconjugated cation.
changes after the phase transition. The pyridine rings are arranged nearly mutually perpendicularly and the proton is localized at ring A, although the N+H bond is markedly elongated (1.17 A); also, the NHN bridge length is the same in the a! andpphases. In the present contribution we wish to discuss the vibrational spectra of both phases. We thought that a more detailed analysis would enable us to clarify the origin of the “anomalous” temperature effect observed in a number of salts containing homoconjugated NHN+ cations. If hindering of rotational motion of ClO*- anions had such a remarkable influence on the temperature effect in the infrared, one could suspect that this phenomenon is related to the lattice dynamics. Thus, it seemed desirable to study the low frequency spectra. EXPERIMENTAL
The IR spectra of polycrystalline samples were studied as Nujol or hexachlorobutadiene mulls as well as in KBr discs on a Perkin Elmer 180 spectrophotometer. The Raman spectra have been measured using both polycrystalline samples and single crystals on a Dilor spectrometer linked to a Tracer TN 1710 modular multichannel computer (Northern Instruments) with a 4880 A line of a Spectra Physics argon laser. 4-Aminopyridine hemiperchlorate was obtained by crystallization from an acetonitrile solution of the 1:l salt and an excess of amine. Monocrystals were grown from acetonitrile by slow evaporation of the solvent at 295 K (cy phase) and 277 K (/3 phase). RESULTS AND DISCUSSION
As far as the internal vibrations of the pyridine ring and the behaviour of the hydrogen bond are concerned, no visible differences between the phases are observed. Moreover, there are no marked changes in the vibrational spectra with respect to a free 4-aminopyridine molecule. This relates both to IR and Raman spectra. However, remarkable differences appear in the range of NH, vibrations of the amino groups. This is due both to the n-electron conjugation and to interaction with Clod- anions. In the IR spectra, in addition to the frequency shift of the NH2 vibration bands, we found a splitting of both the symmetric and asymmetric u(NH,) absorption bands as shown in Fig. 2. It was assumed [l] that the splitting is due to a non-equivalence of the moieties of 4-aminopyridine homoconjugated cation associated with an asymmetric
25
%T
i a
b
3200
3300
3LOO
3500 cm-1
Fig. 2. Comparison of IR spectra in the range of NH, stretching bands: (a) (Yphase (---) (-) supercooled to 93 K; (b) fi phase (---) 290 K, (-_) 93 K.
300 K,
proton density distribution within the bridge. However, one cannot neglect the second factor affecting the behaviour of NH2 groups, i.e. the environmental effect. In the case of the cr phase, neutron diffraction studies [ 11 indicate that the NH, groups attached to the pyridine ring B are packed closer to the oxygen atoms than are those attached to ring A. Hence, in the (x phase, the v,(NH,) bands ascribed to the B ring are more distinctly shifted towards the long wavelength range and possess higher intensity. In Raman spectra the intensity ratio, in agreement with expectation, is the opposite. In the p phase, close to the phase transition temperature, the high frequency component disappears and a single band of enhanced intensity occurs. We think that as a result of the compensation of internal and external factors, overlapping of the bands ascribed to A and B rings takes place. Of some importance seems also to be the influence of a non-symmetric interaction between NH2 groups and ClO,- anions on coupling between both N-H bonds. Further decrease of temperature again causes the appearance of splitting and at low temperatures we observe two sharp peaks separated by 21 cm-l. This splitting can just be regarded as originating from the two different terminal NH:, groups. The evolution of changes taking place in the range of v,(NH,) symmetric vibrations is shown in Fig. 3. One should point out the intensity changes of the components in the transition point and the changes in the band widths. In the case of the supercooled (Y phase no change in the position of the v, (NH,) band is observed on cooling but only better separation of peaks and
26
-
a
3462
3f62
b
c
31
d
e
\
3, cm”
Fig. 3. Evolution of the V,(NH,) vibration band shape and the a++/3 phase transition: (a) 316 K; (b) 292 K; (c) 288 K; (d) 210 K; (e) 180 K. In (b) the evolution of the spectrum close to the transition temperature is shown.
