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Thermal Stability of Crystalline Complexes of Pyromellitic Dianhydride with Polycyclic Aromatic Hydrocarbons
Thermochimica Acta Elsevier Publishing Company, Amsterdam Printed in Belgium
THERMAL STABILITY OF CRYSTALLINE COMPLEXES OF PYROMELLITIC DIANHYDRIDE WITH POLYCYCLIC AROMATIC HYDROCARBONS F. PELIZZA, F. CASELLATO, and A. GIRELLI Stazione sperimentale per i Combustibili, 20097, San Donato Milanese (Italy)
(Received January 18th, 1972) ABSTRACT
The thermal stability of crystalline complexes formed by pyromellitic dianhydride (PMDA) with twenty polycyclic aromatic hydrocarbons was studied by differential scanning calorimetry in a closed system. All the complexes undergo congruent melting. The melt mantains the characteristic colour of the crystalline complex, thereby suggesting the survival of the chargetransfer interaction between PMDA and hydrocarbon. The melting process breaks down the crystal lattice, but does not cause a substantial "separation" of the two components of the complex. Melting temperature, enthalpy and entropy were determined. In a number of cases, reversible crystal transitions were observed. INTRODUCTION
Pyromellitic dianhydride (PMDA) forms charge-transfer (donor-acceptor) complexes with a number of polycyclic aromatic hydrocarbons. Evidence of this type of association was given by IR and UV-visible spectroscopy 1 • 2 • The stoichiometry of the complexes was determined by UV analysis. Complexing takes place between PMDA and the hydrocarbon with a 1:1 stoichiometry, except for the complexes with 1,2,5,6-dibenzoanthracene and 3,4,8,9dibenzopyrene, which have a PMDA:hydrocarbon stoichiometry of 2:1. The preparation of these complexes in the presence of a proper solvent is generally very easy. For a number of hydrocarbons, merely contact in the solid state with PMDA is sufficient to induce complexing. As in most cases the sublimation temperature of the hydrocarbon is lower than the melting point of the complex, and the thermal study was performed by differential scanning calorimetry in sealed pans. The present work was carried out to determine the temperature, enthalpy, and entropy of melting of the complexes. The calorimetric investigation was also aimed at ascertaining the possible occurrence of crystal transitions, previously observed by microscopy for complexes of polycyclic hydrocarbons with PMDA 1 and with other electron acceptors 3 • Thermochim. Acta, 4 (1972)
135
EXPERIMENTAL
Apparatus and procedure
A Perkin-Elmer Model 1B differential scanning calorimeter (DSC) was used for this work, operating between 30°C and the temperature of complete melting of the complex. Samples were placed in sealed aluminium pans for volatile materials. The enclosure surrounding the sealed pan was mantained under a dynamic nitrogen atmosphere throughout the experiments. The heating rate was chosen at 16 °C/min, and the sensitivity of the instrument was set at 4 mcalfs. The peak areas of the DSC curves were measured by a planimeter. The instrument was calibrated with reference to the melting temperature and enthalpy of an indium standard sample (m. p. 156 oc, AH = -6.79 calfg). The temperatures corresponding to each transition were determined using the criterium of peak maximum, corrected for thermal lag. A segment having a slope equal to that of the ascending side of the indium melting-peak was drawn from the maximum to the peak base line 4 as shown in Fig. 1. The temperature was then read at
1
.
~
. 0
l
l
!ll9l
•
_j_ 210
cl
b)
I)
220
230
JJ
-" 271.2\.
