ARTICLE pubs.acs.org/JPCC
Enthalpies of Formation and Reaction of Two PCBM Fullerene Derivatives Melchor Martínez-Herrera and Aaron Rojas* Departamento de Química, Centro de Investigacion y de Estudios Avanzados del IPN. Av. Instituto Politecnico Nacional 2508, Mexico D.F. C.P. 07360, Mexico
bS Supporting Information ABSTRACT: Although thermochemical information is useful for understanding the structural characteristics of fullerenes, experimental data of standard enthalpy of formation and reaction of these fascinating carbon cages remain undetermined. As a contribution to the filling of this gap, the standard molar enthalpies of formation at the condensed state, ΔfHm(cr), of the methanofullerene derivatives [6,6]-phenyl C61 butyric acid methyl ester (60PCBM) and [6,6]phenyl C71 butyric acid methyl ester (70PCBM) were determined to be 2055.2 ( 13.7 and 2238.4 ( 19.2 kJ 3 mol-1, respectively. These values were calculated based on the energy of combustion, which was measured by microcombustion calorimetry. From these values and the standard molar enthalpies of formation of C60 and C70 , the enthalpic contribution of the functionalization of both fullerenes was calculated as Δ func H m [C 60 (cr) f 60PCBM(cr)] = -237.9 kJ 3 mol-1 and ΔfuncHm [C70(cr) f 70PCBM(cr)] = -299.1 kJ 3 mol-1. Meanwhile, the standard enthalpies associated with the functionalization reaction are ΔrHm [C60(cr) f 60PCBM(cr)] = 27.3 kJ 3 mol-1 and ΔrHm [C70(cr) f 70PCBM(cr)] = -33.8 kJ 3 mol-1. These results show that the functionalization of C70 to obtain 70PCBM produces a greater enthalpic change than the functionalization of C60. Thus, the formation of 70PCBM is favored over 60PCBM by approximately 61 kJ 3 mol-1. Theoretical gas-phase enthalpies of formation of 60PCBM and 70PCBM were derived from the enthalpies of reaction, computed at the B3LYP/3-21G*, SVWN5/6-31G, and B3LYP/6-31G* level of theory, and compared with experimental values.
’ INTRODUCTION The interesting chemical and physical properties of fullerenes C60 and C70, the most representative among these clusters, offer high potential in many different fields, particularly in biological chemistry and in the science of materials.1 These applications, however, have been severely hampered because of the wellknown lack of solubility of these fullerenes in polar solvents. About 20 years ago, Wudl and co-workers synthesized the first methanofullerene by reacting C60 with diphenyldiazomethane to produce a bridged adduct, C61Ph2.2 Later, the reaction was also shown to proceed with functional groups on the phenyl rings of the diphenyldiazomethane, opening the possibility to produce functionalized fullerene derivatives.3 The chemical functionalization of fullerenes not only was a solution to the solubility problem but also enabled the properties of fullerenes to be combined with those of other kinds of materials. Examples include the generation of molecules with a high potential as electron acceptors in solar cells, use as building blocks for the syntheses of highly water-soluble dendrimers, and use as a basis for designing symmetrical and stereochemically defined oligoadducts with promising biological and material properties.4-6 In the search for a soluble fullerene for anti-HIV treatment, Wudl and co-workers synthesized the compound [6,6]-phenylC61-butyric acid methyl ester, 60PCBM.7 Although its use as an r 2010 American Chemical Society
anti-HIV treatment has not fully materialized, 60PCBM would later become the fullerene derivative most widely used as an electron acceptor in solar cells, field-effect transistors, and photodetectors. This compound showed superior performance over other systems.8 The functionalization of C70 with the same addend as in 60PCBM enabled the production of [6,6]-phenyl-C71-butyric acid methyl ester (70PCBM), which resulted in a better electron acceptor in the field of molecular electronics and improved on the performance of 60PCBM.8e,9 Although the research in this field has expanded rapidly in different directions, many properties of the fullerenes and functionalized fullerenes remain unknown, such as their thermochemical properties. These data are useful and important for understanding the structural characteristics of these carbon cages. For example, in a previous study on the stability of fullerenes, the experimental standard enthalpy of formation data revealed that these carbon allotropes become more stable as they become larger.10a Additionally, a study on a methanofullerene derivative demonstrated that the direct insertion of a functional group on the sphere of the cluster is an exothermic process and Received: September 10, 2010 Revised: December 6, 2010 Published: December 20, 2010 1541
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Table 1. Mass and energetic quantities for representative experiments of combustion of the PCBM derivatives at T = 303.15 K (po= 0.1 MPa) a
Figure 1. Molecular structures of the PCBM fullerene derivatives studied by microcombustion calorimetry. [6,6]-phenyl-C61 butyric acid methyl ester (60PCBM) and [6,6]-phenyl-C71 butyric acid methyl ester (70PCBM).