200
600
1000
. 1200
3000
Fig. 4. F&man spectra of (a) Band (b ) CYphases of polycrystalline
3400
cm'
4-aminopyridine
hemiperchlorate.
a decrease in the band half-widths. However, for the v,,(NH,) doublet we observe a continuous decrease in the distance between the peaks while cooling. The most significant differences in the Raman spectra between the a! and j3 phases occur in the region of lattice vibrations (see Fig. 4). Firstly, in the LY phase some sort of disorder affecting the ClO,- groups is manifested. The presence of rotating groups causes a distinct smoothing of the discrete character of the spectrum below 120 cm-l. Such an effect has already been observed by Novak and co-workers [ 31 in triammonium hydrogen disulphate. In the p
21
%T i
400
800
1200
1600
2000
3000
Cm-l
I
Fig. 5. Temperature dependence of the IR spectrum of (a) the j3phase at 80 K (---_) (---); (b) the CYphase at 80 K (---) and 300 K (---).
and 280 K
phase an intense band observed at 108 cm-‘, which can be assigned to intermolecular Clod- vibrations, disappears in the cx phase. In the Raman spectra recorded on single a! phase at various orientations one could definitely reveal only two frequencies: 95 and 38 cm-l. Because of low symmetry (space group P2,/n) and birefringence of the crystal a thorough analysis of the polarized spectrum was not possible. However, we can state that only a very weak line at 122 cm-’ can be assigned to the N*. .N bridge vibrations [ 41. This very weak line is also observed in the /? phase as a shoulder on the intense band at 108 cm-‘. On the other hand, the bridge vibrations are clear in the IR spectrum as an intense, broad band at ca. 125 cm-’ both in the p and CYphases without any changes. The disorder related to the motion of ClO,- anions in the (x phase causes field fluctuations which can be responsible for the strange behaviour of this phase. Thus, fast fluctuations of the electric field gradient were observed in the 14NNQR spectra [ 51. In the case of the p phase (after slow cooling of the sample in the cyphase) one observes four sets of signals corresponding to four non-equivalent nitrogen atoms. However, when the sample is cooled quickly to the supercooled phase the frequencies ascribed to the B ring nitrogen atoms are missing. This indicates some disorder in the lattice and can be directly related to the behaviour of the low frequency Raman spectrum. The possible fluctuations of the electric field gradients can proceed via the interaction of
28
anions with NH2 groups and a transfer through the conjugated n-electron system. The difference in the temperature effect on the absorption intensity below 800 cm-l between the a! and /3 phases can also be related to the different behaviour of the the doublet at 2000-2400 cm-’ (compare Figs. 5(a) and (b)). This doublet, which is due to the us (NH+ *. .N ) vibrations, is more sensitive to temperature for the LYphase than for the /3 phase. It implies that at low temperatures the NHN bridge in the ctlphase becomes shorter than in the pphase. We have used this interpretation to explain the temperature effect on the broad protonic band in homoconjugated NHN systems [ 61. However, in the light of peculiarities in the low frequency vibrations and the possible overdamping of N+**N bridge vibrations due to interaction with the environment, an explanation could also be based on the concept of Sokolov [ 71 and Sakun [8] which interprets the shaping and broadening mechanism of protonic absorption bands in terms of the coupling of bridge vibrations with phonons.
REFERENCES 1 J. Roziere, J. M. Williams, E. Grech, Z. Malarski and L. Sobczyk, J. Chem. Phys., 72 (1980) 6117. 2 P. Teulon, R. Delaplane, I. Olovsson and J. Roziere, Acta Crystallogr., Sect. C, 41 (1985) 479. 3 M. Damak, M. Kamoun, A. Daoud, F. Romain, A. Lautie and A. Novak, J. Mol. Struct., 130 (1985) 245. 4 R. L. Dean and J. L. Wood, J. Mol. Struct., 26 (1975) 197,215. 5 S. Greenbaum, personal communication. 6 E. Grech, Z. Malarski, H. Romanowski and L. Sobczyk, J. Mol. Struct., 47 (1978) 317. 7 N. D. Sokolov, Croat. Chem. Acta, 55 (1982) 223. 8 V. P. Sakun, Chem. Phys., 55 (1981) 27.
23 in The Netherlands
VIBRATIONAL SPECTRA AND THE PHASE TRANSITION 4-AMINOPYRIDINE HEMIPERCHLORATE
E. GRECH, Z. MALARSKI
IN
and L. SOBCZYK
Institute of Chemistry, University of Wroctaw, 50 383 Wroclaw (Poland) J. POTIER
and J. ROZIBRE
Laboratoire des Acides Minkraux USTL, 34060 Montpellier CCdex (France) (Received 9 June 1987)
ABSTRACT The IR and Raman spectra of monocrystals and polycrystalline samples of 4-aminopyridine hemiperchlorate have been studied at various temperatures. The phase transition at 290 K is revealed in the behaviour of NH, stretching vibration bands, low frequency vibrations below 130 cm-’ and broad IR absorption in the fingerprint region. The results confirm that the transition is related to a freezing of rotational motion of ClO,- anions which are responsible for a smoothing of the discrete low frequency branch of the Raman spectrum in the (Yphase. As a rule, this phase undergoes supercooling down to liquid nitrogen temperature.