\
247.3
240
250
260
210
210 T'C
Fig. 1. DSC curves of PMDA complexes with (a) naphthalene (3.76 mg); (b) anthracene (4.07 mg); and (c) 1,2,3,4-dibenzanthracene (2.71 mg).
the point where the indium edge and the sample base line intersect. By this procedure, the reproducibility of the transition temperature was ± 0.2 °C. The values of the enthalpy and entropy of melting obtained by this technique on "pure grade" polycyclic hydrocarbons 5 were found to be in good agreement with the values available in the literature 6 • Calorimetry of PMDA-hydrocarbon complexes
Since complexes prepared by contact or suspension procedures are generally impure from excess PMDA or hydrocarbon, the present work was performed on stoichiometric complexes prepared by crystallization from dilute solutions of the components. Possible impurities adhering to the crystals were removed by repeated washings with ethyl ether; the purity of the complexes was checked by IR spectroscopy1'2. A weighted amount of finely ground complex (2-5 mg) was melted as 136
described above. Each sample was submitted to repeated heating-cooling cycles. Heating and cooling cycles were run at the same rate. After cooling to room temperature, the sample was maintained at this temperature until the completion of the recrystallization. The time lag for complete crystallization varied according to the complex tested. This procedure avoids pitfalls due to non-uniform size distribution of the crystals. Due to the practical impossibility of obtaining samples of crystalline complexes having identical granulometric characteristics, the DSC curves of the first melting cycle were not reproducible. With the sample recrystallized after melting in the pan, reproducible data were obtained within the accuracy limits of the instrument. RESULTS AND DISCUSSION
The values of enthalpy, entropy, and temperature of melting of the PMDAhydrocarbon complexes are given in Table I. The values of temperature and enthalpy TABLE I THERMODYNAMIC DATA FOR THE PMDA-HYDROCARBON COMPLEXES Hydrocarbon component of the complex
Ionization potential of the hydrocarbon
Thermodynamic constant of the complex Heat of fusion (kca/ mo/e- 1 )
Entropy of fusion (ca/mole- 1 oK- 1 )
248.1 295.3 303.2 297.3 274.3 284.0 350.0 290.4 288.5 192.8 250.2 270.6 293.1 218.0 235.0 275.8 271.2 218.8 247.3
9.5±0.1 9.5±0.2 10.0±0.2 10.5±0.2 13.9±0.2 14.5±0.4 17.0±0.1 17.2±0.1 17.5±0.4 17.7 ±0.1 19.7±0.2 21.2±0.2 21.3±0.3 22.1 ±0.3 22.4±0.1 22.9±0.4 25.7±0.3 27.6±0.1 28.8±0.4
18.1 ±0.2 16.7 ±0.4 17.5±0.3 18.4±0.3 24.2±0.2 26.0±0.8 27.4±0.2 30.5±0.2 31.1 ±0.8 37.2±0.1 37.5±0.3 38.9±0.4 37.6±0.6 44.9±0.2 44.1 ±0.4 41.7±0.8 47.3±0.6 56.1 ±0.3 55.4±0.8
273.8 275.8 310.8 395.2
8.5 8.2 41.0±0.3
15.6 15.0 70.1 ±0.5
(eV)
3,4,9,10-Dibenzopyrene I ,2,4,5-Dibenzopyrene I ,2,3,4-Dibenzopyrene 1,2,6, 7-Dibenzopyrene 1,2-Benzopyrene Tetracene 1,12-Benzoperylene Pyrene 3,4,8,9-Dibenzopyrene• Phenanthrene 3,4-Benzopyrene Triphenylene Perylene Naphthalene Chrysene 1,2, 7,8-Dibenzanthracene 1,2,3,4-Dibenzanthracene 1,2-Benzanthracene Anthracene 1,2,5,6-Dibenzanthracene Ia Ib