produces a more stable system than cluster and functional group separately. To provide continuity to that work and to determine if the insertion of an identical functional group releases energy independently of the fullerene involved, a thermochemical study was performed to derive the standard enthalpies of formation and reaction of the fullerene derivatives 60PCBM and 70PCBM (Figure 1). The experimental part of this research involved measuring the standard molar energy of combustion and deriving the standard molar enthalpies of combustion and formation of the fullerene derivatives 60PCBM and 70PCBM. These experiments utilized a set of microcombustion bombs associated with a Calvet-type heat-flux microcalorimeter and sample amounts of approximately two milligrams per combustion experiment. This experimental methodology has already been successfully applied to measure the energy of combustion and to derive the enthalpy of formation of other fullerenes.10 From the resulting standard enthalpies of formation of the PCBM derivatives, the enthalpies of the functionalization reaction and the enthalpic contribution of the substituent to the enthalpy of formation of each functionalized fullerene were calculated. Theoretical enthalpies of reaction of C60 and C70 as well as the gas phase enthalpies of formation of PCBMs were determined by DFT at various levels of theory yielding results in concordance with experimental data.
’ RESULTS AND DISCUSSION The masses and energetic quantities involved in representative combustion experiments for 60PCBM and 70PCBM at 303.15 K are shown in Table 1. The internal energy change for the isothermal bomb process at this temperature was calculated as ΔUIBP = AKm, where A was the area of combustion curve and Km was the calorimetric constant of the measurement microbomb. The detailed quantities of mass and energy resulting from each of the combustion calibration experiments to compute km and those concerning the measurements of the standard energy of combustion of the PCBMs at 303.15 K are provided in the Supporting Information. The resulting massic standard energies of combustion, Δcu at T = 303.15 K, for each microcombustion experiment on the 60PCBM and 70PCBM are reported in Table 2. In addition, their respective average values and their associated uncertainties, calculated as the standard deviation of the mean, are also listed. From the massic energies, the respective molar standard energies of combustion, ΔcUm, were derived. These thermochemical
a
60PCBM
70PCBM
m(compound)/mg
1.9646
2.1045
m(Vaseline)/mgb
0.8439
0.9872
m(cotton)/mgc
0.2757
0.2877
A/Jd
-113.758
-125.140
ΔUIBP/Je ΔUign/J f
-114.995 1.335
-126.421 1.275
ΔUW/Jg
0.060
0.066
mΔcu(Vaseline)/J
-39.160
-45.810
mΔcu(cotton)/J
-4.579
-4.778
mΔcu(compound)/J
-69.861
-74.492
Δcu(compound)/(J 3 g-1)
-35560.0
-35396.6
Mass of the methanofullerene derivative. b Mass of vaseline used as a combustion aid. c Mass of cotton used as a fuse. d Area of the calorimetric curve. e Internal energy change for the isothermal bomb process. f Electric energy for the ignition. g Correction to the standard state.
Table 2. Massic Standard Energies of Combustion Resulting from Each Experiment of the Studied PCBMs at T = 303.15 K (po= 0.1 MPa)a Δcu/J g-1 entry
60PCBM
70PCBM
1
-35560.0
-35396.6
2
-35541.0
-35376.8
3
-35532.1
-35394.3
4
-35552.3
-35432.5
5
-35545.5
-35373.7
6
-35544.8
-35391.9
7 8
-35566.2
-35436.6 -35416.9
ÆΔcu/J g-1æ
-35548.8 ( 4.4
-35402.4 ( 8.4
a
The uncertainty associated for each average value is represented by the standard deviation of the mean.