INTRODUCTION
In the studies of homoconjugated cations NHN+ in the solid state, special attention was paid to crystals of 4-aminopyridine hemiperchlorate ( 1) in which asymmetric [NH.. .N] + bridges of length 2.69 A are present [ 11.These bridges are characterized in the IR spectrum by a doublet in the 2000-2400 cm-’ range, and in some cases by a broad absorption in the fingerprint region which is sensitive to temperature. Crystals of 1 grown at room temperature and quickly cooled show a temperature effect consisting of an increase in absorption below 800 cm-l on cooling. However, this effect only appears when the sample is cooled quickly. If a polycrystalline sample is cooled slowly or a crystal grown below 290 K is used, such an effect is not observed. It has been found that 1 undergoes a first order phase transition /%-+a at 290 K. On the grounds of crystallographic studies performed up to now [1,2] it seems that the main difference between these phases lies in freedom of rotational motion of Clodanions. In the high temperature Q!phase the oxygen atoms of the perchlorate anions exhibit very large isotropic temperature factors, whereas the /? phase contains well-ordered tetrahedral perchlorate groups. As far as the structure of the cation shown in Fig. 1 is concerned, it does not undergo any marked
0022-2860/88/$03.50
0 1988 Elsevier Science Publishers
B.V.
24
Fig. 1. Schematic structure of the 4-aminopyridine
homoconjugated cation.
changes after the phase transition. The pyridine rings are arranged nearly mutually perpendicularly and the proton is localized at ring A, although the N+H bond is markedly elongated (1.17 A); also, the NHN bridge length is the same in the a! andpphases. In the present contribution we wish to discuss the vibrational spectra of both phases. We thought that a more detailed analysis would enable us to clarify the origin of the “anomalous” temperature effect observed in a number of salts containing homoconjugated NHN+ cations. If hindering of rotational motion of ClO*- anions had such a remarkable influence on the temperature effect in the infrared, one could suspect that this phenomenon is related to the lattice dynamics. Thus, it seemed desirable to study the low frequency spectra. EXPERIMENTAL
The IR spectra of polycrystalline samples were studied as Nujol or hexachlorobutadiene mulls as well as in KBr discs on a Perkin Elmer 180 spectrophotometer. The Raman spectra have been measured using both polycrystalline samples and single crystals on a Dilor spectrometer linked to a Tracer TN 1710 modular multichannel computer (Northern Instruments) with a 4880 A line of a Spectra Physics argon laser. 4-Aminopyridine hemiperchlorate was obtained by crystallization from an acetonitrile solution of the 1:l salt and an excess of amine. Monocrystals were grown from acetonitrile by slow evaporation of the solvent at 295 K (cy phase) and 277 K (/3 phase). RESULTS AND DISCUSSION
As far as the internal vibrations of the pyridine ring and the behaviour of the hydrogen bond are concerned, no visible differences between the phases are observed. Moreover, there are no marked changes in the vibrational spectra with respect to a free 4-aminopyridine molecule. This relates both to IR and Raman spectra. However, remarkable differences appear in the range of NH, vibrations of the amino groups. This is due both to the n-electron conjugation and to interaction with Clod- anions. In the IR spectra, in addition to the frequency shift of the NH2 vibration bands, we found a splitting of both the symmetric and asymmetric u(NH,) absorption bands as shown in Fig. 2. It was assumed [l] that the splitting is due to a non-equivalence of the moieties of 4-aminopyridine homoconjugated cation associated with an asymmetric
25
%T
i a
b
3200
3300
3LOO
3500 cm-1
Fig. 2. Comparison of IR spectra in the range of NH, stretching bands: (a) (Yphase (---) (-) supercooled to 93 K; (b) fi phase (---) 290 K, (-_) 93 K.
300 K,
proton density distribution within the bridge. However, one cannot neglect the second factor affecting the behaviour of NH2 groups, i.e. the environmental effect. In the case of the cr phase, neutron diffraction studies [ 11 indicate that the NH, groups attached to the pyridine ring B are packed closer to the oxygen atoms than are those attached to ring A. Hence, in the (x phase, the v,(NH,) bands ascribed to the B ring are more distinctly shifted towards the long wavelength range and possess higher intensity. In Raman spectra the intensity ratio, in agreement with expectation, is the opposite. In the p phase, close to the phase transition temperature, the high frequency component disappears and a single band of enhanced intensity occurs. We think that as a result of the compensation of internal and external factors, overlapping of the bands ascribed to A and B rings takes place. Of some importance seems also to be the influence of a non-symmetric interaction between NH2 groups and ClO,- anions on coupling between both N-H bonds. Further decrease of temperature again causes the appearance of splitting and at low temperatures we observe two sharp peaks separated by 21 cm-l. This splitting can just be regarded as originating from the two different terminal NH:, groups. The evolution of changes taking place in the range of v,(NH,) symmetric vibrations is shown in Fig. 3. One should point out the intensity changes of the components in the transition point and the changes in the band widths. In the case of the supercooled (Y phase no change in the position of the v, (NH,) band is observed on cooling but only better separation of peaks and
26
-
a
3462
3f62
b
c
31
d
e
\
3, cm”
Fig. 3. Evolution of the V,(NH,) vibration band shape and the a++/3 phase transition: (a) 316 K; (b) 292 K; (c) 288 K; (d) 210 K; (e) 180 K. In (b) the evolution of the spectrum close to the transition temperature is shown.