7.30 7.41 7.27 7.75 7.73 6.95 7.35 7.72 7.04 8.08 7.37 8.17 7.11 8.16 7.83 7.58 7.60 7.53 7.43 7.58
Coronene
7.65
n•
"PMDA:hydrocarbon stoichiometry of these complexes was 2:1. bDSC curve is irregular, the peak is broad and tailed. Thermochim. Acta, 4 (1972)
137
corresponding to crystal transitions are reported in Table II. Enthalpy and entropy values tabulated refer to moles of complex (PMDA +hydrocarbon). All the data represent the average of results obtained in a series offive tests; the standard deviation was evaluated on the mean value. TABLE II TEMPERATURE (T) AND ENTHALPY (AH) VALUES CORRESPONDING TO CRYSTAL TRANSITIONS OF PMDA-HYDROCARBON COMPLEXES Hydrocarbon component of the complex
Crystal transition Form II
Form I
Tetracene Perylene Pyrene 1,2-Benzopyrene 1,12-Benzoperylene 1,2,7,8-Dibenzanthracene I ,2,4,5-Dibenzopyrene I II 1,2,5,6-Dibenzanthracene Ia lb
T roc)
AH rkcal mo[e- 1 )"
T roc)
AH rkcal mot- 1 )
269.0 261.0 174.3 123.1 211.4 195.1
3.0 2.0 7.0 3.0 3.4 3.0
263.3
1.0
207.0 221.3
1.8 4.3
254.8 254.8
4.0 5.2
•
"±0.1 kcal mol- 1 •
The DSC curves for the crystalline PMDA-hydrocarbon complexes can be classified as follows: (I) curves showing one single peak, corresponding to melting of the complex, as shown in the examples in Fig. I ; and (2) curves showing peaks attributable to crystal transitions before melting (Fig. 2). A more thorough study of the crystal transitions was performed on the complexes of 1,2,5,6-dibenzoanthracene and 1,2,4,5-dibenzopyrene. The first complex was obtained in three different crystal forms 1 indicated by us as Ia, lb, and II. The forms Ia and lb have a 1:1 stoichiometry, form II has a PMDA:hydrocarbon stoichiometry of 2:1. As shown in Table I, form II has a much higher energy of melting than the other two forms. This confirms our previous observation that form II is the more stable one 7 • The complex of 1,2,4,5-dibenzopyrene crystallized in two different forms 2 , I and II. On heating, forms I and II underwent a crystal transition at 207 oc and 221.3 oc, respectively; both forms melted at 295.3 °C (Fig. 3). After cooling, repeated heating of the material gave identical DSC curves showing a crystal transition at 207 oc and melting at 295.3 oc. The DSC curves and the IR spectra are those of form I. This behaviour suggests the existence of a third crystal form, stable at temperatures between 207 oc and the melting temperature of the complex. 138
a)
260 b)
.., 0
.. c:
. 0 X
c)
l ~
200
j_
210
220
f)
290.4
meal ____ _ _ _ _190 ___ _ _ _ _290 _ _ _ _ Joo ___ _ _J _ _ s L_ ____ 110 180 280 rc 190
200
210 250
260
210
2so rc
Fig. 2. DSC curves of PMDA complexes with (a) tetracene (4.44 mg); (b) perylene (4.31 mg); (c) pyrene (4.27 mg); (d) 1,2-benzopyrene (4.09 mg); (e) 1,12-benzoperylene (4.74 mg); (f) 1,2,7,8dibenzanthracene (2.85 mg).
b)
,:[\, ~207 221.3
l
. . 0
"c 0
j
200
210
220
230
---A. 207
1
mgl 5
200
210
280
290
300T"C
-Jl
a)
220
230
280
290
300T"C
Fig. 3. DSC curves of polymorphic forms of the complex of PMDA with 1,2,4,5-dibenzopyrene: (a) form I (2.4 mg); (b) form II (4.0 mg); --melting;---- remelting.