data were related to the ideal combustion reactions C72 H14 O2 ðcrÞ þ 74:5O2 ðgÞ ¼ 72CO2 ðgÞ þ 7H2 OðlÞ
ð1Þ
and C82 H14 O2 ðcrÞ þ 84:5O2 ðgÞ ¼ 82CO2 ðgÞ þ 7H2 OðlÞ
ð2Þ
From eqs 1 and 2, the corresponding standard molar enthalpies of combustion at 303.15 K were calculated using the equation Δc Hm ðnPCBM, 303:15 KÞ ¼ Δc Um ðnPCBM, 303:15 KÞ - 2:5RT
ð3Þ
with n = 60 and 70 for 60PCBM and 70PCBM, respectively. In eq 3, the factor RT was multiplied by the difference of moles in the gas phase of the products and reactant participants in combustion reactions 1 and 2. The standard molar enthalpies of combustion at 298.15 K were derived from the standard enthalpies of combustion at 1542
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303.15 K utilizing the equation Δc Hm ðcr, 298:15 KÞ Z 303:15K ΔCp, m dT ¼ Δc Hm ðcr, 303:15 KÞ 298:15K
ð4Þ
The value of ΔCp,m for the combustion process was calculated from data on the solid-phase molar heat capacity of each PCBM experimentally determined in this work by dsc (see the Supporting Information) and with the molar heat capacities at a constant pressure of 29.387 ( 0.003, 37.220 ( 0.002, and 75.349 ( 0.080 J 3 K-1 3 mol-1 for O2(g), CO2(g), and H2O(l), respectively.12 The corresponding standard molar enthalpies of formation in the condensed state and at 298 K, ΔfHmo(cr), were calculated using eqs 1 and 2 and the standard molar enthalpies of formation of -393.15 ( 0.13 kJ 3 mol-1 for CO2(g), and -285.83 ( 0.04 kJ 3 mol-1 for H2O(l).12 The results of the thermodynamics properties measured and derived from the combustion experiments of 60PCBM and 70PCBM are summarized in Table 3. The dispersions associated with the mean value of each thermochemical quantity represent the overall uncertainty13 and include dispersions from the calibration and the energy of combustion of cotton and vaseline. An analysis of the data in Table 3 showed that the precision in the measurement of the energy of combustion of the PCBMs, which used microcombustion Table 3. Summary of the Thermodynamics Properties,a in kJ 3 mol-1, Measured and Derived for the PCBMs under Investigation 60PCBM
70PCBM
T = 303.15 K ΔcUm (cr) ΔcHm (cr)
-32380.8 ( 10.0 -32387.1 ( 10.0
-36499.4 ( 15.9 -36505.7 ( 15.9
ΔcHm (cr)
-32388.7 ( 10.0
-36507.0 ( 15.9
ΔfHm(cr)
2055.2 ( 13.7
2238.4 ( 19.2
T = 298.15 K
a
The quoted dispersions represent the overall uncertainty.13
techniques and approximately 2 mg samples, was better than 99.94%. On the basis of the differences between the values of standard molar enthalpy of formation in the condensed state reported in Table 3 for each PCBM and the published data of standard molar enthalpy of formation of ΔfHm[C60(cr)] = 2293.1 ( 22.4 kJ 3 mol-1 for C60 and ΔfHm[C70(cr)] = 2537.4 ( 18.7 kJ 3 mol-1 for C70;10b the enthalpic contributions by the functionalization of C60 and C70 were calculated as ΔfuncHm[C60(cr) f 60PCBM(cr)] = -237.9 ( 26.2 kJ 3 mol-1 and ΔfuncHm[C70(cr) f 70PCBM(cr)] = -299.1 ( 26.8 kJ 3 mol -1 . These results showed in both cases a negative enthalpic change by the insertion of a substituent on the sphere of each fullerene. Nevertheless, the enthalpic change produced by the insertion of the addend into C70 to obtain 70PCBM was more exothermic than the same process for C60, resulting for the insertion of an identical functional group, an additional stabilization of 61 kJ 3 mol-1 for 70PCBM with respect to 60PCBM. On the other hand, the enthalpies of reaction, ΔrHm, for the functionalization of C60 and C70 were derived using the model reactions (Scheme 1) reported by Wudl and Janssen for the synthesis of 60PCBM and 70PCBM, respectively.7,9a From the standard enthalpies of formation in the condensed state determined in this work for the PCBM derivatives, the reported data of enthalpy of formation for the fullerenes C60(cr) and C70(cr)10b and the enthalpy of formation of -265.25 kJ 3 mol-1 for 1-phenyl-1-(3-(methoxycarbonyl)propyl)diazomethane, which was estimated also for the condensed state by the groups contribution method,14 the standard enthalpies of reaction were calculated as ΔrHm[C60(cr) f 60PCBM(cr)] = 27.3 kJ 3 mol-1 and Δ r H m [C 70 (cr) f 70PCBM(cr)] = -33.8 kJ 3 mol -1 . Accordingly, the reaction of C70 with 1-phenyl-1-(3-(methoxycarbonyl)propyl)diazomethane to give 70PCBM is energetically favored by ≈61 kJ 3 mol-1 compared to the reaction of C60 to give 60PCBM. Note that these results do not take into account any solvent effect. On the other hand, the theoretical gas-phase enthalpy of formation of 60PCBM and 70PCBM was derived from the theoretically estimated enthalpies of the functionalization reaction of C60 and C70. These values were based on the model reactions