200
600
1000
. 1200
3000
Fig. 4. F&man spectra of (a) Band (b ) CYphases of polycrystalline
3400
cm'
4-aminopyridine
hemiperchlorate.
a decrease in the band half-widths. However, for the v,,(NH,) doublet we observe a continuous decrease in the distance between the peaks while cooling. The most significant differences in the Raman spectra between the a! and j3 phases occur in the region of lattice vibrations (see Fig. 4). Firstly, in the LY phase some sort of disorder affecting the ClO,- groups is manifested. The presence of rotating groups causes a distinct smoothing of the discrete character of the spectrum below 120 cm-l. Such an effect has already been observed by Novak and co-workers [ 31 in triammonium hydrogen disulphate. In the p
21
%T i
400
800
1200
1600
2000
3000
Cm-l
I
Fig. 5. Temperature dependence of the IR spectrum of (a) the j3phase at 80 K (---_) (---); (b) the CYphase at 80 K (---) and 300 K (---).
and 280 K
phase an intense band observed at 108 cm-‘, which can be assigned to intermolecular Clod- vibrations, disappears in the cx phase. In the Raman spectra recorded on single a! phase at various orientations one could definitely reveal only two frequencies: 95 and 38 cm-l. Because of low symmetry (space group P2,/n) and birefringence of the crystal a thorough analysis of the polarized spectrum was not possible. However, we can state that only a very weak line at 122 cm-’ can be assigned to the N*. .N bridge vibrations [ 41. This very weak line is also observed in the /? phase as a shoulder on the intense band at 108 cm-‘. On the other hand, the bridge vibrations are clear in the IR spectrum as an intense, broad band at ca. 125 cm-’ both in the p and CYphases without any changes. The disorder related to the motion of ClO,- anions in the (x phase causes field fluctuations which can be responsible for the strange behaviour of this phase. Thus, fast fluctuations of the electric field gradient were observed in the 14NNQR spectra [ 51. In the case of the p phase (after slow cooling of the sample in the cyphase) one observes four sets of signals corresponding to four non-equivalent nitrogen atoms. However, when the sample is cooled quickly to the supercooled phase the frequencies ascribed to the B ring nitrogen atoms are missing. This indicates some disorder in the lattice and can be directly related to the behaviour of the low frequency Raman spectrum. The possible fluctuations of the electric field gradients can proceed via the interaction of
28
anions with NH2 groups and a transfer through the conjugated n-electron system. The difference in the temperature effect on the absorption intensity below 800 cm-l between the a! and /3 phases can also be related to the different behaviour of the the doublet at 2000-2400 cm-’ (compare Figs. 5(a) and (b)). This doublet, which is due to the us (NH+ *. .N ) vibrations, is more sensitive to temperature for the LYphase than for the /3 phase. It implies that at low temperatures the NHN bridge in the ctlphase becomes shorter than in the pphase. We have used this interpretation to explain the temperature effect on the broad protonic band in homoconjugated NHN systems [ 61. However, in the light of peculiarities in the low frequency vibrations and the possible overdamping of N+**N bridge vibrations due to interaction with the environment, an explanation could also be based on the concept of Sokolov [ 71 and Sakun [8] which interprets the shaping and broadening mechanism of protonic absorption bands in terms of the coupling of bridge vibrations with phonons.
REFERENCES 1 J. Roziere, J. M. Williams, E. Grech, Z. Malarski and L. Sobczyk, J. Chem. Phys., 72 (1980) 6117. 2 P. Teulon, R. Delaplane, I. Olovsson and J. Roziere, Acta Crystallogr., Sect. C, 41 (1985) 479. 3 M. Damak, M. Kamoun, A. Daoud, F. Romain, A. Lautie and A. Novak, J. Mol. Struct., 130 (1985) 245. 4 R. L. Dean and J. L. Wood, J. Mol. Struct., 26 (1975) 197,215. 5 S. Greenbaum, personal communication. 6 E. Grech, Z. Malarski, H. Romanowski and L. Sobczyk, J. Mol. Struct., 47 (1978) 317. 7 N. D. Sokolov, Croat. Chem. Acta, 55 (1982) 223. 8 V. P. Sakun, Chem. Phys., 55 (1981) 27.