Thermochim. Acta, 4 (1972)
139
The observed crystal transitions are not the only ones possible. In fact, such transitions most likely depend on the mutual orientation of the donor and the acceptor molecules during the formation of the complex. In different conditions of preparation, we have occasionally obtained polymorphic forms of the same complex 1 • 2 • 7 • As reported in the Experimental section, we have noticed that, in general, the behaviour of the complexes during the DSC tests does not depend on their thermal history. Therefore we infer that, if the escape of one of the complex components from the system is prevented, coupling of the two components of the complex survives in the melt. This conclusion is supported by the following considerations: (J) above the melting temperature, the base line does not show any noticeable drift attributable to true "decomplexing ", i.e. to dissociation of the donor-acceptor pairs; (2) no peaks attributable to the melting of either PMDA or hydrocarbon are evident; and (3) melting tests on a heated plate, in sealed capillary tubes, have shown that the characteristic colour of the complex (due to the charge transfer interaction) persists in the melted product. The value for the enthalpy of melting, therefore, essentially corresponds to the lattice energy of the crystalline complex. The energy of formation for charge transfer complexes of aromatic hydrocarbons with acceptors different from PMDA, is of the order of8 1-5 kcal mole- 1 • Since such values are much lower than the energy of melting of the crystalline complexes of PMDA (Table I), the possible contribution of the "energy of coupling" to the overall energy of formation of the crystalline complexes should be negligible, anyway. It is therefore obvious that no simple correlation can be observed between the thermodynamic data of the complexes and molecular parameters of the hydrocarbon (such as the ionization potential), which are usually related to the stability of the donor-acceptor association. ACKNOWLEDGEMENTS
This work was supported in part by the Italian Research Council (CNR). A scholarship to F. P. from "Fina Italiana" is gratefully acknowledged. The authors thank Prof. B. Casu for helpful discussion during the preparation of this paper. REFERENCES 1 2 3 4 5 6 7 8
F. Casellato, B. Casu, and A. Girelli, Chim. Ind. (Milan), 53 (1971) 753. F. Casellato, B. Casu, and A. Girelli, Chim. Ind. (Milan), 53 (1971) 909. A. Kofler Z. Elektrochem., 50 (1944) 200. Thermal Analysis Newsletters, Perkin-Elmer, No. 5, 1966. Unpublished results from this laboratory. Wen-Kuey Wong and E. F. Westrum, Jr., J. Chern. Thermodyn., 3 (1971) 105. F. Pelizza, F. Casellato, and A. Girelli, Chern. Ind., (1972) 42. R. Foster, Organic Charge-Transfer Complexes, Academic Press, New York, 1969, p. 210.
140
THERMAL STABILITY OF CRYSTALLINE COMPLEXES OF PYROMELLITIC DIANHYDRIDE WITH POLYCYCLIC AROMATIC HYDROCARBONS F. PELIZZA, F. CASELLATO, and A. GIRELLI Stazione sperimentale per i Combustibili, 20097, San Donato Milanese (Italy)
(Received January 18th, 1972) ABSTRACT
The thermal stability of crystalline complexes formed by pyromellitic dianhydride (PMDA) with twenty polycyclic aromatic hydrocarbons was studied by differential scanning calorimetry in a closed system. All the complexes undergo congruent melting. The melt mantains the characteristic colour of the crystalline complex, thereby suggesting the survival of the chargetransfer interaction between PMDA and hydrocarbon. The melting process breaks down the crystal lattice, but does not cause a substantial "separation" of the two components of the complex. Melting temperature, enthalpy and entropy were determined. In a number of cases, reversible crystal transitions were observed. INTRODUCTION