Scheme 1. Model Reactions Utilized to Calculate the Enthalpy of the Functionalization Reaction of the C60 and C70 to Produce 60PCBM and 70PCBM
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reported by Wudl and Janssen described above but with all reactants and products in the gas phase
propyl)diazomethane, which were estimated by the group contribution method,14 the theoretical gas-phase enthalpies of formation of the PCBMs were derived as Δf Hm ½nPCBMðgÞ, 298 K ¼ Δr Hm ½298 K þ Δf Hm ½Cn ðgÞ, 298 K þ Δf Hm ½diazo compoundðgÞ, 298 K ð9Þ
The optimized geometries and selected bond lengths, bond angles, and dihedral angles of 60PCBM and 70PCBM, in the level of theory B3LYP/6-31G*, are included in the Supporting Information. The DFT electronic energies (ε0), thermal correction to enthalpy (Hcorr), and Gibbs free energy (Gcorr) of the molecules participating in reactions 5 and 6 are also given in the Supporting Information. From these values, the theoretical enthalpy of reaction ΔrHm(298 K) and Gibbs free energy of reaction ΔrGm(298 K) were computed as X ðεo þ Hcorr Þproducts Δr Hð298 KÞ ¼ X ðεo þ Hcorr Þreactants ð7Þ X ðεo þ Gcorr Þproducts Δr Gð298 KÞ ¼ X ðεo þ Gcorr Þreactants
ð8Þ
where Hcorr = [HM(298 K) - HX(0 K)] þ ZPE and Gcorr = Hcorr - TS. From the theoretical ΔrHm(298 K) of each PCBM and the experimental gas-phase enthalpies of formation of 2475.1 ( 22.4 kJ 3 mol-1 for C60,10b 2737.4 ( 18.9 kJ 3 mol-1 for C70,10b and -191.1 ( 10.0 kJ 3 mol-1 for 1-phenyl-1-(3-(methoxycarbonyl)-
where n = 60 or 70. The theoretical ΔrHm(g), ΔrGm(g), and ΔfHm (g) of 60PCBM and 70PCBM calculated from eqs 7, 8, and 9, respectively, at various levels of theory are shown in Table 4. The enthalpies of reaction and Gibbs free energies of reaction showed that the gas-phase reaction of C70 with 1-phenyl1-(3-(methoxycarbonyl)propyl)diazomethane to give 70PCBM is slightly more favored than that for C60 to give 60PCBM on the order of approximately 2.5 and 6.3 kJ 3 mol-1, respectively. Both experimental and theoretical results of ΔrHm obtained in this work suggest that in the solid and gas phases, functionalization of C70 is more exothermic than the equivalent process on C60. This finding is also in agreement with the results obtained in a recent theoretical study performed in the framework of the density-functional theory, where it was shown that some of the C70 reaction sites have larger reaction energies than those in C60.16 To compare the theoretical against the experimental enthalpy of formation in gas phase, data on the enthalpy of sublimation for 60PCBM and 70PCBM were necessary. Because no experimental results were available, these quantities were also estimated by the groups contribution method14 as ΔsubHm(298 K) = 286.7 ( 5.7 kJ 3 mol-1 for 60PCBM and ΔsubHm(298 K) = 319.1 ( 6.4 kJ 3 mol-1 for 70PCBM (see Supporting Information). The uncertainty accompanying each of these values was calculated as approximately 2.0% of the estimated heat of sublimation. From the standard molar enthalpy of formation and enthalpy of sublimation, the experimental gas-phase enthalpies of formation for 60PCBM and 70PCBM were calculated to be ΔfHm(g, 298 K) = 2341.9 ( 14.8 kJ 3 mol-1 and ΔfHm(g, 298 K) = 2557.5 ( 20.2 kJ 3 mol-1, respectively. Table 4 shows that the maximal difference between the experimental and theoretical enthalpies of formation is 230 and 186 kJ 3 mol-1 for 60PCBM and 70PCBM, respectively. In order to elucidate the accuracy of the theoretical calculation procedure applied to the fullerene derivatives, enthalpies of formation of C60 and C70 were also estimated at different levels of theory and are shown in Table 4. For these clusters, deviation with respect to the experimental data goes from 58 to 132 kJ mol-1, being minor to that obtained for the PCBMs. Such smaller deviation can be explained considering that theoretical estimation of the enthalpy of formation of C60 requires the wellestablished experimental data of enthalpy of formation of C70 and vice versa (see Supporting Information). In contrast, for the fullerene derivatives the enthalpy of formation of 1-phenyl1-(3-(methoxycarbonyl)propyl)diazomethane required in the theoretical calculation is not available and was estimated by groups contribution method, introducing then some unaccuracy in the theoretical enthalpy of formation in gas phase.
’ CONCLUSION Utilizing a microcalorimetric combustion technique, it was possible to measure the energy of combustion of the fullerene 1544
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Table 4. DFT Enthalpies of Reaction, Gibbs Free Energies of Reaction and Theoretical and Experimental Enthalpies of Formation of 60PCBM and 70PCBM (kJ 3 mol-1) B3LYP/3-21G*
SVWN5/6-31G
B3LYP/6-31G
B3LYP/6-31G*
ΔrHm(g)
-158.9
-150.1
-146.9
ΔrGm(g) ΔsubHm(estimated)a
-138.8
-138.4
-132.4
ΔfHm(g)
2125.1
2133.9
2137.2
2111.8
Δ(exp-theor)c
216.8
208.0
204.7
230.1
C60 f 60PCBM
experimental 60PCBM
-172.3 286.7 ( 5.7a
C70f70PCBM
2341.9 ( 14.8b 70PCBM
ΔrHm(g)
-160.3
-153.1
-149.2
ΔrGm(g)
-153.8
-145.0
-138.4
-174.7 319.1 ( 6.4a
ΔsubHm(estimated)a ΔfHm(g)
2386.0
2393.3
2397.2
2371.7
Δ(exp-theor)c
171.5
164.2
160.3
185.8 C60
C60 ΔfHm(g)
2533.0
2574.7
2588.0
2568.7
Δ(exp-theor)c
-57.9
-99.6
-112.9
-93.6
ΔfHm(g)
2669.9
2621.2
2605.6
2628.2
67.5
116.2
131.8
109.2
2475.1 ( 22.4 C70
C70 Δ(exp-theor)c
2557.5 ( 20.2b
2737.4 ( 18.9
Estimated from groups contribution method as described in ref 14. b ΔfHm(g)(exp) = ΔfHm(cr) (exp) þ ΔsubHm(estimated). c Difference between experimental and theoretical gas phase enthalpy of formation. a
derivatives 60PCBM and 70PCBM with a precision better than 99.94%. This technique also provided reliable values for the standard enthalpy of formation in the condensed state of these functionalized fullerenes as well as the derived enthalpies of reaction and the enthalpic contribution due to the insertion of an identical group on the C60 and C70. Such results indicate a greater exothermic enthalpic change when C70 is functionalized with the same addend as in C60. This functionalization further stabilizes 70PCBM compared to 60PCBM. If the functionalization process were carried out in the gas phase, by considering the enthalpies of sublimation of these PCBMs, the additional stabilization of 70PCBM remains in that phase. Results on the standard enthalpies of reaction of C60 and C70 also indicated that the formation of 70PCBM was favored over 60PCBM. This finding was supported by the enthalpies and Gibbs energies of reaction that were theoretically estimated.
’ EXPERIMENTAL SECTION Substances. The fullerene derivatives 60PCBM (CAS number 160848-21-5) and 70PCBM (CAS number 609771-63-3, mixture of isomers), for which energies of combustion were measured, were obtained from SES Research. The samples of 60PCBM and 70PCBM had a mass fraction purity by HPLC of at least 0.995 and 0.99, respectively. Both samples were additionally characterized by 1H NMR, electrospray and mass spectrometry, as well as UV-vis and IR spectroscopy. The 1H NMR spectrum of 70PCBM showed three signals for the methoxy groups (ratio ≈8:81:11) at δ = 3.48, 3.64, and 3.71, respectively, which indicated the presence of three isomers. These results were in agreement with those reported by Janssen and co-workers.9a The major isomer of 70PCBM was the chiral derivative methyl 5-(30 H-cyclopropa[8,25] (C70-D5h(6) [5,6]fellerenyl)-5-phenylpentanoate17 produced by 1,3-dipolar addition to the most
reactive C(8)-C(25) double bond.16 In minor proportion, there are two achiral stereoisomers of methyl 5-(30 H-cyclopropa[9,10] (C70-D5h(6))[5,6]fellerenyl)-5-phenyl pentanoate.17 Microcombustion Calorimetry. The massic energy of combustion Δcu(cr) of each PCBM derivative was measured with a microbomb set associated with a Calvet-type heat-flux microcalorimeter (C80 from Setaram) operating isothermally at 303.15 K.10 The sensors of the microcalorimeter involved two fluxmeters with a detection limit of 2.0 μW and a calorimetric resolution of 0.1 μW, which are assembled inside a calorimetric block with a temperature control of at least (0.01 K. Design and adaptation of the microbomb set to the microcalorimeter have already been described.18 Before measuring the energy of combustion of the PCBMs, the measurement microbomb was calibrated