Pyromellitic dianhydride (PMDA) forms charge-transfer (donor-acceptor) complexes with a number of polycyclic aromatic hydrocarbons. Evidence of this type of association was given by IR and UV-visible spectroscopy 1 • 2 • The stoichiometry of the complexes was determined by UV analysis. Complexing takes place between PMDA and the hydrocarbon with a 1:1 stoichiometry, except for the complexes with 1,2,5,6-dibenzoanthracene and 3,4,8,9dibenzopyrene, which have a PMDA:hydrocarbon stoichiometry of 2:1. The preparation of these complexes in the presence of a proper solvent is generally very easy. For a number of hydrocarbons, merely contact in the solid state with PMDA is sufficient to induce complexing. As in most cases the sublimation temperature of the hydrocarbon is lower than the melting point of the complex, and the thermal study was performed by differential scanning calorimetry in sealed pans. The present work was carried out to determine the temperature, enthalpy, and entropy of melting of the complexes. The calorimetric investigation was also aimed at ascertaining the possible occurrence of crystal transitions, previously observed by microscopy for complexes of polycyclic hydrocarbons with PMDA 1 and with other electron acceptors 3 • Thermochim. Acta, 4 (1972)
135
EXPERIMENTAL
Apparatus and procedure
A Perkin-Elmer Model 1B differential scanning calorimeter (DSC) was used for this work, operating between 30°C and the temperature of complete melting of the complex. Samples were placed in sealed aluminium pans for volatile materials. The enclosure surrounding the sealed pan was mantained under a dynamic nitrogen atmosphere throughout the experiments. The heating rate was chosen at 16 °C/min, and the sensitivity of the instrument was set at 4 mcalfs. The peak areas of the DSC curves were measured by a planimeter. The instrument was calibrated with reference to the melting temperature and enthalpy of an indium standard sample (m. p. 156 oc, AH = -6.79 calfg). The temperatures corresponding to each transition were determined using the criterium of peak maximum, corrected for thermal lag. A segment having a slope equal to that of the ascending side of the indium melting-peak was drawn from the maximum to the peak base line 4 as shown in Fig. 1. The temperature was then read at
1
.
~
. 0
l
l
!ll9l
•
_j_ 210
cl
b)
I)
220
230
JJ
-" 271.2\.
\
247.3
240
250
260
210
210 T'C
Fig. 1. DSC curves of PMDA complexes with (a) naphthalene (3.76 mg); (b) anthracene (4.07 mg); and (c) 1,2,3,4-dibenzanthracene (2.71 mg).
the point where the indium edge and the sample base line intersect. By this procedure, the reproducibility of the transition temperature was ± 0.2 °C. The values of the enthalpy and entropy of melting obtained by this technique on "pure grade" polycyclic hydrocarbons 5 were found to be in good agreement with the values available in the literature 6 • Calorimetry of PMDA-hydrocarbon complexes
Since complexes prepared by contact or suspension procedures are generally impure from excess PMDA or hydrocarbon, the present work was performed on stoichiometric complexes prepared by crystallization from dilute solutions of the components. Possible impurities adhering to the crystals were removed by repeated washings with ethyl ether; the purity of the complexes was checked by IR spectroscopy1'2. A weighted amount of finely ground complex (2-5 mg) was melted as 136
described above. Each sample was submitted to repeated heating-cooling cycles. Heating and cooling cycles were run at the same rate. After cooling to room temperature, the sample was maintained at this temperature until the completion of the recrystallization. The time lag for complete crystallization varied according to the complex tested. This procedure avoids pitfalls due to non-uniform size distribution of the crystals. Due to the practical impossibility of obtaining samples of crystalline complexes having identical granulometric characteristics, the DSC curves of the first melting cycle were not reproducible. With the sample recrystallized after melting in the pan, reproducible data were obtained within the accuracy limits of the instrument. RESULTS AND DISCUSSION