in energy. The microbomb's calorimetric constant, km, was determined by six combustion experiments with approximately 4 mg of standard reference NIST 39j benzoic acid to yield an average value of km = 1.010883 ( 0.000184. The average electric firing energy required for ignition of the sample was independently determined as 1.335 ( 0.029 J from five experiments. This value was also obtained by the C80 microcalorimeter and was taken into account in both the calculation of the calorimetric constant of the microbomb and the calculation of the energy of combustion of each PCBM. Each sample of the PCBM used in a combustion experiment was pelletized using a Parr pellet press and a 2.38 mm stainless steel punch and die set. Combustion experiments with pellets of approximately 2.0 mg were carried out under 3.04 MPa of oxygen and T = 303.15 K. To promote the total oxidation of the sample, solid Vaseline was used as combustion aid and unburned traces were not detected by visual inspection of the microbomb crucible following the combustion. The standard energy of combustion of Vaseline Δcuo = -46403.7 ( 6.0 J 3 g-1 was measured previously using a macro static-bomb calorimeter.19 The methodology 1545
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The Journal of Physical Chemistry C applied in the flushing to ensure absence of nitrogen and filling with oxygen at 3.04 MPa for the measurement and the reference microbomb as well as the period of thermal stabilization and data acquisition have been described in detail elsewhere.10b In a first step, combustion experiments with both PCBM fullerene derivatives were carried out on the commercial samples without additional purification. The resulting enthalpy of formation of 70PCBM derived from energy of combustion was lower than that of 60PCBM, in disagreement with the known fact that the enthalpy of formation for fullerenes increases with the number of carbon atoms. Scanning experiments from 298.15 to 473.15 K on the samples of 60PCBM and 70PCBM were carried out using a differential scanning calorimeter (PerkinElmer DSC-7). The resulting thermal curve of 70PCBM showed a signal at approximately 423 K, which could have corresponded to residual solvent. Meanwhile, the curve of 60PCBM did not show any signal at that temperature. To eliminate impurities, the 60PCBM and 70PCBM samples were kept in a stove under a residual pressure of ∼1 Pa and T = 453.15 K for 15 h. Additional scanning experiments on the samples after this treatment did not show any thermal signal arising from the presence of an impurity. After this purification procedure, a new set of combustion experiments was carried out on the purified sample of 70PCBM with a resulting energy of combustion and an enthalpy of formation higher than that of 60PCBM. In contrast, the energy of combustion of 60PCBM remained unchanged before and after the drying treatment, suggesting that the purification procedure eliminated solvent residues and this treatment did not affect in any other way the value of the energy of combustion of these fullerene derivatives. The mass of the pellet of the PCBM and all the substances involved in each microcombustion experiment were measured using a Mettler-Toledo UMX2 microbalance, sensitive to 0.1 μg (precision, (0.1 μg). Mass corrections on apparent mass and corrections to the standard state were performed with computer software written in the laboratory based on the Washburn corrections.20 Densities, massic heat capacities, data of (∂U/ ∂P)T of the methanofullerenes, cotton, and Vaseline included in the combustion experiments and the energy of combustion of auxiliary materials are listed in the Supporting Information. All calculations of the molar masses were based on data of atomic masses recommended by the IUPAC.21 Detailed quantities of mass and energy involved in each combustion experiment for the PCBMs at 303.15 K are also given in the Supporting Information.