The values of enthalpy, entropy, and temperature of melting of the PMDAhydrocarbon complexes are given in Table I. The values of temperature and enthalpy TABLE I THERMODYNAMIC DATA FOR THE PMDA-HYDROCARBON COMPLEXES Hydrocarbon component of the complex
Ionization potential of the hydrocarbon
Thermodynamic constant of the complex Heat of fusion (kca/ mo/e- 1 )
Entropy of fusion (ca/mole- 1 oK- 1 )
248.1 295.3 303.2 297.3 274.3 284.0 350.0 290.4 288.5 192.8 250.2 270.6 293.1 218.0 235.0 275.8 271.2 218.8 247.3
9.5±0.1 9.5±0.2 10.0±0.2 10.5±0.2 13.9±0.2 14.5±0.4 17.0±0.1 17.2±0.1 17.5±0.4 17.7 ±0.1 19.7±0.2 21.2±0.2 21.3±0.3 22.1 ±0.3 22.4±0.1 22.9±0.4 25.7±0.3 27.6±0.1 28.8±0.4
18.1 ±0.2 16.7 ±0.4 17.5±0.3 18.4±0.3 24.2±0.2 26.0±0.8 27.4±0.2 30.5±0.2 31.1 ±0.8 37.2±0.1 37.5±0.3 38.9±0.4 37.6±0.6 44.9±0.2 44.1 ±0.4 41.7±0.8 47.3±0.6 56.1 ±0.3 55.4±0.8
273.8 275.8 310.8 395.2
8.5 8.2 41.0±0.3
15.6 15.0 70.1 ±0.5
(eV)
3,4,9,10-Dibenzopyrene I ,2,4,5-Dibenzopyrene I ,2,3,4-Dibenzopyrene 1,2,6, 7-Dibenzopyrene 1,2-Benzopyrene Tetracene 1,12-Benzoperylene Pyrene 3,4,8,9-Dibenzopyrene• Phenanthrene 3,4-Benzopyrene Triphenylene Perylene Naphthalene Chrysene 1,2, 7,8-Dibenzanthracene 1,2,3,4-Dibenzanthracene 1,2-Benzanthracene Anthracene 1,2,5,6-Dibenzanthracene Ia Ib
7.30 7.41 7.27 7.75 7.73 6.95 7.35 7.72 7.04 8.08 7.37 8.17 7.11 8.16 7.83 7.58 7.60 7.53 7.43 7.58
Coronene
7.65
n•
"PMDA:hydrocarbon stoichiometry of these complexes was 2:1. bDSC curve is irregular, the peak is broad and tailed. Thermochim. Acta, 4 (1972)
137
corresponding to crystal transitions are reported in Table II. Enthalpy and entropy values tabulated refer to moles of complex (PMDA +hydrocarbon). All the data represent the average of results obtained in a series offive tests; the standard deviation was evaluated on the mean value. TABLE II TEMPERATURE (T) AND ENTHALPY (AH) VALUES CORRESPONDING TO CRYSTAL TRANSITIONS OF PMDA-HYDROCARBON COMPLEXES Hydrocarbon component of the complex
Crystal transition Form II
Form I
Tetracene Perylene Pyrene 1,2-Benzopyrene 1,12-Benzoperylene 1,2,7,8-Dibenzanthracene I ,2,4,5-Dibenzopyrene I II 1,2,5,6-Dibenzanthracene Ia lb
T roc)
AH rkcal mo[e- 1 )"
T roc)
AH rkcal mot- 1 )
269.0 261.0 174.3 123.1 211.4 195.1
3.0 2.0 7.0 3.0 3.4 3.0
263.3
1.0
207.0 221.3
1.8 4.3
254.8 254.8
4.0 5.2
•
"±0.1 kcal mol- 1 •
The DSC curves for the crystalline PMDA-hydrocarbon complexes can be classified as follows: (I) curves showing one single peak, corresponding to melting of the complex, as shown in the examples in Fig. I ; and (2) curves showing peaks attributable to crystal transitions before melting (Fig. 2). A more thorough study of the crystal transitions was performed on the complexes of 1,2,5,6-dibenzoanthracene and 1,2,4,5-dibenzopyrene. The first complex was obtained in three different crystal forms 1 indicated by us as Ia, lb, and II. The forms Ia and lb have a 1:1 stoichiometry, form II has a PMDA:hydrocarbon stoichiometry of 2:1. As shown in Table I, form II has a much higher energy of melting than the other two forms. This confirms our previous observation that form II is the more stable one 7 • The complex of 1,2,4,5-dibenzopyrene crystallized in two different forms 2 , I and II. On heating, forms I and II underwent a crystal transition at 207 oc and 221.3 oc, respectively; both forms melted at 295.3 °C (Fig. 3). After cooling, repeated heating of the material gave identical DSC curves showing a crystal transition at 207 oc and melting at 295.3 oc. The DSC curves and the IR spectra are those of form I. This behaviour suggests the existence of a third crystal form, stable at temperatures between 207 oc and the melting temperature of the complex. 138
a)
260 b)
.., 0
.. c:
. 0 X
c)
l ~
200
j_
210
220
f)
290.4
meal ____ _ _ _ _190 ___ _ _ _ _290 _ _ _ _ Joo ___ _ _J _ _ s L_ ____ 110 180 280 rc 190
200
210 250
260
210
2so rc
Fig. 2. DSC curves of PMDA complexes with (a) tetracene (4.44 mg); (b) perylene (4.31 mg); (c) pyrene (4.27 mg); (d) 1,2-benzopyrene (4.09 mg); (e) 1,12-benzoperylene (4.74 mg); (f) 1,2,7,8dibenzanthracene (2.85 mg).
b)
,:[\, ~207 221.3
l
. . 0
"c 0
j
200
210
220
230
---A. 207
1
mgl 5
200
210
280
290
300T"C
-Jl
a)
220
230
280
290
300T"C
Fig. 3. DSC curves of polymorphic forms of the complex of PMDA with 1,2,4,5-dibenzopyrene: (a) form I (2.4 mg); (b) form II (4.0 mg); --melting;---- remelting.
Thermochim. Acta, 4 (1972)
139
The observed crystal transitions are not the only ones possible. In fact, such transitions most likely depend on the mutual orientation of the donor and the acceptor molecules during the formation of the complex. In different conditions of preparation, we have occasionally obtained polymorphic forms of the same complex 1 • 2 • 7 • As reported in the Experimental section, we have noticed that, in general, the behaviour of the complexes during the DSC tests does not depend on their thermal history. Therefore we infer that, if the escape of one of the complex components from the system is prevented, coupling of the two components of the complex survives in the melt. This conclusion is supported by the following considerations: (J) above the melting temperature, the base line does not show any noticeable drift attributable to true "decomplexing ", i.e. to dissociation of the donor-acceptor pairs; (2) no peaks attributable to the melting of either PMDA or hydrocarbon are evident; and (3) melting tests on a heated plate, in sealed capillary tubes, have shown that the characteristic colour of the complex (due to the charge transfer interaction) persists in the melted product. The value for the enthalpy of melting, therefore, essentially corresponds to the lattice energy of the crystalline complex. The energy of formation for charge transfer complexes of aromatic hydrocarbons with acceptors different from PMDA, is of the order of8 1-5 kcal mole- 1 • Since such values are much lower than the energy of melting of the crystalline complexes of PMDA (Table I), the possible contribution of the "energy of coupling" to the overall energy of formation of the crystalline complexes should be negligible, anyway. It is therefore obvious that no simple correlation can be observed between the thermodynamic data of the complexes and molecular parameters of the hydrocarbon (such as the ionization potential), which are usually related to the stability of the donor-acceptor association. ACKNOWLEDGEMENTS
This work was supported in part by the Italian Research Council (CNR). A scholarship to F. P. from "Fina Italiana" is gratefully acknowledged. The authors thank Prof. B. Casu for helpful discussion during the preparation of this paper. REFERENCES 1 2 3 4 5 6 7 8
F. Casellato, B. Casu, and A. Girelli, Chim. Ind. (Milan), 53 (1971) 753. F. Casellato, B. Casu, and A. Girelli, Chim. Ind. (Milan), 53 (1971) 909. A. Kofler Z. Elektrochem., 50 (1944) 200. Thermal Analysis Newsletters, Perkin-Elmer, No. 5, 1966. Unpublished results from this laboratory. Wen-Kuey Wong and E. F. Westrum, Jr., J. Chern. Thermodyn., 3 (1971) 105. F. Pelizza, F. Casellato, and A. Girelli, Chern. Ind., (1972) 42. R. Foster, Organic Charge-Transfer Complexes, Academic Press, New York, 1969, p. 210.
140