’ COMPUTATIONAL DETAILS Density functional theory was applied to estimate the gasphase enthalpy of formation of 60PCBM and 70PCBM. These calculations were based on the model reactions reported by Wudl and Janssen for the synthesis of 60PCBM and 70PCBM, respectively, and involved C60 or C70 and 1-phenyl-1-(3(methoxycarbonyl)propyl)diazomethane. The geometry of all compounds implicated was fully optimized using DFT according to Becke's three-parameter gradient-corrected exchange functional and the Lee-Yang-Parr gradient-corrected correlation functional (B3LYP),22 as well as with the Slater exchange functional and the local spin density functional of Vosko, Wilk, and Nusair (SVWN5)23 using the 3-21G*, 6-31G, and 6-31G* basis set. Vibrational frequencies were calculated at the optimum B3LYP/3-21G*, B3LYP/6-31G, and SVWN5/6-31G molecular geometries obtained using the same basis set. Frequencies were
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not computed at the optimum B3LYP/6-31G* because of computational limitations. Nevertheless, the resulting enthalpy of formation obtained from the enthalpy of reaction at the B3LYP/3-21G*, B3LYP/6-31G, and SVWN5/6-31G level of theory did not change significantly, even when Hcorr was not included. These results showed that it was possible to employ energies at T = 0 K rather than standard enthalpies, eliminating the need for the prohibitively expensive computation of zeropoint energies and thermal corrections at the B3LYP/6-31G*. All calculations were carried out with the Gaussian 98 package.24
’ ASSOCIATED CONTENT
bS
Supporting Information. Tables of physical, chemical, and thermochemical properties required in standard state corrections, detailed mass and energy data involved in each calibration and combustion experiment, selected geometric parameters of PCBMs (B3LYP/6-31G*) as well as DFT electronic energies and thermal corrections, and listing of group contributions for estimation of the enthalpy of sublimation of the PCBMs. This material is available free of charge via the Internet at http://pubs. acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Telephone: þ55 5747 3800. Fax: þ55 5747 33 89.
’ ACKNOWLEDGMENT The authors are grateful to CONACYT (Mexico) for financial support through Grant 104299 and a scholarship for M. Martínez-Herrera. ’ REFERENCES (1) (a) Fullerenes: Synthesis, Properties and Chemistry of Large Carbon Cluster; Hammond, G. S., Kuck, V. J., Eds.; American Chemical Society: Washington, DC, 1992. (b) Fullerenes: Chemistry, Physics, and Technology; Kadish, K. M., Ruoff, R. S., Eds.; Wiley InterScience: New York, 2000. € Science (2) Wudl, F.; Suzuki, T.; Li, Q.; Khemani, C.; Almarsson, O. 1991, 254, 1186–1188. (3) (a) Wudl, F. Acc. Chem. Res. 1992, 25, 157–161.(b) Hirsch, A. In The Chemistry of the Fullerenes; Thieme: Stuttgart, 1994.(c) Diederich, F.; Isaacs, L.; Philp, D. Chem. Soc. Rev. 1994, 243–255. (d) Diederich, F.; Thilgen, C. Science 1996, 271, 317–323. (4) (a) Wudl, F.; Sijbesma, R.; Srdanov, G.; Castoro, J. A.; Wilking, Ch.; Friedman, S. H.; Decamp, D. L.; Kenyon, G. L. J. Am. Chem. Soc. 1993, 115, 6510–6512. (b) Hirsch, A. Adv. Mater. 1993, 5, 859–861. (c) Prato, M. J. Mater. Chem. 1997, 7, 1097–1109. (d) Prato, M.; Rose, T. D. Chem. Commun 1999, 663–669. (e) Nakamura, E.; Isobe, H. Acc. Chem. Res. 2003, 36, 807–815. (5) (a) Bingel, C. Chem. Ber. 1993, 126, 1957–1959. (b) Hirsch, A.; Camps, X. J. Chem. Soc., Perkin Trans. 1 1997, 1595–1596. (c) Wang, G. W.; Zhang, T. H.; Lu, P.; Li, Y. J.; Peng, R. F.; Liu, Y. Ch.; Murata, Y.; Komatsu, K. Org. Biomol. Chem. 2004, 2, 1698–1702. (6) (a) Zhou, Z. L. U.S. Pat. Appl. Publ. 2006, 10 pp.(b) Brettreich, M.; Hirsch, A. Tetrahedron Lett. 1998, 39, 2731–2734. (c) Dugan, L. L.; Turetsky, D. M.; Du, Ch.; Lobner, D.; Wheeler, M.; Almli, C. R.; Shen, C. K. F.; Luh, T. Y.; Choi, D. W.; Lin, T. S. Proc. Natl. Acad. Sci. U.S.A. 1997, 94, 9434–9439. (d) Hwang, K. Ch.; Wang, I. Ch.; Tai, L. A.; Lee, D. D.; Kanakamma, P. P.; Shen, C. K. F.; Luh, T. Y.; Cheng, Ch. H. J. Med. Chem. 1999, 42, 4614–4620. 1546
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(7) Wudl, F.; Hummelen, J. C.; Knight, B. W.; Lepeq, F.; Yao, J.; Wilkins, C. L. J. Org. Chem. 1995, 60, 532–538. (8) (a) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789–1791; (b) Hummelen, J. C.; Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.; Fromherz, T. Appl. Phys. Lett. 2001, 78, 841–843; (c) Waldauf, C.; Schillinsky, P.; Perisutti, M.; Hauch, J.; Brabec, C. J. Adv. Mater. 2003, 15, 2084–2088; (d) Wang, X. H.; Hofmann, O.; Das, R.; Barrett, E. M.; Demello, A. J.; Demello, J. C.; Bradley, D. D. C. Lab Chip 2007, 7, 58–63; (e) Frechet, J. M. J.; Thompson, B. C. Angew. Chem. 2008, 120, 62–82; Angew. Chem., Int. Ed. 2008, 47, 58–77. (9) (a) Janssen, R. A. J.; Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.; Hummelen, J. C.; van Hal, P. A. Angew. Chem. 2003, 115, 3493–3497; Angew. Chem., Int. Ed. 2003, 42, 3371–3375. (b) Yang, Y.; Yao, Y.; Shi, Ch.; Li, G.; Shrotriya, V.; Pei, Q. Appl. Phys. Lett. 2006, 89, 153507–3. (c) Anthopoulos, T. D.; de Leeuw, D. M.; Cantatore, E.; van't Hof, P.; Alma, J.; Hummelen, J. C. J. Appl. Phys. 2005, 98, 054503– 054506. (d) Anthopoulos, T. D.; Woebkenberg, P. H.; Bradley, D. D. C.; Kronholm, D.; Hummelen, J. C.; de Leeuw, D. M.; Coelle, M. Synth. Met. 2008, 158, 458–462. (10) (a) Rojas, A.; Martínez, M.; Amador, P.; Torres, L. A. J. Phys. Chem. B 2007, 111, 9031–9035. (b) Rojas-Aguilar, A.; MartínezHerrera, M. Thermochim. Acta 2005, 437, 126–136. (c) Martínez, M.; Rojas, A. J. Phys. Chem. C 2009, 113, 13527–13531. (d) Torres, L. A.; Campos, M.; Martínez, M.; Rojas, A. J. Chem. Thermodyn. 2009, 41, 957–965. (11) (a) Aihara, J.; Sakurai, A. Int. J. Quantum Chem. 1999, 74, 753– 759. (b) Ahihara, J.; Oe, S.; Yoshida, M.; Osawa., E. J. Comput. Chem. 1996, 17, 1387–1394. (c) Wakabayashi, T.; Achiba, Y. Chem. Phys. Lett. 1992, 190, 465–468. (12) Cox, J. D.; Wagman, D. D.; Medvedev, V. A. CODATA Key Values for Thermodynamics; Hemisphere Publishing Corp.; New York, 1989. (13) (a) Bjellerup, L. Acta Chem. Scand. 1961, 15, 121.(b) Olofsson, G. In Experimental Chemical Thermodynamics; Måansson, M., Sunner, S., Eds.; Pergamon: Oxford, 1979; p 137. (14) Domalski, E. S.; Hearing, E. L. J. Phys. Chem. Ref. Data 1993, 22, 805–1159. (15) (a) Hirsch, A.; Lamparth, I.; Gr€osser, T.; Karfunkel, H. R. J. Am. Chem. Soc. 1994, 116, 9385–9386. (b) Djojo, F.; Herzog, A.; Lamparth, I.; Hampel, F.; Hirsch, A. Chem.;Eur. J. 1996, 2, 1537–1547. (b) Hirsch, A.; Lamparth, I.; Schick, G. Liebigs Ann. 1996, 1725–1734. (16) Ueno, Y.; Saito, S. Physica E 2007, 40, 285–288. (17) Powell, W. H.; Cozzi, F.; Moss, G. P.; Thilgen, C.; Hwu, R. J. R.; Yerin, A. Pure Appl. Chem. 2002, 74, 629–695. (18) Rojas-Aguilar, A.; Valdes-Ordo~nez, A. J. Chem. Thermodyn. 2004, 36, 619–626. (19) Rojas-Aguilar, A. J. Chem. Thermodyn. 2002, 34, 1729–1743. (20) Hubbard, W. N.; Scott, D. W.; Waddington, G. In Experimental Thermochemistry; Rossini, F. D., Ed.; Interscience: New York, 1956; pp 75-148. (21) Wieser, M. E. Pure Appl. Chem. 2006, 78, 2051–2066. (22) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785– 789. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (23) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200–1211. (24) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98; Gaussian, Inc.: Pittsburgh, PA, 1998. 1547
dx.doi.org/10.1021/jp108654x |J. Phys. Chem. C 2011, 115, 1541–1547
